How to calculate cfm through pipe

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Discussion Overview

The discussion revolves around calculating the cubic feet per minute (cfm) of air being pulled through a pipe in a cement distribution terminal, specifically in relation to dust collectors used during the pneumatic loading of bulk powdered cement into ships. Participants explore various methods and instruments for measuring or calculating cfm, considering the presence of dust in the airflow.

Discussion Character

  • Technical explanation
  • Exploratory
  • Debate/contested

Main Points Raised

  • One participant suggests using a nozzle with known dimensions to measure pressure differences in the duct, along with fan coefficients for correlation.
  • Another participant proposes using a pitot tube traverse to measure velocity in the pipe, followed by the formula Q = V*A to calculate cfm.
  • Concerns are raised about the effectiveness of pitot tubes in dusty environments, with one participant noting that they typically do not perform well in such conditions.
  • It is mentioned that orifices and venturis are not suitable for two-phase flows without specific calibration.
  • A participant with experience in environmental engineering describes a method involving a Type-S pitot tube and an oil manometer to measure flow accurately, referencing EPA methods for guidance.

Areas of Agreement / Disagreement

Participants present multiple methods for measuring cfm, but there is no consensus on the best approach, particularly regarding the challenges posed by dust in the airflow. The discussion remains unresolved with competing views on the effectiveness of different measurement techniques.

Contextual Notes

Participants mention various factors that could influence measurements, such as moisture content, barometric pressure, and the need for calibration in dusty conditions. The discussion highlights the complexity of measuring airflow in environments with particulate matter.

kvalrico
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I work at a cement distribution terminal. We load bulk powdered cement pneumatically into ships. Part of this operation is to remove the excess air from the ship by use of dust collectors. I need to know if my dust collectors are pulling enough cfm to keep up with the total cfm going into the ship.

Is there an instrument I can take a reading with to calculate cfm? Or does this need to be crunched through a formula? Is there someone out there that can assist me with this? I can provide whatever info you need to make the calculation, ie pipe diameter, specs on the dust collector fan, length of pipe the collector is pulling through etc. Let me know what is needed.

Thanks,
 
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The easiest thing to do is just to put a nozzle with known dimensions somewhere in the duct and measure the pressure difference across of it. If you know the Cp, Ch, and Cq of your fan you could also just measure the pressure the across and correlate the data.
 
Another option is a pitot tube traverse to get the velocity in the pipe. Then it's simply

Q = V*A
 
FredGarvin said:
Another option is a pitot tube traverse to get the velocity in the pipe.

dust collectors

Pitot tubes typically don't like dust.
 
Topher925 said:
Pitot tubes typically don't like dust.

True. The OP didn't really specify how bad it was, but it stands to reason. However, one never knows until the question has been raised. Technically orifices and venturis are out of scope for two phased flows. It would have to be calibrated for that particular flow.
 
You could always perform the test when there is no dust...
 
I work for an environmental engineering and testing firm. We frequently test at portland cement manufacturing locations. Its a relatively easy test to measure the flow of a dust collector. Typically if you traverse the duct with a Type-S pitot tube and an oil manometer to get an average pressure differential, use the stack diameter, moisture (typically ambient on a dust collector 2-5%), barometric pressure, pitot tube coefficient, etc you can get a very accurate flow measurement. Refer to EPA methods 1 through 4 in Title 40 of the Code of Federal Regulations Part 60.

http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=9b04c253da1c4fbb563e0a72773217d6&tpl=/ecfrbrowse/Title40/40cfr60d_main_02.tpl"
 
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