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What is Superheat?


ACProf  was kind enough to explain the meaning of Superheat for us.  The following text originated from the A/C forum.  The original message and replies appear here.

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.

The article above originated from the A/C forum.  The original message and replies appear here.

Thank you ACProf!









Visitor Comments
  1. Comment #1 (Posted by Ben Moser)
    I do commercial air and have been to several contract classes for copeland and they are adament about superheat being set at 20 degrees and liebert units for server rooms insist on 15 degrees. Loved your article on superheat very enjoyable!!
     
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