Refrigeration Technologies

Refrigeration is the withdrawal of heat from a chamber (refrigeration load) to achieve temperatures lower than ambient temperatures. After heat is withdrawn, it is transferred to a condenser and dissipated to air or water.

The purpose of refrigeration in food processing is to preserve quality and delay spoilage; in volatile organic compounds recovery, is to condense and capture harmful vapor emissions; and in the liquid natural gas industry, to facilitate natural gas storage and transportation.
CCAR was developed as an innovative refrigeration technology for the above industrial applications in the –70°F to –150°F ultra-cool range. Above –70°F, mechanical refrigeration takes over as the predominant technology. Under –150°F, refrigeration is provided by cryogenic methods.

Mechanical Refrigeration

More than 90 percent of U.S. industrial refrigeration is provided by mechanical systems using ammonia as the refrigerant ( Shepherd, Toromont; Anderson, International Institute of Ammonia Refrigeration; Sterling Group (Interview)). Mechanical refrigeration units are dedicated systems, installed at individual industrial facilities and owned and operated by the industrial companies.

Refrigeration is achieved when the refrigerant, circulating in the system, withdraws heat energy from the chamber to be cooled (load). Heat energy (latent heat of evaporation) is absorbed as liquid refrigerant undergoes a phase change to a gaseous state. Systems are composed of four basic elements connected with piping into a closed loop that re-circulates refrigerant (Figure A1).

Figure A1. Conventional Mechanical Refrigeration System

Compressors (generally) use motor-driven rotating impellers to generate gas pressure. Gaseous refrigerant enters the compressor at low pressure and temperature and exits at high pressure and temperature.

Inside condenser coils gaseous refrigerant condenses to liquid state. To facilitate phase change, the condenser dissipates heat energy to ambient air or water. High pressure refrigerant exits at lower temperature.

An expansion valve controls the flow of high pressure liquid refrigerant to the evaporator. As refrigerant passes through the expansion valve it is further cooled by the Joule Thompson effect, the scientific principle that the temperature of a stream is reduced when forced through a narrow nozzle and allowed to expand.

Inside the evaporator, liquid refrigerant vaporizes into a gaseous state. Vaporization requires heat energy, which is extracted from the industrial process load (food items to be cooled). The refrigerant is returned to the compressor to repeat the cycle.

Cryogenic Refrigeration

The cryogenic approach uses very low temperature gases in liquid or solid form such as liquid nitrogen or solid carbon dioxide. Liquid nitrogen is manufactured in large, capital-intensive air separation plants, serving entire geographic regions and hauled to distant points of use in insulated tanks via ground transport. carbon dioxide is manufactured as part of fertilizer production or directly extracted from the ground. Solid carbon dioxide is hauled to points of use via ground transport. The carbon dioxide market is deemed to be approximately twice the size of the liquid nitrogen market. About 25 percent of cryogens in the United States go into food processing applications. (Kiczek, Air Products (Interview)).

At industrial plants, cryogens can be released into freezer coils or into freezer chambers, coming into direct contact with items to be refrigerated.

The cost of cryogenic refrigeration is four times the unit cost of mechanical refrigeration, and cryogenic refrigeration is generally used only for specialty applications requiring temperatures colder than –100°F.

Closed-Cycle Air Refrigeration

CCAR is a new refrigeration technology, combining elements of mechanical and cryogenic refrigeration in an innovative manner. It operates on the reverse Brayton Cycle and uses dry, high pressure air as the working fluid. It is configured as a closed system to avoid the need for continuous moisture removal from makeup air; that is, moisture would freeze on turbine blades and ice particles could damage the rotating equipment. The high-pressure working fluid is in a gaseous state throughout the system, and, unlike mechanical refrigeration, phase change and the latent heat of evaporation are not utilized. For a functional representation of the CCAR system, see Figure A2.

Figure A2. CCAR System

As with mechanical refrigeration, the compressor raises air pressure and temperature. The cooling system acts like a water-cooled condenser to remove the heat of compression. As an additional step to pre-cool the air stream, air passes through a core heat exchanger to give up heat energy to the cold air stream returning from the load exchanger.

Pre-cooled high-pressure air then enters the expander,which is a rotating machinery that looks like a small turbine. It is used to extract energy from the air stream by reducing pressure, thereby resulting in a temperature drop from –82°F to –105°F. The air stream applies a forceto the expander blades, causing the rotation of the compander shaft and providing some of the power requirements of the compressor. The compressor’s remaining power requirements are metby an electric motor, geared to the compander shaft.

Cold air from the expander flows to the load exchanger. If the load is adjacent to the CCAR unit, then the load exchanger can directly cool the space to be refrigerated. If the process load is hundreds of feet away (as in many food industry applications), then a low pressure secondary loop is used to connect the CCAR unit to the process load. The reason for the secondary loop is economics. It is too expensive to run thick-gauge stainless steel piping to deliver high-pressure cold air to distant refrigeration loads.

In the load exchanger, cold air picks up heat energy from process load or from the secondary loop (in contact with process load) and exits as return-air at –90°F. Return air is taken through the core heat exchanger and is returned to the compressor to repeat the cycle.

Using modified design parameters, the expander can produce temperatures as low as –184°F. However, at temperatures colder than –150°F, CCAR is no longer cost competitive with cryogenic systems.

Kodak Demonstration

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

• Unit output was specified at 50 tons of refrigeration. 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 coefficient of performance level, consistent with coefficient of performance levels of conventional mechanical refrigeration units. The coefficient of performance COP measures the relative efficiencies of different refrigeration cycles. At –100°F, a temperature level that conventional mechanical refrigeration units cannot reach, the unit operated at the 0.66 coefficient of performance design point.
• With 40 percent turndown (load reduction), CCAR unit efficiency (coefficient of performance) decreased by only 3 percent. Comparable 40 percent turndown of a conventional mechanical refrigeration unit resulted in 37 percent efficiency reduction.
• At less than 85 decibels, Occupational Safety and Health Administration requirements were met.

The overall assessment by Kodak Project Manager (W. Klumpp) was that “CCAR met or exceeded all acceptance criteria, as set forth in our contract” and that the test was a successful demonstration of CCAR’s technical feasibility.

CCAR Configuration in Food Processing

Air Products has developed a standard CCAR unit for food industry applications, sized at nominal 200 tons of refrigeration capacity, optimizing cost-performance relationships. The unit is pre-assembled and skid mounted. It has a footprint of 12 by 40 feet and can be placed outside food processing plants to save plant space. It weighs about 125,000 pounds.

Food processing refrigeration loads (high volume items, moving on continuous belts) may be hundreds of feet from the CCAR unit. Hence, food industry applications typically require a secondary loop that can efficiently transport cooling to process loads. Unlike the high pressure (1200 psig) CCAR loop, the secondary loop is operated at low pressures (around 10–15 psig) using smaller diameter and thinner wall piping. Secondary loops deliver refrigeration to process loads through heat transfer coils (HX1 and HX2) to spiral freezers, tunnel freezers, and other “enabling devices.” Figure A3 displays this configuration.

Figure A3. Food Processing Plant CCAR Configuration

For initial CCAR systems (to be installed in further-processed food plants), Air Products plans to use ammonia as the secondary loop heat transfer fluid. The choice of ammonia creates some limitations. It becomes highly viscous at –90°F and cannot be used under –95°F. Hence, CCAR effective temperature range, with an ammonia-based secondary loop, is reduced from its full economic potential of –150°F down to –95°F. Ammonia is also toxic and this compromises CCAR’s environmental and safety “credentials.”However the impact is one of appearance more than substance, as the CCAR secondary loop is hermetically sealed under only one atmosphere of internal pressure (in contrast to mechanical refrigeration systems, where ammonia is under higher pressures, causing potential leakages at compressor shaft seals). As part of a continuing CCAR technical development effort, Air Products is evaluating alternative heat transfer fluids to replace ammonia, including D-LimoneneTM, a harmless chemical made from citrus extracts.

Food Freezing Systems

Chilling and freezing products in further-processing plants takes place in a variety of freezing systems, otherwise known as “enabling devices” (Valentas and Rotstein, 1997).

When coupled with mechanical refrigeration, the refrigeration effect is transmitted from the ammonia (working fluid) through heat transfer coils (HX) to an air stream that circulates throughout the freezing chamber by the action of fans (Figure A4).

Figure A4. Food Processing Plant with Mechanical Refrigeration.

Freezing systems or chambers, used with mechanical refrigeration, include:

• Spiral freezers, for large volume production where food items are placed on continuous metal conveyor belts, passing through the chamber via a spiral path.
• Tunnel freezers, for both small and large production volumes. Small volume operations can grow by installing additional modules. Product moves through the freezing chamber on a continuous linear conveyor.
• Impingement freezers, increasing heat transfer rates by higher air velocity. Process is more energy intensive but still less expensive than cryogenic freezing. Over the last five years, impingement freezing (augmenting conventional mechanical refrigeration systems) was developed to achieve improved quality and yield and reduced dehydration of hamburger patties.
• Fluidized bed freezers are a specialized tunnel freezer where high velocity air flow is directed upward through perforations in the belt, causing small unwrapped products of uniform size and shape (fries, peas, beans, etc.) to be suspended in the air. Freezing is faster and more uniform.

When coupled with cryogenic refrigeration (utilizing liquid nitrogen or carbon dioxide), freezers are customized so that vaporized liquid nitrogen or carbon dioxide come into direct contact with food items to extract large amounts of heat (see Figure A5).

Figure A5. Food Processing Plant with Cryogenic Refrigeration.

Entering spiral and tunnel freezers, cryogens pass through throttling valves so as to expand to atmospheric pressure and vaporize. Liquid nitrogen, vaporized to cold nitrogen gas, is sprayed into freezers to freeze food items via direct contact. Nitrogen gas (initially at –320°F) moves through progressively warmer zones and may be re-circulated throughout the freezer prior to being vented. Freezers can operate at temperatures as low as –150°F to –200°F. carbon dioxide passes through the throttling valves to be sprayed on food products as a mixture of solid (dry ice) and vapor. The sublimation of solid carbon dioxide to vapor provides part of the refrigeration effect, with the remainder deriving from cold vapor entering the chamber directly.

When coupled with hybrid cryomechanical refrigeration, specialized freezer systems are needed. Cryomechanical freezing starts with liquid nitrogen or carbon dioxide being used to freeze the crust and to seal the surface of food items so as to prevent loss of internal moisture. Next, food items are moved to another freezer where cold air from mechanical refrigeration system completes the freezing process. The complexity of multiple freezers, conveyors, and cryogen storage facilities significantly increases cryomechanical costs over conventional mechanical refrigeration. However, improved food quality and reduced yield loss compensate for higher freezing costs. Table A1 compares CCAR with alternative systems.

Notes: L, low; I, intermediate; H, high. Cryogenic and CCAR rankings are based on sale of refrigeration-type service contract.
Source: Valentas and Rotstein, 1997.

Plant Throughput and Refrigeration Capacity

For a very large further-processing poultry plant, throughput can be as high as 20,000 pounds per hour. The plant would typically run for two shifts or 16 hours per day, five days per week. Prior to freezing and packaging the product, the meat would be marinated, cooked, chilled (to facilitate slicing), sliced, for instance, into fajitas strips, mixed with other meal items, such as vegetables and oils, and cooked.

About 200 BTU of heat content would be withdrawn from each pound of cooked hot product to lower its temperature to 10°F. Handling 20,000 pounds per hour would require a refrigeration system producing 4 million BTU/hour and a plant capacity of 334 tons of refrigeration (Roberts (Interview)). Two standard CCAR units of 200 tons would be installed to support this level of production.

For a large poultry processing plant, we assumed 10,000 pound per hour production and the installation of one 200-ton CCAR unit.

Innovations From CCAR Development

To make CCAR a practical refrigeration alternative, efficiency and reliability levels had to be improved and costs reduced relative to the cost of cryogenics. The coefficient of performance (COP) is the key measure for efficiency. To achieve COP levels in the 0.66–0.75 range, CCAR process optimization required using

• High pressure (1,200 psig) air, in combination with –150ÞF temperatures and 30,000 rpm compander shaft speeds. In combination, these were challenging “step out” conditions, requiring significant technical advances.
• Single wheel compressor and single wheel expander designs, compared to more expensive cryogenic systems with multi-staged compressors and expanders.
• Low compression ratios (compressor output to expander output) of 1.6:1 compared with cryogenic machines operating at ratios of 8:1.
• Ultra low leakage seals, to prevent high pressure air escaping at the surface of the compressor and expander shaft at more than two standard cubic feet per minute.
• High efficiency aluminum plate fin core heat exchanger with 2°F to 3°F close approach temperature delta; that is, 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.

Some of these innovations, developed to address the CCAR “step-out” conditions, have potential usefulness to other industrial applications and represent opportunities for cross-industry knowledge diffusion.

Improved Shaft Seals

Seals are devices that prevent the leakage of fluids along a rotating shaft, when the shaft extends out from the housing enclosure, containing pressurized fluids.

In the case of a CCAR unit, the expander and compressor are mounted on a common rotating shaft but are enclosed in separate housing. Seals are needed to prevent high pressure air from escaping along the rotating shaft from the expander and compressor housing. Even modest air leakage from a pressurized CCAR unit will cause significant degradation of system performance and efficiency.

Shaft seals usually consist of an elastomer ring bonded to a metallic ring that is a press (tight) fit in the hole of the housing through which the shaft extends. The sealing effect is provided by a lip on the elastomer ring, pressed snugly around the shaft by a helically wound garter spring. When properly designed and installed, the lip rides on a film of lubricant about 0.0001 inches thick. If improperly installed, then the lubricant film can become too thick and the shaft will leak. If the film becomes too thin, then the lip gets hot, and the seal may fail.

Dry gas seals (DGS) address these problems by providing a clearance maintaining mechanism. One face of DGS is etched with spiral contours or grooves. This changes the pressure distribution or repulsive forces between seal faces, tending to counteract out of spec increases or decreases in face clearance. DGS can maintain non-contacting and non-wearing operations despite vibration, temperature variations, and axial shaft motion.
DGS operations depend heavily on properties of the sealed process gas. The presence of solid or liquid impurities may cause DGS face contamination and damage, even disintegration of the rotating seal face. DGS also has very high first cost, inhibiting industry adoption of this step-out sealing technology, for high speed, smaller OD shaft, turbo-machinery.

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)).

High Pressure Core Heat Exchanger

Heat exchangers are devices that transfer heat from a hot to a cold fluid. The barrier between the two fluids is a metal wall, such as that of a tube or pipe. In many engineering applications it is desirable to increase the temperature of one fluid while cooling another. This double action is economically accomplished by coils, evaporators, condensers, and coolers that may all be considered heat exchangers.

Heat exchangers are designed with various flow arrangements. The concentric tubes design uses one pipe placed inside another. Cold fluid flows through the inner tube and the warm fluid in the same direction through the annular space between the outer and the inner tube. Heat is transferred from the warm fluid through the wall of the inner tube (the so-called heating surface) to the cold fluid. Concentric tube heat exchangers can also be operated in counter-flow, in which the two fluids flow in parallel but opposite directions.

The shell and tube design utilizes a bundle of tubes through which one of the fluids flows. These tubes are enclosed in a shell with provisions for the other fluid to flow through the spaces between the tubes. In most designs of this type, the free fluid flows roughly perpendicular to the tubes containing the other fluid in what is known as a cross flow exchange.

The plate-fin design uses metal sheets brazed together into internal channels to carry warmer fluid stream, which is to be cooled. Fins, brazed to the outside surface of these channels, facilitate faster and more efficient heat transfer to the cold fluid stream on the outside of these channels.

CCAR uses a high efficiency aluminum plate fin core heat exchanger, fabricated by Chart Heat Exchangers (CHE). CHE has the normal capability of fabricating units up to 1,700 psig for a single stream with lower pressure specifications for the other streams (that is, 50–500 psig). The CCAR core heat exchanger was a “step-out” technology in as much as it specified high pressure tolerance (1,200 psig) for all streams. This required that CHE develop new shop practices and fabrication standards for brazing heavier metal stock in complex configuration. (Markusen, Chart Heat Exchangers (Interview)).

CHE sells high pressure heat exchangers primarily in the petrochemical, air separation, and natural gas industries. This market represents estimated worldwide sales of $10–20 million per year, and CHE is the only U.S. supplier with approximately 50 percent worldwide market share. Based on improvements in shop and fabrication practices, deriving from the CCAR experience, CHE estimates 2–5 percent annual increases in market share or $200,000 to $1 million additional revenues for this U.S. corporation (Markusen, Chart Heat Exchangers (Interview)).

Improved Casting Technology

In a casting process, molten metal is forced into a mold and allowed to harden.

Die casting is a mass production process for forming metal objects by injecting molten metal under pressure (from a plunger or piston) into dies or molds.

Investment casting is a high precision technique for forming metal shapes, involving the following process. A gelatin mold is formed around a solid sculptured form. The mold is removed (in two or more sections) from the sculptured form, and the inside of the mold is filled with wax or coated with a layer of wax of the same thickness as that desired for the final casting. Then the outer gelatin mold is removed, and a second mold, of heat-resisting clay, is formed around the wax shell, the interior of which is filled with a clay core. In the “burnout phase” the mass is baked, hardening the clay and melting the wax, which runs off through openings in the outer mold. Then the hardened mold is packed in sand, and molten bronze is poured through the openings to fill the space vacated by the lost wax. The mold is then broken, and the bronze form remains. In modern foundries, plastics are used in place of the wax. The process is used for manufacturing small parts that require minutely precise details.

To fabricate mold prototypes for the CCAR expander wheel, Air Products Cryomachinery Laboratory used investment casting, in conjunction with rapid prototyping.

Conventional prototyping involves fabricating three dimensional (3-D) models from two-dimensional drawings, using subtractive machining processes such as milling, turning, or grinding metal parts. Conventional prototyping requires significant investment in hard tooling, the production of which is time consuming and expensive. Hard tooling is also inflexible. It cannot easily accommodate design changes typical during the design and development process.

Rapid prototyping automates the fabrication of 3-D models. A computer translates the information form CAD drawing into slices of a 3-D object and passes the information to a prototype machine (PM). The PM transforms this information into solid objects by using lasers to shape the physical layers of the 3-D prototype from plastics or powdered metals (Bylinsky, 1998). Rapid prototyping dramatically cuts the time from drawing board to market and provides flexibility for an evolutionary design and development process at lower costs (Technology Review, 1998).

To fabricate prototype molds for the CCAR expander, the prototyping machine used a new advanced material (Quick Cast honeycomb structure) instead of plastic or powdered metal. “Quick Cast build styles create quasi hollow parts from specially formulated resin. The liquid resin is drained from the interior regions of the part, leaving a pattern that is approximately 80 percent hollow with a honeycomb configuration. This structure facilitates the internal collapse of the part, without damage to the shell, during the burnout phase of the investment casting process” (http://www.3d-cam.com).

The molds for CCAR expander were fabricated using 3-D 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)).