Lesson 7 - Refrigeration Theory and Cycle SAQ

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a) What is the vapour compression cycle?

b) Describe the components of the OVC cycle, and state the purpose of each one.

c) Using specific enthalpy, discuss what each component does to the primary refrigerant in the OVC cycle.

Response:

a) The ordinary vapour compression (OVC) cycle is a phase change cycle that the primary refrigerant goes through in order to transfer heat from the source to the sink.

b) The OVC cycle consists of an evaporator, followed by a compressor, and then a condenser and an expansion valve. The primary refrigerant goes through each piece of equipment continuously in a cycle. The purpose of each component is as follows:

• Evaporator – withdraws heat from the source, which evaporates the refrigerant from a liquid/vapour mixture to a saturated vapour at constant pressure.

• Compressor – compresses the gas to a higher pressure, changing it from a saturated vapour to a superheated vapour.

• Condenser – releases the heat to the sink by condensing the refrigerant to a saturated liquid at a constant pressure.

• Expansion valve – reduces the vapour to a lower pressure (and temperature), changing the saturated liquid to a mixture of liquid and vapour.

c)

• Evaporator – changes a liquid/vapour mixture to a saturated vapour at constant pressure, increasing the specific enthalpy of the primary refrigerant.

• Compressor – changes a saturated vapour to a superheated vapour, which increases the specific enthalpy of the primary refrigerant.

• Condenser – changes a superheated vapour to a saturated liquid, which decreases the specific enthalpy of the primary refrigerant.

• Expansion valve – changes a saturated liquid to a mixture of liquid and vapour (ideally). The specific enthalpy of the primary refrigerant does not change during this cycle step.