calculation of friction losses 3

I am studying electrical engineering and work on a project regarding solar water pumping. To dimension the pump and the solar array I need to calculate the total head (of existing pipe grids).
I have to calculate the pipe friction losses and I'm searching for a suitable equation. For laminar condition I use following equation to calculate the friction factor: f = 64/RE.
As I have no experience on this field there are some questions:

How important is the roughness factor of the pipe for the friction losses in turbulent flow conditions? Is it negligible?

Which effects have the different roughness factors on the total head? Can I use one general roughness factor for all used materials (piping for village water supply, livestock watering and irrigation)?

I'm grateful for every hind I get!

What are suitable equations for practical calculations?

 

[insideman]

For conditions away from laminar flow, use the following equation.

Head loss = f(l/d)/(v^2/2g)

f is determined from a chart relating loss to roughness and Reynolds number. I can scan and Email you a copy if you wish.

However, for most cases f = .02 will serve nicely.

"Minor losses" from elbows, tees, valves, etc wil frequently completely overshadow the pipe friction loss.

 

[DiaphragmPumps]

Try the following links, they ought to help

Regards

http://www.efunda.com/formulae/fluids/calc_pipe_friction.cfm#calc

http://www.mainind.com/design/

http://www.monachos.gr/greek/calculators/Darcy_weisbach.htm

http://www.geocities.com/Eureka/Concourse/3075/hazen2.html

http://www.freecalc.com/fricdia.htm

http://www.freecalc.com/fricfram.htm

 

[turulato]

If you are working with a pressurized system there is no head, you just calculate friccion losses. At which case you just need a recirculation pump (like the big skyscrapers use) then you would just need to calculated the liquid volume in the system and the velocity at which the liquid would be moving thru the solar panel.
Other than that, stick with Insideman's formula.

 

Thank you all for your advices!

But there is still a problem: the calculation is part of a computer-program.
This program should help non-experts (like farmers) to develop there own PV water pumping system. It includes the calculation and a simulation-part. Therefore I can't use the chart to obtain the friction factor. I have to choose an average factor or calculate it in "real-time". Also there is no constant flow due to the power supplied by pv-modules.
My predecessor has suggested to use the Blasius equation for RE > 2316:

f = 0.316/RE^0.25

in books this equation is said to be accurate for smooth pipes and RE between 5000 and 10^5.
Using this equation for RE near to 2316, I calculated a friction factor of 0.045. That's double of the value you suggested.

So I don't know if I can use this equation. What do you think?

Does anyone know another way to determine f?

 

[butelja]

If you want a rigorous solution for turbulent flow, use the Colebrook equation for the friction factor. It has the disadvantage of requiring solution via itteration. This is not a problem if you are using it within a computer program. The Colebrook equation is used to generate most, if not all, of the Moody diagrams you see in print.

 

[TD2K]

Butelja's solution is the best, I've set this up in my spreadsheets to calculate the friction factor for my flow cases. Once I get into laminar flow, I use f = 64/Re as discussed above. Granted, where laminar flow starts is a little hard to predict, I typically use turbulent flow for anything over an Re of 2000 just to be on the safe side of my pressure drop calcs.

If you don't want to go through an interative solution, there are several approximations that will yield acceptable results over a specific range of Reynold's number. One recommendation is:

http://sellensr.me.queensu.ca/sellens/441/losses.htm

I used to have several papers that discussed various correlations and their accuracy, I'll have to see if I can dig them out from my files at home.

 

[TD2K]

I FOUND IT!!

Sorry, usually my filing system doesn't work so well. This was from an article in Chemical Engineering magazine (the date didn't copy unfortunately).

The equation unfortunately is a little ugly to try and enter here:

I'll scan it and it can be downloaded from:

http://briefcase.yahoo.com/testdog2000

 

[mgp]

I once made these two VBA functions for use in an excel spreadsheet. The first calculates Reynolds no, the second the friction factor (using the result of the first)
The input variables I think speak for themselves.

Maybe they could be of use to you?


Dim FrictionFactor As Variant
Dim Testvalue As Variant
Dim Stopvalue As Variant
Dim MediumVelocityTest As Variant
Dim ReynoldsNumber As Variant


Public Function ReynoldsNo(PipeID, MediumVelocity, MediumDensity, Viscosity_dyn)
ReynoldsNo = MediumVelocity * PipeID * MediumDensity / Viscosity_dyn
End Function


Public Function FrictionFactorMoody(PipeID, RoughnessFactor, ReynoldsNumber)
If ReynoldsNumber < 2000 Then
FrictionFactor = 64 / ReynoldsNumber
Else
FrictionFactor = 0.12
counter = 0
Do
Testvalue = FrictionFactor
FrictionFactor = 1 / (-2 * (1 / Log(10)) * Log((RoughnessFactor / (3.7 * PipeID)) + 2.51 / (ReynoldsNumber * (Testvalue) ^ 0.5))) ^ 2
Stopvalue = Abs(Testvalue - FrictionFactor)
counter = counter + 1
Loop Until Stopvalue < 0.000001
End If
FrictionFactorMoody = FrictionFactor
End Function


Regards
Mogens

 

[KernOily]

Man, you guys need a life... ;-)

I thought I was the only one that read technical papers at night... My wife thinks I'm crazy... Thanks!
Pete
P. J. (Pete) Chandler, PE
Principal Engineer
Mechanical, Piping, Thermal, Hydraulics
Processes Unlimited International, Inc.
Bakersfield, California USA
pjchandl@prou.com

 

Thanks allot!
The article you sent me was very helpful!!
I think I use the most ugly equation (equation (2)). This means only 4 lines of code.
Now I need the roughness of the different pipe-materials. (Especially for plastic-pipes)
The books I found in the library contain no tables.
Does anybody now links or other sources for this information?
Also would be great to get the loss coefficients for fittings valves etc..
I found some tables but they all show different values.

Again thanks allot for your help!!

 

[TD2K]

stkl, post your email address, I'll scan and email you a copy. Sort of surprised you didn't find anything in your library, any good on hydraulics will have roughnesses for various materials. The only ones I've had problems finding is for things like rubber hose and fiberglass piping.

 

[TD2K]

I did a bit of searching on the web this morning. Most plastic piping is sized using the manning equation rather than versions of the Darcy Weisbach equation (which uses a relative roughness to come up with the friction factor). The Manning n factor for clean plastic pipe is 0.01 to 0.009 but this is clean pipe in the lab. There is quite a bit of discussion on the web about whether this is appropriate for the piping after it has been in service (note, n for concrete pipe is typically between 0.012 to 0.013 and flow is directly porportional to n).

I would take plastic pipe somewhere between clean commercial steam pipe (e = 0.00015 ft) and tubing (e = 0.000005 ft) where e is the absolute roughness but a lot of it depends on how clean your water is, treatment, etc because fouling or deposits in the pipe will increase the effective roughness and decrease the diameter (and changes in the diameter affect the pressure loss proportional to the 5th power, that is,
d1/d2 is proportional to (dP1/dP2)^5.

 

[TD2K]

Drat, make that (d1/d2)^5 is proportional to dP1/dP2

 

[jmw]

Is this site any use?

http://www.connel.com/freeware/scripts.shtml

 

[quark]

TD2K!

I opened your briefcase and found valuable stuff inside. (taken enough care to wipe out all the finger prints) I think they are and will be of great help to me.

I appreciate very much of your efforts.

Regards,

 

Hello TD2K,
May be good books on hydrailics have roughness of materials, but all these books seam to be on loan. Again I tried to find some in the library and found (after a long search) one book. It has a short table with roughness of some materials.
There is only one value for glass and plastic together. It is 0.0008 in..
It has no value for aluminum.
If you have a more detailed table, please let me know.

I'm very grateful for your help!!