static pressure calculation

[ferracon]

I need to find the static pressure of my supply end of a HVAC project, so i can size my fan. there are three 11.5"X7.5" grates can anyone give me some insite to equations and frictional aspects.

 

[cbiber]

Look up "minor losses" in the pipe-flow section of your fluids book. dp = K * dynamic head.
K = minor loss coefficient; depends on open area ratio for your grate, at least.
dynamic head = (1/2) rho * V^2
rho = density of fluid (air)
V = average velocity -- use velocity approaching the grate unless the data you find for K specifies otherwise.

Using V = Q/A lets you build a resistance curve for fan selection. Q is volume flow rate and A is the grate overall area. Watch the units, it's easy to get tangled.

Are the three grates in parallel or series?

If series, add the pressure drops. If parallel, assume evenly distributed flow between them, and calc the pressure drop based on 1/3 of the total flow rate.

 

[lilliput1]

If you really want to learn how to do it right you have to read SMACNA Duct Design Manual or ASHRAE Fundamentals. The pressure drop components you have to account for are:
entrance loss; duct friction for straight run; elbows; transition at fan inlet; fan entrance loss; transition at fan discharge; straight run through main of diverging tee; branch of diverging tee; volume damper; discharge loss through discharge grille.
In addition make sure to include in equipment pressure drops if any like, dirty filters; heating coil; cooling coil; room pressurization; flex duct; air terminal box; reheat coil etc.
Usually pressure drops are like what cibber said but in correct units:

pressure drop = coefficient x velocity pressure

in wg PD = coefficient x (fpm/4005)^2

The SMACNA duct design manual tabulates the coefficient for various fittings.
For low velocity ductwork, supply duct is typicall sized at 0.1" wg/100 ft of duct up to maximum 2000 FPM. Low velocity exhaust or return is sized at 0.08" wg/100 ft of duct to 0.1 " wg/100 ft of duct.