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Originally Posted by David N.
The coolant system is pressurized to 13 PSI? Isn't 1 atm ~15 PSI, where water boils at 100 degrees C? Seems like 13 would have it boil even lower.
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Hi,
Well, you're both right and wrong. 1 ATM is equal to 14.5 PSI (standard) at Sea Level, and
pure water does indeed boil at 100°C. But we're not boiling water here, we're talking about a 50/50 mix of Anti-Freeze and Water which boils at 106°C at 1 ATM.
All pressure measurements are made using 1 ATM as the starting point (because you have no pressure until you exceed this), so technically, the system pressure is 13PSI above atmospheric pressure, or 27.5 PSI above a vacuum - 0PSI, but again, only at Sea Level.
The Cooling System uses pressure to further raise the boiling point of the Coolant. Just as the boiling temperature of water is higher in a pressure cooker, the boiling temperature of Coolant is higher if you pressurize the system. If the system is pressurized to 13PSI, this adds about another 25°C to the boiling point totalling 131°C or 267.8°F.
There is one more reason to pressurize the system. As the Coolant Pump Impeller turns, it creates bubbles by adding energy into the Coolant and causing dissolved gasses to precipitate. You do not want Air Pockets (bubbles) collecting around Water Jackets because little or no heat transfer will take place. By pressurizing the system, you decrease the ability of the dissolved gasses in the Coolant to form bubbles. Hope this helps...
Happy Motoring!... Jim'99