Open-drive centrifugal chillers are able to use an oil-management system that can make oil loss a relatively minor issue at low-lift conditions. Learn more about oil-management systems, as well as compressor aerodynamics.

Oil-Management System. Low-lift conditions also impact a chiller’s oil-management system. For example, hermetic chillers under low-lift conditions could lose oil at an increasingly faster rate through the motor bearings and refrigerant-gas seals. When this happens, large amounts of oil will enter the refrigerant circuit and migrate to the evaporator.

Normally, excess oil should reside in the oil sump of the compressor. But if oil is in the evaporator instead, the chiller may shut down on a low-oil-pressure safety. In addition, the excess oil in a flooded evaporator migrates to the top layer of tubes. But this is where the best heat transfer (refrigerant boiling) occurs. Therefore, when refrigerant in this area is displaced by oil, heat transfer and chiller efficiency suffer significantly.

Open-drive centrifugal chillers are able to use an oil-management system that can make oil loss a relatively minor issue at low-lift conditions. For example, an oil-eduction system can be employed to separate oil from the refrigerant in the evaporator and return it to the oil sump. With this type of system, more oil stays in the sump, permitting the chiller to operate effectively at low-lift conditions.

Compressor Aerodynamics. The design of a centrifugal chiller’s compressor also is critical to low-lift performance. That is because a centrifugal compressor operates most efficiently when the tip speed of its impeller is optimized for the application.

In a direct-drive compressor where the impeller is directly connected to the motor, the only way to adjust tip speed is by changing the size of the impeller. Because there are relatively few impeller sizes to choose from, compressor tuning is limited and performance suffers accordingly.

By contrast, in a gear-drive compressor, tip speed is a combined function of impeller size and gear ratio. With multiple impeller combinations per compressor size, and multiple gear combinations per impeller size, it is easier to select a gear-drive compressor that will match the low-lift application’s requirements most efficiently.

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