The evaporative pressure or suction pressure is an important factor that affects the performance of a vapor compression system. Let the suction pressure be reduced from P1 toP1‘ on the P-h diagram. Its effect will be.
- Decrease in refrigerating effect from (h1-h4) to (h1‘-h4)
- Increase in the specific volume of vapour from v1 to v1‘.
- Increase in compressor work from (h2-h1) to (h2‘-h1‘) due to increase in pressure ratio.
- Decrease in volumetric efficiency due to increase in the pressure ratio.
- Decrease in COP of the system.
The decrease in suction pressure (evaporative pressure) leads to the decrease of COP. Therefore it is desired to have the highest suction pressure ie. consistent with the requirement of temperature and load at the evaporator.