The role of the lubricant chemistry in a R-744 refrigeration plant oil - return and lubricity. comparison between two chemistries using data from a real plant.

The lubricant plays an important role in the operation of a refrigeration plant, and understanding the effect of the lubricant is paramount. Since the lubricant is exposed directly to R-744, the solubility has to be balanced between enhancing lubricant flow when in the system so that oil return is sufficient for reliable operation, versus reduction in viscosity in the compressor due to R-744’s properties as an effective solvent. In addition to the lubricating properties, lubricants must also exhibit excellent chemical and thermal stability.
There are two main barriers for the expansion Southward in Europe of R-744 refrigeration systems: the higher operating pressures (related to design robustness), and the maximum discharge temperature allowable by the lubricant to avoid its thermal breakdown. Two chemistries are available to the contractors for R-744 refrigeration plants design and operation: POE (Polyol-Ester) and PAG (Poly-Alkylene-Glycol). In this study, benefits and drawbacks of each chemistry will be explained, as well as practical points identified to help the technicians to select and work with the best operating conditions, including the maximum allowable discharge temperature.
Using commercial grades properties and technical data for both ISO VG68 PAG and ISO VG85 POE, key properties (viscosity and percentage of R-744 dissolved in the oil) have been determined in each of the system components (gas cooler, evaporator, liquid reservoir, crankcase). This will enable the technicians to understand and take better decisions with the knowledge of what will happen once in operation.