Weird Duct Velocity/Pressure readings

[mjpetrag]

My apologies in advance, as this might be trivial, but I can't seem to figure this out.

I have 5 14' long vertical round ducts (~11-14" dia) venting into a common header. 2 of the 5 ducts are closed with blast gates and capped upstream. I first took a pitot tube downstream of the blast gates into each of the ducts to get a velocity pressure reading.

My high pressure line was going into line connected to the parallel and opposite direction of the air flow. My low pressure line was going into the line perpendicular to the flow. The pressure readings were taken from a magnehelic gage from 0-1" wg. So my pitot tube set up is correct.

However when I went to take readings on the closed ducts (downstream of the blast gate), I still got a velocity pressure reading. So I took a calibrated anemometer to it and still got a reading (~2500 FPM on a 12" round duct). Please note again that this line is also capped upstream.


I also took the pitot tube and disconnected the static line to get a stagnation pressure. Then I disconnected the stagnation line to get a static pressure. My stagnation was lower than my static. The pitot tube inlet was facing the parallel and opposite direction of flow. Any help will be appreciated.

-Mike

 

[ChrisConley]

Why were you measuring air velocity in a duct that is capped on both ends?

Is the system short on air?

 

[hoodguy]

I have some quick thoughts.

Is your pitot tube is plugged? This happens when measuring dirty airflows such as industrial ventilation systems.

Are there other branches feeding into the system that you can't see? Are you sure you fully understand the system?

I don't know what you mean by stagnation pressure. Disconnecting the tube perpendicular to the duct will give you static pressure (SP). Disconnecting the tube parallel to the duct will give you total pressure (VP + SP), and disconnecting neither will give you velocity pressure (VP).
VP will always be a positive number.Static pressure can be + or -. Total pressure can be + or -.

I don't know if this helps or not.

Peter Ott

http://www.benham.com

 

[ChasBean1]

You know there's no net flow coming through the two that you mention so I'm not sure why you'd be compelled to take readings on them(?)

Here's what I think: you have flow through the other three, correct? Is it substantial?

Anyway, air whizzes by the dead ducts from the outlets of the other three because they vent into a common header. You can get a whole bunch of air current in the idle ducts. Air going past the opening will create a vacuum at one part of the hole via venturi effect, drawing air out of the dead duct which will have to be replaced by the same airstream going past the latter (more downstream) part of the opening.

This will create a whole lot of circulation, although a net flow of zero in the two capped ducts. This could also explain the reversal that seems to be indicated in your last paragraph.

CB