Screw compressors are used in middle capacity chillers, mostly starting from 70 ton to 300 ton. These are positive displacement machines. Refrigerant is compressed between two screws. Refrigerant pressure increases while passing through these screws. This may be single screw or twin screw compressor.
Capacity control is through slide valve mechanism. Hot gas bypasses to suction side through this slide valve under part load conditions. Part load performance of screw compressor chiller is characterized by low energy consumption. Noise level reduces at part load operation.
Compressor can also be operated with VFD to obtain step less capacity control. Motor RPM modulate with change in demand. Proportionately energies consumed are reduced at low loads.
Multiple compressors may be used for one chiller. These provide option of multiple circuits. This is important in providing reliable operation. If any component in one circuit fails, second circuit keeps on running. Fifty percent capacity will be available under this scenario. Multiple compressors improve part load performance of the chiller.
Few moving mechanical parts are involved in screw compressor operation. Less wear and tear occurs during chiller operation. Few mechanical breakdowns are experienced with screw compressors. In this compressor two rotors mesh with each other. When main shaft is rotated, it compresses refrigerant.
Part load efficiency of screw compressors is better than the other types within their operating range. These chillers are compact in size. Machine foot print is small. It can be transported from standard door width and height. This is inherently balanced machine. Excessive vibration is not experienced. There are very few chances of vibration transmission with screw type chillers. Due to these advantages chillers can be placed at any location, close to occupied areas or in basements. Issues of noise and vibration to public areas with screw chillers are not as serious as in case of centrifugal type chiller machines.
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