A refrigerant blend is two or more refrigerants mixed together to form a new refrigerant. All 400 Series refrigerants (e.g., R-404A, R-448A, R-449A) are classified as blended refrigerants. One of the properties of blended refrigerants is that when they change state from a liquid to a vapor, each of its components evaporates at different rates, causing the composition to be altered during the phase change. Not all 400 Series refrigerants behave the same; for some, the composition change is severe, and for others, less so.

Today it is common practice to remove a 400 Series refrigerant from a cylinder in its liquid phase to prevent a potential change to its composition. Adding liquid refrigerant to an operating system can present an issue for a service technician. On some systems, the only low-side access port available is directly at the compressor body. Adding liquid refrigerant at this location or any other low-side port can lead to bearing washout or liquid refrigerant entering the compression chamber of the compressor, both of which can cause internal compressor damage.

When adding liquid refrigerant at this location — or any low-side access — it is recommended to slowly throttle in the refrigerant. There are adapters available that can be added to a refrigerant cylinder to allow the refrigerant to be metered as it leaves the cylinder. Some technicians will use the valve on their manifold gauge to throttle in the refrigerant. They will partially open the valve to allow the refrigerant pressure feeding the low side to be about 10 psi above the current operating suction pressure. They will then frequently close the valve to avoid introducing an excessive amount of saturated or liquid refrigerant into the system at once. Care must be taken when using this method, as it is possible to add too much saturated or liquid refrigerant into the compressor and cause compressor damage.

On systems with a receiver and king valve, there is an alternate method of adding liquid refrigerant directly to a system. With the system running, front seat the king valve and add liquid refrigerant directly into the liquid line. With the king valve front seated, the liquid line pressure will drop below the refrigerant pressure in the cylinder, allowing liquid refrigerant to directly enter the liquid line. A relatively large amount of liquid can quickly be added to the system using this method; however, it will be difficult to determine when enough refrigerant has been added. You will need to frequently stop the charging, put the king valve back to a mid-seated position, and allow the system to operate to a steady state before measuring the subcooling level of the refrigerant leaving the condenser. If the system has a liquid line sight glass, you can also use it as a guide, but it is best to rely on condenser subcooling for an accurate charge.

There are several variables to consider when adding liquid refrigerant to an operating system. Not every system is the same, so one method will not work on every system. Always think before adding the liquid refrigerant into an operating system to ensure you will not cause any compressor damage.