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Home » Refrigerant changes and lubricants – Part 2

Refrigerant changes and lubricants – Part 2

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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. In Part 2 of this topic, we look at oil types and properties.

Recap of question from Part 1: Refrigerant changes and lubricants

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, continuing from the last issue of RACA Journal; until the phaseout of CFC, HCFC 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).

Mineral oils could remain in use with some of the Hydro Carbon refrigerants. Much work has been done to find acceptable lubricants to use with HFC/HFO/HC and natural refrigerants.

No ice formation at bottom: refrigerant is lighter than oil (R717). This can be clearly seen by indication of the refrigerant causing ice formation on top and the oil at the bottom causing no ice formation. Image supplied

Which oil types are used for refrigeration systems?

  • Poly alkylene glycols (PAG)
    These refrigeration oils have a high viscosity index and thermal stability. However, PAG is hygroscopic, which means it can attract water. PAG is soluble with ammonia.
  • Polyol ester compressor oil (POE)
    High-performance Polyol Ester (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.
  • Polyalkyleneglycol (PAG) compressor oils
    Synthetic Polyalkylene Glycol based (PAG) lubricant suitable for specific refrigeration applications.

    Polyalkylene Glycol–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 (auto-motive 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
    Naphtenic oils have a low pour point, which make them more suitable at lower temperatures. Naphtenic oil is derived from crude oil and is processed using a separation process removing all contaminants.

Properties of lubricating oils

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.

Lubrication is the separation of moving parts by a film of oil. The closer the parts are to each other, the tighter the fit, the more emphasis there must be on the role of lubrication. It is safe to say that all high-quality oils do lubricate satisfactorily.

Refrigerant oils are described by the following characteristics:

  • viscosity
  • floc point
  • dielectric strength
  • neutralisation number
  • flash point
  • fire point.
  • 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 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 very small 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 of 0.1 in. between two poles immersed in the oil. Recommended dielectric strength is a minimum of 25 kV.
  • Neutralisation number: 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, 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. These substances are known as organic acids. Mineral acids, if present, are detected by the neutralisation number test. 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 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 two 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 R-13, R-14, ammonia, and carbon dioxide.

As a matter of interest: of the refrigerants listed in the last two groups, only ammonia and carbon dioxide are lighter than oil. They float on top when the oil/refrigerant mixture separates into two phases.

Gean, with regards to selection; as I have said before, 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.

Gean, thank you for the question, I hope that this helps you moving forward.

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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