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

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Discussion Overview

The discussion revolves around calculating the flow rate (CFM) through a system with varying inlet and outlet areas, specifically focusing on the implications of fan specifications and system design. The context includes theoretical considerations and practical applications related to airflow dynamics.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • John presents the known parameters of the system, including inlet and outlet areas and fan specifications, and asks for the CFM through the system and the equations needed for the calculation.
  • One participant suggests that the CFM is already provided in the problem statement as 270 cfm.
  • Another participant argues that the 270 cfm rating does not guarantee that amount will be exhausted due to potential restrictions caused by the inlet and outlet sizes, indicating that pressure may build up if the airflow is choked.
  • A further suggestion is made to consider fan curves and estimate the pressure drop through the system to better understand the flow dynamics.
  • A request is made for a sketch of the system to aid in the discussion.

Areas of Agreement / Disagreement

Participants do not reach a consensus; there are competing views regarding the interpretation of the fan specifications and the actual flow rate through the system, with some emphasizing the importance of system design and restrictions.

Contextual Notes

Limitations include the lack of specific details about the system design, such as the exact configuration of the inlet and outlet, and the absence of fan curve data, which may affect the calculations and understanding of airflow.

JohnPinnacle
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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
 
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The answer appears to be given in the problem statement: 270 cfm.
 
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.
 
You'll need to look at fan curves and estimate pressure drop through your system then.

Do you have a sketch you can upload?
 
Last edited:

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