Cooling large buildings typically requires the use of air- or water-cooled chillers that produce chilled water, which then cools the air. About 39% of buildings over 100,000 square feet use chilled-water systems employing various refrigeration compressor designs.
Selecting the right chiller and compressor requires a specifying engineer to determine the building’s cooling load and the proper chiller capacity¹. Calculations are also done to determine the return on investment between different systems by comparing the energy cost per ton of refrigeration along with the operational costs.
When buying a new chiller, specifying engineers and facility owners naturally focus on efficiency ratings to estimate the chiller’s annual energy costs. In addition to the chiller’s performance ratings, however, there is another variable worth considering: How will the chiller and the chiller’s compressors maintain their rated performance in real-world operating conditions years after the purchase has been made? A significant factor that affects chiller performance over time is the oil used by the chiller’s compressor.
This article discusses the impact of oil on chiller compressors and examines the performance of variable-speed centrifugal semi-hermetic compressors with oil-free magnetic bearings over a 10-year period in two real-world applications.
Consequences of Oil Fouling
Oil is used to form a seal that prevents refrigerant from returning to the suction port as well as lubricate the compressor bearings, gears and shaft seals. While necessary for operation, the oil over time becomes entrained in the refrigerant and circulates throughout the system. Eventually, the oil coats the heat exchanger tubes, which creates a thermal barrier that degrades efficiency — a problem known as “oil fouling.”
A number of independent third-party studies detail the consequences of oil fouling, which include:
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