Conditions for Highest COP:
Effect of Evaporator Pressure
Assume a simple saturation cycle 1-2-3-4 along Freon 12 as the refrigerant as indicated in given fig. for operating conditions of tk = 40°C & t = - 5°C.
Now assume a change in the evaporator pressure equivalent to a decrease in the evaporator temperature to - 10°C. The changed cycle is indicated as 1'-2'-3'-4' in fig.
Therefore It is seen that a drop in evaporator pressure equivalent to a drop of 5°C in saturated suction temperature enhance the volume of suction vapour and therefore reduced the capacity of a reciprocating compressor and enhance the power consumption per unit refrigeration.
![797_Conditions for Highest COP.png](https://www.expertsmind.com/CMSImages/797_Conditions%20for%20Highest%20COP.png)
Effect of Evaporator Pressure
It is observed that a reduction in evaporator temperature results in :
(a) Reduction in refrigerating effect from (h1 - h4) to (h1' - h4')
(b) Enhancement in the particular volume of suction vapour from v1 to v1'
(c) Reduction in volumetric efficiency, because of enhancement in the pressure ratio, from ?v to ?v '
(d) Enhancement in compressor work from (h2 - h1) to (h2' - h1') because of enhancement in the pressure ratio with change from steeper isentropic 1-2 to flatter isentropic 1'-2.
Since
. . . (2.1)
and
W * = m* w . . . (2.2)
Expressions for the dependence of unit power consumption and capacity can now be written as:
![1134_Conditions for Highest COP2.png](https://www.expertsmind.com/CMSImages/1134_Conditions%20for%20Highest%20COP2.png)