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spray wash header design calculations
4

thread378-30321

spray wash header design calculations

spray wash header design calculations

(OP)
I am designing a spray wash system for use underneath a belt filter.  The system will have one inlet and go around the perimeter of the machine on three sides with an additional branch across the width of the machine in the center.  It should look like a backwards E with the inlet on the top left corner.  The machine is approximately 7' wide by 60' long, and the pipe will have flat fan spray nozzles aimed at the floor approximately every 2 feet.  This results in 40 odd nozzles supplied by one inlet at about 30psig and 20GPM.

My question is about how to perform calculations to ensure that all nozzles will operate effectively.  Do I need to do a pressure drop calculation between each nozzle adjusting for the flowrate as I go or is there some rule of thumb I can follow?

Thanks in advance.

Brandon

RE: spray wash header design calculations

The pressure drop for 20 gpm is about 1.8 psi per 100' for 1 1/2" sch 80 pipe.  For 2" pipe, you are looking at 0.4 psi.  

I'd likely run 2" pipe for the first 10 nozzles or so, drop to 1 1/2" for the next 20 nozzles and then 1" for the last 10 nozzles.  That should keep the dP through your header pretty low (less than a couple of psi) and keep the flows through each nozzle fairly constant.  Of course, you could just run all 2" pipe also.

the difference in flow from one nozzle with 30 psi water supply pressure and the last one with 28 psi water pressure isn't that much (0.5 gpm versus 0.48 psi).

RE: spray wash header design calculations

3
The main thing is you need to make sure that the pressure drop across the nozzles is very large compared to the drop across the length of the spray header.  If it's not, then you will get a bunch of water out the first nozzles and the nozzles farther downstream will just go tinkle.  The way you do this is by using an area reduction factor.  The usual numbers I've seen are that the sum of the cross-sectional areas of all the nozzles should be about 10% of the cross-sectional area of the pipe.

Thanks!
Pete

RE: spray wash header design calculations

(OP)
Thanks for the above tips.  They have definitely shed light on what I need to be thinking about.

I am also concerned about the velocity in the pipe.  With a 1.5" pipe (sch 40 galv.), I am only getting a linear velocity of 3.15fps, and with a 2" pipe, it is only 1.91fps.  I know that typically, linear velocity should be between 5 and 12 fps.  However, with 1" pipe or smaller I have the problem of too much drop through the pipe as well as an area smaller than the total area of the nozzles.  There are 43 nozzles nominally 1/4" or 1/8" in size.  I am not exactly sure how close the i.d. of the nozzle is to the nominal size, though I am checking into that.  Any comments on the velocity issue and how to balance it with the sum of the nozzle areas?  Thanks for all your help thus far.

Brandon

RE: spray wash header design calculations

Flow velocity guidelines are "rules of thumb" to start with, before considering pressure drop.  For critical systems, pressure drop requirments override the velocity guidelines.

RE: spray wash header design calculations

The low velocities are not a problem in this case and in fact, are needed to avoid excessive pressure drop through your header that would result in uneven water distribution.

RE: spray wash header design calculations

Suggest you invest in AFT's Fathom hydraulic software. It has spray headers as a standard junction. refer www.aft.com.

I have just used it to design a system for spreading reuse water from a sewage plant over a vcery large area. I did the deign in less than a day. It paid for itself in time alone.

RE: spray wash header design calculations

THE RULE OF THUMB THAT I HAVE USED IS THAT THE PRESSURE DROP OF THE SPRAYS SHOULD BE 10% OF THE PRESSURE DROP OF THE HEADER.

THIS HAS WORKED ON STEEL PLANT.

I DONT KNOW WHERE IT CAME FROM SO I HAVE NO REFERANCES

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