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Sizing Inlets and Outlets For Manifolds Correctly

03/28/2017 8:13 AM

I usually use the cross sectional area for the larger inlet and divide the smaller outlets to determine how many lines the larger hose can supply. Is this the correct way?

Also, if i was to use a smaller inlet for more hoses how would i determine the pressure or flow rate loss?

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#1

Re: Sizing inlets and Outlets for Manifolds Correctly

03/28/2017 8:21 AM

That's OK for low flow rates. Need to do more complex calcs for higher flow rates, including the viscosity (and probably density) of the fluid (not my field).

Plenty of calculators on-line. (e.g. http://www.tlv.com/global/TI/calculator/water-flow-rate-through-orifice.html).

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#7
In reply to #1

Re: Sizing inlets and Outlets for Manifolds Correctly

03/29/2017 12:13 PM

http://www.freecalc.com/fricfram.htm is a pretty good calculator that allows you to add fittings, and other adjustments,

You need to calculate losses for each pipe diameter by itself, branch & main, start with one pressure/flow you need at the usage point, and add to it the losses of each pipe section, to determine your pump pressure capacity needed.

The pressure loss through your header varies as the flow drops off, calculate each section with the correct flow, until you get the feel for the difference.

When calculating two different size branch flow Tees (like 2" main & 1/2" branch, I use the smaller tee size to give conservative results.

Usually a header is sized with a much larger section area than required, to provide a uniform pressure to each branch, no matter where it connects on the header pipe. The supply pipe to the header is sized for low (economic) friction losses, usually 3-6 fpm for design.

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#2

Re: Sizing Inlets And Outlets For Manifolds Correctly

03/28/2017 10:57 AM

As a consequence of Darcy-Weisbach, equivalent main diameter = (∑i=1n di2.5)0.4.

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#3
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Re: Sizing Inlets And Outlets For Manifolds Correctly

03/28/2017 11:50 AM

Will this provide me with what a smaller feed line will provide to the individual smaller hoses on the manifold?

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#5
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Re: Sizing Inlets And Outlets For Manifolds Correctly

03/28/2017 12:48 PM

No, because the equation cannot tell whether there is a blockage or a leak.

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#4

Re: Sizing Inlets And Outlets For Manifolds Correctly

03/28/2017 12:47 PM

A <...flow rate loss...> is really a "leak".

<...the cross sectional area for the larger inlet and divide the smaller outlets to determine how many lines the larger hose can supply. Is this the correct way?...>

It takes no account of the friction losses in individual branches. Analogy time: if were to wire 8 different value 0.5W resistors between the + node and the - node in an electrical circuit one wouldn't expect the same current to flow in each of them when one switched it on.

<...determine the pressure...loss...> By iterative calculation. The Darcy-Weisbach equation will do it, though many candles will be burnt doing this sort of thing by hand. Measuring it or using network analysis software would be two improvements.

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#6

Re: Sizing Inlets And Outlets For Manifolds Correctly

03/29/2017 8:19 AM

You do not give us the fluid. For hydraulic systems the general rule is by the flow rate/speed:

suction - 5 fpm

pressure - 20 fpm

return - 10 fpm

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#8
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Re: Sizing Inlets And Outlets For Manifolds Correctly

03/29/2017 12:56 PM

Fps, rather than fpm.

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