Chapter 13 Rotary Air Compressors – Thermal Engineering

Chapter 13

Rotary Air Compressors

13.1 ❐ INTRODUCTION

In a rotary compressor, the compression of air is achieved due to the rotating blades fitted in a rotor. It requires less starting torque as compared to reciprocating compressors because of direct coupling with the prime over. Usually, rotary compressors operate at high speed and supplies higher quantity of air than reciprocating compressors. Rotary compressors may be classified as: (a) Roots blower, (b) Vane type compressor, (c) Lysholm compressor and (d) Screw compressor.

13.2 ❐ WORKING PRINCIPLE OF DIFFERENT ROTARY COMPRESSORS

In positive displacement type rotary compressors, the air is compressed by entrapping it between the reduced space of two sets of engaging surfaces. The pressure rise is either by backflow of air, as in the case of roots blower, or by both squeezing action and backflow of air, as in the case of vane type.

13.2.1 Roots Blower or Lobe Compressor

The schematic arrangement of a roots blower is shown in Fig. 13.1(a). It consists of two rotors that are driven externally. One rotor is connected to the driver and the second rotor is gear driven from the first in the opposite direction. The lobes of the rotors are of epicycloids, hypocycloid, or involute profiles to ensure a seal between the high and low pressure regions at all angular positions. A small clearance provided between the rotors and the cylinder surface to reduce the wear reduces the efficiency of the compressor due to leakage.

The volume Vs of air at atmospheric pressure p1 is entrapped between the left hand rotor and the cylinder casing. The volume of air once entrapped does not decrease from entry to exit, and therefore, there is no rise of pressure till the exit port is uncovered. As the exit part opens, some high pressure air from the receiver rushes back and mixes with the air volume Vs irreversibly until the pressure is equalised. The resulting pressure after mixing will be the receiver pressure p2. The air is then transferred to the receiver. This happens four times in one revolution in case of two-lobed rotor and six times in case of three-lobed rotor. The pV diagram for the roots blower is shown in Fig. 13.1(b).

Figure 13.1 Roots blower

where i = number of lobes

Let V1 = volume of air handled per minute at p1 and T1.

Then work done per minute is,

If the compression is isentropic, then ideal work required per minute is,

The efficiency of roots blower

where rp = is the pressure ratio.

The roots blower is used to supply air from 0.15 to 1500 m3/min with pressure ratio up to 3.6 per blower. Rotational speeds up to 12,500 rpm are used and can be directly coupled to a steam turbine or a gas turbine shaft without any intermediate gearing.

Figure 13.2 Vane type compressor

13.2.2 Vanes Type Blower

The schematic diagram of a vane type blower is shown in Fig. 13.2(a). It consists of a rotor located eccentrically in a cylindrical outer casing. The rotor carries a set of spring-loaded vanes in the slots of the rotor. The air of volume V1 at atmospheric pressure p1 is entrapped between two vanes. As the rotation proceeds counter clockwise, the entrapped air is first compressed reversibly to Va. Afterwards, the compression continues to Vc (say) and then to Vd. After this, the delivery port is uncovered and the irreversible compression takes place from pressure pd to p2. The pV diagram for the compression process is shown in Fig. 13.2(b). The work done per revolution with i vanes is given by:

The vane blowers require less power than the root blowers for the same capacity and pressure rise. They are used to deliver up to 150 m3/min of air at pressure ratio up to 8.5. The speed is limited to 3000 rpm.

13.2.3 Lysholm Compressor

The Lysholm compressor is shown in Fig. 13.3. Its principle of working is similar to that of the roots blower. The air is admitted through one end of the compressor and trapped between the helical rotors and the casing. The screw action of the rotors displaces the air axially. It produces constant compression internally. Its main disadvantage is mechanical complexity.

Figure 13.3 Lysholm compressor

13.2.4 Screw Compressor

The screw compressor may be single helical or double helical. The advantage of double helical is that they are balanced axially. The air is carried forward to the discharge along the rotor in pockets formed between the teeth and the casing as shown in Fig. 13.4. The principle of working is similar to that of the roots blower.

Figure 13.4 Screw compressor

Example 13.1

Free air of 25 m3/min is compressed from 1 bar to 2.5 bar. Calculate the indicated power required if the compression is carried out in (a) roots blower and (b) vanes blower. Assume that there is 20% reduction in volume before the backflow occurs. Also, calculate the isentropic efficiency in each case.

Solution

The pV diagrams for roots blower and vanes blower are shown in Fig. 13.5(a) and (b), respectively.

Figure 13.5 p−V diagrams: (a) Roots blower, (b) Vanes blower

  1. Roots blower,
  2. Vanes blower,

    I.P. if whole compression is carried out isentropically,

    Isentropic efficiency of roots blower

    Isentropic efficiency of vanes blower

Example 13.2

A rotary vanes blower works between the pressure limits of 1 bar and 1.8 bar, and gives 5 m3/min of free air delivered when running at 240 rpm. Determine the power required to drive the blower when (a) ports are so placed that there is no internal compression and (b) when the ports are so placed that there is 50% pressure rise due to internal adiabatic compression before backflow occurs. Assuming mechanical efficiency to be 98%, calculate the blower efficiency.

Solution

The pV diagrams without and with internal compression are shown in Fig. 13.6(a) and (b), respectively.

Figure 13.6 p−V diagrams: (a) Without internal compression, (b) With internal compression

  1. Without internal compression,
  2. With internal compression,

    For minimum power requirement, the whole compression should be a reversible adiabatic process.

    Actual I.P. = Ideal I.P. × ηmech

        = 5.333 × 0.98 = 5.226 kW

    Isentropic efficiency without internal compression =

    Isentropic efficiency with internal compression =

13.3 ❐ COMPARISON OF ROTARY AND RECIPROCATING COMPRESSORS

The comparison of rotary and reciprocating compressors is given in Table 13.1.

 

Table 13.1 Comparison of rotary and reciprocating compressors

Example 13.3

Free air of 30 m3/min is compressed from 101.3 kPa to 2.23 bar in roots blower. Determine the power required and the isentropic efficiency.

Solution

The p − V diagrams for roots blower is shown in Fig. 13.7.

Figure 13.7 p V diagrams

V1 = 30 m3/min, p1 = 101.3 kPa
p2 = 2.23 bar

Refer Fig. 13.8.

Indicated power, I.P. =

Isentropic indicated power, (I.P.)isen =

Isentropic efficiency, ηisen =

Summary for Quick Revision

  1. Rotary compressors are of positive displacement type in which air is compressed by entrapping it between the reduced space of two sets of engaging surfaces.
  2. The pressure rise is either by backflow of air (as in roots blower) or by both squeezing action and backflow (as in vanes blower).
  3. Roots blower

    Work done per minute, Wroots = (p2p1)V1

    Roots efficiency, ηroots =

    where V1 = volume of air handled per minute

    rp = pressure ratio

  4. Vanes type blower

    Work done per revolution =

    where i = number of Vanes

    pd = maximum isentropic pressure

  5. Indicated power, I.P. = kW

Multiple-choice Questions

  1. The efficiency of a roots air blower is (rp = pressure ratio)
  2. In a roots blower with two lobes, the high pressure air is delivered in one revolution
    1. Once
    2. Twice
    3. Thrice
    4. Four times
  3. The efficiency of vane type air compressor as compared to roots blower for the same pressure ratio is
    1. Less
    2. More
    3. Same
    4. May be less or more
  4. The pV diagram shown in Fig. 13.8 below is for

    Figure 13.8 p-V diagram

    1. Roots blower
    2. Vanes blower
    3. Centrifugal compressor
    4. axial compressor
  5. Roots blower is an example of
    1. Reciprocating (positive displacement) compressor
    2. Rotary (positive displacement) compressor
    3. Centrifugal compressor
    4. Axial compressor

Review Questions

  1. What are the types of rotary compressors?
  2. Draw the pV diagram for a roots blower.
  3. Which rotary compressor is used for steam/gas turbines?
  4. What do you mean by a positive displacement compressor?

Exercises

13.1 What are the various types of compressors?

13.2 What is a positive displacement compressor?

13.3 Explain the working of a roots blower.

13.4 Derive an expression for the work done and efficiency of a roots blower.

13.5 Explain the working of a vane type blower.

13.6 Explain the working of a screw compressor.

13.7 Compare rotary and reciprocating compressors.

ANSWERS TO MULTIPLE-CHOICE QUESTIONS
  1. a
  2. d
  3. b
  4. b
  5. b