Blocked pipe = increased air velocity?

[radonbc]

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. ?

 

[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?

 

[radonbc]

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 a lot 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.

 

[russ_watters]

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.

 

[JBA]

What type of velocity measurement instrument are you using?