Bruce Nelson, Colmac Coil President, has written a new design guide for CO₂ evaporators. As a natural refrigerant, ammonia exhibits excellent operating efficiencies; however, its toxicity poses risks to workers and products in the event of a large leak. CO₂ is another natural refrigerant which can be applied in refrigeration systems and has the added benefit of being non-toxic and non-flammable. Therefore, there is growing interest in applying CO₂ in industrial refrigeration systems. The process of designing evaporators for use with CO₂ is similar to that of ammonia; however, certain aspects of the design require special attention, namely,material compatibility, working pressures, and heat transfer and fluid flow properties.

Selection of materials which are compatible with CO₂ may be different compared to ammonia. Materials which can be exposed to ammonia may not be able to withstand corrosion from CO₂ under certain conditions. For example, carbon steel will typically not corrode in ammonia systems, but if there is a sufficient amount of water in a carbon steel CO₂ system, the piping will be susceptible to corrosion due the formation of carbonic acid.

The working pressures in CO₂ systems are much higher than in ammonia systems at similar temperatures, requiring special attention to the tube wall thickness specified for evaporators. Heat transfer and pressure drop characteristics are very different for CO₂ compared to ammonia and require that the evaporator manufacturer adjust circuit length appropriately. For these andother reasons, CO₂ evaporators must be built by special standards. The CO₂ Evaporator Design Guide addresses these issues and illustrates the proper design of CO₂ evaporators.

CO2EVAPORATOR DESIGN

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