Many people ask for assistance in the understanding of theoretical and practical aspects of the industry. We are going back to basics as I have questions coming in that indicate that the basic understanding necessary to work in the industry is not in place.

Grant Laidlaw is currently the owner of the Air Conditioning and Refrigeration Academy (ACRA) in Edenvale. He holds a Bachelor of Business Administration and an associate degree in educational administration. He has a National Technical Diploma and completed an apprenticeship with Transnet. He has dual-trades status: refrigeration and electrical. He has been involved with SAIRAC for over two decades and served on the Johannesburg committee as chairman and was also president between 2015 and 2018. Currently he is the SAIRAC national treasurer.

 

Ryan asks: Good day Grant, we have issues when servicing refrigeration systems relating to compressor oil. It is often difficult to get information on specific compressor oil requirements. What is so special about refrigeration/air conditioning oils? Your input would be appreciated. Thanks.

Hi Ryan, it is always best to 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 if available.

What is so special about refrigeration compressor oils you ask. Well, let us look into this in more detail. 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 and performs 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.

Other oils used to lubricate motor vehicle engines, hydraulic systems, etc. do not have the correct properties and as such are not suitable for refrigeration systems. For example, the hydraulic oils used in a forklift will never be exposed to the low temperatures found in a low temp evaporator.

In the past most of the lubricants for refrigeration systems have been mineral oil, which is relatively inexpensive, has desirable solution characteristics with the traditional refrigerants, is reasonably stable and has been widely available. With modern refrigerants we have modern oils available, although mineral oils remain in use.

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

Mineral oils could remain in use with some of the hydrocarbon refrigerants.

Care must be taken when filling oil into a compressor that only the correct oil is used. In refrigeration systems oil circulates with the refrigerant. As the temperature and pressures vary greatly in the system, refrigeration 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 into valve gear, pistons, etc, due to the gas delivery temperature in the compressor

At low temperature and pressures:

  • The oil must remain fluid so that it can still lubricate
  • The oil must be dry and not contain wax that 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 (i.e. it must be compatible with the refrigerant and the system components such as gaskets and seals). Mineral oils were used for HCF’s refrigerants but are not compatible with the new refrigerants due to their low solubility factors. Polyester oils are mostly used for the HFC refrigerants – however, the oil is hydroscopic and will readily absorb water.

The oil must be chemically stable

  • The oil must be very dry – contain the minimum amount of water
  • The oil must be miscible with the refrigerant
  • It must be specially made for refrigeration applications
  • It must have the right viscosity
  • It must have good wear characteristics
  • It must have chemical, thermal and hydrolytic stability at both low and high temperatures

The selection of the refrigeration oil depends on the operating conditions of the system, the type of refrigerant (HCFCs require mineral oil and HFCs require 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.

  • Consequences of using the wrong oil:
  • Due to bad mixing properties, oil returned 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

Let us look at which oil types are typically used for refrigeration systems:

Polyester compressor oil (POE): for refrigeration compressors and systems. Specifically designed for use with ozone-friendly HFC refrigerant fluids; formulated from synthetic Polyol esters to provide outstanding lubricity and wear protection, as well as chemical and thermal stability. Compatible with R23, R134a, R404a, R407c, R410a, R410b, R417a, R422a, R422d, R427a, R507, R507a.

Alkyl Benzene compressor oils (AB): Extreme performance synthetic lubricant for refrigeration compressors with and without ammonia refrigerant. Wax-free for excellent low-temperature fluidity and evaporator efficiency. Outstanding thermal/ oxidative and chemical stability for long oil life, extended drain intervals, less routine maintenance. Compatibility with seals previously used with mineral lubricant for limited risk of oil leakage.

Polyalkylene glycol (PAG) compressor oils: Synthetic PAG lubricant suitable for specific refrigeration applications. PAG–based shear-stable lubricant with outstanding resistance to thermal degradation and the formation of sludge and deposits. Suitable for HC (hydrocarbon) or CO2 refrigerant fluid applications. Compatible with R290, R600, R600a, R134a (automotive compressor applications) and R1234yf.

Polyalphaolefin (PAO): These refrigeration oils have a good chemical and thermal stability. PAO also has excellent viscosity temperature characteristics. The miscibility with refrigerant is low, which is why the major use of PAO in refrigeration is in applications where miscibility is not a concern. PAO can cause seals to shrink.

Mineral oils: Naphthenic oils have a low pour point, which make them more suitable at lower temperatures. Naphthenic oil is derived from crude oil and as processed using a separation process removing all contaminants.

Remember, that you should use only the lubricant recommended by the compressor manufacturer. Oil producers and compressor manufacturers go to great lengths and expense to develop and test new lubricants. There are tests for viscosity, floc point, pour point, flash point, etc. You should understand the terms and why they are important.

 

Viscosity

Viscosity means how thick or thin an oil is at a given temperature, and how readily it flows at that temperature. The oil becomes more ‘fluid’ (it flows more easily) as the temperature increases, and more ‘viscous’ (thick) at low temperatures. A standard of 37.7°C (100°F) has been set as the temperature at which most oil viscosities are measured.

The importance of viscosity lies in the selection of a range that provides proper lubrication under all conditions. It must also allow for the diluting effect of the refrigerant. Refineries can supply viscosity ranges to meet any specification. When in doubt about proper oil viscosity, consult the manufacturer’s recommendations.

 

Floc point

All mineral oils contain paraffin wax – some more than others. Even those that are called ‘wax-free’ contain minute amounts of wax. The floc test determines the measurable temperature at which wax separates from an oil. The small amounts of wax in ‘wax-free’ oils will settle out in a mass (flocculate) only when oil/refrigerant mixtures are at very low temperatures.

In general, the solubility of a material (its ability to be dissolved in a solvent) decreases as the temperature of the solvent decreases. In the same way, as the temperature of an oil/refrigerant mixture is lowered, the solubility of the paraffin wax decreases. If there is more wax present in an oil/refrigerant mixture than can remain in solution at a given temperature, precipitation will take place.

 

Dielectric strength

Dielectric strength is a measure of the resistance of an oil to an electric current. It is expressed in thousands of volts (kilovolts, or kV) of electricity required to jump a gap between two poles immersed in the oil. Recommended dielectric strength is a minimum of 25kV.

Rough Guide for refrigeration oil usage

Rough Guide for refrigeration oil usage. Supplied by Grant Laidlaw

Neutralisation number

Neutrality is a difficult thing to visualise. Chemically, it is something that is neither alkaline nor acidic. The concept of neutrality may take on more meaning if you think of it in terms of drinking water. Household ammonia is alkaline and vinegar is acidic. By contrast, ordinary drinking water is neutral. Nearly all lubricating oils contain materials of varied and uncertain chemical composition, which react with alkali. A low neutralisation number indicates the absence of corrosive mineral acids.

 

Flash and fire point

If you heat an oil in an open container and pass a small flame over the surface of the oil, at some point (temperature) there will be a flash of fire on the surface as the oil vapour burns. The temperature at which this occurs is called the flash point. If you continue to heat the oil further, it will reach a point (temperature) which it can be ignited and will continue to burn. This temperature is called the fire point.

 

Solubility

Oil in a refrigeration system must do more than simply meet the normal requirements of lubrication. It must be able to mix with the refrigerant and travel through the system. Therefore, you must consider the solubility relationships between refrigerants and oils.

These relationships fall into three groups:

Those that mix in all proportions with oil in the range of temperature operation. These include ethane, propane, isobutane and the other hydrocarbons.

Those that separate into phases in the operating range, depending on temperature, pressure, and type base stock (chemical category) of the oil. These include R-21, R-22, R-114, and R-115.

Those that are insoluble in oil in the operating temperature range. These include ammonia, and carbon dioxide.

Of the refrigerants listed in the last two groups, only ammonia and carbon dioxide are lighter than oil. and float on top when the oil/refrigerant mixture separates into two phases.

Ryan, I hope that this helps with your understanding of refrigerant oils. I will continue with lubricants in the next issue. Incorrect oil will almost certainly cost you a compressor.

REFERENCES:
1. ACRA