Negative duct fitting coeffecient

In summary, some fittings in the ASHRAE fitting database have negative duct fitting coefficients, which may indicate a pressure regain. However, there is confusion as to whether this regain is static, as the formula for total pressure suggests it can only decrease as it travels through the duct. This contradiction has led to uncertainty about the meaning of the negative coefficient. The question also arises about whether the inlet and outlet sizes affect this coefficient, with some tests showing a negative result for both same and different sizes.
  • #1
Great Smoky
8
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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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  • #2
Are the inlet and outlet the same size?
 
  • #3
I have tried a range on inlet and outlet sizes. Some with the same and some with different sizes and a negative coefficient result.
 

FAQ: Negative duct fitting coeffecient

1. What is a Negative Duct Fitting Coefficient?

A Negative Duct Fitting Coefficient is a term used in fluid mechanics to describe the resistance of a duct to airflow. It is a negative value that indicates a decrease in pressure within the duct due to turbulence or other factors.

2. How is the Negative Duct Fitting Coefficient calculated?

The Negative Duct Fitting Coefficient is calculated using the ratio of the actual pressure loss in the duct to the theoretical pressure loss, multiplied by -1. It can also be calculated using experimental data and various mathematical models.

3. What factors can affect the Negative Duct Fitting Coefficient?

There are several factors that can affect the Negative Duct Fitting Coefficient, such as the shape and size of the duct, the type and velocity of the fluid flowing through the duct, and the roughness of the duct's surface. Other factors include the presence of obstacles, bends, and transitions within the duct.

4. Why is it important to consider the Negative Duct Fitting Coefficient in fluid mechanics?

The Negative Duct Fitting Coefficient is important because it affects the accuracy and efficiency of fluid flow calculations. Failure to account for this coefficient can result in errors in pressure drop calculations, which can impact the performance and design of duct systems.

5. Can the Negative Duct Fitting Coefficient be negative in a real-life scenario?

Yes, the Negative Duct Fitting Coefficient can have negative values in real-life scenarios. This is because there are various factors that can contribute to a decrease in pressure within a duct, such as turbulence, eddies, and changes in direction or diameter. These factors can result in a negative pressure drop, leading to a negative coefficient.

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