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

HVAC Control in the New MillenniumHeat EnergyHeat is a form of energy that is in transit between its source anddestination point. Heat energy only exists while flowing between thesource and destination.Heat energy is similar to work energy, which is another form ofenergy in transit. The main difference is that heat energy is energy intransit as a result of temperature differences.British Thermal UnitThe British thermal unit is a unit of heat or heat energy. Since 1929British thermal units have been defined as one Btu being equal to251.996 IT (International Steam Table) calories, or 778.26 feet-pound ofmechanical energy units (work). Since one IT calorie equals 1/860 of awatt-hour, one Btu is equivalent to about 1/3 watt-hour.Prior to 1929, one Btu was defined as the amount of heat necessaryto raise 1 pound of water 1°F. Because of some difficulty in determiningthe exact value of a Btu, it was redefined in terms of a more fundamentalelectrical unit.Heat and WorkEnergy can be defined as the ability to do work or move against aresistance. Work can be defined as the overcoming of resistance througha certain distance using the expenditure of energy.Work is measured by a standard unit called the foot pound. Thisis defined as the amount of work done in raising 1 pound the distanceof 1 foot, or in overcoming a pressure of 1 pound through a distance of1 foot.A relationship between work and heat exists and is referred to asthe mechanical equivalent of heat. This states that one unit of heat isequal to 778.26 foot-pounds of work.The relationship of the mechanical equivalent of heat was firstestablished by experiments conducted in 1843 by Dr. Joule of Manchester,England. He determined by experiment that when 772 foot-poundsof energy had been expended on one pound of water, the temperatureof the latter had risen one degree.©2001 by The Fairmont Press, Inc. All rights reserved.