“In the first stage, the suction gas is raised to about 5 to 6 Mpa (intermediate pressure). And inner pressure of the shell case is intermediate pressure to minimize gas leakage between compressing rooms and the inner space of the shell case.”

The researchers stated that the compressor “achieved high efficiency and high reliability. This could facilitate the development of environmentally friendly home-use refrigerators and air conditioners.”

Reciprocating Units

Next, engineers at Shizuoka University looked at CO2 in reciprocating compressors. They assembled a prototype and noted that piston rings “functioned well as a means of sealing in the CO2 under high-pressure operation.

“However, some leakage flowing backward through the valves existed, and it affected performance of the compressor with a relatively small stroke volume.” So, “some oil was mixed into the suction vapor intentionally. The oil was supplied from an oil reservoir having a function of oil separator in the discharge line of the compressor.”

While such a compressor does work, the researchers said it is necessary “to take great care of the estimation of the clearance volume and the valve leakage.”

Scroll Prototype

There was also research done by Matsushita with hermetic scrolls using CO2. The company built a prototype similar to a medium-capacity scroll. Displace-ment volume was set to obtain a capacity range of 2.5 to 5.0 kW when operated by a 30- to 60-Hz ac inverter.

The company reduced the “wrap height in proportion to the displacement volume. The volume ratio was modified by changing the number of wrap windings to take into consideration the smaller compression ratio” of CO2 vs. R-410A refrigerant.

A key finding: “The volumetric efficiency and the compressor efficiency were improved as operating speed increased. Although the volumetric efficiency of a CO2 scroll compressor was expected to be substantially lower than that of an R-410A compressor due to the large pressure difference between suction and discharge, the difference in fact remained extremely small.”

Semi-Hermetic Research

Finally, there was research on two-stage semi-hermetics done by SINTEF Energy Research in Norway. The researchers developed a series of single- and two-stage compressors with two cylinders running at nominal speeds of 2,900 rpm.

According to a report, “Measurements on the two-stage CO2 compressors show that very good efficiencies can be achieved at low-temperature applications compared to a one-stage compressor.

“Using a two-stage compressor also increases the flexibility in the overall system design.”

Publication date: 10/02/2000