CO2 As Refrigerant: The Transcritical Cycle

By Jeff Staub, Bjarne Dindler Rasmussen, Max Robinson
January 20, 2004

In three previous articles, we have presented an overview of CO2 (R-744) as a refrigerant, its applications in industrial refrigeration, and a case study of a CO2 cascade system in a European supermarket. (See “CO2 in Refrigeration Applications,” Oct. 6, 2003; “CO2 in Industrial Refrigeration,” Nov. 3, 2003; and “CO2 is Keeping Supermarkets Cool,” Dec. 8, 2003.) The systems presented were all subcritical — that is, the refrigeration cycles were entirely below the critical point of CO2. Now, after a brief review of CO2 characteristics, comparing subcritical and transcritical cycles, we will present a functioning transcritical system with a hermetic CO2 compressor and discuss design considerations.

CO2 is a component of our atmosphere that is essential to life. It has no ozone depletion potential and insignificant global warming potential, so CO2 has no regulatory liability, as do HFCs. There is no need to account for the amount used, and it does not need to be reclaimed. (Characteristics of CO2, compared with those of R-134a and R-404A, are shown in Table 1.)

Other principal benefits of CO2 are that it is a natural substance, it is cheap, readily available, not poisonous in any common concentration, and nonflammable. At prices a bit over $1 per pound, it is truly an inexpensive refrigerant. The grade used must be dry, but it can presently be obtained in 99.9-percent purity from companies that supply welding gases, with a 20-pound tank selling for about $21.