Heat flow in a Duct

In any air-conditioning with participation of the pipeline system, and remains, the introduction of heat due to air movement in ducts between the coils and supply grids, when the air is lower than the room temperature. If the ducts are located throughout most of its length in the conditioned space, then of course, this absorption of heat does not affect the overall load and can often be left without attention.

More often, however, power supply, ducting must pass through the premises which do not have air conditioning. In these conditions, the heat absorbed by the air in the duct system can be considered as an additional load on the refrigeration equipment. The increase in temperature in the duct system of cooling, for installation depends on the following factors:

The temperature of the space air duct, through which passes
Air velocity through the air duct
Thickness and type of insulation, if any

The first factor is determined by the temperature difference between the air on both sides of the wall of the chimney. The dew point of the surrounding air duct can also have some influence on the heat pickup, as condensation on the surface of the duct transfers heat of vaporization of the air flowing through the system. At high air velocities in accordance with the acoustic requirements the plant can be used not only for the economy of sheet metal, the material used and to reduce the heat pickup in the ducts.

Amount of heat absorbed by a unit area of a sheet-metal pipeline transportation of chilled air is almost directly proportional to the temperature difference in the atmosphere around the pipelines and cooled air, regardless of the speed of the latter. Heat pickup rate will depend in part and exterior of the air duct and air traffic, if any, in the space through which the duct passes.

Heat leakage in Btu / hour for one square meter per degree temperature difference for uncovered galvanized steel air duct will be between 0.5 and 1.0, the average value of 0.73. The rate of leakage, of course, will be greatest at the beginning of the duct and will gradually decrease, as the air temperature rises. Floor duct with the equivalent of 0.5. rigid sheets and sealing cracks tape will reduce the average level of heat pickup per square meter of surface per hour 0,23 BTU / degree difference.

So, the designer of the Central block of the system, shall comply with the following:

Locate equipment as close as possible to the conditioned space as possible.
Use duct high speeds, and as a practical, given the level of acoustic space and operational characteristics of the fans.
Insulate all supply air ducts with a salary of not less than 0.5. strict isolation, and seal cracks tape.