In Mechanical Engineering, How do you determine what suction pressure should be in R22 air conditioning compressors suction line?

You should measure the conditioned air temperature and subtract twenty five then using a pressure chart convert to psig. For example if the conditioned air is75 degrees farenheight then subtract twenty five or thirty so you get40 to45 degrees farenheight then the psig should be between70 to75 psig.

it is decided on the basis of the refrigerant used and the amount of charge.