Measuring Air velocity in a AC Grill

[adnanhb]

Hello everybody,

For commissioning an AC system, which our company recently finished installing, we need to measure the air flow and air velocity in the diffusers and the grills. We bought a flow hood from shortridge to measure the air flow. We also bought a pitot tube to measure the air velocity. Per the user manual from shortridge, the instrument is capable to measuring air velocity using the pitot tube. But our customer is adamant about using a anemometer to measure the air velocity. Now, I am not 100% sure that a pitot tube indeed can be used to measure the air velocity in a grill. From what I read on the internet, pitot tubes can measure velocity in a duct.

Can somebody please confirm whether pitot tubes can indeed be used to measure air velocity in the grills (granted the instrument, which in our case is shortridge ADM-850L,supports measuring the air velocity). A technical explanation would be very helpful since we can communicate the same to our customer.

Thanks a bunch.

 

[Compositepro]

There are many types of anemometers (device to measure wind speed). Pitot tube is one type. Pitot Tubes measure velocity at a small point, so for duct velocity measurements readings are taken at, say, nine points across the duct and averaged. Vane type anemometers read larger areas so they intrinsically average the velocity (sort of). High precision measurements are normally done with Pitot tubes, but it does take some skill to use them correctly.

 

[MintJulep]

Pitot tubes need a uniform flow parallel to the axis of the tube to measure accurately.

This probably doesn't exist at a grill.

 

[Tmoose]

I think the ASHRAE standards cover that in a very practical way

 

[Drazen]

pitot tubes are not for grilles, they are almost all the time used to verify ahu units capacity.

 

[DRWeig]

Use your flow hood to measure flow volume. If applied properly, it's quite accurate. It passes the captured air through an averaging-traverse array of sensors, not just a single pitot tube.

Divide the flow (cubic feet per minute) by the free area of the diffuser you just measured (in square feet). The answer is average velocity at the plane of the diffuser.

If customer demands an anemometer, you'll have to make yourself a sampling grid for each diffuser and come up with an average. Your flow hood and the flow/area calculation is more reliable in my opinion.



Good on ya,

Goober Dave

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[Shophound]

Just curious why the customer is so adamant about knowing the velocity at the grills. Is he/she concerned about noise?

I'd go with what DRWeig said above, and if the grills are new (and you didn't toss the cut sheets) perhaps you can get the aK of each grill and then derive the FPM of each grill.