PDA

View Full Version : Duct Taps?


LeadHead
08-24-2010, 04:47 PM
Can anybody tell me why you should not tap your pipe branches out of the top or bottom of your rectangular trunk? I've always tapped the sides for aesthetic reasons but I had a couple of guys tell me that the pressure will be different out of the top and bottom. Is this true? If so, WHY?

Bud
08-25-2010, 03:24 AM
In one of my favorite books "How to design Heating - Cooling Comfort Systems" there is a chapter "Air and How To Move It" The references shown and explained all demonstrate side takeoffs. That's in a perfect world. The important thing to remember is that the duct is part of a system, including all take offs, dampers, runs, registers. And knowing what the pressure drop is across the elbows / takoffs.

There is not one reference in the book (that I can find) that one shouldn't use top or bottom takeoffs in a system.

Back in the day - you would see main branches with divided flow fittings and or you would see a lot of main duct runs inculde reduced side takeoffs as the further down the main you went, they'd take off the side and reduce the width of the main duct and move on...they were pretty impressive and to understand the work that went into these systems is something else.

Understand the system resistance / friction and in your takeoffs throughout the system too, and it's obvious you won't get a larger centerline radius in a top or bottom takeoff to reduce the resistance.

striker12300
08-25-2010, 05:33 AM
Everything I have read or seen doesn't say anything about not going out the top or bottom. I just looked in the smacna, HVAC system duct designs book and every example they show has the takeoffs coming out the top of the duct. I would think were the duct run is and whether or not its going to be concealed has a lot to do with it. If you go out the top, you gain headroom by going between the joists.

tinknockingnick
08-25-2010, 09:13 AM
tapping from the side of a duct with a round take off adds 30ft of equivelant length to the branch line run. rectangular take off from the side adds only 10ft of equiv. length.

a round take off from the top of a trunk adds 50ft of equiv. length, while a rect. one adds only 40.

a sq to rd take off from the side adds 10ft of equiv. length while one from the top adds 40ft.

ref. Unit 37, pg894 refrig and A/C tech. volume 5

tinner104
08-25-2010, 05:14 PM
It makes no difference which side the tap comes off of. The pressure, exerted by the air, inside the duct is equal on all sides within the duct run. Now the air does move faster along sides that are larger versus a shorter side, but this is a result of friction slowing the air down as it drags along the sides of the duct; it does not effect the total amount of air pressure within the duct. No one side of the duct has more or less air pressure since air exerts pressure equally in all directions.

tinknockingnick
08-26-2010, 10:03 AM
it makes plenty of difference. true the internal static pressure will be equal all around, but its the velocity pressure that has the more difficult time being lead out the takeoff from the top due to gravity and turbulance. usually the side of the duct is the shorter dimension which will give you less friction loss and can funnel air out easier to the takeoff because the take off is usually just as big as the duct height give or take some inches. compare that to the wider horizontal area which gives more friction loss and is considerably wider than the takeoff offering a more difficult time for the air to be diverted to the branchline.

Rob
08-26-2010, 05:56 PM
Can anybody tell me why you should not tap your pipe branches out of the top or bottom of your rectangular trunk? I've always tapped the sides for aesthetic reasons but I had a couple of guys tell me that the pressure will be different out of the top and bottom. Is this true? If so, WHY?

The pressure is no different.
The resistance factor is what's different.
Generally speaking, a top take off has twice the resistance factor as a side take off.
The equivalent length of a top take off is 20 feet.
The equivalent length of a side take off is 10 feet.
In an extended plenum system, the closer you are to the unit, the greater the resistance, so the equivalent lengths of each are increased.
Nothing wrong with using top take offs, as long as the system is sized accordingly.
Besides, I can't imagine how you could install residential systems without them.:)