USING PURE R-22 IN A SYSTEM DESIGNED FOR R-12.

This note addresses possible implications of using pure R-22
in a system designed for R-12 refrigerant.

* R-22 is a much higher pressure refrigerant than R-12.

Static pressures & Temps
Temp	R12-PSIG R22-PSIG
70F	70 	 121
116F	150	 245

A 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 out
a fragile evaporator, blow out plug or other weak spot.

When operating, R-12 high sides run at 150-175 PSIG, and R-22 runs
at 230-280 PSIG range.. Again danger of blowing out safety relief
and 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 less
piston displacement than (typically 3/5) an R-12 compressor for
the same service class, so they result in equal capacity and
motor amperage draw.  Using an R-12 compressor with R-22 in
the same service class, will result in motor overloading
and burnout as the motor will be overworked at roughly 5/3
of it's rated capacity.

It may be possible to use an EPR (evaporator pressure
regulator) or system "unloading" devices to allow the
compressor to run at 3/5 of capacity (with R-22) so the
net capacity is the same. EPRs or unloaders will hurt total
system efficiencies some though. They may also raise
the compression ratio (see section on R-22 high heat of compression).
Failure to control the evaporator pressure can easily
result in an evaporator which is much too cold and
cause freeze-ups..

It also may be possible to use a compressor designed for
High temp (airconditioning), R-12, to be used with R-22
in low temp service (freezers below 0F). The "lower" the
temp of service class, the larger the piston displacement
for a given horsepower rating.  However, using R-22 in
low temp applications may cause other problems (see the section 
in R-22 high heat of compression).

Ancient open "belt drive" compressors, may have the pulley
on the motor made larger, to slow down the RPM (by 3/5)
of the compressor  so it has the same capacity as it
did 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 refrigerant
starts to occur.  R-22 freezers have long been plagued
with "coking" of the compressor valves, the compressor discharge
line turning black and carbonizing, and all this mess working 
its way to the expansion device, etc. Acids formed from the
refrigerant breakdown eat away at the motor winding insulation
and form "sludge". R-502 was invented to help overcome the
high 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) rated
at about 1/10 of total system capacity, may be used to inject
liquid refrigerant directly into the suction line at a point
near the compressor, to provide pre-cooling of the suction
vapor. Each degree of drop in the suction gas typically also
drops 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 is

In short, R-22 does not dissolve in mineral oils as well as
R-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, to
increase 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, the
compressor may run out of oil, and fail mechanically from lack
of 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