HVAC Control in the New Millennium.pdf - HVAC.Amickracing

Heat, Ventilation and Damper Control Trendsing low pressure in the evaporator. The required pressure on the highside takes place by compression using a diffusion passage.Central SystemsA central air-conditioning system is generally designed for assemblyin the field rather than in the factory as a unit. One condensing unitcan service several rooms at a lower cost than a number of self-containedunits serving single rooms. Central air-conditioning systems provideair ducts to the various rooms along with one or more exhaust fansto remove any undesired proportion of the air.MixingA mixing chamber is usually incorporated in the system for thepurpose of recirculating a portion of the air within the building in orderto reduce the load and operating costs. In the summer the cooled air andin the winter the heated air is mixed with the fresh air drawn in fromoutside, so that the load is reduced considerably.Some fresh air is drawn in and mixed with the recirculated air. Thisis accomplished with a mixing damper. The mixing chamber must beairtight so that little or no air leakage occurs.Heating CoilsIn cold weather regions a heating or tempering coil is used towarm the intake air. This is used when the outside air is at such a lowtemperature that the heat from the recirculated air may not be sufficientto meet the requirements of the load.Tempering heaters must be used wherever cold conditions arefound. In some cases, these conditions may not occur every year or forany length of time. The tempering heater is designed to prevent coldincoming air from causing the water spray to freeze.There are several different types of heaters available. The electricheater requires little space but is expensive to operate unless electricityis available at very low rates.©2001 by The Fairmont Press, Inc. All rights reserved.