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

Heat, Ventilation and Damper Control TrendsDry-bulb and Wet-bulb TemperaturesDry-bulb temperature is the temperature that an ordinary thermometerwill measure. Wet-bulb temperature is the temperature atwhich the air becomes saturated when moisture is added without anychange in the heat. If the bulb of an ordinary thermometer is surroundedwith a moistened wick, placed in a current of air, and superheated withwater vapor, the measurement obtained will be at some point below thedry-bulb temperature. This minimum reading is the wet-bulb temperatureof the air.Wet-bulb DepressionOutdoor air in the summer is rarely fully saturated, there is usuallya respectable difference between the dry- and wet-bulb temperatures.This difference between the two temperatures is known as the wet-bulbdepression. If the dry-bulb temperature reading is 85 and the correspondingwet-bulb temperature is 65, the wet-bulb depression is 20.Temperature-humidity IndexThis is an index of the human discomfort resulting from temperatureand moisture. It is found by adding the dry- and wet-bulb temperaturereadings, multiplying the sum by 0.4, and adding 15. About 10% ofthe population are uncomfortable before the index passes 70, more than50% are uncomfortable after it passes 75, and almost all are uncomfortableat 80 or greater.Psychrometric ChartsThe different methods of treating air can be shown on a psychrometricchart. The chart is a graphical representation of the fundamentalmathematical relationship dealing with the thermodynamic propertiesof moist air entering and leaving a heating or cooling unit.Three basic formulas are used to determine the following quantities:©2001 by The Fairmont Press, Inc. All rights reserved.