While both carbon dioxide (CO2) and water vapor (H2O) share some similarities as ideal gases, there are specific reasons why CO2 is used in refrigerators instead of water vapor:
Phase Change Temperatures: One of the primary reasons is the phase change temperatures of the two substances. CO2 can undergo a phase change from gas to liquid and vice versa at relatively low temperatures, making it suitable for refrigeration applications. The critical temperature of CO2 is about 31.1°C (87.98°F), which means it can be readily condensed into a liquid by moderate cooling. On the other hand, water vapor has a much higher critical temperature of 374°C (705.2°F), requiring significantly higher cooling temperatures to condense it into a liquid, making it impractical for typical refrigerator systems.
Pressure-Temperature Relationship: CO2 has a more favorable pressure-temperature relationship for refrigeration. It operates at relatively low pressures and temperatures compared to water vapor, which makes it easier and safer to handle within a refrigeration system. Water vapor, due to its high critical temperature, would require extremely high pressures to be used effectively in refrigeration, which could pose safety risks and complicate the design and manufacturing of refrigeration systems.
Environmental Impact: CO2 is considered a more environmentally friendly refrigerant compared to many other commonly used refrigerants. It has a low global warming potential (GWP) and ozone depletion potential (ODP), which are critical factors in modern refrigeration systems to minimize their impact on climate change and the ozone layer.
Efficiency and Performance: The thermodynamic properties of CO2 make it a suitable refrigerant, providing good energy efficiency and performance in refrigeration systems. It can efficiently absorb and release heat during the phase change, leading to effective cooling capabilities.
Compatibility with Components: CO2 is compatible with many standard refrigeration components, which simplifies the design and construction of refrigeration systems. It doesn't require specialized materials or components like some other refrigerants might.
While both CO2 and water vapor have ideal gas properties, the practical considerations related to phase change temperatures, pressure-temperature relationships, environmental impact, and overall system performance make carbon dioxide a more suitable choice for refrigeration applications. However, it's worth noting that water vapor has its uses in certain specialized applications, but as a refrigerant in typical household and commercial refrigeration, CO2 is the preferred choice.