When you move air through a duct, it rubs on the side of the duct and through friction builds up resistance to the flow. The rougher the duct, the more friction and resistance.
This resistance to the flow of the air is called static pressure. Not only does friction in the duct produce static pressure, but every shape transaction, elbow, branch, also adds static pressure, so if you add them all up in the system, you have total back pressure.
Designing a system to minimize these pressure losses in the system is a good first step, but after that is done a fan must be selected that will move the CFM that you require at the total static pressure of the system. All fans are not created equal.
Fans with a simple propeller on them will move the most air for the least horsepower, but will produce almost no static pressure. When you see their CFM rating it is usually given in free air.
Centrifugal fans require more horsepower to move the same amount of free air, but they will operate at elevated static pressures. It varies by design, but for instance a 4” Vortec fan produces about 2” of Water column static pressure and a 6” about 2.28”.
A cubic inch of water weighs .0361 pounds per cubic inch, so 2.28” WC equates to about .082 pounds per square inch. Not very much until you put it over a large area, and then for instance a 6’8” X 30” door has 196.8 pounds of pressure pressing against it.
The amount of air a specific fan design will put out is directly proportional to its RPM. A fan that puts out 100 cfm at 1000 rpm would put out 120 cfm at 1200 rpm.
The additional horsepower necessary to achieve the 20% increase however is equal to the cube of the increase, or 72.8% increase in horsepower and amperage draw.
In addition increases in static pressure draw additional horsepower at the rate of the square of the increase. A 20% increase in static pressure would therefore draw an extra 44% horsepower.
What that means of course is that the penalties for not designing a clean low backpressure system are severe, so it behooves us to pay close attention.
The larger the duct for a given flow, the lower the static pressure and the smoother the duct walls the lower the static pressure.
The straighter the duct and least amount of fittings and transitions, the lower the static pressure.
Anytime there is turbulence in a duct or the air speeds up or slows down, it raises static pressure.
That means that to keep the air moving uniformly, when you pull branches off the main duct, you must reduce the main duct size where it continues, so that its cross sectional area plus the cross sectional area of the branch equal the cross sectional area of the original main duct or is as close as one can come to it using standard ducting sizes.
Most ventilation designers use programs or ventilation slide rules, but the way I calculate the sizes on a simple calculator is as follows:
Duct diameter in inches squared X .7854 equals the duct area in square inches.
For a transition duct size, duct Diameter 1 plus duct Diameter 2 = the area of the main duct.
Divide the area of the main duct by .7854 and then compute the square root for the diameter of the main duct.
For instance using six inch ducts, 6” X 6” X .7854= 28.27 square inches of cross sectional area.
2 X 28.27 square inches = area of main duct or 56.55 square inches.
56.55 divided by .7854= 72
The square root of 72 = 8.49 “, or the desired main trunk size
The design of fittings has a significant impact on static pressure. The following two attachments show proper fitting design: