Refrigeration oils and their purpose of use
Refrigeration oils are necessary to ensure the flawless operation of the compressor. There is only one compressor type that does not require lubrication, the turbocompressor.
The purposes of oil:
- reducing friction
- reducing the noise made by moving parts
- reducing internal refrigerant leaks
- compressor cooling
- carrying the leak detection agent
Unfortunately, some of the oil is expelled as vapour or small droplets into the refrigeration plant’s piping with the refrigerant. This is not harmful if the oil concentration remains low. It actually slightly improves the power of the heat exchangers. That said, the introduction of excess oil into the machinery is obviously nothing but a detriment to its operation.
As a result, refrigeration systems must always be built in such a way that as little oil as possible enters the circulating pipes. Moreover, the structure must ensure that the trace amounts of oil that enter the system are returned to the compressor through the circulation.
Refrigeration oils suitable for various refrigerants
Oils suitable for CFC and HCFC refrigerants
- Mineral oils
- Synthetic oils
- Semi-synthetic oils
Oils suitable for HFC refrigerants
- Polyolester oils or POE oils or colloquially “ester oils”
- PAG oils for car air conditioning system
Oils suitable for ammonia
- Mineral oils
- Synthetic P-type oils
Do you want to know more?
Read Bitzer raport: General aspects on refrigerant development (table, source)
Mineral oils (previously the most common oil)
Mineral oils used to be the most common oil type used in conjunction with old CFC and HCFC refrigerants.
The oil is manufactured from raw oil by means of vacuum distillation and purification. It is also affordable due to the low degree of processing. The lubrication qualities are limited in extreme conditions, however. The viscosity of mineral oil changes dramatically as a function of temperature, which is why the oil’s optimal operating range is rather narrow. The viscosity of mineral oils drops at high temperatures, which deteriorates the lubricating properties. At low temperatures, there may also be some oil return issues due to the decreased dissolving of the lubricant and refrigerant.
Synthetic oils / fully synthetic oils
Synthetic oils are made from natural gas by polymerising ethene gas or through the refinement and conversion process. This enables the molecular chains of the synthetic to be more homogenous than in mineral oils that are manufactured directly from raw oil. Synthetic oil is better at withstanding demanding conditions, such as high final compression temperatures and low evaporation temperatures.
The mixture of a mineral oil and synthetic oil is called a semi-synthetic or part synthetic oil. These cannot be used with current HFC refrigerants. Mineral oils dissolve fully with old R11 and R12 refrigerants and they are efficiently returned to the compressor. In the context of other CFC and HCFC refrigerants, the dissolution is only partial. This means that the return of the oil must be ensured in refrigeration plants.
Semi-synthetic oil can be regarded as a compromise in terms of price and properties.