Reducers

[tinnerjohn]

While making a couple rectangular duct reducers, I started thinking. Is there a rule of thumb or formula for the angle or length of the taper? Is a short or long taper better? Does it really matter? I'm talking residential,under 1000 CFM. John

[wmckane]

I know that a lot of people would say it's not so important, and I've seen some frightening examples of that, but my feeling is that any reducer/transition should be as long and gradual as the space allows. That's the problem with residential, though---usually very tight conditions.

Bill

[tinnerjohn]

Thats been my thought too. In this last case I had some strange OA lengths (12 3/8" and 14 !/4") because I used some cutoffs that were long enough to get the whole part out of. I seem to remember reading somewhere that plenum Takeoffs should have a 45 degree angle. John

[wmckane]

We always made 45* takeoffs and I know that a lot of prefab residential and commercial fittings are too. But then there are the straight ones. My assumption, and experience in a hospital setting, is that in a variable speed or constant velocity situation the degree of takeoff isn't a big factor. However, it's still necessary to reduce the trunk size as takeoffs are installed. Rob or others, thoughts?

Bill

[Rob]

Great topic.
It took me a while to find these.
They are from the system design manual.

[Rob]

Originally Posted by tinnerjohn

I seem to remember reading somewhere that plenum Takeoffs should have a 45 degree angle. John

We make our plenum take offs with the opening 1.5 times the area of the duct.
Depending on the rise and the width, the bottom angle can get quite steep.
If we don't like the angle (maximum 45 degrees) , we'll make the take off longer.
Sometimes have to cheat a bit on the depth if space is limited.

[tinnerjohn]

Thanks Rob. I'm going to try that 1.5 times on the next job. The prefab takeoffs are 10" at the plenum X 8" duct, but when time allows, I prefer to make as many of my fittings as possible. Thats actually been a selling point on a few jobs. It seems fabbing duct and fittings is a dying art, and some customers appreciate someone who does it! Plus, it seems like I'm getting a little anal in my old age and the mass produced fittings just don't cut it. John

[Bradleyc1982]

Originally Posted by Rob

Great topic.
It took me a while to find these.
They are from the system design manual.

How much do you reduce by in a long section of duct to keep the pressure constant?? And how often? Do you reduce by 100mm or 50mm?? What is the best book available to show this? Thanks guys

[ductworker]

I am not certain about low velocity systems/domestic. But any duct fitting with a slope angle of more than 24 degrees (1:2.4) will put extra resistance in the duct. Sure at times this is impossible and not practical, so that is why the system will require a fan to overcome the duct resistance. Try and keep all rectangular, square and circular offsets and reducers max 24 degree slope and
bends to halve the duct cheek (Rect.duct)dimension, circular duct the Dia.

[tinstructor]

Originally Posted by ductworker

I am not certain about low velocity systems/domestic. But any duct fitting with a slope angle of more than 24 degrees (1:2.4) will put extra resistance in the duct. Sure at times this is impossible and not practical, so that is why the system will require a fan to overcome the duct resistance. Try and keep all rectangular, square and circular offsets and reducers max 24 degree slope and
bends to halve the duct cheek (Rect.duct)dimension, circular duct the Dia.

> I would like to offer a calculation to use to insure that you don't exceed the 24 degree slope on reducers/transitions.(this is NOT an endorsement of any friction/resistance formula. Merely a formula to easily obtain answers for the above)

> Without boring you with the Trig.calculations,it is this. Multiply the reduction by 2.25. The centerline length of the fitting must be AT LEAST this amount.

> Example: a flat on one side reducer goes from 16" to 10". That is a 6"reduction on one side. multiply the reduction by the 2.25 factor. the result is the minimum length for the fitting,13.5" ( 2.25 x 6=13.5)

>>>NOTE (this factor,of 2.25 is only for a criterion of 24 degrees)