Several customers want to do VFD on big-ass dust collector fans. I need a
measuring device that can be inserted into a duct intake or test port and
These ducts are at least 60" round and the systems have airflow at the fan
of 150,000 cfm. We need to verify what slowing those fans down does to the
CFM on all the ductwork branches.
Is there an electronic instrument out there today capable of doing that? I
can do it other ways, like using duct pressure and trying to extrapolate the
data, but I'd really like to verify it with a REAL CFM reading.
Ummm... I think that 'BAC' on the side stands for 'Baltimore
Air Coil', not 'Big Ass Collector', Jake :-)
Regardless of where the transducer is ( tip integrated or
remote ). BTW - placement is VERY critical. I've been on jobs where
the flow was ACTUALLY 10,000 CFM or whatever, but the system read
'zero' or even 'negative', due to incorrect sensor placement.
You need to be at least 6 duct diameters downstream from any
turns or transitions ( so, 30 feet in your case ) - more is better -
and if you can place multiple sensors and average them, so much the
better. Turbulance, and variations in the laminar flow can throw any
given spot 'out'.
You could go with something like http://www.ntktech.com/hfd.htm , a
Duct Mount Anemometer, but it's STILL subject to the issues above.
You need to traverse the duct, if putting a hood on the oulets
is not an option.
One of our newer members may have something to add on the
On Fri, 18 Mar 2005 01:58:35 GMT, pjm@see_my_sig_for_address.com
Oh - you said 'hand held'. OK, same thoguhts apply as re : measruing
position, but ...
http://www.grainger.com/Grainger/wwg/start.shtml and search
Good luck with that. I can verify that slowing down the fan will
reduce cfm in each of those branches, and I can do so without even
having taken measurements :)
The relative flow rates, which you seem to be interested in, will not
necessarily remain proportional over changes in fan speed, especially
if equal friction wasn't a consideration in the system's design. If
turning vanes aren't present then you have even more problems
introduced by turbulence, a science that for all practical purposes
doesn't yet exist.
If proportional flow rates are what you are in fact attempting to
check, then get yourself some good duct design software and do it the
Try a TSI Velocicalc. I have one and I love it. This is a hot wire
annemometer with other options, dependong on the model. If you enter
the duct size, whether round or rectangular, and do the traverse, it
will give you actual CFM. It does all the math for you to eliminate
math mistakes. Mine also has pressure ports for hooking up a pitot
tube, good of there is dust in the duct when you test, still does the
math. Dust will ruin the thernal sensor over time, Mine also measures
differentioa bressure across coils etc. And it measures wet bulb, dew
point and relative humidity in a duct. Telescoping probe i 40" long,
so you will have to measure from both sides of the duct, unless you can
buy a 60 inch pitot tube. TSI do not sell pitot tubes but dwyer does.
Very versatile instrument, I love vaving mine. The model I have was
about $2100.00. Not cheap, but worth it.
What model are you using? I looked at TSI's web site and they have an array
of stuff. Is it the 'multi-parameter' model line?
Maybe a rotating van model would do the job as I do have duct sizes and
might be able to calc it easily enough. Looks like these instruments have
the capability of logging/download so dumping the data into a spreadsheet or
utility program might be OK.
The prices don't look too bad in their on-line store, either.
The airflow in these systems are gonna be dirty and quite likely above 225
(f) most of the time if I'm measuring under operating conditions (which I
should be !). They typically feed wet 'scrubbers'.
If you've got more info, I'd appreciate it.
You will have trouble getting a rotating vane anemometer inside a duct.
The thermo-anemometer probe will be ruined by long exposure to dust.
Use a pitot tube, hooked to the TSI.
If you get a cheap instrument, your results will be less accurate. If
you want to guess, skip the instruments and just guess.
I will look up the model number of my TSI and post it later.
Rotating vamne anemometers have their bearings wear out after prolonged
use and lose accuracy. They can also block the duct. Rotating vane
anemometers are best used to measure uniform flows to or from grilles.
I don't want a 'cheap' instrument... as a matter of fact, I don't think I
have any now (-;.
I'm just looking for the best instrument for the job is all. If I'm going to
ruin either type, I might as well ruin the one that's less expensive (not
I'll have to do some calcs on the bigger diameters to see if the rated
measurement flow would still be OK in the main system.
150,000 cfm will pull your hard-hat off!!
Dust removal systems usually require very high velocities in the duct
to carry the dust. Dust won't hurt the pitot tube. and neither will
the high velocities. My instrument is a model 8386 from TSI.
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