Pressure Drop/Head Loss calculation

[PS]

Hello Professionals,

I have come across two methods to determine the frictional pressure drop of FITTINGS (Elbows, Tees, etc.) in chilled water piping. One is to use equivalent length of fittings and the other is by using the local frictional coefficient of fittings. Which one of these methods are preferred most? Because I'm getting two different values as pressure drops by the methods.

Example:
If Black Steel Sch 40 pipe of Diameter 100mm is used for a flow 208 USGPM, fluid velocity = 1.6 m/s, pressure drop = 250 Pa/m, then:

(i) Equivalent Length Method:
An elbow, r/D =1, (90deg) is having an equivalent length of 3.4 m (ASHRAE Fundamentals 1997, Chapter 33, Table 6, Pg: 33.6) . Hence the total pressure drop in the fitting is 3.4*250 = 850 Pa

(ii) Friction Co-efficient Method:
For the same fitting, the friction co-efficient/ K-factor is found to be 0.31. (ASHRAE Fundamentals 1997, Chapter 33, Table 2, Pg: 33.2). The fluid velocity is given to be 1.6 m/s. Then the pressure drop is found by the relation:
dP = (K-factor)*(Fluid Density)*(square of fluid velocity)/2
dP = (.31*1000*1.6*1.6)/2 = 400 Pa.

Would anyone please suggest which one of the methods is the best to proceed? Or else to be conservative, is it wise to always calculate by two methods and take the higher drop?

Thanks in advance for the support.

 

[ProcessHVAC]

Both work well as long as you use high quality data.

 

[dbill74]

I personally use equivalent length method, it is simply easier and faster to calculate.

Why you're getting about half the pressure drop with the other method? I bet you have an error your calculations, which easily happens when you mix systems of units.

 

[MintJulep]

dP = (.31*1000*1.6*1.6)/2 = 400 Pa.

Units do not work.

 

[lilliput1]

I prefer using equivalent length because after totaling up pipe lengths and equivalent length I multiply them by 1.15 safety factor, use the pressure drop in ft of water/100 ft, then add all the pertinent equipment pressure drops (valves, strainers, coils, etc) without applying the safety factor to the equipment pressure drops, to come up with total head loss.
Note also 2017 ASHARE Fundamentals IP edition Table 6 Page 22.7 has ASHRAE Research K values at 4 fps, 8 fps and 12 fps. Interpolating K for the desired elbow gives 0.360715 vs 0.31 in table 4
I prefer to be safe to factor in additional fitting that may be required to clear obstructions and allow for pipe corrosion.

 

[PS]

Thanks a lot @ProcessHVAC, @dbill74, and lilliput1.

I've read in various books and found that for pipe fittings, the ratio of equivalent length to diameter remains constant. And, this value for standard, r/D=1, Sch 40 Black Steel Pipe, 90deg elbow is 20.

i.e. (Le/D)=20. So the equivalent length of Dia 100mm elbow is Le = 20*.1 = 2 m.

But, ASHRAE suggests 3.4 m equivalent length for the same elbow.

To be conservative, I can go ahead with the ASHRAE data. But, this puts me to think always to find values from different sources and compare them. Then proceed with the bigger number, to be in safer side. I'm really confused in selecting source to read data.

Kind attention @MintJulep:
Thanks for your kind response. But, I'm NOT understanding which units you are talking about. Please do explain.

 

[MintJulep]

The units associated with the numbers in your equation.

They do not equal Pa.

 

[Jams_imported]

I think.
dP = (K-factor)*(Fluid Density)*(square of fluid velocity)/2
this formula also for calculating pressure drop in the straight pipe.
since here we do not consider any data related to the bend or variation in the cross section of the fittings.

waiting for the reply

 

[PS]

@Jams:

Thanks for your reply.

There are two types of losses in pipes: Major losses and Minor Losses.
Major Losses are due to the frictional drag between the walls of the pipe and fluid layers. This loss is significant only in straight pipes and often NEGLIGIBLE in pipe fittings such as elbows, tees, etc..
On the other hand Minor losses are due to the dynamic changes in the fluid flow such as change velocity or change in the direction. This loss is significant in pipe fittings (Elbows, Tees, Reducers, etc.)and it is almost ZERO in straight pipes.

The equation "dP = (K-factor)*(Fluid Density)*(square of fluid velocity)/2" is the minor (dynamic) loss equation and therefore, one should NOT use for finding pressure loss straight line pipes.

I hope you understood.