Charging by weight is used if a complete charge is to be added to the system and the weight of the charge is known. Liquid charging an empty system (Figure SP-4-6) is much faster than charging with vapor refrigerant because the density o f liquid refrigerant is much greater than that of vapor refrigerant. The result is that the same charging hoses deliver much more liquid refrigerant than vapor to the system over a fixed period o f time. Because the liquid charging method is faster and more economical, it is sometimes used to recharge a small system (5 pounds of charge or less) that contains a partial charge. In this case, all the remaining charge in the system is recovered, then the system is recharged with the total amount.
Charging by weight is the only method that can be used to charge heat pump units operating in the heating mode.
Liquid charging is performed with the compressor off and the proper charge weighed in to the high side of the system using an accurate charging scale (Figure SP-4-6). Liquid refrigerant is charged through the opened liquid line service valve. The suction line service valve is closed. Never charge liquid refrigerant into the low side of the system. Compressor failure can result from the compressor trying to compress liquid refrigerant that may have entered the compressor.
Under most conditions, the entire charge will flow into the system in the liquid state. A t other times, the flow of liquid refrigerant may slow down to a trickle or stop before the entire charge can be weighed in. Should this happen, stop liquid charging and close the high-side service valve. Then, turn on the compressor and vapor-charge the remainder of the required charge into the low side of the system using the weigh-in method (Figure SP-4-7).
