elbow construction technique vs radius of bend

[James Baker SD]

I am slowing assembling my ducting and trying to keep the static pressure losses as low as possible. The nordfab pieces I am using have 3 types of 90 degree elbows. The standard elbow (1.0CLR), a longer radius elbow (1.5CLR) and a still longer radius (2.5CLR). Simple to rule out the 1.0CLR unless nothing else will physically fit in a particular location.

My question concerns the other two. The 1.5 CLR (normally it would be worse) is formed from two halves of pressed steel with a pair of welded seems running PARALLEL to the air flow. The 2.5 CLR is formed of 10 short segments of tube thus having 9 welded seems running ACROSS the air flow. Without sophisticated test equipment, which is likely to have the lower status pressure loss? Will the shallower curve of the 2.5 CLR more than make up for the extra "roughness" of the inside wall? I always thought the main reason that flex hose was so bad was how much turbulence it creates inside.

James

[ian maybury]

Maybe take a look James at the losses for each type of bend. Google 'engineering data Cincinatti Fan', open the .pdf for the manual and go to the section listing these.

They have an R= 2D 90 deg elbow as causing the same pressure drop as 7ft of straight duct (you can extract pressure drop/100ft run of duct for various diameters and air speeds in the table a bit further on), and an R= 2.5D as causing 6ft. i.e. there's minimal difference between the two - so a rough inside could even tip it the other way.

An R= 1.5D elbow is quite a bit more restrictive at 12ft.

My take on this issue is that it's primarily the layout that determines the type and number of bends needed. Looking at photos of installations suggests that it's thinking the layout through before cutting metal that matters most - that's being pig headed and refusing to rush to installation until we truly have an optimum arrangement for our situation that truly minimises the number of bends required. You see so many using what look very like superfluous bends - especially around cyclones.

One way of looking at it is that once the layout is decided our choice as to how many bends (and the required radii) becomes quite highly constrained - we don't have that much choice...

ian

ps envious of your Nordfab ducting :)

[Michael W. Clark]

I would probably try to use the 2.5D where possible, but it is likely you will need some 1.5D as well due to space restrictions. Are you sure the 10 segment elbows are welded? This sounds like a lot of labor for duct this small. Maybe they are crimped?