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Calculation of CFM discharge from an opening of a duct collar

Dear sir,

I would like to know while designing of duct how will i calculate the cfm Airdischarge in units like CFM through a collar opening?
What is the formula for giving the collar size for air discharge.
Lets for Example I have a AHu of Capacity 1200 CFM
I have to distribute the total cfm equally through duct on three seperate room . then what will be the duct collar opening for each room so that each room will get 400 cfm each?


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  • weathermaker Jun 20, 2009

    I want the formula for calculating the size for air distribution from duct collar.



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There's a thumb rule in A/C air distribution. At 1 TR load, the air supply is about 400 CFM and the ideal diffuser size is 16" x 16" with an effective flow area of about 70%. Branch duct size for this outlet should be around 12" x 12".

Posted on Jun 20, 2009

  • Boyet Soriano
    Boyet Soriano Jun 22, 2009

    I assumed that you have an engineering background. Designing a simple centralized A/C system is a complicated job. Its not simply deviding the the total air volume flow to the number of rooms you want to supply air. You need to know the exact A/C load for each room, the contaminants present, the amount of fresh air to supply to ventilate the rooms, the orientation of the rooms, and you need to have a stong knowledge of air psychrometry. Anyway if you really insist in knowing the equation for air flow in a circular duct, the equation is Q = f x Ai x V, where Q is in CFM, Ai (inside cross sectional area of duct, V is the velocity of air in FPM, and f is the flow factor. In ducts the ideal V is 1700 FPM w/ f = 0.75. For air diffusers V is 400 FPM w/ f = 0.85 to .90. Let me remind you theat the air flow from the outlet of the fan to the farthest air outlet will experience a pressure drop . Your calculations should consider this factor. You may assume an equal pressure drop to each outlet. Goodluck to your design!


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Dear sir please forward to me air conditioning grill cfm formula,
how is measure this calculation .
Best Regards

Posted on Nov 10, 2011


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