Pressure balance at duct junctions

[davidyu2404]

Is pressure balance at duct work junction required only for equal friction method? I used equal velocity method to size my duct, do I need to balance pressure at junctions?

 

[Drazen]

balance is a must, otherwise your ducting will not function as designed because air will always follower lower resistance path until pressure in all branches equalize, and, without balancing it will equalize by itself, meaning - you will not get your design flows.

 

[davidyu2404]

Thanks Drazen. That's what I thought too. I was reading ASHARE Handbook-Fundamentals and there is a section where the diameter of one of the duct is decreased incrementally until the pressure at the junction is balanced. But how do you calculate the pressure after each branch? Is it static pressure?

 

[davidyu2404]

I tried the Darcy Colebrook equation to calculate static pressure loss after each branch but it doesn't seem to give me the right result.

 

[Drazen]

yes, that's static pressure.

you can make calculations manually using ashrae handbook, but there are precise while cheap software packages like DuctNet.

 

[willard3]

In order to calculate pressure drop accurately, you will have to calculate both velocity and static pressure losses.

 

[davidyu2404]

Thanks guys! So to clarify: I will need to calculate static pressure losses for each branch. Static pressure loss at each branch is consisted of frictional losses (major head due to viscosity) and dynamic losses (minor head due to fittings)?

 

[dbill74]

The only duct pressure loss you really need to concern yourself with is the loss to the most remote diffuser/grille.

Trying to "balance" a system in the design phase with duct sizing is not economical. It will take a lot of time and effort on your part, the ductwork will NOT be installed EXACTLY as you designed, require lots of custom duct sizes and lots of custom fittings and transitions.

You are better off sizing your duct work (mains and branches) to maintain approximately 0.08-0.1" wg / 100 feet using "off-the-shelf" duct sizes and your ductulator. So a balancing agent can 'balance' the system in the field, specify manual balancing dampers at branches and take-offs. Then do a minimum number of pressure drop calculations to find the most remote diffuser and size your fan to provide sufficient pressure at that point. Everything upstream of that will then work itself out.