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Old 09-25-13, 08:59 AM   #38
stevehull
Steve Hull
 
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The flow through each of the parallel circuits (each of the legs) will only be the same, if the diameter of the supply conduit is slightly larger than any of the parallel legs.

Here is why.

Imagine a flow circuit with the parallel "legs" (all the same diameter) being vertical and a supply manifold "header" is at the top of each and a return manifold is on the bottom. The parallel circuit is fed water from the upper left.

If the manifold and each of the legs are the same diameter, then the parallel legs closest to the inlet get the most water flow (heat). The further you are from that inlet, then the lesser the flow. This is because there is a pressure drop along that manifold.

The reason for this is that there is a finite resistance in the manifold "header" tubing. But if you put it in slightly larger in diameter, then the pressure drop along the length is negligible compared to any one parallel resistance.

In physiology, this is why supply arteries (aorta for example) are always larger than the downstream arteries (femoral, iliac, renal, etc). The downstream arteries are all in parallel. Such a system keep the pressure at each downstream artery virtually the same and thereby flow through any one organ system is simply regulated by the resistance of the arterioles (smallest arteries) in that organ. This is an example of a constant pressure system with flow being regulated simply by resistance and being directly proportional to changes in pipe diameter.

If the supply manifold (aka "header") is the same size as the parallel circuits, increasing pressure in the system somewhat negates the pressure drop along the manifold pipe. But in general, radiant floors are low pressure systems.

Flow always goes into the parallel circuit with the lowest resistance (electricity or water). In the above parallel system, there will be flow in each of the parallel legs, but the flow will be highest in the legs that are closest to the inlet supply.

By increasing the manifold diameter slightly, you decrease the resistance in the manifold markedly. Only a 11% increase in diameter will decrease resistance in half (double the flow). This is because flow is proportional to radius to the forth power.

Having a larger manifold diameter also allows you to have parallel circuits of differing lengths have similar (not identical) flows.

You can model this as either a constant flow inlet - or constant pressure inlet system, but the results will be the same.


Steve
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Last edited by stevehull; 09-25-13 at 01:29 PM..
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