Arkansas HVACR NewsMagazine November 2019

S tate, National, Chapter News Tech News

radiator and the temperature and velocity of air moving across the skin has a direct correlation to convective heat loss. Increasing the rate of air across the skin will increase the rate of evaporation . The least understood principle of body heat rejection is radiation . These properties of physics all play a role in day to day challenges for the HVAC contractor and are a major key to comfort. For the above reasons, terminal device location is critical and often missed completely. Grille performance or placement is viewed as a secondary factor dependent upon duct design when the exact opposite is true. Efficient system operation depends on properly placed terminal devices. Without proper placement we will lose substantial capacity. An often-made mistake is simply attaching a new variable speed air handler or furnace to existing duct systems. While installing a new air handler or furnace might seem to be the right thing to do, without adjustments, 80% of the time we overblow the duct system and transitions threatening biological growth and placing mechanical strain on the outdoor unit. For this discussion we will focus on terminal devices. Our first step for designing duct is application. The quality of and type of

Registers & Grills:

Terminal devices and the hard work they do……

. 08 = Weight of a cubic foot of air at sea level

Every HVAC contractor should remember the number as it is the basis for everything we do. .08 lbs. per cubic foot x 400 cubic feet per minute = 32 pounds per minute 32 pounds per minute x 60 minutes = 1,920 lbs. per hour or just under one ton of air. (Multiply x number of system size / tonnage) Fairly basic math, but three in four contractors pay little attention due to the belief, “ the first guy did it right ”. We must double check calculations for computing air flow, select proper equipment, adjust selected equipment to local conditions and evaluate grille or terminal device placement prior to installation. Why? As humans we need very little air to sustain life. During normal respiration we breath one cubic foot of air a minute. If we are to condition the air for comfort, much more air is necessary, and several properties of science demand our attention. Our skin serves as the body’s