7tea1

What causes the cooling system to actually pressurize?

I never thought about it!!! I just know it does, the cap assures the pressure does not go above a set spec, the hoses need to ge in good shape, the pressure raises the boiling point, etc. BUT.............how does it pressurize :confused:

Help me save face :D I told her it's hard to explain, but i'd get back to her.
(She didn't fall for it)

rpounds

As water heats, it expands. This expansion causes a pressure build up in the cooling system.

OatBoy

the heat

shoptek

My Mrs. throws me a ringer every now and again. Some are so "easy" that you have to actually think about it.
Like the others said heat expands the water, therefore the pressure rises since it is a sealed (hopefully) system.


[Modified by shoptek, 9:57 PM 10/28/2002]

jerryp58

The "ideal gas law" is PV/T=k (and I think it applies to liquid too), where:

P = pressure
V = volume
T = temperature
k = a constant of proportionality

So, if I remember correctly, you can just think of it as:

Pressure is directly proportional to Temperature.
Pressure is inversely proportional to Volume.
Temperature is directly proportional to Volume.


So, given no change in volume, if you increase temperature, then you increase pressure (decrease temp, decrease press).

When the car is running the temp increases, so the pressure increases. Volume would increase too (and it does which is why you push some fluid into the overflow tank). When the car is shut off, the system cools, pressure drops, and the volume decreases (fluid is drawn back into the system).

I don't know if all that's right, but I think it sounds nice :yesnod:

rainman69

Yup. Pressure due to expansion and nowhere to expand to. Yah, I'm just a geek. :cheers:

PRNDL

Where are all the Physic-ists? :) Does the coolant actually expand when heated? I woulda thought it is the steam part that creates the pressure. Either way, it is the heat. I am just wondering if the coolant actually expands. I didnt think water expanded when hot. you gotta freeze it to make it expand! MJ

rpounds

Nope. Coolant expands when heated. Unless the coolant is boiling, there should be no steam. Plus, the cooling system is pressurized specifically to prevent boiling . . .

mp77

When you take a cold shower, do you see any expanding going on. :lol:
Seriously though, the more pressure you have on liquid raises its boiling point. Water at atmospheric pressure(like in a pot on a stove)boils around 212 degrees. Raising the pressure raises its boiling point. By having a sealed system in your car it prevents boiling. :cheers:

JB

I'm confused (what can I say, I was an English major). So water expands when it freezes, expands when it's heated, but doesn't compress? I would have thought was the steam that caused the pressure, and the pressure that cause the overflow, not the physical expansion of the liquid. Do you actually have to have boiling to have steam?

JB


[Modified by JB, 9:49 PM 10/28/2002]

feb18blacksunday

Ethelene Glycol ie. coolant raises the boiling temp and lowers the freezing temp of water. When the boiling pt or freezing pt is increased the expansion qualities of water are changed the more coolant the less expansion. Thats what happens when your prestone is messured during cold weather.


:chevy :chevy :chevy

Desertdawg

Just tell her that the phitzenfelter valve is what causes the pressure...... :D


[Modified by Desertdawg, 9:23 PM 10/28/2002]

Stevo81

I think water contracts as it cools, then expands at freezing.
And you don't need boiling temperature water to get steam. As soon as you set water out in dry air, some water molecules start to become water vapor. We call it steam when we can see it, but it is there as soon as the water is warming up. My 2 cents... :conehead

81Vette_by_eric

You guys are nuts. :lol: :lol:

At any rate, yes water expands when frozen into a solid state. One of the few substances on earth that does. Odd little fellow. Most things contract when cold. As the water heats up, its not really expanding, the molecules are just moving faster and need more space to move in, so it pulls air into it, hence water boils. Where did you think all that air in boiling water came from. Yes, there is some air in the water, just not a lot. Any rate, as the water is heated in the engine/raditor, the molecules are heated and what to move faster, but since there is nowhere to go, it creates pressure.

:bs

Or something like that. If you don't agree, its because i am a accounting major. But it all boils down to simple math. :D

Sting Raycin

No, boiling coolant does not cause the system pressure. If your coolant boils you are in BIG TROUBLE......The system is pressurized in order to raise the boiling point of the coolant. This can raise boiling point by up to 45 degrees F. The spring on your radiator cap determines the maximum system pressure.
The pressure IS caused by heat expanding the coolant. The reason steam comes from the radiator when the cap is opened is because the boiling point is instantly lowered and the coolant (which can be at 250 degrees) boils in the open air.
Well that's about all I have to say about that.

Shark Racer

Water is a very ionic compound, and forms rather tight bonds in its liquid state. (Oxygen attracts the hydrogen molecules.) However, when it's frozen, it forms a sort of lattice, which stretches the molecules out to a large degree than normal. When it's heated, the molecules move about more quickly, eventually to the point where the hottest evaporate.

PV=nRT, is an equation I can't quite remember the purpose of. R is a constant, n is the number of mols of the subtance, T is temperature, pressure/volume are P/V. Increase in volume, assuming that nRT remains constant, will cause a decrease in pressure. (Volume and pressure are inversely proportional). Increase pressure with constant volume/mols and temperature will increase, same goes for increasing volume with constant pressure/mols. Increase temperature with constant volume/mols, pressure goes up. Increase temp w/ constant pressure/mols, volume goes up.

Just restating what's above. That was from AP Chemistry. :crazy:

-Steve

*edit* PV=nRT, or PEVNERT, (or PERVERT) as it's referred to, is the ideal gas law spoken of above.


[Modified by Pacin'California, 12:07 AM 10/29/2002]

Turbo-Jet

Water is a quirky substance because it expands when it solidifies. In fact, 99.999999% of all other known substances will contract when cooled. This characteristic of water is very peculiar, and we are lucky it is this way because life on earth would not exist otherwise, according to my former thermodynamics professor. His theory is based on the fact that water will expand when frozen, making it less dense and therefore buoyant in liquid water. If water did not do this, it would sink. The result is that solid ice would always be found in the bottom of oceans and lakes because it would never get the chance to melt. If this were the case, then it would have been almost impossible for life to evolve in the ocean as algae, and then crawl out of the ocean, and so on…

Cool eh!




[Modified by Turbo-Jet, 1:43 AM 10/29/2002]

Frank75

I don't think that the ideal gas law applies to liquids. I always thought the system was pressurized because there is always a small amount of air - just look under the rad cap and it's very hard to completely bleed a system of air. Maybe glycols expand as they're heated too?

Turbo-Jet

Correct, the ideal gas law applies only to simple compressible gases. It works for water vapor.

joe73vette

I guess an mechanical engineer has to put this straight:
Everything expands as it is heated, and contracts as it cools! Solids, liquids, and gasses. The amount of change depends on the substance.
However, water (ice) is odd in that as it cools from 0*C to -4*C it expands! After that anomaly, it contracts again as it cools.
Boiling is not the same as evaporation. Boiling takes a large amount of energy, and the temperature does not increase at all until the liquid is all turned to gas (steam). There is no air in steam, just water vapor. Evaporation is just some surface molecules escaping into the air.
Same thing for melting and freezing: They happen at a particular temperature and are an actual change of state, the molecules don't behave the same after the change.

Technically, the water in your cooling system doesn't expand, except to bulge the hoses. After that, the volume is constrained and the hotter, more excited molecules cause the pressure to increase. Eventually something has to give, usually the cap, so then the volume increases (inot the overflow).
For older systems, there is an air pocket in the radiator, and the expanding water compresses the air, which makes room to keep the liquid in the radiator.

Joe