S R C C Alternatives to H F s in the efrigeration and Air Conditioning Sector ection 01 35 Th e need to ensure lubrication and oil transport creates a challenge in RAC systems as the lubricating properties will vary greatly, depending on operating conditions. It is therefore not uncommon that compressor manufacturers will recommend different oils for different applications. Neglecting the importance of oil properties has resulted in a large number of failures in systems that use alternatives to R-22 including HFCs, service blends and hydrocarbons. There have been serial problems in new HFC and new hydrocarbon systems, as well as in connection with different service blends. The oil behaviour in the refrigeration systems is often significantly more complex and challenging than the refrigerant’s behaviour. When a system is modified, it is important to consider how this will impact lubrication and oil transport. When there is information from the compressor and/or oil manufacturer, this should be taken into account. System manufacturers should be consulted if they are aware of any problems with their particular system although some do not consider it to be in their own interest to extend the life of old (often inefficient) equipment or to assume responsibility for advice that will involve some risk of problems. Ester oils and HFCs act as a solvent in the system and there is a risk that debris and contaminants that have accumulated in the system over time are dissolved TEC HNICAL INFORMATION S ome key properties are listed below: ■■Lubricating properties are important to create a reliable system with a long life. The lubrication is strongly affected by the refrigerant in the system when there is a miscible oil-refrigerant mixture. The miscibility is important due to the requirement to have acceptable oil transport throughout the system. The amount of the refrigerant dissolved in the oil is affected by pressure and temperature, which means that the lubrication varies with the operating conditions. Sometimes compressor manufacturers require different oil based on the operating conditions. The miscibility of the refrigerant in oil is an important factor to provide the right lubrication properties when the refrigerant is dissolved in the oil at all pressures and temperatures that can occur in the compressor’s operating envelope. Also, the CFC or HCFC refrigerant by itself affects the lubrication properties as the chlorine from the CFC or HCFC acts as an anti-wear additive. ■■Chemical stability in the system in the presence of refrigerant and all materials that could be used over the whole operating envelope. This must also take into account the presence of moisture and contaminants that can occur in a refrigeration/air conditioning system. This is a key concern as contaminants drastically decrease stability. Poor practices applied during manufacturing, installation and servicing results in a decreased life expectancy for the system. Unless practices are improved, the phase out of ODS will result in an increased failure rate as the new oils are more sensitive to poor handling. On the other hand, with the introduction of good servicing practices, the life of RAC systems can often be many times what was expected with CFC or HCFCs and old servicing methods. ■■Miscibility - Oil transport. As there will always be a small amount of oil that leaves the compressor with the compressed refrigerant it is essential to ensure that it comes back. If the oil is not transported around the system, the compressor will eventually run out of oil and fail. Oil in the heat exchangers will also have a negative impact on the heat transfer and decrease the energy efficiency. Normally the oil transport problem will be critical at low evaporation temperatures and low capacity and could occur after a long time when the system operates under specific conditions. E Elastomers compatibility■■ with oils is an important issue, in particular in old systems where O-rings and other seals can be affected by a change of oil. There are some commonly used materials like Viton™ (a trade name of special fluoropolymer offered by DuPont) that are not recommended together with POE and PAG oils. If these oils are introduced in systems with unsuitable elastomers, the risk of increased leaks is obvious. These elastomers are often used in gaskets/seals in solenoid and other valves. They can also be used on sight glasses applied in liquid lines and receivers. There are also elastomers used in the service ports (so-called Schrader valves) and in the caps for these. As a rule these caps and/or cap seals should be replaced when refrigerant and oil are replaced with new combinations that can be more aggressive to elastomers.