fan static pressure

Presumably, you need to find and use the fan operating curve, and match that against the resistance imposed by your plenum and ducting

TTFN

FAQ731-376

Yep...exactly. Anyone got any resources for calculating pressure from duct work & plenum?

Find a mechanical engineer who is familiar with turbomachinery and air systems. If you try to fake it here, you won't like the consequences.

You should probably have a fire engineer looking at this too. They would establish the actual removal criteria with respect to the considered fire scenarios and establish the amount of air you need to exhaust.

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.

Not trying to fake anything here. We've got a duct run of 5' with no turns, nothing fancy. While I don't do this often I feel like with a good resource I can figure it out.

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.

You typically need a data source, like the ASHRAE handbook or SMACNA that will provide inlet and exit losses, any reducing or diverging transition loss.

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.

In most jurisdictions, smoke control systems are prescribed in local codes, which follow the NFPA fire codes. First, you should ask the fire marshall which NFPA code applies, and obtain a copy. I believe it is NFPA 92. You may also search "smoke control" in your local code.

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.

I find your dilemma (the specific question) a little perplexing.

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.