Flow rate (CFM) in a different inlet are & outlet area

[JohnPinnacle]

Hello,

Here are the knowns:
Inlet open area = 17 inches^2
Outlet open area = 30 inches^2
Internal Fans = 270 cfm (3 fans @ 90 cfm each)

What is the CFM through the system and what's the equations used to figure this out?

Thanks a ton!
John

 

[russ_watters]

The answer appears to be given in the problem statement: 270 cfm.

 

[JohnPinnacle]

The fan spec is 90cfm per fan. That doesn't mean that 270cfm (3 fans) is what will exhaust out the rear of the unit. If I have a huge fan inside a box and a very little inlet or outlet, the fan will be choked and pressure will build due to the open air restriction.

 

[russ_watters]

You'll need to look at fan curves and estimate pressure drop through your system then.

Do you have a sketch you can upload?

 

[pmacias]

Dear John,

Thank you for your question regarding flow rate (CFM) in a system with different inlet and outlet areas. To determine the CFM through the system, we need to use the equation Q = AV, where Q is the flow rate (CFM), A is the cross-sectional area of the opening, and V is the velocity of the fluid passing through the opening. In this case, the fluid is air and the opening is the inlet and outlet of the system.

Using the given information, we can calculate the flow rate through the system by first determining the velocity of the air passing through the inlet and outlet. We can do this by rearranging the equation to solve for V: V = Q/A. Plugging in the values, we get a velocity of 15.88 feet per minute (fpm) for the inlet and 9 fpm for the outlet.

To determine the total CFM through the system, we need to add the individual flow rates of each fan. Since there are 3 fans at 90 cfm each, the total CFM through the system is 270 cfm.

I hope this helps answer your question. Let me know if you need any further clarification or have any other questions.

Best,