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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..
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
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
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
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!
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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