Class 4 (4th Engineer) MEP 📅 Dec 2024

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-off 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 on the compressor suction side (3)

Reference Answer

### (a) Reasons for Foaming in Refrigeration Compressor Crankcase
Foaming in a compressor crankcase is the violent formation of bubbles in the lubricating oil, primarily caused by liquid refrigerant rapidly boiling out of the oil solution when the crankcase pressure drops. This phenomenon is a direct result of refrigerant miscibility with lubricating oil.
**Mechanism:**
1. **Refrigerant Migration (Off-Cycle):** During compressor shutdown, the crankcase cools. If it becomes the coldest point in the system, the pressure difference causes refrigerant vapour to migrate from warmer sections (like the evaporator and condenser) to the crankcase.

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