Chapter 3 Steam Generators – Thermal Engineering

Chapter 3

Steam Generators

3.1 ❐ INTRODUCTION

A steam generator, popularly known as a boiler, is a closed vessel made of high-quality steel in which steam is generated from water by the application of heat. The water receives heat from the hot gases formed by burning fuel through the heating surface of the boiler. Steam is mainly required for power generation, process heating, and space heating purposes.

3.2 ❐ CLASSIFICATION OF STEAM GENERATORS

Steam generators are classified based on the following:

  1. Water tube and fire tube: In a water tube boiler, water or steam flows through the tubes and heat is supplied to the external surface. Babcock and Wilcox and Stirling boilers fall under this category.

    In a fire tube boiler, hot gases flow through the tubes and water circulates outside the tubes. There may be one large tube surrounded by water as in a Cornish boiler, or two large tubes surrounded by water in a big tube of water as in the case of Lancashire boiler. There may be many smaller tubes through which hot gases pass and are surrounded by water as in the case of vertical Cochran or locomotive boiler.

  2. Natural/forced circulation: Natural circulation of water takes place by natural convection currents produced by application of heat as in the case of Lancashire boiler and Babcock and Wilcox boiler.

    In forced circulation, the fluid is forced ‘once through’ or controlled with partial recirculation as in the case of LaMont boiler, Velox boiler, and Benson boiler.

  3. Horizontal, vertical, or inclined: The disposition on the principal axis of the boiler decides whether the boiler is horizontal, vertical, or inclined.
  4. Single/multiple tube(s): A boiler may have only one fire/water tube or multiple tubes.
  5. Stationary/mobile: Boilers are called stationary (land) or mobile (marine and locomotive). Stationary boilers are used for power plant steam generation. Mobile boilers are portable boilers.
  6. Internally/externally fired: A boiler is said to be an external combustion boiler when combustion takes place outside the region of boiling water. A boiler is said to be an internal combustion boiler if the furnace region is completely surrounded by water-cooled surface as in the case of Lancashire boiler.
  7. Source of heat: Boilers may be classified according to the source from which heat is supplied to water for evaporation. For example, coal fired, oil fired, gas fired, electrical energy, or nuclear energy.
3.3 ❐ COMPARISON OF FIRE TUBE AND WATER TUBE BOILERS

 

Table 3.1 shows the comparison of fire tube and water tube boilers.

 

Table 3.1 Comparison of fire tube and water tube boilers

3.4 ❐ REQUIREMENTS OF A GOOD BOILER

A good boiler should possess the following qualities:

  1. It should be able to generate steam at the required pressure and quantity with minimum fuel consumption.
  2. Its initial cost, installation, maintenance, and operational cost should be as low as possible.
  3. It should occupy a small floor area and should be light in weight.
  4. It should be able to meet the fluctuating demands of steam without fluctuations in steam pressure.
  5. All parts of the boiler should be easily accessible for cleaning, inspection, and maintenance.
  6. It should have minimum joints to avoid leakage of steam.
  7. Its erection time should be reasonable with minimum labour.
  8. The velocities of water and flue gas should be high for better heat transfer with minimum pressure drop through the system.
  9. There should be no deposition of mud and foreign materials inside the surface and soot deposits on the outside surface of heat-transferring parts.
  10. The boiler should conform to safety precautions as laid down in the Boiler Regulations.
3.5 ❐ FACTORS AFFECTING BOILER SELECTION

The factors affecting boiler selection are as follows:

  1. Power to be generated, that is, steam to be raised per hour
  2. Working pressure
  3. Initial capital investment
  4. Facilities available for erection
  5. Availability of floor area
  6. Location of power house
  7. Availability of fuel and water
  8. Probable load factor
  9. Operating and maintenance cost
  10. Accessibility for inspection, cleaning, and maintenance
3.6 ❐ DESCRIPTION OF BOILERS

3.6.1 Fire Tube Boilers

Lancashire Boiler

It is a stationary, fire tube, horizontal straight tube, internally fired, natural circulation boiler. Its normal working pressure range is up to 15 atmospheres and evaporative capacity up to approximately 8000 kg/h. The size varies from 7−9 m in length and 2−3 m in diameter.

The three sectional views of the Lancashire boiler are shown in Fig. 3.1. The gaseous products of combustion formed as a result of burning of fuel on the grate move along the furnace tube or main flue. They are deflected up by the brick wall bridge. After reaching the back of the main flue, they deflect downwards and travel through the bottom flue. The bottom flues are situated below the water shell. These flue gases heat the lower portion of the shell. After travelling from the back to the front, these gases bifurcate into two separate paths in the side flues. They travel from the front to the back again in the main flue from where they discharge to the chimney and atmosphere. The flues are built of ordinary brick-work and lined with refractory material or bricks.

Figure 3.1 Lancashire boiler

Superheater

The superheater used with Lancashire boiler is shown in Fig. 3.2. The arrows show the path of the gases, when the superheater is in commission. 

When superheated steam is to be produced, the dampers are closed and opened so that the flue gases pass on to the superheater flue at the back and travel towards the bottom flue. In this case, valve 2 is closed and valves 3 and 4 are opened so that steam from the shell passes to the superheater and after getting superheated, they are passed to the main pipe 1. If the superheated steam is not required, valve 2 is opened and valves 3 and 4 are closed and the damper to the superheater flue is closed and the other damper is opened, thus bypassing the superheater and steam is passed through valve 2 to main pipe 1.

Figure 3.2 Superheater

Cornish Boiler

In construction and appearance, it is similar to the Lancashire boiler except that here, instead of two flue tubes, only one is used. Its capacity and working pressure range is low. Its shell is usually 4−8 m in length and 1.25−1.75 m in diameter. Its working pressure is about 11.5 bar and can raise steam up to 1350 kg/h.

Locomotive Boiler

The locomotive boiler shown in Fig. 3.3 is an internally fired horizontal multi-tubular portable fire tube boiler. It is specially designed to cope up with sudden variations in demand of steam even at the cost of efficiency. The pressure range is up to about 21 bar and can raise up to 55−70 kg per square metre of heating surface per hour.

Cochran Boiler

The Cochran boiler, shown in Fig. 3.4, is a multi-tubular, internal furnace vertical fire tube boiler having several horizontal fire tubes. Its normal size is about 2.75 m shell diameter and 6 m height of the shell. The steaming capacity is about 3500 kg/h.

The flame and hot gases produced as a result of burning fuel on the grate, rise in the combustion furnace which is dome shaped. The unburnt fuel, if any, will be deflected. The gases are also deflected back and pass through the flue to the fire tubes from where they pass out to smoke box and hence to chimney.

The water circulation is shown by arrows. Water flows down by cooler wall of the shell and rise up past the fire tubes by natural circulation due to convection currents.

Figure 3.3 Locomotive boiler

Figure 3.4 Cochran boiler

3.6.2 Water Tube Boilers

For pressures above 10 bar and capacities in excess of 7000 kg of steam per hour, the water tube boiler is used almost exclusively.

Babcock and Wilcox Boiler

Figure 3.5 shows a stationary type Babcock and Wilcox boiler. It consists of a large number of parallel tubes inclined at an angle which varies from 5° to 15° to the horizontal which connect the uptake header with the downtake header. These are connected to the shell having a substantial quantity of water in it. The uptake header is connected to the shell, through a short tube, whereas a long tube is employed to connect the downtake header with the shell. The coal is fed through the fire hole on to the chain grate stoker. The velocity of the chain is adjusted so as to ensure complete combustion of coal by the time it reaches the other end of the grate. The flue gases first rise up, move down, and rise up again due to the presence of the baffles. The hot water and steam moisture rise up through the uptake header into the boiler shell, where steam separates from water and collects in the steam space. The cold water flows down into the tubes through the downtake header. Hence, a continuous circulation of water is maintained by the convection currents set up.

Superheater

A set of superheater tubes is provided to superheat steam which enters these tubes from the steam space in the boiler shell. The superheated steam can be taken out to the steam stop valve, through the steam pipe. The lower part of the downtake header has a mud box attached to it to collect the sediments. The damper chain controls the quantity of air flowing through the boiler. Two soot doors are provided for internal cleaning of boiler.

Figure 3.5 Babcock and Wilcox boiler

Figure 3.6 Stirling boiler

Stirling Boiler

Figure 3.6 shows the Stirling boiler, which comes in a water tube type in which three steam drums are connected together by the banks of bent water tubes. These tubes stand nearly in a vertical position near the rear end while they slope steeply over the fire at the front end. The baffles deflect the products of combustion from the furnace over the banks of tubes into the mud drum. The steam generated collects in the steam spaces of the three drums. The steam can be raised at a working pressure of 60 bar with the temperature as high as 450°C. An evaporation capacity of 50,000 kg/h has been achieved with a Stirling boiler.

3.7 ❐ HIGH PRESSURE BOILERS

In all modern power plants, boilers raising steam at pressures greater than 100 bar are universally used. These are called high pressure boilers. They offer the following advantages:

  1. The efficiency and the capacity of the plant can be increased as reduced quantity of steam is required for the same power generation if high pressure steam is used.
  2. The forced circulation of water through the boiler tubes provides freedom in the arrangement of furnace and water walls in addition to the reduction in the heat exchange area.
  3. The tendency of scale formation is reduced due to high velocity of water.
  4. The danger of overheating is reduced as all the parts are uniformly heated.
  5. The differential expansion is reduced due to uniform temperature and this reduces the possibility of gas and air leakages.

3.7.1 Boiler Circulation

There are four types of boiler circulation as follows.

  1. Natural circulation: Water circulates naturally in a boiler when its density in one part of the circuit is less than that in another part at the same level. The boiler drums, tubes, and water wall make up the multi-passage circuit. During steam formation in a simple drum, the water near the sides of the drum gets heated and forms steam bubbles, lowering its density. The water in the drum centre being heavier displaces the lighter water-bubble mix and, in turn, gets heated up. This sets up a constant circulation that releases steam into the upper drum region.

    Similarly, in a water tube circuit, the water from the drum flows through the downcomer tube to the bottom of the heated riser. Heat forms steam bubbles in water passing through the riser, lowering the density of the mixture. The heavier downcomer water displaces the riser mixer to establish continuous flow. The force of gravity to produce flow in a natural circulation comes from the difference between the densities of the fluids in the downcomer and riser portions of the circuit. The natural circulation is depicted in Fig. 3.7(a).

  2. Forced circulation: Forced circulation in various circuits of boiler units is produced by mechanical pumps as depicted in Fig. 3.7(b).
  3. Once-through forced circulation: This type of circulation, as depicted in Fig. 3.7(c), receives water from the feed supply, pumping it to the inlet of heat-absorbing circuit. Fluid heating and steam generation take place along the length of the circuit until evaporation is completed. Further process through the heated circuits results in superheating the vapour.
  4. Once-through with recirculation (forced): The ‘recirculating’ forced-circulating type unit has water supplied to the heat absorbing circuits through a separate circulating pump. The water pumped is considerably in excess of steam produced and requires a steam-and-water drum for steam generation. This type of system is depicted in Fig. 3.7(d).

3.7.2 Advantages of Forced Circulation Boilers

The advantages of forced circulation boilers are as follows:

  1. Smaller bore and therefore, lighter tubes
  2. Absence from scaling troubles due to high circulation velocity
  3. Lighter for a given output
  4. Steam can be raised quickly and load fluctuations met rapidly
  5. Uniform heating of all parts eliminates the danger of overheating
  6. Greater flexibility in layout of boiler parts
  7. Boiler can be operated at desired conditions
  8. Boiler can be started rapidly

Figure 3.7 Types of boiler circulation: (a) natural circulation, (b) forced circulation, (c) once-through, (d) once-through with recirculation (force)

3.7.3 LaMont Boiler

This is a forced circulation boiler whose arrangement of water circulation and different components are shown in Fig. 3.8. Small diameter tubes are used in the evaporating section which gives flexibility in placing the heat transfer surfaces. The drum may be placed well away from the furnace. These boilers have been built to generate 45−50 tonnes of superheated steam per hour at a pressure of 150 bar, 500°C.

The feed water from the hot well is supplied to the boiler through the economiser. A pump circulates the water through the evaporator tubes and a part of the vapour is separated in the separator drum. The centrifugal pump delivers the water to the headers at a pressure above the drum pressure which distributes the water through the nozzle into the evaporator. The steam separated in the boiler is further passed through the superheater and then supplied to the prime mover.

Figure 3.8 LaMont boiler

3.7.4 Benson Boiler

The major drawback in the LaMont boiler is the formation and sticking of bubbles in the inner surface of the heating tubes. These attached bubbles reduce the heat flow and steam generation as it offers higher thermal resistance as compared to water film. If the boiler pressure is raised to critical pressure (221.2 bar), the steam and water would have same density, and therefore, the danger of bubble formation can be completely eliminated.

The arrangement of the boiler components is shown in Fig. 3.9. The feed water is passed through the economiser and the radiant evaporator where a major part of water is converted into steam. The remaining water is evaporated by hot gases in the convective evaporator. The supercritical pressure steam is then passed through the superheater after which it is supplied to the prime mover.

The maximum pressure and temperature of steam obtained from this boiler are 500 bar and 650°C, respectively, and generating capacity of 150 tonnes/h. The major difficulty in the operation of this boiler is the deposition of salt and sediments on the inner surface of water tubes.

3.7.5 Loeffler Boiler

The major difficulty of Benson boiler is avoided in the Loeffler boiler by preventing the flow of water into the boiler tubes. Most of the steam is generated outside from the feed water by using a part of the superheated steam coming out from the boiler. The arrangement of different components and steam circulations is shown in Fig. 3.10.

The pressure feed pump draws the water through the economiser and delivers to the evaporator drum. About 65% of the steam coming out of superheater is passed through the evaporator drum to evaporate the feed water coming from the economiser. This steam is drawn by the steam circulating pump and passed through the radiant superheater and then convective superheater.

Figure 3.9 Benson boiler

Figure 3.10 Loeffler boiler

Loeffler boilers are available with generating capacity of 94.5 tonnes/h and operating at 140 bar. This boiler can handle high drum salt concentration without trouble.

3.7.6 Schmidt-Hartmann Boiler

The arrangement of this boiler is shown in Fig. 3.11. In the primary circuit, a feed pump supplies water to drum through the economiser, which, in turn, discharges saturated steam to a convective superheater and then to load. A closed secondary circuit heated by furnace resembles a sectional header boiler. This section supplies the heated vapour in a coil to evaporate the main feed water. It has a tube evaporating section. These boilers are built for pressure ranging from 35−125 bar.

3.7.7 Velox Boiler

The arrangement of a Velox boiler is shown in Fig. 3.12. The pressurised combustion chamber (furnace) of this boiler uses low excess air and has high heat transfer rates. The air for the furnace is supplied by the air compressor. After heating water and steam, the combustion gases pass through a gas turbine to atmosphere. This gas turbine drives the axial flow compressor that raises incoming combustion air from atmosphere to the furnace pressure. The other important components are feed pump, economiser, steam separating section, water circulating pump, and convective superheater. This boiler is built for pressure ranging from 70−80 bar and 500°C.

Figure 3.11 Schmidt-Hartmann boiler

Figure 3.12 Velox boiler

3.7.8 Once-through Boiler

The arrangement of this boiler is shown in Fig. 3.13. It has inclined coils arranged in a spiral. Forty coils are paralleled around the furnace. Steam generated in the headers flows into headers and then to the convective superheater. Other essential components are the feed pump and economiser.

3.8 ❐ CIRCULATION

The flow of water and steam within the boiler circuit is called circulation. The insulated downcomers carry water from the steam drum to the header and are located outside the furnace. The risers are located inside the furnace and carry steam to the steam drum from the header. Risers installed all around the four-walls of the furnace carry away the heat from the furnace. A simple downcomer−riser circuit connecting a drum and a header is shown in Fig. 3.14.

Downcomers are fewer in number and bigger in diameter, whereas risers are more in number and smaller in diameter. Downcomers are meant to make the water fall by gravity. Bigger the diameter, lesser the pressure drop due to friction, since pressure drop is inversely proportional to the tube diameter. The pressure drop is given by

where D = diameter of downcomer, L = length of downcomer, ρ = density of water, v = velocity of water, f = Darcy’s friction factor. So, the downcomers are made bigger in diameter.

Risers absorb heat from the furnace. For the same total cross-sectional area, the smaller the diameter, the larger the surface area exposed to hot gas for heat transfer. Therefore, the risers are of smaller diameter, as compared to that of downcomers, and more in number.

Figure 3.13 Once-through boiler

Figure 3.14 Downcomer-riser circuit

3.9 ❐ STEAM DRUM

The purposes of drum used in boiler are as follows:

  1. To store water and steam sufficiently to meet varying load requirements.
  2. To aid in circulation.
  3. To separate vapour or steam from water-steam mixture, discharged by the risers.
  4. To provide enough surface area for liquid vapour disengagement.
  5. To maintain a certain desired ppm in the drum by phosphate injection and blow down.

A mixture of water, steam, foam and sludge is discharged into the drum by the risers. Steam must be separated from the mixture before it leaves the drum. Any moisture carried with steam to the superheater tubes contains dissolved salts. In the superheater, water evaporates and the salts remain deposited on the inside surface of the tubes to form a scale, which is difficult to remove. This scale reduces the rate of heat absorption which ultimately leads to the failure of the superheater tubes by overheating and rupture. The superheater tubes are exposed to the highest steam pressure and temperature on the inside and the maximum gas temperature on the outside. The superheater tubes are made of very costly material and utmost care should be exercised so that no damage is done to them by the excessive moisture in steam. Some of the impurities in steam may be vaporised silica, which may cause turbine blade deposits.

No vapour bubbles should flow along with saturated water from the drum to the downcomers. This will reduce the density difference and the pressure head for natural circulation. The bubbles tending to flow upward may also impede the flow in the downcomer and thus affect circulation. The drum has to secure moisture-free steam going to superheater and bubble-free water going to the downcomers as shown in Fig. 3.15.

Figure 3.15 Functions of steam drum

3.9.1 Mechanism of Separation of Moisture in Drum

The separation of steam from steam-water mixture can be done by the following methods:

  1. Gravity separation: At low pressures up to 20 bar, gravity separation is used if sufficient disengaging surface is provided, as shown in Fig. 3.16(a). Water separates out by gravity from steam-water mixture, being heavier. But at high pressure, separating force due to gravity is low and it is difficult to achieve adequate separation, and there is considerable moisture carried with steam.

    Figure 3.16 Steam drum separation: (a) Gravity separator, (b) Baffle and screen, (c) Cyclone and scrubber

  2. Mechanical separators
    1. Baffles and screens: Baffle plates shown in Fig. 3.16(b) act as primary separators. They change or reverse the steam flow direction, thus assisting gravity separation, and act as impact plates that cause water to drain off. Screens made of wire mesh act as secondary separators where the individual wires attract and intercept the fine droplets. The accumulating water drops then fall by gravity back to the main body of water.
    2. Cyclone separators: These separators shown in Fig. 3.16(c) utilises the centrifugal forces for separation of two-phase mixture, which is entered tangentially to direct the water downward and to make the steam flow upward. The steam then goes through the zigzag path in corrugated plates, called the scrubber or dryer, on the way out to help remove the last traces of moisture. Finally, perforated plates or screens under the drum exit provide the final drying action.

The internal details of a controlled circulation boiler are shown in Fig. 3.17.

Figure 3.17 Drum internals of a controlled circulation boiler

3.10 ❐ FLUIDISED BED BOILER

Fluidised bed boilers produce steam from fossil and waste fuels by using a technique called fluidised bed combustion (FBC).

3.10.1 Bubbling Fluidised Bed Boiler (BFBB)

A schematic diagram of BFBB is shown in Fig. 3.18. In this boiler, crushed coal (6−20 mm) is injected into the fluidised bed of limestone just above an air-distribution grid at the bottom of the bed.

The air flows upwards through the grid from the air plenum into the bed, where combustion of coal occurs. The products of combustion leaving the bed contain a large proportion of unburnt carbon particles which are collected in cyclone separator and fed back to the bed. The boiler water tubes are located in the furnace.

Since most of the sulphur in coal is retained in the bed by the bed material used (limestone), the gases can be cooled to a lower temperature before leaving the stack with less formation of acid (H2SO4). As a result of low combustion temperatures (800−900°C), inferior grades of coal can be used without slagging problem and there is less formation of NOx. Cheaper alloy materials can also be used, resulting in economy of construction. Further economies are achieved since no pulveriser is required. The volumetric heat release rates are 10 to 15 times higher and the surface heat transfer rates are 2 to 3 times higher than a conventional boiler. This makes the boiler more compact.

Figure 3.18 Bubbling fluidised bed boiler

3.10.2 Advantages of BFBB

  1. The unit size and hence capital cost are reduced due to better heat transfer.
  2. It can respond rapidly to changes in load demand.
  3. Low combustion temperatures (800−950°C) restricts the formation of NOx pollutants.
  4. Fouling and corrosion of tubes is reduced considerably due to low combustion temperatures.
  5. Cost of coal to fine grind is reduced as it is not essential to grind the coal very fine.
  6. Low grade fuels and high-sulphur coal be used.
  7. Fossil and waste fuels can be used.
  8. Combustion temperature can be controlled accurately.
3.11 ❐ BOILER MOUNTINGS

Different fittings and devices necessary for the operation and safety of a boiler are called boiler mountings. The various boiler mountings are as follows:

  1. Water- level indicator
  2. Pressure gauge
  3. Steam stop valve
  4. Feed check valve
  5. Blow-down cock
  6. Fusible plug
  7. Safety valve: spring loaded, dead weight, lever type
  8. High steam and low water safety valve.

3.11.1 Water Level Indicator

The water-level indicator indicates the level of water in the boiler constantly. Every boiler is normally fitted with two water-level indicators at its front end. Figure 3.19 shows a water-level indicator used in low pressure boilers. It consists of three cocks and a glass tube. The steam cock D keeps the glass tube in connection with the steam space and cock E puts the glass tube in connection with the water space in the boiler. The drain cock K is used to drain out the water from the glass tube at intervals to ascertain that the steam and water cocks are clear in operation. The glass is generally protected with a shield.

For the observation of water level in the boiler, the steam and water cocks are opened and the drain cock is closed. The rectangular passage at the ends of the glass tube contains two balls. If the glass tube is broken, the balls are carried along its passage to the ends of the glass tube and flow of water and steam out of the boiler is prevented.

3.11.2 Pressure Gauge

The pressure gauge is used to indicate the steam pressure of the boiler. The gauge is normally mounted in the front top of the steam drum. The commonly used pressure gauge is the Bourdon type pressure gauge shown in Fig. 3.20. It consists of an elastic metallic tube of elliptical cross-section bent in the form of circular arc. One end of the tube is fixed and connected to the steam of the boiler and the other end is connected to a sector wheel through a link. The sector remains in mesh with a pinion fixed on a spindle. A pointer is attached to the spindle to read the pressure on a dial gauge.

Figure 3.19 Water-level indicator

Figure 3.20 Pressure gauge

When high pressure steam enters the elliptical tube, the tube section tends to become circular, which causes the other end of the tube to move outward. The movement of the closed end of the tube is transmitted and magnified by the link and sector. The sector is hinged at a point on the link. The magnitude of the movement is indicated by the pointer on the dial.

3.11.3 Steam Stop Valve

The function of the stop valve is to regulate the flow of steam from the boiler to the prime mover as per requirement and shut off the steam flow when not required.

A commonly used steam stop valve is shown in Figure 3.21. It consists of a main body, valve, valve seat, nut, and spindle, which pass through a gland to prevent leakage of steam. The spindle is rotated by means of a hand wheel to close or open.

3.11.4 Feed Check Valve

The function of the feed check valve is to allow the supply of water to the boiler at high pressure continuously and to prevent the back flow of water from the boiler when the pump pressure is less than boiler pressure or when the pump fails.

A commonly used feed check valve is shown in Fig. 3.22. It is fitted to the shell slightly below the normal water level of the boiler. The lift of the non-return valve is regulated by the end position of the spindle which is attached to the hand wheel. The spindle can be moved up or down with the help of hand wheel which is screwed to the spindle by a nut. Under normal conditions, the non-return valve is lifted due to the water pressure from the pump and water is fed to the boiler. If the case pump pressure falls below boiler pressure, the valve is closed automatically.

Figure 3.21 Steam stop valve

Figure 3.22 Feed check valve

3.11.5 Blow-Down Cock

The function of the blow-down cock is to remove sludge or sediments collected at the bottommost point in the water space in a boiler, while the boiler is steaming. It is also used for complete draining of the boiler for maintenance.

A commonly used type of blow-down cock is shown in Fig. 3.23. It consists of a conical plug fitted accurately into a similar casing. The plug has a rectangular opening which may be brought into the line of the passage of the casing by rotating the plug. This causes the water to be discharged from the boiler. The discharge of water may be stopped by rotating the plug again. The blow-down cock should be opened when the boiler is in operation for quick forcing out of sediments.

3.11.6 Fusible Plug

The main function of the fusible plug is to put off the fire in the furnace of the boiler when the water level in the boiler falls below an unsafe level and thus avoid the explosion which may occur due to overheating of the tubes and shell. The plug is generally fitted over the crown of the furnace or over the combustion chamber.

A commonly used fusible plug is shown in Fig. 3.24. It consists of a hollow gun metal body screwed into the fire box crown. The body has a hexagonal flange to tighten it into the shell. A gun metal plug is screwed into the gun metal body by tightening the hexagonal flange formed into it. There is yet another solid plug made of copper with conical top and rounded bottom. The fusible metal holds this conical copper plug and the gun metal plug together due to depressions provided at the mating surfaces.

During normal operation, the fusible plug remains submerged in water and on no account, its temperature can be more than the saturation temperature of water.

The fusible metal is protected from direct contact with water or furnace gases. When the water level in the boiler shell falls below the top of the plug, the fusible metal melts due to overheating. Thus, the copper plug drops down and is held within the gun-metal body by the ribs. The steam space gets communicated to the fire box and extinguishes the fire. The fusible plug can be put into position again by interposing the fusible metal.

Figure 3.23 Blow-down cock

Figure 3.24 Fusible plug

3.11.7 Safety Valves

The function of a safety valve is to prevent the steam pressure in the boiler exceeding the desired rated pressure by automatically opening and discharging steam to atmosphere till the pressure falls back to normal rated value. There are three types of safety valves—spring loaded (Ramsbottom) type, dead weight type, and lever type.

  1. Spring-loaded safety valve: The spring-loaded safety valve of the Ramsbottom type is shown in Fig. 3.25. The spring holds the two valves on their seats by pulling the lever down. The lever is provided with two conical pivots, one integrally forged with the lever and the other pin connected to one end. The upper end of the spring is hooked to the lever midway between the two pivots. The lower end is hooked to the shackle fixed to the valve chest by studs and nuts. The shackle and the lever are also connected by two links, one end of which is pin-jointed and the other end has a slot cut into it to allow for the pin to slide in it vertically, thus allowing the lever to be lifted and retaining the link connection. These links are provided as a safety measure in case the spring breaks, the lever should not fly off. The lever projects on one side for manual operation to check the satisfactory working of the device.
  2. Dead weight safety valve: In this type of device, as shown in Fig. 3.26, the steam pressure in the upward direction is balanced by the downward force of dead weights acting on the valve. It is generally used on low capacity boilers like the Lancashire boiler. The bottom flange is directly connected to seating block on the boiler shell.
  3. Lever safety valve: A typical lever safety valve is shown in Fig. 3.27. The lever is the second kind with effort in the middle of fulcrum and load. It is suitable for stationary boilers.

Figure 3.25 Spring loaded safety valve

Figure 3.26 Dead weight safety valve

Figure 3.27 Lever safety valve

3.11.8 High Steam and Low Water Safety Valve

The functions of water safety valve are of two, namely to blow out if the steam pressure is higher than the working pressure and to blow out steam when the water level in the boiler is low. It is perhaps the most important mounting on the boiler. Figure 3.28 shows the details of Hopkinson’s high steam and low water safety valve.

Figure 3.28 High steam and low water safety valve

F1 is the fulcrum, W the weight suspended on one end of the lever, and W is the adjustable balance weight on the other end, which can be fixed in position on the lever 2 by a set screw. A predetermined force is exerted through the strut 1 on the outer valve.

The arrangement of the inner valve with spindle 4 and a dead weight 8 acts as dead weight safety valve. However, the load on the outer valve is both due to dead weight and strut thrust. If the pressure in the boiler exceeds the limit, the valve lifts and the steam escapes to waste.

On one end 9, of lever 6−9 with fulcrum F2, is attached a float 10 and on the other end is fixed a balance weight 7. When the float 10 is submerged in water, the lever is balanced about fulcrum F2. As soon as the float is uncovered, it gets unbalanced and tilts the lever to the right. This lever has a projecting knife edge which is clear of the collar 5 when the float is submerged. As soon as it tilts towards right, it establishes contact with the collar and pushes the spindle 4 up, thus raising the inner valve from its seat. Thus steam starts escaping out through the waste pipe. A drain connection 11 is provided to drain off the condensed steam.

3.12 ❐ BOILER ACCESSORIES

These are the appliances installed to increase the overall efficiency of the steam power plant. The various accessories installed are pressure reducing valve, steam trap, steam separator, economiser, air preheater, superheater, feed pump, and injector.

The functions of the various accessories are as follows:

  1. A pressure reducing valve maintains constant pressure on its delivery side with fluctuating boiler pressure.
  2. The function of steam trap is to drain of water resulting from the partial condensation of steam from steam pipes and jackets without allowing steam to escape through it.
  3. The function of the steam separator is to separate suspended water particles carried by steam on its way from the boiler to the prime mover.
  4. The function of an economiser is to recover some of the heat carried away in the flue gases up the chimney and utilise for heating the feed water to the boiler.
  5. An air preheater recovers some portion of the waste heat of the flue gases by preheating the air supplied to the combustion chamber.
  6. Superheaters are used to increase the temperature of steam above its saturation temperature.
  7. The function of a feed pump is to pump water to the water space of the boiler.
  8. The function of an injector is to feed water to the boiler with the help of a steam jet.

We shall discuss the air preheater, economiser, and superheater in detail.

3.12.1 Air Preheater

In the air preheater, air is heated by the heat carried away by the flue gases and goes as a waste through the chimney. It is situated between the economiser and the chimney. Air preheaters are of three types—tubular, plate type, and regenerative.

In the tubular type of air preheater, shown in Fig. 3.29, the air passes down outside the tubes and the flue gases through the tubes before going to the induced draught fan at the base of the chimney. Baffles are provided across the tubes to make the air follow a zigzag path a number of times to utilise more heat of flue gases.

In the plate type of air preheater, there are alternate chambers provided by the plate surfaces for the flow of flue gases upwards to the induced draught fan and for the flow of combustion air down of combustion air downwards.

In the regenerative type, the air preheater chamber is divided into two rotating compartments. The heating surfaces are made up of a series of corrugated sheets between which the hot flue gases pass upwards on one side and the combustion air passes down on the other side. Due to the rotation of pre-heater at a very low speed, the heater surface comes in contact with combustion air and the cooled surface is again heated by the flue gases. This alternate heating and cooling of heating surface is called regenerative heating and is more effective than other types.

Figure 3.29 Tubular air preheater

3.12.2 Economiser

The Green’s economiser shown in Fig. 3.30 is used for boilers of a medium pressure range up to about 25 bar. It consists of groups of vertical cast iron pipes fitted at their two ends to cast iron boxes, at the top and bottom. All the tubes are enclosed within the brick work of the economiser. There are two pipes, C and D, outside the brick work E. Rows of top and bottom boxes are connected to these top and bottom pipes. The feed water is pumped to the bottom pipe D. From here, it goes to the bottom boxes and rise up in the groups of vertical pipes into the top boxes. From the top boxes, it goes to the pipe C from where it passes to the water space of the boiler through the check valve. F is a stop valve for entry and G is a stop valve for exit of the water from the economiser. S is a safety valve. These tubes are continuously scraped by scrapers J to remove soot collected on them due to the passage of flue gases through the economiser. A pair of scrapers is connected by a chain passing over a pulley such that one scraper comes down and the other attached to the same chain goes up. L is the soot chamber where soot is collected and removed through a soot door.

Figure 3.30 Economiser

3.12.3 Superheater

The superheater commonly used in a Lancashire boiler is shown in Fig. 3.30. It consists of two headers and a set of superheater tubes made of high-quality steel in the form of U-tube. It is located in the path of furnace gases just before the gases enter the bottom flue. The amount of hot gases passed over the superheater tubes should be in proportion to the amount of superheated steam passing through the tubes. Otherwise, the tubes would be over heated. To avoid this, the hot gases are diverted as shown in Fig. 3.31. The superheater is put out of action by turning the damper upward to the vertical position. In this position of the damper, the gases coming out from the central flue pass directly into the bottom flue without passing over the superheater tubes.

For getting superheated steam, the valves A and B are opened and valve C is closed. The damper is kept open as per the quantity of steam flowing through the pipe. For this position, the flow direction of steam is shown in Fig. 3.31. If wet steam is required, valves A and B and the gas damper are kept closed, and valve C is kept open. In this case, steam comes directly from the boiler through valve C. By adjusting the gas damper, the temperature of steam coming out of superheater is always maintained constant, irrespective of the amount of steam passing through the superheater.

Figure 3.31 Superheater

3.13 ❐ STEAM ACCUMULATORS

Boilers are not always required to supply steam for economic or rated load but also required to meet the fluctuating demand of load, that is, peak fractional load. The boiler takes an appreciable time to change the rate of steam generation but the demand for a considerable change of power may occur instantly if the boiler could meet such a sudden increase in load; there is always the problem of disposing the steam once the peak has been passed. Moreover, in meeting the fluctuating load, the capacity and efficiency of boiler are reduced. To meet these requirements, it is a general practice to incorporate steam accumulators to equalise the load on the boilers. Thus, the steam accumulators acting as a thermal flywheel store energy during periods when the output of the boiler exceeds the demand and restores or supplies back the energy when the demand is more than the output of the boiler.

The steam accumulators are of two types, namely variable pressure system (Ruth’s accumulator) and constant pressure system (Kiesselbach accumulator).

3.13.1 Variable Pressure Accumulator

This accumulator (Fig. 3.32) was first introduced by Dr Ruth, a Swedish technician, in 1916. It consists of a steel shell filled with about 99% water under low pressure required by the low turbine or process work.

During charging, high pressure steam from the boiler enters the pipe C through the value V1. Under this condition, the non-return valve provided in pipe C is closed and the high pressure steam enters into the shell through the non-return valve B. Specially shaped nozzles surrounded by pipes are employed to discharge the steam silently into the water and thereby bring about condensation. This increases the pressure in the accumulator. The more the pressure rises in the accumulator, the greater becomes the thermal storage capacity.

Figure 3.32 Ruth’s accumulator

During the discharging process (the demand of steam is more than output of boiler), the pressure in the pipe C falls below the pressure of water in the vessel when the low pressure mains valve is in communication with the industrial plant. In such circumstances, the pressure in the accumulator is also lowered, whereas the temperature remains same as before. This causes evaporation of water into steam, thereby increasing the pressure and reducing temperature until equilibrium condition between temperature and pressure is again attained. The non-return valve B is closed and steam passes to the low pressure turbine or industrial plant through the dome E and the non-return valve A. When the output of the steam equals the consumption of steam, there is no flow into and out of the accumulator.

Ruth’s accumulator gives a large discharge over a short period and a small discharge over a large period. This accumulator is widely used in mines and steel works where the reciprocating steam engines direct their intermittent exhausts into a low pressure receiver which is placed in series with accumulators. Low pressure turbines consume the steam supply from the accumulators.

The main advantages of this system are as follows:

  1. Increases the efficiency of boiler
  2. Increase in rate of evaporation, less repairs, and economy in fuel
  3. Rapid pick up
  4. Improvement in boiler load factor

3.13.2 Constant Pressure Accumulator

Figure 3.33 shows a constant pressure accumulator. This is used in conjunction with small water tube boilers, it stores heat in water during charging period (i.e., when the demand is less) but delivers water (not steam) during discharge period (i.e., when the demand is more than out-put). Therefore, this is essentially a feed-water accumulator.

In this system, the rate of firing of the boiler is maintained constant but the quantity of feed is varied. When there is fall in the demand for steam, an extra quantity of feed water is circulated through the boiler where the feed-water temperature rises to that of steam, and after that the surplus water is passed into the accumulator.

Figure 3.33 Kiesselbach accumulator

3.14 ❐ PERFORMANCE OF STEAM GENERATOR

3.14.1 Evaporation Rate

The quantity of water evaporated into steam per hour is called the evaporation rate. It is expressed as kg of steam/h, kg of steam/h/m2 of heating surface, kg of steam/h/m3 of furnace volume, or kg of steam/kg of fuel fired.

Equivalent Evaporation

It is the equivalent of the evaporation of 1 kg of water at 100°C to steam at 100°C. It requires 2257 kJ.

Factor of Evaporation

The ratio of actual heat absorption above feed-water temperature for transformation to steam (wet, dry, or superheated) to the latent heat of steam at atmospheric pressure (1.01325 bar) is known as a factor of evaporation.

Let h = specific enthalpy of steam actually produced

hf = specific enthalpy of feed water

hf (atm) = specific enthalpy of evaporation at standard atmospheric pressure

ms = actual evaporation expressed in kg/kg of fuel or kg/h of steam

me = equivalent evaporation expressed in kg/kg of fuel or kg/h

Then, the equivalent evaporation = actual evaporation × factor of evaporation

where F = factor of evaporation

h1 = hf1 + xhfg1 for wet steam actually generated

= hg1 for saturated steam actually generated

= hsup for superheated steam actually generated.

3.14.2 Performance

The performance of boiler may be explained on the basis of any of the following terms:

  1. Efficiency: It may be expressed as the ratio of heat output to heat input.
  2. Combustion rate: It is the rate of burning of fuel in kg/m3 of grate area/h.
  3. Combustion space: It is the furnace volume in m3/kg of fuel fired/h.
  4. Heat absorption: It is the equivalent evaporation from and at 100°C in kg of steam generated/m2 of heating surface.
  5. Heat liberated: It is the heat liberated/m3 of furnace volume/h.

3.14.3 Boiler Thermal Efficiency

It is the ratio of heat absorbed by steam from the boiler per unit time to the heat liberated by the combustion of fuel in the furnace during the same time.

where s = mass of steam generated in kg/h

f = mass of fuel burned in kg/h

C.V. = calorific value of fuel in kJ/kg

3.14.4 Heat Losses in a Boiler Plant

  1. Heat used to generate steam
    Q1 = ms (hhf1)
  2. Heat lost to flue gases: The flue gases contain dry products of combustion and the steam generated due to the combustion of hydrogen in the fuel.

    Heat lost to dry flue gases,

    where mg = mass of gases formed per kg of fuel

    cpg = specific heat of gases

    Tg = temperature of gases, °C

    Ta = temperature of air entering the combustion chamber of the boiler, °C

  3. Heat carried by steam in flue gases

    where ms1 = mass of steam formed per kg of fuel due to combustion of H2 in fuel

    hf1 = enthalpy of water at boiler house temperature

    hs1 = enthalpy of steam at the gas temperature and at the partial pressure of the vapour in the flue gas

  4. Heat loss due to incomplete combustion: If carbon burns to CO instead of CO2, then it is known as incomplete combustion. 1 kg of C releases 10,200 kJ/kg of heat if it burns to CO, whereas it releases 35,000 kJ/kg if it burns to CO2. If the percentages of CO and CO2 in flue gases by volume are known, then

    where CO, CO2 = % by volume of CO and CO2, respectively, in flue gases

    C = fraction of carbon in 1 kg of fuel

    Heat lost due to incomplete combustion of carbon per kg of fuel,

     

  5. Heat lost due to unburnt fuel
    Q5 = mf1 × C.V.

    where mf1 = mass of unburnt fuel per kg of fuel burnt.

  6. Convection and radiation losses: Heat unaccounted for due to convection and radiation losses,

    where Q = mf × C.V.

    = heat released per kg of fuel

3.14.5 Boiler Trial and Heat Balance Sheet

There are three purposes of conducting the boiler trial.

  1. To determine and check the specified generating capacity of the boiler when working at full load conditions.
  2. To determine the thermal efficiency of the plant.
  3. To draw up the heat balance sheet so that suitable corrective measures may be taken to improve the efficiency.

The following measurements should be observed during the boiler trial.

  1. The fuel supplied and its analysis.
  2. Steam generated and its quality or superheat.
  3. Flue gases formed from exhaust analysis.
  4. Air inlet temperature and exhaust gases temperature.
  5. Volumetric analysis of exhaust gases.
  6. Mass of fuel left unburnt in ash.
  7. Feed-water temperature.

The heat balance sheet is a systematic representation of heat released from burning of fuel and heat distribution on minute, hour or per kg of fuel basis. A pro forma for heat balance sheet is given in Table 3.2.

 

Table 3.2 Heat balance sheet for boiler

Example 3.1

A boiler generates 4000 kg/h of steam at 20 bar, 400°C. The feed-water temperature is 50°C. The efficiency of the boiler is 80%. The calorific value of fuel used is 44,500 kJ/kg. The steam generated is supplied to a turbine developing 450 kW and exhausting at 2 bar with dryness fraction 0.96. Calculate the fuel burnt per hour and turbine efficiency. Also find the energy available in exhaust steam above 50°C.

Solution

From steam tables at 20 bar, 400°C, h1 = 3245.5 kJ/kg

Enthalpy of feed water at 50°C, hf = 209.3 kJ/kg

Fuel used per hour =

Enthalpy of exhaust steam at 2 bar, 0.96 dry,

h2 = hf2 + x2hfg2 = 504.7 + 0.96 × 2201.9 = 2618.5 kJ/kg

Enthalpy drop in turbine, ∆h = h1h2 = 3245.5 − 2618.5 = 627.0 kJ/kg

Turbine efficiency,

Energy available in the exhaust steam above 50°C = h2hf = 2618.5 − 209.3 = 2409.2 kJ/kg

Example 3.2

The following readings are taken during trial on a boiler for 1 h:

Steam generated = 5000 kg

Coal burnt = 650 kg

CV of coal = 31,500 kJ/kg

Dryness fraction of steam entering the superheater = 0.90

Rated boiler pressure = 10 bar

Temperature of steam leaving the superheater = 250°C

Temperature of hot well = 40°C

Calculate: (a) equivalent evaporation per kg of fuel without and with superheater, (b) thermal efficiency of the boiler without and with superheater, and (c) amount of heat supplied by the superheater per hour.

Solution

Given that ms = 5000 kg/h, mf = 650 kg/h, C.V. of fuel = 31,500 kJ/kg, x = 0.90, p = 10 bar

Equivalent evaporation,

where ms = mass of steam generated per kg of fuel =

  1. Without superheater,

    At 10 bar, hf 1 = 762.8 kJ/kg, hfg1 = 2015.3 kJ/kg

    At 40°C, hf = 167.6 kJ/kg

    With superheater,

    At 10 bar, 250°C, hsup = 2942.6 kJ/kg

  2. Thermal efficiency,

    Without superheater,

    With superheater,

  3. Heat supplied by the superheater per hour = ms [(1 − x)hfg1 + hsup]
    = 5000 [(1 − 0.9) × 2015.3 + 2942.6] = 15,720,650 kJ/h

Example 3.3

The following data refer to a boiler trial:

Feed water = 700 kg/h

Feed-water temperature = 25°C

Steam pressure = 15 bar

Steam temperature = 300°C

Coal burnt = 90 kg

CV of coal = 30,500 kJ/kg

Ash and unburnt coal in ash pit = 4 kg/h with C.V. = 2200 kJ/kg

Flue gas formed = 20 kg/kg of coal burnt

Flue gas temperature at chimney = 300°C

Ambient temperature = 30°C

Mean specific heat of flue gases = 1.025 kJ/kgK

Calculate (a) the boiler efficiency, (b) the equivalent evaporation, and (c) the percentage heat unaccounted for.

Solution

From steam tables, at 15 bar, 300°C, hsup = 3038.9 kJ/kg

At 25°C, hf = 108.77 kJ/kg

Heat absorbed by 1 kg of feed water to transform into steam at 300°C and 15 bar

= hsuphf = 3038.9 − 108.77 = 2930.13 kJ/kg
  1. Boiler efficiency,
  2. Equivalent evaporation,

    Heat carried away by flue gases = mg cpg (TgTa)

    = 20 × 1.025 × (300 − 30) = 5535 kJ/kg of coal
  3. Heat unaccounted for per kg of fuel
    = CV of fuel − heat in steam − heat in flue gases − heat in ash pit
    = 30500 − 22790 − 5535 − 97.8 = 2077.2 kJ/kg

Example 3.4

The following observations were made during a boiler trial:

Mass of feed water and its temperature = 640 kg/h, 50°C

Steam pressure = 10 bar

Fuel burnt = 55 kg/h

HCV of fuel = 44,100 kJ/kg

Temperature of flue gases = 300°C

Boiler house temperature = 30°C

Dryness fraction of steam = 0.97

Heating surface of boiler = 18.6 m2

Specific heat for flue gases = 1.1 kJ/kgK

Specific heat for superheated steam = 2.1 kJ/kgK

Composition of liquid fuel used by mass: C = 85%, H2 = 13%, and ash = 2%

The flue gas analysis by volume done by using Orsat apparatus:

CO2 = 12.5%, O2 = 4.5%, and N2 = 83%

Calculate (a) the equivalent evaporation per kg of fuel per m2 of heating surface area per hour, (b) the boiler efficiency, and (c) the draw the heat balance sheet on 1 kg of fuel basis. Take the partial pressure of steam in exhaust gases = 0.07 bar.

Solution

  1. Equivalent evaporation,
    h1 = hf1 + x1hfg1

    At 10 bar, hf1 = 762.6 kJ/kg, hfg1 = 2013.6 kJ/kg

    For feed water at 50°C, hf = 209.3 kJ/kg

  2. Boiler efficiency,
  3. Heat used to generate steam per kg of fuel

    Air supplied per kg of fuel burnt =

    Dry flue gases formed per kg of fuel burnt = 17.1 + 0.85 = 17.95 kg

    Heat carried by dry flue gases = mg × cpg (TgTa)

    = 17.95 × 1.1 (300 − 30) = 5331 kJ/kg of fuel

    Moisture formed per kg of fuel = 0.13 × 9 = 1.17 kg

    Heat carried by moisture in the flue gases per kg of fuel

    = 1.17 [hg + cps (TsupTs) − hf] at 0.07 bar
    = 1.17 [2572.6 + 2.1 (300 − 39) − 125.8]
    = 3504 kJ

    Heat unaccounted for = 44,100 − (29,166 + 5331 + 3504) = 6099 kJ

Heat balance sheet:

Example 3.5

The following data was recorded during a boiler trial for 1 h:

Steam generated = 4500 kg, Steam pressure = 10 bar gauge

Record of throttling calorimeter:

Steam pressure = 30 mm of Hg above barometer reading.

Steam temperature = 105°C

Barometer reading = 735 mm of Hg

Specific heat of superheated steam = 2.1 kJ/kgK

Feed-water temperature = 58°C

Temperature of steam leaving the superheater = 202°C

Coal fired = 450 kg, HCV of coal = 35,700 kJ/kg

Flue gas temperature = 260°C, Boiler house temperature = 30°C

Ultimate analysis of dry coal:

C = 82 %, H2 = 14 %, and ash = 4%

Volumetric analysis of dry flue gases by Orsat apparatus:

CO2 = 12 %, CO = 1.5 %, O2 = 7%, N2 = 79.5 %

Specific heat of dry flue gases = 1.0 kJ/kgK

Partial pressure of steam going with flue gases = 0.07 bar

Moisture content of coal at the time of feeding into the boiler = 4%

  1. Calculate the heat absorbed by the superheater per minute.
  2. Draw up the heat balance sheet on the basis of 1 kg of wet coal fired.

Solution

Quality of steam generated by the boiler:

Pressure of steam after throttling =

Absolute pressure of steam generated =

Total heat in steam before throttling at 10.98 bar = Total heat in steam after throttling at 1.02 bar

hf1 + x1 hfg1 = (hf2 + hfg2) + cps (Tsup2Ts2)

or 780.936 + x1 × 2000.7 = 2675.696 + 2.1 (105 − 100)

or x1 = 0.952

Heat absorbed in the superheater per minute:

Heat generated per kg of coal fired:

  1. Hg = Content of dry coal × HCV
    = (1 − 0.04) × 35,700 = 34,272 kJ/kg
  2. Heat absorbed in boiler and superheater per kg of coal to generate steam
    = 10 [2781.6 + 2.1 (202 − 184) − 4.187 × 58] = 25,765.8 kJ/kg
  3. Air supplied per kg of coal burned

    Dry flue gases formed per kg of coal fired

    = 14 + (1 − 0.04) × 0.82 = 14.787 kg/kg of coal
  4. Heat carried away by dry flue gases per kg of coal burned
    = 14.787 × 1.0 (260 − 30) = 3401.0 kJ/kg of coal
  5. Moisture carried away by flue gases per kg of coal burned
    = Moisture in the fuel + Moisture formed due to combustion of H2

    Heat carried by moisture going with flue gases

    = 0.299 [hg + cps (TsupTs)] at 0.07 bar
    = 0.299 [2572.6 + 2.1 (260 − 39)] = 908 kJ
  6. Part of carbon burnt to CO =

    C.V. when C burns to CO2 = 35,000 kJ/kg, and when burns to CO = 10,250 kJ/kg

    Heat lost due to incomplete combustion = 0.0875 (35,000 − 10,250)

    = 2165.5 kJ/kg

    Heat balance sheet on the basis of 1 kg of coal burned is as follows:

Example 3.6

A power plant producing 125 MW of electricity has steam condition at boiler outlet as 110 bar, 500°C and condenser pressure is 0.1 bar. The boiler efficiency is 92 % and consumes coal of calorific value 25 MJ/kg. The feed-water temperature at boiler inlet is 150°C. The steam generator has risers in the furnace wall 30 m high and unheated downcomers. The quality at the top of the riser is 8.5 % and minimum exit velocity of mixture leaving the riser and entering the drum is required to be 2 m/s. The risers have 50 mm outside diameter and 3 mm wall thickness. Neglecting pump work and other losses, estimate (a) the steam generation rate, (b) the fuel burning rate, (c) the evaporation factor, (d) the pressure head available for natural circulation, (e) the circulation ratio, (f) the number of risers required, and (g) the heat absorption rate per unit projected area of the riser.

Solution

Refer Fig. 3.34.

Given that P = 125 MW, p1 = 110 bar, t1 = 500°C, p2 = 0.1 bar, ve = 2 m/s

ηb = 92 %, C.V. = 25 MJ/kg, tfw = 150°C, xtop = 0.085

From steam tables:

At p1 = 110 bar, 500°C, h1 = 3361 kJ/kg, s1 = 6.540 kJ/kgK

p2 = 0.1 bar, hf2 = 191.83 kJ/kg, hfg2 = 2392.8 kJ/kg

sf2 = 0.6493 kJ/kgK, sfg2 = 7.5009 kJ/kgK

At tfw = 150°C, hf5 = 632.2 kJ/kg

s1 = s2 for isentropic process

6.540 = 0.6493 + x2 × 7.5009

or x2 = 0.7853

h2 = hf2 + x2 hfg2 = 191.83 + 0.7853 × 2392.8 = 2070.89 kJ/kg
h3 = h4 = hf2 = 191.83 kJ/kg
  1. Let s = mass flow rate of steam in kg/s

    Then s (h1h2) = P

    or s (3361 − 2070.89) = 125 × 103

    or s = 96.89 kg/s

  2. Boiler efficiency,

    Figure 3.34 Temperature-entropy diagram

    or

    Rate of fuel burned, f = 11.495 kg/s

  3. Evaporation factor =
  4. Pressure head available for natural circulation,

    ∆p = gH (ρfρm)

    At 110 bar, vf = 0.001489 m3/kg, vf = 0.015987 m3/kg

    vtop = 0.001489 + 0.085 (0.015987 − 0.001489) = 0.002721 m3/kg

    p = 30 × 9.81 (671.6 − 519.53) = 44.754 kPa or 0.44754 bar

  5. Circulation ratio, CR =
  6. di = d0 − 6 = 50 − 6 = 44 mm

    Rate of steam formation in riser,

    Number of risers =

  7. At 110 bar, hfg = 1255.5 kJ/kg from steam tables.

    Heat absorption rate per unit projected area of the riser

Example 3.7

An oil fired boiler uses 52 kg/h of oil having an HCV of 44,900 kJ/kg and composition/kg: C = 0.847, H2 = 0.13, S = 0.0125, and generates 635 kg of steam/h at 10.5 bar pressure from feed water supplied at 338 K. The flue gases having dry gas analysis/unit volume: O2 = 0.043, CO2 = 0.124, N2 = 0.833, and specific heat 1.005 kJ/kg-K, leave the boiler at 635 K. The pressure and temperature of steam after throttling in a throttling calorimeter is 1.15 bar and 398 K, respectively. Taking the partial pressure of steam in flue gases as 0.07 bar and the specific heat of superheated steam = 2.1 kJ/kg-K, determine (a) the equivalent evaporation/kg of fuel and (b) boiler efficiency.

Draw up a complete heat balance sheet. Take boiler room temperature = 234 K. [IES, 2000]

Solution

Given: f = 52 kg/h, HCV = 44,900 kJ/kg, s = 635 kg/h, p1 = 10.5 bar,

Tf = 338 K, p2 = 1.15 bar, Tsup = 398 K, pvs = 0.07 bar, Tg = 635 K,

Ta = 234 K, cps = 2.1 kJ/kgK, cpg = 1.005 kJ/kgK,

Fuel: C = 0.847, H2 = 0.13, S = 0.0125 per kg

Flue gas: O2 = 0.043, CO2 = 0.124, N2 = 0.833

Throttling calorimeter:

hf1 + x1 hfg1 = hf2 + hfg2 + cps (TsupTs2)

From steam tables, we have

At p1 = 10.5 bar, hf1 = 772.2 kJ/kg, hfg1 = 2007.7 kJ/kg

At p2 = 1.15 bar, hf2 = 434.0 kJ/kg, hfg2 = 2247.5 kJ/kg, ts2 = 102.05°C

or Ts2 = 375.05 K

772.2 + x1 × 2007.7 = 434.0 + 2247.5 + 2.1 (398 − 375.02) = 2729.76

or

Dryness fraction of steam = 0.975

  1. Equivalent evaporation:
    h1 = hf1 + x1 hfg1 = 772.2 + 0.975 × 2007.7 = 2729.7 kJ/kg
    At Tf = 338 K, hf = cpw (Tf − 273) = 4.187 (338 − 273)
    = 272.1 kJ/kg

    hfg(atm) = specific enthalpy of evaporation at standard atmospheric pressure of 1.01325 bar

        = 2256.9 kJ/kg

    Equivalent evaporation,

     

  2. Boiler efficiency,

    Heat balance sheet

    1. Heat used to generate steam per kg of fuel =
    2. Air supplied per kg of fuel burned =

      Dry flue gases formed per kg of fuel burned, mg = 17.242 + 0.847 = 18.089 kg

      Heat carried away by dry flue gases = mg × cpg (TgTa)

      = 18.089 × 1.005 (635 − 234) = 7253.7 kJ/kg of fuel
    3. Moisture formed per kg of fuel, mw = 0.13 × 9 = 1.17 kg

      Heat carried away by moisture in flue gases per kg of fuel

      = mw [hg + Cps (TsupTs) − hf] at p = 0.07 bar
      = 1.17 [2572.5 + 2.1 (635 − 312) − 163.4] [∴ Ts = 39°C at 0.07 bar]
      = 3087.4 kJ/kg of fuel
    4. Heat un-accounted for = 44,900 − (30,011 + 7253.7 + 3087.4)
      = 4547.9 kJ/kg of fuel

Example 3.8

During a boiler trial, steam was produced at a pressure of 20 bar and superheat temperature of 360°C using feed water at 30°C at the rate of 90000 kg/h. Coal with 3% moisture having calorific value of 34000 kJ/kg of dry coal was burnt at the rate of 1000 kg/h. The temperature of flue gases at the exit was 200°C, whereas inlet air was at 30°C. Coal contains 80% carbon, 6% hydrogen, and rest ash. Volumetric analysis of dry flue gases is: CO2 = 12 %, O2 = 10% and N2 = 78 %. Enthalpy of steam at 20 bar, 360° C = 3159.3 kJ/kg, enthalpy of water at 30°C = 126 kJ/kg. In the flue gas, the enthalpy of steam at exit condition is 2879.7 kJ/kg. Specific heat of gas = 1.01 kJ/kg.

Draw the heat balance sheet.

[IES, 1998]

Solution

Steam condition: 20 bar, 360°C

Feed water: 30°C, 90,000 kg/h

Moisture in coal = 3 %, CV of coal = 34000 kJ/kg of dry coal, f = 1000 kg/h

Exhaust temperature of flue gases = 200°C

Inlet temperature of air = 30°C

Coal composition: C = 80 %, H2 = 6 %, rest is ash.

Volumetric analysis of dry flue gases:

CO2 = 12%, O2 = 10 %, N2 = 78 %

Enthalpy of steam at 20 bar, 360°C = 3159.3 kJ/kg

Enthalpy of water at 30°C = 126 kJ/kg

Enthalpy of steam in flue gases at exit = 2879.7 kJ/kg

Specific heat of gas = 1.01 kJ/kg

Heat supplied by coal per kg, Q = (1 − 0.03) C.V. = (1 − 0.03) × 34,000 = 32,980 kJ/kg

Heat used to generate steam per kg of fuel,

Air supplied per kg of fuel burnt =

Dry flue gases formed per kg of fuel burned, mg = 15.76 + 0.80 = 16.56 kg

Heat carried away by dry flue gases, Q2 = mg × cpg (TgTa)

= 16.56 × 1.01 (200 − 30) = 2843.3 kJ

Moisture formed per kg of fuel = 0.06 × 9 = 0.54 kg

Heat carried away by moisture in flue gases, Q3 = 0.54 × 2879.7 = 1555 kJ

Heat lost to moisture in coal, Q4 = 0.03 × 2879.7 = 86.39 kJ

Q1 + Q2 + Q3 + Q4 = 28,144 + 2843.3 + 1555 + 86.39 = 32628.69 kJ

Heat unaccounted for, Q5 = 32980 − 32628.69 = 351.31 kJ

Heat balance sheet:

3.15 ❐ STEAM GENERATOR CONTROL

The purpose of steam generator control is to provide the steam flow required by the turbine at design pressure and temperate. The variables that are controlled are (i) fuel firing rate, (ii) air flow rate, (iii) gas flow distribution, (iv) feed-water flow rate, and (v) turbine valve-setting.

The key measurements that describe the plant performance are (i) steam flow rate, (ii) steam pressure, (iii) steam temperature, (iv) primary and secondary air flow rates, (v) fuel firing rate, (vi) feed-water flow rate, (vii) steam drum level, and (viii) electrical power output.

The control system must act on the measurement of these plant parameters so as to maintain plant operation at the desired conditions.

We shall discuss only a few basic control systems related to the following:

  1. Feed-water and drum level control: Feed water and steam flow are controlled to meet load demand by the turbine. Therefore, the level of water in the steam drum has to be maintained, normally half-full up to the diametral plane.

    A schematic diagram for the three-element feed-water control system is shown in Fig. 3.35. It comprises a drum-level sensor, feed-water flow sensor, and steam flow sensor.

    Figure 3.35 Schematic diagram for a three-element feed-water control system

    Figure 3.36 Schematic diagram of a steam pressure control system

  2. Steam pressure control: A schematic diagram of steam pressure control system is shown in Fig. 3.36. It maintains the steam pressure by adjusting fuel and combustion air flow to meet the desired pressure. When pressure drops, the flows are increased. A steam pressure sensor acts directly on the fuel flow and air flow controls. A trimming signal from fuel flow and air flow sensors maintains the proper fuel-air ratio.
  3. Steam temperature control: For efficient power plant operation, an accurate control of superheat temperature of steam is essential. The principal variables affecting superheat temperature are (i) furnace temperature, (ii) cleanliness of radiant and pendant superheaters, (iii) temperature of gases entering the convective superheater, (iv) cleanliness of convective superheater, (v) mass flow rate of gases through the convective superheater, (vi) feed-water temperature, and (vii) variation of load on the unit.

A reduction in steam temperature results in loss in plant efficiency. On the other hand, a rise in steam temperature above design value may result in overheating and failure of superheater, reheater tubes, and turbine blades.

The temperature of the saturated steam leaving the drum corresponds to the boiler pressure and remains constant if the steam pressure controls are working properly. It is the superheater-reheater responses to load changes which need to be corrected. Some of the methods to correct them are as follows:

  1. Combined radiant-convective superheaters
  2. Desuperheating and attemperation
  3. Gas by-pass or damper control
  4. Gas recirculation
  5. Excess air
  6. Tilting burners
  7. Burner selection
  8. Separately fired superheater
3.16 ❐ ELECTROSTATIC PRECIPITATOR

The basic elements of an electrostatic precipitator are shown in Fig. 3.37. It comprises two sets of electrodes insulated from each other. The first set is composed of rows of electrically grounded vertical parallel plates, called the collection electrodes, between which the dust–laden gas flows. The second set of electrodes consists of wires, called the discharge or emitting electrodes that are centrally located between each pair of parallel plates. The wires carry a unidirectional negatively charged high voltage current from an external DC source. The applied high voltage generates a unidirectional, non-uniform electrical field whose magnitude is the greatest near the discharge electrodes. When that voltage is high enough, a blue luminous glow, called a corona, is produced around them. Electrical forces in the corona accelerate the free electrons present in the gas so that they ionise the gas molecules, thus forming more electrons and positive gas ions. The new electrons create again more free electrons and ions, which result in a chain reaction.

Figure 3.37 Basic elements of an electrostatic precipitator

The positive ions travel to the negatively charged wire electrodes. The electrons follow the electrical field toward the grounded electrodes, but their velocity decreases as they move away from the corona region around the wire electrodes towards the grounded plates. Gas molecules capture the low velocity electrons and become negative ions. As these ions move to the collecting electrode, they collide with the fly ash particles in the gas stream and give them negative charge. The negatively charged fly ash particles are driven to the collecting plate. Collected particulate matter is removed from the collecting plates on a regular schedule.

The arrangement of an electrostatic precipitator is shown in Fig. 3.38.

Figure 3.38 Arrangement of an electrostatic precipitator

Example 3.9

A boiler generates 8 kg of steam per kg of coal burnt at a pressure of 10 bar, from feed water having a temperature of 65°C. The efficiency of the boiler is 75% and factor of evaporation 1.2. Specific heat of steam at constant pressure is 2.2 kJ/kgK. Calculate (a) the degree of superheat and temperature of steam generated, (b) the calorific value of coal used in kJ/kg, and (c) the equivalent evaporation in kg of steam per kg of coal.

Solution

Given: ms = 8 kg/kg of coal, ps = 10 bar, tfw = 65°C, ηb = 75%, Fe = 1.2, cps = 2.2 kJ/kgK

  1. At p = 10 bar, from steam tables:

    hg = 2778.1 kJ/kg, Ts = 273 + 179 = 452 K

    Also, hf1 = cpw tfw = 4.187 × 65 = 272.155 kJ/kg of feed water

    hfg = 2256.9 kJ/kg at atmospheric pressure.

    Factor of evaporation,

    or

    or 2708.28 = 2505.945 + 2.2 (Tsup − 452)

    or Tsup = 543.97 K or tsup = 270.97°C

    Degree of superheat = TsupTs = 543.97 − 452 = 91.97°C

  2. Boiler efficiency,

    or 0.75 =

    or C.V. = 28,440 kJ/kg of coal

  3. Equivalent evaporation,

Example 3.10

The following observations were made during the trial of a gas-fired boiler of a steam power plant:

Generator output = 50,000 kW

Steam conditions = pressure 50 bar temperature 500°C

Feed-water temperature = 80°C

Steam output = 65 kg/s

Percentage composition of fuel (natural gas), by volume CH4 = 96.5, C2H6 = 0.5 (rest incombustibles)

HCV of gas = 38,700 kJ/m3, at 1.013 bar and 15°C

Gas consumption = 6.5 m3/s

Calculate the boiler efficiency and the overall thermal efficiency based on the lower calorific value of the fuel.

[IES, 1996]

Solution

Combustion equations:

CH4 + 2O2 → CO2 + 2H2O
C2H6 + 2.5O2 → 2CO2 + 3H2O

1 mole of CH4 burns to give 2 moles of water

Therefore, 0.965 moles give 2 × 0.965 × 18 = 34.74 kg of H2O

Similarly 0.005 moles of C2H6 burns to give 3 × 0.005 × 18 = 0.27 kg of H2O

Assuming steam as a perfect gas, 1 mole of gas at 1.013 bar and OC is 22.41 m3

∴ 1 mole of gas at 15°C =

Mass of steam produced,

HCV = LCV + heat in steam

HCV = 38,700 − ms hfg at 15°C

    = 38,700 − 1.481 × 2465.9 [hfg at 15°C = 2465.9 kJ/kg]

    = 35,043.07 kJ/kg

At 50 bar and 500°C, h1 = 3433.8 kJ/kg

At 80°C, hfg for feed water = 334.98 kJ/kg

Enthalpy of steam = h1hfg = 3433.8 − 334.9 = 3098.9 kJ/kg

Boiler efficiency =

 

Overall thermal efficiency =

Example 3.11

In a trial on a boiler fitted with an economiser, the following results were obtained:

The fuel contains 80 % carbon and it does not contain nitrogen.

Calculate (a) the air leakage into the economiser per minute if the fuel used in the boiler is 60 kg/min. (b) the reduction in temperature of the gas due to air leakage if the atmospheric air temperature is 20°C and the flue gas temperature is 350°C.

For air: specific heat, cpa = 1.005 kJ/kgK and for gas, cpg = 1.1 kJ/kgK.

Percentage of incombustible in fuel = 15%

Solution

Air supply per kg of fuel for the gas analysis entering the economiser,

Air supply per kg of fuel for the gas analysis leaving the economiser,

Air leakage into the economiser per kg of fuel burned in boiler

= maomai = 24.7 − 23.4 = 1.3 kg

Air leakage in the economiser per minute = 1.3 × 60 = 78 kg

Mass of flue gas products formed per kg of coal = 23.4 + (1 − 0.15) = 24.25 kg

cpg mg Tg + cpa ma Ta = cpg (mg + ma) Tm

where Tm = temperature of exhaust gases leaving the economiser

1.1 × 24.25 × 623 + 1.005 × 1.3 × 293 = 1.1 (24.25 + 1.3) × Tm

or Tm = 604.9 K or 331.9°C

Drop in temperature of exhaust gases due to air leakage into the economiser

= 350 − 331.9 = 18.1°C
3.17 ❐ DRAUGHT

In order to maintain the continuous flow of fresh air into the combustion chamber, it is necessary to exhaust the products of combustion from the combustion chamber of a boiler. A pressure difference has to be maintained to accelerate the products of combustion to their final velocity and to overcome the pressure losses in the flow system. This pressure difference so maintained is called “draught”.

3.17.1 Classification of Draught

The method of producing draught may be classified as follows:

  1. Natural (or chimney) draught
  2. Artificial draught
    1. Steam jet draught
      1. Induced draught
      2. Forced draught
    2. Mechanical draught
      1. Induced fan draught
      2. Forced fan draught
      3. Balanced draught

3.17.2 Natural Draught

Natural draught is obtained by the use of a chimney. A chimney is a vertical tubular structure of brick, masonry, steel, or reinforced concrete, built for the purpose of enclosing a column of hot gases, to produce the draught. The draught produced by the chimney is due to the density difference between the column of hot gases inside the chimney and the cold air outside.

A diagrammatic arrangement of a chimney of height H in m above the grate is shown in Fig. 3.39.

Pressure at the grate level on the chimney side,

p1 = patm + γgh

where patm = atmospheric pressure at the chimney top

Figure 3.39 Diagrammatic arrangement of natural chimney draught

γg = specific weight of chimney hot gases.

Pressure acting on the grate on the open (atmospheric) side,

p2 = patm + γah

where γa = specific weight of air outside the chimney.

Net pressure difference causing the flow through the combustion chamber, static draught,

Static draught can be measured by a water manometer

Let hw = difference in water level in the two legs of U-tube manometer, mm

Then

Equating the above two equations, we get

Δp = hw = (γaγg)H

3.17.3 Height and Diameter of Chimney

It can be assumed that the volume of combustion products is equal to the volume of air supplied when both reduced to the same temperature and pressure conditions.

Let ma = mass of air supplied per kg of fuel

Tg = average absolute temperature of chimney gases

Ta = absolute temperature of atmospheric air

Density of air at atmospheric conditions,

Density of hot gases at temperature, Tg,

Let Δp be equivalent to H1 metre height of burnt gases.

Then

The velocity of gases passing through the chimney is given by,

If the pressure loss in the chimney is equivalent to a hot gas column of height h1, then

where

    = 0.825 for brick chimney

    = 1.1 for steel chimney

Mass of gas flowing through any cross-section of chimney is given by,

mg = Avρg kg/s

3.17.4 Condition for Maximum Discharge Through Chimney

Neglecting losses in chimney,

Density of hot gases,

Mass of gases discharged per second, mg = Avρg

where

The value of mg will be maximum when , as Ta and ma are fixed.

Maximum discharge,

3.17.5 Efficiency of Chimney

A certain minimum flue gas temperature is required to produce a given draught with a given height of chimney. Therefore, the temperature of flue gases leaving the chimney in case of natural draught has to be higher than that of flue gases temperature in the case of artificial draught.

Let Tn = absolute temperature of flue gases leaving the chimney to create the draught of hw mm of water.

Tm = absolute temperature of flue gases leaving the chimney in case of artificial draught.

cpg = mean specific heat of flue gases

Extra heat carried away per kg of flue gases due to higher temperature required in natural draught = cpg (TnTm) as Tn > Tm

Draught pressure produced by natural draught system in m of hot gases,

Maximum energy given by H1 to one kg of air at the cost of extra heat

Efficiency of chimney,

where Tn = Tg

Generally ηch < 1%

3.17.6 Advantages and Disadvantages of Natural Draught

Advantages of natural draught are as follows:

  1. No power is required to produce draught.
  2. The life of chimney is quite long.
  3. The chimney does not require much maintenance.

Disadvantages of natural draught are as follows:

  1. There is very low efficiency.
  2. It requires a tall chimney (30.48 m minimum).
  3. Efficiency is dependent on atmospheric temperature.
  4. There is no flexibility on draught.

3.17.7 Draught Losses

The causes of the losses in draught are as follows:

  1. Frictional resistance offered by the gas passages during flow of gas.
  2. Loss near the bends in the gas flow circuit.
  3. Frictional head loss in different equipment, for example, grate, economiser, superheater, etc.
  4. Heat lost to impact velocity to flue gases.

The total loss is nearly 20% of the total static draught produced by chimney.

3.17.8 Artificial Draught

A draught produced by artificial means which is independent of the atmospheric conditions is called artificial draught. It gives greater flexibility to take the fluctuating loads on the plant. The artificial draught may be of mechanical type or of steam jet type. If the draught is produced by a fan, it is called fan (or mechanical) draught, and if it is produced by a steam jet, it is called steam jet draught. Steam jet draught is used for small installations such as locomotives, whereas mechanical draught is invariably used in central power stations.

Figure 3.40 Forced draught

Forced Draught

In a forced draught system, a blower is installed near the base of the boiler and air is forced to pass through the furnace, flues, economiser, air-preheater, and to the stack. It is called forced positive draught system because the pressure of air throughout the system is above atmospheric pressure and air is forced to flow through the system. The arrangement of the system is shown in Fig. 3.40. A stack or a chimney is also used to discharge gases high into the atmosphere for better dispersion of ash particles and pollutants.

Induced Draught

In this system, the blower is located near the base of the chimney. The air is sucked into the system by reducing the pressure through the system below the atmospheric pressure. The induced draught fan sucks the gases from the furnace and the pressure inside the furnace is reduced below that of the atmospheric pressure, thus inducing the atmospheric air to flow through the furnace. The draught produced is independent of the temperature of hot gases. Therefore, the gases may be discharged as cold as possible after recovering as much heat as possible in the economiser and the pre-heater. The arrangement of the system is shown in Fig. 3.41.

Figure 3.41 Induced draught

3.17.9 Comparison of Forced and Induced Draughts

The comparison of forced and induced draughts in given in Table 3.3.

 

Table 3.3 Comparison of forced and induced draughts

3.17.10 Comparison of Mechanical and Natural Draughts

The comparison of mechanical and natural draughts is given in Table 3.4.

 

Table 3.4 Comparison of mechanical and natural draughts

3.17.11 Balanced Draught

A balanced draught is a combination of forced and induced draughts. The forced draught overcomes the resistance of the fuel bed and therefore sufficient air is supplied to the fuel bed for proper and complete combustion. The induced draught fan removes the gases from the furnace, maintaining the pressure in the furnace just below atmospheric pressure. This prevents to blow-out of flames and leakage of air inwards, when the furnace doors are opened. The balanced draught is shown in Fig. 3.42.

Figure 3.42 Balanced draught

3.17.12 Steam Jet Draught

A steam jet draught of the induced type is shown in Fig. 3.43. A steam nozzle located near the smoke box induces the flow of gases through the tubes, ash pit, grate, and flues. It is generally used for a locomotive boiler.

The steam jet creates pressure below that of the atmosphere in the smoke box. The advantages of steam jet draught are as follows:

  1. Very simple and economical
  2. Low-grade fuel can be used
  3. Forced type system keeps the fire bars cool and prevents the adhering of clinker to them
  4. Maintenance cost is nil
  5. Occupies minimum space

However, it cannot be started until high pressure steam is available.

Figure 3.43 Steam jet draught

Example 3.12

A boiler is equipped with a chimney of 25 m height. The ambient temperature is 27°C. The temperature of flue gases passing through the chimney is 300°C. If the air flow through the combustion chamber is 20 kg per kg of fuel burned, then find the following:

  1. Theoretical draught in cm of water
  2. The velocity of flue gases passing through the chimney if 50% of theoretical draught is lost in friction at grate and passage.

Solution

Given that H = 25 m, Ta = 27 + 273 = 300 K, Tg = 300 + 273 = 573 K, ma = 20 kg/kg fuel

  1. Equivalent gas head,

  2. Available head, Hav = 20.476 × 0.5 = 10.238 m

    Velocity of flue gases,

Example 3.13

Determine the height of the chimney required to produce a draught equivalent to 2 cm of water if the flue gas temperature is 240°C and ambient temperature is 25°C. The minimum amount of air per kg of fuel is 18 kg.

Solution

Given that hw = 2 cm of water, Tg = 240 + 273 = 513 K, Ta = 25 + 273 = 298 K, ma = 18 kg/kg fuel

Example 3.14

Find the mass of flue gases flowing through the chimney when the draught produced is equal to 2 cm of water. The temperature of flue gases is 305°C and the ambient temperature is 27°C. The flue gases formed per kg of fuel burned are 20 kg. The diameter of the chimney is 2 m. Neglect losses.

Solution

Given that hw = 2 cm, Tg = 305 + 273 = 578 K, Ta = 27 + 273 = 300 K,

1 + ma = 20 kg/kg of fuel.

Example 3.15

The height of a chimney used in a plant to provide natural draught is 15 m. The ambient air temperature is 20°C. (a) Find the draught in mm of water when the temperature of chimney gases is such that the mass of gases discharged is maximum and (b) if temperature of gases should not exceed 330°C, then find the air supplied per kg of fuel for maximum discharge.

Solution

Given that H = 15 m, Ta = 20 + 273 = 293 K, Tg = 330 + 273 = 603 K

  1. (hw)max =
  2. For maximum discharge,

    or 1.029 ma = ma + 1

    or ma = 34.47 kg/kg of fuel

Example 3.16

A boiler is equipped with a chimney of 30 m height. The ambient temperature is 25°C. The temperature of flue gases passing through chimney is 300°C. If the air flow is 20 kg/kg of fuel burnt, then find (a) draught produced and (b) the velocity of flue gases passing through chimney if 50% of the theoretical draught is lost in friction.

Solution

H = 30 m, Ta = 273 + 25 = 298 K, Tg = 273 + 300 = 573 K, ma = 20 kg, mf = 1 kg

  1. Draught produced,
  2. Equivalent gas head,

    Available head, Hav = 0.5 H1 = 12.469 m

    Velocity of flue gases, v =

Example 3.17

A boiler uses 18 kg of air per kg of fuel. Determine the minimum height of chimney required to produce a draught of 25 mm of water. The mean temperature of chimney gases is 315°C and that of outside air 27°C.

Solution

Given: ma = 18 kg, mf = 1 kg, hw = 25 mm of H2O, Tg = 273 + 315 = 588 K,

Example 3.18

With a chimney of height 45 m, the temperature of flue gases with natural draught was 370°C. The same draught was developed by induced draught fan and the temperature of the flue gases was 150°C. The air flow through the combustion chamber is 25 kg per kg of coal fired. The boiler house temperature is 35°C. Assuming cp = 1.004 kJ/kg K for the flue gases, determine the efficiency of the chimney.

Solution

Given that H = 45 m, Tgn = 273 + 370 = 643 K, Tgi = 273 + 150 = 423 K

mg = 25 kg/kg of coal fired, Ta = 273 + 35 = 308 K, cpg = 1.004 kJ/kgK

Example 3.19

A boiler is provided with a chimney of 24 m height. The ambient temperature is 25°C. The temperature of flue gases passing through the chimney is 300°C. If the air flow through the combustion chamber is 20 kg/kg of fuel burnt, then find (a) the theoretical draught in cm of water and (b) the velocity of flue gases passing through the chimney if 50% of theoretical draught is lost in friction at grate and passage.

Solution

Given that H = 24 m, Ta = 273 + 25 = 298 K, Tg = 273 + 300 = 573 K

ma = 20 kg/kg of fuel burnt,

  1. Equivalent gas head,

Example 3.20

The following data relate to a boiler using induced draught system:

Length of the duct carrying flue gases = 150 m

Mean size of the square duct = 75 cm2

Mean flue gas velocity in the duct = 900 m/min

Mean temperature of gases passing through duct = 227°C

Plenum pressure = 15 cm of water

Atmospheric pressure = 75 cm of Hg

Number of 90° bends = 4

Number of 45° bends = 4

Loss of draft in every 90° bend = 0.1 cm of water

Draft available from chimney = 1.5 cm of water

Fuel bed resistance for under fired stoker = 10 cm of water

Fan efficiency = 60%

Motor efficiency driving fan = 92.5%

Characteristic gas constant for gases = 0.294 kJ/kgK

Friction factor, f, corresponding to round section duct = 0.006

Assuming that a 45° bend equal one-half of 90° bend and that the resistance offered by square duct is 20 % greater than a similar round duct, determine the following:

  1. Draft to be produced due to friction in square cross-section duct in mm of water
  2. Draft due to velocity head of flue gases in mm of water
  3. Total draft for the boiler
  4. Draft to be produced by induced fan
  5. Power required to drive fan

    [IES, 2011]

Solution

Given that L = 150 m, f = 0.006, As = 75 cm2, vg = 900 m/min, C = 1.2, Δp = 15 cm of water, tg = 227°C, patm = 75 cm of Hg, ηf = 0.6, ηm = 0.925, Rg = 0.294 kJ/kgK

  1. Frictional head loss in duct carrying flue gases,

    Equivalent diameter of round duct,

    Chimney draft produced, pc = 15 mm of water or 15 × 9.81 = 147.15 N/m2

    Atmospheric pressure,

    Pressure of hot flue gases, pg = patmpc = 0.9992 × 105 − 147.15

    = 99,844.6 N/m2

    Density of hot flue gases,

    Frictional head loss in square duct in terms of mm of water

    = 344 mm of water

    Plenum pressure drop in duct = 150 mm of water

    Draft to be produced due to friction in duct = 344 + 150 = 494 mm of water

  2. Draft due to velocity head of flue gases:

    4 bends of 90° =

    4 bends of 45° = 0.5 × 41.28 = 20.64 m of hot flue gases

    Total draft due to velocity head = 41.28 + 20.64 = 61.92 m of hot flue gases

    Loss of draft due to 90° bends = 4 × 1 = 4 mm of water

    Loss of draft due to 45° bends = 2 mm of water

  3. Fuel bed resistance for under fired stoker = 100 mm of water

    Total draft for the boiler = 344 + 150 + 42 + 4 + 2 = 542 mm of water

  4. Draft to be produced by induced fan, h = 542 + 100 = 642 mm of H2O
  5. Volume flow rate of flue gases through the duct, = As × vg
    = 75 ×10−4 × 15 = 0.1125 m3/s

    Power required to drive fan =

Example 3.21

In a steam power plant, the steam generator generates steam at the rate of 120 t/h at a pressure of 100 bar and temperature of 500°C. The calorific value of fuel used by steam generator is 41 MJ/kg with an overall efficiency of 85 %. In order to have efficient combustion, 17 kg of air per kg of fuel is used for which a draught of 25 mm of water gauge is required at the base of stack. The flue gases leave the steam generator at 240°C. The average temperature of gases in the stack may be taken as 200°C and the atmospheric temperature is 30°C. Determine (i) the height of stack required, and (ii) the diameter of stack at its base. Take the following steam properties for solution: h = 3375 kJ/kg, hf = 632.2 kJ/kg.

[IAS, 2010]

Solution

Given that ms = 120 t/h, p = 100 bar, t = 500°C, CV = 41 MJ/kg, ηoverall = 85%, m = A/F = 17 kg/kg of fuel, hw = 25 mm, tg = 240°C, tgs = 200°C, tatm = 30°C, h = 3375 kJ/kg, hf = 632.2 kJ/kg

  1. Density of air,

    Density of flue gases,

    Δp = 103 × ghw × 10−3 = gH (ρaρg)
    25 = H(1.165 − 0.652)

    Height of stack, H = 48.73 m

  2. Velocity of gases passing through the chimney,
    s × h = f × CV × η0

    Mass of air, a = 17 × f = 54.878 kg/s

    Mass of flue gases, mg = f + a = 3.228 + 54.878 = 58.106 kg/s

Summary for Quick Revision

  1. A boiler is a closed vessel in which steam is produced from water by combustion fuel at the desired temperature and pressure. It consists of a fire place and a steam-raising vessel.
  2. In a fire tube boiler, hot gases flow through the tubes and water is heated to raise steam outside the tubes. Cochran, Lancashire, Cornish, Locomotive, and Scotch boilers are fire tube boilers.
  3. In water tube boilers, water flows through the tubes and hot gases heat water from the outside to raise steam. Babcock and Wilcox and Stirling boilers are water tune boilers.
  4. High pressure boilers work up to 200 bar pressure and raise steam from 30−650 tons/h. Generally no drum is used in these boilers.
  5. Features of boilers are summarised as follows:
  6. Critical and supercritical boilers work at more than 221.2 bar pressure.
  7. LaMont, Benson, Loeffler, Schmidt–Hartmann, and Velox are high pressure boilers.
  8. Boiler mountings are different fittings and devices necessary for the operation and safety of a boiler. The various boiler mountings are: Water level indicator, pressure gauge, steam stop valve, feed check valve, blow down cock, fusible plug, and safety valve.
  9. The functions of various boiler mountings are as follows:
    1. Water-level indicator: To indicate the level of water in the boiler.
    2. Pressure gauge: To indicate steam pressure of the boiler.
    3. Steam stop valve: To regulate the flow of steam from the boiler to the prime mover as per requirement.
    4. Feed check valve: To allow the supply of water to the boiler at high pressure continuously and to prevent the backflow of water from the boiler when the pump pressure is less than the boiler pressure.
    5. Blow–down cock: To remove sludge or sediments collected at the bottommost point in the water space of boiler, when the boiler is working.
    6. Fusible plug: To put off the fire in the furnace of the boiler when the water level in the boiler falls below an unsafe level.
    7. Safety valve: To prevent the steam pressure in the boiler exceeds the desired rated pressure.
  10. Boiler accessories are appliances installed to increase the overall efficiency of the steam power plant. The various accessories are as follows:
    1. Pressure-reducing valve, steam trap, steam separator, economiser, air preheater, superheater, feed pump, and injector.
  11. The functions of various accessories are as follows:
    1. Pressure-reducing valve: To maintain constant pressure on delivery side with fluctuating boiler pressure.
    2. Steam trap: To drain-off water resulting from the partial condensation of steam without steam to escape through it.
    3. Steam separator: To separate suspended water particles carried by steam on its way from the boiler to prime mover.
    4. Economiser: To recover some heat carried away by flue gases and use for feed-water heating.
    5. Air preheater: To recover some heat of flue gases by preheating the air supplied to the combustion chamber.
    6. Superheater: To increase the temperature of steam above its saturation temperature.
    7. Feed pump: To pump water to the water space of the boiler.
    8. Injector: To feed water to the boiler with the help of a steam jet.
  12. Evaporation rate is the quantity of water evaporated into steam per hour.
  13. Equivalent evaporation is the equivalent of 1 kg of water at 100°C to steam at 100°C. It requires approximately 2257 kJ of heat.
  14. Factor of evaporation is the ratio of actual heat absorption above feed-water temperature for transformation to steam to the latent heat of steam at atmospheric pressure.
    Equivalent evaporation = actual evaporation × factor of evaporation
  15. Boiler performance parameters:
    1. Efficiency =
    2. Combustion rate =
    3. Combustion space =

      Heat absorption is the equivalent evaporation from and at 100°C in kg of steam generated per m2 of heating surface

    4. Heat liberated =
  16. Boiler efficiency

    where s = mass of steam generated in kg/h

    f = mass of fuel burned in kg/h

    C.V. = calorific value of fuel, kJ/kg

  17. Boiler power =
  18. Heat balance sheet for boiler:
    1. Heat supplied by fuel, Q = mf × C.V.
    2. Heat used to generate steam, Q1 = ms × (hhf1)
    3. Heat carried away by flue gases, Q2 = mgcpg (TgTa)
    4. Heat carried away by steam in flue gases, Q3 = mst × (h1hf1)
    5. Heat lost due to incomplete combustion,
    6. Heat lost due to unburnt fuel, Q5 = mf1 × C.V.
    7. Unaccounted for heat, Q6 = Q − (Q1 + Q2 + Q3 + Q4 + Q5)
  19. Draught is the small pressure difference maintained in the boiler to exhaust the products of combustion form the combustion chamber.
  20. The draught may be natural draught produced by a chimney or artificial draught produced by a steam jet or mechanically.
  21. Steam jet draught may be induced or forced type.
  22. Mechanical draught may be induced, forced and balanced type.
  23. In the induced draught, the blower is located near the base of the chimney, whereas, in the forced draught, the blower is installed near the base of the boiler.
  24. Natural draught:

    Static draught, Δp = hw = (γaγg) H

  25. Natural draught in terms of height of burnt gases:
  26. Velocity of hot gases passing through the chimney,

    where

    = 0.825 for brick chimney.
    = 1.1 for steel chimney.
  27. Chimney diameter,
  28. Condition for maximum discharge through chimney.
    (H1)max = H

    Maximum discharge, (mg)max =

  29. Chimney efficiency,

    where Tn = Tg and Tm = absolute temperature of flue leaving the chimney in case of artificial draught.

    ηch is generally less than 1%.

Multiple-choice Questions

  1. Consider the following statements:

    Blowdown is necessary on boilers, because

    1. The boiler water level is lowered rapidly in case it accidentally rises too high.
    2. The precipitated sediment of sludge is removed while the boiler is in service.
    3. The concentration of suspended solids in the boiler is controlled.

    Of these statements:

    1. 1, 2, and 3 are correct
    2. 1 and 2 are correct
    3. 3 alone is correct
    4. 1 and 3 are correct
  2. Once through boiler is named so because
    1. Flue gas passes only in one direction
    2. There is no recirculation of water
    3. Air is sent through the same direction
    4. Steam is sent out only in one direction
  3. Economiser is generally placed between
    1. Last superheater/reheater and air-preheater
    2. Air-preheater and chimney
    3. Electrostatic precipitators
    4. induced draft fan and forced draft fan
  4. Attempering is
    1. Reduction of steam pressure
    2. Reduction of steam temperature
    3. Discharge of flue gases at certain height
    4. Conditioning of disposable fluids for minimum pollution
  5. In fluidised bed combustion velocity of fluid is proportional to (r = radius of the particle) as
    1. r
  6. Which one of the following is the steady flow energy equation for a boiler?
    1. Q = (h2h1)
    2. Ws = (h2h1) + Q
  7. The output of a boiler is normally stated as
    1. Evaporative capacity in tonnes of steam that can be produced from and at 100°C
    2. Weight of steam actually produced at rated pressure in tonnes per hour
    3. Boiler horse power
    4. Weight of steam produced per kg of fuel
  8. Which one of the following is the fire-tube boiler?
    1. Babcock and Wilcox boiler
    2. Locomotive boiler
    3. Stirling boiler
    4. Benson boiler
  9. Which one of the following factors has the maximum effect on the equivalent evaporation of a boiler?
    1. Steam generator pressure
    2. Feed-water inlet temperature
    3. Heating surface of the boiler
    4. Quality of steam produced
  10. Consider the following:
    1. Safety valve
    2. Steam trap
    3. Steam separator
    4. Economiser

    Among these, the boiler accessories would include

    1. 1, 2, and 3
    2. 2, 3, and 4
    3. 1 and 4
    4. 1, 2, 3, and 4
  11. Match List I and List II and select the correct answer using the codes given below the lists:
    List I List-II
    A. Babcock and Wilcox 1. Forced circulation
    B. Lancashire 2. Fire tube
    C. LaMont 3. Water tube
    D. Cochran 4. Vertical

    Codes:

            A   B   C   D

    1.  1   2    3    4
    2.  2   3    4    1
    3.  3   2    1    4
    4.  2   4    1    3
  12. Consider the following statements:
    1. Boiler mounting are mainly protective devices.
    2. Steam stop valve is an accessory.
    3. Feed-water pump is an accessory.

    Of these statements:

    1. 1, 2, and 3 are correct
    2. 1 and 2 are correct
    3. 2 and 3 are correct
    4. 1 and 3 are correct
  13. Match List I with List II select the correct answer using the codes given below the list:
    List I List-II
    A. Soot Blower 1. Removal of solids from boiler drums
    B. Electrostatic precipitator 2. To clean the tube surfaces of fly ash
    C. Blow down 3. Cleaning of fluid gas
    D. Zeolite 4. Air cleaning
    5. Water purification

    Codes:

            A   B   C   D

    1.  2   4    3    5
    2.  1   3    2    5
    3.  3   2    1    4
    4.  2   3    1    5
  14. In forced circulation boilers, about 90% of water is recirculated without evaporation. The circulation ratio is
    1. 0.1
    2. 0.9
    3. 9
    4. 10
  15. Given that h is draught in mm of water, H is chimney height in meters, and T1 is atmospheric temperature in K, the maximum discharge of gases through a chimney is given by
    1. h = 176 T1/H
    2. h = H/176.5 T1
    3. h = 1.765 H/T1
    4. h = 176.5 H/T1
  16. The excess air required for combustion of pulverised coal is of the order of
    1. 100 to 150%
    2. 30 to 60%
    3. 15 to 40%
    4. 5 to 10%
  17. Consider the following:
    1. Increasing evaporation rate using convection heat transfer from hot gases.
    2. Increasing evaporation rate using radiation.
    3. Protecting the refractory walls of the furnace.
    4. Increasing water circulation rate.

    The main reasons for providing water wall enclosures in high pressure boiler furnaces would include:

    1. 2 and 3
    2. 1 and 3
    3. 1 and 2
    4. 1, 2, 3, and 4
  18. Consider the following statements:

    Expansion joints in steam pipelines are installed to

    1. Allow for future expansion of plant.
    2. Take stresses away from flanges and fittings.
    3. Permit expansion of pipes due to temperature rise.

    Of these statements:

    1. 1, 2, and 3 are correct
    2. 1 and 2 are correct
    3. 2 and 3 are correct
    4. 1 and 3 are correct
  19. In high pressure natural circulation boilers, the flue gases flow through the following boiler accessories:
    1. Superheater
    2. Air heater
    3. Economiser
    4. ID fan

    The correct sequence of the flow of flue gases through these boiler accessories is:

    1. 1, 3, 4, 2
    2. 3, 1, 4, 2
    3. 3, 1, 2, 4
    4. 1, 3, 2, 4
  20. Consider the following components:
    1. Radiation evaporator
    2. Economiser
    3. Radiation superheater
    4. Convection superheater

    In the case of Benson boiler, the correct sequence of the entry of water through these components is:

    1. 1, 2, 3, 4
    2. 1, 2, 4, 3
    3. 2, 1, 3, 4
    4. 2, 1, 4, 3
  21. Coal fired power plant boilers manufactured in India generally use
    1. Pulverised fuel combustion
    2. Fluidised bed combustion
    3. Circulating fluidised bed combustion
  22. Match List–I (name of the boiler) with List–II (special features) and select the correct answer using the code given below the lists:
    List I List-II
    A. Lancashire 1. High pressure water tube
    B. Cornish 2. Horizontal double fire tube
    C. La Mont 3. Vertical multiple fire tube
    D. Cochran 4. Low pressure inclined water tube
    5. Horizontal single fire tube.

    Codes:

            A   B   C   D

    1.  2   5    1    3
    2.  2   4    3    1
    3.  1   5    2    3
    4.  5   4    1    3
  23. Which of the following safety devices is used to protect the boiler when the water level falls below a minimum level?
    1. Water-level indicator
    2. Fusible plug
    3. Blow-off cock
    4. Safety valve
  24. Which of the following form part (s) of the boiler mountings?
    1. Economiser
    2. Feed check valve
    3. Steam trap
    4. Superheater

    Select the correct answer using the codes given below:

    Codes:

    1. 2
    2. 1 and 3
    3. 2, 3, and 4
    4. 1, 2, 3, and 4
  25. Which of the following power plants use heat recovery boilers (unfired) for steam generation?
    1. Combined cycle power plants
    2. All thermal power plants using coal
    3. Nuclear power plants
    4. Power plants using fluidised bed combusting

    Select the correct answer using the codes given below:

    1. 1 and 2
    2. 3 and 4
    3. 1 and 3
    4. 2 and 4
  26. Benson boiler is one of the high pressure boilers having
    1. One drum
    2. One water drum and one steam drum
    3. Three drums
    4. No drum
  27. Forced draught fans of a large steam generator have
    1. Backward curved blades
    2. Forward curved blades
    3. Straight or radial blades
    4. Double curved blades
  28. The device used to heat feed-water by utilising the heat of the exhaust flue gases before leaving through the chimney is called
    1. Superheater
    2. Economiser
    3. Air preheater
    4. ID fan
  29. Consider the following statements:
    1. Forced circulation is always used in high pressure power boilers.
    2. Soot blowers are used for cleaning tube surfaces at regular intervals.
    3. Electrostatic precipitator is used to remove fly ash from flue gases.

    Which of these statements are correct?

    1. I, II, and III
    2. II and III
    3. I and III
    4. I and II
  30. Once-through boilers operate at
    1. Subcritical pressure
    2. Supercritical pressure
    3. Subcritical as well as supercritical pressures
    4. Critical pressure only
  31. Match List I (components) with List II (functions) and select the correct answer using the codes given below the lists:
    List I List-II
    A. Steam trap 1. Controls steam flow rate
    B. Fusible plug 2. Controls rate of water flow to boiler
    C. Blow-off cock 3. Puts off furnace fire when water level reaches unsafe limit
    D. Feed check valve 4. Removes mud and dirt collected at the bottom of boiler
    5. Drains off water collected by partial condensation of steam in pipes

    Codes:

            A   B   C   D

    1.  5   1    4    2
    2.  1   3    5    4
    3.  5   3    4    2
    4.  1   2    5    4
  32. Which of the following sequences indicates the correct order for flue gas flow in the steam power plant layout?
    1. Superheater, economiser, and air preheater
    2. Economiser, air preheater, and superheater
    3. Air preheater, economiser, and superheater
    4. Economiser, superheater, and air preheater
  33. Which one of the following statements is not correct?

    In a fluidised bed boiler,

    1. The combustion temperatures are higher than those in the conventional boilers
    2. Inferior grade of coal can be used without slagging problems
    3. The formation of NOx is less than that in the conventional boilers
    4. The volumetric heat release rates are higher than those in the conventional boilers
  34. Match List I (boilers) with List II (type/description) and select the correct answer using the codes given below the lists:
    List I
    (Boilers)
    List II
    (Type/Description)
    A. Lancashire 1. Horizontal straight tube, fire-tube boiler
    B. Benson 2. Horizontal straight tube, water-tube boiler
    C. Babcock and Wilcox 3. Bent tube, water-tube boiler
    D. Stirling 4. High pressure boiler

    Codes:

            A   B   C   D

    1.  4   2    1    3
    2.  1   4    2    3
    3.  4   2    3    1
    4.  1   4    3    2
  35. A device which is used to drain off water from steam pipes without escape of steam is called
    1. Steam separator
    2. Steam trap
    3. Pressure reducing valve
    4. Injector
  36. Blowing down of boiler water is the process to
    1. Reduce the boiler pressure
    2. Increase the steam temperature
    3. Control the solids concentration in the boiler water
    4. Control the drum level
  37. Consider the following statements regarding fluidised bed combustion boilers:
    1. The combustion temperatures are low, around 900°C.
    2. The formation of oxides of nitrogen is low.
    3. It removes sulphur from coal during combustion process.
    4. It requires high quality of coal as fuel.

    Which of these statements are correct?

    1. 1, 2, 3 and 4
    2. 1, 2 and 3
    3. 2, 3 and 4
    4. 1 and 4
  38. The correct gas flow path in a typical large modern natural circulation boiler is
    1. Combustion chamber – reheater – superheater – economiser – air preheater – ID fan – electrostatic precipitator – stack
    2. Combustion chamber – superheater – reheater – economiser – air preheater – electrostatic precipitator – ID fan – stack
    3. Combustion chamber – reheater – superheater – air preheater – economiser – electrostatic precipitator – ID fan – stack
    4. Combustion chamber – superheater – reheater – economiser – air preheater – ID fan – electrostatic precipitator – stack
  39. The draught in locomotive boilers is produced by
    1. Chimney
    2. Centrifugal fan
    3. Steam jet
    4. Locomotion
  40. Consider the following statements regarding waste-heat boilers:
    1. Waste-heat boilers are placed in the path of exhaust gases.
    2. These are fire tube boilers.
    3. The greater portion of the heat transfer in such boilers is due to convection.

    Which of the statements given are correct?

    1. 1, 2, and 3
    2. 1 and 2
    3. 2 and 3
    4. 1 and 3
  41. Which of the following statements is not true for a supercritical steam generator?
    1. It has a very small drum compared to a conventional boiler
    2. A supercritical pressure plant has higher efficiency than a subcritical pressure plant
    3. The feed pressure required is very high, almost about 1.2 to 1.4 times the boiler pressure
    4. As it requires absolutely pure feed water, preparation of feed water is more important than in subcritical pressure boiler
  42. Which one of the following is the correct path of water flow through various components of boiler of a modern thermal power plant?
    1. Economiser – boiler drum – water walls – boiler drum – superheater – turbine
    2. Economiser – boiler drum – water walls –superheater – turbine
    3. Economiser – water walls – boiler drum – superheater – turbine
    4. Economiser – water walls – superheater – turbine
  43. Consider the following:
    1. Superheater
    2. Economiser
    3. Air pre-heater
    4. Condenser

    Which of these improve overall steam power plant efficiency?

    1. Only 1, 2, and 3
    2. Only 2 and 3
    3. Only 1 and 4
    4. 1, 2, 3, and 4
  44. Consider the following:
    1. Injector
    2. Economiser
    3. Blow-off clock
    4. Steam stop valve

    Which of the above is/are not boiler mountings?

    1. Only 1
    2. Only 1 and 2
    3. 1, 2, and 3
    4. 2 and 4
  45. The draught produced by the chimney is called
    1. Natural
    2. Induced
    3. Forced
    4. Balanced
  46. The draught produced by a steel chimney as compared to that produced by a brick chimney for the same height is
    1. More
    2. Less
    3. Same
    4. Unpredictable
  47. Artificial draught is produced by
    1. Induced fan
    2. Forced fan
    3. Steam jet
    4. Both (a) and (b)
  48. The draught in locomotive is produced by
    1. Forced fan
    2. Induced fan
    3. Steam jet
    4. Chimney
  49. The artificial draught produces
    1. Less smoke
    2. More draught
    3. Less chimney gas temperature
    4. All of these
  50. The chimney efficiency is approximately
    1. 10%
    2. 5%
    3. 2%
    4. 0.25%
  51. The pressure at the furnace is minimum is case of
    1. Forced draught
    2. Induced draught
    3. Balanced draught
    4. Natural draught

Explanatory Notes

  1. 14. (c)

Review Questions

  1. What is a steam generator?
  2. How do you classify steam generators?
  3. What are the merits of water tube boilers?
  4. What are the demerits of fire tube boilers?
  5. List the requirements of a good boiler.
  6. Enumerate the factors affecting boiler selection.
  7. What are the advantages of high pressure boilers?
  8. List the advantages of forced circulation boilers.
  9. What is a boiler mounting? List the various boiler mountings.
  10. What are the functions of boiler accessories?
  11. Name the various accessories used on a boiler.
  12. Define equivalent evaporation and factor of evaporation.
  13. Define boiler efficiency.
  14. What are the purposes of carrying out boiler trial?
  15. Differentiate between natural and artificial draught.
  16. Describe the consumption features of a Lancashire boiler. How can superheated steam be produced with this boiler?
  17. Describe the construction features of a Cochran boiler. Discuss its working briefly.
  18. When is a water tube boiler exclusively used? Discuss the construction and working of Babcock and Wilcox boiler.
  19. With the help of a neat sketch, describe a Loeffler boiler. What is the working pressure of such a boiler?
  20. Draw a schematic flow diagram of a modern steam generator. Discuss the installation of connection and radiation superheaters and their exit temperature response with varying steam flow.
  21. With the help of a neat sketch, discuss the working principle of Velox high pressure boiler.

Exercises

3.1 In a boiler trial, 1360 kg of coal was consumed in 24 hours. The mass of water evaporated was 13600 kg and steam pressure was 7.5 bar. The feed-water temperature was 35°C. The calorific value of 1 kg of coal is 30000 kJ/kg. Calculate the boiler efficiency and equivalent evaporation.

[Ans. 87.31%, 11.6 kg]

3.2 A boiler generates 8.5 kg of steam per kg of coal burned at a pressure of 13.5 bar from feed water having absolute temperature of 350 K. The boiler efficiency is 70% and factor of evaporation 1.17. Taking specific heat of steam at constant pressure to be 2.1 kJ/kgK, calculate (a) the degree of superheat and temperature of steam generated, (b) the calorific value of coal, and (c) the equivalent evaporation.

[Ans. 84°C, 550.4°C, 32070 kJ/kg, 9.94 kg]

3.3 The data obtained during a boiler trial is as follows:

Duration = 1 hour

Steam generated = 35,500 kg

Steam pressure = 12 bar

Steam temperature = 250°C

Temperature of water entering economiser = 17°C

Temperature of water leaving economiser = 77°C

Oil burnt = 3460 kg

CV of oil = 39,500 kJ/kg

Calculate (a) equivalent evaporation, (b) thermal efficiency of plant, and (c) percentage of heat energy of fuel energy utilised by the economiser.

[Ans. 13.01 kg, 74.4%, 6.52%]

3.4 The following data relate to a boiler trial:

Coal burnt per hour = 6750 kg

Moisture content in coal = 2%

HCV of coal = 35,490 kJ/kg

Ultimate mass analysis of dry coal: C = 84%, O2 = 4%, ash = 8%

Dry flue gas analysis by volume: CO2 = 9.5%, O2 = 10.48%, N2 = 80.02%

Temperature of flue gases = 305°C

Room temperature = 32°C

Specific heat of flue gases = 1 kJ/kgK

Specific heat of steam in flue gases = 2 kJ/kgK

Mass of steam generated at 16.5 bar from feed water at 47°C = 60,500 kg/h

Draw up the heat balance sheet per kg of dry coal. Assume that air contains 23.1% by mass of oxygen.

Ans.

3.5 Calculate the height of chimney required to produce a draught equivalent to 1.7 m of water if the fuel gas temperature is 270°C and ambient temperature is 22°C and minimum amount of air per kg of fuel is 17 kg.

[Ans. 33.45 m]

3.6 A boiler is equipped with a chimney of 30 m height. The flue gases passing through the chimney are at temperature of 228°C, whereas the atmospheric temperature is 21°C. If the air flow through the combustion chamber is 18 kg/kg of fuel burnt, find the theoretical draught produced in mm of water and velocity of flue gases if 50% of theoretical draught is lost.

[Ans. 1.37 cm of water, 13.45 m/s]

3.7 Calculate the mass of flue gases flowing through the chimney when the draught produced is equal to 1.9 cm of water. Temperature of flue gases is 290°C and ambient temperature is 20°C. The flue gases formed per kg of fuel burnt are 23 kg. Neglect the losses and take the diameter of the chimney as 1.8 m.

[Ans. 38 kg/s]

3.8 A chimney is 42 m high and the temperature of hot gases in the chimney is 310°C. The temperature of outside air is 35°C. The furnace is supplied with 20 kg of air per kg of fuel burnt. Calculate the draught in cm of water and velocity of flue gases.

3.9 In a chimney of height 45 m, temperature of flue gases with natural draught is 365°C. The temperature of waste gases by using artificial draught is 130°C. The temperature of outside air is 35°C. If air supplied is 20 kg per kg of fuel burnt, determine the efficiency of chimney. Take cpg = 1.004 kJ/kgK for flue gases.

3.10 Calculate the thermal efficiency and equivalent evaporation of a boiler for the following data:

Steam pressure = 10.5 bar

Steam temperature = 250°C

Feed-water temperature = 35°C

Water evaporated = 10 kg/kg of coal burnt

Calorific value of coal = 33,400 kJ/kg

3.11 The following data refers to a steam power plant.

Height of Chimney = 35 m

Draught = 18 mm of water gauge

Temperature of flue gases = 370°C

Boiler house temperature = 32°C

Determine the quantity of air used per kg of fuel burnt in the boiler.

3.12 In a boiler trial, the following observations were made:

Steam pressure = 10 bar, steam generated = 550 kg/h

Fuel used = 66 kg/h, Moisture in fuel = 1.5% by weight

Weight of dry flue gases = 8.5 kg/kg of fuel, LCV of fuel = 34 MJ/kg

Temperature of flue gases = 320°C, Boiler house temperature = 27°C

Feed-water temperature = 50°C, Specific heat of flue gases = 1.1 kJ/kgK

Dryness fraction of steam = 0.95

Draw up the heat balance sheet for the boiler.

3.13 The following data was obtained during a boiler trial:

Feed-water temperature = 45°C, feed water supplied = 4200 kg/h

Steam pressure = 10 bar, dryness fraction of steam = 0.98

Coal fired = 450 kg/h, CV of dry coal = 37,500 kJ/kg

Moisture in coal = 4%, temperature of flue gases = 280°C

Boiler house temperature = 15°C, barometer pressure = 1 bar,

Ultimate analysis of coal:

C = 86 %, H2 = 4 %, Ash = 5 %, incombustible matter = 5%

Volumetric analysis of dry flue gases:

CO2 = 10.4 %, CO = 1.2 %, O2 = 9 %, N2 = 79.4 %

Draw the heat balance sheet on 1 kg of coal used basis.

ANSWERS TO MULTIPLE-CHOICE QUESTIONS
  1. b
  2. b
  3. a
  4. b
  5. c
  6. b
  7. b
  8. b
  9. a
  10. a
  11. c
  12. c
  13. d
  14. c
  15. c
  16. d
  17. c
  18. c
  19. d
  20. c
  21. a
  22. a
  23. b
  24. a
  25. c
  26. d
  27. a
  28. a
  29. a
  30. d
  31. c
  32. a
  33. d
  34. d
  35. b
  36. c
  37. b
  38. b
  39. c
  40. b
  41. c
  42. c
  43. a
  44. b
  45. a
  46. a
  47. d
  48. c
  49. d
  50. d
  51. c