Class 4 (4th Engineer) MEP 📅 Apr 2025

Exam Question

(a) What are the reasons for foaming in refrigeration compressor crankcase? (4)

(b) Describe the causes for the following in the domestic refrigeration system:

(i) Frequent cut-in and cut-out of the compressor. (3)

(ii) Lub oil low pressure cut-out of the compressor. (3)

(iii) High pressure cut-out of the compressor. (3)

(iv) Frosting in the compressor suction side. (3)

Reference Answer

### (a) Reasons for Foaming in Refrigeration Compressor Crankcase (4 Marks)
Foaming is the phenomenon of rapid formation of bubbles of refrigerant gas within the lubricating oil in the compressor crankcase. It is caused by the sudden drop in crankcase pressure upon compressor start-up, which causes the refrigerant dissolved in the oil to flash into vapor.
**Fundamental Principle:**
The solubility of refrigerant in lubricating oil is directly proportional to pressure and inversely proportional to temperature. During the compressor's off-cycle, the refrigerant migrates to the coldest part of the system, which is often the crankcase, and condenses. It is then readily absorbed by the cool lubricating oil under the system's equalized pressure.

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