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

Heat, Ventilation and Damper Control TrendsLatent Heat of FusionThe change of a substance from a solid to liquid or from a liquidto a solid involves the latent heat of fusion. One pound of water at atemperature of 32°F requires the extraction of 144 Btu to cause it tofreeze into solid ice at 32°F. All solid substances have a latent-heat valueand the amount required to convert it, or affect a change of state, iscalled the latent heat of fusion. This heat cannot be measured with athermometer since the heat is absorbed or expanded in the change ofstate process.Latent Heat of EvaporationThe change of a substance from a liquid to a vapor or from a vaporto a liquid involves the latent heat of evaporation. The conversion of 1pound of pure water at 212°F to steam at 212°F requires 970 Btu at thepressure of the atmosphere found at sea level. After the 970 Btus havebeen used, the further addition of heat serves only to heat the steam.SuperheatSuperheat is the sensible heat that is absorbed by a vapor or gasnot in contact with its liquid. It does not follow the temperature-pressurerelationship and refers to the sensible heat absorbed by the vaporraising the temperature of the vapor or gas without any appreciablechange in pressure.A gas is usually considered as a vapor in a highly superheatedstate or as a vapor not near its condensing point. Water in the air thatis close to the condensing point is water vapor. Since as superheat issensible heat, its effect can be measured with a thermometer and as atemperature rise. A 10°F superheat means a vapor has absorbed enoughheat to raise the vapor temperature 10°F above the temperature of vaporizingliquid.When a gas is condensed to a liquid, the change of physical stateis always accompanied by the giving up of heat. This takes place in thecondensing unit of an air conditioner due to the mechanical work exertedon the gas by the compressor.©2001 by The Fairmont Press, Inc. All rights reserved.