Why does a cooling system need to be pressurized?

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ISX
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Sorry about posting my pointless crap, I just wonder a lot about things lol.

I realize the cooling system is pressurized to keep the coolant from boiling, but what if you never see over 190F, ever. Would the pressurized cap have any benefit in that case? Someone told me once about how even though you might never see it on the gauge, there are hot spots you don't see that could be above the coolant boiling point, but it doesn't boil because of the pressure. I don't know what to think of that. Any insights on this?
 

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AlephZero
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The cooling system needs to be sealed, for the obvious reason.

When the water gets hot, the water vapor creates an additional pressure in the air space in the header tank. In fact water will boil when the vapor pressure is the same as the air pressure (about 14 to 15 psi at sea level).

So, the system don't "need to be" pressurized for some reason. Rather, it is impossible fora sealed system NOT to pressurize itself when it heats up.

If the system ran at 190F with no boiling but was not pressurized, it would lose water by evaporation. In other words "steam" would be coming from it, just like if you heated up a pan of water on a stove but kept it below boiling point.

IF the system is working properly, there should not be any local boiling from hot spots, because the water circulation should be fast enough to prevent that. Of course if the system is blocked up by corrosion and other gunk, local boiling may occur.

Local boiling sometimes occurs after the engine is switched off, because the water pump is not circulating the coolant but there is still heat in the engine block. Modern cars have electric fans to cool the radiator and help the water circulate by convection, to stop this from happening.

In a car, it would not be cost effective to make a very high pressure cooling system that could run way above 212F without the water boiling, because it would need a stronger and heavier radiator, more complicated clips for the cooling hoses, etc. Radiator caps are usually designed to "blow" at about 14 psi, whch allows the water to reach about 190F. On the other hand, water cooled nuclear reactors may be designed to run at very high pressures, with no boiling at temperatures up to about 2000F.
 

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