air conditioning is provided by central plant elsewhere in the building). Coils are typically manufactured from copper for the tubes, with copper or aluminum fins to aid heat transfer. Cooling coils will also employ eliminator plates to remove and drain condensate. The hot water or steam is provided by a central boiler, and the chilled water is provided by a central chiller. Downstream temperature sensors are typically used to monitor and control "off coil" temperatures, in conjunction with an appropriate motorized control valve prior to the coil. If dehumidification is required, then the cooling coil is employed to over-cool so that the dew point is reached and condensation occurs. A heater coil placed after the cooling coil re-heats the air (therefore known as a re-heat coil) to the desired supply temperature. This process has the effect of reducing the relative humidity level of the supply air. In colder climates, where winter temperatures regularly drop below freezing, then frost coils or pre-heat coils are often employed as a first stage of air treatment to ensure that downstream filters or chilled water coils are protected against freezing. The control of the frost coil is such that if a certain off-coil air temperature is not reached then the entire air handler is shut down for protection. Humidifier Humidification is often necessary in colder climates where continuous heating will make the air drier, resulting in uncomfortable air quality and increased static electricity. Various types of humidification may be used: Evaporative: dry air blown over a reservoir will evaporate some of the water. The rate of evaporation can be increased by spraying the water onto baffles in the air stream. Vaporizer: steam or vapor from a boiler is blown directly into the air stream. Spray mist: water is diffused either by a nozzle or other mechanical means into fine droplets and carried by the air. Ultrasonic: A tray of fresh water in the airstream is excited by an ultrasonic device forming a fog or water mist. Wetted medium: A fine fibrous medium in the airstream is kept moist with fresh water from a header pipe with a series of small outlets. As the air passes through the medium it entrains the water in fine droplets. This type of humidifier can quickly clog if the primary air filtration is not maintained in good order. Mixing chamber In order to maintain indoor air quality, air handlers commonly