I am curious as to what makes freon good for air conditioning systems. I have read that it heats up nicely when compressed and cools off nicely when it expands, or is it the other way around?
I want to know what is happening at the molecular level that gives it these properties.
Thanks!
think of heat as water. you can use a sponge to soak up a spill then ring it out in a bucket. thats how freon works. in an air conditioner the freon is compressed. it gets warm from the heat being confined to a small area and air is blown over the pipe it is in to cool it down. then the air is blown outside. then the freon is allowed to expand. the heat is now streched over a greater area and is there for less concentrated. meaning the freon is cooler now. air is blown over it and cooled down. and blown in to your house. in theory you could use just about any gas or liquaid like freon.
one adendum i would to make is that while you could use many things like freon few things work as good as freon.
To answer the specifc question asked, Freon is used because of its high compressibility and relative non-toxicity. The amount of cooling, as stated by various gas laws, is proportional to the change in volume, so the more you can compress a gas, the more cooling power you get when it’s allowed to expand. Freon and many other hydro- chloro- and fluorohydro-carbons excel in this area. Some commercial freezers use ammonia as the coolant, but given it’s noxious nature and relatively high toxicity, it’s not used in residential installations.
True, but there’s another important factor: Freon has a convenient boiling point.
This is true, but it should be kept in mind that time is also a factor. If I quickly compress a gas, it will heat up. If I leave the gas in the compressed state, the temperature must eventually decrease until it’s in equilibrium with the environment. The opposite is true for expanding a gas: If I quickly decompress a gas, it will get cold. If I leave it the decompressed state, the temperature must eventually increase until it’s in equilibrium with the environment.
So freon is highly compressible. So what makes it more compressible than other gases such as oxygen, nitrogen, etc. ?
The word Freon® is a registered trademark, owned by Du Pont, for fluorinated hydrocarbons.
“Compressible” in this thread means “a really nice boiling point”. The compression is coming from being converted into a liquid via pump pressure.
What do you mean by a “a really nice boiling point?” What characteristic of a Freon molecule accounts for this nice boiling point?
The “Freons” are carbon compounds of flouring, chlorine and/or bromine with thermodynamic and physical properties making them desiralble as refrigerants and propellants.
They are relatively non-toxic, inert or non-reactive with most materials and have very useful thermodynamic properties.
All refrigerants provide cooling when allowed to expand into a low pressure area, the evaporator. The low pressure vapor absorb heat and is removed by a ‘pump’ or compressor that discharges it into a high pressure high temperature area, the condenser, where it is cooled and converted back to a liquid state. The cooled liquid, at high pressure, passes through an expansion valve into the low pressure area, the evaporator, where it absorbs heat to continue the cycle.
The temperatures and pressures at each stage of the cycle determine the preference of one refrigerant over another depending on the specific application.
Jodocus asked (stated).
“I want to know what is happening at the molecular level that gives it these properties.”
Perhaps a physical chemist can answer that.
What difference does it make?
Why the specific interest?
The answer to the OP is here
In short the previously used materials were toxic, Freon is not. Freon was cheap, thought to be harmless, and easy to make. From the A/C makers point of view, what’s not to like?"
Except for that pesky hole in the Ozone layer, Freon is pretty wonderful stuff. :smack:
On side benefit of using R-12 was the temperature / pressure relationship is 1:1 for most temps. In otherwords, if the temp of a tank of R12 is 70F its pressure is 70psi.
What? so what happens at temperatures lower than zero? Negative pressure?
Freon works just like any other gas. No different. You can use other gases which have appropriate boiling/pressure characteristics. You can use propane and it will refrigerate fine. Not good is you have leaks though. You can use ammonia and plenty of industrail plants use just that but I can tell you from personal experience that breathing the stuff is no fun and when the stuff permeates a whole plant you are going to be breathing it for a while. But it is used often as it is cheap and well known.
If instead of having a cycle where the evaporator was at 0F and the condenser at 140F you wanted to cool from (say) 300F down to 70F then water might make a very good refrigerant.
I’ll hazard a lightly-researched guess and say the reason certain elements are useful in coolling systems is electronegativity, i.e. the tendency of an atom to grab free electrons (and take on an overall negative charge). The elements in column 17 of the periodic table are called the halogens and include flourine and chlorine. The elements in this group have seven electrons in their outmost shells, and since eight represents a stable ideal (elements that naturally have eight electrons in their outmost shells make up the noble gasses), the halogens are eager to grab electrons whenever they can. As a dise note, having just two electrons in the outmost shell is another ideal arrangement, so the metals in the eleventh column (copper, silver and gold) are also eager to grab free electrons whenever they can to make their single outermost electron less lonely.
Here’s where the speculation starts: high electronegativity means that when you shoot a bunch of excited (i.e. hot) electrons at a electronegative substance, they all get grabbed and the overall energy of the system decreases. Thus, coolants made up of halogens (combined into compounds known as freons) absorb energy rapidly, as do heat sinks made up of copper, silver or gold.
I’m not myself a physicist, and I’m sure a more accurate description of the phenomena is forthcoming. In any case, flourine is happy to grab an electron becuase that makes it look like noble neon, chlorine wants an electron so it can look like noble argon, etc. Freon, as any of several mixtures of these and similar elements (with a bit of carbon and hydrogen to hold the structure together) chases electrons like Garfield going after lasagna[sup]*[/sup] and thus heat transfer is very rapid and efficient.
[sup]*[/sup] Sorry, my wife thought that was gangbusters.
boiling point is affected by many things. for something inert (like propane) the only factor is molecule size, bigger molecules will have higher boiling point. but something that tends to form hydrogen bonds like water will have an uncharacteristically high boiling point for such a small molecule. so its basically two things - the kinds of bonds that the liquid forms and the size of molecules.
to build a refrigeration system you want something that can absorb a lot of heat without changing its temperature very much, otherwise you would have to move enormous amounts of the substance to maintain adequate heat flow.
the way to do this is to EVAPORATE the stuff. evaporation or condensation involves transferring massive amounts of heat energy, this is true for any substance not just freon, but like others have said it has a convenient boiling point. basically if we’re talking A/C then the warm air in your room should boil off the freon. obviously you could not boil off water at room temperature.
No, I don’t think that has anything to do with. Once elements are bound up in stable compounds, they generally have their valence shell requirements filled. This is why they “like” forming compounds in the first place. Take water for example. The hydrogens each fill their valence shells (which want two electrons) by sharing one electron each with the oxygen atom. The oxygen atom fills it’s valence shell (which has only six electrons, but wants eight), by sharing the two electrons from the hydrogen atoms.
another reason why you want to have a phase change is this :
you have two reservoirs, a high pressure and a low pressure one and you have the freon circuilating from one to another. you must pump power into the system somehow, or else nothing will be happening really, common sense.
you pump energy into this sytem because you move more volume to the high pressure reservoir than you have coming back, so you’re constantly pushing uphill. WHY ? becaue you are moving gas one way and liquid back, liquid takes much less space.
the energy you expend this way is ultimately used to move heat against the gradient from your cold room to the hot street.
First off Q.E.D. Auto air conditioners ( the users of R-12) never have the evaporator get below 22-24F, as an absolute lowest temp otherwise freezing would occur. You will note I said most temps.
Now on to Freon and A/C systems.
The thing that makes A/C systems work is changes of state. A whole bunch of heat is required to evaporate a liquid, and a whole bunch is given off when vapor condenses back into a liquid.
For example (because I know the numbers by heart) If you take one pound of water at 60F and raise it temp to 212 it will take 152 British thermal units. To take one pound of water (liquid) @ 212F and turn it into one pound of steam at 212F requires another 970 BTU! The amount of energy required to change the state far exceeds what it takes to heat the water. When the pound of steam is returned to liquid 970 BTU is given off.
So in an A/C system the Freon is compressed, making it a high temp/ high-pressure gas. From there it goes to the condenser (in front of the radiator when heat is given off and the Freon is returned to a liquid state. Still high pressure and still hot. Depending on the system and the ambient temp maybe 250 PSI
In front of the of the passenger compartment the Freon gets to a restriction (either fixed or variable) and the pressure of the Freon Drops to about 24-36 PSI. This makes the Freon a very cold liquid. A set of coils inside the passenger compartment (called the evaporator) has the warm air from the passenger compartment blown across it. The heat in the air is passed into the Freon causing it to evaporate. The Freon then returns to the inlet of the compressor and the cycle starts again.
The above is a very simplified explanation of how A/C works; I have to get to the airport and don’t have time for more right now. I will post more later.
We all know how dangerous that 1920’s style stuff can be!
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The reasons freons are used:
Low boiling point CF[sub]2[/sub]Cl[sub]2[/sub] is -29.8C
Low viscosity
Low surface tension
High density
Non-toxic
Non flammable
Odourless
Chemically inert
Thermally stable
Halogens such as Cl and F are very electronegative. They form strong bonds with the C, thus making stable compounds.