Building Design and Construction Handbook - Merritt - Ventech!

HEATING, VENTILATION, AND AIR CONDITIONING 13.33
Air filters come in a number of standard sizes and thicknesses. The filter area
should be such that the air velocity across the filters does not exceed 350 ft/min
for low-velocity filters or 550 ft/min for high-velocity filters. Thus, the minimum
filter area in square feet to be provided equals the airflow, ft 3 /min, divided by the
maximum air velocity across the filters, ft/min.
Most air filters are of either the throwaway or cleanable type. Both these types
will fit a standard filter rack.
Electrostatic filters are usually employed in industrial installations, where a
higher percentage of dust removal must be obtained. Check with manufacturers’
ratings for particle-size removal, capacity, and static-pressure loss; also check electric
service required. These units generally are used in combination with regular
throwaway or cleanable air filters, which take out the large particles, while the
charged electrostatic plates remove the smaller ones. See also Art. 13.6.
13.6 DUCT DESIGN
After air discharge grilles and the air handler, which consists of a heat exchanger
and blower, have been located, it is advisable to make a single-line drawing showing
the duct layout and the air quantities each branch and line must be able to carry.
Of the methods of duct design in use, the equal-friction method is the most
practical. It is considered good practice not to exceed a pressure loss of 0.15 in of
water per 100 ft of ductwork by friction. Higher friction will result in large power
consumption for air circulation. It is also considered good practice to stay below a
starting velocity in main ducts of 900 ft/min in residences; 1300 ft/min in schools,
theaters, and public buildings; and 1800 ft/min in industrial buildings. Velocity in
branch ducts should be about two-thirds of these and in branch risers about onehalf.
Too high a velocity will result in noisy and panting ductwork. Too low a velocity
will require uneconomical, bulky ducts.
TABLE 13.6 Diameters of Circular Ducts in Inches Equivalent to Rectangular Ducts
Side 4 8 12 18 24 30 36 42 48 60 72 84
3 3.8 5.2 6.2
4 4.4 6.1 7.3
5 4.9 6.9 8.3
6 5.4 7.6 9.2
7 5.7 8.2 9.9
12 10.7 13.1
18 12.9 16.0 19.7
24 14.6 18.3 22.6 26.2
30 16.1 20.2 25.2 29.3 32.8
36 17.4 21.9 27.4 32.0 35.8 39.4
42 18.5 23.4 29.4 34.4 38.6 42.4 45.9
48 19.6 24.8 31.2 36.6 41.2 45.2 48.9 52.6
60 21.4 27.3 34.5 40.4 45.8 50.4 54.6 58.5 65.7
72 23.1 29.5 37.2 43.8 49.7 54.9 59.6 63.9 71.7 78.8
84 39.9 46.9 53.2 58.9 64.1 68.8 77.2 84.8 91.9
96 49.5 56.3 62.4 68.2 73.2 82.6 90.5 97.9