expansion cap question.

[Northern Bongolow]

http://www.igmaynard.co.uk/bongo/forum/ ... ap#p534470.

in the above link to how the cap works by steve widdowson the animation shows how the system relaxes when cooling. via the inner spring assisted valve. this allows the internal pressure to fall by the spring setting (one pound) at a time until all the poss pressure is gone and then lets the system rest at minus one pound when cold.

me and another are having a pm discussion on the finer points of this and we cannot agree. one of us says that if the system is hot and sitting at 10 pounds positive pressure inside the coolant system that for the system to relax/cool that a pressure of 10 pounds PLUS is needed to open the return pressure spring therefore allowing the valve to open, and allow the pressure to fall until it closes again.

the other point of view is that a vac of one pound is enough during cooling to overcome the 10 pounds of pos pressure enabeling the system to relax even though the system may still be hot and sitting at a pos pressure of say 8-9 pounds.

please help we are both pig headed and wont back down. who is right,and who must buy the beer.

[widdowson2008]

http://www.igmaynard.co.uk/bongo/forum/ ... ap#p534470.

in the above link to how the cap works by steve widdowson the animation shows how the system relaxes when cooling. via the inner spring assisted valve. this allows the internal pressure to fall by the spring setting (one pound) at a time until all the poss pressure is gone and then lets the system rest at minus one pound when cold.

me and another are having a pm discussion on the finer points of this and we cannot agree. one of us says that if the system is hot and sitting at 10 pounds positive pressure inside the coolant system that for the system to relax/cool that a pressure of 10 pounds PLUS is needed to open the return pressure spring therefore allowing the valve to open, and allow the pressure to fall until it closes again.

the other point of view is that a vac of one pound is enough during cooling to overcome the 10 pounds of pos pressure enabeling the system to relax even though the system may still be hot and sitting at a pos pressure of say 8-9 pounds.

please help we are both pig headed and wont back down. who is right,and who must buy the beer.

'if the system is hot and sitting at 10 pounds positive pressure inside the coolant system that for the system to relax/cool that a pressure of 10 pounds PLUS is needed to open the return pressure spring therefore allowing the valve to open, and allow the pressure to fall until it closes again'
I think this one is right Ady.
So who buys the beer?

[Northern Bongolow]

system at 10 pounds positive, and switched off so cooling, and pressure dropping.
so when does the cap vent inwards to relax the system.
is it at just below 9pounds positive pressure. (1 pound vac)
or.
when all the 10 pounds pos has gone, and the system is at minus 1pound (spring pressure).

sorry steve, cannot reveal which one i am, but the other party is coming round today to press his point further so need to know

[Doone]

So it could be said you're under a bit of pressure to sort this out quickly then Ady...

[dave_aber]

The inner lid (IMO) won't move until the pressure inside the tank is less than the external atmospheric pressure, minus the spring pressure.

S0,

internal = 10psi above atomospheric - it stays shut
internal = 9psi above atomospheric - it stays shut
internal = 8psi above atomospheric - it stays shut
internal = 7psi above atomospheric - it stays shut

etc

internal = atomospheric - it stays shut

internal = Atmospheric minus spring pressure - it opens

i.e. On many cycles of run, stop, overnight, it may not ever open, depending on the atmospheric pressure.

However, if the external pressure is (relatively) high, and the internal is (relatively) low, it will draw air in at the safest place to equalise the pressure.

IMO of course.

[Northern Bongolow]

just a tad lorna . how are you ?.

spot on dave. .
because as the pressure drops and temps decrease, the coolant expands to neutralise/fill and vac that is forming,until the coolant has fully expanded then it may go neg internal pressure.

thanks to everyone.
looks like i am buying the beer

[Driver+Passengers]

because as the pressure drops and temps decrease, the coolant expands to neutralise/fill and vac that is forming,until the coolant has fully expanded then it may go neg internal pressure.

My tuppence...

As the coolant cools, both coolant and air contract so as to lessen the overall volume of the system, while the hoses relax - as the temperature and volume drop, so too does the pressure. I'm sure the air in the header tank, under decreasing pressure will expand (though there isn't much of it anyway), but the coolant shouldn't expand at all - you can't compress a liquid, I don't think??

A vac only forms when the temp/volume drop to such an extent that the pressure is negative w.r.t. atmosphere. Only then does the 'inlet' come into play.

looks like i am buying the beer

Not just beer - 4 Yorkshire puds, double helping of meat and all you can eat veggies, thanks!

[Northern Bongolow]