What does superheat mean, it is not a term used in automotive hvac, which I am certified in. On a car compressor the superheat switch shuts down the compressor clutch.
Why don't home a/c have a provision to make sure the evaporator does not go below freezing, like automobile systems have .therm sw, poa valve ect..
Thanks For Your Help! Robert Banks ASE Master Automobile Tech firstname.lastname@example.org
From: MAGARJ@cadlab.eas.pdx.edu (Jeff Magar)
Well let's see, superheat means that a fluid has fully gone into the vapor state, this means that there is no fluid left, only vapor.
>Why don't home a/c have a provision to make sure the evaporator does not >go below freezing, like automobile systems have .therm sw, poa valve ect..
Usually the temprature in the suction line is monitored and determins the flow rate of the refregerent at the TXV, this keeps the "evaporator" from freazing. I dont know about car A/C systems but I hope this helps you.
ME Major @ PSU (someday I'll have a real job!)
From: email@example.com (paul milligan)
I couldn't tell you about cars, but superheat is quite simply the number of degrees a condensable gas is above it's boiling point at it's current pressure.
Sub-cooling is the exact opposite. That 34 degree ice water you drink has 178 degrees of subcooling ( which would be a bit excessive in most A/C systems :~) )
At atmospheric pressure, 212 degree steam has no superheat, 213 degree steam has 1 degree superheat, etc. Change the pressure, and the boiling point changes, therefor the temperature at which superheat will be one degree changes.
It's important because A ) much more energy is transferred in the change of state than in merely heating or cooling a liquid or gas, so it's important to change ALL the liquid to gas, and superheat is a way to talk about how far beyond that point ( as a fudge factor, kind of ) the system is, and B ) Compressors make very poor liquid pumps, and tend to break when asked to fill that role. Superheat is a way of talking about a safety margin that helps assure you don't have liquid coming back into the compressor.
~~>Why don't home a/c have a provision to make sure the evaporator does not go below freezing, like automobile systems have .therm sw, poa valve ect..
Many do - generally small inexpensive cap tube thermostats added in the circuit to interrupt 'Y', called freeze-stats.
Now it's your turn - what's a POA valve ? : Paul
Robert Banks wrote :
A POA Valve is a pilot operated absolute valve. It is in the suction line. It keeps the evap pressure at about 30 psi so it is cold , but not cold enough to freeze up and restict air flow (ice).
If you had a GM car in the late 60's-70's it had one. They were about 8 in. long, and went bad often. When the POA became bad they turned into a expensive orfice tube. Most cars are orfice tube systems now. (GM uses a variable displacment compressor) Neat!
Thanks Paul for your super heat explanation.
Robert Banks ASE L1 Master Tech
Another thing to consider is the effect of superheat downstream of the compressor. The compressor adds superheat to the refrigerant vapor due to the heat of compression. This heat has to be removed before the vapor can start recondensing. This heated vapor has a very poor heat transfer coefficient (less than 10) so if too much superheat is supplied to the vapor on the suction side of the compressor, something extra must be done on the condensing side, such as making the condenser surface larger or by increasing the cooling capacity of the cooling medium. In steam systems, superheat is frequently removed by spraying water into the steam just before a condenser. Of course, this won't work in an AC system.
-- Charles J. Sterner, ChE, P.E.
From: firstname.lastname@example.org (George Goble)
It has been done.. The "Hy-save??", people who make the liquid line booster pump, to keep flash gas out of liquid lines, have one of these also...Uses their same liquid line pump to spray condensed and subcooled refrigerant into the condenser inlet or the compressor discharge line.
From: email@example.com (Joseph Huber)
In article <3rhigq$11dg@ns3-1.CC.Lehigh.EDU> a120@Lehigh.EDU writes: >Another thing to consider is the effect of superheat downstream of the compressor. The compressor adds superheat to the refrigerant vapor due to the heat of compression. This heat has to be removed before the vapor can start recondensing. This heated vapor has a very poor heat transfer coefficient (less than 10)
This is not necessarily true for all condensers. In shell-and-tube refrigerant condensers with the condensation on the shell side, if the tube wall temperature is below the saturation temperature of the refrigerant (which it usually is), the phenomenon known as "wet wall desuperheating" will usually occur. The incoming superheated vapor condenses immediately without having to go through a "desuperheating" section. The heat transfer coefficient for this process is within a few percent of the two-phase coefficient.
Because the tube wall temp is usually below the saturation temp in shell-and-tube refrigerant condensers, wet wall desuperheating is generally assumed in condenser design. In the tests I've run on shell-and-tube condensers with sight glasses, I have always observed condensation on the top tube row.
-- Joe Huber firstname.lastname@example.org 817-557-3186
From: email@example.com (Jonathan M. Elson)
You SHOULD have been taught about this. Superheat is the heat that the refrigerant picks up after evaporating, so that it contains more heat than it took to boil it. Therefore, the refrigerant is now heated above saturation for its current pressure. A little superheat is necessary to keep the liquid from entering the compressor, causing damage. Too much superheat overloads the condenser with heat to be rejected, and effectively reduces the capacity of the compressor, because the superheated gas has less density.
: Why don't home a/c have a provision to make sure the evaporator does not : go below freezing, like automobile systems have .therm sw, poa valve ect..
Home A/C units have constant-speed compressors (except newest hi-eff units), whereas auto A/C has compressor speed at the mercy of the driver's foot. The compressor runs too slow when idling, and too fast when cruising on the highway. So, some method of controlling compressor output must be employed. Everything from suction throttling, short-cycling the compressor, variable-speed drives, variable-displacement compressors have been tried. A home A/C has constant speed compressor and a constant-speed evaporator fan, and so freeze-ups are usually the result of low refrigerant charge, clogged filter or thrown evap fan belt. If your humidity goes through the roof, however, the best calculations of the system designer could be thrown off.
[ Editor's note - There were several divergent threads on aspects of superheat going on at the same time, so please forgive me if I get them mixed up a bit. Some really great discussions, though :~) ]