Chilled-Water VAV Systems - HVAC.Amickracing

Primary System ComponentsFigure 10. Chilled-water cooling coilairflow inairflow outwater outwater inIn most applications, the cooling coil also dehumidifies as water vapor in theair condenses on the cold fin surfaces of the coil. This water then drainsdown the coil surfaces, drops into the drain pan located beneath the coolingcoil, and is piped away by the condensate drain line.The coil tubes are usually constructed of copper, and the coil fins ofaluminum. For some applications, coils may use copper fins or amanufacturer may cover the coil surfaces with a coating to minimizecorrosion.Selection of the cooling coil impacts the cost of installing, operating, andmaintaining both the VAV air-handling unit and the chilled-water system. Forexample, the amount of material used to construct the coil—overall size,number of tubes, number of fins—determines the initial cost: more materialincreases the cost. But the size of the cooling coil also dictates the weight andfootprint of the air-handling unit: the larger the coil, the larger the AHU mustbe to house it. A larger AHU may require a larger mechanical room (reducingusable or rentable floor space), limit access for service, impact the amount ofstructural support needed, or challenge the arrangement of ductwork andpiping.Because the cooling coil is also part of the air distribution system, itsgeometry—size, number of rows, fin spacing, and fin profile—contributes tothe airside pressure drop and affects the energy used and sound generatedby the fans. A larger AHU will typically result in a lower airside pressure dropthrough its components, which can reduce fan energy (see example inTable 4).Finally, because a chilled-water cooling coil is also part of the chilled-watersystem, its geometry contributes to the waterside pressure drop and affectsthe energy used by the pumps. And, the extent to which coils raise thechilled-water temperature dramatically affects both the installed cost ofchilled-water piping and pumping energy. Coil performance can eveninfluence the efficiency of the water chiller. For further discussion, see“Chilled-Water System,” p. 79.16 Chilled-Water VAV Systems SYS-APM008-EN