The Compass DeRose Guide to Air supply for disaster shelters

This page was written by Steven J. DeRose, and was last updated on 2003-03-22.

This page covers specific calculations for airflow in various size pipes and ducts.

This information was found in several sources, including:

Thomas J. Glover, Pocket Ref. Littleton, Colorado: Sequoia Publishing, Inc., 2002. isbn 1885071337 (an incorrect or obsolete ISBN is printed on my copy).
Cresson H. Kearny and Eugene P. Wigner, Nuclear War Survival Skills: Updated and Expanded 2001 Edition. Oregon Institute of Science & Medicine. 2nd edition, May 1999. ISBN: 094248701X

(specific table of airflow rates and pipe capacities, as well as information on UV, HEPA, and activated charcoal filtering, are coming shortly)

Basic formulas

The air volume through a pipe in cubic feet per minute is

60 * V * A,

where V is the air's velocity along the pipe in feet per second (see below), and A is the cross-sectional area of the pipe in square feet.

The cross-sectional area for a rectangular pipe is the width times the height. The cross-sectional area for a circular pipe is 3.14 times the radius squared (the radius is the distance from center to edge, or 1/2 the diameter).

The velocity of air moving along a pipe, in feet per second, is

sqrt( 25000 * D * P / L )

where D is the pipe diameter in inches; L is the length of the pipe in feet; and P is the pressure loss.

Pressure loss hurts airflow. If it exceeds the original pressure going into the pipe, you won't get any airflow. If you force air through a pipe faster, the pressure loss goes up even faster, so it soon becomes a losing proposition:

Doubling the velocity quadruples the pressure loss
Doubling the pipe diameter cuts the pressure loss in half
Thus, if you want to double the velocity but keep the pressure loss unchanged, you have to quadruple the pipe diameter.

Every bend or irregularity in the pipe makes the pressure loss worse. This is not accounted for here: these tables assume a simple stright pipe.

Because of the serious pressure loss that occurs at sharp bends, use as few and gradual bends as possible. If you need to get a pipe shifted to the side or up or down, don't do it with a 90-degree jog over and a 90-degree jog back; do it with 2 45-degree bends instead, and save about half the pressure loss. Even better, try to plan the pipe's route so you can avoid the bend entirely.

Also be sure that joints don't create narrow points in the pipe -- a coupler should surround the pipe, not wedge within it.

So, for several different velocities and pipe diameters, here are the pressure loss values:

Air velocity in ft/second	Air velocity in ft/minute	Pipe diameter in inches
Air velocity in ft/second	Air velocity in ft/minute	1	2	4	6	10
1	60	0.0004	0.0002	0.0001	0.00007	0.00004
2	120	0.0016	0.0008	0.0004	0.0003	0.00016
5	300	0.01	0.005	0.0025	0.0017	0.001
10	600	0.04	0.02	0.01	0.0067	0.004
15	900	0.09	0.045	0.0225	0.0015	0.009
20	1200	0.16	0.08	0.04	0.027	0.016
25	1500	0.25	0.125	0.0625	0.0417	0.025
30	1800	0.36	0.18	0.09	0.06	0.036

Vacuum systems, such as for dust control in workshops, run at more like 70fps.

So what size pipe do I need?

Assuming you're not planning to expend an enormous amount of power to force high-pressure air along, but just using some kind of low-pressure hand or electric fan, here are the air volumes you can expect to get through various sizes of pipe, in cubic feet per minute.

(coming soon...)

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