What would the effects be if an r-22 system were to be evacuated and filled with R134a?
What is the reason that this equipment can not handle this gas?

The boiling points of the 2 are completely different.(-15 for 134 and -41 for R-22). In order for the system to maintain the correct temps/pressure, the refrigerant must be compatible. 134 A operates at higher pressure/temps than R-22 and will not cool, as the pressure in the system is directly related to temperature.

Very good answer but the kicker for me is the polyester oil used with the 134a its not compatible with the mineral oil used with r-22 systems.. turns it into sludge..

ISCEON 59
R417A Advantages
• The only HFC based R-22 replacement that is compatible with both synthetic and mineral oils (MO,AB,POE).
• Proven compatibility with all standard equipment components and materials used in R-22 manufactured systems.
• Suitable for use in scroll, screw, rotary and reciprocating compressors.
• Closely matches R-22's operational pressures.
• Can be used in a wide range of traditional R-22 temperature applications, and beyond, while providing adequate oil return and system capacity.
• Avoids the need for unnecessary oil changes and only requires minor system adjustments or modifications.
• Zero ODP (ozone depletion potential) HFC refrigerant.
• Improved COP (coefficient of performance), NU-22 has a lower global warming impact compared to R22, as indicated by NU-22's TEWI (total equivalent warming impact) measurement.
• Non-toxic and non-flammable (ASHRAE classification A1).
• Compatible with all standard refrigerant oils, NU-22 can drastically reduce the level of waste oil handling and disposal normally associated with using refrigerants requiring an oil change.
Performance Specs:
Boiling point
-41.8°C (-43.2°F)
Critical temperature
90.5°C (194.9°F)
Critical pressure
559 psia
Density (saturated vapor)
@ boiling point temp.
0.2447 lb/cf
Density liquid @ 25°C (77°F)
72.85 lb/cf
Heat of vaporization
@ boiling point temp.
89.42 BTU/lb
Quality Specs:
Max. moisture
0.001%vol
Max. non-condensable
1.5%vol
High boiling impurities
0.01%vol
Safety Specs:
ASHRAE designation
R417A
ASHRAE safety classification
A1
Max recommended exposure limits
1000 ppm
Refrigerant - NU22 (R417A)
Part
• Compatible with all standard refrigerant oils.
• ASHRAE Designated.
• R22 like operating characteristics.
• Exceptionally Low TEWI.
10% Energy Reduction Over R22.
• Zero Ozone Depletion.
• No Expensive Retrofits.
• Can Be Topped Off Repeatedly.
• ASHRAE Rated A1.
Non Toxic - Non Flammable

Can an R-134A AC unit be installed with a drained, flushed, and evacuated EXISTING R-22 evaporator coil?