Rusty Walker with Hill-Phoenix comes on and talks about CO₂ triple and critical points. He also covers some best practices for refrigeration techs working with CO₂.

The triple point is the temperature and pressure at which a substance can exist in all three phases of matter. CO₂ has a very high triple point, and CO₂ refrigeration equipment can reach its triple point during operation, unlike most other refrigerants. Solid CO₂ is dry ice, and it sublimates by becoming a gas and bypassing the liquid CO₂ phase under low-pressure conditions. Therefore, the relatively high pressure applied in a CO₂ refrigeration system keeps the refrigerant in a liquid state. We want to avoid reaching the triple point because solids can cause restrictions.

The critical point is the point at which a substance becomes a supercritical fluid and loses its pressure-temperature relationship due to densities equalizing. CO₂ has a low critical point, only 87 degrees Fahrenheit. So, CO₂ refrigeration systems will have supercritical or transcritical CO₂ in their systems. You cannot calculate superheat under these circumstances, and you cannot condense supercritical fluid. So, you need to send the supercritical fluid through a gas cooler to reduce the temperature before it can change state.

Critical and triple points are important to keep in mind when working on a CO₂ system. You want to control pressure to steer clear of the triple point and understand the necessity of gas cooling when dealing with supercritical fluid. Remember: all of the basic laws of thermodynamics still apply.

Rusty and Bryan also discuss:

• CO₂ leak detection
• Bars and pressure conversions
• Supercritical fluid as a solvent
• Avoiding triple point on a service call
• Recommended equipment and practices for working on CO₂ systems
• Vacuum
• Booster system piping and brazing