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A PC case is basically a rectangular pathaway for air to flow through but at the places where the air enters and exits there are solid plastic or metal panels commonly with meshes and little holes.

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When a fan is rated at 50cfm or 80m3/h at 100% speed by how much is this figure decreased when taking into account the characteristics of the volume of the case?

Is it 20% or 80% for example?

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    $\begingroup$ The volume of the case will have nothing to do with the airflow. The resistance to airflow will and this will typically be determined by the minimum cross-sectional area of the airflow path. For a given airflow rate the volume of the case will determine the number of air changes per unit time. Can you reformulate your question? $\endgroup$
    – Transistor
    Commented May 9, 2020 at 12:44
  • $\begingroup$ What is the minimum cross-sectional area of the airflow path? How can I look this up or estimate it? What is the number of air changes per minute? $\endgroup$
    – Vulkan
    Commented May 9, 2020 at 12:50

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When a fan is rated at 50cfm or 80 m3/h at 100% speed by how much is this figure decreased when taking into account the characteristics of the volume of the case?

Is it 20% or 80% for example?

The volume of the case will have nothing to do with the airflow. The resistance to airflow will and this will typically be determined by the minimum cross-sectional area of the airflow path.

What is the minimum cross-sectional area of the airflow path?

If it's not given you would have to work it out by measurement.

For a given airflow rate the volume of the case will determine the number of air changes per unit time.

What is the number of air changes per minute?

If the volume of the cabinet is, for example, 0.5 m3 and the airflow is 80 m3/h then the number of air changes per hour will be given by $ n = \frac f v = \frac {80}{0.5} = 160 $ (where f is the airflow and v is the volume. 160 changes per hour is almost 3 changes per minute.

Any filters or blockages will reduce the airflow.

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  • $\begingroup$ What is the formula for resistance affecting airflow? $\endgroup$
    – Vulkan
    Commented May 9, 2020 at 13:40
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    $\begingroup$ engineerlive.com/content/… $\endgroup$
    – Transistor
    Commented May 9, 2020 at 13:57
  • $\begingroup$ engineering.stackexchange.com/questions/35665/… $\endgroup$
    – Vulkan
    Commented May 9, 2020 at 21:15
  • $\begingroup$ Also, how are you that good at finding these kind of answers? Do you use Google? $\endgroup$
    – Vulkan
    Commented May 9, 2020 at 21:15
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    $\begingroup$ Most of it is basic understanding of physics. I'm an electrical engineer working in industrial automation so I get paid to think about stuff - although this question is outside my normal work. Everything in my answer should be fairly obvious now that you've thought about it. I didn't have to look that up. The article I linked in the comments I found searching for "practical calculation for axial fan and airflow". Read, think, read, think, read, ... $\endgroup$
    – Transistor
    Commented May 9, 2020 at 21:35

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