A rendering plant flue velocity has to be in excess of 45Ft/sec but is measured at 220 degrees C.
How will I convert the reading?

Will it make a big difference as it seems to be heavy in water vapour?

Being measured with a Pitot Tube.

Thanks
Tom

 

> Will it make a big difference as it seems to be heavy in water vapour?

The Brits published a paper entitled "Problems with Pitots: ISSUES WITH FLOW MEASUREMENT IN STACKS"

http://tinyurl.com/qxr55j8

"The over-all conclusion that can be drawn from these reviews is that there are still significant uncertainties in the understanding of Pitot tube performance under extreme stack conditions.

One of the restrictions stated for the use of Pitot tubes is "density of the stack gas to be approximately the same as that of air."

Although water likely has an impact on the viscosity factor, the conclusion was "it is unclear whether the presence of significant levels of dust and water vapour will significantly affect the operation of Pitot tubes in industrial stack environments" and "there is little evidence of experimental investigations of the effects of dust and water vapour on the Pitot tube calibration."

 

Thanks David,

I was hoping it would be a bit simpler, I assumed it would be a regular event in Industry!

Re the Temperature. As it is measured at 200C which is 433C absolute and NTP is at 21C i.e. 294C absolute. If reduced to NTP would it contract to 294/433 of its volume and hence 45 ft /min measured at 200C is only 45*0.679 I.E. 30.6 ft/min.

Or have I simplified mother nature too much?

Tom

 

Tom,

I didn't pay attention to your numbers the first time around, but when I did the 2nd time, I found your velocity number doesn't make any sense.

I seem to recall from some years back when I did some duct velocity work that the minimum measurable velocity with a pitot was around 800 fpm, yet you're talking 45 fpm, better than an order of magnitude lower.

My memory can be faulty, so I looked up a velocity vs pressure drop chart on the Dwyer Instruments site and it shows a DP of 0.01 inches w.c. indicating a velocity of ~530 fpm at 400 Deg F (200 Deg C = 392F). The charts bottom out at 400 fpm, nothing lower than 400 fpm.

Where'd you get 45 (forty five) feet per minute with a pitot tube? What's the DP at 45 fpm with a pitot tube? It doesn't make sense. Is there now an electronic DP that works down below 0.01" wc? Is the sensing diaphragm a foot in diameter?

A quick look at the same velocity vs DP chart shows that at increased temperature, any given DP will produce a higher velocity.

At 70 Deg F, a 0.01"wc drop is ~ 400 fpm
At 400 Deg F, a 0.01"wc drop is ~ 525 fpm

The temperature correction is not a straight ratio of absolute temperatures (by the way, I get 200 Deg C = 473 Deg K, not 433C). Any of the tutorials on the web show the velocity calc with density being in the divisor.

FYI, the density includes barometric pressure, which if this for compliance purposes, you might consider factoring in a barometric reading.

David

 

The fluid velocity listed is 45 fps (2700 fpm), believe you will easily get a measurement.

Since a velocity measurement is being sought, the guy simply wants the measured pressure from a pitot tube.

 

Re Compensation for non standard Temperatures when measuring Flue velocities etc (want max ejection speed!)

Page 7&8 of this covers it very well. Is straightforward but nice to be able to quote a ref. in a dispute.

https://www.gov.uk/government/publications/m2-monitoring-of-stack-emissions-to-air

Tom Pen