Header Tank

[teun]

More simple:http://www.toyne.org.uk/docs-cooling.html

[flyerncle]

Exactly what Ginge said,on the Audi tank the cap is the pressure relief valve and it has a channel to vent it out of the bottom of the filler neck.
The reason the system is under pressure is to hold the coolant against the cylinder walls and all the other metal components in the engine to transfer the heat to the water otherwise it would boil away from the metal and no cooling would result and as others have found out the hard way boiling water is not funny under pressure.
The small top pipe on said tank is the return to vent air/water,without it there is no curculation.

[spud69]

I may be wrong Paul but i had always presumed, also, that it was so the operating temperature could be higher when under pressure without boiling, upto 105deg.

Maybe Bull but that's what i had thought.....

AndyH

[gingea1pom]

Andy,

You are correct. If you increase the pressure you increase the boiling point.

The opposite is also correct. If you boil a pan of water up on Everest it would boils at a much lower temp than at sea level.

[flyerncle]

And if you add antifreeze/coollant it really bollox's the whole theory ! And the boiling point etc. I will leave thermodynamics to the relavent heating engineer