Relationship between Duct Static Pressure and AHU supply fan speed
Relationship between Duct Static Pressure and AHU supply fan speed
(OP)
Hi, I would like to determine the new AHU supply fan speed by assuming the duct static pressure drop from 1.5" to 0.75". What equation I should use for a rough estimation? Thanks.





RE: Relationship between Duct Static Pressure and AHU supply fan speed
Is this a residential application? This is what you need to know. TOTAL static pressure which is the return static pressure PLUS the supply static pressure. With that value you can go to the manufacturers specs to find out the CFM for the given fan speed. Typically the schematic diagram will tell you which wire is low, med, high etc. assuming the basic rull of thumb for 400 cfm per ton. If you have a 2.5 ton system you would want 1000 cfm. If the fan speed with the static pressure gived you 1000 cfm plus or minus 10 percent, your in the ballpark. Some manufacturers may state 350 cfm per ton or whatever. So.... go figure out what fan speed you are set on.... then look it up in the manufacturers specs... and see what your cfm is. If your cfm is to low for the size of the unit, increase fan speed. too high.... decrease fan speed. Then once you adjust, recheck your static pressures and verify correct airflow in cfm.
(typically increasing fan speed increases static pressure and vice verse. Make sure before you start you have a clean coil and clean filters.)
RE: Relationship between Duct Static Pressure and AHU supply fan speed
RE: Relationship between Duct Static Pressure and AHU supply fan speed
" The system curve static pressure varies in proportion to the CFM ratio squared. The fan curves are proportional to the rpm ratio"
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BOTH are proportional to RPM squared,or CFM squared.
RE: Relationship between Duct Static Pressure and AHU supply fan speed
RE: Relationship between Duct Static Pressure and AHU supply fan speed
I think ASHRAE also has them.
RE: Relationship between Duct Static Pressure and AHU supply fan speed
ME's use different units, but the relationships are the same.
RE: Relationship between Duct Static Pressure and AHU supply fan speed
Bubblehead Squared!
P.S. Brian2903 sorry if I lead you astray.... these guys are WAY outta my league. I am still semi noob league.
Good job guys.... maybe I will learn something visiting this site!
RE: Relationship between Duct Static Pressure and AHU supply fan speed
RPM2/RPM1=square root of(p2/p1), although if you could get fan curves you would get more accurate results.