Duct Problems

[genduct]

Sounds like you have no static pressure. Too many openings or too many air exits to allow for static pressure to build up and push the air out of the closest registors. Close down some of the dampers until you get static pressure in the trunk. I Should think you would need a 1/2 inch or so of pressure at 2/3 down the trunk run, on a unit that size.

But don't you agree that close to the unit discharge we have all velocity pressure and won't have any static til the airstream slows down and we see the static regain we are looking for?

[genduct]

I kinda missed the turbulence word first time around. That being said, I have seen it many times with high velocities that the air just rushes on by the first branch.

A number of years ago we installed a make-up air unit for an industrial printing press. The mechanical insisted on a high velocity supply since the run was short and the we had plenty of fan on this extended plenum kind of design. well the air was moving around 1800 FPM and actually sucked air in on the first supply register. Just thought of that looking at your post. Funny huh?

[synergy]

Static pressure should be at its higest at the fan exist and at its lowest at the end of the run, this is due to friction loss through the duct. The duct internal area and the number of exits need sized and balanced so as the air supplied to the duct is slightly higher than the air leaving the duct. This creates static pressure. Static pressure forces the air out of the registors. With out static pressure, the air will travel down the duct and the velocity pressure will force the air out at the end runs only. You need to have a pressurised duct to get even air flow to all supplies. How much static varies from type of system.

[second opinion]

But don't you agree that close to the unit discharge we have all velocity pressure and won't have any static til the airstream slows down and we see the static regain we are looking for?

Velocity pressure only exists with static pressure.

You can have static without velocity though.

[genduct]

Velocity pressure only exists with static pressure.

You can have static without velocity though.

I agree with your quote but without motion what would be the point of a system that wasn't delivering anything?

Me and Mr Bernoulli Think of TOTAL Pressure the same way you think of WATTS You know Volts time Amps. Key being the watts stay the same and as the velocity slows the static builds. Now that doesn't mean there is all velocity pressure at the discharge just mostly velocity pressure.

That's my understanding of the Physics of the situation. If you see any flaws or have a simpler explanation, you know me, I am always open to being educated

[PPettigrew]

Is this a new unit? Has the unit and duct been sized correctly? Has it been balanced?