Hotel Service Unit 11 Refrigeration Q7 – Answer

  1. (a) State FIVE properties of a refrigeration oil.
    (b) Describe the operation of an Oil Separator in a refrigeration system.

(a) Properties of Refrigeration Oil (5 marks)

  1. Low pour point – so it remains fluid at evaporator temperatures.
  2. Good lubricating qualities – to reduce wear in the compressor.
  3. Chemical stability – must not react with refrigerant or decompose at operating temperatures.
  4. Low foaming tendency – prevents foam formation in the compressor crankcase.
  5. Good dielectric strength – important when used with motor-compressor units to avoid electrical tracking.

(Other acceptable answers: miscibility with refrigerant when required, non-corrosive, low moisture content.)

(b) Operation of an Oil Separator in a Refrigeration System (5 marks)

  • Located in the discharge line after the compressor.
  • Refrigerant gas leaving the compressor carries fine droplets of lubricating oil.
  • Inside the separator:
    • Gas velocity is reduced and flow is directed through baffles or mesh screens, causing oil droplets to coalesce.
    • Oil collects at the bottom of the separator.
  • A float-operated valve automatically returns the separated oil through a small line back to the compressor crankcase.
  • The now oil-free refrigerant vapour continues to the condenser.

Purpose:

  • Prevents oil from circulating through the system where it could foul heat exchangers (reducing efficiency).
  • Ensures compressor crankcase maintains correct oil level for lubrication.