A/C theory question

[GetMore]

I was told by a guy that sells and installs aftermarket A/C systems that it's a good idea to run the high pressure line (from the compressor to the evaporator) and the return line (from the evaporator to the condenser) next to each other and even to wrap them together with some kind of insulation.
The idea actually is to warm up the freon going into the evaporator and to cool down the now vaporized freon going to the condenser.

I know that offhand that sounds like a bad idea: You would be warming the freon that is supposed to cool the interior.
However, I think it is supposed to help keep the temperature from varying as much.

I don't know if it increases efficiency or not. Can anyone tell me if the compressor hase to work just as hard to condense cool freon as it does for warm freon?
Is it possible that this makes for a more efficient system?

I'm thinking that the compressor will have to run more, but might take up less power.

Does anyone know what this would actually accomplish?

[ProTouring442]

The refrigerant goes into the compressor as a low-pressure gas, and comes out as a high-pressure gas. The condenser then changes this to a high pressure liquid. At the expansion tube, the high-pressure liquid is then released into the evaporator. Optimally, with R-12, you want it to remain a liquid during its trip through the evaporator, as it absorbs heat from the interior. With R-134A, you want it to remain a liquid through most of the evaporator, but not all. By the time it leaves the evaporator, the low pressure liquid will have heated up due to absorbing interior heat, and turned into a low pressure gas again.

Warming the high-pressure liquid on the way to the evaporator makes no sense, as you want it to be as low a temperature as possible so it can absorb the most heat. Consequently, I think your man may be a bit out on a limb!
Shiny Side Up!
Bill

[GetMore]

Okay, first I learned that the compressor is before the condenser. I thought it was after, but thinking about it now, it makes more sense.

However, I'm not sure about the freon remaining liquid through the evaporator. It seems to me that it wouldn't do anything if this were the case. As I understand it, it is the phase change that causes the temperature drop. If the freon is still liquid in the evaporator then it won't lower the temps, especially since it's got to be warmer than the air temp.
Can you elaborate? Maybe I still don't understand A/C?

One other thing: He may have said that doing this would help eliminate icing. I really don't remember for certain.

The other thing I'm thinking is that maybe the freon gas coming out of the evaporator is cooler than the liquid freon in the lines? In this case then you'd actually be cooling the freon going to the evaporator.

The liquid freon wouldn't be any cooler than ambient temperature coming out of the condenser, while the freon gas in the evaporator should be close to freezing (for water), and can't get any warmer than ambient.

[ProTouring442]

Okay, first I learned that the compressor is before the condenser. I thought it was after, but thinking about it now, it makes more sense.

However, I'm not sure about the freon remaining liquid through the evaporator. It seems to me that it wouldn't do anything if this were the case. As I understand it, it is the phase change that causes the temperature drop. If the freon is still liquid in the evaporator then it won't lower the temps, especially since it's got to be warmer than the air temp.
Can you elaborate? Maybe I still don't understand A/C?

My understanding is that it is the release of pressure that drops the temperature of the refrigerant, and it is the absorption of the interior heat that causes it to become a gas. I barely passed physics in highschool, so I may be a little wrong on this one.


One other thing: He may have said that doing this would help eliminate icing. I really don't remember for certain.

Icing of the evaporator is an undercharged situation. When the evaporator ices, it is because the low side pressure has dropped too low allowing the refrigerant to drop below 32 degrees.


The other thing I'm thinking is that maybe the freon gas coming out of the evaporator is cooler than the liquid freon in the lines? In this case then you'd actually be cooling the freon going to the evaporator.

Now here is a point I hadn't considered. The outlet side of the evaporator should be somewhere around the inlet temperature, and would be capable of absorbing more heat. However, it also seems to me that if this were a good idea, manufacturers would already be doing it. In other words, I don't know why, exactly, it would be a bad idea, but I still think it is. I'll have to look more into this one.

Shiny Side Up!
Bill

[GetMore]

I can be a bit of a geek, so I do remember something from HS physics regarding phase changes.
There is a certain amount of energy required to change a compound from on phase to the next. It takes 1 calorie to raise one gram of water one degree celcius, but it takes 540 (yes, 540!) calories to change 1 gram of 100 degree celcius water to 100 degree steam. (I had to look the numbers up.)
The reverse is true as well. Condensing 100 degree steam to 100 degree water releases 540 calories per gram.

Freon is not as efficient, it has a lower heat of vaporization.

Either way, it is when the freon changes from liquid to gas (due to both the release of pressure and the addition of heat) that it cools down.


Now, here's the response (edited) I got when I asked on another forum:
The problem is you can superheat the refrigerant coming back to the compressor and get it so hot that the temps/pressure damage it.

They make accumulators to serve the same purpose, and that would be to ensure no liquid back to the compressor...
As far as subcooling the liquid goes...if the system is that desperate get another (larger) system. There is nothing free.
If you put heat into the refrigerant it cost to get it out...through compression.

you sub cool liquid out of condenser.... and superheat gas out of evaporator...

The trick is to maximize subcooling and minimize superheat without damaging anything. The optimium superheat is 0 degrees F...but most tend to go with 6-12 degrees for safety. If any hotter, damage the compressor...if any less take the risk of liquid getting back to compressor...damage the compressor....
If you could get 0 degree superheat out of the evaporator and not damage the compressor it would be "optimum".

[Twentyover]

The TXV causes isenthalpic expansion of the refrigerant. It enters the TXV as a sub-cooled liquid, and exitas the TXV in 2 phases, approximately 80% by volume is gas, 20 % liquid; about 20% gas by weight, about 80% liquid. Numbers are WAAAAY approximations.

Heat is removed from the air going through the evap, that heat is used to boil the remaining liquid refrigerant. Refrigerant should leave the evap 8-10 degrees superheated (8-10 degrees above boiling temperature of the refrigerant at that evap out pressure)

Evap freez is caused by a failure of the freeze protection circuit, either a suction throttling valve or a thermistor that senses temperature. A low charge will yield lower evap temps, but freeze protection should still work even with a low system charge.

Remeber that your superheated gas is 40 degrees or so, and your subcooled liquid may be at 150 degrees, so don't let the names throw you. Heat you add after exiting the evap is not going to change performance, but heat you remove before the TXV will improve performance.

As long as you get gaseous refrig to the compressor, and the compressor is carrying oil in the sump, or the refrigerant has oil entrained in it, the compressor wont's car if it's 6 or 12 or 20 dgrees of superheat. Pump usually has a thermal limirtter switch to prevent frying due to low oil flow (usuallu consitions that provide high superheat.)

Liquid slugging in both positive displacement and centrifigual compressor s is a problem. Optimum performance is a mixture of gas and liquid coming out of the evap, which CCOT systems do have, and then use the accumulator as a boiling chamber to prevent liquid going to the compressor.

[dhondagod]

Either way, it is when the freon changes from liquid to gas (due to both the release of pressure and the addition of heat) that it cools down.




Let me try. The temperature DROP in the refrigerant occurs when the high-pressure, high-temperature coming from the condensor passes through the expansion valve/orifice tube/txv or whatever type of "flow-control" your paticular system uses.

Its the PRESSURE DROP that causes the temperature drop in the refrigerant. (anytime you have a drop in pressure you have a drop in temperature)

When the liquid, high temp refrigerant passes through the "orifice" and into the evaporator, it goes from 150-250psi liquid(approx) to 25-35psi liquid. R-12 has almost a 1-1 temp/pressure relationship meaning if you have a pressure of 25psi the temp of the refrigerant is approx 25deg f.

As the refrigerant passes through the evap core as a low-pressure LIQUID it absorbs the heat from the air blowing past the evap core. This heat cases the LIQUID refrigerant to boil into a gas. At this "change of state" from a liquid to a gas the heat is absorbed into the refrigerant and carried out of the passenger compartment by the low-pressure GAS and back to the compressor on the engine. If you have ANY liquid refrigerant coming OUT of the evap then your losing efficiency as the liquid refrigerant dosent carry any heat with it. (overcharge, malfunctioning txv/expansion valve etc)

The compressor re-compresses the refrigerant and sends it through the condensor where the "change of state" from a gas to a liquid occurs again. This time however the air blowing through the condensor absorbs the HEAT from the high temp/pressure GAS and causes it to condense back to a liquid.

Clear as mud? :banghead:

Chris :hmm:

[GetMore]

It's Easter that has to do with resurection, not Christmas, but what the heck. ;)

Chris, I follow everything you said. I do have to disagree with you in part on the pressure/temperature relationship. In gasses it is accurate, but fluids and solids don't change temperature due to pressure. At least that's the science that I learned in school.

[barno68]

Refrigerant systems work due to latent heat of vaporization, it is the theory of energy conservation in the form of heat transfer due to pressure changes. Pretty much what people have been replying, just wanted to through out what the process occuring after the THX is called.

[MonzaRacer]

I was told by a guy that sells and installs aftermarket A/C systems that it's a good idea to run the high pressure line (from the compressor to the evaporator) and the return line (from the evaporator to the condenser) next to each other and even to wrap them together with some kind of insulation.
The idea actually is to warm up the freon going into the evaporator and to cool down the now vaporized freon going to the condenser.

I know that offhand that sounds like a bad idea: You would be warming the freon that is supposed to cool the interior.
However, I think it is supposed to help keep the temperature from varying as much.

I don't know if it increases efficiency or not. Can anyone tell me if the compressor hase to work just as hard to condense cool freon as it does for warm freon?
Is it possible that this makes for a more efficient system?

I'm thinking that the compressor will have to run more, but might take up less power.

Does anyone know what this would actually accomplish?

nothing but impracticality and a lot of plumbing issues.
basicly every liquid has a point where it vaporizes (ie the latent heat of vaporization)not what your AC system does is this contained within it is a specific anount of fluid(refrigerant) uper pressure.

Now the system works like this, the compressor compresses the returning vapor from the evaporator into what is called a the superheat cyccle and passes it into a high pressure/high temperature gas that goes to the condenser and is cooled and condensed into a high pressure liquid, which goes through the high side or liquid line going to the evaporator.
Now you will have either a accumulator/drier in the low side or a drier in the high side line and goes to the control either a thermal expansion valve,hblock or orrifice tube. This "sprays"the high pressure liquid into the evaporator which as it changes from a liquid to a gas as it absorbs heat from the evaporator fins/tubes.
then the low side or suction side pulls the heated (the heat removed from the interior)gas into the compressor and then goes through the compression/superheat cycle and it starts all over.
tying them together would do nothing.
Best thing to do is make sure you have proper condenser fan moving air over the condenser and getting a properly certified AC tech/shop evac andrecharge it properly and dont try to use any drop ins, if its R12 get it converted leaglly.
I have done thousands of conversions and have had no issues todate.
I also have helped peopl tie different parts together to make a workable system (ie gm/ford/chry parts all intermingled.
If you know how to work with the parts it can be done.

[LS6 Tommy]

To answer the original question, tying the liquid & suction lines together is done in low temp refrigeration. It has little to no effect on A/C. It's done to keep capillary tube metering devices from freezing in low temp applications.

No disrepect inteneded to any individuals, but there's a lot of semi-correct info here, too. I would advise sticking with having a system done by certified personnel.

I could point out incorrect statements that have been made here & on other A/C posts, but I'm not trying to flame anyone or start arguments.

Tommy

[Mjwhoopie]

W O W!!!
There is a lot of technical jargan thrown around here, and not nessarilly all is it's correct context.

There is some correct info mixed in for good meassure.

AC operation/theory is Basicly fairly simple.

When a gas is compressed, it's temperature rises.
When a gas's pressure is reduced it's temperature drops.
If you cool or reduce the temperature of a compressed gas, and then rapidly reduce it pressure it's temp will drop below the original or ambiet temperature.

If you want to study the Physics of it all Google "Bernoulli's Principal"

In Automotive AC systems the choice of this gas is Freon. Currantly R-134a.
R-134a Boils @ -15Degrees F.
It is in a liquid state above 75psi.

You can also Google " R134a" to get it's specs and more in depth info.

This applies like this:
The compressor compress the gas into a liquid.
The liquid travels to the condensor to have it's temp reduced.
The liquid then flows to the evaperator, Where a flow control device (Expansion vale, Orifice tube, ECT) allows the reduced temp liquid to rapidly expand back to a gas form.
As the gas flows thru the Evaperator the interior air is passed over it, exchangeing heat.
The gas then flows back to the compressure to start the cycle all over again.
I hope this helps!
Mike

[LS6 Tommy]

Mj, I'm not trying insult or flame, but there's some things wrong with your post.

Freon is not a type of refrigerant. It's a brand name. 134a is Tetrafluoroethane, a haloalkane refrigerant.
The boiling point of 134a is dependent on pressure. It boils at -15.34* F at atmospheric pressure (14.7 Psia). Increasing the pressure only condenses it to a liquid if the temperature remains constant.

The refrigerant cycle goes like this:

The refrigerant leaves the compressor as a high temp, high pressure, superheated gas.
It enters the condenser where it is desuperheated, condensed & subcooled into a high pressure, low temperature liquid. It does drop pressure a little.
The subcooled, high pressure refrigerant enters the metering device where it's pressure is dropped & it starts to boil, becoming a low temperature, low pressure saturated gas.
Next it enters the evaporator, where it is exposed to the heat of the air & the boiling increases, removing the heat from the air. It continues boiling until it becomes a low pressure, superheated gas.
It enters the compressor where it is compressed to a high temperature, high pressure superheated gas & the cycle starts all over.

Tommy