1. Equipment in VCR
Room

3. A Positive Displacement Compressor
P M V Subbarao
Associate Professor
Mechanical Engineering Department
IIT Delhi
Small Things will have Huge Vigor………….

4. Reciprocating Compressors
The cylinder walls are cooled :
Air cooled: Circumferential Fins.
Water Jacket: Annular Jacket.
Its close to Isothermal but not Isothermal
Let us say polytropic.
1 < n < g

5. Pressure Volume Diagram
Indicated work done on the gas per cycle:
= area abcd
= area abef +area bcoe – area adof
Work input

6. Ideal Gas Equation Polytropic process:
The actual power input to the compressor is larger than the indicated power, due to
the work necessary to overcome the losses due to friction.
Shaft Power = Indicated Power + Friction Power

7. Indicated Power

8. Condition for Minimum Work
The work done on the gas is given by the area of the indicator diagram.
The work done will be minimum when the area of the diagram is a minimum .
The height of the diagram is fixed by required pressure ratio.
The length of the line da is fixed by the cylinder volume.
The cylinder volume is fixed by the required induction of gas.
Only the line which can influence the area of the diagram is line ab.
The shape of line ab depends on the value of index n.

9. The indicated work done when the gas is compressed isothermally is:
Area ab1cd = area ab1ef +area b1c0e - area ad0f
Area ab1ef =
Indicated work per cycle for isothermal compression:
For an ideal gas under going an isothermal process:

10. Various Compression Process on T-s Diagram
Isothermal Power
Various Compression Process on T-s Diagram

11. Actual Reciprocating Compressor
Clearance is necessary in a compressor to give mechanical safety to the working parts.
Clearance is also required to provide necessary space for valve operations.
General clearance volume for good quality compressors is about %.

12. Compression and Re Expansion

13. The indicated work done:
Area abcd = area abef -area cefd
Indicated Power

14. The mass delivered per unit time can be increased by designing the
machines to be double acting.

15. Real Indicator Diagram

16. Volumetric Efficiency of A Compressor
One of the effects of clearance is to reduce the induced volume of gas to a less than that of the swept volume.
Volumetric efficiency: the volume of gas handled by a compressor measured at the free air pressure and temperature, divided by the swept volume of the cylinder. Or
The mass of gas delivered, divided by the mass of gas which would fill the swept volume at the free air conditions of pressure and temperature.
The volume of air dealt with per unit time by an air compressor is quoted as Free Air Delivery (FAD).
FAD is rate of volume flow delivered, measured at the pressure and temperature of the atmosphere.

17. Mass delivered per cycle, if FAD per cycle is V, at pcyl and Tcyl.
The mass required to fill the swept volume, Vs at patm and Tatm is:
Isothermal Efficiency of A compressor = Actual power consumption/Isothermal power consumption

18. Volume induced = Va-Vd = Vs+Vc-Vd

19. Volumetric efficiency decreases with increasing pressure ratio.
Temperature of the gas after compression, T2 is high.
Isothermal compression minimizes the work input.

20. High Pressure Compressors

21. High Pressure Compressors

22. High Pressure Compressors

23. High Pressure Compressors
Higher the compression ratio, higher is the value of vd.
For p4 > p3 > p2 ….
Vd’’ > Vd’ > Vd…
For a given cylinder volume, the amount of induced air decreases with increasing pressure.
Leads to under utilization of cylinder volume.
Loss of volumetric efficiency.

24. Multi Stage Compression
Distribute the compression between two or more cylinders.
The delivery volume of low pressure cylinder enters as induced volume of high pressure cylinder.
Vb-Vc becomes Va’’ – Vd’.
Total loss of volume = loss volume in LP cylinder + Loss of volume in HP cylinder.
Vd-Vc + Vd’ + Vc’

25. Multistage Compression with Inter cooling

27. Roots Blower P2
Work done per cycle: P1 V

28. Vane Type Blower