Refrigeration practice: In a vapour-compression system, where should the oil separator be installed to return lubricating oil to the compressor without carrying it through the condenser and evaporator?

Introduction / Context:
Compressors require lubrication, and a small fraction of oil inevitably travels with the refrigerant. Excess oil circulating through heat exchangers degrades heat transfer and can pool in the evaporator. An oil separator removes oil from the discharge gas and routes it back to the crankcase.

Given Data / Assumptions:

• Standard vapour-compression layout: compressor → condenser → expansion device → evaporator → compressor.
• Oil separator is a mechanical device that coalesces and separates oil droplets.
• Objective: minimize oil carryover through the condenser/evaporator.

Concept / Approach:
Place the separator where oil concentration is highest and refrigerant is superheated gas at high pressure: immediately downstream of the compressor on the hot gas line. Separated oil is returned via a controlled return line to the compressor sump, often by differential pressure.

Step-by-Step Reasoning:

Verification / Alternative check:
Equipment manuals and piping schematics consistently show separators on the discharge line; liquid lines do not carry bulk oil.

Why Other Options Are Wrong:

• Before compressor or after evaporator (suction): gas is cool and oil tends to return naturally; separators are ineffective here.
• Liquid-line locations contain mostly liquid refrigerant, not oil mist.

Common Pitfalls:
Misplacing the separator on the suction line or omitting proper oil return control can starve the compressor of lubrication.

Final Answer:
Between the compressor and the condenser (on the hot discharge line)