Question

For a vapour compression refrigeration cycle using a typical refrigerant, such as R134a, describe, in no more than 200 words, the role of the throttle. Include in your answer the description of the change through the throttle and what condition the refrigerant is expected at the beginning and end regarding saturation conditions and dryness fraction. Any words over the 200 word limit will not be considered. [4]

Answer 1

Throttling in refrigeration cycle is done by throttle device.Throttle device can be valve or capillary tube having small area of flow.When the refrigerant pass through the throttle valve its pressure reduces due to friction.

Refrigerant exit as a high pressure saturated liquid from the condenser.The condition of refrigerant before throttle device is a high pressure satured liquid.After that it enters as a high pressure saturated liquid into the throttling device ,its pressure decreases due to the friction and small area of throttling device.Due to decrease in pressure temperature its temperature also decreases. Refrigerant exit from throttle device as a low pressure low temperature and as partial liquid and vapour i.e very wet vapour which enters the evaporator.Pressure reduces so that it can vaporize at the reaquired temperature in the evaporator.

The throttling  device also controls the flow of refrigerant. When refrigeration load is more that means more refrigerant is required which is done by the throttle valve by increasing the flow of refrigerant. When refrigeration load is less that means less refrigerant is required which is controlled by throttle valve by decreasing the flow of refrigerant.

Throttling process is a isoenthalpic process i.e. enthalpy remains constant.