Determining target subcooling

[dirtydeeds2301]

I'm trying to come up with the formula used to determine target subcooling for an air conditioner.After several inquiries to major manufacturers, no one can seem to explain to me how the 10-15° target is determined. I already know how to find existing subcooling. I'm looking for the formula that determines the target for a subcooling chart. Input please?

 

[chicopee]

I know of no magic formula that will give you the numbers that you have shown. You should be familiar with the different refrigeration cycles and their related refrigerants, be familiar with the refrigerants thermodynamics properties being used when analyzing these cycles, know the limitations and applications of the refrigeration and air conditioning systems. Sure, when you analyze the thermodynamics cycles, there will be formulas and tables that you must use, however, the temperature limits used in these cycles will be dependent on locations (northern or southern hemisphere, subtropical regions, high altitude areas, dry desert areas, etc) and their applications( such as in a cooler or freezer, air conditioning,).

 

[MintJulep]

You want to ensure that there will be at least some sub-cooling at the expansion device inlet considering:

Longest allowed liquid line run.
Highest allowed elevation differential between condenser and evaporator.
A bit of undercharge.
A reasonable assumption of high-load and high ambient operation conditions (not design condition).

Consider also that likely don't have control of the compressor's maximum flow rate, so playing with the amount of sub-cooling is your way to balance compressor capacity and system capacity.

Balance that against:
Coils are expensive and you don't want to buy a bigger coil than needed.
Refrigerant is expensive and you don't want to have to fill up that big coil