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

Heat, Ventilation and Damper Control TrendsHeat LeakageThe thermal properties of a building affect the design of the airconditioning and heating systems. The rate of heat loss through walls,floors, and ceilings is important when calculating the heat and/or coolingload required for a building or area. Air leakage will take placethrough various apertures, such as cracks around windows and doors,through walls, ducts and chimneys.Most of the components of the cooling load vary greatly during a24-hour period and economic considerations should be the determiningfactor in the selection of equipment for heating and cooling season operations.Heat leakage is usually given in Btu per hour per degree Fahrenheittemperature difference per square foot of exposed surface (Btu/square foot/hour°F).Estimates of the maximum probable heat loss for each room orspace should include:• losses through confining walls, floors, ceilings, glass, or other surfacesand• infiltration losses due to leaks through cracks and crevices arounddoors and windows.The heat leakage through walls, floors, and ceilings depends on thetype and thickness of the insulating material used. The general formulafor heat leakage is:H = KA(t 1 - t 2 )where H = heat requiredK = heat-transfer coefficient, Btu/square foot/hour/°FA = area, square feett 1 - t 2 = temperature gradient through wall, °FFor example, suppose you wish to calculate the heat leakagethrough an 8-inch concrete wall having an area of 100 square feet if theinside temperature is 70°F and the outside temperature is 10°F.The heat-transfer coefficient of a plain 8-inch concrete brick wall is©2001 by The Fairmont Press, Inc. All rights reserved.