Measuring Velocity in a Pipe

[formsand]

Hello everybody!
First, I want to thank everyone for the help that they give to these forums, I can't remember how often just browsing through threads gives me an idea or sparks a solution.

My question is this: I want to measure the velocity of air flow through a milling system. Specifically, this would be the air flow after the product has been milled and leaves the output of the mill (in most cases we will be milling wheat.) Normally, if it was just air or a similar fluid, this would be a trivial problem with an easy solution. We are aiming for ~4000 fpm velocity.

What I am worried about is how the flour will affect the measuring device. I have thought about using a hotwire anemometer like this:

http://www.amazon.com/Amprobe-TMA-2...1369158953&sr=8-1&keywords=hotwire+anemometer

to gauge the velocity. This would entail installing it into a short section of piping and installing that pipe after the mill. Am I correct in thinking the flour will mess with the measuring device?

I also thought about a venturi meter, but since this is a system where we want maximum product movement, restricting the flow is an issue. Is there a way that we can install a venturi meter in parallel with the piping?

I am just trying to gather some ideas here, any help is appreciated. As usual for most requests, the cheaper the solution the better

Thanks!
Aaron

 

[LittleInch]

You could try an ultrasonic doppler shift meter which has no probe and is supposed to be good for dirty air and fluids as it actually needs something in the fluid to defect sound back off Try

http://www.omega.co.uk

/ prodinfo / flowmeters. They have a good guide. Go for anything which doesn't have an internal probe as this could easily foul. Also I thought flour in air was explosive so best to avoid any non explosion proof sensor...

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[dicksewerrat]

The manual for your Amprobe toll states. Do not use in explosive environment or dust.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[MikeHalloran]

Isn't flour dust dangerous?
I'd be cautious about introducing anything electric or electronic.

This will probably require regular cleaning, but should give useful data:

http://www.dwyer-inst.com/Product/AirVelocity/AirVelocity/Meters/Model460

Mike Halloran
Pembroke Pines, FL, USA

 

[zdas04]

The problem I see with any solution is the variable density. If you could count on the amount of wheat in the air stream then it would be reasonable to use just about any flow measurement device. If you are trying to get an order of magnitude kind of number, then I think you would have better luck with a venturi meter than any of the alternatives. An orifice or v-cone meter will tend to accumulate and separate the solids in the downstream eddy patterns. The venturi meter is a smooth flow profile that shouldn't have that problem. The high-dollar ultra-sonic or corriolas meters would not give you any better measurement for a lot more money.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[formsand]

Thank you guys for all of your input! And thank you a lot dick for saving some time with the anemometer
Flour dust is dangerous, so as a precaution I guess it would be better to not stick anything directly into the particle stream.

zdas, do you think there will be any reduction in flow velocity in the fluid from the venturi meter that would skew the results?

 

[Compositepro]

A Pitot tube should work and you can apply a purge air flow to keep it from plugging. You also have to collect your flour somewhere. Why not measure flow after your bag house (or whatever).

 

[zdas04]

No, the permanent pressure drop in a vernturi meter is very small, so you won't get much change in velocity (a slight increase, but it will be very slight.

I wouldn't use a pitot tube. They have to be calibrated for a specific gas and a specific pressure/temperature. It sounds like you will have considerable variability and most of the time the numbers you would get from a pitot tube would just be numbers (not much relationship to flow).

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[MikeHalloran]

David, he said he's looking for velocity, not mass flow.

Given that, maybe a pitot tube would be an acceptable choice.




Mike Halloran
Pembroke Pines, FL, USA

 

[zdas04]

You cannot measure fluid velocity. The best you can do is infer a volume flow rate and convert that into a bulk velocity. That technique doesn't capture a velocity distribution, but getting one of those is pretty hard. The problem with the pitot tube is a small change in the mix of stuff going past it can be a huge change in the inferred flow rate (and therefore velocity).

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[racookpe1978]

I'd see the problem with a pitot tube being the sensing holes in the tube: both reference and high pressure point would clog at different times and at different rates.

Too bad a flywheel or flapper would not work: The energy of the blowing dust turns the wheel (or forces the flapper panel up) directly proportional to the mass of dust and speed of the dust.

then gain, if the flapper were directly over top of a 90 degree mitered joint with the exit turning down into the next pipe section, the dust would force the flapper "up" and the deflected dust and air would be blown "down" the vertical leg and away from the flapper so it would not jam up the flapper and horizontal inlet piping section ....

 

[formsand]

Thanks for the input guys. Looks like I will be exploring putting in a venturi meter, we'll see how it works out.
Thanks for all of the help!