Ducts and Duct Sizing

Ducts are passageways used for conveying air from a furnace or cooling unit to the rooms and spaces within a structure. The duct­work is also used to return the air to its source for recirculation.

The air is distributed to the rooms or spaces by means of grilles, registers, and diffusers. The size and location of the supply-air out­lets and the return-air inlets are determined by such design factors as: (1) use (heating and cooling), (2) air velocity, (3) throw, (4) drop, and (5) desired distribution pattern. These and other aspects of air outlet and air inlet design and selection are considered more thor­oughly in Chapter 7 of Volume 2, “Ducts and Duct Systems.”

Designing and sizing a duct system is a compromise between the requirements and limitations of both the structure and the heating and cooling system. Duct sizing will be affected by a number of variables, including (1) the architectural design of the structure,

(2) space limitations, (3) the required air supply, (4) the allowable duct air velocities, (5) the desired noise level, and (6) the capacity of the blower. Adequately sized ducts represent the best possible com­promise among these variables.

Accuracy in estimating the resistance to the flow of air through the duct system is important in the selection of blowers for applica­tion to such systems. Resistance should be kept as low as possible in the interest of economy. However, underestimating the resistance will result in failure of the blower to deliver the required volume of air. The various calculation methods used for sizing ducts are given in Chapter 7 of Volume 2, “Ducts and Duct Systems.”

A number of precautions should be taken in the design of a duct system. For example, careful study should be made of the building drawings with consideration given to the construction of duct loca­tions and clearances. Other recommendations that should be consid­ered when designing a duct system can be summarized as follows:

1. Keep all duct runs as short as possible, bearing in mind that the air flow should be conducted as directly as possible between its source and delivery points, with the fewest possi­ble changes in direction.

2. Select locations of duct outlets so as to ensure proper air distribution.

3. Provide ducts with cross-sectional areas that will permit air to flow at suitable velocities.

4. Design for moderate velocities in all ventilating work to avoid waste of power and reduce noise.

Posted in Audel HVAC Fundamentals Volume 1 Heating Systems, Furnaces, and Boilers