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

HVAC Control in the New Millenniumapproximately 0.50 and substitution of values in the above equationgives:H = 0.50 × 100(70 - 10) = 3000 Btu/hourNatural VentilationThe natural forces that move air through and out of buildings includewind forces and temperature differences between the inside andthe outside. Air movement can be caused by either of these forces actingalone or in combination, depending on atmospheric conditions, buildingdesign, and location.The use of natural forces for producing ventilation can involve thefollowing conditions:• average wind velocity,• prevailing wind direction,• seasonal and daily variations in wind velocity and direction, and• local wind interference by buildings and other obstructions.Without obstructions to change the wind’s direction, the movements ofthe airstream along with the pressure, remain constant.Air leakage occurs through the building’s cracks or openings. Thecold and heavy air enters at low levels and pushes the warm and lightair out at higher levels. The same draft takes place in a chimney.Temperature-difference ForcesThe draft in a stack or chimney is known as the flue effect. The flueeffect of a stack is produced in a building when the outdoor temperatureis lower than the stack temperature. It is due to the difference in weightof the warm column of air in the stack and the cooler air outside. Theflow due to the flue effect is proportional to the square root of the drafthead, and is given by the following approximate relation:Q=9.4Ah t ± t o©2001 by The Fairmont Press, Inc. All rights reserved.