What is SUPERHEAT??

[ACProf]

This question comes up every so often on the board when someone uses the word SUPERHEAT in a response about Thermostatic Expansion Valve (TXV)operation. Unless you are in the field of Thermodynamics, or went specifically to an Air Conditioning School, what superheat is is generally a mystery. I was asked in another posting by a frequent contributor to explain in some detail what superheat really is.

If you are not familiar with the basic names and operation of an AC system or don't care to know what superheat is, STOP HERE and go on to another post. Knowledge at this level is not necessary to diagnose and repair an AC system. I've condensed this tutorial down to as few words as possible and still present as simply as possible in general terms what SUPERHEAT is and why it is important in the design and operation of AC systems..
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Superheat measurement is the only completely accurate way to determine if an airconditioning system is charged with the exactly correct amount of refrigerant and is operating at peak efficiency. Superheat measurements are most commonly used on HVAC home and commercial systems to determine the correct refrigerant charge. HVAC systems have a known speed and volume output compressor, permanently adjusted orifice system (called a capillary tube system), known heat load, and constant airflow over the evaporator and condenser. The constant superheat temperature value will be determined mainly by the refrigerant charge in the system. None of these are true on an automotive system making superheat determination more difficult, and not widely used. . Although not complicated to determine, it does require a few laboratory quality pressure and temperature measurement devices.

Superheat is very near and dear to me because it is the reason, as a teenager, that I got the left side of my face and ear pretty severely burned by steam expelled from opening an overheated car radiator.

Because everybody knows about water, I'll use water to explain the principle of superheat and then apply it to AC evaporator operation.

Everything exists in one three states, solid, liquid, or gas (vapor). Water can exist in all three states, depending on its temperature and pressure. Let's start with water as a liquid. How hot can water get? Does boiling water get hotter if you turn up the stove heat? Answer: NO. WATER (liquid) at sea level, can only get to 212 degrees F. If more heat is added it starts to boil and turn 212 degree water (liquid) into 212 steam (vapor). Turning up the heat more only makes water boil FASTER into steam, but as long as both, water and steam exist, they both stay at 212 degrees.

Water requires a lot of absorbed heat energy just to turn into steam at the boiling temperature. The heat absorbed by the water to change into steam (vapor) doesn't raise the temperature of the water or the steam. This is called LATENT HEAT (hidden heat) because in spite of all the heat absorbed no temperatures were raised. This law of thermodynamics is what AC systems are designed on.

Once ALL the water (liquid)) has turned into steam (vapor), and ONLY steam exists, if we continue to add heat, the STEAM temperature will rise above the boiling point. This amount of this temperature rise above the boiling point is called SUPERHEAT! The very presence of a temperature greater than the boiling point (that's the superheat temperature) means that all liquid has been converted to a vapor. Obviously (but VERY IMPORTANT) then, if a vapor contains superheat, its guaranteed to all be in the vapor condition!

About the car radiator. This particular car radiator had a 13 pound pressure cap on it. This added pressure will cause the boiling point of water to rise, and the water to remain a liquid well above its regular boiling point temperature. THAT's WHY pressure cookers cook food faster. The food inside is in water that is actually much hotter than 212 degrees. Same in the radiator. The 13 pounds of pressure causes the water in the radiator to stay liquid at temperatures well over 230 degrees.

Not many teenagers know about superheat or raised boiling points. Soooo, the car was overheating and I slowly removed the cap to take a look. I knew that sometimes the radiator spews a little steam and hot water as the pressure is relieved so I wiggled the cap first. - nothing happenned - good. I twisted the cap the rest of the way off and as I lifted it up, the cap seal to the radiator broke loose. Uh Oh!

Well, with the pressure in the radiator now gone, the water reverts to boiling at around 212 degrees. However, the water in the radiator is still 230 degrees. This water is SUPERHEATED 18 degrees above its new boiling point!!! WHAT HAPPENS??? It wants to be steam! ALL the water in the radiator IMMEDIATELY turned to steam and blew out of the radiator like a jet engine. I jerked away, but it still got me!! LESSON LEARNED THE HARD WAY!!! As mentioned earlier, (and getting more IMPORTANT) SUPERHEATED substances MUST be in a vapor state.

Finally, how does this SUPERHEAT relate to air conditioning.

The whole reason air conditioning works is because liquid refrigerant with a very low boiling point entering the evaporator is made to boil to vapor by absorbing heat from the air passing over the evaporator coils. The air forced through the evaporator is now leaving with less heat (say-cooler) than when the air went in.

At this point, as mentioned earlier with water, the temperature of the vapor will be the same as the boiling liquid it came from (remember latent heat). This also means that the temperature of the evaporator output tube will be at the same temperature as the input tube.

However, a serious problem may still exist, if not all the liquid actually gets boiled into vapor, and some liquid leaves the evaporator. The liquid will damage the compressor. This is where SUPERHEAT comes in. The TXV will insure that all the liquid is boiled by limiting the incoming refrigerant flow so that the outlet temperature is higher than the inlet temperature. When the evaporator output tube is slightly warmer than the input side, this INSURES that there is a SUPERHEAT temperature and everything leaving the evaporator WILL be vapor. HOW does the TXV know how to do that?? Oh Yah, the TXV temperature sensing bulb that is attached to the evaporator tubing near the discharge end!

The temperature bulb "reports back" to the TXV what the outlet temperature of the evaporator is by way of pressure in the bulb's tubing. The TXV already "knows" what the boiling point temperature of the refrigerant is (by way of the low side entry pressure). These two pressures are on opposite sides of the diaphragm that operates the flow control valve. The TXV diaphragm movement is factory adjusted to allow just enough enough refrigerant into the evaporator to guarantee complete boiling to vapor and an additional amount of temperature rise above boiling (THE SUPERHEAT!!) guaranteeing all the liquid refrigerant has boiled and no liquid to escape remains. The TXV then, as a side benefit, will always allow in as much refrigerant as possible, while maintaining the superheat temperature to prevent liquid discharge. That's MAXIMUM EFFICIENCY of the evaporator. Too little (starved TXV) or too much (flooded evaporator) refrigerant charge will interfere with the TXV's operation and results in less than perfect operation.

Typical SUPERHEAT for auto systems is 3-5 degrees. On Home and Commercial Units, it can be over 10 degrees.

Too bad Detroit replaced this little jewel with an Orifice tube that does NONE of these things. THAT'S WHY there is an accumulator on the end of the evaporator on an O-tube system -- to catch the liquid that gets in but doesn't get boiled because there is no SUPERHEAT control.

Side note 1- for you Suburban and most DUAL-AIR system owners. The REAR system generally has a TXV and the front system has an O-tube. THAT'S WHY the rear system seems to work better all the time, compared to the front system. It IS working better!!!!

Side Note 2 - (A CAUTION ACTUALLY) I have heard that there have been several cases where water put in a glass measuring cup and heated in a microwave didn't boil and became superheated. When the person disturbed the cup, the entire contents of the cup turned to steam and severly burned the person's hands and arms. Sounds plausible. To avoid this, it is recommended to put a spoon (metal's ok) in the cup before heating. It will cause boiling to occur instead of superheating.

[Duane in Japan]

Thank you very much, I understand the pressure differential (or temp) diaphram in the TXV, it may be very similar to an aircraft pressure differential when adjusting cabin pressure as opposed to outside pressure at 30,000 feet, it self regulates based on what you set the cabin altitude as manually.

I hate to sound dumb or cheap, my microwave is over 20 years old and still does ok, although it will not pop popcorn anymore, I cannot put a spoon in it, there will be a lightning storm inside, did the technology change now, I need a new microwave anyways.

May I have your permission to copy and reuse this theory in the future. I also always thought that steam just above the water was hotter similar to infra-red being warmer than red, visible light, just outside the spectrum, latent heat, thats the answer once and for all, I wont question a guy who has had a revelation.

We have also noticed newer TXV with no temp bulb or capillary tubes only a very short capillary tail that seams to be useless, just a cap that was soldered closed for whatever is in the diaphram. Is this another subject ?????? Sorry.

[ACProf]

Duane,

My posting here are public domain.

[Duane in Japan]

We recieved a very nice set of wedding dishes with real 14k gold inlay, the gold gave me a few flowery tatoos on my hands sometimes, and a few minor light shows. I was pretty sure smooth metal could go in similar to the wrap around a microwavable burrito with the browner option in the package.

Aneroid sound close. Spring loaded diaphram, adjustable. Been too many years since my Air Force training seminar.

[Tom Greenleaf]

Thanks Prof for the great explanation. Duane and Prof. My micros will take metal but not the fancy ware with gold or silver trim finer stuff as it marks is and does spark when doing so. Metal is suggested in cookbooks like using foil to stop overcooking of certain foods with protrusions. All micros I have are older and can't speak for ones less that about 15 years old,

T

[cm5400]

I saw the superheated water "myth" on Mythbusters. Totally confirmed that it can and does happen. The water did not look like it was boiling but as soon as it was disturbed, it violently exploded into steam. Pretty scary stuff.

[ACProf]

[Warren Willingham]

An old seat of the pants installer/ tech gets confused around you rocket scientists. Nice job, guys! marv, I'll try to get this into the knowledge base. I'll probably need Chris Bede to help. He's a pretty smart feller, kinda like y'all and our DBACKS manager, Bob Melvin!! Had to get that in. Ha.

[johnandcats]

I also hear a lot about subcoolong in the HVAC world. Can you explain briefly the difference?

Thanks!

[ACProf]

I'll consider posting that in the Knowledge base.

[Fordadicted]

umm .... I doubt many average joes understand what was posted .

Basically ... in the evaporator .... ( the part in the dash where the fan blows through ) the line going into it has a certain temperature .... lets say 150 degrees .... this is called the liquid line , or high side line .

The line comming out of the evaporator will be slightly hotter because it has absorbed the heat from the air inside the car ( the suction line ... the bigger one ) so its probly 165 degrees .

( Yes the refrigerant absorbs the heat from the air .... strange huh )

So we say that the line going in was hot .... and now the one comming out is now hotter ... so essentially in technicians terms its superheated .

[Duane in Japan]

I disagree here my friend, the outlet of the evap is cold to the touch but none the less, the freon inside is carrying heat away from the evap.

Look at a map of an AC system, compressor pumps out hot vapor, the condensor condenses the hot vapor into a hot liquid (subcooling, but not cold), the TXV converts the liquid into a cold vapor and the cold vapor carries heat away from the evap back to the compressor.

The way I undetstand superheat is that the far end of the TXV will not allow liquid freon pass back to the compressor via regulation of the liquid that is entering by compaing the inlet to the outlet (regulation). The fan blowing warm air across the evap will vaporize (boil, by raising the temp of the freon liquid, if any, at this point, the liquid is no longer pressurized to raise its boiling point or to force vapor molecules back into liquid form) the small amount of liquid that got past the inlet of the TXV. This rapid expansion of liquid into a vapor form is what makes anything (liquid to gas, or gas to a rapid flowing vapor) cold. Worse case, there could be a flooded evap but the superheat regulation will not allow liquid to pass, eventually the flooded evap will evaperate back into a gas when the car is shut off. (pressures equalize)

AC units of the past used propane liquid and vaporized it to make it cold. When you pop a tire bead on a tire machine and the tire still has some tire pressure in it, white vapor comes out of the tire when you bust the bead, of course the air in the tire has moisture so that is the rapidly expanding moist air you see and it is cold air.

How much of that is wrong??????

[Fordadicted]

umm ... Duane ... I just picked a number out of the air ... 150 .... for easy comprehension and math .

Im a MVAC tech therefore dont charge by superheat and subcool . The guys that do central air and commercial rooftop systems use that . So I dont know the exact temps on the evap , largely because most are well hidden inside the dash ... its probly close to 35 degrees .

But the way I described it in lamens terms is right on .

If you really want to know the exact temps I can crank up my personal car and check the temps on the lines .

The gas comming from the compressor is probly 200 degrees , and then the liquid line from the condensor is probly 170 degrees ( subcooling would be 30 degrees ) and when it hits the TXV it probly drops well over 100 degrees ( bigtime subcooling ) * wink *

Then as it hits the evap theres superheat as it goes through the boiling process and absorption . Im gonna say its near 35 degrees ... and comming out is probly 45 ( superheat would be 10 degrees )

[Atomic Punk]

Ford

I think you need to get a infra thermometer and take readings from your own system. If you go to the knowledge base and read the original post from AC Prof you wil see that the inlet and outlet temp of a correctly work evaporator will be the same. The TXV compares the temps of the inlet and outlet (by pressure) and when the outlets pressure becomes higher, the TXV will open slightly and allow more refrigerant in to bring the temp (pressure) down and the reverse if temp gets to low. Your condensor is out in the open, so you should be able to take readings easliy, i don't think that your condensor will have any where near a 30 degree drop. (i wish they would).
As for what Duane said-- he was basilly correct except he contridicted himself. In one sentence he said that the TXV converted a high pressure liquid to a low pressure vapor, but then he was correct when he said that the evap took in a low pressure liquid and converted it to a gas by boiling .

[Fordadicted]

Atomic .... uhh .... if the temps are the same on the inlet and outlet of the Evap .... then the refrigerant isnt absorbing the heat from the air inside the vehicle . Thats impossible .

Im not sure why you would try and find false statements in what I type .... I have been doing AC work for almost 20 years . Granted I work on a fleet of trucks .... and I dont see all kinds of different makes and models .... but AC is AC is AC

; )

[ACProf]

Uh, Fordaddicted....

I know a lot of 20 year mechanics. Several are very good at turning bolts, they have lots of experience. One of them dropped out of school after eighth grade! He only buys service manuals with lots of pictures. He can't tell you how to change inch-pounds to foot-pounds, or what the difference is. He doesn't know what temperature pure water in a radiator with a 15 pound cap will boil at. (Incidentally, the answer is about 257 degrees). But if you need an alternator changed, or brakes relined, he's the one to do it. He doesn't claim to be a know everything mechanical engineer.

I'd suggest strongly you open a Physics book or principles of Air Conditioning, and look up "latent heat of vaporization" under the topic of PHASE CHANGE, before you make any more statments about things being impossible. By the way, for the average joe, a Phase Change is a change in state of a substance from, like, liquid to vapor.

If you don't have access to a physics book, browse this link.
http://www.physchem.co.za/Heat/Latent.htm#vaporization

Its a college level physics site that might be more toward the average joe with just pictures and no big words:

Be sure to check out the blue italiczed definition at the bottom of the picture.

[Duane in Japan]

I know they are two different style systems but I know a guy who can service a OT & accumulator system by feel, when the accumulator gets cold (instantly), its full. Again I need to do more studying on all this theory stuff. Also a drop of 50 ~ 55 degrees is about max on a condensor. Any more is a sign of a restriction.

[Fordadicted]

Prof ..... we arent talking about latent heat of vaporization ( although its part of the process ) .... we are talking about Superheat .... and how its acheived .

You simply cannot have a properly working AC system without the evaporizing process in the evaporator .... period .... and this happens with temperature change / phase change .

Superheat is "temperature above the evaporating temp." at a given pressure . When the refrigerant crosses through the metering device ...its pressure drops ....so does its temp ....and then additional heat that it takes on in the evaporator ( sucking heat out from the air inside the vehicle ).... headed down to the compressor ..... is superheat.

We agree so far ?

The amount of superheat tells you how well the evaporator is working, or how well the evaporation process is taking place inside the evaporator. A superheat reading outside of certain range tells you that the refrigerant isn't being evaporated, or making a phase change from liquid to vapor the way it should be. There are many possible causes, a few being over or under-charged system, restrictions, poor airflow across the evaporator, metering device problems, etc.

ALL .... Im telling you ALL Commercial HVAC techs are trained to charge systems by superheat , and subcool temps .

If they take a evaporator superheat reading and its 0 ..... they have a problem .

I think the only way there cant be NO superheat , is if the refrig in the evap is the exact same temp as the air crossing the evap ..... which tells me theres no need for AC to begin with ; )

[Fordadicted]

I know they are two different style systems but I know a guy who can service a OT & accumulator system by feel, when the accumulator gets cold (instantly), its full. Again I need to do more studying on all this theory stuff. Also a drop of 50 ~ 55 degrees is about max on a condensor. Any more is a sign of a restriction.

thats a backyard way ..... but I do that too sometimes when I dont know how much a vehicle takes .

When charging ... slowly .... watch the accum to see when it starts to sweat . Its very close to being full charged . Now check vent temps , and if you have under 40 degrees .... STOP .

[ACProf]

Mr. Ford,

Atomic .... uhh .... if the temps are the same on the inlet and outlet of the Evap .... then the refrigerant isnt absorbing the heat from the air inside the vehicle . Thats impossible .

I think the only way there cant be NO superheat , is if the refrig in the evap is the exact same temp as the air crossing the evap ..... which tells me theres no need for AC to begin with ; )

And you'd be wrong on both counts.

I think you have superheat confused with latent heat. Latent heat is what makes air conditioning possible, not superheat.

Your second comment, first:
Superheat has NOTHING to do with the temperature of the air across the evap and everything to do with the exiting refrigerant temperature compared to the entering refrigerant temperature. If the in and out are the same temperature, no superheat exists (regardless of what the air temperature is), but maximum heat absorption has occurred through vaporization of the refrigerant and so to, maximum cooling. Since superheat (temp above boiling temp) can only occur after the refrigerant has absorbed all the heat from the air the liquid can hold just to get to vapor, superheat does not increase cooling, but its presence does insure that complete vaporization has occurred and the maximum cooling situation has been reached.

Now about that impossible statement:
[Refrigerant entering as a liquid and leaving as a vapor AT THE SAME TEMPERATURE has absorbed more than 95% of heat absorbing capability of the refrigerant. That my friend, is Latent heat of vaporization. Heat absorption that you said is impossible! Its NOT impossible, and is actually the basis of how AC makes cold air come out of the vents.

In other words, according to your "impossible" logic then, if we had an evaporator in a VERY HOT environment, that had 212 degree water (a liquid) enter it, and 212 degrees steam (a vapor) leave it, you'd say that no heat was absorbed in the process of turning the water into steam and no cooling has taken place. Not hardly!

Secondly, AC systems cool perfectly without superheat! BUT, without any superheat , there remains a possibility that not all of the liquid was turned to vapor and some liquid could find its way out to the compressor. The HVAC techs worry A LOT about this condition!!! It overloads electrically driven compressors and burns up the motor in them. MVAC techs need not worry as much, since accumulators collect any of the evaporator liquid overflow output problem on orifice systems, and in TXV systems, the valve restricts the flow of refrigerant to insure 3-5 degrees of superheat on the output.

Incidentally, it might not be as obvious, but auto orifice systems DONT HAVE ANY SUPERHEAT! The orifice just allows exactly the same cruise condition best guess amount of refrigerant into the system in all heat conditions. Sometimes too little, sometimes too much, sometimes just right. You can tune the refrigerant to the best amount by setting approximate cruise RPM and add refrigerant until the evaporator outlet is the same temperature as the input. That's what DUANE was trying to tell you.

That's the best orifice systems can get for maximum heat absorbtion and greatest cooling. Add more refrigerant, - no additional cooling - but liquid comes out and makes the outlet COLDER than the input. Reduce the amount, the outlet gets hotter (superheat) but insures no liquid gets out, but the cooling suffers too.

I don't have to defend or validate these facts of thermodynamics with either my age or AC experience. I'll let any AC book that contains a "theory of air conditioning" do that.