# Duct Design ( Looking for some advice)

joe shmo
Posers and Puzzles 30 Apr '14 00:55
1. joe shmo
Strange Egg
30 Apr '14 00:55
I have to design a Duct system at work, The duct work basically looks like a 2 pronged fork. Something like this below ( please bare with me on this representation).

......| <--Inlet
....____ <-- Plenum
../........\
..|........|<--Duct outlets

On the left fork of the duct there will be 1 outlet, on the right fork there will be 2 outlets ( one on each side of the duct). All outlets will be slots along the length of of the section shown as "Duct outlets" above, and will all have the same cross-sectional area.

I am attempting to design the system such that the flows from each outlet are essentially equal. ( ie. 1/3 of the total system flow ).

I am attempting to generate a system curve from this configuration, by plotting the head loss, as a function of the volumetric flowrates Q_i, for each leg of the duct.

From continuity I know that in my left most duct (1) that

Q1 = 1/3*Qtot eq(1)

and, so the right most duct (2) must be

Q2 = 2/3*Qtot eq(2)

Using the Darcy-Weisbach equation ( temporarily ignoring component losses, for simplicity in getting my point across )

Head Loss (Q) = f1*1/(2g)*L1/D1*Q1^2/A1^2 + f1*1/(2g)*L1/D1*Q2^2/A2^2 eq(3)

Sub eq(1) & eq(2) ---> eq(3)
Assume round duct of Diameters D1 and D2 respectively

After simplification

H = ( f1*32/(9*pi^2)*1/g*L/D1^5 + f2*8/(9*pi^2)*1/g*L/D2^5)*Q^2 eq(4)

Fixing D1 in the above equation, I will Plot H(Q) for several specified D2, thus giving me a series of system curves based on a specific D2, over the entire domain of Q.

These can then be intersected with a specific fan curve ( giving a series operation points), and a duct size "D2" can then be selected from the the point of operation which corresponds to the desired system output.

My question to you: Is this a valid approach?
2. sonhouse
Fast and Curious
14 May '14 17:263 edits
Originally posted by joe shmo
I have to design a Duct system at work, The duct work basically looks like a 2 pronged fork. Something like this below ( please bare with me on this representation).

......| <--Inlet
....____ <-- Plenum
../........\
..|........|<--Duct outlets

On the left fork of the duct there will be 1 outlet, on the right fork there will be 2 outlets ( one on ea ...[text shortened]... which corresponds to the desired system output.

My question to you: Is this a valid approach?
Joe, our resident air engineer here said there is the bible of duct work called Manual D, here is a bit about it, not sure if you can get it free online, he said it cost's about 100 dollars but it does all the heavy lifting, you don't need to solve equations for ductwork:

And this:

http://www.wrightsoft.com/products/right-d.aspx

Our engineer said just put in three ducts but use adjustable dampers on each outlet, that way you can adjust them till you get equal flow and not have to worry about design issues and equations.
3. sonhouse
Fast and Curious
14 May '14 19:23
Here are some words from an email from Pat, our ductwork engineer:

The bible for practical ductwork sizing is the ACCA Manual D. It gives airflow parameters for all common ductwork and fittings. It also gives a method for calculating total friction loss.

The more abbreviated design method would be to look in a ductwork or A/C supply catalog and pick some fittings that fit the size of the airflow source. (Southwark Metal, Philadelphia, or JohnstoneSupply.)

To get 3 equal flows in 3 equal sized outlets, the ductwork should have equal path lengths and minimum turbulence. As you go further from the source, there will be less air volume and less velocity. If these 3 outlets are spaced far apart, it is often helpful to reduce the size of the trunk to speed up the air going to the last outlet. But, for every outlet, bend, or size change there is turbulence, volume or flow reduction which also makes balancing difficult.

I suggest that each outlet have a damper to help balance the airflow. I don't see round 3-way sheetmetal fittings in the catalogs, so I would use two 2-way Wyes to do the split and do the final balance with dampers in the ductwork.
4. joe shmo
Strange Egg
14 May '14 22:53
Wow, thank you, and please thank your resident HVAC engineer for me. I greatly appreciate it, and I will certainly look into purchasing the manuals!
5. sonhouse
Fast and Curious
15 May '14 15:133 edits
Originally posted by joe shmo
Wow, thank you, and please thank your resident HVAC engineer for me. I greatly appreciate it, and I will certainly look into purchasing the manuals!
What state do you live in, if I might ask? There might be local ordinances that supersede some of the stuff in the manuals, not sure but that might be the case. There certainly is some of that going on in electrical codes.

Looks to me like the best part of the advice from Pat is to use variable dampers on each outlet, allowing you to adjust the flows and balance the whole deal a lot easier.

Do you have proper instruments that measure air flow? My guy here has air flow meters and uses laser temperature probes to measure the duct outlet temperatures to see if the heating and air conditioning systems are working correctly. I think he said they generally like to see something like 50 or 51 degrees F coming out of the vents for air conditioning and 80ish for heating.

In case you are light on instrumentation, here is a link (just at random) that sells various kinds of airflow meters:

http://www.instrumart.com/categories/5618/air-velocity-meters-anemometers?gclid=CLKjiNycrr4CFW4R7AodUgIAIQ&gclsrc=aw.ds

You can see you can get some for under 200 bucks and some more serious ones in the 4000 dollar area. For you I would suspect the cheapo one would work just fine.

As it turns out, here at my company, we had extreme issues with just that area, one of my semiconductor manufacturing machines, called a Sputtering tool, was installed in what used to be a loading dock! Can you imagine a worse place for a machine that has strict temperature requirements? If it got lower than 60 degrees F, some dam oring in the vacuum seal went and the vacuum inside went from 1 E-7 range, a very nice number and sometimes even lower, like 6E-8 range! But get the machine to 60 and all of a sudden its 2 or 3 orders of magnitude higher, which sucked because it didn't suckđź™‚

So I spent a lot of time troubleshooting with helium leak checkers to find the bad oring and maybe found it but it has turned to summer here in the east and won't be seeing 50's for months now so don't know if I nailed it or not till then.

At the other end of the heat spectrum, my computer craps out if it get up to 80 degrees F internally so I had to add a personal chiller to that dam computer would you believe, I adjust the thing closer or farther from the comp to keep it at around 70 degrees where it is happy.

So we had a lot of ductwork to do, adding heating and cooling elements which was no easy trick because we have only X amount of heating and cooling for our labs and Pat had his hands full borrowing from peter to pay paul!

I think we finally convinced our new (thankfully) CEO that we need some serious attention to the heating and cooling system.

They know that now, we have rubbed that in the face of management, especially after the sputtering machine debacle!

Now they KNOW we are going to need a LOT more cooling on the roof since we are expanding our cleanroom to hold a lot more machines like that that really add to the heat load of the whole building.

Another hitch in our plans. which I discussed yesterday:

Our outside power transformer taking in the 13,000 volt line to 480 and then to 240/208/120 etc, is only rated at one quarter megawatt.

We are pushing the hell out of that limit already and for instance, we have a huge air compressor that costs the company 3000 bucks a month just for electricity and so we installed 'baby' air compressors for just three machines in R&D and some production to handle just those so we can shut down the big guy at night saving quite a bit of money.

Those baby compressors just can't handle it, not reliable enough. They are less than 200 bucks each but we have gone through four of the dam things, and now I am pushing for a larger one, like you see at gas stations, much more reliable but of course closing in on a thousand bucks a pop.

The owner of our company didn't mind that but the power limit is crunching us and we have to find 208 3 phase and something like 25 amps to run it and that pushes us ever closer to our 1/4 megawatt limit AND it is going to cost another thou for the electricians to wire it up properly.

So now it is a 2000 dollar project, where we had been spending 200.

But now we have already spend 800 on those stupid baby compressors that keep dying, so I think management can see the light at the end of THAT particular tunnel.

The problem for us with that 1/4 meg limit is the power company refuses to give us a larger transformer, the bastids!

So I told that to the owner of our company and told him it might take upper management words to THEIR upper management to get us a bigger transformer.

The expansion we are contemplating will run us out of power in 6 months the way it is now so it is a serious issue!
6. AThousandYoung
08 Jun '14 23:02
You'll need a beak, two feet, some wings, feathers and a quack.
7. sonhouse
Fast and Curious
14 Jun '14 14:51
Originally posted by AThousandYoung
You'll need a beak, two feet, some wings, feathers and a quack.
My wife just fired her quack....
8. AThousandYoung