High velocity rectangular elbow pressure drop
High velocity rectangular elbow pressure drop
(OP)
I have been assigned to an air handler replacement project serving a lab of a hospital. The system is 100% OSA, dual-duct and has approx 16,500 cfm at 60 deg supply air to the space. The new system will supply 20,000 cfm and possibly 52-54 deg air range.
At that cfm, the lab will have about 1.5 cfm/SF and the duct velocity will be approx 4000 FPM. This is like a dust collection system. The exist TSP is 7.5" and calcs indicate 3.5-4" ESP for known losses like filters, coils, etc. The new ESP may be 6" using fan laws plus losses due to duct tie-in since this unit is going on the roof.
My question is fitting losses. ASHRAE gives a constant of .12 times the velocity pressure. So that puts me at .12" SP for an elbow since my VP is unity. This doesn't seem close at all.
I have tried Carrier data books and this seems to be all in the same range.
I found a mfr (Aero Dyne) which states their PD is .17" at 2500 FPM. I can use fan laws to est SP drop at 4000 fpm.
Anybody here have any data or experience on rectangular fitting pressure drop for my situation. I'm hesitant on using turning vanes at this velocity not just for SP drop but permanent deformation or blow-out.
At that cfm, the lab will have about 1.5 cfm/SF and the duct velocity will be approx 4000 FPM. This is like a dust collection system. The exist TSP is 7.5" and calcs indicate 3.5-4" ESP for known losses like filters, coils, etc. The new ESP may be 6" using fan laws plus losses due to duct tie-in since this unit is going on the roof.
My question is fitting losses. ASHRAE gives a constant of .12 times the velocity pressure. So that puts me at .12" SP for an elbow since my VP is unity. This doesn't seem close at all.
I have tried Carrier data books and this seems to be all in the same range.
I found a mfr (Aero Dyne) which states their PD is .17" at 2500 FPM. I can use fan laws to est SP drop at 4000 fpm.
Anybody here have any data or experience on rectangular fitting pressure drop for my situation. I'm hesitant on using turning vanes at this velocity not just for SP drop but permanent deformation or blow-out.





RE: High velocity rectangular elbow pressure drop
Where noise is a concern, we limit rectangular ductwork to 2000 FPM and use round or flat oval ductwork up to maximum 4000 FPM. We also provide sound attenuator (packless type in hospitals) at the supply & return air duct.
RE: High velocity rectangular elbow pressure drop
The cold deck duct is 50x16.
I'll check SMACNA tables.
RE: High velocity rectangular elbow pressure drop
I find SMACNA's loss calculator one of the best. I suspect there charts work pretty well too.
RE: High velocity rectangular elbow pressure drop
For Single thickness vanes w/ vane radius 2" and vane spacing 1.5", 2500 FPM, Loss Coef = 0.20
For Single thickness vanes w/ vane radius 4.5" and vane spacing 3.25", 2500 FPM, Loss Coef = 0.22
For Double thickness vanes w/ vane radius 2" and vane spacing 1.5", 2500 FPM, Loss Coef = 0.40
For Double thickness vanes w/ vane radius 2" and vane spacing 2.25", 2500 FPM, Loss Coef = 0.49
For Double thickness vanes w/ vane radius 4.5" and vane spacing 3.25", 2500 FPM, Loss Coef = 0.23
Fitting loss in. wg. = Loss Coef C x Vp
Where Vp = (FPM/4005)^2 inches wg.
RE: High velocity rectangular elbow pressure drop
I just don't agree with those figures.
For Single thickness vanes w/ vane radius 4.5" and vane spacing 3.25", 2500 FPM, Loss Coef = 0.22
VP=.38 at 2500 fpm
PD=.38x.22=.09" ........ I just don't see that.
Aero Dyne vanes are max is 3000 FPM also
RE: High velocity rectangular elbow pressure drop