Choosing The Right Lubricant For VLT Refrigeration
Choosing the right lubricant in low temperature refrigeration is as critical as choosing the right fuel for your car’s engine. Choosing the wrong fuel might work well for the short term, but could also have dire consequences in the long term. The same holds true for refrigeration lubricants. Before the Montreal Protocol lubricant selection was simple, you had two classes of lubricants. Refrigeration grade mineral oils and alkylbenzenes worked extremely well with the CFCs and HCFCs that were used. The two groups could be mixed and substituted and in most cases you would not notice any change. The only application which required differentiation was in systems operating at temperature below -60° F (-51°C). In these systems Alkyl Benzene was the preferred lubricant.
After the Montreal Protocol several new classes of refrigerants such as HFCs, FCs and interim blends of HFCs and HCFCs were developed. Mineral oils and alkylbenzenes were ineffective with the new refrigerants. Synthetic lubricants such as Polyol esters (POE) and Polyalkylene glycol’s (PAG) were found to work effectively. The primary difference with these new lubricants is that they are miscible with the new refrigerants.
Miscibility and solubility between the refrigerant and lubricant determine how the two will behave throughout the system. Miscibility is defined as the ability of two liquids to mix together to form a single liquid phase. Solubility is the ability of a gas to dissolve into a liquid.
Miscibility is a function of temperature and concentration. Over the operating range of a system, a lubricant may exhibit one of three different forms of miscibility. Completely miscible is where the refrigerant and oil mix to form a single phase over the applied temperature range. Partially miscible is where the lubricant and liquid refrigerant form a single phase over a limited temperature range. The maximum/minimum temperature at which the lubricant and the refrigerant separate into two phases is called the critical solution temperature (CST). Immiscible is when the oil and refrigerant do not mix and stay in distinct two phases.
Miscibility Examples
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Complete |
Freon R-12(CFC) / BVA 3 (MO) Suva 134a (HFC) and RPOE LT 32 (POE) |
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Partial |
Freon R-22 (HCFC) / BVA 4 (MO) Suva R-134A (HFC) / RPOE 68 (POE). |
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Immiscible |
Suva 134A (HFC) / BVA 3 (MO)
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Miscibility plays a critical role in VLT applications and using the wrong lubricant can create many problems over the life of the system. In the past the only lubricant used for VLT applications was alkyl benzene. Alkyl benzene is a fairly specific composition and the various refrigeration grades commercially available exhibited the approximately the same miscibility characteristics with low temperature refrigerants. The same cannot be said about POEs.
Every manufacturer of POEs uses different building blocks and additive packages in the formulation of their product. Miscibility characteristics of the same ISO grade with low temperature HFC refrigerants can range from immiscible with one manufacturer to completely miscible with the other.
The problem with immiscible and insoluble lubricants seems to be more pronounced in flooded liquid evaporators then in direct expansion or dry evaporators. In flooded evaporators, the turbulence may be enough to move the oil back to the crankcase. At the same time this turbulence may result in the immiscible lubricant being brought into contact and freezing on the surface of the evaporator tubing.The formation of an immiscible lubricant film greatly reduces heat transfer by creating a thin coating on the evaporator wall.
In DX evaporators an immiscible fluid will separate into two layers if the product does not efficiently flow out of the evaporator at the same rate which it enters, heat transfer will be affected as well as oil return. . In addition to oil logging reducing heat transfer, it may also starve the compressor of lubricant. Using a lubricant such as RPOE LT 32 with Suva 95 (R408B) in VLT applications is a excellent way to eliminate potential low temperature problems.
