Physics Help Forum Calculating equal flow through parallel flow pipes?

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 Mar 27th 2018, 05:50 PM #1 Junior Member   Join Date: Mar 2018 Posts: 3 Calculating equal flow through parallel flow pipes? Hello everyone, I am hoping some here might be able to help with what I am hoping is a simple problem for you all. Thank you for any help I get with this ahead of time.... I am looking to run water through a series of parallel water pipes and am looking to achieve equal flow through all sections of the pipe system,. The fluid circulating will be water, looking to use 4" diameter pipes. I attached a very crude drawing so you get the idea. I am taking a wild guess and assuming that either the diameter of each cross pipe will need to be larger and larger the further it moves away from inlet OR the diameter of the inlet header will need slowly transition to a larger and larger diameter as it reaches the last cross pipe. Is this correct? If so how do I calculate this? Is there a online calculator, or a simple formula a layman could work through? Thank you all for your help, I appreciate it. Fernando Attached Thumbnails
 Mar 27th 2018, 10:59 PM #2 Senior Member   Join Date: Apr 2017 Posts: 410 The first question is are these pipes arranged as in the picture , like a ladder going up against gravity ? Or are they all laying on a horizontal surface ?? This will make a difference... I'll assume all pipes are horizontal ... I think you mean you want to have equal flow through all of the 5 cross pipes . how to do this will depend on how fast the flow is ...Generally the best way is to have the two main pipes bigger than the cross pipes . With slow flow if the main pipes are 4" then the cross pipes can be 3" ... there really is no need to vary the diameter of the main pipes , unless there is extreme distance between each cross pipe ... For a more accurate reply give lengths and spacing of pipes , flow rate , and which pipe do you want to be 4" .. If you explain what this is for I'm sure I could offer other ideas .
 Mar 27th 2018, 11:13 PM #3 Junior Member   Join Date: Mar 2018 Posts: 3 Thank you for the response, they will be perfectly level in a horizontal position. This is a very crude example, it will be more than 5 cross pipes, and I can vary the diameter of the pipes as need, making the headers larger as you suggested. The cross pipes will be about 50 feet long each and each header will be about 70 feet long. There will be roughly 14 cross pipes and the minimum diameter will be 4". Each header(inlet and outlet) will be roughly 70 feet long. The flow rate I really do not know to be honest at this point. I hope this is enough info. I appreciate it.
 Mar 27th 2018, 11:44 PM #4 Senior Member   Join Date: Apr 2017 Posts: 410 The total length of one header will be 70 ft .... 14 cross pipes 50 ft long 4" diam Again flow rate and hence pressure drop is all important to give an accurate answer , but I would guesstimate the header needs to start at a diameter of a few feet for fast flow ... 6" for low flow rate ... everything depends on flow rate .
 Mar 28th 2018, 03:33 AM #5 Senior Member   Join Date: Apr 2015 Location: Somerset, England Posts: 995 Here is the issue Water in a pipe flows from higher pressure towards lower pressure, not the other way round. So to continue to flow along a pipe the pressure must decrease in the direction of flow. I have shown some simplified pressure numbers to illustrate this and what it means for a ladder of pipes. In particular for water to have inlet and outlet positioned as the OP wants, The pressure must decrease from A to E and further decrease in the last cross pipe to F. To flow out in the direction shown (F to J) there must be a similar steady pressure drop along the outflow pipe. But this means the cross pressures are steadily mounting so the pressure drop AJ is 9 times that of the pressure drop EF in the example. Attached Thumbnails
 Mar 28th 2018, 06:53 AM #6 Junior Member   Join Date: Mar 2018 Posts: 3 The flow rate will be slow, I guess this is not an easy question to be solved then? What would you guys suggest or who would you suggest that would be able to calculate this for me? An HVAC engineer/ designer? I don't really know who I would contact to help determine these sizes. Definitely beyond me. Would this be a similar design the way the main trunk and air flows through the duct work of a house to get equal flow to each room. The main trunk is very large, and the further away from the source the cross section of the duct work gets smaller and smaller to keep pressures up so the flow is equal, correct. Any ideas? Thank you guys Fernando
Mar 28th 2018, 08:56 AM   #7
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Join Date: Apr 2015
Location: Somerset, England
Posts: 995
 Originally Posted by clone477 The flow rate will be slow, I guess this is not an easy question to be solved then? What would you guys suggest or who would you suggest that would be able to calculate this for me? An HVAC engineer/ designer? I don't really know who I would contact to help determine these sizes. Definitely beyond me. Would this be a similar design the way the main trunk and air flows through the duct work of a house to get equal flow to each room. The main trunk is very large, and the further away from the source the cross section of the duct work gets smaller and smaller to keep pressures up so the flow is equal, correct. Any ideas? Thank you guys Fernando
I am simply trying to get you to think about what you are asking because some of it is a physical impossibility.

For instance the flow in pipes AE and FJ cannot be constant since fluid enters pipe FJ at G, H and I and leaves pipe AB at B, C and D.

Also I hope you have now realised that you cannot have equal sized pipes and equal flow volumes because of the varying pressure drops.

This can be alleviated by a different configuration geometry of the pipework.

Secondly there is the question of why are you sending this fluid round a ladder network?

Is this a heating system (it would be a very inefficient cooling system) or is it a fire sprinkler or what?

If the pipes are actually carrying and distributing heat, not water, then you will not need or want equal flows since you also have to take temperature drops into account as well as pressure drops.

 Mar 28th 2018, 08:33 PM #8 Senior Member   Join Date: Apr 2017 Posts: 410 I agree with studiot it is impossible to get exactly the same flow ... because the pressure can never be as high away from the source of water as it is for the cross pipe nearest to the source .... But with slow flow rates and big header pipes you can get the flow in the cross pipes almost the same , such that you would never notice the difference , within a few % of each other .... Tell us what you are doing ... I have sleepless nights wondering If you had the flow exiting in the other direction , exiting at the lower right corner , entering at the upper left of your diagram , this will achieve just what you want every crosspipe would have exactly the same flow rate. Last edited by oz93666; Mar 28th 2018 at 08:40 PM.

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