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Air flow (eg in a duct) has a dynamic pressure, and when the air flow goes through e.g. a 90°-bend the bend has a dynamic-loss-coefficient.
The dynamic-loss-coefficient is used to calculate the total-pressure-loss by the formula:
Total Pressure loss = C * Velocity Pressure
(Velocity pressure is a different name for dynamic pressure. Both Total Pressure loss and velocity pressure are in Pascal, while dynamic-loss-coefficient C doesn't have a unit.)

For most bends/structures C is less than 1.
What I don't understand is what a C greater than 1 actually represents/means in the real world?
(Does it mean the air turns backwards?)

(Because if you subtract the total pressure loss from the dynamic pressure it would be negative.)

(Since this also has practical relevance there are several organizations empirically determining/collecting these values (e.g. ASHRAE and SMACNA), and in most cases C<1, but there are quite a few cases where C>1. E.g. an example of an fitting by SMACNA and an example by ASHRAE. (Edit: Heres an image of the definitions and formulas used by SMACNA.)
But the examples are just for illustration and don't matter in themselves, i'm just interested in what happens to the airflow when C>1.)

Regards

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  • $\begingroup$ It means nothing special. You are only calculating the loss due to bends, fittings etc. and that loss can be any multiple of dynamic/velocity pressure. Larger the loss (larger the $C$) larger the pressure difference needed to maintain a particular flow rate. $\endgroup$
    – Deep
    Commented Sep 28, 2023 at 13:42

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