Oil Talk – Short #167

In this short podcast episode, Bryan gets into some oil talk, covering some common refrigerant oil terms and types.

Esterification is the process by which organic acid and alcohol come together to form polyolester (POE) oil and water. Hydrolysis refers to the decomposition of a substance when it comes into contact with water; when POE mixes with water, it will break down into esters, organic acids, and alcohol. Once POE oil undergoes hydrolysis, the process can't be reversed to get the same original oil.

POE oil is also hygroscopic; hygroscopicity refers to the ability of the oil to absorb moisture. Miscibility refers to the ability of an oil to mix with refrigerant and be carried with it. In the context of refrigerant oil, “polar” refers to a molecular structure with an uneven distribution of electrons; oils with polar structures attract water molecules. Solubility refers to how well one compound can dissolve into another.

Mineral oil is a product of the distillation of crude oil and was common in systems that used CFC and HCFC refrigerants. Mineral oil isn't as miscible with new refrigerants that lack a chlorine molecule. Alkylbenzene (AB) is a synthetic oil used in some commercial refrigeration systems that is compatible with mineral oil. Polyolester (POE) oil is one of the most common synthetic oils we use in systems that use HFC refrigerants; its main downside is its high hygroscopicity and tendency to undergo hydrolysis. Polyalkylene glycol (PAG) oil is common in automotive A/C systems (R-134A) and is more hygroscopic than POE oil but does not undergo hydrolysis. Polyvinyl ether (PVE) oil is used as an alternative to POE oil; it is more hygroscopic but does not undergo hydrolysis.

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In this short episode, Bryan clears up the confusion behind pumping vs. compression.

Pumping is usually associated with liquid. Liquid, like vapor, is a fluid, but it’s a lot denser and is essentially incompressible because the molecules are close together. Liquid volume varies based on temperature and density, but we can’t squish the molecules together. That’s why compressors with very little axial and radial compliance fail when flooded; they can’t compress the liquid. A circulator pump in a hydronic system is an example of a pump that pushes liquid along, but the pressure difference across the pump is minimal compared to that of a compressor; the same is true of liquid vs. vapor CO2.

However, vapor molecules are far apart and can be squished together to take up a smaller volume. That’s why vapor can go through compressors, which take a fluid of one volume and apply pressure to shrink it to a smaller volume, raising the temperature in the process. Pneumatics, as opposed to hydraulics, utilize compression to move vapor around in a circuit. Recovery machines need to be able to compress vapor refrigerant and pump liquid into the recovery tank.

Compressors and pumps can either be positive displacement or non-positive displacement systems. A positive displacement system creates a trap and forces a fixed amount of fluid through a system, like a scroll compressor or a circulator pump. Refrigerant volumes may vary because vapor density varies with temperature and pressure (the volume flow is the same, but the mass flow varies). A condensing fan motor, on the other hand, is dynamic and varies the volume of air it moves in response to pressure, as the mechanism relies on velocity and kinetic energy to move air.

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