How to calculate cfm through pipe

  • Thread starter kvalrico
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    Cfm Pipe
In summary, the dust collector is not pulling enough cfm to keep up with the total cfm going into the ship. This could be because of the orifice or venturi size, or the fan's specifications. A pitot tube test could be used to determine the flowrates.
  • #1
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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  • #2
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.
 
  • #3
Another option is a pitot tube traverse to get the velocity in the pipe. Then it's simply

[tex]Q = V*A[/tex]
 
  • #4
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.
 
  • #5
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.
 
  • #6
You could always perform the test when there is no dust...
 
  • #7
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" [Broken]
 
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1. How do I calculate the CFM (cubic feet per minute) through a pipe?

To calculate the CFM through a pipe, you will need to know the pipe's diameter, length, and pressure drop. The formula for calculating CFM is: CFM = (0.785 * (pipe diameter in inches)^2 * velocity in feet per minute)/ 144. The velocity can be calculated by dividing the flow rate (in gallons per minute) by the cross-sectional area of the pipe (in square feet).

2. What is the pressure drop and why is it important in calculating CFM?

The pressure drop is the decrease in pressure as the fluid (air or liquid) flows through the pipe. It is important in calculating CFM because it affects the velocity of the fluid, which is a crucial factor in the CFM equation. The higher the pressure drop, the lower the velocity and therefore, the lower the CFM.

3. Can I use the same formula to calculate CFM for both air and liquid in a pipe?

No, the formula for calculating CFM is different for air and liquid. For air, the formula is: CFM = (1.08 * (pipe diameter in inches)^2 * velocity in feet per minute)/ 144. For liquid, the formula is: CFM = (0.408 * (pipe diameter in inches)^2 * velocity in feet per minute)/ 144.

4. How does the length of the pipe affect the CFM?

The longer the pipe, the lower the CFM. This is because the longer the pipe, the more friction the fluid encounters as it flows through, resulting in a higher pressure drop and lower velocity. Therefore, it is important to consider the length of the pipe when calculating CFM.

5. Are there any online calculators or tools available to help calculate CFM through a pipe?

Yes, there are many online calculators and tools available that can help you calculate CFM through a pipe. These calculators typically require you to input the diameter, length, and flow rate of the pipe, and they will calculate the CFM for you. However, it is always recommended to double-check the results with the formula to ensure accuracy.

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