Explore the heart of refrigeration systems with this insightful presentation on refrigerant compressors. Learn about different types of compressors, their working principles, and gain practical insights through numerical examples. Whether you're a student or a professional in the field, this slide deck provides valuable knowledge in the realm of HVAC and refrigeration technology

1. Welcome To
Heating, Ventilation, Air Conditioning and Refrigeration Engineering
• Topic : Refrigerant compressor
PROF. R.S. KOLHE
ME (HEAT POWER)
BE (MECHANICAL)

2. Thermal Design of Refrigeration System
Components
• Compressor : Characteristic curves of reciprocating & Centrifugal
compressors, sizing of reciprocating compressor
• Evaporator : Standards & Codes, Performance analysis of Dx evaporator,
• Condenser: Standards & Codes, air-cooled condenser, shell & tube
condenser and evaporative condenser.
• Expansion Devices : Standards & Codes, Operating Characteristics,
Liquid Charge in the Sensing Bulb , Hunting of Thermostatic Expansion
Valve
• Cooling Tower: Types & design of cooling towers, cooling tower thermal
performance, tower Efficiency.

3. Introduction
• A refrigerant compressor, as the name indicates, is a machine used to
compress the vapour refrigerant from the evaporator and to raise its
pressure so that the corresponding saturation temperature is higher
than that of the cooling medium.
• It also continually circulates the refrigerant through the refrigerating
system.
• Since the compression of refrigerant requires some work to be done on
it, therefore, compressor must be driven by some prime mover.

4. Classification of Compressors
1. According to the method of compression
(a) Reciprocating compressors,
(b) Rotary compressors, and
(c) Centrifugal compressors.
2. According to the number of working strokes
(a) Single acting compressors, and
(b) Double acting compressors.
3. According to the number of stages
(a) Single-stage (or single-cylinder) compressors, and
(b) Multi-stage (or multi-cylinder) compressors.
4. According to the method of drive employed
(a) Direct drive compressors, and
(b) Belt drive compressors.
5. According to the location of the prime mover
(a) Semi-hermetic compressors (direct drive, motor and compressor in separate housings),
(b) Hermetic compressors (direct drive, motor and compressor in same housings

5. Important Terms
1. Suction pressure. It is the absolute pressure of refrigerant at the inlet of a compressor.
2. Discharge pressure. It is the absolute pressure of refrigerant at the outlet of a
compressor.
3. Compression ratio (or pressure ratio). It is the ratio of absolute discharge pressure to
the absolute suction pressure. Since the absolute discharge pressure is always more than
the absolute suction pressure, therefore, the value of compression ratio is more than
unity. The compression ratio may also be defined as the ratio of total cylinder volume to
the clearance volume.
4. Suction volume. It is the volume of refrigerant sucked by the compressor during its
suction stroke. It is usually denoted by Vs
5. Piston displacement volume or stroke volume or swept volume. It is the volume
swept by the piston when it moves from its top or inner dead position to bottom or outer
dead centre position. Mathematically, piston displacement volume or stroke volume or
swept volume,

6. Important Terms
5. Piston displacement volume or stroke volume or swept volume. It
is the volume swept by the piston when it moves from its top or inner
dead position to bottom or outer dead centre position. Mathematically,
piston displacement volume or stroke volume or swept volume,
D = Diameter of cylinder, and L = Length of piston stroke.
6. Clearance factor. It is the ratio of clearance volume (vc) to the piston
displacement volume (Vp). Mathematically, clearance factor,

7. Important Terms
• Compressor capacity. It is the volume of the actual amount of refrigerant
passing through the compressor in a unit time. It is equal to the suction
volume ( v s ). It is expressed in m3 /s.
• Volumetric efficiency. It is the ratio of the compressor capacity or the
suction volume Vs to the piston displacement volume (Vp).
Mathematically, volumetric efficiency,

8. Reciprocating Compressors
• The compressors in which the vapour refrigerant is compressed by the
reciprocating (i.e. back and forth) motion of the piston are called
reciprocating compressors.
• These compressors are used for refrigerants which have comparatively low
volume per kg and a large differential pressure, such as ammonia (R-717),
R-12, R-22, and methyl chloride (R-40).
• The two types of reciprocating compressors in general use are single acting
vertical compressors and double acting horizontal compressors. The single
acting compressors usually have their cylinders arranged vertically, radially or
in a V or W form.
• The double acting compressors usually have their cylinders arranged
horizontally.

9. Single Stage, Single Acting Reciprocating Compressor

10. Work Done by a Single-Stage
Reciprocating Compressor
Two Important Cases Of Work Done:
1. When there is no clearance volume in the cylinder, and
2. When there is some clearance volume.

11. Work Done by a Single-Stage, Single Acting
Reciprocating Compressor without Clearance Volume

12. • P1 = Suction pressure of the refrigerant (before compression
• V1=Suction volume of the refrigerant (before compression
• T1= Suction temperature of the refrigerant (before compression)
• P2, V2 and T2 = Corresponding values for the refrigerant at the
discharge point i.e. after compression, and
• r = Compression ratio or pressure ratio, p2 I p 1

13. Work done during isothermal compression

14. Work done during polytropic compression
( pvn= Constant)

17. Work done during isentropic compression

18. Power Required to Drive a Single-Stage Reciprocating
Compressor

19. Problem :
A single-stage reciprocating compressor is required
to compress 1.5 m3/min of vapour refrigerant from
1 bar to 8 bar. Find the power required to drive the
compressor, if the compression of refrigerant is
1. isothermal;
2. polytropic with polytropic index as 1.12; and
3. isentropic with isentropic index as 1.31.

23. Work Done by Reciprocating Compressor with Clearance Volume

25. Volumetric Efficiency of a Reciprocating Compressor
• volumetric efficiency of a reciprocating compressor is the ratio of actual
volume of refrigerant passing through the compressor per cycle (Vs) to the
stroke volume of the compressor (V P ).
• Mathematically, volumetric efficiency,

27. Performance Characteristics of Refrigerant Reciprocating
Compressor
• The performance of a refrigerant reciprocating compressor is
measured in terms of its refrigerating capacity, brake power (B.P.) per
tonne of refrigeration and total brake power.
• The two important parameters in design of a refrigerant reciprocating
compressor are evaporator (or suction) and condenser temperatures.
• Effect of these two parameters on the refrigerating capacity and B.P.
are discussed below:

28. Effect of suction temperature on compressor
refrigerating capacity
• Refrigerating capacity of a reciprocating
compressor decreases with the decrease in
evaporator or suction temperature.
• It is due to the fact that at low suction
temperature , the vaporising pressure is low
and therefore the density of suction vapour
entering compressor is low.
• Hence the mass of refrigerant circulated
through the compressor per unit time
decreases with the decrease in suction
temperature for a given piston
displacement.
• Qe=Mr* re

29. Effect of suction temperature on compressor B.P.
• The effect of suction temperature on the
compressor B.P. per tonne of
refrigeration and total B.P. is shown in
Fig.
• We see from the figure that the
compressor B.P. per tonne refrigeration
decreases with the increase in suction
temperature, when the condenser
temperature remains the same.
• But the total B.P. of the compressor is
dependent on the following two factor
• (a) work of compression, and (b) mass of
refrigerant circulated per minute.

30. Effect of suction temperature on compressor B.P.
• With the decrease in suction
temperature, the work of compression
increases and the mass of refrigerant
circulated per minute decreases.
• The decrease in the mass of refrigerant
circulated outweighs the increase in work
of compression per kg.
• Thus, the net effect is to decrease the
total B.P. of the compressor with the
decrease in suction temperature.

31. Effect of condenser temperature
• The effect of condenser temperature
on the compressor refrigerating
capacity, B.P. per tonne of
refrigeration and the total B.P. is
shown in Fig.
• The suction temperature is kept
constant in all the cases.
• The increase in condenser
temperature results in decrease of
compressor refrigerating capacity,
increase in B.P. per tonne of
refrigeration and the total B.P.

32. Centrifugal Compressors
• This compressor increases the pressure of
low pressure vapour refrigerant to a high
pressure by centrifugal force. The centrifugal
compressor is generally used for refrigerants
that require large piston displacement and
low condensing pressure, such as R-11 and
R-113
• However, the refrigerant R-12 is also
employed for large capacity applications and
low temperature applications.

33. Centrifugal Compressors
• A single stage centrifugal compressor, in its simplest form, consists of an impeller to
which a number of curved vanes are fitted symmetrically, as shown in Fig.
• The impeller rotates in an airtight volute casing with inlet and outlet points.
• The impeller draws in low pressure vapour refrigerant from the evaporator. When the
impeller rotates, it pushes the vapour refrigerant from the centre of the impeller to its
periphery by centrifugal force.
• The high speed of the impeller leaves the vapour refrigerant at a high velocity at the
vane tips of the impeller.
• The kinetic energy thus attained at the impeller outlet is converted Inlet into pressure
energy when the high velocity vapour refrigerant passes over the diffuser
• The volute casing collects the refrigerant from the diffuser and it further converts the
kinetic energy into pressure energy before it leaves the refrigerant to the evaporator

34. Comparison of Performance of Reciprocating and Centrifugal
Compressors
• Centrifugal compressors have high efficiency over a
large range of load and a large volumetric
displacement for its size, as compared to
reciprocating compressors.
• In addition to this, the centrifugal compressor has
many other desirable features.
• The most important feature is the flat head capacity
characteristic as compared to reciprocating
compressor, as shown in Fig.
• From the figure, we see that for a centrifugal
compressor, evaporator temperature changes only
from 2°C to 7.5°C when the load changes from 100
to 240 tonnes of refrigeration, whereas the
evaporator temperature changes from -11 c to 6°C
for the same load change for a reciprocating
compressor.

35. Capacity variation with condensing temperature.
• for a centrifugal compressor, there is a
reduction in power requirement with the
increase in condensing temperature.
• There is a reduction in refrigerating capacity
of the centrifugal compressor with the
increase in condensing temperature as shown
in Fig.
• Hence there is no overloading of the motor as
the condensing temperature increases.

36. Effect of speed on the capacity and power.
The effect of speed on the refrigerating capacity and power requirements is
shown in Fig. (a) and (b) respectively for both the centrifugal and reciprocating
compressors

37. Determine the size of cylinder of double acting air
compressor of 30kw indicated power in which air is
drawn in at 1 bar and compressed to 15 bar according
to law pv^1.25= constant . RPM=280 Piston
speed=200 m/min volumetric efficiency =0.82

40. A single stage double acting compressor of 62.5 kw
IP at 180 RPM take air at 1 bar and deliver at 10 bar
assume process is according to PV^1.35= constant.
find diameter and stroke of Cylinder
Take Piston Speed = 200 m/min and volumetric
Efficiency =90%

43. An R-22 hermatic (Directly coupled motor) reciprocating
compressor with 4% clearance is to be design for 6 TR capacity at 40
C evaporating and 400 C condensing temperature. The compression
index may be taken as 1.15. Number of cylinder selected are 2 and
mean piston speed as 3 m/sec approximately .Take L/D= 0.8.
Pressure drop at suction and Discharge valve may be assume as 0.2
and 0.4 bar respectively. Determine
1) power consumption of compressor
2) cop of cycle
3) volumetric Efficiency of compressor
4) Bore and Stroke of cylinder

65. References
1) REFRIGERATION AND AIR CONDITIONING BY R.S.KHURMI

66. Have a Good Day
66