View Full Version : Square Duct Question
09-20-2005, 02:07 PM
I am a licensed hvac contractor. When I bought my house, the attic was full of flex. I'm wanting to tear this all out & put in a square trunk line that runs the length of my house & just do round take offs to the boots. My downstairs A/C is a 3 1/2 ton with a 4 ton blower on it. I have done a few commercial jobs that have square duct in them & the prints always call for those conical spin ins with the scoop. My question is if the square duct is pressurized can I come off of it with air tights or do I need that scoop on every take off to get my airflow?
Sized per manual D,from http://www.acca.org ,the "scoops are not needed and they increase the ESP.So if you use them add them in to the Total Equivelent Length.
09-20-2005, 02:50 PM
If your trunk is dead-ended like alot of them that I see your'e probably going to be alright with airtights & dampers.If trunk is not dead-ended I'd use scoops.I studied ductwork design till I found out it was way over my head.Dead endinding, if I remember right,(don't hold me to it)engineered ductwork is a no-no.I'm really not sure.But here's an interesting phenomenon--. Years ago before tubeless tires we had tube deflator which would suck a tube flat as a flitter by blowing compressed air across the tip of the valve stem at a 90 degree angle.We even had an automotive A.C. evac tool that worked much the same way.No pump and it worked-still amazes me.Is it possible that this physical phenomenon could take place in ductwork? It's O.K. to laugh,I know I'm Ignorant,but I still want to know.Sorry I couldn't answer your question but this still kinda picks my only brain cell.
09-20-2005, 07:48 PM
I prefer to use the square to round top and side takeoffs as they have more of a cutout area (depending on brand) and taper down to desired pipe diameter...adj. round are next best surface area....any time you can have a bigger exit from the duct and taper down, especially in a 90 degree bend, the better.
09-20-2005, 09:17 PM
We use whats called an S-5 for take off's. The part that goes into the duct is 7", the exit is 6" (can get any size you want though) and are made in 3 pieces so you can adjust them like an elbow. Seem to flow as good as needed in almost all cases.
I wouldn't worry about spinin's, these are basically the same thing as a bell mouth spin in, but much faster and easier to put in.
The venturi effect (I believe is what it's called when you blow perpendicular to a tube, and it lowers the pressure). Next time you're drinking from a straw blow across the top. You should be able to spray the person sitting across from you lol. But since duct is under pressure (when sized right) the air will follow the path of least resistance, which almost always is out of the take offs and out of the registers.
09-20-2005, 11:32 PM
amickracing--WOW! Your brief but informative post was great.I can't tell you how much it confirmed many personal theorys in several different feilds I have ponderd.I'm so analytical it drives me crazy.I have to know how everthing works down to the atomic level before I feel confident at what I'm working on.My Brain cell was so happy It did a backflip while reading your reply.I love to learn from intelligent people.I've come to admire a lot of you!Thanks Guys.
Low PD takeoff are nice,but they don't reduce the TEL very much.
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