MIL-HDBK-1003/3
Section 6:
AIR DISTRIBUTION
6.1
Duct Design for HVAC Systems
6.1.1
Sizing General. See Figure 2 for duct sizing. ASHRAE
Handbook, Fundamentals recognizes three methods of sizing
ductwork: the equal friction method, the static regain method,
and the T-method. For design of small simple systems, the equal
friction method will suffice. Use the static regain method for
VAV design (refer to Appendix C).
6.1.2
Equal Friction Method Sizing. Select a constant
pressure loss in inches of water per 100 foot length of duct from
the preferred part of Figure 2. The preferred part of Figure 2
is between 0.08 and 0.6 inches of water per 100 feet friction
loss for air quantities up to 18,000 cfm, and between 1800 fpm
and 4000 fpm for air quantities greater than 18,000 cfm. Use low
velocities and a low friction drop for small projects, or where
ductwork is cheap and energy is expensive. For systems of 18,000
cubic feet per minute and over, use a friction loss of 0.08 and
velocities of 1800 to 3000 feet per minute. After sizing the
entire system at the selected unit pressure drop, go back and
adjust velocities and pressure drops in the shorter branches to
equalize the pressure drops at each duct branch junction. The
designer must observe the recommended permissible room sound
pressure levels for various applications discussed in NFGS-15895,
Ductwork and Ductwork Accessories.
6.1.3
Ductwork General
6.1.3.1
Round Ducts. Use round ducts wherever possible. Under
normal applications, the minimum duct size shall be 4 inches in
diameter. Use smooth curved elbows as much as possible. If
these are not available, use three-piece elbows for velocities
below 1600 feet per minute and five-piece elbows for velocities
above 1600 feet per minute. The throat radius shall not be less
than 0.75 times the duct diameter.
6.1.3.2
Rectangular Ducts. Use a minimum duct size of 6 inches
by 6 inches. Where possible, keep one dimension constant in
transitions and do not make transitions in elbows. Make
transitions in sides and bottom of the duct keeping top level to
maintain maximum clearance above ceiling. The transition slope
shall be 30 degrees on the downstream. Where ductwork is
connected to equipment fittings such as coils, furnaces, or
filters, the transition shall be as smooth as possible. Drawings
shall indicate ductwork pitch, low spots, and means of disposing
of the condensate. Elbows shall be smooth, with an inside radius
of 1.0 times the width of the duct. Where space constraints
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