Calculating equal flow through parallel flow pipes?

Mar 2018
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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
 

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Apr 2017
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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 2018
5
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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.
 
Apr 2017
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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 .
 
Apr 2015
1,035
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Somerset, England
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.
 

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Mar 2018
5
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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
 
Apr 2015
1,035
223
Somerset, England
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 2018
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I understand what you are saying. Parallel flow, ladder design to reducing the size of pump needed to circulate the water. This is a preheater/precooler for an vac system in a new home been constructed. Ground temps 8' below at a constant 12 degree Celsius where I am from. This in the summer will be circulated through an coil that has been installed in the furnace for precooling, and in the winter will be used to precondition the air that feeds my HRV I the home.

I was originally going to build in wth all 4" pipe, it will be installed below basement slab when they are excavating. But I then realized that most likely the cross sections furthest away from the inlet/out will have the most restriction and hence will not get much flow through them.

Thats when I thought it might be wise to balance the flow so to speak.

I am not able to, but if I was able to install some type of flow valves in each cross section I would be able to get equal flow most likely, but these can be installed in a buried system like this.

I appreciate the help, if this seems to hard/ or impossible to figures out I may just stick with all 4" piping I guess at this point.

Let me know your thoughts.

Fernando
 
Apr 2017
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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.
 
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Mar 2018
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That is frustrating I did respond with what this was for but it did not post, I will retype it.

To answer the previous question...

1. I decided on the parallel flow to reduce the size of the circulating pump needed as opposed to a series flow design

Quick overview of what this is for, it is a passive geothermal system to preheat/precool air going into a heat exchanger coil before my HRV on a new construction home. The pipe will be buried 8 feet under ground where ground temps are a constant 12 degrees Celsius up here in Canada all year round. Water will be used in the tube not air as I want heat sink effect of the water as opposed to air.

I like the idea of simply locating the outlet opposite end of the design like you mention. Seems like a very simple solution to my problem.

If I did that, would it be best to simply increase the diameter of both the inlet and outlet header, keeping all the cross sections at the same 4" diameter pipe?

Also the only question I would have with this suggested setup would be if there would be any issues extending either the inlet or outlet so I can have both pipes coming out in the same location in the basement? So they are not on opposite ends of the basement.

Thank you for the suggestion and advice.