The use of carbon dioxide as a refrigerant continues to draw attention. Every few months, the list of places where the refrigerant is being talked about and tried grows. The ways the refrigerant can work with various compressors is also being explored.

It came up about four years ago at the IKK Expo in Germany. Then the refrigerant turned up in several experiments at Herrick Laboratory on the Purdue University campus in West Lafayette, Ind. Then the ammonia refrigeration industry talked of the use of CO2 in conjunction with ammonia in cascade systems.

CO2 has also established a beachhead at the United States’ biggest HVACR show, the Air-Conditioning, Heating, Refrigerating Exposition (AHR Expo), in technology from the U.S. Army.

A small Environmental Control Unit (ECU) was first shown at the most recent expo. It was developed a few years earlier. Since its development, two additional units have been produced. One has an automotive belt-driven compressor, and the other has a hermetic rotary compressor.

Development was under the direction of John Manzione, chief of the special project team, and the project engineer was Frank Calkins. Also assisting were Robert Bruce, Byrd Pritchett, and Nick Johnson. Work took place at Fort Belvoir in the Washington, D.C. area.


Making such technology a reality requires some trial and error, but also demonstrates persistence. Here is how Calkins described the project.

“In 2000, the U.S. Army Communications and Electronics Command’s Research, Engineering and Development Center developed the first prototype packaged-unitary compact CO2 heat pump built within the physical constraints of existing Army ECUs.

“The performance of this unit has been further improved by the addition of an internal suction line heat exchanger. The unit is lighter in weight than the current military standard unit. When tested under a range of military operational conditions, it achieved a higher coefficient of performance (COP) but a lower capacity than the military standard unit. The reduced capacity was due to an undersized compressor.

“An existing 9,000-Btu/hr Military Standard ECU was modified to produce the CO2 prototype. The maximum system design high-side pressure is 2,250 psi. And as a first prototype, the ECU system was designed utilizing a safety factor of four times working pressure, which is common for Army systems.

“The basic CO2 ECU consisted of a compressor, a gas cooler, an expansion device, an evaporator, and a four-way valve. A Modine suction line heat exchanger was added to allow for additional capacity at high ambient conditions. A suction line heat exchanger transfers heat from the supercritical fluid at the gas cooler outlet, to the cool vapor at the evaporator outlet, thus reducing the temperature of the refrigerant before expansion. The lower temperature at the expansion valve results in more liquid refrigerant available for evaporation in the evaporator and a higher capacity.

“A two-stage, rotary Sanyo compressor was used and the heat exchangers were parallel-flow, multiple-slab, micro-channel aluminum coils manufactured by Hydro Aluminum. These heat exchangers are smaller and lighter than the copper tube aluminum fin heat exchangers currently used in the Army systems and produce a much lower air-side pressure drop. This lower air-side pressure drop allows for the use of much lighter axial fan assemblies with smaller motors, a key factor in the improved COP.”

Calkins noted that the CO2 unit weighs 127 pounds, while an R-22 ECU weighs 170 pounds.

Reasons Behind CO2

There would appear to be two reasons for this — and other efforts — to work with CO2.

One is a sense of uncertainty about the long-term viability of HFC refrigerants. They appear safe for now in the United States, but they are on shaky ground in other parts of the world, especially Europe.

A second reason might be a more idealistic one. There are always those in the HVACR industry who want to do things differently and try something else, in the hopes that the eventual result will be a better mousetrap — in this case a more energy-efficient, environmentally correct, cost-effective way to create necessary cooling.

That may be a long way off when it comes to CO2, but the fact that there are always those trying to achieve such challenging goals, means that it just might happen.

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Peter Powell is refrigeration editor. He can be reached at 847-622-7260, 847-622-7266 (fax), or

Publication date: 04/07/2003