Step One: Compare Systems

When teaching students about CO₂, the first step should be to compare the basic refrigeration cycle with the CO₂ system. With their much higher pressures, CO₂ systems may seem daunting to students, which is why every effort must be made to explain the similarities between the two systems.

First, cover the basic refrigeration cycle using HFC/HFO refrigerants and the associated high- and low-side pressures that cause change of state. Then, walk students through the entire cycle for a CO₂ system, comparing the pressures in each system type. Explain what must be done to provide a sealed environment that produces sufficient liquid to do the work of removing heat from a conditioned space so it can be rejected into the outdoor ambient or reclaimed in some way to heat water or be used for another purpose.

One difference between an HFC unit and a CO₂ booster unit is the rack system — with the HFC refrigerant system, the low- and medium-temperature rack systems are not combined. With the CO₂ booster, both the low- and medium-temperature loads can be supplied by the same booster rack.

 

Step Two: Emphasize Safety

The next step in teaching students about CO₂ is to talk about safety. In any service or repair refrigeration work, emphasize the necessity for personal protective equipment, including gloves, glasses, and safety shoes. The safety instructions for working with CO₂ are the same as they are for other refrigerants:

• Prevent liquid from reaching the compressor;
• Provide a pressure relief valve (PRV) for overpressure protection;
• Prevent excessive vibration;
• Ensure proper alignment of piping to prevent connection stress;
• Protect external surfaces from corrosion; and
• Take precautions to avoid refrigerant contact with skin due to risk of frost or burn.

It cannot be overstated that students must know and understand how to work with CO₂ safely. Because CO₂ has no odor, a gas detection device is paramount. Warning signs — and sometimes respirators — will be installed wherever CO₂ systems are located. The equipment, too, comes with safety features, such as high-pressure relief valves (HPRVs). Most importantly, those working on CO₂ units must follow the rules and keep their eyes open.

 

Step Three: PID Diagram

The piping and instrument diagram (PID) is a roadmap that identifies every component, sensor, and safety and bypass valve in a working CO₂ system, so it is important that the service technician understands how to navigate it. For instance, the included engineering notes in this troubleshooting tool even explain what type of piping should be used, as well as the bypass locations, allowing technicians to only shut down that part of the system that needs repair.

 

Step Four: Learn the SOO

Students need to understand the sequence of operation (SOO), so they know what to expect when the system actually starts to cool. Take for example, a fairly simple refrigeration system, where the medium-temperature compressors push the gas up to the gas cooler. From there, it goes to the high-pressure expansion valve (HPEV), which brings the pressure (and temperature) down. When the pressure is low enough, it changes to a liquid and is stored in the flash tank.

 

Step Five: Discuss Major Components

Be sure to allocate time to discuss the construction, installation, operation, and basic troubleshooting of all the components and the role they play in a system. This includes the CO₂ compressor, temperature sensors, HPEVs, pressure sensors, and HPRV valves.

 

Step Six: Identify Best Practices

It is important for students to understand the best practices that should be employed when installing, starting up, and commissioning a CO₂ system. Under this general topic, instructors must stress the tasks that are similar to those involved with installing and servicing HFC systems. However, students should understand that all tools, test devices, and valves must be pressure-rated for use on CO₂ equipment.

Piping, brazing, and evacuation practices are virtually the same, but charging must be more deliberate at the flash tank. For example, the service technician must be careful to avoid the triple point. This means that only CO₂ vapor should be introduced into the system until the pressure rises above 75.1 psia. If liquid CO₂ is introduced into the system before this pressure is achieved, dry ice can form in the sealed system. Solids in a sealed refrigeration system are never a good idea.

When charging the system, a safe pressure to begin the switchover from vapor to liquid charging could be in the 100 to 150 psia range — a range in which no dry ice can form. If the rack being started is a medium-temperature/low-temperature (MT/LT) transcritical booster system, the MT compressor(s) must be started first, giving the MT loads time to stabilize. Then the LT compressor(s) can be brought online. Once the mid-tank sight glass clears, the startup technician can begin checking the loads as they begin pulling down to the set temperature.

Note that the controllers for CO₂ systems are quite complex, and they will likely be handled by an HVACR controls specialist or integrator. The installing technician will need to verify that all temperature and pressure sensors are located properly throughout the system and that the leak detection probes are properly placed and wired. Students will also need to learn about the different types of copper that may be used on the same job — sometimes, even stainless steel piping is employed on the same rack.

Learning about refrigerants such as CO₂ in the classroom will teach students why each component does what it does, and overall, that makes for better troubleshooters and better servicers of the equipment. Still, there’s simply no substitute for experience. While we, as instructors, can philosophically talk about installing a unit, on-the-job training is essential, and it is the final best practice for all of our students to follow.