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

Heat, Ventilation and Damper Control TrendsThe value of 772 foot-pounds is known as Joule’s equivalent. Morerecent experiments have given higher figures, and the value of 1 Btu =778.26 foot-pounds is used today.PsychrometryPsychrometry is that area of physics that involves the measurementand determination of atmospheric conditions, including the effectsof moisture mixed with the air. In air-conditioning systems the dry airand water vapor composing the atmosphere are understood to be separateentities and each has its own characteristics. This water vapor is notdissolved in the air, but it serves to moisten the air.Air and Water Vapor MixturesWater vapor is the gaseous form of water at a temperature belowthe boiling point of water. It is the most variable part of the atmosphere.At certain temperatures and barometric pressures it is extremely unstablein either gaseous or liquid form. This is shown by the rapid formationand disappearance of clouds and fog.Water vapor makes up about 3% of the total air by volume in hot,humid weather and about 1/3 to 1% of total air by volume in dry, coldweather. Water vapor is steam at very low temperatures and it acts likesteam at these low temperatures.DehumidifyingThe air becomes humidified when moisture is added to it and isdehumidified when moisture is removed. The addition of moisture toair and the removal of moisture from air are important to air conditioning.An excess or deficiency of moisture has a noticeable effect on comfortand depends on the relative humidity.Relative HumidityRelative humidity is defined as the ratio of the quantity of vaporactually present in the air to the greatest amount possible at any given©2001 by The Fairmont Press, Inc. All rights reserved.