While refrigerants are essential for a refrigeration system’s cooling properties, a dedicated refrigeration oil is vital for correct functionality of the compressor. Lubrication solutions for refrigeration compressors cut back friction, mitigate wear and work as a seal between both the low- and high-pressure sides.
Although the core function of a refrigeration oil is to lubricate, it must also be capable of withstanding extreme temperature differences and be compatible with any refrigerant at any given time. In this in-depth blog, we will take a closer look at refrigeration systems, refrigerants and the type of oils used, as well as the important properties they must possess to perform their role. Read on to learn more.
How do refrigeration systems operate?
A typical refrigeration cycle is performed by first circulating, then evaporating and condensing a refrigerant within a closed system. Refrigerant evaporation happens at low temperature and pressure, while the condensation process takes place at high temperature and under high pressure. As a result, heat transfer is possible from low temperature areas to areas of high temperature.
First, hot compressed vapour will enter the condenser. Here, it is cooled and then condensed into a liquid form, causing heat to generate. The refrigerant enters the refrigeration system’s compressor during the vapour phase and is then compressed to a much higher pressure. This results in a far higher temperature. Once cold, the mixture then enters the evaporator, and the liquid part of the refrigerant mixture is then evaporated.
The condensed liquid refrigerant now flows through the system via an expansion valve and the pressure is effectively reduced. This reduction in pressure results in the evaporation of part of the liquid refrigerant. As a result, the temperature of the liquid-vapor refrigerant mixture is lowered.
What types of refrigerants are utilised in refrigeration systems?
Chloro-Fluoro-Carbons (CFCs) are the first group of refrigerants used and are considered the most harmful to the environment. Examples include R11, R12 and R13, among many others. Hydro-Chloro-Fluoro-Carbons (HCFCs) are another type of refrigerant that are safer, still slightly harmful to our environment. Examples of HCFCs include R21 and R123. Hydro-Fluoro-Carbons (HFCs) are not harmful to the atmosphere refrigerants or the ozone layer. Finally, Ammonia is sometimes used and, while it is eco-friendly, it is exceptionally toxic to people.
Requirements of refrigeration oils
To be effective in refrigeration systems, oils used must have specific qualities that allow them to help equipment function effectively. The following is a list of the key requirements of a refrigerant oil.
Dedicated refrigeration oils must be able to function properly over a broad temperature range. Process end compression temperatures within refrigeration compressors can hit top temperatures of 180ºC. As a result, refrigeration oil must have thermal stability. If an oil used contains volatile oil fractions, its lighter ends will enter the compressor system in vapour form, where they will condense and decrease the heat transfer efficiency, while simultaneously increasing the viscosity of the oil.
To avoid a chemical reaction with the dedicated refrigerant, the refrigeration oil used must be chemically stable.
Refrigerant to oil solubility
Oil and refrigerant mixtures may be partly insoluble or soluble. Total solubility helps lubrication, although it can lead to substantial viscosity drops within the compressor that can cause increased friction and undue wear. To work effectively, the oil-refrigerant solution must have a viscosity that is adequately high to provide efficient lubrication and sealing inside of the compressors it serves.
When liquids boil, the extra heat is employed to increase the molecule’s kinetic energy, until they cannot remain in a liquid phase and start to form into a vapour. This occurs at a specific pressure, but if this pressure is increased, the boiling point will also increase.
The relationship between boiling temperature and pressure for saturated conditions is dictated by the “vapour pressure diagram”.
What types of oil types are utilised in refrigeration systems?
There are four different kind of refrigeration oils that are typically used in refrigeration systems. These are as follows:
Alkylbenzenes (ABs) are refrigeration oils with good chemical and thermal stability. They are highly miscible with refrigerants and are entirely soluble with both PAO and mineral oils. This makes it possible to create mixtures to improve lubricant properties.
Polyalphaolefin (PAO) also have a good thermal and chemical stability. PAO also offers outstanding viscosity temperature attributes. It has low miscibility with refrigerants and, as a result, it is mainly used when miscibility is not an issue. PAO can lead to seals shrinking, but this issue is solved when it is mixed with AB refrigeration oil.
Polyol ester or POE refrigeration oil is perhaps the most common synthetic lubricant that is used with HFC type refrigerants.
Finally, Polyalkyleneglycols (PAG) are refrigeration oils with thermal stability and high viscosity index. Hygroscopic, they attract water and are soluble with ammonia.