# Blocked pipe = increased air velocity?

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1. Feb 8, 2016

Blocking the intake of an air flow station I built results in the FPM shooting up along with the vacuum. ie: wide open both ends 4" pipe with inline fan reads . 5" wc / -190 pa / 3700 fpm on instruments monitoring intake pipe. Blocking the intake increases both the vacuum to -470 pa and increases the velocity to 5400 FPM.
MY QUESTION: Is their a conversion factor to plug in at the beginning of a CFM equation to compensate for vacuum on the intake side. ie: .0873(4"pipe) x 3700 fpm = 323 cfm This answer seems like it should be smaller using the same equation with blocked intake but it actually increases unless some allowance for the blocked intake is factored. ??????

2. Feb 8, 2016

### JBA

Are you talking about a partially or a completely blocked inlet; and, how and where are you measuring your velocity and where are you measuring your vacuum?

3. Feb 8, 2016

The flow station was between the damper/closure and the fan on the intake side. The measurements increase as the rate of blockage increases from partial to near full. ie: increasing damper angle, then blocking intake. It is alot of work to change things around to measure velocity on the exhaust side so I was wondering if there was a conversion that factored in the higher vacuum readings.

4. Feb 8, 2016

### Staff: Mentor

If I'm understanding your post correctly, you should be using the velocity pressure in the velocity calculation. It's total pressure minus static pressure.

5. Feb 8, 2016

### JBA

What type of velocity measurement instrument are you using?