Compressors. An air compressor is a mechanical device that produces compressed air i.e. which increases the pressure of the air above the atmospheric pressure as per the requirement and stores it in a high-pressure vessel. In an air compressor, normal atmospheric air is sucked and compressed continuously. Compressors are devices for the compression and delivery of gases. They are widely used as separate units and as important parts of different types of heat engines. Compressors are driven by different types of prime movers (electric motors, steam and gas turbines, diesel engines, etc).

1. COMPRESSORS
30 AUGUST
2019

2. INTRODUCTION
• A compressor is the heart of a refrigeration system. Hence, it the
costliest component of a refrigeration.
• It regulates the flow of a refrigerant in a refrigeration plant by
pumping action.
• This refrigerant compressor takes in refrigerant gas at low pressure
(from the evaporator), compresses it and then delivers it as a high-
pressure gas to the condenser.
• The absolute pressure of a refrigerant at the inlet of a compressor
is known as the suction pressure; and the absolute pressure of a
refrigerant at the outlet of a compressor is known as the discharge
pressure.
• The most commonly used compressors in the modern vapour
compression system are the reciprocating, rotating screw,
centrifugal and scroll compressors.
• Each application prefers one or another due to size, noise, and
efficiency and pressure aspects.

3. Functions and Types of refrigeration
compressor
• Remove vapour from the evaporator and reduce it to a point at
which desired evaporating temperature can be maintained.
• Increasing pressure of the vapour to levels high enough such that
saturation temperature become higher than the temperature of the
cooling medium.
Types of compressors
• Reciprocating - workhorse of refrigeration industry. Consists of a
piston moving back and forth in a cylinder with suction and
discharge valves arranged to allow pumping to take place,
• Screw, centrifugal, and vane compressors – they use rotating
elements, the screw and vane compressors are positive-
displacement machines, and centrifugal compressors operate due
to centrifugal force.

4. Performance of Reciprocating Compressors
• Important performance characteristics of a compressor are
listed below.
 The most important ones are the refrigeration capacity and
power requirement. These characteristics of a compressor
operating at constant speed are controlled largely by the
suction and discharge pressures.
 Actual volumetric efficiency.
 Clearance volumetric efficiency (ideal).
 Power.
 Refrigeration capacity.
 COP.
 Discharge temperature.
 Compression efficiency.

5. Volumetric efficiency

6. Volumetric efficiency

7. Volumetric efficiency

8. Volumetric efficiency

9. Volumetric efficiency

10. Volumetric efficiency
• Cylinder heating of the suction gas reduces the volumetric efficiency, since
immediately upon entering the cylinder the gas is warmed and expanded.
• The specific volume of the gas inside the cylinder consequently higher
than when entering the compressor, which is the position on which the
volumetric efficiency is based.
• All these result in a lower actual volumetric efficiency than is predicted by
re-expansion of clearance gas alone.

11. VOLUMETRIC EFFICIENCY BASED ON
CLEARANCE VOLUME
* Defined by:
ηcv = 1 – m(R1/n – 1)
where m = percentage clearance volume
R = compression ratio
η = isentropic compression exponent
= 1,15 for R22
= 1,25 for NH3

12. Volumetric and mass flow rate

13. Volumetric and mass flow rate
• The curves for volumetric efficiency and mass flow rate with the
evaporating temperature at a certain condensing pressure are shown in
Figure below.
• The volumetric efficiencies are calculated based on the percent clearance
m and applied to a compressor operating with a condensing temperature.
• When the suction pressure and discharge pressure are the same (same
evaporating and condensing pressure), the volumetric efficiency is 100%.

14. Power requirement

15. Power requirement

16. Implications of Power Curve
• Most systems operate on the left side of the curve
• During the period of pulldown of temperature following start-up with a
warm evaporator, however, the power requirement passes through its
peak and may demand more power than the motor, which is selected for
design conditions, is capable of supplying steadily.
• Sometimes motors have to be oversized just to take the system down
through the peak in the power curve.
• To prevent oversizing the motor, the suction pressure is sometimes
reduced artificially by throttling the suction gas until the evaporator
pressure drops below the peak in the power curve.
• During regular operation heavy refrigeration loads raise the evaporating
temperature, which increases the power requirement of the compressor
and may overload the motor.

17. Power requirement
• Compressor work and mass flow
rate decreases as the
condensing temperature
increases.
• Power increases to a peak and
then begins to drop off.
• From the standpoint of power
and efficiency, a low condensing
temperature is desirable, thus the
condenser should use the coldest
air or water available.
• The condenser should operate
with maximum airflow or water
flow.

18. Refrigeration Capacity

19. Refrigeration Capacity
• Both refrigerating capacity
and the refrigerating effect
decrease with increasing
condensing temperature
• The refrigerating capacity
drops rapidly on an
increase in condensing
temperature.

20. COP and volume flow rate per kilowatt of
refrigeration

21. Compression and Mechanical Efficiency

22. Compressor Discharge Temperature

23. EXAMPLE OF VOLUMETRIC EFFICIENCY
CALCULATION
A 6-cylinder reciprocating R22-compressor operating
between -20°C saturated suction and 45°C saturated
condensing temperatures has a cylinder bore of 72 mm
and a stroke 65 mm. It runs at 1750 rev/min, has a
refrigeration capacity of 95 kW and a clearance volume
of 4,5 %. Drive power is measured at 35,6 kW. Assuming
an ideal cycle, calculate :
(a) the clearance volumetric efficiency ;
(b) the actual volumetric efficiency ; and
(c) The compression efficiency.

24. SOLUTION
(a) Clearance volumetric efficiency:
ηvc = 100 – m(Vsuc/Vdis – 1)
ηvc = 1 – m(R1/n – 1) = 1 – 0,045[(1721/245)1/1,15 – 1)
= 80 %
(b) Compressor displacement:
Vpiston = (π/4) X D2 x L x N x n
Vpiston = (π/4) x 0,0722 x 0,065 x (1750/60) x 6
= 0,0463 m3/s From refrigerant tables :
hv = 1198,24 kJ/kg and hf = 977,30 kJ/kg
Hence, Refrigeration Effect Δh = 220,94 kJ/kg
Mass flow rate of refrigerant m = Q/Δh = 95/220,94
= 0,430 kg/s

25. SOLUTION
Actual volumetric flow rate is therefore:
Vactual = 0,430 kg/s x 0,092 m3/kg = 0,0396 m3/sec
Hence, the actual volumetric efficiency is:
(0,0396/0,0463) x 100 = 85,5%
(c) Compression efficiency :
Actual work of compression Wactual = hdischarge – hsuction
Isentropic work of compression Wisentr = m x Δhisentr.compr.
hdischarge = 1252 kJ/kg
hsuction = 1198,24 kJ/kg
Hence, Δhisentr.compr.= 1252 – 1198,24 = 53,76 kJ/kg
Wisentr.= 0,43 x 53,76 = 23,12 kW
ηc = (23,12/35,6) x 100 = 24,3 %