Relationship between Duct Static Pressure and AHU supply fan speed

[Brian2903]

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.

 

[Bubbleheadski]

You need to give more information.
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.)

 

[lilliput1]

You have to solve this graphically to find the intersection of the fan curve (varies for each speed) with the system curve. The fan & system curves are plotted with the vertical scale the static pressure in inches wg and the horizontal scale in CFM. The system curve static pressure varies in proportion to the CFM ratio squared. The fan curves are proportional to the rpm ratio. Note however that in your case you have to clarify why the duct static pressure dropped from 1.5" to 0.75". Is this at the same CFM & the drop is attributed to less drop from the air path, then you are talking about 2 different system curves. You have to use the correct one and plot it against the fan curve at various rpm till you come up with the intersection of the (2) curves at the design CFM.

 

[zekeman]

Lillput quote:
" 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.

 

[quark]

CFM is proportional to RPM and static pressure is proportional to RPM squared, so static pressure is proportional to CFM squared. Both are one and same.

 

[willard3]

Called the fan affinity laws or fan laws and I get them from Buffalo Forge "Fan Engineering", chapt 12.

I think ASHRAE also has them.

 

[willard3]

I was trying to think where a civil eng would find this information....pump affinity laws are the same as fan affinity laws.

ME's use different units, but the relationships are the same.

 

[Bubbleheadski]

Wow.... ok I will shut up now....
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!

 

[zekeman]

Translating all the above, a first estimate would be
RPM2/RPM1=square root of(p2/p1), although if you could get fan curves you would get more accurate results.