772 ENGINEERING THERMODYNAMICS
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- The various parts of a simple vapour compression cycle are : Compressor, Discharge line (or hot gas line),
Condenser, Receiver tank, Liquid line, Expansion valve, Evaporator and Suction line. - If the vapour is not superheated after compression, the operation is called ‘Wet compression’ and if the
vapour is superheated at the end of compression, it is known as ‘Dry compression’. Dry compression, in
actual practice is always preferred as it gives higher volumetric efficiency and mechanical efficiency and
there are less chances of compressor damage. - p-h chart gives directly the changes in enthalpy and pressure during a process for thermodynamic analysis.
- When suction pressure is decreased, the refrigerating effect is decreased and work required is increased.
The net effect is to reduce the refrigerating capacity of the system and the C.O.P. - The overall effect of superheating is to give a low value of C.O.P.
- ‘Subcooling’ results in increase of C.O.P. provided that no further energy has to be spent to obtain the extra
cold coolant required. - The refrigerating system should always be designed to operate at the highest possible vaporising tempera-
ture and lowest possible condensing temperature, of course, keeping in view the requirements of the
application. - ‘Volumetric efficiency’ is defined as the ratio of the actual volume of gas drawn into the compressor (at
evaporator temperature and pressure) on each stroke to the piston displacement. If the effect of clearance
alone is considered, the resulting expression may be termed ‘clearance volumetric efficiency’. The expres-
sion used for grouping into one constant all the factors affecting efficiency may be termed ‘total volumetric
efficiency’. - Clearance volumetric efficiency, ηcv = 1 + C – C
p
p
d
s
F n
HG
I
KJ
1/
where, C = Clearance volumeSwept volume
pd = Displacement pressure
ps = Suction pressure.
- Total volumetric efficiency,
ηtv =^1
1
+−F
HG
I
KJ
L
N
M
M
O
Q
P
CC P××
p
p
p
p
T
T
d
s
n
c
s
s
c
/
where subscript ‘c’ refers to compressor cylinder and ‘s’ refers to the evaporator on the suction line just
adjacent to the compressor.
Objective Type Questions
Fill in the blanks :
- ...... means the cooling of or removal of heat from a system.
- Most of the commercial refrigeration is produced by the evaporation of a liquid ......
- ...... is the ratio between the heat extracted and the work done.
- ...... =
Actual C.O.P
Theoretical C.O.P.. - The C.O.P. for Carnot refrigerator is equal to ......
- The C.O.P. for a Carnot heat pump is equal to .....
- The C.O.P. for a Carnot refrigerator is ...... than that of Carnot heat pump.
- The C.O.P. of an air refrigeration system is ...... than a vapour compression system.
- In a refrigeration system the heat rejected at higher temperature = ...... + ......
- Out of all the refrigeration systems, the ...... system is the most important system from the stand point of
commercial and domestic utility.