HVAC Air-Duct Systems


Air ducts for transmission of air in a forced-air heating, ventilation, or air-conditioner system must be carefully designed from the standpoint of economy, as well as for proper functioning. When designing air ducts, the following methods may be used:

? Compute the total amount of air to be handled per minute by the fan, as well as the fractional volumes composing the total, which are to be supplied to or withdrawn from different parts of the building.
? Locate the supply unit in the most convenient place and as close as possible to the center of distribution.
? Divide the building into zones, and proportion the air volumes per minute in accordance with the requirements of the different zones.
? Locate the air inlets or outlets for supply and recirculation, respectively. At the positions so located on the building plans, indicate the air volumes to be dealt with. The position of the outlets and inlets should be such as to produce a thorough diffusion of the conditioned air throughout the space supplied.
? Determine the size of each outlet or inlet based on passing the required amount of air per minute at a suitable velocity.
? Calculate the areas, and select suitable dimensions for all branch and main ducts. Do this based on creating equal frictional losses per foot of length. This involves reducing the velocities in smaller ducts.
? Ascertain the resistance of the ducts that sets up the greatest friction. In most cases, this will be the longest run, although not so invariably. This will be the resistance offered by the duct system as a whole to the flow of the required amount of air.
? Revise the dimensions and areas of the shorter runs so that the ducts themselves will create resistances equal to the longest run. This will cut down the cost of the sheet metal, and the result will be just the same as if dampers were used. Too high a velocity, however, must be avoided.
? To compensate for unforeseen contingencies, volume dampers should be provided for each branch.

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