a. Check the nameplate on the unit to determine the exact amount of charge. This figure is reflected in ounces. (Some larger commercial units indicate this figure in pounds [fig. 64b]. In this case, the refrigerant tank must be weighed. Subtract the amount of charge and then leave the tank on the scale. Charge the unit until the correct amount of refrigerant leaves the tank.) A more precise method is to use an electronic charging scale (See fig. 75c).
b. Fill the charging cylinder with the exact amount of refrigerant as described earlier and in figure 80.
c. If there is no service valve on the suction line, connect a piercing valve and leave it closed.
d. Connect the charging cylinder to the gauge manifold and to the piercing valve as shown in figure 80 and leave it closed.
e. Turn on the unit.
f. Turn on the piercing valve, the compound gauge, and the valve on the charging cylinder until the Freon in the charging cylinder gets to zero on the scale of the charging cylinder. Then,
1. Shut off the charging cylinder valve.
2. Shut off the piercing valve and the valve on the compound gauge.
3. Remove the compound gauge hose from the suction line.
4. Remove all of the hoses and put the cap on the piercing valve after wrapping its threads with Teflon tape.
When installing a piercing valve, tighten the screws gently and evenly. Do not overtighten. Also be careful when removing the manifold gauge lines. Any movement of the valve on the tubing may cause a leak. All this information is given in the instructions with piercing valves. Be sure to read it.
Commercial units can be charged by the high-side method. Connect the manifold gauge to a refrigerant container (never use a disposable container with this method, as it may explode). Then connect the gauge to the suction and discharge service valves as shown in figure 80a.
a. Using a service wrench, turn the stem on the discharge service valve all the way clockwise with the refrigerant container upright. (See figs. 101 and 102.)
b. Run the compressor no longer than a few seconds, just enough to build a pressure in the refrigerant container, about 35 to 40 psi above the pressure in the condenser.
c. Turn off the compressor and turn the refrigerant container upside down to allow liquid Freon flow from the cylinder. Read the printed instructions on the refrigerant cylinder for the type of Freon you are using first.
d. Turn the stem on the discharge service valve counterclockwise until you hear a gurgling sound (which means liquid refrigerant is flowing into the receiver). Read page 117 before using this method.