Negative duct fitting coeffecient

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Negative duct fitting coefficients in the ASHRAE fitting database suggest that certain fittings may produce pressure regain, which raises questions about whether this regain is static. The confusion arises from the formula P(total)=C*P(velocity), indicating that total pressure should decrease as air travels downstream from the fan. This creates a perceived contradiction regarding the nature of negative coefficients. Additionally, the discussion touches on variations in inlet and outlet sizes, with negative coefficients observed in both scenarios. Clarification on the implications of negative coefficients in duct design is sought.
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In the ASHRAE fitting database, there are negative duct fitting coefficients. I assume this means that that particular fitting will produce a pressure regain. However, I am unsure that it would be a static regain because:

the formula is P(total)=C*P(velocity), and P(total) only can only drop as it travels thru a duct (downstream of the fan). Is this a contradition?

So I am confused of what the negative means because of this contradiction. Anybody know?
 
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I have tried a range on inlet and outlet sizes. Some with the same and some with different sizes and a negative coefficient result.
 

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