With the cost of refrigerants what they are today, it makes more sense than ever to take the time to have a good estimate of how much is required for the system. If you're short by 100-200 lbs the few hours it takes won't seem so terrible at all. The math really isn't so difficult. It usually take a bit more effort to gather the needed info than to tally it all up.
If you have the original store legend available, much of what you need will be listed in there like line sizes, length of runs case and condenser model numbers, etc. If there's no legend at the store, the owner's should have a copy or contact the rack manufacturer. They will have one.
The
Honeywell Refrigerant Piping Manual has a good section on the weight of refrigerant per foot as well as data for receiver capacities. (See p.11-12) For a typical store you'll only be concerned with about 7 pipe sizes...ie, 1/2", 5/8", 7/8, 1 1/8", 1 3/8", 1 5/8" and 2 5/8". One shortcut in the math is to calculate only the liquid lines, total them up and then add a factor of about 5% to account for the vapor in the discharge and suction lines.
The case and condenser manufacturers will typically state the normal operating charge for their equipment in their literature. When I used to do such estimates I usually figured the condenser's fully flooded charge and the receiver at 25% as the worst case scenario for operating charge, but if you have split condensers figure 50% condenser flooding.
I used to scoff at Dad for accounting for how much solder each joint required when he did estimates for market installations. Now I realize his wisdom because it paid off twofold. One to help win the bid and the other to help insure he made money if he got it.
