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

Heat, Ventilation and Damper Control TrendsA warm-air heating system is one in which the air is heated in afurnace and circulated through the rest of the structure either by gravityor motor-driven centrifugal fans. A system in which air circulation dependsprimarily on mechanical means for its motive force is called aforced warm-air heating system. Some gravity warm-air systems usefans to supplement the gravity flow.Forced warm-air heating systems are often classified according tothe type of duct arrangement used. The two basic types of duct arrangementused are:• perimeter duct systems and• extended plenum duct systems.A perimeter duct system has the supply outlets located around theouter edge of the structure. Perimeter duct systems include perimeterloopduct systems and radial-type perimeter duct systems. In the perimeter-loopsystems, the ducts around the perimeter are connected in aloop while in the radial system there is no loop with only the radialfeeder ducts from the furnace unit.An external plenum system has a large rectangular duct whichextends out from the furnace plenum in a straight line down the centerof the building. Smaller supply ducts connect the plenum to the heatemitting units.Year-around Air ConditioningYear-around air conditioning systems warm the air during thewinter and may also wash, filter, and humidify it. During the summer,the air-conditioning system will cool the air and perform humidificationor dehumidification as required. A year-round air-conditioning systemcan eliminate the dust, soot, and germs from the air entering the intakeducts. It may increase the humidity in winter and decrease the relativehumidity or moisture content of the air in summer. Circulation of theconditioned air also takes place. Figure 1-2 shows two arrangements ofequipment for year-around air-conditioning.Several types of service techniques can be used including gas-compression,absorption and steam-jet vacuum. Gas-compression is mostcommon and consists of a refrigeration system employing a mechanical©2001 by The Fairmont Press, Inc. All rights reserved.