*3.4.4. Chillers*

There are several ways to regulate system cooling capacity. (1) On–off control: This is the simplest way to control the capacity, but can cause frequent short cycling of compressors, which is detrimental to compressor performance. (2) Unloading control: This is often used for reciprocating compressors with multiple cylinders. (3) Slide valve control: This is often used for a screw compressor, which can adjust the compressor capacity with a wide range. (4) Hot gas bypass: This is not an efficient way as a mixture of hot and cold refrigerant is used. (5) Digital compressor: This is developed by Emerson for scroll compressors. The cooling capacity can be varied from 10% to 100%. (6) Variable speed compressor: This provides a smooth system capacity modulation with a wide range and is more energy efficient. Some chiller manufac‐ turers already produced chillers with variable speed scroll compressors or variable speed screw compressors.

For a VFD-equipped compressor, the compressor speed is often modulated to maintain the supply water or supply air temperature set point.
