Head Loss from rubber tubing and pipe fittings

[Brown23]

Hey,
I have been told to size a pump for a closed loop system and have been trying to get the head loss calculations right, but I keep finding different references for equivalent feet of head loss from various pipe fittings and through rubber tubing, the loss for the copper pipe seems fairly consistent at 10' of head per 100' of pipe. Can any of you direct me to a site or table that has the right numbers for equivalent head loss through pipe fittings for 3/4" pipe (elbows, tees, unions, etc.) and 3/4" rubber tubing with an ID of 1/2"?

 

[MikeHalloran]

For short tubing runs, the ID of the barbed nipple that the hose attaches to will tend to govern the flow; treat it like an orifice. Yeah, it's a bitch to get the information unless you have the actual part in your hand.

Similarly for pipe fittings; smallest ID tends to limit the flow no matter what else is attached. ISTR that normal cast elbows are usually taken as ~50 diameters of straight pipe. Drilled elbows and branch flow in all tees are much worse. You can find tables for most fttings; just watch out for 'king nipples' and other odd fittings.






Mike Halloran
Pembroke Pines, FL, USA

 

[LittleInch]

Your pressure losses will be dominated by the rubber hose and high friction fittings such as the nipples and flow through the branch of any tees, sharp elbows etc.

Bear in mind that the friction losses for the same flowrate are in the order of Di1/Di2 ^5. Hence difference between 0.75" ID to 0.5" ID is in the order of 8 times more pressure loss.

I can only asume you have a fixed flow as you don't quote it, but plainly pressure losses vary in proportion to the flowrate^2 for the same size pipe.

If in doubt go for a bigger centrifugal pump, some sort of control valve or variable speed and a flow meter to monitor your flow rate.

Good luck.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Brown23]

Thanks for the tips, the flow rate is constant and needs to be at 5gpm. The loop won't have any branches and the only tee connection will be for the expansion tank.

Can anyone recommend a good rubber tubing? I need it in the loop because it connects the stationary section on the ground with the solar cell which tracks the sun. The biggest concern I have about it is that the tubing needs to withstand temperatures of possibly 180 to 200 degrees Celsius.

 

[MikeHalloran]

If your working fluid is water, you'll need 'steam hose', and your tracker will need a sturdy drive, because steam hose is heavily reinforced.



Mike Halloran
Pembroke Pines, FL, USA

 

[Brown23]

The fluid is a 50/50 water/glycol mix (antifreeze), so if I'm thinking right rubber hose would work fine. The type I'm looking at currently is weather resistant EPDM tubing, mainly because of its temperature rating.

 

[tkall]

I work with small diameter pipe often and was not satisfied with the "one size fits all" minor loss tables. The internal design of fittings varies from one manufacturer to the next and if the system response is controlled or potentially controlled by minor losses the tables are not adequate. If knowing is important (it was to me) I suggest you set up a bench test and measure the loss coefficient of individual fittings and hose. I did this, spent days at it, and was quite surprised by the results.

 

[katmar]

Lots of good advice given above. With the low Reynolds numbers you are working at and with these pipe materials you are safely in the "smooth pipe flow" regime so you do not have to fuss too much about the roughness of the piping.

The resistance factors for pipe fittings vary with both the pipe size and the Reynolds Number (and of course with the fitting geometry). In your case with small diameters and low Re Nos your resistance factors could easily be double the "one size fits all" tables. This type of question comes up fairly often and I have tried to answer a range of them in a document available at

http://www.katmarsoftware.com/articles/pipe-fitting-pressure-drop.htm

Unfortunately in trying to answer all the questions the document got a bit long, but you can skim-read through to the relevant bits. The references for the methods are given at the end.

@tkall - are you able to publish your test results? It would be very interesting to compare them with those from a state of the art method like Darby's 3-K.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[tkall]

katmar:
Good article, I enjoyed it. As far as publishing goes...I just don't have the time because I am spread so thin. I see you write software. I was writing a program and during the field testing I encountered some screwy results and figured it was time to quantify the minor losses of the fittings more carefully. I built a test apparatus and measured pressure loss as a function of velocity for several different fittings and diameters. In a few instances, the standard tabulated values were dead on, generally, however, the minor loss coefficients I measured differed significantly. A difference worth noting was whether the fitting was discharging freely into the atmosphere or was connected to a length of pipe. At any rate, I coded the measured loss coefficients, recalculated the hydraulics and tested the system. These results were right on.

Someday I would like to spend more time and really dig into the question but something else always begs for my time.