57' over heating

[Paul J.]

c1 61 vacuum tank color

If the rubber seal on the cap shows a mark where it seals on the radiator lip it (the cap) needs replaced.
Paul

[Doug Flaten]

Re: 57' over heating

That is an excellent point Duke. Just installing a 15 psig cap does not mean the system pressure will opearate at that pressure rating. It is the maximum pressure rating that it will let the system operate at before burping fluid. The additional pressure delays the boiling point of the fluid. I don't have the charts in front of me for glycol mixtures but I have a couple of figures for 100% water.
At 14.7 psia (0 psig or no radiator cap), The boiling point is 212 F. If you boil a pot of water that is not covered, the temp of the water will go to 212 deg F and remain there until all of the water is boiled off. The temp does not rise above 212 deg F.
A 15 psig cap would raise the absolute pressure to 27.7 psia, the BP would be about 245F. So if you cap off a pot and heat the water to 245 deg F., it would exert a pressure of about 15 psig on the lid and the pot.
If the temperature was only raised to 220F or if your cooling system could stabilize at 220F, it would only be exerting about 4.6 psig on the sytem due to the temperature increase. There would be slightly higher pressure at the discharge of the pump to account for the friction in the system.
Doug

[Duke W.]

Re: 57' over heating

I forgot to mention my data was for a 50/50 glycol mix, so it increases the boiling point about 20 F at 15 psig relative to straight water, and this delta probably holds for a 7 psi cap too.

Duke

[Doug Flaten]

Re: 57' over heating - Add On

Since the glycol has a higher boiling point, it will operate at a lower pressure than 100% water does at a given temperature. For example, At 220 degrees F, A 50/50 mixture of coolant will only exert a vapor pressure of about 15 psia or less than 1 psig on the system compared to the 4.6 psig for 100% fresh water that I posted above.

However, In the above discussion, I failed to take into account the thermal expansion of the fluid due to the change in density of the coolant with the increasing temp. The charts indicate that a 50/50 mixture will expand about 6.5% going from 0 deg F to 220 deg F. Factors that would affect how much pressure is generated are the coolant system capacity and the size of the surge tank as well as the cold fill level of the coolant in the tank. My math may not hold up to cross examination, but I calculated an estimate of about 5.7 psig due to thermal expansion alone.
Doug

[Doug Flaten]

Re: 57' over heating - Add On

I re-examined my math, and the volume change in the surge tank vapor space could easily be reduced by 50%. That would give a pressure change over 28 psig which would put you operating at the full 15 psig of a higher rated cap.
I'm not sure I did the math correctly though.
Doug