Self-Balanced Distribution Duct Plenum

[ChrisConley]

This is a little obscure request, but I'm hoping someone out there can point me in the right direction.

I'm trying to design a self balanced air distribution plenum by pressurizing a piece of ductwork and cutting holes of increasing size to deliver the same volume of air to each hole as we travel down the duct.

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I've seen this concept in open spiral duct applications, and fabric duct application, I'm just attempting to translate it to rectangular duct with rectangual openings.

Anyone have a formula, or design guide that would point me in the path to layout the pattern of holes?

 

[willard3]

You could do this with static regain design and then you could vary any of the parameters, ie, length between holes, size of holes and etc.

 

[berkshire]

If the duct is parallel there will be virtually no static regain, you have to calculate the drop in static pressure after each hole. Then calculate the flow at the new lower static pressure.
B.E.

 

[willard3]

If the CFM changes at an outlet and the duct size stays the same, velocity pressure is converted to static pressure and the static pressure stays the same.

Total pressure changes/decreases along the length, but static pressure stays the same.

 

[ChrisConley]

Thanks all, I ended up doing what berkshire mentions. I calculated each length of duct and fitting loss and then iteratively manipulated sizes to create an approximately equal pressure drop to each runout.

 

[berkshire]

ChrisConley (Mechanical)
Willard was right I should have said total pressure, but you figured out what I was trying to say.
B.E.

 

[chicopee]

ChrisConley, it is highly unlikely, you will get what you want, no matter how well you design the system, that's why dampers are used.

 

[AbbyNormal]

there was just a thread in here about doing this on spiral.

sometimes the air immediately upstream does not like to exit the first holes, likes to blow straight down the duct

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[berkshire]

Abbynormal,
Isn't that why "sugar scoops" were developed, to stop that?
B.E.

 

[AbbyNormal]

Air scoops or extractors help.

I was troubleshooting a commercial system once, drilled a hole in supply duct on discharge, blower casing door, return duct. Figured I could check some statics, see what was happening in supply duct, return duct and what it was taking to pull air through the filters/coil.

Got a rediculously low TSP on the fan.

Turns out there was some turbulance on the discharge, was getting a negative supply static, "no cold air" was coming out of the hole I drilled. 2 feet down the line was the discharge air sensor so I pulled that out, got a decent reading, and a correct TSP that jived with what the air balance report on the grilles said and what the fan curve and rpm were saying.

Sometimes air dos not want to do the samething we want it to do.

Take the "V" out of HVAC and you are left with a HAC(k) job.