Refrigerant 134a enters the evaporator of a refrigeration system as a saturated

Question

Refrigerant 134a enters the evaporator of a refrigeration system as a saturated liquid-vapour mixture at 140 kPa. The refrigerant absorbs 180 kJ of heat from the cooled space, which is maintained at - 10 degree C, and leaves as a saturated vapour at the same pressure. Determine (a) the entropy change of the refrigerant, (b) the entropy change of the cooled space, and (c) the total entropy change of this process. Go back to Problem 5: If the cooled space temperature were lowered to -20 degree C, what should be done to the evaporator pressure? (Should it be increased, decreased, or kept at 140 kPa?) Explain in very few words.

Explanation / Answer

During the evaporation process taking place there will be only phase change at constant saturation pressure as well as at constant saturation temperature

At this region two points have to be noticed they are

1) Degree of freedom is one

2) Pressure is dependent on temperature and vice versa

From from point 2) change in pressure is directly proportional to change in temperature

in the given problem temperature is lowered ,

To compensate this temperature change we need to have simlar change in pressure

it means pressure has to decrease from 140 kpa to lower value than 140kpa

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