Duct pressure drop --- 12" spiral ruduced to 8" and then blown back up to 12"

[BronYrAur]

Can someone help me out with a friction loss calculation? I have a 12" medium pressure spiral duct upstream of a VAV box. It will carry 1,300 CFM. The duct needs to transition down to 8" spiral in order to pass through a hole in a structural beam. It can then go right back up to 12"

Not sure how to calculate the pressure drop of going down to 8" and then back up to 12"

Thanks for your help.

 

[EnergyProfessional]

ASHRAE fundamentals has equation for flex duct and fittings. This will depend on how the felx duct is stretched. Pressure drop will be very high since 12" already is small for 1300 cfm.

 

[BronYrAur]

just to reiterate, I am talking about hard spiral duct not flex. The transition pieces could be fabricated at any angle desired. I just have to reduce my 12 inch run down to 8 inch in order to get through the beam. I can then go back up to 12 inch.

 

[berkshire]

Bron Yr Aur,
The attached chart may help , but I suspect you may already have that.

Your duct is going to go from 113sq inches to 50sq inches, your head loss is going to be 8" per hundred feet until you hit that reducer, then to keep the same volume of air you are going to have to increase the duct velocity to 3700fpm. Or lose the volume to 580 cu ft. per minute to keep the same velocity. Is there any where else in the beam where you can drill a couple of additional holes to bring your total volume back up? One other suggestion is a booster fan, but that is going to be noisy for that volume of air through that size of hole.
B.E.

You are judged not by what you know, but by what you can do.

 

[317069]

Read ASHRAE or any other fluid mechanic book, engineer has to continue studying all his life

 

[BronYrAur]

I'm looking for a little more help than to just find the answer myself in ASHRAE. I have been sizing ductwork my entire career, but I have never run across this particular situation before.

 

[willard3]

What's going to change is the velocity pressure which will effect the pressure drop.

 

[katmar]

Assuming that you use 30 degree (i.e. 15 degree half angle) tapered reducers and a 3 ft section of straight 8" pipe, and it is all made out of relatively smooth material you can expect a pressure drop of about 0.4 inch WG. Depending on the thickness of your wall you may be able to reduce the length of the straight section. If the straight section is only 1 ft long, together with the reducers which will each be about 8" long, the whole arrangement would be less than 3 ft long and the pressure drop would be about 0.35 inch WG.

Is the hole in the beam limited in width and height? Would it be possible to use a rectangular duct 8" high but 14" wide to give approximately the same flow area as the 12" duct? I guess the square to round transition pieces could be expensive to fabricate.

I can't get my calculations to match berkshire's value of 8 "WG per 100 ft of 12" duct. I get closer to 0.4 "WG/100ft with my software and with his chart. I have assumed that the 12" and 8" are the IDs of the ducts. If the IDs are actually less then of course the pressure drops will be a bit higher.

There was a very useful HVAC manual posted by bimr in a recent thread. See bimr's first post on Nov 17 in

http://www.eng-tips.com/viewthread.cfm?qid=374928

See page 5-47 of that manual for the reducer values - but I have used my own software to get the numbers above so using this manual would be a good crosscheck.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[dynamo78]

Sorry Berkshire but cant reconcile your numbers in my duct calcultions.Can you please expound on htat more.
Thanks

 

[dbill74]

Bron,
Do you have a copy of the SMACNA HVAC systems Duct Design manual. Formulas for calculating the losses in the adapters can be found in chapter 14.

Just to see what I would get, I ran thru your exercise. The pressure loss in the reducing fitting will be approx 0.01" wg; on the enlarging side will be 0.21" wg. Treat these like you would any other fitting, add them to your total pressure losses. Using 5x diameter or 40" of 8" dia. duct to allow the flow to stabilize before going back to 12" dia, 2.4"/100ft loss from my ever handy ductalator, I estimate your total loss to be 0.3" wg.

Hope this helps.
B

 

[BronYrAur]

The beams have round holes 8" in diameter, so my duct ID would be a little less. I ran the numbers in that manual and came up with around 0.45" net loss. I had about 0.8" of loss through the contraction and a regain on 0.35 through the expansion. That is close to your numbers, katmar. Thanks for pointing me to that manual. With a short piece of 8" duct to get through the hole, I'm at 0.5" overall net drop.

 

[berkshire]

Dynamo78,
Sorry that was a fat finger error on my chart, the 12" diameter line is hidden under a bunch of other numbers, I followed the wrong line to the edge of the chart. it should have been .4
B.E.

You are judged not by what you know, but by what you can do.

 

[rogzog]

Perhaps you could fabricate two "Y" fittings to transition from 12" round to two 8" round or smaller, and pass through the beam with two 8" round or less dia. if needed and on the other side go back to a single 12" round.

 

[berkshire]

BronYrAur,
Can you get more than one 8" hole in the beam, or are you stuck with that hole? Alternatively, Is there enough ceiling height for an equal area transformer? I am presuming there is not, because as long as you have been on this forum you do not ask these kind of questions.
B.E.

You are judged not by what you know, but by what you can do.

 

[BronYrAur]

Thanks everyone for your replies. We had a structural engineer look at the beams, and we can increase the holes sizes in order to fit the larger duct through.

 

[bwilliams86]

Another solution could have been a round to square transition.....an 8" x 14" rectangle would produce almost equivalent head losses per distance to the 12" dia duct.