Quote:
Originally posted by msv
First WB in the loop will get the highest pressure and the last WB will have the lowest pressure, right?
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Not true, I don't think. (EDIT: I suppose it is true actually, though it doesn't affect performance as explained below)
Try thinking about it this way.
Think of a pressure vs flow rate curve for a pump. As the pressure drop of the system connected between the pump's outlet and inlet is increased the flow rate wil decrease, following the curve described above.
When you're thinking about the pressure drop of a system, it makes more sense to think of the pressure drop of the whole system as a sum of the pressure drop of the individual components.
The pressure drop vs flow rate curve for a restriction, eg block/rad, will be a non linear curve increasing from the origin (zero flow at zero pressure drop) as increased pressure drop results in increased flow rate.
To combine pressure drop for a number of components, the pressure drops at a given flow rate are simply cumulative. In other words, the pressure drop of a whole system with a flow rate of x gpm, is the sum of the pressure drops for all of the components at x gpm. In other words, if you imagine all of the pressure drop vs flow rate curves, (pd on vertical axis) they can be stacked up one on top of another, resulting in a steeper curve. This makes sense as more blocks means more pressure is required to maintain the same flow rate.
So you now have a pressure vs flow rate curve for the pump, and the same for the system connected between the outlet and the inlet. From this, we can predict the flow rate through the system.
The important thing to realise here is that the flow rate is the same through the whole system. So consider a water cooled dual processor rig, with two identical cpu blocks in series. These blocks "should" have the same pressure drop vs flow rate curves. Now, since the flow rate through each of the blocks is the same, the pressure drop across the blocks must also be the same, provided they are the same blocks. The key point here is that the PRESSURE DROP is the same, NOT the absolute pressure. But it is the pressure DIFFERENCE across the block which results in flow, and not the actual value of pressure. So while the value of pressure decreases from a maximum absolute pressure at the pump outlet to a minimum absolute pressure at the pump inlet, it is not true that the second of two identical blocks in series will suffer due to the presence of the first block before it. In terms of pressure that is. It is obvious and well known that the coolant entering the second block will be at a marginally higher temperature than the coolant entering the first block, due to the thermal energy picked up while flowing through said first block.
If you want to use the electrical analogy again, think about it this way, consider an electrical component connected between +5V and GRND, and then consider the same component connected between GRND and -5V. Is there any difference, NO, because they still have the same potential difference across them, resulting in the same current flow. It is exactly the same with water blocks, except the relationship between pressure drop and flow rate is often more complicated than the relationship between potential difference and current, (often linear, whereas flow vs pressure tends not to be).
The key point to remember is that liquid flows down a pressure gradient, much like current flows down a potential gradient.
So infact, it is quite possible for a leak sprung in any part of the loop where the pressure has dropped to below that of atmospheric, for air to be drawn in rather than water leak out, since the pressure difference between the atmosphere and the loop, will cause air to flow down the gradient, ie into the loop.
This then gets complicated as you are adding more matter into a closed loop, so in theory the pressure in the loop will rise until the pressure at the point of the leak will is the same as atmospheric. What happens then, I don't know. It is possible there will be an alternation of air bubbles leaking in followed by water leaking out. But that last bit is just guess work.
Hope this helps
8-ball