Seven Keys to Servicing CO2 Systems

This blog summarizes an article from our most recent E360 Outlook, entitled “Keys to Servicing CO₂ Systems.” Click here to read it in its entirety.

From a service technician’s perspective, CO₂ has unique performance characteristics and operating peculiarities that dictate system design and impact maintenance requirements. Following are seven key considerations to be aware of when servicing CO₂ systems.

1. 1. Low critical point (subcritical vs. transcritical) —R-744 has a relatively low critical point (1,055 psig or 87.8 °F) that determines its modes of operation. Subcritical mode refers to systems operating in regions with colder climates and lower ambient temperatures where the refrigeration cycle takes place below 87.8 °F. Transcritical mode takes place above this point (also referred to as supercritical) such as in warmer regions or periods during the summer heat.
   2. Higher operating pressure —one of the common reservations when using CO₂ is its relatively high operating pressure. But, it’s important to realize that high pressure only takes place in the beginning stages of the refrigeration cycle while the rest of the refrigeration cycle operates at pressures like that of a traditional R-410A high-side system. Stainless steel piping is typically used to handle these pressures, although high-pressure ferrous alloy copper piping has recently been introduced.
   3. High triple point (possibility of dry ice formation) —triple point is the point at which the three phases of CO₂ coexist (60.4 psig or -69.8 °F). While the temperature seems low, the pressure is relatively high by refrigerant standards. As the pressure approaches that point in CO₂ systems, the refrigerant will turn to dry ice (an unusable state that’s neither a vapor nor a liquid). This can occur during maintenance when a contractor mistakenly thinks the lines are clear, taps the system and discovers the formation of dry ice.
   4. System charging —the high triple point affects R-744’s charging procedures. After pulling a vacuum, the internal pressures of the system will be well below 60.4 psig. Since standard atmospheric pressure is 14.696 psig, the process cannot start with liquid charging. Instead, contractors must vapor-charge the system (roughly to around 145 psig), and then wait until the system has equalized with 145 psig of vapor before charging with liquid.
   5. Managing scheduled shutdowns and power outages — when a CO₂system shuts down for longer periods of time, pressures will build more quickly than in an HFC system. To preserve the system charge, the most reliable method is to install a generator with a standby condensing unit. When the power goes out, the generator powers a condensing unit that has a loop within the flash tank (i.e., receiver) designed to cool the volume of liquid within the tank and keep pressures down.
   6. Resumption of power —the electronic expansion valve (EEV) on every CO₂ case utilizes a stepper motor or a pulse-width modulated type of valve. When the power goes out, the stepper motor is frozen in that exact position, leaving the system’s CO₂ evaporators susceptible to flooding. R-744 naturally migrates quickly to these cold evaporators, and when the system resumes, this can cause considerable damage to compressors. To avoid this, liquid line solenoids placed upstream of the EEV, supercapacitors or battery backups are often used on case controls to force the valves closed during a power outage.

Form a refrigerant plan — managing CO₂ is different from what contractors may be accustomed to with traditional HFCs. Operators and contractors alike need to understand the local codes for storing R-744 cylinders (inside or outside the building), and develop an appropriate strategy.