Pressure Cap 101 Tutorial

[ArtJ]

Hi guys

I recall we had a discussion regarding operation and pressure ranges of
various freshwater cooling radiator caps a while back.
I have noticed antifreeze backing up into the overflow plastic bottle after
30 minute runs. It never runs back in until the cap is removed. The
bottle by necessity is mounted outboard and higher than the fill cap.

What range of cap should we use for fresh water cooling on the A 4?

How do we identify rating of a specific cap?

What is the physics behind it?


Thanks and Regards

Art

[sastanley]

Art, I am not an expert, but here is what I know.

When water is under pressure, it raises the boiling point of the water. I do not think boiling water transfers heat very well, so when using it in a HX or radiator, we need to keep it from boiling. For every 1 PSI you put the water under pressure, it raises the boiling point xx°F. I don't know what it is, maybe 3 or 4 degrees??

At atmospheric pressure at sea level, we all know water boils at 212°F. So, for a 7 PSI cap you would raise the boiling point of the water maybe 20 degrees or so. Since I don't know how much per PSI, this is just a guess.

For our A4 purposes this is how I think it works. As the water heats & expands, it forces the spring up on the pressure cap a bit & pushes the water/coolant to the overflow bottle...it is supposed to suck it back in as the coolant cools and creates a vacuum, pulling the previously expanded water in the overflow bottle back into the HX.

Ok..I've exahusted all of my knowledge. I personally haven't seen much change in the water level in my overflow bottle, however, I occasionally have seen water move thru the clear tubing, so there is something moving.

I think the cap Moyer sells with their HX is 3 or 4 PSI...so not much.

[Dave Neptune]

Just boiling

Art, you do pose a good question. Personally on most anything I have ever run that does not have a boiling (excessive heat) issues I use "ZERO" preassuer . There is no need for the preassure unless boiling is an issue, the preassure just raises the boiling temp and does nothing for the actual cooling. I use a std radiator cap so the system is sealed, and drill a small hole throught the preassure "valve" so the espansion tank still functions and this keeps the system full and purged of air. In this way hoses and gaskets are under far less strain to remain sealed and if you do spring a leak it6 will dribble instead of being forced out via system poraessure.

Dave Neptune

[ndutton]

Also consider the condition of the cooling system castings after a lifetime of salt water. For example, applying pressure to a thin manifold could finish it off.

The raw water cooling system was unpressurized and it worked fine for decades.

[ArtJ]

Thanks for the info guys

Possibly I am mistaken, but didn't the very early cars have huge radiators
which on later cars became smaller when pressurized radiators were invented?
Doesn't this imply that pressurizing the radiator means that a smaller
radiator which is pressurized can outperform a larger unpressurized radiator?


Regarding the cap on our motors. I understand that the pressure cap allows
pressure to be released when specified temperature is exceeded. But how
does the cap allow antifreeze back in?
My cap ( new from Indigo a couple years ago) doesn't seem to allow the
antifreeze to reenter the heat exchanger until the cap is removed. This
occurs despite the fact that my overflow plastic bottle is higher than the
top of the exchanger fill cap.

What experiences do others have with antifreeze and caps>

Best Regards

Art

[jhwelch]

I don't have a cap; my overflow bottle is plumbed directly into the sealed heat exchanger via a tiny hose.

[ArtJ]

I don't have a cap; my overflow bottle is plumbed directly into the sealed heat exchanger via a tiny hose.

Hi Jhwelch
What type of heat exchanger do you have ? Is it the MMI exchanger, or some
other brand? Did you seal it off, or was it sealed by the mfr?

Best Regards

Art

[jhwelch]

I have a Sen-dure heat exchanger, custom made by them. It's two pass and has four fittings for the water hoses and a much smaller fitting for the hose that runs to the overflow bottle.

Some years ago I took my old one (about 10 years older than the boat!) to their offices in Ft. Lauderdale where the chief engineer measured it, asked me a few design questions, and later shipped the new one up to Titusville.

Due to space constraints in my engine box I had to replace what I had with the exact same size and shape.

-Jonathan

[romantic comedy]

Water will boil at a higher temperature at a higher pressure. The higher the pressure the higher the boiling point. The ability of the water to remove t he heat is not that different at the different temperatures. We do need liquid water, not steam.

In cars the water(coolant) heats and expands and some goes out the tube to the overflow bottle. When it cools, the coolant gets sucked back in.

Modern cars are designed to run hotter. Our engines are designed to run much colder, so we never approach the boiling point of water. It is more efficient to run hotter though.

[hanleyclifford]

Art - The clue to this lies in Tom Stevens' remark about an air leak in the hose from the overflow tank to the exchanger. Look at your cap to see that the check valve is free. When the system cools and tries to draw coolant back into the exchanger it must draw only antifreeze and no air, kinda the reverse thinking for a syphon break. Of course first prize is having the overflow tank higher than the exchanger but it can be lower provided the line is absolutely air tight. Hope this helps. Regards, Hanley

[ArtJ]

How can the check valve in the cap be checked?
Appologize for the dumb
question in advance.

Regards

Art

[hanleyclifford]

It sounds like both Dave and Jonathan are running zero pressure systems with sufficient capacity and "head" to contain even a heated and expanded body of coolant, a la Model T or A. Great if you can pull it off without loosing the cookies.

[Dave Neptune]

Zero still sucks

I have run zero preassure recovery for years on many cars and FWC boats. The expansion/overflow tank is still functioning to keep the system below the cap at capasity! The only change is that Zero preassure still expands and contracts into&out of the expansion tank it just uses the expansion and does not require additional preassure to function or "suction" to vent it back to the system via a preassure cap.
The function and operation are the same just no additional preassure and if sealed can be above or below as per Hanley's comment regarding being "sealed" (no leaks) behind the expansion tank.

Dave neptune

[jhwelch]

My expansion bottle has two little hoses. One runs directly to the heat exchanger. The other I have run vertically up the inside of a bulkhead. This allows for air to enter and leave the system. At rare intervals this hose has slumped down or otherwise gotten too low and I have found some coolant was lost from motoring when heeled over. Otherwise no problems from my zero pressure system.

The failed heat exchanger I took out of the boat some years ago (it was made around 1957, and the boat was made in 1969) was set up the same way, so presumably the previous owners were happy with that setup.

-Jonathan

[ArtJ]

It seeems as though the two hose system must work on gravity with
the plastic bottle mounted higher than the engine.
IT seems that the
seal on the ordinary pop off top would not be air tight as sold by MMI and not
need the extra hose?
Does anyone have suggestions on how to test BOTH sides of the
pressure cap?

What pressure cap is sold with the MMI Exchanger?


Thanks and Regards

Art