The emergence of HFC refrigerants has added a new set of confusing chemical names and properties with which the HVAC/R technicians must be familiar. One of the more confusing developments in the refrigeration industry in recent years has been the plethora of impossibly difficult to pronounce polys used for refrigerant oil with the new HFC refrigerants. There are several synthetic lubricants with multisyllabic names that begin with poly: polyalkylene glycol, polyol ester, and polyvinylether. Polyol ester is actually “short” for neopentyl polyol ester. To make discussion of these chemicals possible, they are generally referred to by three letter abbreviations: PAG, POE, and PVE. Polyalkylene glycol(PAG) is widely used in car air conditioning systems, but is generally not used in hermetic and semi-hermetic systems because of its lower electrical resistance. Currently, polyol ester, POE, is the primary lubricant in hermetic and semi-hermetic systems using HFC refrigerants. POE has been used for years as a lubricant in jet engines. One advantage POE has in refrigeration is that it is compatible with a wide range of refrigerants including CFCs, HCFCs, and HFCs. Another important characteristic is relatively high electrical resistance, making POE superior to PAG for hermetic and semi-hermetic systems. A major disadvantage is that POE is very hygroscopic; it readily absorbs water. Water is never welcome in a refrigeration system (unless the system is a lithium bromide chiller). However, water in a system with POE is a particularly bad set of circumstances. To understand why this is such a problem you need to understand how POE is made. POE is made by a reaction with acid and alcohol which yields water and POE. Like many chemical reactions, it can be reversed. Add water and heat, and POE reverts to acid and alcohol. The hydrolysis of POE creates goo that clogs metering devices, filters, screens, and oil holes. Filter driers are absolutely essential for all systems with POE because of this added water hazard. Even deep evacuation will not adequately dehydrate a POE system if there is already any water in the POE. Some links to sites that discuss these lubricants are PAG, POE, and PVE.