Nov 15, 2018
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:
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