Ventilation balance with different pressure drop and flow requirements

Question

Ventilation balance with different pressure drop and flow requirements

xj25 (Electrical)

(OP)

29 Oct 16 07:34

Hello,

We have to integrate a ventilation system with two branches. We have a detailed calculation for each branch and it shows that pressure drop curve vs flow is higher in one of them.

Additionally, the flow requirement is that main flow has to be diverted 70% for the branch with higher pressure drop, so some kind of balancing device is needed.

After some work with our CFD consultant and Fan supplier we have two proposals for the balancing, but I am not very confident with any of them.
See file attached.

Any comment or suggestion will be welcomed

http://files.engineering.com/getfile.aspx?folder=f3977030-4518-4e13-9fbc-c9

xj25 (Electrical) · Answer 1 · 2016-10-29 07:37:57

BTW I proposed to put the deflector in mid position and use a perforated plate to increase the pressure drop in upper branch, but our fan supplier didn´t agree with that.

xj25 (Electrical) · Answer 2 · 2016-11-03 08:30:18

See CFD flow field with proposal deflector position, any comment?

Thanks

lilliput1 (Mechanical) · Answer 3 · 2016-11-07 03:41:28

This is a nonstandard fitting not sanctioned by SMACNA. Typically branch tees are designed to size the inlet such that each will have the same velocity by adjusting the cross sectional area to correspond with the design CFM. The tee would branch off with radius elbow or square vaned elbow (use single thickness vanes which has less pressure drop than single vaned elbows. The outlet size of each branch is enlarged to the size appropriate for the deasign pressure drop in inches water gauge per 100 ft of duct length. Also each outlet branch is provided with manual volume damper with construction appropriate for the design pressure construction. What you have to remember is that to balance the flow the pressure drop from the fan discharge to each outlet terminal has to be the same because all the outlet discharges to athmospheric pressure. Those branches with pressure drop less than design would need branch damper and terminal outlet damper throttled to kill the excess pressure. Damper location is also important in that they serve to limit the downstream pressure so that the downstream duct construction class can be set and specified.
On the return & OA intake side the pressure drop from each inlet to the fan suction must be the same for balanced flow (since each inlet is at atmnospheric pressure). Similarly balancing dampers would be required and tees similarly constructed.

xj25 (Electrical) · Answer 4 · 2016-11-07 08:03:57

If I undersood well the key is " What you have to remember is that to balance the flow the pressure drop from the fan discharge to each outlet terminal has to be the same because all the outlet discharges to athmospheric pressure. Those branches with pressure drop less than design would need branch damper and terminal outlet damper throttled to kill the excess pressure."

That´s what I tried to express with using a custom adjusted "perforated plate" downstream the T junction to match both pressure drops, so using it as damper. am I right?

The installation is a retrofit for a railway vehicle, so a lot of non standards solutions have to be used due to space constraints.

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Ventilation balance with different pressure drop and flow requirements