Bret, you've got some good points. But you've also got some misconceptions.
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Originally Posted by screamingchief
Also there is the fact that the whole entire cooling system is'nt under the same exact pressure,as a matter of fact pretty much half the system is acutally under negative pressure,and then there are pressure drops across the radiator core as the coolant cools to account for,so that nifty science lab experiement means next to nothing in terms of how all that handy physics knowledge relates to a cooling systems actual operation.
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First, there's no such thing as "negative pressure". There is gauge pressure, absolute pressure and there is vacuum. Check it out.
If you think that half of the system is under "negative pressure", then why don't you go for a high-speed run, pull over and then remove the radiator cap with the engine running. After all, that's the low-pressure side of the system, right? Whether or not there is air in the system, there is always positive pressure in a properly operating system when it's above ambient temperature. Expansion of the coolant is what causes the pressure. Any air in the system is compressed the same as if it went through an air compressor. The reason why systems can operate at 220-230 degrees and not have any steam is because they are under pressure. If the suction side of the pump was under vacuum, the water would flash into steam and the pump would become vapor bound. Trust me, it's a fact and it completely conforms to the LAWS of physics. That's what causes cavitation in a pump suction. It can happen at very low temperatures.
If you think that the suction hose can collapse because of vacuum, that's not exactly true. What you are calling negative pressure is, now read this carefully, IN RELATION TO THE PUMP DISCHARGE PRESSURE. There are times when the coolant flow, because of radiator restriction, etc, can not keep up with the pump speed. If there are no voids to fill then the pump will slip and steam bubbles may form at the suction eye. If there is air, it may be further compressed and the lower suction pressure in the hose can cause collapse. That's why the hose may collapse when you rev the engine up, until the water can rush in to fill the void. Most of the times I've seen this happen is when the radiator cap is off. If you think about it, you'll probably recall that the coolant level in the radiator dropped when you revved the engine, as well. That shows that the system is not full. IF THE SYSTEM IS CLOSED AND FULL, THE HOSE SHOULDN'T COLLAPSE BECAUSE THERE WILL BE NO VOIDS TO FILL AND A LIQUID CAN NOT BE COMPRESSED. A properly operating and sized system will not have the hose collapse.
You're right, the system is dynamic and, at times can replenish itself when the engine is running. That would happen if it cooled enough to require water due to shrinkage. Generally speaking, though it won't do that.
Water at atmospheric pressure boils at 212, right? Does that mean that you can open your cap at 200-degrees? I wouldn't advise it because the system is under pressure. What causes the pressure? Even though the temperature is too low to boil and make steam, It's under pressure due to expansion. As you said, the hoses will absorb some of the pressure and acting somewhat like a balloon force some of the hot liquid out of the system if the pressure is relieved. Any air COMPRESSED in the system compounds that action. You can open the system, but you have to vent off that pressure before you remove the cap. Right??? That's true even though the cap is the low-pressure side.
As far as contradictions? Let's just say that air will always flow upward in the system, If there is a clear path for it to do so. That's true whether the system is operating or not. If there is a void with no outlet at the top, the air will be trapped. Maybe I didn't explain myself very well, but there really aren't any contradictions if you read it with an open mind.