3-Vapour Compression Systems
3-Vapour Compression Systems
3-Vapour Compression Systems
Of
Refrigeration & Air-Conditioning
Vapour Compression Systems
Prepared By
Brij Bhooshan
Asst. Professor
B. S. A. College of Engg. And Technology
Mathura, Uttar Pradesh, (India)
Supported By:
Purvi Bhooshan
Please welcome for any correction or misprint in the entire manuscript and your
valuable suggestions kindly mail us brijrbedu@gmail.com.
1. Calculate the power required for two compressors in an ammonia system that
serves a 30 tonne evaporator at ‒28.9°C. The system uses a direct contact cascade
condenser, and the condenser temperature is 37.8°C. The following values for
ammonia are available:
(1) Properties of superheated Ammonia
Pressure Entropy Temp Enthalpy
kgf/cm 2 kcal/kg-K °C kcal/kg
4.13 1.3772 48.9 376.9
14.00 1.27 87.8 386.6
14.00 1.3772 150 426.4
(2) Saturated Ammonia
Saturation Saturation Enthalpy Entropy
Temp Press kcal/kg (Vapour)
°C kgf/cm2 hg hf kcal/kg-K
-28.9 1.22 336 11.9 1.3772
30.56 4.13 345 43.7 1.27
37.8 14.0 351.6 86.2 1.171
If instead of two-stage compression a single-stage is used, calculate the power
required. Also compare the COP of one-stage and two-stage systems.
2. A vapour compression refrigerator uses methyl chloride and operates between
pressure limits of 180.54 kgf/m2 and 986.3 kgf/ m2. At entry to the compressor the
tion on
50°C 100°C
Temp. Pressure
t P vf vg hf hg sf sg h s h s
kJ/kg-
C bar m3/kg kJ/kg kJ/kg-K kJ/kg kJ/kg kJ/kg-K
K
–15 2.36 1.52 0.509 112.3 1426 0.457 5.549 1543 5.963 1656 6.3
35 13.5 1.7 0.096 3417.5 1476 1.282 4.93 1616 5.368 1744 5.7
Take:
Volumetric efficiency of compressor = 0.65;
Speed of compressor = 1200 rpm;
Stroke bore ratio of compressor = 1.2;
Adiabatic efficiency of compressor = 0.85;
Mechanical efficiency of compressor = 0.95;
8. A refrigerator with evaporator and condenser temperatures of -10 °C and 32 °C
uses R11. The vapour is superheated by 8 °C at entry to the compressor and the
superheating is obtained by undercooling the liquid before entry to evaporator.
Assuming isentropic compression, determine the power required for a cooling load
of 350 kJ/s, the overall efficiency for power use being 75%.
Discuss about the choice of the type of compressor for this plant.
Properties values are tabulated below.
Temp Specific Volume of Enthalpy kJ/kg Entropy of vapour Specific heat
°C vapour m3/kg liquid vapour kJ/kg K of kJ/kg K
-10 0.61141 191.44 384.64 1.7023 0.562
32 0.13104 227.96 406.11 1.6807 0.620
11. Calculate the (i) c.o.p. (ii) power requirement and (iii) cooling capacity of a CO 2
compressor working between 22.68barand 64.32 bar pressures. The liquid in the
condenser gets cooled, by another system, to 15°C before entering the expansion
valve. The system is assumed to work on wet-compression with x = 0.9. The
compressor data: stroke volume = 500 c.c; rpm = 500 and ηvol = 0.85.
Property values are: sp. heat of CO2 vapour = 2.4 kJ/kg°C.
P Enthalpy (KJ/kg) v (m3/kg) S (KJ/kg°C) t
bar Liq. Vap. Liq. Vap. Liq. Vap. °C
22.68 49.62 322.86 0.00101 0.0166 0.1976 1.2567 -15
50.92 127.75 308.08 0.00130 0.0066 0.4697 1.0959 15
64.32 164.17 283.63 0.00147 0.0042 0.5903 0.9912 25
12. A vapour compression refrigerator works between pressure limits of 10 bar and 3
bar. The working fluid is dry at the end of compression and there is no
undercooling before the expansion valve. If refrigerant flow rate is 10 kg/min,
determine (i) COP and (ii) the capacity of the refrigerant.
Table for properties of the refrigerant is as under:
Pressure Saturation Liquid Latent Liquid
temperature heat heat entropy
bar °C kJ/kg kJ/kg kJ/kgK
10 25 298.90 1165.94 1.1242
3 - 10 135.37 1297.68 0.5443
13. In a refrigeration system of 10 TR cooling capacity using CHCIF2, the evaporator
and condenser temperatures are -10°C and 45°C respectively.
Properties of CHCIF2 at saturation are:
Temperature Sp. Volume Enthalpy Entropy
m3/kg kJ/kg kJ/kg-K
°C vg hf hg sf sg
-10 0.0654 34.25 247.37 0.1374 0.9473
45 0.0133 101.76 261.95 0.3662 0.8697
Consider standard vapour compression cycle with inlet to compressor as saturated
vapour and inlet to expansion valve as saturated liquid. Assume that vapour may
be treated as perfect gas in desuperheating process with average specific heat of
0.9335 kJ/kg-K. Show the cycle on T-s and p-h diagrams. Find:
(i) compressor outlet temperature and enthalpy,
(ii) mass flow rate of refrigerant,
(iii) work requirement,
(iv) condenser heat rejection,
(v) COP and
Temp (°C) -5 0 5 10
Pressure (bar) 2.43 2.928 3.5 4.146
22. A commercial refrigerator with refrigerant 134a as the working fluid keeps a space cooled at
-30°C. It rejects heat to cooling water that enters the condenser at 18°C and at the rate of
0.25 kg/s and it leaves at 26°C. The refrigerant enters the condenser at 1.2 MPa and 65°C
and it leaves at 42°C. The inlet state of compressor is 60 kPa and -34°C. It gains a net heat
of 450 W from the surroundings. Sketch T-S diagram and determine,
(i) refrigeration toad
(ii) COP
(iii) Theoretical maximum refrigerant toad for the same power input to the
compressor.
Given properties
60 𝐾𝑃𝑎
ℎ−34°𝐶 = 230.03 kJ/kg, ℎ1200
65°𝐶
𝐾𝑃𝑎
= 295.16 kJ/kg,
1200 𝐾𝑃𝑎
ℎ42°𝐶 = 111.23 kJ/kg, 𝑥111.23 = 0.47.
60 𝐾𝑃𝑎
hf hg sf sg vg
(0°C) (bar) (kJ/kg) (kJ/kg K) m3/kg
1 4.816 198.83 404.99 0.996 1.753 0.0487
45 17.290 256.40 417.31 1.187 1.693 0.0133
Table 1 Saturated Values at 17.290 bar:
Temperature Specific enthalpy Specific entropy
0°C (kJ/kg) (kJ/kg K)
65 436.27 1.751
70 440.77 1.764
75 445.21 1.777
32. A simple R-12 plant is to develop 5 tonnes of refrigeration. The condenser and
evaporator temperatures are to be 40 °C and -10 °C respectively. Determine the
following:
(i) Refrigerant flow rate
(ii) Volume flow rate handled by the compressor in m3/s
(iii) Compressor discharge temperature if the enthalpy of refrigerant at
compressor exit is 209.41 kJ/kg
(iv) Pressure ratio
(v) Heat rejected to the condenser in kW
(vi) Flash gas percentage after throttling
(vii) COP
(viii) Power required to drive the compressor.
Saturated values of properties of R-12:
Sp. enthalpy Sp. enthalpy Sp.
Temperature Pressure Volume
(Saturated liquid) (Saturated vapour) entropy
(°C) (bar) h f (kJ/kg) hg (kJ/kg) (kJ/kg-K) (m3/kg)
10 2.1912 — 183.19 0.7019 0.077
40 9.6066 74.59 203.981 — —
Enthalpy values of vapour refrigerant at 9.6066 bars:
Temperature (°C) Sp. enthalpy (kJ/kg)
40 203.981
50 210.950
33. An ammonia refrigerator operates on the simple vapour compression cycle. The
pressure of liquid ammonia before throttling is 12.3 kgf/cm2 and its temperature is
27.2°C. The evaporator pressure is 2.53 kgf/cm2. Ammonia gas leaves the
evaporator at 9.5°C. The power input to the compressor is 2.12 kW and the mass
flow rate of ammonia is 27 kg/h. Determine the dryness fraction of the gas after
throttling, the heat absorbed per hour in the evaporator and the coefficient of
performance of the refrigerator. Sketch the cycle on the T-s and the p-h diagrams.
Properties of ammonia are given below:
Temperature Pressure Saturation enthalpy, kcal/kg
°C kg/cm2 liquid gas
13.8 2.53 27.5 341
+31.1 12.30 77.5 351
Specific heat of liquid at 12.3kgf/cm2 may be taken to be 1.14 kcal/kg°C and the
specific heat of superheated vapor at 2.53 kgf/cm2 may be taken to be 0.6
kcal/kg°C.
53. An R717 based vapour compression refrigerating machine works between 38°C and
-20°C temperature. The ammonia leaves the compressor dry and saturated. Liquid
ammonia is undercooled to 30°C temperature inside the condenser, before
throttling. Find the theoretical COP of the machine. The Cp of saturated ammonia
liquid is 4.91 kJ/kg-K. If net refrigeration required is 25.0 TR, find the mass flow
rate of ammonia in kg/h, assuming relative COP 0.75.
The saturation properties of R717 are given in the following Table:
t,°C Liquid Vapour
h, kJ/kg s, kJ/kg-K h, kJ/kg
38 380.78 1.6134 1489.36
20 108.55 0.6538 1437.68
54. An air-conditioner with refrigerant R134a as the working fluid is used to keep a
room at 26°C by rejecting the waste heat to the outside air at 34°C. The room is
gaining heat through the walls and the windows at a rate of 250 kJ/min while the
heat generated by the appliances in the room amounts to 900 W. An unknown
amount of heat is also generated by the people in the room. The condenser and
evaporator pressures are 1200 kPa and 500 kPa respectively. The refrigerant is
saturated liquid at the condenser exit and saturated vapour at the compressor
inlet. If the refrigerant enters the compressor at a rate of 100 L/min and the
isentropic efficiency of the compressor is 75%, determine the (i) temperature of the
refrigerant at the compressor exit, (ii) rate of heat generation by the people in the
room, (iii) COP of the air-conditioner, and (iv) minimum volume flow rate of the
refrigerant at the compressor inlet for the same compressor inlet and exit
conditions.
Properties of R134a:
At 500 kPa : h = 259.3 kJ/kg, vg = 0.4112 m3/kg, sg = 0.924 m3/kg,
At 1200 kPa: hf = 117.77 kJ/kg