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Figure 2. Schematics of a spray freezing process

The direct contact of the sprayed cold gases and the blasting action of the fans on the food enables the product to reach the freezing state in a few minutes. The heat transfer methods used are conduction and convection heat transfer.

Commercial spray freezing units are available as small to very long tunnel freezer to fit almost every need. The cryogenic gas (liquid nitrogen or CO2) blows down the cold gas on top of the open trays containing food or the food is placed directly on the conveyor belt, thus freezing the product faster. Food product thickness is directly proportional to the freezing time and the heat transfer efficiency. The thicker the food product the longer the freezing time. The cold gas can be directed in either direction of the moving trays or food product. The cryogenic gas is sprayed in fine drops of liquid nitrogen or as solid particles of CO2» commonly called snow flakes, on top of the open trays or directly on the food product to be frozen. This tunnel freezing system is a flexible method that is used to freeze chicken, fish, meat, pizza, bread, pies, cakes, entries and other processed foods on open trays or packaging containers. Moisture losses could be 1% or less depending of the freezing time.

2.2.3. Cryogenic Air-freezing System. This new air-freezing technology is used to freeze food products simulating the freezing effect that cryogenic liquid and gases have. This freezing blasting system uses compressed fresh air as a refrigerant This new COLDBLAST™ system takes air from the environment and cools it through a three stages compression process, heat exchanger and a Turbo-expander (12). The cold air temperature reached is -25(FF. Cold air at a - lOOT is returned to the heat exchanger to be used as coolant, and the warm air is released to the atmosphere at 99®F (See figure 3).

The cold air is fed into a tunnel or spiral freezer to quickly freeze the food product. This procedure reduces moisture losses to a minimum of 0.5% of product weight. The heat transfer method used is convection heat transfer (13). This air freezing system is environmentally safe and eliminate safety and residual concerns related to the ammonia and/or a CFC's system. The system advantages are similar to the benefits of the cryogenic freezing with liquid nitrogen and carbon dioxide gas. Compression, expansion and heat exchanger process unit operations are involved in this cooling-freezing method.

Figure 3. Schematics of a Coldblast System™. (Courtesy of Air Products and Chemicals, Inc.)

This air freezing system can be used for meat, fish, poultry, pizzas, processed and prepared food and baked goods. The use of cleaning in place, CIP, systems, defrosting units, air-filter and temperature control devices could be implemented in the freezing system to improve efficiency and savings.

2.3. Mechanical-Cryogenic Freezers

The frozen industry has taken advantage of the benefits of the cryogenic liquid and gases and the mechanical freezing equipment to improve the energy consumption, freezing times, equipment flexibility, food production rates, finished product quality and manufacturing costs. Combination of the best of the mechanical and cryogenic technologies has allowed the frozen food industry and freezing equipment manufacturers to develop new freezing equipment alternatives to improve finished product quality, procedures and capabilities. An overview of the cryogenic-mechanical freezer consists of freezing the food products using different systems that complement each other.

2.3.1. Use of Cryogenic Elements in the Integration of Mechanical and Cryogenic Freezing Methods Into a Single Freezing Unit. This system is used for its versatility, space requirement, capability, minimum freezing residence time to reach freezing equilibrium and for the high volume of frozen food products produced. The method consists in integrating the cryogenic elements and a mechanical freezer into a single processing unit with two major steps. Step one is a fast surface product frozen, and step two is an equilibrium frozen product temperature. Liquid nitrogen, LN2, and carbon dioxide, CO2, are the cryogenic mediums used for cryogenic freezing.

• Carbon Dioxide Freezing System, CO2. The carbon dioxide cryogenic and mechanical freezers system is based on the physical properties of using pressurized liquid carbon dioxide. Carbon dioxide is stored as a liquid generally at a pressure of 300 psig. The liquid carbon dioxide is transferred to the mechanical freezer and released to atmospheric pressure generating an intensive cooling effect. The mechanical freezer could be spiral, tunnel, belt or spray.

The carbon dioxide snow produced is applied on top of the food and quickly develops a frozen crust in the processed food surface, and consequently, ice crystal formation and moisture losses are minimized. The high air-cold gas velocities, generated by the fans, complement the convection freezing process. The IQF product reaches its freezing equilibrium later in the tunnel or spiral freezer (See figure 4.). The cryogenic carbon dioxide system is very flexible and it is currently used by the Food Industry to freeze a wide variety of food products, such as chicken, fish, hamburgers, bake goods, cookies, meat patties, pizza, vegetables, entree, ethnic foods, etc. The heat transfer methods used are conduction and convection heat transfer.

Figure 4. Schematics of a Cryogenical-Mechanical Freezer. (Courtesy of Liquid Carbonic Corp., Chicago)

• Liquid Nitrogen (LN2) Freezing System. LN2 works as follows: the food product is dispensed by gravity or by a vibratory conveyor into a liquid nitrogen bath, which is at its boiling temperature at atmospheric pressure of -32(FF (-l^^C), and remain there for a few seconds reducing the dehydration losses. The LN2 level is automatically adjusted depending on the food product components, thickness, water content, water and LN2 boiling activity, etc. The food product's surface is instandy frozen and by action of an open mesh conveyor belt, the frozen food product is transferred from the cryogenic bath to a mechanical freezer, (It could be spiral, tunnel, belt or spray freezer), for product freezing temperature equilibrium.

The use of manifold spray systems and fans in the mechanical freezer help to improve the heat transfer and equipment efficiency. The heat transfer methods used are conduction and convection heat transfer. Overall freezing residence time is reduced to a few minutes. Product through put and equipment efficiency are high and the maintenance required is minimum. The space required by this single integrated unit is very small compared with other freezing systems. Storage tanks and continuous liquid nitrogen feed system are needed to supply the cryogenic liquid to the processing line.

To increase the dual freezing system efficiency, the sensible heat of the liquid nitrogen transformed in gas could be recycled back to the mechanical freezer and blown at high speed on the food product. The refrigeration media used in the secondary freezer could be based on an ammonia cooling system or a sub-cooled closed loop system.

3. NEW TECHNOLOGIES / APPLICATIONS / EFFICIENCIES

From the prior description and definition of the different methods and equipment used in freezing food products, it is apparent that in the 21st century most of the new technologies will be based on improving equipment efficiency, reducing energy costs, maximizing the advantages of ammonia as refrigerant, cryogenics, cooling efficiencies, dealing with the world's environmental concerns, identifying equipment flexibility and maximizing technical resources.

One of the biggest challenge of the freezing and refrigeration industry is the research and development of new alternatives in the refrigeration media equipment that are economical and efficient. The marketing future of a new self contained refrigeration media for the frozen food and refrigeration industry (other than CFC's) that could be compiled in a single small unit, is unlimited. The application opportunities are tremendous because it could be used in the home, supermarkets, transportation and distribution systems, warehouses refrigerators and freezers. A few ideas currently developed and potential opportunities for the freezing and the refrigeration manufacturing industry in foods and equipment are described below:

3.1. Cleaning in Place, CIP, System

The implementation of CIP systems into freezing units presents an opportunity to reduce costs of overall sanitation and freezing equipment's start-ups. The CIP systems could be automated, designed or retrofitted in tunnels or spiral freezers. A few of the advantages of a well designed freezer CIP system are:

Easily reach, sanitize and clean the mesh belts spiral conveyor, coils, fans, walls and floor of the freezer. Computerized CIP system controls the water temperature, consumption, and pressure, allowing savings of 1/2 to 2/3 of the water consumed with a manual freezer wash down. The CIP computer control system is automated by using programmable logic control systems, specially designed to any freezing equipment need. The use of a CIP recirculating system maximizes the utilization of water and chemicals. Recirculating 500 gallons of water, with a CIP system, provides the effect of using 9000 gallons of water in a one hour cleaning period (14). Savings in energy, sanitation crew, maintenance, chemicals and cleaning time. Updated CIP options could be accommodated to old freezing systems.

3.2. Preventive Freezer Maintenance and Computer Control

Energy consumption savings, Refrigerators account for 20% of the electricity consumed in US homes (15), improvements in equipment efficiency, low maintenance costs, and higher through put, could be achieved by implementing preventive maintenance, freezer manufacturing Technical Services programs, the use of self-defrosting system, variable speed drive motors, and the installation of process temperature sensors and computerized control automation systems.

3.3. Technological Improvements in New Valves, Spray-manifold, Cryogenics Gases Recirculation and Supply Systems

• Technological improvements in new valves and manifold systems used in cryogenic freezers allow better efficiencies and application of the cryogenic liquid and gases to pre-cool the food product or as a coolant in the freezer equilibrium layers or chambers.

• Improvements in equipment efficiency and capacity, by modifying the recirculating designs and systems, that maximize the utilization of the cryogenic gases' sensible heat generated during the immersion processes.

• Cold CO2 supply system for large refrigeration uses that reduces cryogenic consumption.

(Strata-Cold™) (16). This supply system is used when large volumes of cryogenic refrigeration are needed. The storage vessel system, at -5CPF, (Standard systems store LCO2, liquid carbon dioxide, at (FF), and its controls generates 15% more snow by delivering colder liquid CO2 to the refrigeration process.

The system works based on the density of the LCO2 at different temperatures. The cold CO2 at -5CFF is more dense than the LCO2 at 0®F, consequently, it remains in the bottom of the storage vessel. The cold LCO2 is fed to the refrigeration application as required. The LCO2 at 0®F is cooled to-5(PF and returned to the storage vessel (16). Applications for this system are: Food mixing, pneumatic cooling, food freezing, refrigerated trucking and freight car, and crusting.

3.4. Technological Improvements in Freezer's Wall Insulation

The improvement in the freezer insulation material will increase the operating efficiency of refrigerators and freezers for home or industry, will reduce its overall weight and may make it portable and easy to relocate according to the equipment and food process manufacturing needs and space availability. Wall insulation has been improved by developing a fiber glass vacuum panel and it could be used in refrigerators and freezer doors, adding more available space for storage. Currendy Owens-Corning has a fiber glass insulation concept that gives six times the R value as that of the urethane foam insulation (17).

3.5. Technological Improvements in Equipment Design and Flexibility.

One current technological advance in equipment design is the self-stacking freezing belt spiral unit This new freezing equipment does not need the steel structure required to support the spiral freezing belt and it works on a no-tension principle. In this new design, the tier is placed above another and the self stacking turns and work as a single unit. In this equipment improvement, there are not stationary parts in the freezing zone, consequently its freezing maintenance is minimum. The GYRoCOMPACT System ™ is a very flexible unit that operates with ammonia. The width and height of the belt self-stacking unit could be changed without changing the freezer structure because there are not stationary parts (18,19).

3.6. Technological Improvements in Low-Temperature Mechanical Refrigeration System

This refrigeration system uses convection heat transfer to obtain faster freezing, reduce moisture losses, freezing costs and freezing time during its operation. This Low Temperature "nonexpendable" CO2 Mechanical Refrigeration System is an alternative to the current ammonia mechanical refrigeration system. The system operates at -55aF, internal chamber freezing temperature, compared with -35SF in typical ammonia systems (20). This system could use the ammonia system to condense the vapor CO2 to a liquid.

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