Duct Static Regain at a Velocity Decrease
The static pressure a duct RECOVERS when the air slows at a size increase -- the basis of the static-regain design method. Velocity pressure VP = (V/4005)^2 in w.c. (the 4005 is the standard-air constant), and when a larger downstream duct drops the velocity, part of the lost VP converts back to static: static regain = R x (VP_up - VP_down), recovery factor R ~ 0.75 (0.5-0.9 by fitting). Dropping 2,000 to 1,500 fpm (VP 0.249 to 0.140) at R = 0.75 regains ~0.082 in w.c. -- pressure the fan need not supply. Sizing each section so its regain offsets its friction holds static constant along the trunk. If the velocity INCREASES the result is negative -- a static LOSS (flagged). The 4005 assumes standard air (altitude/temperature shift it); the fitting sets R, and SMACNA/ASHRAE and the engineer govern.
Formula and source
vp = (V/4005)^2 in w.c. (standard air); static_regain = recovery_factor x (vp_upstream - vp_downstream). Recovery factor ~ 0.75 (0.5-0.9 by fitting). A negative result is a static loss (velocity increased).
Duct static-regain method and the standard-air velocity-pressure constant (SMACNA HVAC Duct Design / ASHRAE Fundamentals), by name; the fitting quality sets the recovery factor and the engineer of record governs the design.
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