Water feels cold on your skin because it takes away heat (540 cal/gram) as it evaporates. Water vapor from your breath fogs and warms a cold glass as liquid water beads condense on the cold surface. Refrigerators and air conditioning units repeat these phase transitions, evaporation of liquid to gas and condensation of gas to liquid, in a continuous cycle to carry heat from an evaporator inside a compartment to be released at a condensor outside.
Air conditioners and refrigerators work by evaporating refrigerant liquids in a sealed heat exchanger (evaporator - some look like a car radiator) to cool air. Refrigerant liquids, like Freon (and the new R-134) are chloroflorocarbons (CFCs) with very low boiling points (-30°C) that remove large amounts of heat as they quickly evaporate. The refrigerant vapors can then be pressurized with a refrigeration compressor and sent to another sealed heat exchanger (condensor) to cool and condense the hot gases back to liquid form. The high pressure liquid refrigerant is ready to squirt into the evaporator for additional cooling on demand.
Automotive Air-conditioning System
The following description of refrigerator repair is applicable to home and automobile air conditioners - the basic systems are the same. These general instructions are presented for educational purposes only. This description refers to Freon (R-12) but your system may utilize other refrigerants, such as R-22 (many air conditioners) or R134 (more environmentally safe Freon replacement). Keep in mind that sections 608 and 609 of the EPA Clean Air Act prohibit venting of any CFCs to the atmosphere.
Refrigeration pumps (compressors) are the simple but mysterious black canister heard vibrating behind the bottom back service panel of your refrigerator. Inside the black canister is an electric motor driving a small rotary piston pump. The pump and motor are suspended on springs (to reduce noise) in a pool of refrigerator oil inside the canister. Freon gas enters the interior canister space through a large low pressure copper tube from the evaporator. The pump inside the canister compresses the Freon gas and sends it through a smaller high pressure copper tube to cool and liquefy in the condensor heat exchanger coils. The liquid Freon is sent to a strainer/dehydrator/tank (drier) where it is filtered and passed over beads of a drying agent to remove any water. Finally, the liquid Freon is released into the evaporator, by a thermostatically controlled expansion valve, to cool the refrigeration compartment as needed.
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Equipment Required: Manifold/Gage Set, Vacuum Pump, Freon Line Valves, New Compressor, New Drier, Solder, Torch, Freon, Freon Storage Bottle, Ice Water, Freon Pump.
The refrigeration system is a closed circuit. If there is ever a leak in the system, cooling stops, the refrigeration compressor continues to run, refrigerator oil is lost and air gets into the system. The motor and piston pump overheat. Water from the air can combine with Freon in the overheated system to make acids that will damage other parts of the system.
If you catch the problem early, the refrigeration system might be repaired easily. The first step is to tap into the refrigeration system. Some systems are designed with service ports (Schroeder valves) built into the large (low pressure) and small (high pressure) copper Freon lines going to and from the compressor. Otherwise, self-tapping valves can be installed onto the lines (available at appliance parts stores near you). Connect the appropriate flexible lines from your manifold gage set to the low and high pressure refrigeration lines. Read the Freon charge pressures on the high and low pressure lines - they should read about 50 psi each. If you have this pressure, you probably still have a full Freon charge. Turn on the refrigerator to start the refrigeration pump. The high pressure line should climb to about 120 psi or more and the low pressure line should reach at least 30 psi. If the Freon charge is gone or the compressor is not working, these pressures will not be reached. The refrigeration system must be discharged, parts replaced and the system recharged. If significant amounts of Freon remain in the system, the gas must be collected for proper disposal. This is done by pumping the Freon gas from the system into a specially adapted propane cylinder sitting in ice water. The pressurized Freon turns back to liquid in the cold propane cylinder for recycling.
Freon Vacuum Pump/Compressor and Freon Recycle Canister
An inexpensive Freon vacuum pump/compressor can be made using an old compressor from a junk refrigerator. Cut the large and small Freon lines to the compressor (after confirming the Freon on the junk refrigerator has been previously recycled). Disconnect the 120volt-ac lines that power the compressor. Remove the compressor from the junk refrigerator, preferably retaining with the rubberized frame mount. Connect a replacement 120V power line, on/off switch and electrical plug to the compressor. Crimp and solder to seal the copper high and low pressure Freon lines. Attach self-tapping valves to establish standard Schroeder connectors on the lines. You now have a low volume vacuum pump capable of drawing vacuum down to less than 1/10 atmosphere (100 mbar or 2 psig) at the large tube. The pump is capable of producing more than about 150 psi at the small tube. This pump can be used to liquefy Freon gas or to test refrigeration systems for leaks.
Freon can be recycled from systems you service. Cut the flame tip off an old propane torch head and solder a Schroeder valve fitting in its place. Put the modified torch head onto an empty propane canister and pull all gases from the canister with a vacuum pump. Connect the high pressure side of the refrigerator to the low pressure side of your Freon pump through the gage manifold set. Connect the high pressure side to the Freon pump to the propane canister sitting in a pail of ice water. Start the Freon pump. The pressure reading of the refrigerator high pressure line will drop as the propane canister becomes heavy with condensed Freon liquid. Close the propane canister and deliver it to an air conditioning professional for recycling.
If you found the refrigerator pump does not work, it can be replaced. New pumps may be purchased for about $100 from an appliance repair parts shop. Installation is the opposite of removal. Mount the new refrigerator pump on its rubberized mounting brackets within the refrigerator frame. Match up the high pressure and low pressure Freon lines and solder them in place. Reconnect the 120 volt ac power lines. The receiver/dryer must be replaced after exposure to the humidity in air. Cut the old dryer out of the high pressure line and solder a new drier in place.
Vacuum Pump and Gage/Manifold Set
To test the repaired refrigeration system for leaks, see how well it can hold a vacuum. Connect a vacuum pump through a gage/manifold set, to the refrigeration system through the Schroeder valves in the high and low pressure lines. Hold a vacuum on the system for some hours to evaporate any water present in the system. Finally, close the manifold valves to seal the system. Shut off the vacuum pump. Observe the manifold gages over time. If the system holds a vacuum over night, there are no significant leaks.
Evacuate the system over night with the vacuum pump before recharging the refrigeration system to ensure there is no water in the system. Remove the vacuum pump and check the system ability to hold a vacuum in case there is a leak. To ensure no air will be in the refrigeration system, the system should be under vacuum before addition of Freon. To recharge the system, attach a Freon supply to the center line of the manifold/gage set, the low pressure manifold line must be connected to the low pressure refrigeration line and the high pressure manifold line must be connected to the high pressure refrigeration line. All manifold lines must be closed and the gages reading low pressure. Power up the refrigerator to start the refrigerator compressor, then open the manifold low pressure valve. Freon will evaporate rapidly from the Freon supply as it is drawn in to the low pressure side of the refrigeration system. Pressure approaches 130 psi on the high pressure side of the refrigeration system as liquid refrigerant fills the dryer. The refrigerator/freezer food compartments become chilled as liquid Freon fills the evaporator. Shut off the Freon supply and close all valves including the self-tapping Schroeder valves on the low and high pressure refrigeration lines.
The refrigerator repair description above is applicable to automobiles. Repair is often simpler because the refrigeration system components are more accessible in an engine compartment. The refrigerator pump powered by a belt driven from the engine crank shaft. The condenser is in front of the engine radiator. Air is blown across the evaporator under the dashboard and out air ducts to cool the passenger compartment.
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