Mods, this might go in General Questions, but I’m asking for opinions/advice so I figure it goes here. Move it if you wish.
So I’m looking on a DIY discussion board and finding out that, for each 90-degree turn in my ducting, I’m effectively adding the air-resistance of 15 more feet of straight* ducting. So, naturally, they all suggest avoiding those turns as much as possible when planning my duct layout.
And, while I happened to be talking to the building engineer where I work, I mentioned my duct-work planning and he also emphasized avoiding 90-degree bends if at all possible. So I asked, “Well, what if I put in a 45-degree bend at either end?”
“Oh,” he nodded enthusiastically, “That should be fine.”
Huh? What? The elbow connectors# are adjustable from straight to right-angle bends. What if I put in three 30-degree bends? Aren’t the bend-angles additive? Isn’t the flow-resistance cumulative, as well? Can someone explain (in layman’s terms) why not?
–G?
- I comprehend the gist of the problem with flexible (corrugated) ducting; I’ll be avoiding that stuff, as well.
Yeah, I know they’re more expensive than straight tubing, but that’s not my point.
The actual pressure loss across an elbow is a function of the loss coefficient and the velocity pressure. The velocity of the air is dependent on the cross section of the duct and the flowrate. The loss coefficient is a function of the duct geometry. Elbows with a larger inside radius have lower losses than a short radius or mitered corner. The trade off is the a large radius elbow takes up more space and uses more sheet metal than a smaller elbow. Turning vanes can lower loss coefficients in elbows. And yes, pressure loss is cumulative.
BTW, it’s been a while since I have calculated duct pressure drops, but if I am looking at my SMACNA reference correctly two 45s would seem to be slightly worse that a single 90, with a single 45 having 60% of the pressure loss of a 90.
You are right, they all add up. Smooth, long radius bends help some but you are not going to find them in consumer grade furnace duct work.
Dennis
Dag’s first paragraph strained my comprehension up to the last couple sentences, so it took me a while to let it all simmer until I got it. That’s my problem, though, not his fault. His second paragraph is much more clear to me.
I’ve got plenty of space up above my ceiling and I’m doing the work myself. However, while I’m a bit reluctant to cut-and-connect too much ducting, a pair of 45’s or a trio of 30s doesn’t sound too daunting. Then again, I figure if I start messing with six 15-degree bends in a big gentle curve, I might as well be using flexible duct.
Dag, where’s that SMACNA reference you were looking at? Is it a chart? I’d rather not have to wrestle with the mathematics of figuring out duct diameters and turn radii or such. I consider myself numerophobic. 
–G?
Head loss through fittings (i.e., bends, restrictions, valves, etc.) scales with the square of the fluid velocity. The scaling factor is specific to each fitting. You can see some loss coefficients here*. According to that page, a 90-degree elbow has a coefficient of 1.5 and a 45-degree has 0.4. A pair of 45’s will be almost twice as efficient - that is, will reduce the pressure of the air flowing through the duct half as much - as a single 90.
*Those are for pipe or tubing and not ductwork, but I bet you could find some duct-specific numbers that are consistent with these.
Here’s a duct-specific table. If you’re using sharp turns, two 45’s are 20-25% more efficient, but if you use rounded bends with larger radii it makes bigger difference.
It’s a HVAC systems duct design book. Unfortunately the math is hard to get away from. It’s not particularly difficult to calculate, but you can’t get away from it. However, I will post some links later that may be helpful in showing what I am talking about. Bear with me, it may be a day or so before I get to my desktop. Copying and pasting links is dreadful on my phone.
Well, I actually found the page from the SMACNA bookI was looking at. the lower the value of C, the lower the pressure loss through the fitting. The bigger the sweep (larger value of R/D) the lower the loss. So a fitting like thishas a lower loss than a fitting like this (ignore the duct diameters, I am only showing these for general geometry). If you do go with an adjustable fitting, note that they will leak more air due to their construction, so you probably want to seal the seams with a mastic like this. Seal all the seams that you can get to, if your duct isn’t already sealed.
The two 45s just generally become a swept 90. The harsher the turn the higher the loss and two 45s at the same bend rate as a 90 will increase the radius of curvature just by adding the the straight portions of the fittings. Like others have quantified with number the effect isn’t huge and you also get some losses just due to each fitting’s connections so a swept 90 is generally better than two sharp 45s.