35 degree bend pressure drop calc
35 degree bend pressure drop calc
(OP)
Hello,
can someone give me the prompting on which formula must be used to calculate the pressure drop through the non-standard angle bend. (assume) I know r/d ratio and angle as well.
"Crane" recommends to use the following formula, but it looks strange:
Kb=(n-1)x(0.25 x Pi x ft x r/d + 0.5K) + K
where n is the number of 90 deg bends, but for eg. 35 deg bend it makes 35/90=0.3889. So, I should substitute this value in place of "n" ???
thanks everyone in advance
can someone give me the prompting on which formula must be used to calculate the pressure drop through the non-standard angle bend. (assume) I know r/d ratio and angle as well.
"Crane" recommends to use the following formula, but it looks strange:
Kb=(n-1)x(0.25 x Pi x ft x r/d + 0.5K) + K
where n is the number of 90 deg bends, but for eg. 35 deg bend it makes 35/90=0.3889. So, I should substitute this value in place of "n" ???
thanks everyone in advance





RE: 35 degree bend pressure drop calc
90o std. elbow: 30
45o std. elbow: 16
90o long radius elbow: 20
for street elbows:
90o: 50
45o: 26
For a square corner elbow: 57
For a close pattern return bend: 50
A smaller-than-45o non-standard smooth elbow would have a smaller value than a 45o std. elbow, thus, using the later, one would be on the safe side. True ?
I know the above is not what you expressely wanted. It's only an idea that may result in saving time.
RE: 35 degree bend pressure drop calc
I have the Crane Techinical Paper 410-C at hand and can not find the formula you are referring to. In Appendix A-27 the Resistance of bends greater than 90 degrees is based on the formula:
L/D = Rt + (n-1) (Rl + Rb/2)
where:
n = total number of 90° bends in the coil
Rt = total resistance due to one 90° bend, in L/D
Rl = resistance due to lenght of one 90° bend, in L/D
Rb = bend resistance due to one 90° bend, in L/D
Though I can see similarities in the formula, I see no need to derive the formula. The CRANE formula is used to calculate the total equivalent lenght of 2 or more elblows in succesion (ie. elbow+elbow+elbow). As the total pressure loss is less than the sum of the pressure loss for all the 90° elbows.
However, what you try to say or do is this.
If I have an elbow with a 35° bend instead of a 90° bend, can I prorate the pressure loss using the "n" value.
The answer is NO as the formula pertains to a whole number of 90° elbows and not a fraction. Incidently if you plug in n = 35/90 = 0.389 then the first part of you equation becomes negative. Depending on your other variables Kb can be negative too, which is not possible.
Please refer to pages 2-12 and 2-13 of the Crane book and you will see that various investigators demonstrated that the Kb coefficient is actually not that well defined with respect to r/D.
So, what value to use in your case.
I would use an equivalent length for a 45° bend as suggested by 25362 and be done with it.
The answer will be close and it will save you time.
Krossview/OK
RE: 35 degree bend pressure drop calc
But it's correct in this case or not? That's the main question!
thanks,
RE: 35 degree bend pressure drop calc
You are correct in saying the results of the L/D formula will still be positive and indeed it is less then Rt.
I did a quick table with n = 35/90 and various r/D ratios as per Crane's graph on page A-27.
However, I was actually refering to your original formula and Kb is negative with an r/D = 18, K = 10 and Ft = 1.
I still believe the answer is not correct as the formula applies to more then one bend in succesion and not the fraction of one bend. I am not going to split hairs about finding an exact value as these formula's are based on emperical findings. One can spend a lot of time on this without gaining much value in accuracy.
BTW, can you tell me on what page of the Crane book you found the formula for Kb as I would like to understand the background for this now.
Krossview
RE: 35 degree bend pressure drop calc
RE: 35 degree bend pressure drop calc
Thanks for your help TD2K
RE: 35 degree bend pressure drop calc
RE: 35 degree bend pressure drop calc
I'm not going to claim I have the absolute latest edition but I have the twenty fifth printing, 1991. You really should up date your copy!
RE: 35 degree bend pressure drop calc
(on the base of Crane).
This works for R/D = 1.5 (R - radius of bending, d - diameter of the pipe), e.g. R=1.5D (D - external diameter)
Ang = angle of bend (no less than 30 deg);
St = 90 deg - a constant;
Ratio=Ang/St;
K=(Ratio-1)x(0.25 x Pi x ft x R/d + 7f) + 14f
Where "K" is taken as for 90 deg bend, "f" is a friction factor, d - internal diameter of pipe