6.(a) State FOUR properties of refrigeration oil.(4)
(b) Describe the operation of an Oil Separator in a refrigeration system.(4)
(c) State reasons for the employment of an oil separator in refrigeration vapour compression units.(2)
a) Properties of Refrigeration Oil (4)
- Low pour point – to remain fluid at low evaporator temperatures.
- High dielectric strength – to provide insulation where the oil may come into contact with motor windings.
- Low miscibility with refrigerant – particularly with ammonia, so that separation is effective.
- Good lubricating properties – to reduce friction and wear of compressor moving parts.
(b) Operation of an Oil Separator in a Refrigeration System (4)
- The oil separator is fitted in the discharge line between the compressor and the condenser.
- Hot discharge gas carrying entrained oil enters the separator and is slowed down, causing oil droplets to separate out by:
- Impingement (striking baffles),
- Change of direction, and
- Reduction in velocity.
- The separated oil collects at the bottom of the separator.
- A float-operated valve automatically returns the oil through a small return line to the compressor crankcase, under pressure differential.
- The cleaned refrigerant gas then flows onward to the condenser.
(c) Reasons for Employment of an Oil Separator (2)
- To prevent excessive oil circulation through the system, which can reduce heat transfer efficiency in the condenser and evaporator.
- To ensure the compressor maintains a sufficient supply of lubricating oil, preventing wear and damage.