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pressure measurement in air duct system with U-tube manometer
2

pressure measurement in air duct system with U-tube manometer

pressure measurement in air duct system with U-tube manometer

(OP)
Dear all,

I want to measure the air pressure in an air duct system both on the head and suction side of a big air ventilator. The channel is square 1.5 x 1.5 meter, air velocity is around 10 meter/sec. The motor is 100 kiloWatt.

Now I want to apply a good old U-tube manometer to measure the air pressure. Does anybody have experience with this method? How to avoid pressure readings induced by speed (p = 0.5 rho x V^2) and carburetter-like sucction effects?

What pressure readings I can expect (approx), I mean what pressures are typical for this size of ventilators?

Please let me know,
MVD

RE: pressure measurement in air duct system with U-tube manometer

I hve done this a few times but only needed relitive nos. Suggest that you contact ASHRAE and maybe they could help.

RE: pressure measurement in air duct system with U-tube manometer

Find a Dwyer catalog and buy a static pressure tip.  The A-303 model even has a magnet on it so you don't have to hold it while getting your measurement.  These tips are made to cancel any venturi effects, etc.  I suppose they have a website too.

The catalog also has good technical sections on how to take various velocity and pressure measurements.  As for guessing what you'll get for pressure measurements, not enough info.

RE: pressure measurement in air duct system with U-tube manometer

MVD,

I agree with BRT549.  You only need a static pressure gauge for the application.  At the velocity/area values provided I would only expect pressures in the range of 0-10 inches of water.

Pharo of Thermo

RE: pressure measurement in air duct system with U-tube manometer




static pressure measurements can be made with a connection that is flush with the plenum wall.

There are also special insert fixtures that accomplish the same.

You can also take a pitot reading and subtract out the velocity term.

Manometers are fine but have been replaced by electronic types.

If you are in a hurry a plastic tube filled with water works great. In practice the velocity effects are not that great unless to direct the tube suarely into the flow.





RE: pressure measurement in air duct system with U-tube manometer

(OP)
Dear All,

You are great!

Thanks for all the information. I'm going for the web-link you gave me. Thanks for all the tips.

: )
MVD

RE: pressure measurement in air duct system with U-tube manometer

(OP)
For anybody curious; the air pressures I measured where:

Pressure head: +9 cm water
Pressure suction -3 cm water

Cheers,

RE: pressure measurement in air duct system with U-tube manometer

Those numbers sound reasonable.  What are you going to do with the data?

RE: pressure measurement in air duct system with U-tube manometer

(OP)
Hi,

I'm checking the air mass balance of a sausage drying tower. Mass In = Mass OUT and the air mass flow [kg/sec] is dependend on pressure , temperature and the volume flow [m3/sec].

Here's some equations I used:

http://home.hccnet.nl/m.dijk/pressure_drop_calculator/calcs/calcs.html

(=Physical Properties of Water and Air.)

Greetings from Holland,
MVD



RE: pressure measurement in air duct system with U-tube manometer

Sounds very interesting.  I enjoy doing air balances in the fertilizer plant I work in.  The web link looks useful, and I'm adding it to my "toolbox".

Greetings from Wyoming, USA.

BRT-549

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