Elbow Radius

[Dan Case LR]

I know long-radius elbows are the best thing to use for dust collection. I have a couple of places in the ductwork I'm installing where reality and theory collide and the result ain't pretty. In at least one of those locations, there is no choice but to use a 1x radius elbow.

For all the discussion and writing on the topic, I can't find anything that clearly quantifies the performance difference between different radii and gives usable data to factor into my system calculations ('just don't use them' doesn't cut it here). Can anyone point me to a definitive reference that shows losses for various radii in and airflow system?

Thanks!

D.

[Carl Weber]

Dan, I am by no means an expert. I was also curious. So I Googled and this is what I found. Losses look to be decreased half again when the Radius of the 90o greater then the duct diameter. Also included below is an example and reasoning for the friction in the curved transition. (it is for fluid but same principals should apply) http://www.engineeringtoolbox.com/minor-loss-air-ducts-fittings-d_208.html http://nptel.ac.in/courses/112104118/lecture-37/37-2_losses_pipe_bends.htm:

Minor Loss - Head or Pressure Loss in Air Duct Components - can be expressed as
hminor_loss = ξ v2/ 2 g (1)
where
hminor_loss = minor head loss (http://www.engineeringtoolbox.com/minor-pressure-loss-ducts-pipes-d_624.html) (m, ft)
ξ = minor loss coefficient (http://www.engineeringtoolbox.com/minor-pressure-loss-ducts-pipes-d_624.html)
v = flow velocity (m/s, ft/s)
g = acceleration of gravity (http://www.engineeringtoolbox.com/accelaration-gravity-d_340.html) (m/s2, ft/s2)

Minor loss can also be expressed as pressure loss (http://www.engineeringtoolbox.com/minor-pressure-loss-ducts-pipes-d_624.html) instead of head loss.
Minor loss coefficients for different components common in air duct distribution systems


Component or Fitting

Minor Loss Coefficient
- ξ -



90o bend, sharp

1.3



90o bend, with vanes

0.7



90o bend, rounded (http://www.engineeringtoolbox.com/minor-loss-air-ducts-fittings-d_208.html)
radius/diameter duct <1

0.5



90o bend, rounded
radius/diameter duct >1

0.25



45o bend, sharp

0.5



45o bend, rounded
radius/diameter duct <1

0.2



45o bend, rounded
radius/diameter duct >1

0.05



T, flow to branch
(applied to velocity in branch)

0.3



Flow from duct to room

1.0



Flow from room to duct

0.35



Reduction, tapered

0



Enlargement, abrupt
(due to speed before reduction)
(v1= velocity before enlargement and v2 = velocity after enlargement)

(1 - v2 / v1)2



Enlargement, tapered angle < 8o
(due to speed before reduction)
(v1= velocity before enlargement and v2 = velocity after enlargement)

0.15 (1 - v2 / v1)2



Enlargement, tapered angle > 8o
(due to speed before reduction)
(v1= velocity before enlargement and v2 = velocity after enlargement)

(1 - v2 / v1)2



Grilles, 0.7 ratio free area to total surface

3



Grilles, 0.6 ratio free area to total surface

4



Grilles, 0.5 ratio free area to total surface

6



Grilles, 0.4 ratio free area to total surface

10



Grilles, 0.3 ratio free area to total surface

20



Grilles, 0.2 ratio free area to total surface

50




310910

[mike mcilroy]

If you go to Grizzly's site and click on one of their cyclones you can get the instruction manual which has a chart like the one above but also with wyes and flex of different diameters. also has the formula for calculating the SP of your branches.

[Michael W. Clark]

Industrial Ventilation, A Manual of Recommended Practice for Design, published by ACGIH is the text most of the industry uses and refers to. There are also some HVAC reference books that can be useful too and the NYB Engineering Letters.

I assume you are wanting to know the relative difference in static pressure losses for a 1D elbow versus a 2D elbow. It depends on the elbow whether it is smooth or gored (sectioned) and number of gore sections. I think you are using the adjustable elbows, so lets assume a 5-section gore elbow. A 1D elbow has a loss factor of 0.33VP and a 2D elbow has a loss factor of 0.19VP. The VP is velocity pressure which is related to duct velocity. If you know your duct velocity, you can calculate the VP, VP = (Velocity / 4005)^2 for standard air. For example, VP = 1"wg for 4000 FPM duct velocity. Your elbow losses would be 0.33"wg and 0.19"wg for the 1D and 2D elbows, respectively at 4000 FPM duct velocity.

How this affects collection and system performance will depend on your fan capability and the losses of the rest of the duct run.

Mike

[Ole Anderson]

For electrical we have Julie, for dust collection we have Michael. Thank God for the Creek!