Class 2 (2nd Engineer) MEKM 📅 Jan 2025

Exam Question

With respect to the refrigeration system on board vessels, answer the following:

(a) Why are some TEVs fitted with an external equalising connection? (6)

(b) What is the purpose of a back pressure valve. What will be the effect if it leaks? (5)

Reference Answer

### (a) Why TEVs are fitted with an External Equalising Connection
A Thermostatic Expansion Valve (TEV) controls the rate of refrigerant flow into the evaporator to maintain a pre-set superheat at the evaporator outlet. This is achieved by balancing three forces on its diaphragm:
1. **Opening Force:** The pressure of the gas in the sensing bulb (P_bulb), which is strapped to the evaporator outlet pipe.
2. **Closing Force 1:** The pressure of the refrigerant inside the evaporator (P_evap).

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