Most of the refrigeration systems used for air-conditioning are vapor compression systems. Because of the increase in the energy cost of natural gas in the 1980s, the application of absorption refrigeration systems has dropped sharply. Refrigeration systems used for air-conditioning classified mainly in the following categories: direct expansion systems and heat pumps, centrifugal chillers, screw chillers and absorption systems. A heat pump in the form of a packaged unit is also a heat pump system. The refrigeration capacity of a centrifugal chiller is controlled by modulating the refrigerant flow at the centrifugal compressor. There are mainly two types of capacity controls: varying the opening and angle of the inlet vanes and using an adjustable-frequency AC inverter to vary the rotating speed of the centrifugal compressor. A screw chiller or a helical rotary chiller is a refrigeration machine using a screw compressor to produce chilled water. A factory-fabricated and assembled screw chiller itself is also a screw vapor compression refrigeration system.