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Refrigerant changes and lubricants

By Grant Laidlaw

There are many changes as we move into a new natural refrigerant era. You will find that we have almost as many new oils on the market as there are refrigerants.

Gean asks:  Grant,  with all the changes with regards to refrigerants how does this affect the lubricants in the systems, aside from the older systems, are we even doing mineral oils?

Hi Gean, let’s begin with the obvious: let the compressor manufacturers specify what oil to use in any application. If you are unsure of what oil to use, call the compressor manufacturer or use the specification data along with their performance curve literature.

Having said that let us move on to the refrigerant oils.

In refrigeration systems the purpose of the oil is to provide adequate lubrication between metal surfaces. Oil often serves as a seal to prevent refrigeration from leaking between high- and low-pressure regions of the compressor, on crankshaft seals and in the automotive sector in coating the rubber hoses. The oil can perform a cooling function that is significant in some compressors. In addition, oil can form an integral part of capacity control on larger compressors. Some oil will leave the compressor during the cooling cycle and circulate with the refrigerant through the system. The system should be piped to return this oil back to the compressor on a continual basis.

Also read: Refrigerant changes and lubricants – Part 2

Refrigeration oils must have the correct properties required by the system. Other oils, such as those used to lubricate motor vehicle engines, or hydraulic systems, for example, do not have the correct properties and as such are not suitable for refrigeration systems. Care must be taken to use the correct oil when filling oil into a compressor. In refrigeration systems oil circulates with the refrigerant through the system. As the temperature and pressures vary greatly in the system, the oil must have very special characteristics.

Some of these are:

  • At high temperature and pressures:
    • The oil must maintain its lubrication properties at condensing conditions.
    • The oil must not carbonise onto the valves, pistons, or other components due to the gas delivery temperatures in the compressor.
  • At low temperature and pressures:
    • The oil must remain a fluid so that it can still lubricate.
    • The oil must not contain wax, which can form deposits at system operating temperatures.
  • At all temperatures and pressures:
    • The oil must not react chemically with the refrigerant and any other materials used in the system. It must be compatible with the refrigerant and the system components, gaskets, and seals. Mineral oils were used for HCFC refrigerants but are not compatible with the HFC refrigerants due to their low solubility factors. Poly Ester oils are mostly used for the HFC refrigerants. (However, the oil is hygroscopic and will readily absorb water).
    • Some HC refrigerants may be used with mineral oils.
    • The oil must be very stable so that it lasts a long time.
    • The oil must be very dry (contain the minimum amount of water).
    • The oil must be miscible with the refrigerant.

The selection of the refrigeration oil depends on the operating conditions of the system, the type of refrigerant (HCFC which requires mineral oil and HFC which requires polyester oil) as well as the type of compressor.

Lubricating oils for refrigeration compressors are specifically designed for that application and selection of these lubricants requires special attention and knowledge. Do not mix minerals and polyester oils, as they are not compatible.

The phase-out of CFCs, HCFCs and the latest phase-down of HFCs along with a reduction in global warming gas emission has led to numerous refrigerant/lubricant combinations. Many of these new refrigerants are requiring new lubricants in order for the plant to function correctly.

The essential properties of refrigeration oil are as follows:

  • It must be specially made for refrigeration applications
  • It must have the right viscosity
  • It must have good wear characteristics
  • It must be very dry and not contain wax
  • It must be miscible with the refrigerant
  • It must have chemical, thermal, and hydrolytic stability at both low and high temperatures

Traditionally the lubricant for refrigeration systems has been mineral oil. Mineral oil is relatively inexpensive, has desirable solution characteristics with the traditional refrigerants, is reasonably stable and has been widely available.

Consequences of using the wrong oil:

  • Due to bad mixing properties oil return to the compressor is reduced resulting in insufficient lubrication of the compressor.
  • Reduced heat transfer in the evaporator and the condenser resulting in reduced system efficiency.
  • Excessive foaming of the oil, which can result in liquid slugging.
  • Formation of sludge and coke, causing blockages and corrosion.

Until the phase-out of CFC refrigerants, mineral oils were the primary oils used in refrigeration systems. However, mineral oils are not suitable for use with the HFC / HFO refrigerants. The main reason for this incompatibility is the insufficient lubricity (lubricating ability) and miscibility (mixing ability) of mineral oils. Mineral oils could remain in use with some of the Hydrocarbon refrigerants.

Much work has been done to find acceptable lubricants to use with HFC/HFO refrigerants.

Which oil types are used for refrigeration systems?

Gean, thank you for the question, I will continue on the topic of types of oils in the next issue of the RACA Journal.

References:

  1. SARACCA
  2. ACRA
  3. SANS 10147

Thank you for all your questions. Send your problems (and sometimes your creative solutions) to acra@netactive.co.za with ‘Solutions Page’ in the subject line. You may include pictures.

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