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# HVAC duct pressure drop vs. ac unit S.P. rating.(2)

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 flyinghigh (Structural) 30 Apr 12 18:31
 A rooftop HVAC unit on a 2-story office building is rated at 2400 cfm at a static pressure of 0.6"wcg. The conditioned space is on the 1st floor.  The first duct segment is 30x12 inches metal and contains at least 3 elbows.  The estimated equivalent length Le is 125 ft.  The pressure drop is calculated to be 0.08"wc/100 ft, or 10"wc total for this first segment.  Since the 10"wc drop is way more than the 0.6"wc output of the ac unit, it does not seem possible this system would work; but it does.  What am I missing?
 Compositepro (Chemical) 30 Apr 12 19:41
 It will operate where the blower curve (flow rate vs. pressure) intersects the system curve. This will be less than 2400 cfm (and therefore also less than 10" wc).
 (2)  urgross (Mechanical) 1 May 12 8:18
 Your arithmetic is poor. The drop would be 1.25 * 0.08 = 0.1 IWC.
 willard3 (Mechanical) 1 May 12 10:17
 Hire an Engineer familiar with ventilation and air systems.
 BronYrAur (Mechanical) 1 May 12 16:33
 Did your question get answered?  urgross is correct that the pressure drop in the duct you described is only 0.1 IWC, not 10.  Remember that your drop of 0.08 IWC is per 100'.  You calculated it per foot.

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