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

Heat, Ventilation and Damper Control Trendsthe medium’s temperature. An example of this is solar radiation.Radiation, as a heat loss, involves the throwing out of heat in rays.The heat rays proceed outwards in straight lines and the intensity of theheat radiated from any one source becomes less as the distance from thesource increases.The amount of heat loss through radiation depends upon the temperatureof the surrounding surfaces. The colder these surfaces are, thefaster and greater will be the heat loss from a heated object. If the surfacesare warmer than the heated object, then heat will be radiated fromthese surfaces to the object.Radiation, conduction, and convection are often interrelated. Heatfrom a burning fuel passes to the metal of the heating surface by radiation,through the metal by conduction, and is transferred to water byconvection. The circulation needed for convection is caused by the variationin the weight of the water due to temperature differences. The waternext to the heating surface receives heat and expands becoming lighter.It rises displacing the colder and heavier water above.Mean Radiant TemperatureThe mean radiant temperature of the surfaces of an enclosure isimportant when dealing with heat loss by radiation. The mean radianttemperature (MRT) is the weighted average temperature of the surroundingsurfaces.The significance of the mean radiant temperature becomes apparentwhen compared with the clothed body of an adult at 80°F or 26.7°C.If the MRT is below 80°F, the human body will lose heat by radiation tothe surfaces of the enclosure. If the MRT is higher than 80°F, the oppositeeffect will occur.Sensible HeatSensible heat is a form of heat that causes a change in the temperatureof a substance and can be measured by a dry-bulb thermometer.There is no change in moisture content. It is referred to as sensible heatbecause it can be sensed by instruments or the touch. When the temperatureof water is raised from 32 to 212°F, an increase in the sensibleheat takes place since these are dry-bulb measurements.©2001 by The Fairmont Press, Inc. All rights reserved.