boiling point of water...cooling systems

I think it has to do with that and the water pump not moving enough water through the radiators.

 

I think that this is the pressure at which the rad cap will safety valve out extra pressure. Though higher pressures will cause higher boiling temperatures, I don't think that letting vapor out at a lower pressure will reduce the boiling point since you are venting out to a higher pressure atmosphere.

Now if your car was at elevation that is a different story.

heat exchage is most likely your problem, not enough of it.

 

Isn’t that 0.9 bar ABOVE atmospheric pressure?

 

Yes it should be 0.9 bar ABOVE the atmosperic pressure which leads to a boiling temperature of 119°C or 246F (@ 1.9b absolute)

 

Surely you are not running just pure water? What about the ethylene glycol?

And I don't get this "venting out to a higher atmospheric pressure", either...I thought venting only works from high pressure to low pressure?

I thought that most cars nowadays could probably tolerate coolant temps in the range of 230 - 235 degrees or more before venting into the catch tank.

 

The cap adds pressure to the ambient pressure (varies with your elevation.) A 1 bar cap at sea level at normal barametric pressure would have 14.7psi + 14.7psi = 29.4psi of absolute pressure. Pure water would boil at about 249F in this situation.

 

I think he means that the cap will hold against net pressure in the cooling system of 0.9bar. The outside ambient pressure is apx 1.0bar. Thus, a 0.9bar cap holds the cooling system to 1.9bar. The forces on the cap would be ballanced if the atm is 1.0bar and the total vapor pressure of the system is also 1.0bar (ie it's cold and you've just closed the cap). As the system heats, the pressure inside increases. The cap's seal will hold until there is a net pressure differential of 0.9bar, then, as designed, it blows. Because the coolant was in the liquid state under >1.0bar at a temperature above it's boiling point at 1.0bar, it will boil almost explosively when the pressure is rapidly reduced to 1.0bar (say, if you uncapped a hot system) as the liquid rapidly undergoes a first order phase transition to gas.

Edit - I need to learn to type faster.

 

It's about as basic as physics gets.

 

The cap valve is most certainly operating against exterior pressure.


Physics 1A

 

If you lived in a vacuum, this would be true. The only reference pressure available to the cap is the external, ambient pressure. It holds the contents at a fixed pressure above this. Using the one bar cap above, at 4000 ft altitude, atmospheric pressure is about 12.7psi. 12.7psi plus 14.7psi for the cap means that the absolute pressure in your cooling system is 27.4psi, and your pure water will boil at about 245F.

 

Because every pressure gage you own is calibrated to read 0 at ambient pressure, since all of them use ambient pressure as a reference. This is true for your tire pressure gages, the gage on your shop compressor, your blood pressure gage, your A/C manifold gage, etc. All of them. Even your vacuum gage. Just because your tire pressure gage reads zero before it is held up to the tire stem does not mean that there is no air pressure around you. It would be difficult to breath if that were the case.


(OK, I happen to own one gage that reads 0 at a pressure of absolute 0, though it's never read that low. It's a specialty instrument you are unlikely to have.)

 

redline makes a cool product its called water wetter, put that in with the next coolant change, they advertise a 20 degree drop in tempurature,

supposedly it transfers more heat than water alone can id say check it out, i havent checked but royal purple may also make something similar

 

Why are we trying to reinvent the wheel?

The car called for a .9 when built and was superceeded to a 1.1.

If either of those will not contain the coolant when the motor is running it is overheating.

It really is that simple.

 

It subjects the pump to a couple of pounds more pressure so iffy seals fail sooner and increases the max temp a little.

Air space....just like every other water cooling system ever made.

 

I don't understand this piece of data. What is your source of information for this?

Water, when heated, expands by some percentage of the volume. The pressure generated can be many thousands of PSI, or nothing at all, based on the expandability of the container holding it. In the case of an automobile, any expansion which drives the pressure over the cap pressure simply dribbles off into the expansion tank, where it is recovered back into the radiator when the engine cools back off.

 

16cm?! Are you certain? I have always heard it should be 6cm.

 

Must be a misprint, at 16cm the tank would be empty.

6cm is about right, a bit over 2 in. from the bottom of the neck.