How Cooling with CO2 Works
Liquid CO2 (L-CO2) is a popular refrigerant and is also known as Refrigerant R-744 according to ASHRAE (Formerly the American Society of Heating, Refrigerating and Air Conditioning Engineers). Most of the widely used methods of process cooling involve the vaporization of a liquid to a gas.
Processes like water cooling, air cooling, and thermoelectric cooling would be exceptions, but the cooling process for environmental testing generally involves a phase change, that is the cooling media changes from one state to another, typically from liquid to gas. This phase change process releases what is known as latent heat, the energy that is tied up in the media keeping in the liquid (or solid) phase. This absorption of heat is in addition to the heat that is removed simply by the warmer item warming up the cooler cooling media.
Cryogenic cooling with CO2 utilizes a special case of phase change. Because of its chemical properties, it has no liquid state below 75 psi. It is said to have a triple point instead, where it can simultaneously exist in all three states, liquid, gas, and solid. Most elements or chemical compounds have a triple point at some combination of pressure and temperature ( for example water has a triple point at .088 atmospheres and 0.01 Degrees C.) For CO2 the triple point happens in the range releasing to atmosphere CO2 of normal tank temperatures and pressures. What this means for cooling purposes is that when liquid CO2 is precisely introduced to the system and the pressure is dropped dramatically such as at the nozzle of a spray gun or cooling injector tube on a temperature chamber or thermal platform (cold plate), the liquid quickly turns to dry ice snow, solid-state CO2. As the dry ice warms up, the resulting phase change is the direct change from solid to gas, called sublimation. There is a great release of the latent heat as the CO2sublimates.
Because CO2 readily goes into the solid state for cooling processes, it can sometimes be troublesome if there is some problem that might cause too much coolant to be expanded into a solid at once and cause blockage of the cooling system. It is very important to design CO2 supply and control systems so that the cross-section of the plumbing line never (expands and subsequently) restricts. This will invariably result in dry ice blockage.
Due to the lower costs to capture and compress CO2, it is generally commercially cheaper and for that reason can be a preferable coolant. If your cooling requirements are not as extreme (above -50C), CO2 can be a better choice. A properly designed and maintained system will not suffer blockages due to the sudden solidification of liquid CO2.
High pressure (900 psi) CO2 is generally a little less efficient due to the lower heat of vaporization but it can be a better choice when usage is intermittent as it stores indefinitely at room temperature. Low-pressure CO2(~300 psi) stored in vacuum insulated bottles is often more cost-efficient than LN2. Low-pressure CO2 will store somewhat longer than LN2 because it is stored at about -18C inside the tank as opposed to -190C. By comparison, LN2 can go colder and has greater heat removing capability below -60C, CO2 can be cheaper in larger-scale usage due to its slightly lower volume cost and slightly longer storage times. A couple of further advantages to CO2 cooling is that since it is not nearly as cold upon liquid delivery there, is less loss and consumption delay due to vaporization in the hose. With less vaporization in the hose, delivery time is more predictable upon startup and after longer pauses in usage.
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