The third and final type of compressor, a centrifugal compressor, operates on a different compressor philosophy that relies on dynamic compression to compress and raise the refrigerant pressure. Screw compressors also are generally louder than scroll compressors, with higher noise levels in the lower frequencies octave bands. Screw chillers typically have very good full-load and part-load kW/ton efficiencies. It is important to note that screw chillers have only one compressor, so a loss in the compressor would cause a complete loss in chiller capacity. Screw compressors have the ability to vary the cooling output capacity from 100% to 20% via the use of a slide vane to limit refrigerant delivery to the compressor and provide a smooth, modulating transition between capacities. Screw compressors are available in sizes up to about 500 tons. Once the capacity exceeds the size of multiple scroll compressors, typically four to six 30-ton scroll compressors, chillers use screw compressors. Failure of one compressor will cause a loss of capacity but will still allow the chiller to remain in service and provide partial cooling output. For example, an 80-ton chiller may have four 20-ton compressors, with two compressors on each refrigerant circuit. Although the multiple compressors may be a disadvantage for capacity control, generally they are piped with multiple refrigerant circuits which provide some system redundancy. The disadvantage is that chiller capacity control is provided as stepped control instead of modulating control. As the capacity increases, the chillers increase the quantity of scroll compressors, typically of equal sizes to provide the total chiller capacity required. Scroll chillers start as low as 20 tons and increase in size to approximately 200 tons. Most small chilled water plants-up to approximately 200 tons in capacity-use scroll compressors for production of chilled water. These devices trap refrigerant vapor and compress it by gradually reducing the volume to increase the pressure. Positive displacement compressors operate with two mechanical devices, such as scroll- or screw-shaped rotors.

The first category of chillers is defined by the method used to compress the refrigerant.

Basic chiller training