Exam Question

(a)
(b) The vapor-compression refrigeration system operates on a cyclical process. The refrigerant, initially a low-pressure, low-temperature gas (1) in the evaporator, absorbs heat from the space to be cooled, causing it to evaporate. This low-pressure vapor is then drawn into a compressor (2), where it is compressed to a high-pressure, high-temperature gas. This hot, high-pressure gas enters the condenser (3), where it releases heat to the surrounding environment, condensing into a high-pressure liquid. Finally, the high-pressure liquid passes through an expansion valve (4), undergoing a significant pressure drop, causing some of it to flash into vapor and a significant temperature reduction. This low-pressure, low-temperature mixture then enters the evaporator, restarting the cycle.
Operating temperature is adjusted primarily through two methods:
A refrigerant flow control valve: (often a solenoid valve) regulates the refrigerant flow rate into the evaporator. Reducing the flow reduces the amount of refrigerant evaporating, thus increasing the evaporator temperature (and therefore the space temperature). Increasing the flow has the opposite effect. A thermostatic expansion valve automatically adjusts the flow rate based on evaporator temperature.