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# How much CFM do I need?

#1
03-07-17, 11:44 AM
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How much CFM do I need?

I'm trying to determine how much CFM I need in a 4" duct in order to achieve a capture velocity of 75 FPM 12" away from the source of pollution (a hot plate). The 4" duct will have a 14" conical hood on the end. I've already calculated the CFM required at the hood face, but need to determine what velocity I need inside the duct to pull this amount of volume at the hood face. Here is what I have thus far, using Q = V (10*x*x+A)

Q, Volume of airflow in CFM
V, Centerline Velocity at X distance from hood in FPM
x, Distance from hood in ft
A, Area of hood opening in ft[SUP]2[/SUP]

For V = 75 FPM (recommended flow for exhausting soldering fumes), x = 12", and A = 1.07 (area of 14" round hood)...

Q = 75(10*1*1 + 1.07)
= 830 CFM

I'm not sure I'll be able to pull this amount of volume, but regardless, if I need 830 CFM at the face of the hood, how much CFM do I need in the 4" duct? I imagine it's a function of the hood opening, the duct opening, and the distance between the two? I'm looking for an equation basically, disregarding any losses.

Last edited by mossman; 03-07-17 at 12:21 PM.
#2
03-07-17, 12:21 PM
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The duct in the building is a 4" round duct, so there's no way we'll be able to get this kind of air flow. What is the maximum CFM we can effectively pull through a 4" duct that is about 40 ft long?

#3
03-07-17, 12:38 PM
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depends on the size of your motor and fan. Just put in a 10HP exhaust fan to pull 6000cfm on 8in duct for a fume table.

#4
03-07-17, 12:42 PM
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normal cfm on an HVAC system would be about 35cfm

#5
03-07-17, 12:53 PM
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Let me rephrase, is there any limitation as to how much volume can be pulled through a 4" duct? I realize the more flow the more losses, but is there any limitation aside from imploding the duct? I'm obviously exaggerating, but I need to know if we need to increase the size of our ductwork or if we can simply beef up the size of our fan.

#6
03-08-17, 02:42 PM
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bigger fan will move more air.