Two common refrigerants, ammonia and R-12, are compared on the basis of energy efficiency, superheat rejection, superheat temperatures, and high-pressure heat exchange area. Ammonia has the same energy efficiency as R-12 when used in a simple single-stage vaporcompression cycle. Because of its high thermal conductivity, ammonia requires less high-pressure heat exchange area than R-12. The ammonia becomes much more superheated than R-12, which can lead to excessive temperatures in the compressor. This problem can be eliminated by recycling liquid ammonia to the compressor, which keeps the temperatures low and improves energy efficiency. This recycle will be discussed in Parts II and III of this three-part series.