USING PURE R-22 IN A SYSTEM DESIGNED FOR R-12.This note addresses possible implications of using pure R-22in a system designed for R-12 refrigerant.* R-22 is a much higher pressure refrigerant than R-12.Static pressures & TempsTemp	R12-PSIG R22-PSIG70F	70 	 121116F	150	 245A common "low side" design (test) pressure is 150 PSIG for evaporators,especially in cars. On a hot conditions, a (non running) R-12 system,charged with R-22 could easily exceed these pressures and blow outa fragile evaporator, blow out plug or other weak spot.When operating, R-12 high sides run at 150-175 PSIG, and R-22 runsat 230-280 PSIG range.. Again danger of blowing out safety reliefand other weak spots.* R-22 is a much higher "capacity" refrigerant than R-12.A compressor made for R-22 for a given temperature use (low, med,or high temp), at a given horsepower rating, will have much lesspiston displacement than (typically 3/5) an R-12 compressor forthe same service class, so they result in equal capacity andmotor amperage draw.  Using an R-12 compressor with R-22 inthe same service class, will result in motor overloadingand burnout as the motor will be overworked at roughly 5/3of it's rated capacity.It may be possible to use an EPR (evaporator pressureregulator) or system "unloading" devices to allow thecompressor to run at 3/5 of capacity (with R-22) so thenet capacity is the same. EPRs or unloaders will hurt totalsystem efficiencies some though. They may also raisethe compression ratio (see section on R-22 high heat of compression).Failure to control the evaporator pressure can easilyresult in an evaporator which is much too cold andcause freeze-ups..It also may be possible to use a compressor designed forHigh temp (airconditioning), R-12, to be used with R-22in low temp service (freezers below 0F). The "lower" thetemp of service class, the larger the piston displacementfor a given horsepower rating.  However, using R-22 inlow temp applications may cause other problems (see the section in R-22 high heat of compression).Ancient open "belt drive" compressors, may have the pulleyon the motor made larger, to slow down the RPM (by 3/5)of the compressor  so it has the same capacity as itdid with R-12.* R-22 has a much higher "heat of compression" than R-12.For high compression ratios (e.g. a freezer or low temp service),R-22 heats up much more than R-12 does during compression. It fact it (in the compressor discharge valves) may easily exceed 300F, above this temp breakdown of the refrigerantstarts to occur.  R-22 freezers have long been plaguedwith "coking" of the compressor valves, the compressor dischargeline turning black and carbonizing, and all this mess working its way to the expansion device, etc. Acids formed from therefrigerant breakdown eat away at the motor winding insulationand form "sludge". R-502 was invented to help overcome thehigh heat of compression of R-22 in low temp service.A technique called "liquid injection" also can help here.A small hand operated expansion valve (needle valve) ratedat about 1/10 of total system capacity, may be used to injectliquid refrigerant directly into the suction line at a pointnear the compressor, to provide pre-cooling of the suctionvapor. Each degree of drop in the suction gas typically alsodrops the discharge temp a degree also.* Refrigerant metering device may need to be changed out.* R-22 is less miscible in R-12 mineral oil than R-12 isIn short, R-22 does not dissolve in mineral oils as well asR-12 does, so there may be problems returning oil to the compressor,if the refrigerant is changed from R-12 to R-22.Systems  designed for R-22, use smaller diameter suction lines, toincrease gas velocities to help "drag" the oil back to the compressor.If this isn't done, poor oil return, may result in oil "logging"in the evaporator, impairing efficiency, and in severe cases, thecompressor may run out of oil, and fail mechanically from lackof lubrication.  Since may systems do not have an oil sight glass,this problem is often discovered only after it has already happened,when it is too late.This problem may be combated in several ways:1) use smaller diameter suction lines to get better oil return.2) change the mineral oil to "AB" (Alkylbenzene) oil which has   better miscibility in R22.3) add 5% or so of propane, isobutane, to provide for oil return.   R-406A uses this approach. (4% isobutane)--ghg