DUCT CONSTRUCTION

In this section, basic sheet metal ducts, both round and rectangular, are discussed. Emphasis is placed on layout and pattern requirements. Then fiberboard duct construction and its use are discussed.

Round Duct

Straight sections of round duct are usually formed from sheets rolled to a proper radius and assembled with a longitudinal grooved seam Each end of a round section is swaged and assembled with the larger end of the adjoining section butting against the swage. Sections are held together by rivets, by sheet metal screws, or by solder. Where solder is not used, duct tape or liquid rubber (duct sealer) should be used as a covering at all joints. Rectangular ducts are generally constructed by bending corners and by grooving along the longitudinal seam. The duct system should be constructed in a way that avoids abrupt changes in size,

Figure 13-2.-Residential duct system.

direction, or other resistance conditions that can create unnecessary noise and reduce the air volume. The normal noise level of air flowing through a duct depends on the velocity of the air moving through the duct. This can be further reduced by lining or covering the duct with soundabsorbing material. The exterior of ducts that carry conditioned air can be covered with heat insulation materials to prevent heat transfer between ducts and the surrounding air. All materials used for duct lining and coverings must be noncombustible.

Ducts should be constructed for easy maintenance. They should have access plates or doors included to facilitate cleaning and inspection. It is important that the correct size duct (as specified on the prints or drawings) be used for the construction of the duct system. The amount of air to be carried depends on the size of the duct. This determines the pressure loss in the system-the larger the quantity of air moving through a duct of a given cross-sectional area, the greater the friction loss. Similarly, with a given quantity of air to be delivered, the friction loss increases in inverse proportion to the sizes of ducts provided to carry the air. Therefore, the power required at the fan for delivering a given quantity of air increases rapidly as the duct size is decreased. It is important to bear these facts in mind when it is necessary to replace or to change sections of ducts. The same size new duct should be used unless proper design provisions are made for a change in size.

Rectangular Duct

Straight sections of rectangular duct are normally formed by personnel in the Steelworker rating. This is normally accomplished on bending-brake type of equipment. Then the rectangular ductwork is joined together as mentioned earlier.

Straight sections of ducts can usually be laid out without a pattern. However, elbows, transitions, jump fittings, and so forth, require a pattern. While Steelworkers perform the task, you, as a planner, need to be aware of the time required to draw and fabricate the required patterns. Also bear in mind that if this is a one-time job, you can make the pattern of paper or cardboard. When there are large numbers of the same size and dimension fittings to be constructed, you should make the pattern of sheet metal.

[ Back ] [ Home ] [ Up ] [ Next ]

Privacy Statement - Press Release - Copyright Information. - Contact Us - Support Integrated Publishing