The refrigeration industry is going through a period of change thanks to a raft of innovations and technological developments and updates to safety and environmental legislation, according to Rainer Lange, Mobil SHC brand advisor - EAME.
For refrigerants the attention continues to focus on the environmental impact that they can have on the Ozone layer and a user's carbon footprint. As a result, companies are turning towards "natural" refrigerants, such as ammonia, carbon dioxide (CO2) and hydrocarbons, or the newer hydrofluoroolefin (HFO) technologies in an effort to reduce their environmental impact.
Benefits of CO2 as refrigerant
CO2 was widely used as a refrigerant fluid in the early twentieth century; however, its popularity declined with the development of chlorofluorocarbons (CFCs). But the use of CO2 as an industrial refrigerant fluid has witnessed a resurgence. The reason for this comeback is that it has excellent heat transfer properties and a high volumetric cooling capacity, allowing for installations of relatively small cooling units. And with a much lower environmental impact than CFCs, the reason for its increasing popularity is clear. However, there is a number of key lubrication challenges associated with CO2 as a refrigerant fluid that need to be addressed before equipment based on this technology can operate at peak performance.
The lubrication challenge
The two key issues surrounding the lubrication of CO2-based systems are the elevated operating pressures and CO2's high solvency, which takes place in subcritical cascade systems and transcritical high pressure applications. The daily need for evaporator defrosting with hot CO2 places high loads and stresses on bearings and other moving parts of the compressor. Its high solvency and solubility in some polyolester-based (POE) lubricants also needs to be taken into account as it leads to potential excessive viscosity reduction as well as possible CO2 outgassing on bearing surfaces, disrupting lubricant films. The impact of these issues varies from an improper sealing of clearances and a potential loss of compression ratio, to insufficient lubrication leading to excessive bearing wear, potentially resulting in compressor damaged.
Next generation lubricants
ExxonMobil identified the need for a CO2-miscible refrigeration lubricant that delivers step-out performance. It therefore developed Mobil SHCTM Gargoyle 80 POE - a high performance oil designed specifically for refrigeration compressors using carbon dioxide. The product has been formulated using an innovative polyolester technology (POE) to provide outstanding lubricity, wear protection and chemical and thermal stability.
The potential for CO2-based systems is strong and operators can now be confident that there is a lubricant option to help maximise this promise.
