Another team of Purdue engineers studied semi-hermetic CO2 compressors. They noted, “The transcritical cycle technology using carbon dioxide as the refrigerant has recently received increased attention as a possible replacement for vapor-compression-cycle technology using fluorocarbon-based refrigerants.” They further noted that “Most of the major automotive manufacturers have carbon dioxide prototype [A/C] systems, and several new and innovative designs for heat exchangers, compressors, and valves have emerged from studies in this area.”

Moreover, they said applications for such compressors are being looked at for “environmental-control units, which are packaged air-to-air air conditioners that are used in cooling of mission-critical electronics and personnel.”

A number of engineers from Daikin in Japan looked at hermetic swing compressors using CO2. “The density of CO2 is high and its refrigeration capacity per unit volume is large,” they reported. “It means that the displacement of CO2 compressors becomes small and, as a result, the influence of leakage of capacity becomes larger and the volumetric efficiency tends to be lower. In addition, its operating pressure is high and the differential pressure is large. Due to a large differential pressure, leakage increases and to maintain the reliability becomes difficult.

“Therefore, we focused on a swing compressor that can maintain high efficiency and reliability even under the larger differential pressure.”

As a result of their efforts, “We achieved high efficiency and reliability of CO2 compressors by optimizing the design of a swing compressor for minimization of leakage and reviewing the thickness of parts that receive a large pressure difference. These compressors are mounted on CO2 heat pump water heaters that are already commercialized and contribute to energy savings through a high energy efficiency of COP 3 or higher.”

Researchers from Danfoss in Denmark looked at low-capacity hermetic-type compressors as a way to deal with the fluid properties of CO2. “A number of compressors were assembled and tested in various running conditions,” according to the published report. “Tests exceeded more than 1,300 hours with individual compressors at various operating conditions up to 160 bars as high pressure.”

At the time of the presentation, “No critical wear was detected on any compressor performance, as well as the noise and vibration levels.” The report added that the study was continuing in an effort “to further improve the performance.”

Publication date: 09/02/2002