In any air-conditioning installation involving a duct system, invariably there is an accession of heat by the moving air in the ducts between the coils and supply grilles when air is supplied below room temperature. If the ducts are located through much of their length in the conditioned space, then, of course, this heat absorption has no effect on the total load and frequently may be disregarded.
More frequently, however, the supply ducts must pass through spaces that are not air- conditioned. Under these conditions, the heat absorbed by the air in the ducts can be regarded as an additional load on the cooling equipment. The temperature rise in a duct system of a cooling installation depends on the following factors:
? Temperature of the space through which the duct passes
? Air velocity through the duct
? Type and thickness of insulation, if any
The first factor establishes the temperature differential between the air on either side of the duct walls. The dew point of the air surrounding the duct may also have some effect on the heat pickup, as condensation on the duct surface gives up the heat of vaporization to the air passing through the system. The highest air velocities consistent with the acoustic requirements of the installation should be used, not only for economy in the sheet metal material used, but also to reduce the heat pickup in the ducts.
The amount of heat absorbed by a unit area of sheet-metal duct conveying chilled air is almost directly proportional to the temperature difference between the atmosphere surrounding the duct and the chilled air, irrespective of the velocity of the latter. The heat pickup rate will be influenced somewhat by the outside finish of the duct and by the air motion, if any, in the space through which the duct passes.
Heat leakage in Btu per hour per square foot per degree difference in temperature for uncovered galvanized iron ductwork will be between 0.5 and 1.0, with an average value of 0.73. The rate of leakage, of course, will be greatest at the start of the duct run and will gradually diminish as the air temperature rises. Covering the duct with the equivalent of 0.5-in. rigid insulation board and sealing cracks with tape will reduce the average rate of heat pickup per square foot of surface per hour to 0.23 Btu per degree difference.
To summarize, the designer of a central unit system should observe the following:
? Locate the equipment as close to the conditioned space as possible.
? Use duct velocities as high as practical, considering the acoustic level of the space and operating characteristics of the fans.
? Insulate all supply ducts with covering equivalent to at least 0.5 in. of rigid insulation, and seal cracks with tape.
