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

1. Apr 28, 2017

### MiketheMuman

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?

2. Apr 28, 2017

### Staff: Mentor

Welcome to PF!

Conservation of mass for incompressible air says the cfm has to be the same.

3. Apr 28, 2017

### MiketheMuman

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)?

4. Apr 29, 2017

### Staff: Mentor

I suppose it would, but I'm not clear on what you mean or what you are after. Could you upload a diagram?

5. Apr 30, 2017

### yangshi

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.

6. May 1, 2017

### RogueOne

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 thats your flow rate for the room as well, unless there is other HVAC that we don't know about yet.

7. May 3, 2017

### MiketheMuman

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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