Don Gillis with Emerson joins us on the podcast to teach us the basics of CO₂ as a refrigerant. He explains how it works and its applications.

Carbon dioxide is a colorless, odorless gas that is becoming an important refrigerant for commercial refrigeration (R-744). It is desirable because it has a low critical point and high triple point, so we can use subcritical (below the critical point) and transcritical (above the critical point) CO₂. Carbon dioxide also has a very low global warming potential (1), is inexpensive, and is very efficient at transferring heat.

Above the critical point, we see transcritical fluid, which is a high-pressure fluid. Below the critical point, you get lower pressures. We don't see CO₂ in our everyday air conditioners because it doesn't have the typical pressure-temperature relationship above the critical point (over ~88-degree ambient conditions). It is also more common in regions with colder ambient conditions like Canada.

We rarely encounter the triple point in other refrigerants, but it is crucial in CO₂ refrigeration. The triple point is the temperature and pressure at which a substance can exist as a solid, liquid, and gas. The triple point of carbon dioxide is very high, so we can come across it in normal equipment operation. We don't want dry ice in the system, so we want to charge the CO₂ system with our pressure well above the triple-point pressure.

Don and Bryan also discuss:

• John Gorrie's original machine
• Recovery (or lack thereof)
• Sublimation of dry ice (solid to vapor CO₂)
• Risk of asphyxiation in confined spaces
• Leak detection
• Saturation and operation pressures of CO₂ compared to HFCs
• Liquid vs. gas tanks
• Piping and fittings
• CO₂ grades and moisture content
• Sales and distribution

 

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