according to ASHRAE Handbook, HVAC Systems and Applications. The
static regain method should be used for sizing supply ducts in a
VAV system (refer to Section 6 and Appendix C for additional
information). Minimum rectangular duct size is 6 inches by 6
inches and minimum round duct size is 4 inches diameter. Round
duct is preferred because of reduced noise, pressure loss, and
leakage. In general, try to size low velocity ducts in a range
of .05 to .08 inch static pressure drop per 100 linear feet of
ductwork. For large duct systems, the designer should iterate
the design by doing optimization to ensure lowest life cycle
cost. Additional information on duct design is given in Section
6. For industrial ventilation duct design, refer to
MIL-HDBK-1003/17, Industrial Ventilation Systems.
b) HVAC Duct Construction. Duct construction shall
follow Sheet Metal and Air Conditioning Contractors' National
Association (SMACNA) standards. On drawings, note the SMACNA
pressure, seal, and leak classifications required. In
specifications, note the duct tests required. See Figure 10 for
18.104.22.168 Restriction on Use of Ductwork. Do not use underground
ductwork because of health risks associated with
soil-incorporated termiticides such as chlordane and with soils
containing radon gas. In addition, the following ductwork
construction is prohibited:
Sub-slab or intra-slab HVAC system ducts.
b) Plenum type sub-floor HVAC systems, as defined in
the Federal Housing Administration (FHA) minimum acceptable
construction criteria guidance.
c) HVAC ducts in contact with the ground within an
enclosed crawl space.
d) Other HVAC systems where any part of the ducting is
in contact with the ground.
Industrial Ventilation and Exhaust Systems. For design
of industrial ventilation and exhaust systems, use the following
as appropriate: American Conference of Governmental Industrial
Hygienists (ACGIH) Handbook, Industrial Ventilation - Manual of
Recommended Practice; and MIL-HDBK-1003/17, Industrial
Ventilation Systems. If the system conveys vapors, gases, or
smoke; use the equal friction or static regain method for design.
If the system transports particulates, then velocities shall be
sufficient to transport the particles.