

5.6 Oil and refrigerants

Mineral oil is the compressor lubricant used with CFCs and HCFCs. Due to the different properties of HFCs, mineral oil cannot be used as a lubricant for HFC systems. For example, the miscibility and solubility of HFC and mineral oil differ from those of mineral oil and CFCs or HCFCs. If HFCs and mineral oils were used together, both the oil return to the compressor and the heat transfer in the evaporator and condenser would be impaired.

To achieve an acceptable miscibility between the refrigerant and the lubricant, polyolester oils (POEs) are normally used with HFCs. Polyvinyl ethers are also being introduced in some applications. In automotive air conditioning systems, polyglycol (PAG) is used by most manufacturers, but some manufacturers recommend POE for retrofits. POEs are very hygroscopic, and will therefore absorb water if given the chance. If too much humidity enters the oil, there is a risk of acid formation, which could attack the components of the system. Polyolester oils used with the new refrigerant alternatives also seem to have a cleaning effect. This could lead to a possible problem in converted (retrofitted) systems. Deposits are dissolved and collect in the expansion valve, with consequent blocking of the refrigerant flow. Installing a proper and efficient filter drier in the liquid line can prevent both the humidity and the blocking problem.

When converting an existing system to HFCs, the mineral oil is removed and replaced by polyolester oil. If too much mineral oil is left in the system after converting, it may be deposited in the evaporator and impair the boiling heat transfer.

A refrigerant mixture consists of two or more components. The extent to which these dissolve in the compressor oil varies for the different components. In systems where the refrigerant charge is small in comparison to the oil volume, this difference in solubility may lead to problems. For example, if a mixture has a small percentage of one component that dissolves more readily in the oil than the other components, this will influence the physical properties and thereby the system performance.​​
