Trouble finding Gas flowmeter

[Dryno]

We have an application to measure inert gas flow in a 36" duct at just under 300 SCFH. There is flammable trace dust, which also melts at about 210F. So we're concerned about using a 'hotwire-type' sensor because the dust will melt onto the heated RTD causing measurement errors. We contacted Rosemount about their annubars, but got a response saying the flow was too low; they need roughly 450 SCFH. We cannot increase flow nor reduce the duct size. Also, the R.H. is somewhere about 90%. I've heard that heated sensors don't do well with saturated streams.

Any suggestions are appreciated!

 

[JLSeagull]

Check to see of the Thermo Brandt averaging pitot assembly is suitable.

 

[controlnovice]

A few ideas:

- Thermal Mass meter: Not sure if this is what you meant about 'hotwire-type'. It seems odd that a thermal mass meter would have a temperature that high. But it can be affected by coatings.

http://www.us.endress.com/eh/sc/ame.../contentview/0E87BA0E0D8F10EDC12568CC0037B5FD

- Can you reduce the duct diameter just in the space you need to measure? This will open the possible selection of meters.

- Target meter: Problem is accuracy and rangeability. Unfortunately, I can't find manufacturers now.

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This is normally the space where people post something insightful.

 

[JLSeagull]

Most thermal mass flow meters are available for use with hydrocarbon vapors. They may use RTD or thermister elements. Select the flow element option for the hazardous area classification that applies to the flammable dust.

 

[dcasto]

if you only could afford a size reduction for a venturi.

 

[dcasto]

http://www.solartronisa.com/downloads/isa106.pdf

Heres a nice list I found of solutions.

 

[JLSeagull]

As Mobrey products are now owned by Emerson, perhaps your Micro Motion or Rosemount peddeler may sell the Solartron products these days.

 

[Ashereng]

For low flow rates, I am thinking:
- thermal mass meters (which you already ruled out)
- annubar/pitot tube (which Rosemount already ruled out)
- turbines (which may be ruled out since you are 36" duct)
- clamp-on ultrasonic (need to determine maximum diamters)

Instead of reducing the duct, can you split the 36" duct into 4 10" ducts and then re-join? This way, you may be able to try using the turbines and ultrasonics.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[Scotsinst]

Although it would be too expensive, you would need at least 13 10" ducts to achieve the same area as a 36" line. Any solution depends on the acceptable pressure loss of the application.

I can't think of any reasonable solution. The flow is so low compared to the duct size that it is a challenge.

 

[danw2]

Dryno,

I'm curious to know how you establlished the current flow rate of 300 SFCH.

Dan

 

[Dryno]

Many thanks for all of the input and suggestions. As it turns out, there are two mistakes in the process data. We (instrumentation) were given the wrong gas density. The mass flow rate was given in PPH, so the incorrect density ( by more than an order of magnitude) resulted in grossly incorrect flow. I compounded the problem by using units of SCFH when I should've used SCFM. The corrected flow rate is ~17500 SCFM. Yes, I know; that's a huge difference.

This opens up many possibilities, including mass flowmeters, if their heated wire/RTD is below 210 deg. F. I have contacted FCI, Rosemount, and Kurz; they are working on it.

Thanks again for your suggestions and (danw2) for questioning the data!

 

[Ashereng]

How about vortex meter?

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[dcasto]

clamp on ultrasonic

 

[danw2]

Are you sure your new flow rate isn't really 17500 SCFH? Dan