US04CICV01/ US04CICH02
Unit -3 Dr. N. K. Patel
Natubhai V. Patel College of Pure & Applied Sciences
S. Y. B.Sc. Semester-4
Industrial chemistry/ IC (Vocational)
USO4CICV01/US04CICH02: Chemical Plant Utilities
UNIT – 3
SYLLABUS
Compression equipment, Reciprocating compressor, Work of single stage reciprocating
compressor, Effect of clearance, Volumetric efficiency, Multistage compression,
Refrigeration, COP & refrigerating effect, Industrial refrigerants, Carnot and other refrigeration
cycles.
REFRIGERANT COMPRESSORS
3.0 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, a compressor must be driven by
some prime mover.
Since the compressor virtually takes the heat at a low temperature from the
evaporator and pumps it at the high temperature to the condenser, therefore it is often
referred to as a heat pump.
3.1 CLASSIFICATION OF COMPRESSORS
The compressors may be classified in many ways, but the following are important from
the subject point of view
3.1.1 According to the method of compression
(a) Reciprocating compressors
(b) Rotary compressors
(c) Centrifugal compressors
3.1.2 According to the number of working strokes
(a) Single acting compressors
(b) Double acting compressors
3.1.3 According to the number of stages
(a) Single stage (or single cylinder) compressors
(b) Multi-stage (or multi-cylinder) compressors
3.1.4 According to the method of drive employed
(a) Direct drive compressors
(b) Belt drive compressors
3.1.5 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)
3.2 IMPORTANT TERMS
The following important terms should be clearly understood at this stage
3.2.1 Suction pressure
It is the absolute pressure of refrigerant at the inlet of a compressor.
3.2.2 Discharge pressure
It is the absolute pressure of refrigerant at the outlet of compressor.
3.2.3 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
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Unit -3 Dr. N. K. Patel
clearance volume.
3.2.4 Suction volume
It is the volume of refrigerant sucked by the compressor during its suction stroke. It is
usually denoted by Vs.
3.2.5 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,
Vp = x D2 x L
Where, D = diameter of cylinder
L = length of piston stroke
3.2.6 Clearance factor
It is the ratio of clearance volume (Vc) to the piston displacement volume (Vp).
Mathematically, clearance factor,
C=
3.2.7 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 (Vs). It is expressed in m 3 /s.
3.2.8 Volumetric efficiency
It is the ratio of the compressor capacity or the suction volume (Vs) to the piston
displacement volume (Vp). A good compressor has a volumetric efficiency of 70-80%.
Mathematically, volumetric efficiency,
ηv =
3.3 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 reciprocating compressors are available in sizes as small as 1/12 kW which are used in
small domestic refrigerators and up to about 150 kW for large capacity installations.
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 V or W form. The double acting
compressors usually have their cylinders arranged horizontally.
3.4 SINGLE STAGE SINGLE ACTING RECIPROCATING COMPRESSOR
Figure: 1 Principle of operation of single stage, single acting reciprocating compressor
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Let us consider that the piston is at the top of its stroke as shown in Figure: 1(a). This is
called top dead centre position of the piston. In this position, the suction valve is held closed
because of the pressure in the clearance space between the top of the piston and the
cylinder head. The discharge valve is also held closed because of the cylinder head pressure
acting on the top of it.
When the piston moves downward (i.e. during suction stroke) as shown in Figure: 1 (b)
the refrigerant left in the clearance space expands. Thus the volume of the cylinder (above
the piston) increases and the pressure inside the cylinder decreases. When the pressure
becomes slightly less than the suction pressure or atmospheric pressure, the suction valve
gets opened and the vapour refrigerant flows into the cylinder. This flow continues until the
piston reaches the bottom of its stroke (i.e. bottom dead centre). At the bottom of the stroke,
as shown in Figure: 1(c), the suction valve closes because of spring action. Now when the
piston moves upward (i.e. during compression stroke) as shown in Figure: 1 (d), the volume of
the cylinder decreases and the pressure inside the cylinder increases. When the pressure
inside the cylinder becomes greater than that on the top of the discharge valve, the
discharge valve gets opened and the vapour refrigerant is discharged into the condenser
and the cycle is repeated.
It may be noted that in a single acting reciprocating compressor, the suction,
compression and discharge of refrigerant takes place in two strokes of the piston or in one
revolution of the crankshaft.
In a double acting reciprocating compressor, the suction and compression takes
place on both sides of the piston. It is thus obvious, that such a compressor will supply double
the volume of refrigerant than a single acting reciprocating compressor (neglecting volume
of piston rod).
There must be a certain distance between the top of the piston and the cylinder
head when the piston is on the top dead centre so that the piston does not strike the cylinder
head. This distance is called clearance space and the volume therein is called the
clearance volume. The refrigerant left in this space is at discharge pressure and its pressure
must be reduced below that of suction pressure (atmospheric pressure) before any vapour
refrigerant flows into the cylinder. The clearance space should be a minimum.
The low capacity compressors are air cooled. The cylinders of these compressors
usually have fins to provide better air cooling. The high capacity compressors are cooled by
providing water jackets around the cylinder.
3.4.1 Work done by a single stage reciprocating compressor
We have already discussed that in a reciprocating compressor; the vapour
refrigerant is first sucked, compressed and then discharged. So there are three different
operations of the compressor. Thus we see that work is done on the piston during the suction
of refrigerant. Similarly, work is done by the piston during compression as well as discharge of
the refrigerant. It may be noted that the work done by a reciprocating compressor is
mathematically equal to the work done by the compressor during compression as well as
discharge minus the work done on the compressor during suction.
Here we shall discuss the following two important cases of work done:
 When there is no clearance volume in the cylinder
 When there is some clearance volume
3.4.1.1 Work done by a single stage, single acting reciprocating compressor without
clearance volume
Consider a single stage, single acting reciprocating compressor without clearance as
shown by the p-v and T-s diagrams in Figure: 2.
Let
P1=suction pressure of the refrigerant(before compression)
V1= suction volume of the refrigerent(before compression)
T1 = suction temperature of the refrigerent(before compression)
P2,V2 and T2 = corresponding values for the refregerant at the discharge point i.e.
after compression
r = compression ratio or pressure ratio, p2/p1
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Figure: 2 p-v and T-s diagrams for a single stage reciprocating compressor
As a matter of fact, the compression of refrigerant may be isothermal, polytropic, or
isentropic (reversible adiabatic). Now we shall find out the amount of work done in
compressing the refrigerant in all the above mentioned three cases.
a) Work done during isothermal compression
We have already discussed that when the piston moves from the top dead centre
(point A), the refrigerant is admitted into the compressor cylinder and it continues till the
piston reaches at its bottom dead centre (point B). Thus the line AB represents suction stroke
and the area below this line (i.e. area ABB'A') represents the work done during suction stroke.
From the figure, we find that work done during suction stroke,
W1=Area ABB’A’ = p1v1
The refrigerant is compressed during the return stroke (or compression stroke BC 1) of
the piston at constant temperature. The compression continues till the pressure (p2) in the
cylinder is sufficient to force open the discharge valve at C 1. After that no compression takes
place with the inward movement of the piston. Now during the remaining part of the
compression stroke, the compressed refrigerant is discharged to condenser till the piston
reaches its top dead center. The volume of refrigerant delivered (v2) is represented by line
C1D.
From the figure, we find that
 V1 
Work done during compression: W2 = Area B C1 C1' B' = P1V1 loge  
V 2 
Work done during discharge: W3 = Area C1 d A' C1' = P2V2
Work done by the compressor per cycle
W = Area ABC1D
= Area C1 D A' C1' + Area BC1C1' B' - Area ABB' A'
= W3 + W2 - W1
V1
V1
= P2V2 + P1V1 loge  - P1V1 = P1V1loge  
.....(P1V1 = P2V2)
V 2 
V 2 
= 2.3 P1V1log V 1  = 2.3 P1V1 log r
V 2 
W = 2.3 m R T1 log r
 V 1 P2


 r

V
2
P
1


...............
........(P1V1 = mRT1)
b) Work done during polytropic compression: (P Vn = Constant )
The polyfiropic compression is shown by the curve BC in Figure 2.
n
* mR(T 2  T 1)
W
n 1
c) Work done during isentropic compression
The isentropic compression is shown by the curve BC2 in Figure 2. In this case, the
volume of refrigerant delivered (i.e. V2) is represented by the line C2D.
  1 
 1
W
* mcp 
 (T 2  T 1)  mcp(T 2  T 1)

  
This shows that the work done by the compressor per cycle during isentropic
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compression is equal to the heat required to raise the temperature of refrigerant from T 1 to T2
at constant pressure.
3.4.1.2 Work done by reciprocating compressor with clearance volume
In the previous articles, we have assumed that there is no clearance volume in the
compressor cylinder. In other words, the entire volume of the refrigerant, in the compressor
cylinder, is compressed by the inward stroke of the piston. But in actual practice, it is not
possible to reduce the clearance volume ot zero, for mechanical reason. Morever it is not
disirable to allow the piston head to come in contact with the cylinder head. In addition to
this, the passages leading to the suction and discharge valves always contributes to the
clearance volume. In genral, the clearance volume is expressed as somne percentage of
the piston dislplacement.
Now consider a single stage, single acting horizontal reciprocating compressor with
clearance volume as shown by p-v diagram in figure 3
Figure:3 p-v diagram with clearance volume.
p1 = Suction pressure of refrigerant (before compression)
V1 = Total volume of refrigerant in the compressor cylinder (before compression),
T1 = Suction temperature of refrigerant (before compression),
P2, V2 and T2 = Corresponding values at the discharge point 2 (i.e. after compression),
Vc = Clearance volume,
Vs = Actual volume of refrigerant sucked by the compressor
i. e. suction volume = V1-V4
V4 = Expanded clearance volume i.e. volume of refrigerant after expansion.
Vp = Stroke or effective swept volume or piston displacement of the compressor
= V1-V3 = V1-Vc,
n = Polytropic index for compression and expansion, and
r = Compression ratio or pressure ratio (i.e. p2/p1)
We know that when the piston moves from outer dead center (O.D.C.), during the
return stroke (or inward stroke) of the piston, the vapour refrigerant is compressed as shown
by the curve 1-2 in the figure 3. This compression continues till the pressure in the cylinder is
sufficient to force open the discharge valve at point 2. After that, no more compression takes
place with the inward movement of the piston. Now, during the remaining part of the
compression stroke, the compressed refrigerant is discharged to the condenser till the piston
reaches at point 3. At this stage, there will be some refrigerant (equal to the clearance
volume, Vc) left in the clearance space of the cylinder at the discharge pressure P2. This
entrapped refrigerant in the clearance space will now expand when piston moves from inner
dead center (I.D.C.) during some part of the outward stroke of the piston as shown by the
curve 3-4. This expansion continues till the pressure P1 is sufficient to force open the suction
valve at point 4. Now the fresh charge of vapour refrigerant enters at point 4 during the
suction stroke 4-1 at suction pressure P1.
Though the compression and expansion may be isothermal, polytropic or isentropic,
yet for all calculation purposes, it is assumed to be polytropic.
We know that work done by the compressor,
W = Area 1-2-3-4 = Area A-1-2-B-A-4-3-B
Let,
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
n




 p 2   1 
 p 2  n 1 
n
n



xp1v1    1 
xp 4v 4    1
 p1 
 n 1
 p 1 

n 1




n


 p 2  n 1
n

xp1(v1  v 4)    1
  p1 

n 1


…….(p1= p4)
n


 p 2  n 1 
n


xmRT 1    1
  p1 

n 1


Where (V1-V4) is equal to the actual volume and m is the mass of refrigerant sucked
by the compressor per cycle. From the above expression, we see that the clearance volume
does not affect the work done and the power required for compressing the refrigerant. This is
due to the reason that the work required to compress the clearance volume refrigerant is
theoretically regained during its expansion from point 3 to point 4. In the above expression of
work done, we have assumed the same polytropic index of compression and expansion. In
case the polytropic index of compression is and the polytropic index of expansion is n e then
work done by the compressor per cycle,
nc
ne




 p 2  nc 1 
 p 2  ne 1 
nc
ne


W
xp1v1  
1 
xp1v 4  
1
  p1 
 ne  1
  p1 

nc  1




The effect of clearance in reciprocating compressors is to reduce the volume of
vapour refrigerant sucked during suction stroke.
3.4.2 Power required to drive a single stage reciprocating compressor
We have already obtained the expressions for the work done (W) per cycle during
isothermal, polytropic and isentropic compression. The power required to drive the
compressor may be obtained from the usual relation.
P=
=
If N is the speed of the compressor in r.p.m., then number of working strokes per
minute.
Nw = N
...(For single acting compressor)
= 2N
...(For doubling acting compressor)
Since the compression takes place in three different ways, therefore, the power
obtained from different works done will be different. In general, following are the three values
of power obtained.
1. Isothermal power =
2. Indicated power =
3. Isentropic power =
EXAMPLE:1 A single stage reciprocating compressor is required to compress 1.5 m 3/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) Poly tropic with polytropic index as 1.12
3) Iisentropic with isentropic index as 1.31.
Solution: Given : V1= 1.5 m3/min, P1 = 1 bar = 1X105 N/m2, P2 = 8 bar = 8X105 N/m2, n=1.12,
 = 1.31
1. Isothermal compression
We know that work done by the compressor,
8
 P2 
5
 2.3P1V 1 log   =2.3X1X10 X1.5 log   N  m / min
 P1 
1
= 3.45X105X0.9031 = 3.12X105 N-m/min
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Power require to drive the compressor
= 3.12X105/60 = 5200 W = 5.2 kW Ans.
2. Polytropic compression
We know that work done by the compressor
n


n
 P 2  n 1 


* P1V 1    1
 P1 

n 1


1.12

*1*105*1.5
1.12  1
1.12


1.12 1
8


 
 1 N  m / min
 1 



=14*105(1.25-1) = 3.5*105 N-m/min
Power require to drive the compressor
= 3.5X105/60 = 5833 W = 5.833 kW
3. Isentropic compression
We know that work done by the compressor,




P
2   


* P1V 1    1
 P1 

 1


=

1.311
*1*105*1.5  8  1.31
 1 


 1 N  m / min


= 6.34*105(1.63-1) = 4*105 N-m/min
Power require to drive the compressor
= 4X10560 = 6667W= 6.667kW
3.4.3 Volumetric efficeincy of reciprocating compressor
We have already discussed that the volumetric efficiency of a reciprocating
compressors is the ratio of actual volume of refrigerant passing through the compressor per
cycle (Vs) to the volume of the compressor (Vp). Mathematically, volumetric efficiency,
The p-v diagram of a reciprocating compressor with clearance volume is shown in
figure 4.
v 
vs v1  v4

v p v1  vc
……(i)
Since the refrigerant gases are expanded and compressed in every cycle, therefore,
the curve 3-4 is called re-expansion curve. Applying pvn = C to points 3 and 4,
p3v3n  p4v4 n
Or
p2vc n  p1v4n
1
 p2  n
v4  vc  
 p1 
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Substituting the value of v4 in equation (i)
1
 P2  n
V 1  Vc 

 P1 
v 
V 1  Vc
1
1
 P2  n
 P2  n
V 1  Vc  V 1  Vc 
Vc 


 P1   V 1  Vc  Vc   P1 

V 1  Vc
V 1  Vc V 1  Vc
V 1  Vc
1
 P2  n
 1 C  C 

 P1 
Where,
C= Clearance factor
3.4.3.1 Factors affecting volumetric efficiency of a reciprocating compressor
The various factors which affect the volumetric efficiency are as under
 Effect of clearance factor
 Effect of valve pressure drops
 Effect of heat gain from the cylinder walls and re-expansion index
 Effect of valve and piston leakage
3.4.3.2 Overall or total volumetric efficiency of a reciprocating compressor
The overall or total volumetric efficiency of a reciprocating compressor, considering
all the factors discussed in the previous article, is given by
1
1
p 
 p  nc
  ne
Total v  (1  C )  s   C  p2   0.01 d 
 p1 
 p1 
 ps 
3.5 MULTI-STAGE COMPRESSION
In the previous articles, we have taken into consideration the compression of vapour
refrigerant in a single stage (i.e. single cylinder) compressor. In other words, the vapour
refrigerant is compressed in a single cylinder and then delivered at a high pressure. But
sometimes, the vapour refrigerant is required to be delivered at a very high pressure as in the
case of low temperature refrigerating systems. In such cases, either we should compress the
vapour refrigerant by employing a single stage compressor with a very high pressure ratio or
compress it in two or more compressors placed in series. It has been experienced that if we
employ a single stage compressor for producing very high pressure refrigerant.
3.5.1 Disadvantage of single stage compressor
 The size of the cylinder will be too large.
 Due to compression, there is a rise in temperature of the refrigerant. It is difficult to
reject heat from the refrigerant in the small time available for compression.
 Sometimes, the temperature of refrigerant at the end of compression is too high. It
may heat up the cylinder head or burn the lubricating oil.
 The friction losses and running cost are high.
 The volumetric efficiency is low.
In order to overcome the above mentioned difficulties, the vapour refrigerant is
compressed in two or more cylinders in series with intercooling arrangement between them.
Such an arrangement is known as compound or multi-stage compression.
3.5.2 Advantages of multi-stage compression
Following are the main advantages of multi-stage compression over single stage
compression
 The work done per kg of refrigerant is reduced in multi-stage compression with
intercooler as compared to single stage compression for the same delivery pressure.
 It improves the volumetric efficiency for the given pressure ratio.
 The sizes of the two cylinders (i.e. high pressure and low pressure cylinders) may be
adjusted to suit the volume and pressure of refrigerant).
 It reduces the leakage loss considerably.
 It gives more uniform torque, and hence a smaller size flywheel is required.
 It provides effective lubrication because of lower temperature range.
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 It reduces the cost of compressor.
3.5.3 Two stage reciprocating compressor with intercooler
Figure 5 Two stage reciprocating compressor with intercooler
The schematic arrangement for a two stage reciprocating compressor with water
intercooler is shown in figure 5.
First of all, the vapour refrigerant from the evaporator at pressure p1 and temperature
T1 is sucked by the low pressure (L.P.) compressor at point 1 during its suction stroke. The
refrigerant after compression in the L.P.compressor (first stage compressor) is delivered at
point 2 to the intercooler at pressure p 2 and temperature T2. In the intercooler, the
compressed refrigerant is cooled from temperature T2 to T3 at constant pressure p2. This
cooled refrigerant is now sucked by the high pressure (H.P.) compressor at point 3 during its
suction stroke. Finally, the refrigerant after compression in the H.P. compressor (second stage
compressor) is delivered at point 4 to a condenser at pressure p3 and temperature T4.
REFRIGERATION
3.6 INTRODUCTION
In an air refrigeration cycle, the air is used as refrigerant. In olden days air was widely
used in commercial application because of its availability at free of cost. Since air does not
change its phase i.e. remains gaseous throughout the cycle, therefore the heat carrying
capacity per kg of air is very small as compare to vapour absorbing systems. The air cycle
refrigeration system, as originally designed and installed, are now practically obsolete
because of their low coefficient of performance and high power requirements. However, this
system continues to be favour for air refrigeration because of the low weight and volume of
the equipment. The basic elements of an air cycle refrigeration system are the compressor,
the cooler or the heat exchanger, the expander and the refrigerator.
Before discussing the air refrigeration cycles, we should first know about the unit of
refrigeration, coefficient of performance of a refrigerator and the difference between the
heat engine, a refrigerator and a heat pump.
3.7 UNITS OF REFRIGERATION
The practical unit of refrigeration is expressed in terms of 'tonne of refrigeration' (briefly
written as TR). A tonne of refrigeration is defined as the amount of refrigeration effect
produced by the uniform melting of one tonne (1000 kg) of ice from and at 0°C in 24 hours.
Since the latent heat of ice is 335 kJ/kg, therefore one tonne of refrigeration,
1TR = 1000 x 335 kJ in 24 hours
= 1000x335 = 232.6 kJ/min
24 x 60
In actual practice, one tonne of refrigeration is taken as equivalent to 210 kJ/min or
3.5kW (i.e. 3.5kJ/s)
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3.8 COEFFICIENT OF PERFORMANCE OF A REFRIGERATOR
The coefficient of performance (briefly written as C.O.P.) is the ratio of heat extracted
in the refrigerator to the work done on the refrigerant. It is also known as theoretical
coefficient of performance. Mathematically,
Theoretical C.O.P.= Q/W
Where, Q = Amount of heat extracted in the refrigerator (or the amount of
refrigeration produced, or the capacity of a refrigerator)
W= Amount of work done.
Notes:
 For per unit mass, C.O.P. =q/w
 The coefficient of performance is the reciprocal of the efficiency (i.e. 1/η) of a heat
engine. It is thus obvious, that the value of C.O.P is always greater than unity.
 The ratio of the actual C.O.P to the theoretical C.O.P. is known as relative coefficient of
performance. Mathematically,
Relative C.O.P =
EXAMPLE: 2 Find the C.O.P. of a refrigeration system if the work input is 80 kJ/kg and
refrigeration effect produced is 160 kJ/kg of refrigerant flowing.
Solution: Given: w = 80 kJ/kg ; q = 160 kJ/kg
We know that C.O.P. of a refrigeration system
= q/w = 160/80 = 2 Ans.
3.9 DIFFERENCE BETWEEN A HEAT ENGINE, REFRIGERATOR AND HEAT PUMP
In a heat engine, as shown in Figure 6 (a) the heat supplied to the engine is
converted into useful work. If Q2 is the heat supplied to the engine and Q1 is the heat
rejected from the engine, then the net work done by the engine is given by
WE  Q2  Q1
(a)
(b)
(c)
Figure: 6 Difference between a heat engine, refrigerator and heat pump
The performance of a heat engine is expressed by its efficiency. We know that the
efficiency or coefficient of performance of an engine,
E or (C.P.O) E 
W
Q  Q1
workdone
 E  2
Heat supp lied Q2
Q2
A refrigerator as shown in Figure (b), is a reversed heat engine which either cool or
maintain the temperature of a body (T1) lower than the atmospheric temperature (Ta). This is
done by extracting the heat (Q1) from a cold body and delivering it to a hot body (Q2) doing
so, work WR is required to be done on the system. According to First Law of Thermodynamics,
WR  Q2  Q1
The performance of a refrigerator is expressed by the ratio of amount of heat taken
from the cold body (Q1) to the amount of work required to be done on the system (WR). This
ratio is called coefficient of performance. Mathematically, coefficient of performance of a
refrigerator,
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Unit -3 Dr. N. K. Patel
Q1
Q1

WR Q2  Q1
Any refrigeration system is a heat pump as shown in figure (c), which extracts heat
(Q1) from a cold body and delivers it to a hot body. Thus there is no difference between the
cycle of operations of a heat pump and a refrigerator. The main difference between the two
is in their operating temperatures. A refrigerator works between the cold body
temperature(T1) and the atmospheric temperature(Ta) whereas the heat pump operates
between the hot body temperature (T2) and the atmospheric temperature (Ta). A refrigerator
used for cooling in summer can be used as a heat pump for heating in winter.
In similar way, as discussed for refrigerator, we have
WP  Q2  Q1
The performance of a heat pump is expressed by the ration of the amount of heat
delivered to the hot body (Q2) to the amount of work required to be done on the system
(WP). This ratio is called coefficient of performance or energy performance ratio (E.P.R.) of a
heat pump. Mathematically, coefficient of performance or energy ratio of a heat pump,
(C.P.O) P orE.P.R. 

Q2
Q2

WP Q2  Q1
Q2
 1  (C.O.P.) R  1
Q2  Q1
From above we see that the C.O.P. may be less than one or greater than one
depending on the type of refrigeration system used. But the C.O.P. of a heat pump is always
greater than one.
3.10 AIR REFRIGERATOR WORKING ON REVERSED CARNOT CYCLE
In refrigerating systems, the Carnot cycle considered is the reversed Carnot cycle. We
know that a heat engine working on Carnot cycle has the highest possible efficiency.
Similarly, a refrigerating system working on the reversed Carnot cycle, will have the maximum
possible coefficient of performance. We also know that it is not possible to make an engine
working on the Carnot cycle. Similarly, it is also not possible to make a refrigerating machine
working on the reversed Carnot cycle. However, it is used as the ultimate standard of
comparison.
A reversed Carnot cycle, using air as working medium (or refrigerant) is shown on p-v
and T-s diagrams in Figure 7 (a) and (b) respectively. At point 1, let p1, v1, T1 be the pressure,
volume and temperature of air respectively.
Figure: 7 p-v and T-s diagram of reverse Carnot engine using air as refrigerant
The four processes of the cycle are as follows
3.10.1 Isentropic compression process
The air is compressed isentropically as shown by the curve 1-2 on p-v and T-s
diagrams. During this process, the pressure of air increases from p1 to p2, specific volume
decreases from V1 to V2 and temperature increases from T1 to T2. We know that during
isentropic compression, no heat is absorbed or rejected by the air.
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3.10.2 Isothermal compression process
The air is now compressed isothermally (i.e. at constant temperature, T2-T3) as shown
by the curve 2-3 on p-v and T-s diagrams. During this process, the pressure of air increases
from p2 to p3 and specific volume decreases from V2 to V3. We know that the heat rejected
by the air during isothermal compression per kg of air,
qR  q23  Area2  3  3' 2'
 T3 (s2  s3 )  T2 (s2  s3 )
3.10.3 Isentropic expansion process
The air is now expanded isentropically as shown by the curve 3-4 on p-v and T-s
diagrams. The pressure of air decreases from p3 to p4, specific volume increases from V3 to V4
and the temperature decreases from T3 to T4 We know that during isentropic expansion, no
heat is absorbed or rejected by the air.
3.10.4 Isothermal expansion process
The air is now expanded isothermally (i.e. at constant temperature, T4 = T1) as shown
by the curve 4-1 on p-v and T-s diagrams. The pressure of air decreases from p4to p1 and
specific volume increases from V4 to V1. We know that the heat absorbed by the air (or heat
extracted from the cold body) during isothermal expansion per kg of air,
qA  q41  Area4  1  2' 3'
 T4 (s1  s4 )  T4 (s2  s3 )  T1 (s2  s3 )
We know that work done during the cycle per kg of air
WR = Heat rejected – Heat absorbed = qR  qA  q23  q41
 T2 (s2  s3 )  T1 (s2  s3 )  T2  T1  (s2  s3 )
 Coefficient of performance of the refrigeration system working on reversed Carnot cycle,
qA
q41
Heatabsorbed
(C.O.P.) R 


Workdone
qR  qA q23  q41
T1 ( s2  s3 )
T1


T2  T1  (s2  s3 ) T2  T1
Though the reversed Carnot cycle is the most efficient between the fixed
temperature limits, yet no refrigerator has been made using this cycle. This is due to the
reason that the isentropic processes of the cycle require high speed while the isothermal
processes require an extremely low speed. This variation in speed of air is not practicable.
We have already discussed that C.O.P. of a heat pump,
(C.O.P.) P  (C.O.P.) R  1 
T1
T2
1 
T2  T1
T2  T1
and C.O.P. or efficiency of a heat engine,
(C.O.P.) E 
wR T2  T1  ( s2  s3 ) T2  T1
1



qR
T2 ( s2  s3 )
T2
(C.O.P.) P
3.11 DESIRABLE PROPERTIES OF AN IDEAL REFRIGERANT
We have discussed above that there is no ideal refrigerant. A refrigerant is said to be
ideal if it has all of the following properties
 Low boiling point
 High critical temperature
 High latent heat of vaporization
 Low specific heat of liquid
 Low specific volume of vapour
 Non-corrosive to metal
 Non-flammable and non-explosive
 Non-toxic
 Low cost
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Unit -3 Dr. N. K. Patel
Easy to liquefy at moderate pressure and temperature
Easy of locating leaks by odour or suitable indicator
Mixes well with oil
Ozone friendly
High coefficient of performance
The standard comparison of refrigerants, as used in the refrigeration industry, is based
on an evaporating temperature of -15° C and a condensing temperature of +30° C.





3.12 CLASSIFICATION OF REFRIGERANTS
The refrigerants may, broadly, be classified into the following two groups
Primary refrigerants
Secondary refrigerants
The refrigerants which directly take part in the refrigeration system are called primary
refrigerants whereas the refrigerants which are first cooled by primary refrigerants and then
used for cooling purposes, are known as secondary refrigerants.
The primary refrigerants are further classified into the following four groups:
 Halo-carbon refrigerants or organic refrigerant
 Azeotrope refrigerants
 Inorganic refrigerants
 Hydro-carbon refrigerants
3.12.1 Halo-carbon refrigerants
The American Society of Heating, Refrigeration and Air-conditioning Engineers
(ASHARAE) identify 42 halo-carbon compounds as refrigerants, but only a few of them are
commonly used. The following table gives some of the commonly used halo-carbon
refrigerants.


Refrigerant number
Chemical name
Chemical formula
R - 11
Trichloromonofluoromethane
CCI3F
R-12
Dichlorodifluoromethane
CCI2F2
R-13
Monochlorotrifluoromethane
CCIF3
R-14
Carbentetrafluoride
CF4
R-21
Dichloromonofluoromethane
CHCI2F
R-22
Monochlorodifluoromethane
CHCIF2
R-30
Methylene chloride
CH2CI2
R-40
Methyl chloride
CH3CI
R-100
Ethyl chloride
C2H5CI
R-113
Trichlorotrifluorothane
CCI2FCCF2
R-114
Dichlorotetrafluoroethane
CCIF2CCIF2
R-115
Monochloropentafluoroethane
CCIF2CF3
R- 123
Dichloro-trifluoro-ethane
CF3CHCl2
R- 124
Monochloro-tetrafluoro-ethane
CF3CHClF
R- 134a
Tetrafluoro-ethane
CF3CH2F
R-152a
Difluoro-ethane
CH3CHF2
The halo-carbon compounds are all synthetically produced and were developed as
Freon family of refrigerants. Freon is a registered trade mark of E.I. Du Pont de Nemours and
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Unit -3 Dr. N. K. Patel
Co., America. Most of the halo-carbon refrigerants, are now available from other
manufacturers under various trade names such as Genetron, Isotron etc. The first of the halocarbon refrigerant i.e. R-12 was developed in 1930 by Thomas Midgley. The various halocarbon refrigerants mentioned above are now discussed, in detail, as below:
3.12.1.1 R-12, Dichlorodifluoromethane (CCl2F2):
The R-12 is a very popular refrigerant. It is a colourless, almost odourless liquid with
boiling point of - 290C at atmospheric pressure. It is non-toxic, non-corrosive, non-irritating and
non-flammable. It has a relatively low latent heat value which is an advantage in small
refrigerating machines. The large amount of refrigerant circulated will permit the use of less
sensitive and more positive operating and regulating mechanisms. It operates at a low but
positive head and back pressure and with a good volumetric efficiency. This refrigerant is
used in many different types of industrial and commercial applications such as refrigerators,
freezers, water coolers, room and window air conditioning units etc. Its principal use is found
in reciprocating and rotary compressors, but its use in centrifugal compressors for large
commercial air conditioning is increasing.
R-12 has a pressure of 0.82 bar at -15°C and a pressure of 6.4 bar at 30°C. The latent
heat of R-12 at - 15°C is 159 kJ/kg. The leak may be detected by soap solution, halide torch
or an electronic leak detector. Water is only slightly soluble in R-12. At -18°C, it will hold six
parts per million by mass. The solution formed is very slightly corrosive to any of the common
metals used in refrigerator construction. The addition of mineral oil to the refrigerant has no
effect upon the corrosive action.
R-12 is more critical as to its moisture content when compared to R-22 and R-502. It is
soluble in oil down to -680C. The oil will begin to separate at this temperature and due to its
lightness than the refrigerant; it will collect on the surface of the liquid refrigerant. The
refrigerant is available in a variety of cylinder sizes and the cylinder colour code is white.
3.12.2 Azeotrope Refrigerants
The term 'azeotrope' refers to a stable mixture of refrigerants whose vapour and liquid
phase retain identical compositions over a wide range of temperatures. However, these
mixtures, usually, have properties that differ from either of their components. Some of the
azeotropes are given in the following table.
Refrigerant number
Azeotropic mixing refrigerants
Chemical formula
R-500
73.8% R-12 and 26.2% R-152
CCl2F2/CH3CHF2
R-502
48.8% R-22 and 51.2% R-115
CHClF2/CClF2CF3
R-503
40.1% R-23 and 59.9% R-13
CHF3/CClF3
R-504
48.2% R-12 and 51.8% R-115
CH2F2/CClF2CF3
3.12.3 Inorganic refrigerant
The inorganic refrigerants were exclusively used before the introduction of halocarbon refrigerants. These refrigerants are still in use due to their inherent thermodynamic
and physical properties. The various inorganic refrigerants are given in the following table.
Refrigerant number
Chemical name
Chemical formula
R-117
Ammonia
NH3
R-729
Air
R-744
Carbon dioxide
CO2
R-764
Sulfur dioxide
SO2
R-118
Water
H 2O
These refrigerants are discussed, in detail, as below :
3.12.3.1 R-717 (Ammonia)
The R-717, i.e. ammonia (NH3) is one of the oldest and most widely used of all the
refrigerants. Its greatest application is found in large and commercial reciprocating
compression systems where high toxicity is secondary. It is also widely used in absorption
systems. It is a chemical compound of nitrogen and hydrogen and under ordinary conditions,
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Unit -3 Dr. N. K. Patel
it is a colourless gas. Its boiling point at atmospheric pressure is -33.30C and its melting point
from the solid is -780C. The low boiling point makes it possible to have refrigeration at
temperatures considerably below 0°C without using pressures below atmospheric in the
evaporator. Its latent heat of vaporization at -150C is 1315 kJ/kg. Thus, large refrigerating
effects are possible with relatively small sized machinery. The condenser pressure at 30 0C is
10.78 bar. The condensers for R-717 are usually of water cooled type.
It is a poisonous gas if inhaled in large quantities. In lesser quantities, it is irritating to the
eyes, nose and throat. This refrigerant is somewhat flammable and when mixed with air in the
ratio of 16% to 25% of gas by volume, will form an explosive mixture. The leaks of this
refrigerant may be quickly and easily detected by the use of burning sulfur candle which in
the presence of ammonia forms white fumes of ammonium sulfite. This refrigerant attacks
copper and bronze in the presence of a little moisture but does not corrode iron or steel. It
presents no special problems in connection with lubricant unless extreme temperatures are
encountered. Since the refrigerant R-717 is lighter than oil, therefore, its separation does not
create any problem. The excess oil in the evaporator may be removed by opening a valve in
the bottom of the evaporator. This refrigerant is used in large compression machines using
reciprocating compressors and in many absorption type systems. The use of this refrigerant is
extensively found in cold storage, warehouse plants, ice cream manufacture, ice
manufacture, beer manufacture, food freezing plants etc.
3.12.3.2 R-729 (Air)
The dry air is used as a gaseous refrigerant in some compression systems, particularly in aircraft air conditioning.
3.12.3.3 R-744 (Carbon dioxide):
The principal refrigeration use of carbon dioxide is same as that of dry ice. It is nontoxic, non-irritating and non-flammable. The boiling point of this refrigerant is so extremely low
(-73.60C) that at -150C, a pressure of well over 20.7 bar is required to prevent its evaporation.
At a condenser temperature of +300C, a pressure of approximately 70 bar is required to
liquefy the gas. Its critical temperature is 31°C and triple point is -56.60C. Due to its high
operating pressure, the compressor of a carbon dioxide refrigerator unit is very small even for
a comparatively large refrigerating capacity. However, because of its low efficiency as
compared to other common refrigerants, it is seldom used in household units, but is used in
some industrial applications and aboard ships.
3.12.4 Hydrocarbon refrigerant
Most of hydrocarbon refrigerant are successfully used in industrial and commercial
installation. They possess satisfactory thermodynamic properties but are highly flammable
and explosive. The various hydrocarbon refrigerants are given in the following table
Refrigerant No
Chemical name
Chemical formula
R-170
Ethane
C2H6
R-290
Propane
C3H3
R-600
Butane
C4H10
R-600a
Isobutene
C4H10
R-1120
Trichloroethylene
C2H4Cl3
R-1130
Dichloroethylene
C2H4Cl2
R-1150
ethylene
C2H4
R-1270
Propylene
C3H6
Since the hydrocarbon refrigerants are not commonly used nowadays, therefore,
they are not discussed in detail.
3.13 AIR REFRIGERATOR WORKING ON A BELL-COLEMAN CYCLE
(OR REVERSED BRAYTON OR JOULE CYCLE)
A Bell-Coleman air refrigeration machine was developed by Bell-Caieman and Light Foot
by reversing the Joule's air cycle. It was one of the earliest types of refrigerators used in ships
carrying frozen meat. Figure 8 shows a schematic diagram of such a machine which consists
of a compressor, a cooler, an expander and a refrigerator.
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Figure: 8 Open cycle air Bell-Coleman
Refrigerator
Unit -3 Dr. N. K. Patel
Figure: 9 Closed cycle or dense air
Bell-Coleman Refrigerator
The Bell-Coleman cycle (also known as reversed Brayton or Joule cycle) is a modification
of reversed Carnot cycle. The cycle is shown on p-v and T-s diagrams in figure 10 (a) and (b).
At point 1, let p1,v1 and T1 be the pressure, volume and temperature of air respectively.
(a) p-v diagram.
(b) T-s diagram.
Figure:10 Bell-Coleman cycle.
The four processes of the cycle are as follows
3.13.1 Isentropic compression process
The cold air from the refrigerator is drawn into the compressor cylinder where it is
compressed isentropically in the compressor as shown by the curve 1-2 on p-v and T-s
diagrams. During the compression stroke, both the pressure and temperature increases and
the specific volume of air at delivery from compressor reduce from V1 to V2. We know that
during isentropic compression process, no heat is absorbed or rejected by the air.
3.13.2 Constant pressure cooling process
The warm air from the compressor is now passed into the cooler where it is cooled at
constant pressure p3 (equal to p2), reducing the temperature from T2 to T3 (the temperature
of cooling water) as shown by the curve 2-3 on p-v and T-s diagrams. The specific volume
also reduces from v2 to v3. We know that heat rejected by the air during constant pressure
per kg of air,
Q2-3 = cp (T2-T3)
3.13.3 Isentropic expansion process
The air from the cooler is now drawn into the expander cylinder where it is expanded
isentropically from pressure p3 to the refrigerator pressure p4 which is equal to the
atmospheric pressure. The temperature of air during expansion falls from T 3 to T4 (i.e. the
temperature much below the temperature of cooling water, T3). The expansion process is
shown by the curve 3-4 on the p-v and T-s diagrams. The specific volume of air at entry to
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Unit -3 Dr. N. K. Patel
the refrigerator increases from v3 to v4. We know that during isentropic expansion of air, no
heat is absorbed or rejected by the air.
3.13.4 Constant pressure expansion process
The cold air from the expander is now passed to the refrigerator where it is expanded
at constant pressure p4 (equal to px). The temperature of air increases from TA to T1. This
process is shown by the curve 4-1 on the p-v and T-s diagrams. Due to heat from the
refrigerator, the specific volume of the air changes from v 4 to V 1 . We know that the heat
absorbed by the air (or heat extracted from the refrigerator) during constant pressure
expansion per kg of air is
q4-1 = cp (T1-T4)
We know that work done during the cycle per kg of air
= Heat rejected - Heat absorbed
=Cp(T2-T3)-Cp(T1-T4)
Coefficient of performance,
Heatabsorbed
cp(T 1  T 4)
C.O.P. 

Workdone
cp(T 2  T 3)  cp(T 2  T 3)
(T 1  T 4)

(T 2  T 3)  cp(T 1  T 4)
 T1 
T 4   1
T4 

T2 
 T1 
T 3   1  T 4   1
T3 
T4 
...(i)
We know that for isentropic compression process 1-2,
T 2  p2 
 
T 1  p1 
 

...(ii)
Similarly, for isentropic expansion process 3-4,
 
T 3  p3  
 
T 4  p4 
...(iii)
Since p2=p3 and p1=p4, therefore from equation (ii) and (iii),
T 2 T 3 T 2 T1
 or 
T1 T 4 T 3 T 4
...(iv)
Now substituting these value in equation(i), we get
T4
1
C.O.P. 

T 3  T 4 T 3 1
T4

1
 p3 
 p4 
 
 


1
1
 p2 
 p1 
 
 

1

1
(rp )
 

1
……. V
p 2 p3

p1 p 4
Where,
rp= Compression or expansion ratio =
Sometimes, the compression and expansion processes take place according to the
law pvn = Constant. In such a case, the C.O.P. is obtained from the fundamentals as
discussed below
We know that work done by the compressor during the process 1-2 per kg of air,
w1 
n
n
( p 2v 2  p1v1) 
( RT 2  RT 1)
n 1
n 1
...( pv  RT )
and work done by the expander during the process 3-4 per kg of air,
17
w2 
n
n
( p3v3  p 4v 4) 
( RT 3  RT 4)
n 1
n 1
Net work done during the cycle per kg of air,
 w2  w1 
n
xR (T 2  T 1)  (T 3  T 4)
n 1
The vapour compression refrigeration system is now-a-days used for all purpose
refrigeration. It is generally used for all industrial purposes from a small domestic refrigerator to
a big air conditioning plant.
3.14 ADVANTAGES AND DISADVANTAGES OF VAPOUR COMPRESSION REFRIGERATION SYSTEM
OVER AIR REFRIGERATION SYSTEM
Following are the advantages and disadvantages of the vapour compression
refrigeration system over air refrigeration system:
3.14.1 Advantages
 It has smaller size for the given capacity of refrigeration
 It has less running cost
 It can be employed over a large range of temperatures
 The coefficient of performance is quite high
3.14.2 Disadvantages
 The initial cost is high
 The prevention of leakage of the refrigerant is the major problem in vapour
compression system
3.15 MECHANISM OF A SIMPLE VAPOUR COMPRESSION REFRIGERATION SYSTEM
Figure: 11 Simple vapour compression refrigeration system
Figure: 11 shows the schematic diagram of a simple vapour compression refrigeration
system. It consists of the following five essential parts
3.15.1 Compressor
The low pressure and temperature vapour refrigerant from evaporator is drawn into
the compressor through the inlet or suction valve A, where it is compressed to a high pressure
and temperature. This high pressure and temperature vapour refrigerant is discharged into
the condenser through the delivery or discharge valve B.
18
3.15.2 Condenser
The condenser or cooler consists of coils of pipe in which the high pressure and
temperature vapour refrigerant is cooled and condensed. The refrigerant, while passing
through the condenser, gives up its latent heat to the surrounding condensing medium which
is normally air or water.
3.15.3 Receiver
The condensed liquid refrigerant from the condenser is stored in a vessel known as
receiver from where it is supplied to the evaporator through the expansion valve or
refrigerant control valve.
3.15.4 Expansion valve
It is also called throttle valve or refrigerant control valve. The function of the
expansion valve is to allow the liquid refrigerant under high pressure and temperature to pass
at a controlled rate after reducing its pressure and temperature. Some of the liquid
refrigerant evaporates as it passes through the expansion valve, but the greater portion is
vaporised in the evaporator at the low pressure and temperature.
3.15.5 Evaporator
An evaporator consists of coils of pipe in which the liquid-vapour- refrigerant at low
pressure and temperature is evaporated and changed into vapour refrigerant at low
pressure and temperature. In evaporating, the liquid vapour refrigerant absorbs its latent
heat of vaporisation from the medium (air, water or brine) which is to be cooled.
In any compression refrigeration system, there are two different pressure conditions.
One is called the high pressure side and other is known as low pressure side. The high pressure
side includes the discharge line (i.e. piping from delivery valve B to the condenser),
condenser, receiver and expansion valve. The low pressure side includes the evaporator,
piping from the expansion valve to the evaporator and the suction line (i.e. piping from the
evaporator to the suction valve A)
19