Negative duct fitting coeffecient

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Great Smoky
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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.