Greenheck Fan 240XP-CUb manual Determining Static Pressure Ps, Ductwork

Determining Static Pressure (Ps)

The pressures generated by fans in ductwork are very small. Yet, accurately estimating the static pressure is critical to proper fan selection.

Fan static pressure is measured in inches of water gauge. One pound per square inch is equivalent to 27.7 in. of water gauge. Static pressures in fan systems are typically less than 2 in. of water gauge, or 0.072 Psi. The drawing to the right illustrates how static pressures are measured in ductwork with a manometer.

A pressure differential between the duct and the atmosphere will cause the water level in the manometer legs to rest at different levels. This difference is the static pressure measured in inches of water gauge.

In the case of the exhaust fan at right, the air is being drawn upward through the ductwork because the fan is producing a low pressure region at the top of the duct. This is the same principle that enables beverages to be sipped through a straw.

Exhaust Fan

The amount of static pressure that the fan must overcome depends on the air velocity in the ductwork, the number of duct turns (and other resistive elements), and the duct length. For properly designed systems with sufficient make-up air, the guide lines in the table below can be used for estimating static pressure:

STATIC PRESSURE GUIDELINES

Non-Ducted	0.05 in. to 0.20 in.
Ducted	0.2 in. to 0.40 in. per
	100 feet of duct (assuming duct
	air velocity falls within 1000-1800
	feet per minute)
Fittings	0.08 in. per fitting
	(elbow, register, grill, damper, etc.)
Kitchen Hood Exhaust	0.625 in. to 1.50 in.

Important: Static pressure requirements are significantly affected by the amount of make-up air supplied to an area. Insufficient make-up air will increase static pressure and reduce the amount of air that will be exhausted. Remember, for each cubic foot of air exhausted, one cubic foot of air must be supplied.

Ductwork

To calculate the system losses, one must know the ductwork system configuration (see Ductwork figure).

This duct is sized for air velocities of 1400 feet per minute. Referring to the static pressure chart, that will result in about 0.3 in. per 100 feet. Since we have 10 feet of total ductwork, our pressure drop due to the duct is:

.3 in. x 10 ft. = .03 in.

100 ft.

There is also a 0.08 in. pressure drop for each resistive element or fitting. For this example, there are 5 fittings:

one grill, two duct turns, one damper and louvers in the wall of the office. The total pressure drop for fittings is:

5 x 0.08 in. = 0.4 in.

Therefore, the total pressure drop is:

0.03 in. + 0.40 in. = 0.43 in.

For convenience in using selection charts, round this value up to the nearest 1/8 in., which would be 0.50 Ps.

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