air conditioning cooling
Thursday, September 6th, 2007
whats the basic difination of sub cooling and super heat in air conditioning system?
i m working in air condition company but its make me confusion to understand whats the mean of super cooling and super heat..why it is necessary in airconditioning system,,,
Papa C gives a very good overview of superheat and sub-cool, my hat’s off to you Papa!
You asked for a basic definition and I think I can add just a little more by getting down to those basics. Every liquid has a boiling point where the liquid turns to vapor, and conversely every liquid has a condensing point where vapor and/or gas will turn back into a liquid. Just as water boils into steam (water vapor) at 212 F at sea level and conversely if enough heat is removed from water vapor it will condense back into liquid at the same temperature, refrigerant does the same thing. Different refrigerents will do this same thing at different temperatures and pressures. This is what your pressure/temp. chart for refrigerants tells you. In Papa’s answer above, at 67 PSIG R22 boils at 39 degrees. If you add more heat to the refrigerant you are heating it above it’s boiling point or “super-heating” it. The refrigerant will first turn to vapor and as you superheat it even more, will turn into a gas. Within any kind of refrigeration system (a/c, cooler or freezer) you can measure how much the refrigerant has been superheated in the evaporator the way Papa states above.
On the other hand, Sub-cooling is simply the amount of heat removed below the refrigerants condensing temperature at a particular pressure measured in deg. F. For instance let’s say your compressor is raising the pressure of the R22 in your condenser coil to 240 PSIG, the corresponding condensing temperature is 114 Deg. F. This is the temperature where the superheated gas turns back into a liquid when enough heat is removed. If you continue to remove heat from the refrigerant, you are “sub-cooling” the liquid below it’s condensing temperature. So if the temp. of the liquid line leaving the condenser is 95 Deg. F. you have sub-cooled the liquid 19 Deg. F below it’s condensing temperature at 240 PSIG.
As for why super-heat and sub-cooling is necessary, The more you sub-cool the liquid from the condenser, the more efficient and stable that liquid will be in the run from the condenser to the metering device and will also make the evaporator more efficient at collecting heat. Super-heat on the other hand is a little more critical. Most a/c compressors are depending on the refrigerant coming back to them with some vapor still left (not completely gas) for the purposes of cooling the motor windings. For this reason you want to make sure the refrigerant charge is correct. In a capillary tube or fixed orifice system the refrigerant pressure on the high side will determine the superheat through the evaporator. On a system with a TXV, the TXV determines the amount of super-heat through the evaporator. This is assuming the condenser coil is doing its job and there is solid and sub-cooled liquid refrigerant at the metering device. A typical a/c system will run a super-heat somewhere between 10 and 20 deg. F. Super-heat any lower than 10 means there is very rich vapor coming back to the compressor that can damage bearings and/or valves and super-heat much more than 20 will not provide enough cooling for the compressor windings.
Good luck in the trade! It is a very good one to be in. There are a couple of good real world books on the subject. One is the Modern Air Conditioning Manual and another is Doolin’s Troubleshooting Manual. Both take a real world approach to a/c.
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