Coefficient of Performance:
The Coefficient of performance or COP contains a direct bearing on the running price of the refrigeration system. Greater the magnitude of COP, lower shall be the running cost. As the COP of a refrigeration system is restricted by the Carnot COP, for high operating pressures a multi-stage refrigeration system must be employed. CO2 contain a very low COP. Therefore, it is not appropriate for use as a refrigerant.
Density:
The density of the refrigerant must be as high as possible. In reciprocating compressors, the pressure go up is accomplished through squeezing the entrapped fluid inside the piston-cylinder assembly. Therefore, density selects the size of the cylinder. Again in centrifugal compressors pressure go up is belonged to the density of the vapour. A high value of density results in high pressure goes up.