How CCAR Works

Refrigeration is the withdrawal of heat from items to be refrigerated to achieve lower than ambient temperatures. After heat is withdrawn, it is transferred to a condenser and dissipated to air or water.

Closed-cycle air refrigeration (CCAR) is a new refrigeration technology, combining components from mechanical and cryogenic refrigeration, expanding component capabilities, and integrating components in innovative ways for meeting necessary “step-out” performance conditions. The system uses dry, high-pressure air as the working fluid and is configured as a closed system to avoid the need for continuous moisture removal from makeup air. Moisture freezes, and the resulting ice particles on turbine blades can damage rotating equipment. CCAR avoids this problem through the closed-cycle configuration. Unlike conventional refrigeration systems, high-pressure air is in a gaseous state throughout the cycle, without phase change. Figure 1 indicates key CCAR components and system connections.

Figure 1. CCAR Refrigeration System

A U.S. patent for the basic CCAR technology was issued in 1996 (Miller, Smith, Allam, and Topham, U.S. Patent 5483806, Refrigeration System, January 1996). The Advanced Technology Program (ATP) funded project involved development of technologies for radically improved efficiencies in the expander, compressor, and heat exchanger, as well as advanced system integration, detailed design, fabrication, and a test program (for more details, see Appendix A, “Innovations From CCAR Development.”)

ATP Project History

Concern over the environmental consequences of the widespread use of ozone-depleting chlorofluorocarbons (CFC) and hydro-chlorofluorocarbons (HCFC) sparked efforts to develop environmentally benign alternatives for these common industrial refrigerants. Alternatives include ammonia, propane, and inert gas combinations of argon, krypton, and xenon. However, ammonia is toxic, propane is explosive, and inert gases are unstable mixtures that are substantially more costly than CFC and HCFC.

Air is another alternative working fluid for industrial refrigeration systems. It is environmentally benign, safe to use, and has an unlimited source. Using air for refrigeration is not a new concept. The air refrigeration cycle (the reverse Brayton Cycle) was developed in the nineteenth century and has been used in specialized applications including air conditioning systems for commercial aircraft.

Prior to the ATP-funded CCAR technology, air-based refrigeration systems utilized an open cycle (Verschoor and van der Sluis, TNO Department of Refrigeration & Pump Technology (Interview)) where compressed cold air is blown into a cooling chamber and lost for further use in the cycle. Makeup air is continuously dehumidified and compressed to compensate for the loss of cold air, leading to low system efficiencies and high energy costs.

To reach improved system efficiencies, Air Products and Chemicals, Inc., a major U.S. company active in the food refrigeration industry, undertook the technical development of the open-cycle air system by using complex multi-stage compressors. The open-cycle ColdBlast™ initiative did not fully meet technical, commercial, and revenue expectations. Air Products engineers concluded that an air-based system, if operated at higher pressure and in closed cycle, could reach improved efficiency levels. No such system had existed before, but theoretically, the concept was feasible with the development of new, more efficient components and optimized system operation. They proposed the new approach to management.

Owing to the less than satisfactory ColdBlast™ experience and the project’s high-risk profile compared with alternative R&D opportunities, Air Products management decided to de-prioritize further R&D in this area. This decision also reflected a preference for efficiency improvements of existing products as opposed to the development of radically innovative, longer time-to-market technologies.

Encouraged by the ATP funding opportunity, Air Products reversed its decision and convened a multi-disciplinary team to co-develop and cost share the project with ATP. In partnership with Toromont Process Systems, Inc. (formerly Lewis Energy Systems), Air Products submitted a joint venture proposal to ATP to develop a high pressure, CCAR technology suitable for widespread industrial use.

In its 1995 General Competition, ATP selected the joint venture project for an award. The project encompassed technology development, system integration, fabrication, and demonstration of a CCAR system that would utilize environmentally benign dry air as the working fluid. The core challenge was to greatly improve efficiency of the expander, compressor, and heat exchanger, and optimize system operations. The ATP agreed to cost share $2.1 million of the $4.3 million project, and Air Products and Toromont committed to fund the balance.

Throughout the project, Air Products provided process engineering and technical expertise for pushing system components to step-out performance levels, required by the demanding operating conditions of a high pressure, CCAR system. Toromont provided engineering and technical expertise in the areas of packaged refrigeration, heat transfer, fabrication, and food processing application expertise.

Complementing the skill sets of Air Products and Toromont engineers, specialty contractors were used for compressor shaft seal design (FlowServe), high-pressure heat exchanger design (Chart Heat Exchanger), and innovative casting solutions for the expander turbine (Quick Cast).

Major Innovations

The project was successfully completed in 1999. During the ATP-funded technology development, design, and testing phase, CCAR efficiency and operating reliability levels were improved and costs brought down relative to the costs of cryogenic refrigeration. The key elements of technical progress for improving efficiency, reliability, and costs included:

• Operating at high pressures (1,200 psig), in combination with –150°F temperatures and 30,000 rpm compander shaft speeds. In combination, these were step-out conditions, requiring significant technical advances.
  
• Utilizing a single wheel compressor and expander designs, compared to more expensive cryogenic systems with multi-staged compressors and expanders.
  
• Utilizing a low compression ratio (compressor output to expander output) of 1.6 to 1 compared to cryogenic machines operating at ratios of 8 to 1.
  
• Developing ultra low leakage seals, to prevent high pressure air escaping at the compressor shaft at more than two standard cubic feet per minute.
  
• Developing a high efficiency heat exchanger with no more than 2°F to 3°F temperature difference between high pressure air exiting the cooling system and the return air from the load exchanger.
  

The CCAR test program included bench tests at Air Products’ Cryomachinery Laboratory and a nine-month pilot test program at a Kodak facility in Rochester, New York. The demonstration unit was operated for 6,000 hours and reached or exceeded design specifications.

• Unit output was specified at 50 tons of refrigeration. One ton of refrigeration is a measure of refrigeration capacity sufficient to freeze one ton of water. The plant operated at 60 tons, exceeding the design point by 20 percent.
  
• System reliability was targeted at 95 percent. The plant operated at 98 percent, exceeding expectations by 3 percent.
  
• Refrigeration temperatures were maintained within a close (+/–2°F) band around the –100°F design point.
  
• At –70°F, the demonstration unit achieved a 0.75 COP (coefficient of performance) level, consistent with COP levels of conventional mechanical refrigeration units. The COP measures the relative efficiencies of different refrigeration systems. At –100°F, a temperature level that conventional mechanical refrigeration units cannot reach, the unit operated at a targeted 0.66 COP design point.
  
• With 40 percent turndown (load reduction), CCAR unit efficiency decreased by only 3 percent. Comparable 40 percent turndown of a conventional mechanical refrigeration unit resulted in 37 percent efficiency reduction.
• Operating at less than 85 decibels, CCAR satisfied Occupational Safety and Health Administration equipment noise-level regulations.
  

An overall assessment by the Kodak project engineers was that “CCAR met or exceeded all acceptance criteria” and successfully demonstrated its technical feasibility” (W. Klumpp, Kodak CCAR Demonstration Project Manager, Correspondence to E. Kiczek of Air Products and Chemicals, June 30, 1998).

Some of the innovations developed to address the CCAR step-out conditions have potential usefulness to other industrial applications and represent opportunities for cross-industry technology diffusion. For example:

1. Improved shaft seals. Successful performance of dry gas seals under the severe CCAR operating conditions (the combination of 1200 psig pressure, –150°F temperature, and 30,000 rpm shaft speed parameters) is expected to promote greater industry acceptance of DGS technology (Klossek, FlowServe (Interview)).
   
2. High pressure core heat exchanger. The new high-efficiency aluminum plate core heat exchanger fabricated by Chart Heat Exchangers (CHE) for CCAR has potential applications in the petrochemical, air separation, and natural gas industries. The new shop and fabrication processes employed by CHE for CCAR are expected to result in an increase in market share for this U.S. company.
   
3. Improved casting technology. To fabricate mold prototypes for the CCAR expander wheel, Air Products Cryomachinery Laboratory used three-dimensional rapid prototyping technology with Quick Cast honeycombed advanced materials. This innovative approach significantly reduced the time and cost requirements of building prototypes and facilitated the evolutionary development process for optimizing CCAR performance (Tomasic, Air Products (Interview)).
   

Appendix A provides a more in-depth description of the CCAR technology and technical accomplishments of the ATP project.