LUBRICATION OF REFRIGERATION COMPRESSORS

• Fully synthetic oils (e.g. polyol ester/ POE, or polyalkylene glycol/ PAG) have better thermal and chemical stability, a wider operating temperature range, and better lubricating properties than mineral oils.
• They are compatible with the modern HFC/ HFO refrigerants (e.g. R134a, R407C, R410A) which are not compatible with mineral oils.
• They have a higher viscosity index (less viscosity change with temperature), better low-temperature flow, and better resistance to oxidation and breakdown.
• They provide better protection of the compressor (reduced wear, longer life) and better system performance.

• Oil is carried over from the compressor with the refrigerant discharge because the oil is entrained in the refrigerant vapour/ gas as it leaves the compressor (oil mist/ droplets carried by the high-velocity gas).
• This occurs due to the high gas velocity, the compressor's oil circulation, and the mixing of oil with the refrigerant.
• Oil carry-over is more likely at high load/ high speed, or if the oil level is too high, or if the oil is too light/ foamy.

• An oil separator (oil trap) is fitted in the compressor discharge line. It separates the oil from the refrigerant gas (by centrifugal/ impingement action), collects the oil, and returns it to the compressor sump through a return line (with a float/ level control or a capillary/ orifice). The oil separator ensures the oil is returned to the compressor rather than circulating with the refrigerant.

• Oil in the evaporator coats the heat-transfer surfaces, reducing the heat-transfer efficiency (insulating the surfaces) and reducing the cooling capacity.
• Oil in the evaporator reduces the refrigerant flow/ the effective evaporator volume, affecting the system performance.
• Oil can cause the evaporator to operate at a higher temperature/ pressure, reducing efficiency.
• Oil accumulation can cause the evaporator to flood or the system to malfunction.
• Oil in the evaporator must be returned to the compressor (via an oil return arrangement) to maintain the correct oil level and system performance.