losses in 45deg elbow vs 90deg elbows

[Don Jeansonne]

The tables for static pressure losses show that a 45deg elbow is half the loss of a 90deg elbow. If that is the case, then using 2 45's with a short straight section to lengthen out a 90deg turn doesn't really buy you anything.

Is that the case or am I missing something?

Please enlighten me.

[Michael W. Clark]

Don,
You are exactly right.

The 45s would be advantageous if they allow you to construct a shorter duct run, "cut the corner". This is only because you have less straight duct losses than if you went all the way to the corner. Putting two 45s back to back has the same loss as a 90. Three 30s have the same loss as a 90, Four 15s has the same loss...etc.

Mike

[Ryan Baker]

It depends on the radius of the turn. If the two 45s end up with the same radius as the 90, then you don't gain anything. If putting the two elbows together results in a larger radius turn (with a smooth curve), you will have a gain, which is possible with some kinds of fittings.

[Peter Kelly]

It depends on the radius of the turn. If the two 45s end up with the same radius as the 90, then you don't gain anything. If putting the two elbows together results in a larger radius turn (with a smooth curve), you will have a gain, which is possible with some kinds of fittings.
Exactly why ducting manufacturers sell long sweep elbows versus tight ones. One of the many advantages of using steel pipe over plastic.

[Michael W. Clark]

It depends on the radius of the turn.

The SP loss depends on the radius of the individual elbows, duct roughness, number of sections (gores), etc.. The loss tables published in ACGIH are only published for 90s. If you have a 45, then you have half of a 90 of the same configuration. If you have two identical 45s in your system, when you add them together, you get the same elbow loss as if it was one 90 of the same type.

Duct losses are calculated to the intersecting centerlines of the fittings. This is why you might have slightly less loss with two 45s because the duct length is shorter. (The diagonal is shorter than the sum of the two sides of a right triangle). It takes quite a bit of offset (in feet) to see a significant SP reduction.

The best bet is to run longer radius fittings. A 1.5D radius el (radius = 1.5xdiameter) has 30% less loss than a 1D el of the same type. The incremental cost will be less expensive than purchasing twice the fittings.

Mike

[Jim Andrew]

When I was looking at cyclone systems, I called the sales for Woodsucker, and was informed that straight runs are practically "invisible" to the collector. Yes, elbows do drag down the system, so use as few as possible. And think about a stick being sucked up from your tablesaw, a short 90 will leave that stick stuck in the 90. Better to gradually turn a corner, so the stick goes in the barrel.

[David Kumm]

There are tables to calculate all the losses but velocity really adds resistance. I run my system at at 6-7000 fpm to compensate for older machines and pipe and fitting size become critical. Slowing even a portion of the velocity by using larger pipe and fittings makes a huge difference as every " SP costs lots of CFM. Sometimes that last inch is the difference between great and mediocre. Dave

[Don Jeansonne]

Thanks to all your responses. You cleared up alot of things for me.

[Michael W. Clark]

There are tables to calculate all the losses but velocity really adds resistance.

Absolutely, the SP loss varies with the square of the velocity change (or CFM change for a given duct system). If you double the velocity, your SP loss will be 4X.

Mike