fan static pressure
fan static pressure
(OP)
I've got an "out of the box" situation. We're building a building where there will be fire tests conducted. The fire marshall has requested that an engineer spec the exhaust hood. That will be me. I talked to him because there's really no code requirement for what we're doing. We came to a consensus that we would like to put in a hood that will clear the room of smoke in 60sec. Owners have a fan they'd like to use. It's rated at ___cfm at ___in pressure. How do I calculate the existing pressure? Don't do these calcs often [ever until now]. Thanks.





RE: fan static pressure
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
RE: fan static pressure
RE: fan static pressure
RE: fan static pressure
The fan would need to comply with the various regs associated with fire exhaust fans as well. Highly unlikely that a random fan will fit the bill.
RE: fan static pressure
I've consulted w/ local building department and fire marshall. Together w/ owner, exhaust criteria was determined. That's what we did first after builder was told to have engineer address exhaust.
RE: fan static pressure
The basic method is to calc a velocity pressure from your flowrate and duct area, and then multiply by the tabulated coefficients for the fitting, obtained from the above reference.
Straight duct from a ductulator, or chart in same reference.
Grilles, louvres etc from manufacturers data.
RE: fan static pressure
Fans are rated by external static pressure, which is the static pressure at the inlet and discharge of the fan. Discharge static pressure is controlled by the friction and dynamic losses in the discharge ductwork. For an exhaust fan, that will be the losses from the fan to the outdoors. Inlet static will be negative, ususally, since the fan will necessarily lower the pressure in the room being discharged. Codes require that smoke control systems have openings in the space being exhausted to allow make-up of most or all of the exhaust. These will often be motorized or automatic louvers located in outside walls around the perimeter of the space, near the floor.
Procedures for calculating duct losses can be found in any issue of the ASHRAE Handbook, Fundamentals.
RE: fan static pressure
So correct me if I'm wrong but you should have the following:
To "clear the room" in a minute you would estimate about 5 air changes within that minute. This would get you to about the 95% smoke clear point.
Then the cfm of the fan needed would be the room volume times 5 ACM. So if the room is 20' x 20' with a 10' ceiling (4,000 ft3) you'd want a 20,000 cfm fan.
Assuming you'd done all that correctly, now you would want a make-up pathway on the opposite end of the building that can provide enough open area, without too much restriction, for 20,000 cfm flow.
So what is the question? You mean what negative pressure would you achieve in the room or something about short fan inlet duct? Because if you size the inlet duct (I'd suggest keeping it below about 2,000 feet per minute) and discharge and make-up louvers right, you could spec. a 20,000 cfm fan with pretty low (e.g., 0.5"-0.75") static pressure.