HVAC Air-Duct Calculations

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For the sake of convenience, this section outlines a simplified air-duct sizing procedure, which eliminates the usual complicated engineering calculation required when designing a duct system. Refer to Figs. 3-23 to 3-25, which show the plans of a typical family residence having a total cubic content of approximately 19,000 ft3. It is desired to provide humidification, ventilation, filtration, and air movement to all rooms on both the first and second floors.

The air conditioner located as shown in Fig. 3-23 had a total airhandling capacity of 1000 ft3/min. If the rooms are to have individual air mains, Table 3-3 shows the methods of computing the amount of air to be supplied to each room.

air duct calculation HVAC Air Duct Calculations

air duct layout 1 HVAC Air Duct Calculations

Second column in the table expresses the capacity of the individual room as a percentage of the total volume. For example, 3000 ft3 is 30 percent of the total space (10,000 ft3) to which air mains will lead. Third column indicates the cubic feet of air per minute to be supplied to the individual rooms. These figures are attained as follows. The air conditioner will handle 1000 ft3 of air per minute; 30 percent of this is 300 ft3/min. Similarly, 10 percent is 100 ft3/min, which would indicate the quantity of air to be supplied to the living room and chamber No. 3, respectively. Having thus established the air quantity to be delivered to each of the rooms, the design of the ducts can now be considered.

air duct layout 2 HVAC Air Duct Calculations

For duct sizes, consider the branch duct to both the living room and chamber No. 1 (see Table 3-3). Note that the branch leading into chamber No. 1 handles 150 ft3/min. The duct to the living room handles 300 ft3/min. Obviously, the connecting air main will handle 300 + 150, or 450, ft3/min, in accordance with the foregoing recommendation, allowing a velocity of 600 ft/min for the branches and 700 ft/min for the supply air main. Therefore, the necessary duct areas can be calculated using the following formula:

cal 1 HVAC Air Duct Calculations

cal 2 HVAC Air Duct Calculations

The remaining ductwork may be calculated similarly. It is recommended that the main air supply leaving the unit be the same size as that of the outlet of the unit up to the first branch takeoff. The return main to the unit should run the same size as the inlet of the unit (that is for a distance of approximately 24 in. It should be provided with a large access door in the bottom of this length of the full size duct. Figure 3-26 is helpful as a further simplification in sizing the air ducts.

air duct sizing HVAC Air Duct Calculations

Example It is desired to size a main duct for 250 ft3/min at 500 ft/min velocity. What cross-sectional area is required?
solution Locate 250 ft3/min on the left-hand side of Fig. 3-26. With a ruler or straightedge, carry a line across horizontally to the 500 velocity line and read off on the base line 72 in.2, or 1/2 ft2, which is the area required. All branches, risers, or grilles may be sized in the same manner.

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