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

Heat, Ventilation and Damper Control TrendsAdjustment for air-mass modification may be needed near largebodies of water where air moves from the water over the land in thesummer. On the West Coast, both dry- and wet-bulb temperatures increasewith distance from the ocean. In the region north of the Gulf ofMexico, dry-bulb temperatures increase for the first 200 or 300 miles,with a slight decrease in wet-bulb temperature due to mixing and drierinland air. Beyond the 200- to 300-mile belt, both dry- and wet-bulbvalues tend to decrease at a somewhat regular rate.The difference between large areas of dry surfaces and large areasof dense foliage upwind from the site can account for variations of upto 2°F wet-bulb and 5°F dry-bulb temperature. The warmer temperaturesare associated with dry surfaces.Weather-oriented Design FactorsThe general approach to air-conditioning system design involvesthe computation of a peak design load at a condition established usingone of the frequency of occurrence levels of dry- and wet-bulb weatherdata.During the four warmest summer months, these values occur between2:00 p.m. and 4:00 p.m. The winter statistics occur during the 3coldest months between 6:00 a.m. and 8:00 a.m. The daily dry-bulbvariation will be of the order of the daily range.Statistically, the daily range is the long-term average daily rangefor the warmest month. The daily range is generally greater during clearweather and much less during cloudy or rainy weather.An approximation of the distribution of the hourly dry-bulb temperaturescan be made by using the daily maximum at 3:00 p.m. and theminimum at 7:00 a.m.In the continental or inland areas of the United States, the maximumdry-bulb and maximum wet-bulb temperatures are not coincident.In maritime areas they tend to be coincident.The assumption of dry- and wet-bulb coincidence can result inweather-oriented loads of up to 1/2 greater than might be expected. Onesolution is to determine if the structure is most sensitive to dry-bulb(extensive exterior) or wet-bulb, (outside air ventilation).From several years of local weather records, select days with themaximum wet- and dry-bulb entries. Tabulate the dry-bulb peak with©2001 by The Fairmont Press, Inc. All rights reserved.