The boiling point of water is raised at higher
pressure, with consequent reduction in coolant and the radiator weights and
greater thermal efficiency of the engine due to higher operating temperatures.
For
approximately 10kN/m2 increase in pressure, the boiling point
is raised by 2.5oC. A radiator cap is used to achieve this
special pressure-sealed. The pressure blow-off valve opens when the pressure in
the system exceeds a certain predetermined value, say 50kN/m2.
A vacuum valve is provided to safeguard against the radiator collapse due to vacuum if the steam inside suddenly condenses to form a vacuum.
When the cooling water cools down, its volume decreases and
cooling water in the reservoir returns to the radiator, the reservoir is
usually made of translucent plastic so that it can indicate the level of the
coolant anytime.
An additional advantage of
this system is that being sealed there is no loss of coolant and thus it is
very much suited in deserts. Further, the cooling efficiency of the system is
not decreased at high altitudes where due to low temperature and pressure, the
boiling point is otherwise low.
An important precaution, however, has to be observed in
case of pressure-sealed cooling, while checking the coolant level. The pressure
cap should, initially, be opened only slightly, just sufficient to release
steam and pressure. Only after the complete release, the cap should be opened
fully.
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