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

HVAC Control in the New MillenniumPreheatersA preheater may be used to add heat to the airstream when thetemperature of the incoming air is so low that the tempering heaters onwhich the heating load falls are unable to maintain the required 68 or70°F at the fan.The preheater is used only where extreme temperature conditionsare encountered. In many systems, the tempering heaters are made withenough capacity to carry the heating load.Cooling CoilsThe cooling coils may be chilled by means of refrigerants or apump-forced liquid, such as brine or water. If water of the proper coolnessis obtainable in sufficient quantity, as is the case in many northerncities, it may be employed, since it requires only a pumping operationto lift it out of the ground. Cold water can also be obtained with mechanicalrefrigeration equipment.Air-supply FansSupply fans and blowers are available in various types for specificpurposes. The selection of the proper type is important if the operationis to be economical and efficient. In general, fans can be divided into twotypes: propeller and centrifugal or turbine.Propeller fans are used in short ducts or when there are no ducts.The low speed of the blades near the hub, compared with the peripheralspeed, makes this form of fan unsuitable to overcome the resistanceencountered in long ducts. The air driven by the effective blade areasnear the rim can pass back through the less effective blade areas at thehub than to move against the duct resistance. Propeller fans also tend tocreate more noise.Many air-conditioning installations require long air ducts to carrythe conditioned air to the various areas of the building. Centrifugal andturbine-type fans are employed in these installations.©2001 by The Fairmont Press, Inc. All rights reserved.