A lot of times people say they don't want a particular refrigerant, but then they look at performance and cost, and they change their minds. The majority of jobs we run into today consider two or even three refrigerants. Because a hermetic motor is cooled by refrigerant in the chiller, it gives up a little efficiency compared to an open-drive motor. The combination of more reliable motors and microprocessor controls on chillers has greatly reduced the number of motor burnouts. An open-drive motor can also slightly increase cooling or ventilation load in the machine room.
Centrifugal chillers can have one, two or three stages of compression.
Single-stage compressors are more compact and simpler in design than multistage units. Single-stage compressors have no interstage efficiency losses, as multistage compressors do. But single-stage compressors require geared drives, which add complexity to the chiller design and reduce efficiency. Multi-stage units can be geared or direct drive. Multistage machines offer better stability over a wider operating range.