Charging Methods

Superheat Method for Fixed Oriface/Capillary Tube Indoor Units-

For fixed metering devices such as bullet orifices, pistons, and cap tubes the superheat method will always be the method used. Measuring superheated vapor values at the suction service valve is one of the most important indicators of proper system performance. If the superheat is too high, the evaporator coil will under perform. If too low, there is a risk of damaging the compressor with liquid refrigerant. A fixed metering device is unable to control super heat; therefore, proper superheat values can vary greatly with conditions that affect it, such as outdoor and indoor air conditions. Use the steps outlined in steps 1 thru 6.

Proceed as Follows-
1). Run the unit for at least 10 minutes to allow the system to stabilize. Then take readings of the indoor and outdoor air temperatures and suction pressure. Write them down.

2). On the gauge face, read the equivalent temperature on the R-22 scale next to the suction pressure reading.

3). Using a thermocouple or probe type thermometer read the suction line temperature as it enters the service valve. Make certain that the thermocouple/probe is tight to the copper line.

4). From the thermocouple thermometer reading subtract the suction pressure equivalent temperature (saturated suction).

5). Proceed to the Superheat chart (Fig. 3). Add or remove R-22 to achieve the superheat reading indicated by the chart.
IMPORTANT: When connecting the manifold to the charging bottle be sure to purge the hose of air with a small amount of R-22.

Wait at least 5 minutes between charge adjustments to allow the unit to stabilize.

6). When charge adjustment is complete remove the manifold hoses from the service valves and replace and tighten service port caps.

Chart based on 400 cfm/ton indoor airflow and 50% relative humidity; use only on systems that cool with an fixed oriface or capillary tube.

Sub-cooling Method for Expansion Valve Indoor Units-

An air conditioning system utilizing a thermostatic expansion valve (TXV or TEV) for its metering device will automatically maintain proper evaporator superheat regardless of varying operating conditions. Therefore, when charging a TXV system, the charge is adjusted to assure that there is pure liquid refrigerant, void of any flash gas, entering the expansion device. Verifying a liquid ‘seal’ at the expansion valve can be accomplished by measuring the sub-cooling, (sub-cooling is the sensible heat removed from the liquid refrigerant as it travels through the last passes of the condenser). As with superheat, the proper level of sub-cooling is important. If the sub-cool level is too low or nonexistent, flash gas will form, causing restricted flow through the TXV reducing system capacity. Higher than normal sub-cooling is an indication that liquid refrigerant is "backing up" in the condenser, reducing its effective condensing area. If too high, due to an overcharge, it can lead to high condensing pressure problems.

Proceed as Follows-
1. Measure liquid line temperature and refrigerant pressure at service valve.

2. Determine total refrigerant pipe length and height (lift) if indoor section is above the condenser. Plot the intersection of the two points on Fig. 4 to determine the charging curve to use (Fig. 5).

3. Plot the pressure and temperature on the TXV charging curve, Fig. 5.

4. If lines cross above the curve remove refrigerant; if below curve, add refrigerant.

5. Whenever charge is removed or added, the system must be operated for a minimum 20 minutes to stabilize before additional measurements can be made.