Fundamental Questions about Static Pressure

[j100]

Hello,

I have some questions about static pressure exiting the fan.
I will be referring to the following article for simplicity: "Minimizing System Effect"

http://www.achrnews.com/copyright/cf6d6e506906a010VgnVCM100000f932a8c0____?view=print

1. Is the assumption in the article that the total pressure does not change in the 100% of the effective duct length correct?
2. According to the article, Static Pressure is the pressure that causes the air in the duct to flow, and Velocity Pressure is the pressure that results from the air movement. If the static pressure is low near the fan, how can it cause large Velocity Pressure?
3. If I understand the article correctly, at 100% effective duct length from the fan, the static pressure is regained to 100%. What does this 100% regained static pressure mean (e.g. 100% in reference to what)?
4. At 100% effective duct length the static pressure is the same across the entire cross-section of the duct, correct? At 25% (for example) of at effective duct length, is the static pressure non-uniform across the cross-section?
5. If I measure the static pressure in the area of high Velocity Pressure right outside of the fan, would that pressure be equal to Static pressure at the 100% of the effective duct length from the fan?

I do have AMCA 201: Fans and Systems publication.

Thank You in advance.

 

[willard3]

Start here:

Total pressure = static pressure + velocity pressure

In US units, velocity pressure = (V/4005)*2
V = ft/min = velocity

I think your problem is confusing static pressure for total pressure.

 

[KiwiMace]

The 100% duct is just the length of duct required to achieve maximum static regain. It is arbitrary in this general example, and will be something like 5-10 duct diameters in length in real life.

As pointed out; The static P and velocity P add up to make Total P. One does not make the other. Look at the magnitude of the profile in the diagram, and note how the peak is higher at the fan (and more uneven) and lower at the 100% point (even parabolic profile). This latter point has a lower velocity and more static pressure. The velocity pressure has given up to static pressure.

 

[rmw]

The proper name for the expansion section on the outlet of a fan that converts exit velocity pressure to static pressure as described by Kiwi is an evase'. (the last accented e is pronounced like a long "a".)

It is commonly used in large industrial fans.

However, at that point your static is the highest it is going to be because as the air/gas flows down the duct, there are flow losses and static pressure is lost. As pointed out, if there were no differential in the static pressure there would be no flow.

rmw