is it possible to use R-12 in place of R-22 or vice-versa. and can be mixed in a central plant of having screw/recip compressor.
if possible than in which ratio and with which modification ?
The easy answer is no. Most compressors are designed to compress a specific gas, and are designed for either high, medium, and low design temperatures. Some of the open compressors can handle most all gasses and design temps.
Retrofit or conversion gasses are designed for retrofit of a specific gas only. If you have R-12 systems, retrofit them with a gas designed to replace R-12. Compressor manufacturers will be able to reccommend an acceptable alternative gas.
There are many concerns switching from R-12 to R-22 like, R-22 swells rubber o-rings and rubber hoses, gets hot enough to melt softsolder joints, and distributor restrictors need changed, heat exchangers removed and at low temperatures will need quench valves installed to cool the compressor, not to mention synthetic compressor oil changes, expansion valve, and pressure switch changes. The capacity of a R-22 system when the quench valves opens is little more or worse than a R-12 system. If you can live with all that, it is a great refrigerant for high and medium temperature systems. And in 29 years will be extinct.