Vapor Charging for Proper Subcooling

The thermostatic expansion valve (TXV) maintains a constant superheat over a wide range of load conditions. Because of this, charging using the superheat method cannot be done with systems that contain TXVs or similar devices. Instead, subcooling is used to check the charge. Subcooling is the temperature removed from a refrigerant after all the refrigerant has condensed into a liquid.

The subcooling method (Figure SP-4-11) can be used to check and adjust the charge in an operating system. This method measures the temperature of the refrigerant in the liquid line to determine if the proper quality of liquid refrigerant is being applied to the TXV metering device. If the liquid line temperature is incorrect, it can be changed by adjusting the amount of refrigerant in the system.

The same Required Superheat/Subcooling Calculator shown in the previous example can be used when charging with the subcooling method. The subcooling calculator is on the reverse side (Figure SP-4-12). When used in conjunction with the liquid line temperature and pressure measured in an operating system, this calculator is used to find the required liquid line temperature. Complete instructions for charging by subcooling are printed on the calculator.

A detailed procedure and illustrated examples for charging by subcooling using the Subcooling Calculator are provided later in this section. A brief overview of the procedure and the use of the calculator follows.

After the system has run at least 15 minutes and operation has stabilized, measure the liquid line temperature and pressure. Also, refer to the unit nameplate or the service manual for the unit to find the subcooling temperature required for the system.

The required value of subcooling obtained from the unit nameplate and the liquid line pressure measured on the system are used with the calculator to find the required liquid line temperature (°F). To determine if an adjustment in the system refrigerant charge is needed, the actual liquid line temperature is compared to the required liquid line temperature found with the calculator. A tolerance of ± 3 ° F is allowed before any adjustment is required (Figure SP-4-13).

If the measured liquid line temperature is too high, add refrigerant vapor to lower the temperature in the liquid line. If the measured subcooling temperature is too low, remove (recover) excess refrigerant to raise the temperature in the liquid line. If it is necessary to add or remove refrigerant, the procedure must be repeated each time because the system pressures and temperatures will change, but the required subcooling remains the same.

If a Subcooling Calculator is not available, or you are working on a system that uses a refrigerant other than HCFC-22, the subcooling method of charging can still be used. In these cases, the measured liquid line temperature and pressure can be used along with a standard pressure-temperature chart to find the system subcooling. Using the pressure-temperature chart and the measured liquid line pressure, the saturated temperature of the refrigerant in the liquid line can be found. Then, the subcooling in the liquid line can be calculated by subtracting the measured liquid line temperature from the saturated temperature found with the chart. The calculated value for system subcooling is compared with the required value o f subcooling specified on the unit nameplate or service literature to determine if an adjustment o f the refrigerant charge is necessary. A tolerance of ± 3 ° F is allowed before any adjustment is required.

For example, assume the following:
• The nameplate for an HCFC-22 system shows the required subcooling is 10° F.
• The measured liquid line temperature is 105° F.
• The measured liquid line pressure is 243 psig.
For this example, the pressure-temperature chart shows that the saturated temperature corresponding to the liquid line pressure of 243 psig for HCFC-22 refrigerant is 115° F.

By subtracting the actual liquid line temperature (105° F) from the saturated temperature found with the pressure temperature chart (115° F), the amount of subcooling in the system liquid line is calculated to be 10° F (115° F — 105° F = 10° F).

Since the calculated value of 10° F subcooling is within ± 3 ° F of the required nameplate value (also 10° F), no adjustment is needed.