For calculation of the capacity of any freezer, the necessary holding time is essential. For bulk products such as peas, green beans, French fried potatoes, and fish sticks the capacity of standard equipment is usually specified separately. This is also the case with packaged products that are homogeneous such as spinach purée and fish fillet blocks. For other products holding time must be determined before capacity can be stated.
In the literature many different formulas can be found for accurate determination of holding times. In reality, however, they are of little use, because products differ so much in composition and form that the work to transform the characteristics of a product into mathematical terms is a lot more time-consuming than test freezing of the very product itself. This can usually be done in less time than it takes to analyze the composition of the product.
Of course, such test freezing should be carried out under controlled conditions that do correspond to those that can actually be achieved in production. A suitable pilot freezer for different airflow directions is shown in Figure 19.
MAJOR CONSIDERATIONS IN FREEZER DESIGN Safety of Personnel
The safety of personnel that operate, clean, and service a freezer should be a main consideration in the freezer de-
sign. In too many instances, serious injuries have been the result of unsafe design and operation of freezing machinery.
Mechanical hazards exist in the conveyor drive systems, fans, and other areas. The machinery must be designed so that all areas of a freezer can be easily cleaned and inspected while ensuring that personnel are suitably protected. All drives and fans must be fully guarded so that no worker individual or worker's clothing can reach any part of the machine and get caught or crushed. Emergency stop switches need to be located throughout the machine so that if someone does get caught in the machinery the machine can be stopped quickly to minimize the injury. Fan guards must be designed sufficiently open to remain unrestricted by frost buildup while still preventing personnel from getting their hands or clothing into the fan.
The extreme cold found in most modern freezers represents a hazard to personnel in the form of hypothermia and frostbite. The high air velocities found in efficient freezers that use air as the product heat-transfer medium greatly increases heat transfer and the wind-chill factor, which will quickly freeze exposed skin and draw heat from the body at a very high rate. Exposure to these low temperatures must be limited to a tolerable period, and the fans must be shut off whenever a person needs to enter such a freezer.
Cryogenic freezers must not be entered during operation; even a very brief exposure to the cryogen can quickly cause frostbite as a result of the extremely high heat transfer between the boiling liquid or sublimating solid and warm flesh.
Cryogenic freezers, which generally employ either liquid nitrogen or carbon dioxide in snow form, must be evacuated and refilled with air prior to entry by personnel. The freezers purge all the air out during operation, which results in an oxygen content insufficient to sustain consciousness or life. Such machines must be locked out during operation to prevent access and risk of asphyxiation.
Noise can reach levels that damage hearing in some freezers. Care must be taken to avoid exposure to noise levels above 95 dB by wearing a suitable form of hearing protection such as earplugs or earmuffs.
Noise can be reduced to safe levels by proper selection of fans. Excessive noise is generally a result of using a fan at a pressure above which it is designed to operate, causing cavitation.
The hostile environment in which freezers operate render human surveillance of their operation almost impossible. The machinery therefore must operate in an extremely dependable manner and have a number of detection devices mounted and operational so that if something does go wrong, the damage will be minimized.
The most common problems encountered in freezers are caused by ice accumulation, product jams, and operational errors. Ice accumulation can occur as a result of poor defrost procedure, excessive moisture on the conveyor carrying the product into the freezer, or poor startup and shutdown procedures. Ice can accumulate in locations in the machine where it impedes the safe operation of the freezer by jamming a part of by forcing the conveyor or other parts out of their normal operating position. Product jams can occur because the products to be frozen is improperly loaded onto the conveyor or the conveyor can be blocked by an obstruction in the product path. In either case good operation of the freezer and sound design can alleviate the problem. Operational errors are symptomized by a wide variety of problems. Typical symptoms are frozen conveyors due to improper startup, unbalanced fans, or an ice-plugged coil due to improper defrost.
Excessive pressure in the refrigeration piping and the coils can lead to a failure and subsequent loss of refrigerant and potential safety hazard. During normal operation the refrigeration piping in the freezer is under low pressure, so the risk of a ruptured pipe is small at that time. When a freezer is defrosted, the temperature of the coil is raised in order to melt the frost of the coil. If the refrigeration piping is not properly designed and installed, potentially dangerous pressures can develop in the coil. It is imperative that suitable relief devices be installed in the coil so that if the operator makes an error the pressure of the refrigerant in the coil cannot go above a safe level.
In a typical freezer application the value of the product that passes through the freezer in a period of just a few weeks can exceed the cost of the freezer. It is therefore sound practice to ensure that the product is not damaged or contaminated by the freezer.
Contamination can occur as a result of improper cleaning, debris such as surface coatings, or particles created by wear. Even if the contaminant is not harmful or toxic, its presence can render the product unsalable.
Product contamination can be minimized by sound freezer design in which wear debris cannot be generated in a location where it can get into the product as it is being conveyed through the freezer. Typical sources of contamination are wear debris created between the conveyor belt and the wear strip supporting the belt, dripping of condensed water at entries and discharges from freezers, flaking coatings, and leaking fluids such as hydraulic oil.
Product handling within the freezer can physically damage the product. Symptoms of products that have been damaged within the freezer include clumping together of pieces intended to be separate and free-flowing (IQF), pieces of the product torn from the conveying device as a result of the product being frozen to the conveyor, collision damage as a result of the product itself, and buildup of ice on the product.
Clumping together of product is generally a result of improper handling by the freezer as a result of poor design or improper operation. It is important that relative motion between the pieces to be frozen is maintained while the surface of the product freezes. The relative motion between the particles can be achieved by fluidization, mechanical agitation, or immersion in a boiling liquid that boils at a temperature well below the product freezing point.
In order to avoid damage to a product as it is removed from the conveyor that is transporting it through the freezer, it is essential that the product be solidly frozen and that the product does not adhere to the conveyor excessively.
Excessive adhesion can be avoided by careful selection of the conveyor material or by transporting the product in a manner such that it does not rest on a solid conveyor during the period that the surface of the product is being frozen.
Collision damage is generally a result of excessive loading of product onto the conveyor, which can result in the product colliding with other products inside the freezer or colliding with stationary structures within the freezer. Such collisions frequently result in product jams, which can destroy a considerable amount of product at each occurrence. To avoid such problems even feed equipment should be used.
Ice buildup on products can result in an insightly product. This condition can occur when excessive free moisture is transported into the freezer and is transferred from unfrozen to frozen product in the early stages of freezing. This can be controlled by minimizing the free moisture going into the freezer with the product and by controlling the motion of the product once it is inside the freezer.
Proper hygiene in the freezer environment must be defined by the user. Factors such as the sensitivity of the product to various degrees of contamination must be considered in setting sanitary standards. For example, if a cooked product is frozen that requires no additional cooking prior to consumption, extreme hygienic procedures and very sanitary equipment must be employed because of the high risks to the consumer. Contamination of the product can assume many forms. Typical forms of contamination that must be considered are bacterial, wear debris, pieces of a different product, and foreign debris.
Bacterial contamination is generally the result of bac-terially contaminated food coming in contact with the product and attaching itself. This can occur if the conveying system has not been adequately cleaned and sanitized.
Wear debris will be created in all freezing and processing machinery. Whether or not the product is considered contaminated is a function of how obvious the debris is and what the composition is. Stainless steel rubbing on either a plastic such as polyethelyne or directly on another piece of stainless steel will generate a significant amount of dark-gray powder. If this gray powder is deposited on the product such that it is visible macroscopically, the product is considered contaminated.
Contamination can also occur if previously frozen food or any food particle is deposited onto a product that is being frozen. This can be the result of inadequate cleaning between changes in product changes on the freezer or production line. A clean design and thorough cleaning can alleviate this problem.
Automatic cleaning of freezing machinery permits cleaning with less labor and reduces the need to access difficult areas and dangerous locations while still providing adequate cleaning. Additionally, automatic cleaning reduces labor and the necessary cleaning time (see Fig. 20).
Cleaning solutions must be selected carefully to adequately clean the machinery without damaging the materials or surface coatings used on the freezer. After cleaning, a sanitizing agent is sometimes employed to sterilize the machine.
Hygienic standards demand that all food machinery including freezers be constructed of nontoxic materials that permit use of aggressive cleaning agents such as mild caustics without corroding. Typical materials used presently are 300 series stainless steel, galvanized steel, aluminum, and various food-grade plastics.
Special considerations must be taken into account in selection of materials for specific functions in the freezer.
Surfaces that come in contact with the product must be smooth and totally noncorrosive and not adhere to the
product, either frozen or thawed. Stainless steel and plastics are generally used for product contact.
Materials through which heat must pass must have a high thermal conductivity. Applications that fall under this category are heat-transfer coils, flat metal belts used on contact freezers, and plates used on horizontal and vertical plate freezers.
High-insulation properties are required in practically all freezers to separate the cold environment from the ambient air as well as to prevent the formation of condensation on the warm external walls. Generally the insulating walls are of a panel construction with either metal or fiberglass skins and a plastic low thermal conductivity core. The panel is generally bonded together to give suitable mechanical properties and also to prevent the ingress of moisture. If moisture were to enter the panel, deterioration of the panels thermal and mechanical properties could result because of the freeze-thaw cycles encountered in a typical freezer.
Lubricants selected for food freezer applications must be selected on the basis of both their low-temperature properties and their toxicity. Gearboxes in the freezer must be located so that any leakage of lubricant cannot contaminate the product. Greases and oils used in close proximity to the product or product-carrying surface must be edible as insurance against accidental contamination over the life of the freezer is impossible. Lubricants must remain viscous enough over the entire operating range they are to be exposed (even when the freezer is held at extremely low temperatures without product) to permit dependable operation of the machinery. This generally requires the use of synthetic lubricants at temperatures below 7°C. Lubricants should also have the ability to maintain their properties with considerable moisture content as the cleanup and thawing will result in significant water accumulation in the lubricant. Lubricants must generally be changed at short intervals as a result of water contamination.
Coatings used in freezers must be permanent, or contamination of the product can result. Painted coatings requiring meticulous preparation in application are frequently fragile and require routine maintenance. Metallic coatings such as galvanizing, flame spraying, and plating are generally durable but are not as durable as a component made entirely of noncorrosive material. When coating a material with zinc, consideration should be given the thickness of the coating desired as the zinc will be consumed over time depending on its thickness and the environment. Zinc may not be in contact with the food product.
Materials at freezer temperatures generally undergo large changes in their physical properties, such as brittle-ness and size. The property changes can result in large stress-induced distortions and breakage unless adequately accounted for.
The mechanical efficiency of the freezer is a measure of how much work goes into the mechanical devices within the freezer. The mechanical devices typically consists of conveyor drives, fans, and other powered devices. It is important to realize that any energy put into the freezer either by the work performed in the freezer or by removing heat from the product must be removed by either the refrigeration system or the cryogen. Therefore, a significant multiplier must be applied to the cost of adding extra energy to the freezer environment.
The quantity, types, and power consumption of energy-consuming devices in a given freezer vary greatly between freezer types. Fans are generally the largest power-consuming component in fan-equipped freezers. Fan efficiencies are defined as the ratio of useful fan work to the actual power consumed by the fan motor and can vary from 40 to 70% depending on the selection of the fan. Conveyor drives are generally relatively small power consumers.
Coils are employed in almost all types of freezers that transfer heat by circulating cold air over the product and serve the function of transferring the heat from the air to the refrigerant. The efficiency of such a freezer is greatly influenced by the design of the coil as the design determines the difference between the air and refrigerant temperatures, which has a large bearing on the initial and operating cost of the refrigeration system. Coil efficiency is a function of materials of construction, configuration of the surface and refrigerant pipes, air velocity, and refrigerant circulation.
In addition to heat-transfer considerations, freezer coils must be designed with sanitation, corrosion, frost buildup, and speed of defrosting in mind.
In order to remain operational as long as possible, coils must be designed to accept frost without becoming plugged up prematurely. This is generally done by careful selection of the spaces between the heat-transfer surfaces or by continuous defrosting.
Continuous defrosting can be accomplished by installing multiple banks of coils and shutting off one bank and defrosting it while the others remain operational, blowing the frost from the coils with compressed air during operation, or rinsing the frost from an operating coil with a mixture of glycol and water. In such cases a glycol concentration is used to remove the water. A continuous (automatic) defrosting system is illustrated in Figure 21.
Fans for freezers should be selected to provide the highest possible economy over the life of the freezer taking into account dependability, efficiency, and initial costs.
Fans positioned such that the air flows over them before the coil are subject to large frost buildup. Frost buildup on a fan will result in imbalance, which will impose large stresses on the fan wheel and the motor. The fan must be designed to accept the resultant imbalance without failing.
Fan efficiency is determined by how the fan is selected and applied. Inlet and discharge effects can greatly alter a fan's performance. The air pressure and volume of air that
pass through the fan are normally presented graphically by the fan producer. A fan must be selected to give the correct quantity of air over a fully normal operating range, which can vary significantly from the time when a freezer has been recently defrosted and lightly loaded with product to when the freezer is frosted up and heavily loaded with product.
Fan motors must be selected and designed to take the mechanical stresses imposed on them as well as to power the fan wheel over a wide operating range. Fan motor bearings should be specially lubricated to run freely over a full range of operating temperatures. The bearings need to be adequate to tolerate a significant amount unbalance as discussed before.
Dependable and safe electrical installations require special attention in a freezer environment because of the high moisture and large temperature changes that cause water to migrate and condensate on electrical wiring and components. Electrical devices and wiring need to be protected from moisture by ensuring that they always remain above the dewpoint under all operating conditions and by protecting them from water entry during defrosting and cleaning. Electrical panels external to the freezer need to be protected from the cleanup conditions found in most plants by taking special precautions in sealing and ventilation for cooling.
In freezers employing air as the heat-transfer medium the design of the airflow has a major influence on the performance of the freezer and on the food product quality. The significant factors to consider in regard to airflow are quantity, evenness of distribution over the product, and power consumed by the fans.
The quantity of the air circulated influences the performance of the coils, the change in air temperatures through the freezer, and heat-transfer rates between the air and the product, all assuming that the areas through which the air flows remain constant. An efficient airflow is illustrated in Figure 22.
The distribution of the air over the product should be carefully controlled to give the designed freezing rate uniformly over all the product passing through the freezer.
The deterioration of the airflow through the freezer with increased coil frosting and heavy product loading needs to be taken into account when designing the air system so that the freezer maintains full performance under the full range of operating conditions.
Figure 23. Mechanical refrigeration system: (1) evaporator; (2) compressor; (3) condenser; (4) expansion valve; (5) motor.
Figure 23. Mechanical refrigeration system: (1) evaporator; (2) compressor; (3) condenser; (4) expansion valve; (5) motor.
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