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  #1  
Old 12-12-2003, 05:28 PM
Jodocus Jodocus is offline
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How does freon work?

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!
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  #2  
Old 12-12-2003, 06:10 PM
netscape 6 netscape 6 is offline
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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.
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  #3  
Old 12-12-2003, 06:22 PM
netscape 6 netscape 6 is offline
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one adendum i would to make is that while you could use many things like freon few things work as good as freon.
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  #4  
Old 12-12-2003, 06:31 PM
Q.E.D. Q.E.D. is offline
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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.
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  #5  
Old 12-12-2003, 06:52 PM
Crafter_Man Crafter_Man is offline
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Quote:
Originally posted by Q.E.D.
To answer the specifc question asked, Freon is used because of its high compressibility and relative non-toxicity.
True, but thereís another important factor: Freon has a convenient boiling point.

Quote:
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.
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 itís 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 itís in equilibrium with the environment.
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  #6  
Old 12-12-2003, 07:23 PM
Jodocus Jodocus is offline
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So freon is highly compressible. So what makes it more compressible than other gases such as oxygen, nitrogen, etc. ?
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  #7  
Old 12-12-2003, 07:32 PM
Walloon Walloon is offline
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The word Freonģ is a registered trademark, owned by Du Pont, for fluorinated hydrocarbons.
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  #8  
Old 12-12-2003, 07:38 PM
ftg ftg is offline
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Quote:
Originally posted by Jodocus
So freon is highly compressible. So what makes it more compressible than other gases such as oxygen, nitrogen, etc. ?
"Compressible" in this thread means "a really nice boiling point". The compression is coming from being converted into a liquid via pump pressure.
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  #9  
Old 12-12-2003, 07:57 PM
Jodocus Jodocus is offline
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What do you mean by a "a really nice boiling point?" What characteristic of a Freon molecule accounts for this nice boiling point?
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  #10  
Old 12-12-2003, 08:12 PM
spingears spingears is offline
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Refrigeration 101

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?
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  #11  
Old 12-13-2003, 12:09 AM
Rick Rick is offline
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The answer to the OP is here
Quote:
Refrigerators from the late 1800s until 1929 used the toxic gases, ammonia (NH3), methyl chloride (CH3Cl), and sulfur dioxide (SO2), as refrigerants. Several fatal accidents occurred in the 1920s because of methyl chloride leakage from refrigerators. People started leaving their refrigerators in their backyards. A collaborative effort began between three American corporations, Frigidaire, General Motors and DuPont to search for a less dangerous method of refrigeration.

In 1928, Thomas Midgley, Jr. aided by Charles Franklin Kettering invented a "miracle compound" called Freon. Freon represents several different chlorofluorocarbons, or CFCs, which are used in commerce and industry.
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.

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.
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  #12  
Old 12-13-2003, 12:19 AM
Q.E.D. Q.E.D. is offline
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Quote:
Originally posted by Rick
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?
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  #13  
Old 12-13-2003, 12:33 AM
sailor sailor is offline
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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.
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  #14  
Old 12-13-2003, 12:49 AM
Bryan Ekers Bryan Ekers is offline
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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* and thus heat transfer is very rapid and efficient.



* Sorry, my wife thought that was gangbusters.
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  #15  
Old 12-13-2003, 01:06 AM
vasyachkin vasyachkin is offline
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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.
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  #16  
Old 12-13-2003, 01:08 AM
Q.E.D. Q.E.D. is offline
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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.
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  #17  
Old 12-13-2003, 01:16 AM
vasyachkin vasyachkin is offline
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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.
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  #18  
Old 12-13-2003, 03:44 PM
Rick Rick is offline
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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 donít have time for more right now. I will post more later.
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  #19  
Old 12-13-2003, 04:31 PM
Blown & Injected Blown & Injected is offline
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Quote:
Originally posted by Rick
Several fatal accidents occurred in the 1920s
We all know how dangerous that 1920's style stuff can be!
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[/runs for cover from the mods]
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  #20  
Old 12-13-2003, 07:13 PM
antechinus antechinus is offline
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The reasons freons are used:
Low boiling point CF2Cl2 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.
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  #21  
Old 12-13-2003, 08:51 PM
Jodocus Jodocus is offline
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Is the boiling point of freon and its condensation point the same?
So freon basic boils at a temperature that is optimal for the operating conditions of a typical air conditioner? The lower the boiling point of a gas the better it is at cooling when it expands?
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  #22  
Old 12-13-2003, 09:53 PM
vasyachkin vasyachkin is offline
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jodocus boiling point and condensation point is the same point. BUT this point is a function of pressure. at higher pressure this point is at a higher temeprature. so if in AC condensation occurs at higher pressure than evaporation, then it should also occur at a higher temperature.

as for your last question, the amount of energy transferred is simply a pressure change multiplied by a volume change, does not matter what substance you're compressing the formula is just that.
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  #23  
Old 12-14-2003, 05:07 AM
antechinus antechinus is offline
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Also, fwiw, the high electronegativity of the halogens reduces the attractive forces (dispersion forces) between the molecules of the freon.

So in summary, extremly strong intramolecular forces within and low intermolecular forces between the halocarbon molecules.

The reduced attraction of the halocarbons to other molecules leads to the properties such as low viscosity and low surface tension.
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  #24  
Old 12-14-2003, 05:53 AM
hlanelee hlanelee is offline
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A refrigerant (Freon is a brand name, another example is Genetron) is a substance that can be changed to a vapor by boiling it and back to a liquid by condensing it. R-22, a common refrigerant in the state, has a boiling point of -39F. When it boils it changes state from a liquid to a gas. When this happens it absorbs heat from objects near by, a process called superheat. This gas is compressed which causes the gas to reject its heat and condense back to a liquid, sub-cooling. By the way, propane is very similar to R-22. I have heard that they are interchangable but one of them is flammable.

I refrigerator that uses ammonia does not need electrcity. Evaporation is achieved by applying heat to the liquid and it condenses readily at a lower temperature so there is no need to compress it. It goes through the system by convection. A friend of mine has a small refrigartor that uses ammonia. It is powered entirely by propane. I took it to school, my teacher could not find one to show us.

Source:Refrigeration & Air Conditioning Technology ; William C. Whitman and William M. Johnson
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  #25  
Old 12-14-2003, 02:34 PM
Bryan Ekers Bryan Ekers is offline
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Woohoo! I knew electronegativity was in there somewhere.
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  #26  
Old 12-14-2003, 02:51 PM
sailor sailor is offline
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Quote:
Originally posted by hlanelee
I refrigerator that uses ammonia does not need electrcity.
You are confusing different issues: Ammonia and absortion refrigeration. Different things. Ammonia is also used as a refrigerant in compressor heat pumps and an absorbtion heat pump does not necesarily need to use ammonia.
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  #27  
Old 12-14-2003, 05:31 PM
hlanelee hlanelee is offline
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No Sailor , I am not. I can go right now and put my hands on a small refrigerator, about 2 or 3 cubic feet, that was removed from an RV. It uses ammonia as a refrigerant. A reservior holds the liquid ammonia and a small propane gas flame is used to promote evaporation. There is a prominent warning on the side of the unit that says it contains ammonia. Small absorbtion units use ammonia as an attractant.

Typically larger scale absorbtion chiilers are used where there is a supply of wasted heat. They use a solution of lithium-bromide and water as an attractant. Circulating pumps are used on large units and not compressors.
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  #28  
Old 12-14-2003, 07:57 PM
sailor sailor is offline
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Um, yes, I am quite familiar with both kinds of refrigeration as I have worked with them professionally. I can assure you that (as has been mentioned by several people in this thread) ammonia is often used with compressor driven heat pumps. Please read and understand what I posted and tell me exactly what you believe is wrong. I assert that both types of heat pumps (absorbtion and compressor) can exist which use or not use ammonia. There is nothing special about ammonia. Just because you found an absorbtion type refrigerator which uses ammonia does not mean *only* absorbtion heat pumps use ammonia or that *all* absorbtion heat pumps use ammonia.

So, unless I misunderstood your earlier post, I assert it is mistaken. Or please explain to me how I am misunderstanding it.
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  #29  
Old 12-15-2003, 03:27 AM
hlanelee hlanelee is offline
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After considering your argument, I now understand your point. My intent for using ammonia as an example was to demonstrate that electricity and the use of a compressor were not necessarily needed for the refrigeration cycle to be created. The understood limiting conditions *only* and *all* change the tone of my examples, however I did not use them.

Sailor, I condcede, I will be more careful in future posts. Isn't a free exchange of ideas nice? You have worked on systems that use ammonia as a refrigerant/attractant? I would be too afraid.
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  #30  
Old 12-15-2003, 08:03 AM
sailor sailor is offline
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Ammonia is routinely used as a refrigerant in large industrial installations. You have large compressors in a single location, cooling towers (condensers) and then the liquid, compressed is distributed with pipes all over the plant where it is evaporated as needed and recirculated back.

I worked in the installation of machinery used in the manufacture of carbonated beverages (soft drinks & beer). Our machines required the water to be (among other things) chilled to a given temperature so a chiller was installed right before the machine. These big industrial chillers (10k - 35k l/hr) used the ammonia piped in from the central plant installation.

During the installation, adjustments and maintenance it was not uncommon to have ammonia leaks which would permeate the entire plant and make the air barely breathable for long periods of time. We were expected to grin, bear it and continue with what we were doing.

AFAIK ammonia is the refrigerant of choice for large installations.
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