Quote:
Originally Posted by Darren9 
I can't help but think with that picture and explanation that when you close the loop your just closing in a bit of the atmosphere and it will behave in the same way, open or closed, pump running or not - the pump doesn't provide any of the force necessary to "repel" the atmospheric (or closed loop) pressure, the water does that all by its self. Is a WC pump really powerful enough to compress/expand a fluid by any noticeable amount?
I can't help but think with that picture and explanation that when you close the loop your just closing in a bit of the atmosphere and it will behave in the same way, open or closed, pump running or not - the pump doesn't provide any of the force necessary to "repel" the atmospheric (or closed loop) pressure, the water does that all by its self. Is a WC pump really powerful enough to compress/expand a fluid by any noticeable amount?
You're not wrong about closing a piece of the atmosphere. That's why the amount of air inside the res is so important. If there's no air, the res will act almost identical to a T-res or having no res at all: your only energy loss is from direction (which is tiny by comparison). If there is tons of air, though, then the air has to be at the same pressure as the water (which is the left-over pump pressure after you subtract the loop restriction (this is where the spring analogy comes in). This is why you see the reservoir go way down in loops that haven't been bled: the air is compressing after the pump and decompressing in the reservoir.
When the reservoir is open to the whole atmosphere, that's an insane amount of air to compress, so 100% of the pump's extra pressure is dissipated and the pump inlet has an effective pressure of 0 (since the net pressure of the inlet and outlet from the atmosphere is 0). With a closed loop void of air, water does not compress much at all, so the pump inlet has an initial pressure of 0, but after the pump runs the inlet has a pressure greater than 0 which increases the total pressure and will continue to increase the flow until the loop pressure drop equals the total amount of pressure needed to make the inlet be at pressure 0. You can actually solve for this mathematically using calculus, but I'm lazy.
[Edit]
It's easier to think about the other way: what happens when a loop's restriction exceeds the maximum pressure able to be produced by the pump? It creates a big negative pressure on the backside of the pump that can even make some tubing pinch closed.
Edited by Electrocutor - 9/11/12 at 9:18am
