Air-conditioning is a process that simultaneously conditions air; distributes it combined with the outdoor air to the conditioned space. An air system is also called an air handling system or the air side of an air-conditioning or heating, ventilating, air-conditioning, and refrigerating system. Its function is to condition the air, distribute it, and control the indoor environment according to requirements. A water system consists of pumps, piping work, and accessories. The water system is sometimes called the water side of a central or space-conditioning system. The refrigeration system in the central plant of a central system is usually in the form of a chiller package with an outdoor condensing unit. The refrigeration system is also called the refrigeration side of a central system. Control systems usually consist of sensors, a microprocessor-based direct digital controller, a control device, control elements, personal computer and communication network.