Have CFM at one point, need to find it at another point

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Discussion Overview

The discussion revolves around determining the airflow rate (CFM) in a dust collection system designed for a powder mixing project. Participants explore the implications of airflow conservation principles and the effects of different control volumes on CFM calculations within a tent-like workspace.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant states that conservation of mass for incompressible air implies that CFM should remain constant throughout the system.
  • Another participant questions whether this assumption holds if analyzing a smaller section of the system rather than the entire control volume.
  • A different participant suggests that understanding the size of the pipe and tent is crucial for determining airflow and mentions the potential need for computational fluid dynamics (CFD) to visualize airflow distribution.
  • One participant emphasizes the importance of knowing whether the CFM value is measured or calculated, arguing that without knowing the flow rate into the room, one cannot accurately determine the flow rate exiting the dust collector.
  • Another participant requests clarification and a diagram to better understand the setup and the specific problem being addressed.

Areas of Agreement / Disagreement

Participants express differing views on the assumptions regarding airflow conservation and the implications of measuring versus calculating CFM. The discussion remains unresolved, with multiple competing perspectives on how to approach the problem.

Contextual Notes

Participants have not reached consensus on the assumptions necessary for calculating CFM in this specific setup, and there are unresolved questions regarding the influence of system geometry and measurement methods.

MiketheMuman
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I am designing a dust collection system for a powder mixing project. My current setup is basically a tent (roughly 6ft X 4ft X 7ft) with a dust collector tube hanging from the ceiling (or coming from the floor). I know the CFM, air properties, and pipe area at the end of the dust collector tubing and would like to find the CFM in the work space (roughly the center of the tent). Can someone please offer some help/guidance?
 
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Conservation of mass for incompressible air says the cfm has to be the same.
 
That makes sense. I would think that assumes the control volume includes the entire system (A1 = area of tent, A2 = area of tube). Would conditions change if I am only looking at a small section of the larger "pipe" (roughly same cross-sectional area as tubing)?
 
MiketheMuman said:
That makes sense. I would think that assumes the control volume includes the entire system (A1 = area of tent, A2 = area of tube). Would conditions change if I am only looking at a small section of the larger "pipe" (roughly same cross-sectional area as tubing)?
I suppose it would, but I'm not clear on what you mean or what you are after. Could you upload a diagram?
 
Confused trying to visualize your project. You have a tent with a pipe inside taking dust in and want to figure out how much cfm is through a CV in one portion of the tent? Depends on the how big the pipe is, how big the tent is, etc. Worst case, you'll need to do some basic CFD to get a 3D velocity distribution and get cfm from whatever space you're talking about.
 
If you actually KNOW (Tried-To-Calculate =/= KNOW) the flow rate of the dust collector tube in THAT system, then you already know how many Cubic Feet of air flow through that room per Minute.

If you don't know the flow rate into that room, I have no idea how you even began to calculate the flow rate of the dust collector exiting that room. So, I am assuming that the CFM value that you have in mind is a measured value. Not a calculated value.

If your dust collector's cfm is a calculated value, then its wrong. Unless you got lucky. Because you don't know the flow rate into the room, so you can't know at what pressure the input and the output of air from that room will equalize.
If your dust collector's cfm is a measured value, then that's your flow rate for the room as well, unless there is other HVAC that we don't know about yet.
 
russ_watters said:
I suppose it would, but I'm not clear on what you mean or what you are after. Could you upload a diagram?

Take a look at this. As you know I'm still new to this site. Let me know if you can see this and if it helps illustrate my problem. Thank you!
 

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