Subcooling seems to be a great mystery to a lot of service techs in the field yet, any unit with a TXV type metering device, should be charged by subcooling instead of superheat. Since a TXV has a pre-set superheat (or an adjustable superheat) setting, it will always try to maintain that superheat even if the unit is over or under charged. This is why charging by subcooling is important for units equipped with TXV’s.
Let’s review the process of subcooling first, and then see how to charge by subcooling. The compressor puts out HOT HIGH PRESSURE VAPOR as part of the compression process. This vapor enters the condenser coil, which performs the function its name states, to turn the vapor into a liquid. As the vapor travels through the condenser, heat is removed and the vapor starts to turn to liquid. At some point in the coil, the vapor has been turned to all liquid. This is called saturated liquid and corresponds to the temperature and pressure you read on your gauges. This usually occurs before all the passes in the coil have been used. These extra passes through the coil take the saturated liquid and removes even more heat, or, subcools the refrigerant. This is important, since a metering device must have a continuous column of liquid to operate properly. By subcooling the liquid before it leaves the condenser, we “guarantee” that the liquid will remain liquid all the way to the TXV. If there is insufficient subcooling, the liquid can turn back to a gas (flash gas) and the metering device will not properly meter the refrigerant into the coil.
Now that we understand how subcooling is suppose to work, we can now proceed with how to charge by subcooling. As stated earlier, when your gauge set is attached to the unit, the “head” or liquid pressure you read is the saturated liquid temperature/pressure. Knowing this, and the manufacturer’s recommended subcooling, you can properly charge the unit. You need to strap a thermometer or thermocouple to the liquid line out side the condenser so you can read its temperature and then add or remove charge to get the desired subcooling for that unit.
Let’s look at an example. If you had a “head pressure” of 198 PSIG and looked at a temperature/pressure chart, you would see that the saturated temperature is 100 degrees. If the unit required 10 degrees of subcooling, the thermocouple attached to the liquid line should read 90 degrees (10 degrees “colder” than saturated temperature). Let’s say the thermocouple reads 101 degrees. Do I add or remove refrigerant to get the 90 degrees temperature I need? The key to answering this is to remember that the closer to ZERO or temperatures above the saturated temperature, indicates an undercharge. Think of it this way, since the unit is short of refrigerant, the discharge temperature of the compressor increases because there isn’t sufficient cooling of the compressor. Since the discharge temperature is higher, it will require more of the condenser coil to remove the heat of compression, which causes little or no subcooling. So, we need to add refrigerant to cool the compressor to lower the discharge temperature to get our desired subcooling.
Conversely, if the liquid line were 80 degrees, this would be 20 degrees of subcooling, which is an overcharge. The compressor is cold, the discharge temperature is down, the condenser does not have to remove as much heat, so the refrigerant reaches saturated temperature quicker leaving more coil passes for subcooilng. In order to get the 10 degrees desired subcooling, we would need to recover refrigerant to raise the compressor discharge temperature to require more of the condenser to be used to reach saturated temperature leaving us with the proper amount of passes to subcool the saturated liquid.
If you keep this in mind, properly charging a unit by subcooling will become very easy.