Closed-Cycle, Air-Based Refrigeration System Offers New Capabilities
Air Products and Chemicals, Inc. (Lehigh Valley, PA) believes that it has succeeded where few others have in the history of refrigeration: a commercially viable refrigeration system using air as the working fluid. The company has developed a new, high-pressure compression scheme, and has demonstrated a unit at an Eastman Kodak facility in Rochester, NY, producing 50 tons (210 kW) of refrigeration at -100°F (-73°C). Ultimately, Air Products expects the technology, called CCAR (Closed-Cycle Air Refrigeration) to be attractive to food and pharmaceutical processors, and in process applications where either rapid chilling or low-temperature refrigeration is desirable. CCAR was developed by Air Products and Toromont Process Systems (Houston), with assistance from the Advanced Technology Program of the National Institute of Science and Technology (Washington, DC).
The key innovations, explains Edward Kiczek, product manager, are a combined compressor/expander section (which Air Products calls a "compander"), and a plate-fin heat exchanger where the refrigeration is delivered. The general process is the same reverse Brayton cycle that Air Products uses in its cryogenic air-separation plants worldwide. "Our research showed us that at a sufficiently high pressure, air becomes a fluid with good heat-transfer properties," he notes. The CCAR compressor typically runs at 1,300 psi, although this is not a technical limitation.
After air is compressed, it is cooled, in part by the process air returning from the refrigeration section. Next, it is expanded in the expander section, where some mechanical energy is recovered. The cold working fluid, now at -100°F or lower, passes into an aluminum heat exchanger with a plate-fin core. Process air is refrigerated there, and the working fluid returns to the compressor. The overall Coefficient of Performance (COP) is between 0.75 and 0.8 at a discharge temperature of -60°F. "The process is cost-effective when refrigeration at -70°F or less is needed," says Kiczek. Other benefits are the absence of CFCs or other environmentally harmful refrigerants, and a very small footprint: the Kodak system was mounted on a 12 x 35-ft skid.
William Roberts, Air Products' food processing marketing manager, says that CCAR makes sense in food applications where rapid chilling has product-quality benefits. "A colder refrigeration system allows you to chill faster; when you start with a -100°F refrigeration, you can cut the chilling time in half" relative to conventional refrigeration. (Roberts stresses that operators can choose whatever temperature they like for the final food temperature; it is simply a question of how long the chilling process continues.) Smaller ice crystals are formed, and more water is retained in the frozen product, especially for food products that are cooked and then frozen. The weight of the extra water retained can represent 3-4% of the final product cost—an important economic consideration for narrow-margin products. Roberts sees similar benefits in freeze drying or solvent recovery in pharmaceuticals processing.
Fullscale commercial units are expected in the first quarter of 1999, and will have refrigeration capacities to 200 tons (700 kW). Operating temperatures will be -60°F to -150°F (-50°C to -100°C). The company says that it is investigating colder operation as well as other unit sizes.
By Nick Basta
For more information: Air Products and Chemicals, Inc. 7201 Hamilton Blvd. Allentown, PA 18195. Tel: 800-654-4567, fax: 800-880-5204.