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Duct Leakage Testing and Equation Query 1

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mjkey

Mechanical
Aug 8, 2016
8
Hi all,

I have a question regarding the "pressure" value for the duct leakage equation below (I've copied and pasted this from a paper I was reading):

Qleak = C · As · ∆ p0.65

Qleak is the quantity of leaked air,
C is a constant related to the duct tightness (which relates to the details of manufacture, installation and sealing),
As is the duct surface area,
∆p is the pressure difference between the inside and outside of the duct

They say the delta P value is the pressure difference between the inside and outside of the duct. But the pressure along a duct system varies (rises or drops depending on supply or exhaust). Say at the fan outlet, the pressure is 500Pa and at the index run diffuser outlet, it is say 40Pa. So what delta P value should we be using to determine the maximum Qleak air leakage rate?

Can someone please explain how to use this equation properly, particularly how to determine the delta P value.

Thanks.

 
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I think that equation is for a static pressure test. P would be the static pressure of the test. Otherwise, you are correct that the pressure will change as you travel along the duct. I suppose you could run calculations in discrete steps or come up with a continuous function of how P changes along the duct.
 
>>>>how P changes along the duct.

It's called a pressure plot and can be very helpful in choosing fans and motors.

Also a help for the balancer.
 
The test isn’t done with an active system. They isolate sections of the ductwork, close and cap them, then connect a fan into the sealed duct section and measure how much leaks out via some form of air measuring station at the fan. So it’s a statically pressurized duct section not really subject to losses and changes in pressure since it doesn’t have flowrate or losses in any particular direction.
 
Thanks all for the input.

@GT-EGR: thanks for the information. I think i've got my head around how it works now. Basically each "section" of duct that is to be tested (capped and closed), will be subject to their respective "max" pressures. With the test being a static pressure test to the "max" pressure, it in turn provides some amount of safety factor, as when the system is actually operating it won't be experiencing that "max" pressure.
 
A duct leakage test is usually done at a low pressure - and then they have formulas to then determine how much actual CFM leakage there is at operating pressure.

The leakage test is not intended to test out how the duct holds up relative to its pressure class - that isn’t really tested, more just trusted that the installers followed smacna accordingly.
 
SMACNA - HVAC AIR DUCT LEAKAGE TEST MANUAL can help here I guess.
 
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