Today's freezing equipment can be divided into two main groups: integrated in the processing line and operating in batches.
According to the heat-transfer method, there are basically three main types of equipment:
• Air-blast freezers, which use air for heat transfer. Because air is the most common freezing media, this method of heat transfer has probably the largest range of designs.
• Contact freezers. Heat transfers occurs through conduction. A refrigerated surface is placed in direct contact with the product or package to carry away the heat. Alternatively, the product is immersed in a cold liquid—brine.
• Cryogenic freezers. These freezers use liquefiable gases, nitrogen or carbon dioxide to produce vapors that precool and freeze the products.
Combinations of these heat-transfer methods can be seen in special designs.
The freezing equipment can also be divided into two main groups with reference to the product. Individually quick-frozen (IQF) and packaged products.
All of these methods are used in the food processing industry; however, the preferred systems are those that can be operated in-line, integrated with the processing and packaging operations.
The freezing equipment must be designed to accommodate the three stages of the freezing process and should ideally optimize the total process.
The following design criteria are of special interest (see below under "Major Considerations in Freezer Design"):
Product quality Hygiene
Minimum product losses Reliable and simple operation Simple maintenance Economy, freezing cost
In the following sections the freezing equipment will be discussed with the basis on the method of heat transfer.
Air is the most common freezing medium, and for that reason a number of designs can be found. Even if a storage room should never be considered as a piece of freezing equipment, it is sometimes used for this purpose. However, freezing in a storage room involves so many disadvantages that it should be used only in exceptional cases. The freezing is so slow that the quality of almost all products will be affected adversely.
Sharp Freezer, Blast Room
Basically, a sharp freezer or blast room is a cold storage room that has been especially constructed and equipped to operate at low temperatures for freezing. Even if this room is equipped with extra refrigeration capacity as well as fans for air circulation, there is normally no controlled airflow over the products, and for that reason freezing is normally slow. This type of equipment is, however, still used sometimes for bulk products such as beef quarters, but not for processed food products.
In tunnel freezers refrigerated air is circulated over the product, which is placed on trays or special spacers that stand in or pass through the tunnel in racks or trolleys.
The racks or spacers are arranged to provide an air space between each layer of trays. The racks or trolleys can be moved in and out of the freezer manually or by a fork-lift truck pushed through the tunnel by a pushing mechanism or slide-through. Tunnels are also used for freezing hanging meat carcasses carried on a suspension conveyor or in especially designed racks.
Practically all products can be frozen in a tunnel freezer. Whole, diced, and sliced vegetables may be frozen in cartons or unpacked in a 30-40-mm-deep layer on trays. Spinach, broccoli, meat patties, fish fillets, and prepared foods are frozen in packages in this type of equipment. It is, however, important to recognize that both refrigeration capacity and arrangements for air circulation have been designed for a specific product range. This means that if the tunnel freezer is designed for freezing meat carcasses or bulk meat cartons, the tunnel design is not appropriate for handling unpacked products such as vegetables. The result is often a increase in weight loss that influences both the sensory properties of the product and the economics of the freezing operation.
The flexibility of this type of equipment is balanced by high manpower requirements and a considerable weight loss if improperly used. The high manpower is caused by the handling, releasing, cleaning, and transportation of the trays and of the racks.
The tunnel should always be filled with product in such way that uniform airflow is obtained over all products to be frozen (see Fig. 8).
A certain degree of mechanization is achieved when the racks are fitted with casters or wheels. The racks or trolleys are usually moved on rails by a pushing mechanism often powered hydraulically. Mechanized tunnel freezers are known as push-through tunnels, carrier freezers, and sliding-tray freezers. A typical design is shown in Figure 9.
As in any other tunnel freezer, the products are placed on trays that, in turn, are stacked on trolleys. The tunnel contains one or two rows of trolleys that are pushed forward stepwise by each other on rails in line with the product line. When a trolley leaves the freezer, the frozen prod-
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ucts are removed from the trays and the trolleys are returned to the loading station.
The design and construction of the freezer is common to most tunnel freezers. The evaporation coils are placed on a steel frame standing on the insulated floor. They have a thin spacing, which varies with the depth of the coils. The spacing is wide at the air inlet and narrow at the outlet. This ensures an even frost buildup on the coils without detriment to the airflow. The coils are furnished with liquid and suction headers and arranged for pump circulation of the refrigerant. In some installations arrangements are also made for gravity feed. Standard defrosting is effected by hot gas or by leaving the doors open and running the fans during the night.
The fans circulate the air down between the coil and the wall through the coil, through the trolleys and the products, and then the air is deflected up along the wall and back to the fans above the sheeting.
Tunnel freezers are built for capacities varying from a few hundred kilograms to several tons per hour. The freezing time in this type of equipment varies considerably from a few hours when freezing unpackaged vegetables in a thin layer to 48 h freezing meat carcasses.
The sliding-tray freezer is basically a tunnel consisting of one huge rack accommodating many large trays on each tier. At one end of the construction there is an elevating mechanism that lifts the arriving tray to the top tier, where it is pushed in, forcing all the other trays on this tier to advance one step. The tray at the far end is pushed onto an elevator that brings it down one tier where the tray is entered. Consequently, on every odd tier the trays will be advancing, and on every even tier they will be returning. For each tray that enters, all trays will advance one step.
All mechanisms are usually hydraulically powered. Outside the freezer enclosure automatic loading and unloading of the trays may be arranged.
The freezing trays are in the traveling-tray freezer connected to one or two sturdy roller chains at each end. These are arranged to move the trays forward to a set of sprockets elevating the trays one tier while maintaining their horizontal position. This type is constructed as large as those mentioned previously, but requires more space because the minimum pitch between the tiers is decided by the sprockets, which are 200-300 mm in diameter.
The carrier freezer may be regarded as two push-through tunnels, one on top of the other. At the stop section a row of carriers is pushed forward, while it is returned in the lower section. At both ends there are elevating mechanisms. A carrier is similar to a bookcase with shelves. When it is indexed up at the loading end of the freezer, the products on one tier at a time are pushed off the shelf onto a discharge conveyor.
When the carrier is indexed up the next time, this shelf is level with the loading belt from which new products are transferred to the carrier (see Fig. 10).
The carriers may be designed for almost any pitch between the tiers and for any length and width, giving maximum compactness. The loading and unloading may be manual or fully automatic.
Another automatic freezing tunnel is the reciprocating spiral freezer, which consists of two parallel sets of rails,
one of which is fixed. In between there is a set of movable rails. Initially the products rest on the fixed set. The movable set lifts the products clear of the fixed set, advances one stroke, descends to leave the products resting on the fixed set again, and then returns to the initial position.
Both sets are arranged to form a large spiral, the fixed set supported from an external steel structure and the movable set fitted to a central cylinder. This provides a reciprocating motion around the vertical axis as well as up and down.
The infeed can be arranged very easily for a range of carton sizes provided the utilization of the conveyor area is limited. Small items must be placed on trays that are loaded, unloaded, and transferred from outfeed to infeed separately.
As the total product load is accelerated and decelerated for each stroke, a relatively slow motion is necessitated, making this type of freezer suitable mainly for intermediate and large package sizes.
All freezer designs described above are intended primarily for packaged products. Attempts to freeze fish fillets, meat patties, etc individually on trays invariably have been only moderate successful because of a number of problems, primarily the following:
1. Products stick to the trays. This causes damage and weight losses if products are removed mechanically. An alternative is to heat the trays to release the products. This requires complicated equipment and causes reduced capacity.
2. Hygiene. The trays must be washed after removal of the frozen products if acceptable hygiene is to be obtained.
3. The handling of trays from outfeed to infeed is costly whether it is manual or automatic.
The principal advantages of automatic air-blast freezers in comparison with automatic plate freezers, which are usually the alternative, are the following:
4. Products of much varied thickness may be frozen simultaneously or immediately after another.
5. Products do not need to be square in shape.
The automatic or mechanized tunnel freezer generally has the same advantages and disadvantages as the classical tunnel except that it is slightly better suited as an inline freezer.
Labor costs can be reduced and the flexibility is somewhat better as different products can be handled at the same time by different tracks having different dwell times.
The first belt freezers basically consisted of a wire mesh belt conveyor in a blast room, which satisfied the need for a continuous product flow. In addition to the disadvantage of poor heat transfer in a blast room, many mechanical problems arose.
Modern belt freezers normally utilize vertical airflow, forcing the air through the product layer, which creates good contact with all product particles. A condition is, however, that the product be evenly distributed over the entire belt area. Where the product layer is thin or nonexisting, there is less resistance to the air, which will concentrate to these areas and bypass the thicker product layer. This phenomenon, called channeling, may result in poorly frozen products and thus must be avoided by careful and even spreading of the product across total belt width under all operating conditions. Single-belt freezers designed for freezing unpacked products can be designed to achieve a fluidized freezing.
In order to decrease the necessary floor space of a singlebelt freezer the belts can be stacked above each other as in a multitier belt freezer or a spiral belt freezer. The latter being the most important modern belt freezer equipment. A typical multitier freezer is shown in Figure 11.
A multitier freezer can be used where the factory layout requires a straight-through product flow and where available space is narrow. A freezer consists of three conveyor systems positioned one above the other with fans and coils positioned above the top belt supported by the same steel structure that carries the conveyor system.
Products are fed onto the top belt and transported through the cooling zone into the freezing zone and to the opposite end of the freezer. Here the products are transferred from the top belt to the second via a stainless-steel transfer chute. On a second belt they are conveyed back through the freezing zone to the infeed, where they are transferred to the third belt. The third belt takes the products through the freezing zone and delivers the frozen products to the outfeed, from which the products leave the freezer. This arrangement has the advantage that the product, after being surface frozen on the first belt, may be stacked in a rather deep bed on the lower belts. Thus the total belt area required can be reduced.
By vertically stacking the belt in tiers a minimum of floor space is occupied by the freezer. This is the case in the modern spiral belt freezer (Fig. 12), which maximizes the belt surface area in a given floor space. This is achieved by using a belt that can be bend laterally around a rotating drum. The belt in supported by rails and driven by the friction against the drum. The most advanced and refined spiral freezers operate with a low tension drive system.
The continuous-belt design eliminates product transfer points where product damage can occur within the freezing system. The products are placed on the belt outside the freezer where they can be monitored and will remain in the same spot until leaving the freezer. The flexibility of the belt used allows for more than one infeed and outfeed with one and the same belt, and infeed and outfeed may be arranged in any direction desired to suit the layout of the processing line.
In the most modern version of spiral belt freezers a self-stacking belt—FRIGoBELT—where each tier rests directly on the vertical side links of the tier beneath is used. This construction eliminates the need for rails and runners and allows more tiers of belt to be installed in a given space. The whole stack turns as a unit. Products cannot roll and blow out of the closed freezing zone and cannot become stuck within freezer because there are no stationary structural parts to snag them. Equally important is the improvements of the hygienic conditions of the enclosed product zone. The belt in contact with the product is regularly cleaned and dried in an external washing unit outside the freezer.
The side links of the belt will also serve to channel the vertical airflow in the freezing zone. The air is blown down over the upward-moving products in a countercurrent flow, which is a very efficient form of heat transfer.
The spiral belt freezer provides great flexibility with regard to the product range to be handled. Both unpackaged and packaged products are frozen and typical products are meat patties, fish cakes, fish fillets, and bakery products, which all may be frozen raw or prepared.
Previously freezing of vegetables took place in a plate freezer or tunnel freezer, and the result was more or less a block frozen product that was hard to thaw and rather inconvenient in handling. The use of "cluster busters" in order to obtain a more free-flowing product caused considerable mechanical damage. Belt freezers were introduced soon after World War II, but in order to meet the high freezing demands those freezers became rather huge. In the early 1960s fluidized freezing was introduced after years of experiments and tests, and it was possible for the first time to quick-freeze vegetables individually very fast in a commercial application.
Fluidization occurs when particles of fairly uniform shape and size are subjected to an upward airstream. At a certain air velocity the particles will float in the air-stream, each one separated from the other but surrounded by air and free to move. In this state the mass of particles can be compared to a fluid.
If a mass is held in a container that is fed in one end and the other end is lower, the mass will move toward the lower end as long as more products are added. By utilizing low-temperature air to achieve the fluidization the products are frozen and simultaneously conveyed by the same air without the aid of a conveyor (see Fig. 13).
Figure 13. Fluidized-bed freezer (FLoFREEZE).
Figure 11. Multitier freezer (TRIoFREEZE).
Figure 13. Fluidized-bed freezer (FLoFREEZE).
of belt damage if a dewatering screen is broken down temporarily.
One type of fluidized-bed freezer combines the fluidized bed with a conveyor belt. The freezer operates in two stages: a crust freezing zone and a finishing freezing zone. In the former the product is carried on a fluidized bed that guarantees an efficient heat transfer, a quick crust freezing, and particle separation. The crust frozen product is then conveyed on a belt through the second freezing zone.
The fluidization technique achieves a very efficient air-product contact, which gives heat-transfer rates which are much higher than those for conventional air-blast freezing tunnels or belt freezers. The efficiency of the heat removal can also be seen in the physical dimensions of the equipment, which are generally one-third of the comparable belt freezer.
Figure 12. Spiral freezer (GYROCOMPACT).
The use of the fluidization principle gives a number of advantages in comparison with the use of a belt freezer. The product is always individually quick-frozen (IQF); this applies also to products with a tendency to stick together, eg, French-style green beans, sliced carrots, and sliced cucumber.
The freezer is totally independent of fluctuations in load. If the freezer is partly loaded, the air distribution can be the same as for the full load, ie, with no hazard of channeling. The variability of freezing with products is greatly improved because a deep fluidized bed can accept products with more surface water. Consequently, there is no hazard
The fluidized-bed freezer is in-line equipment suitable not only for vegetables, berries, and other fruits but also for processed products such as French fried potatoes, peeled cooked shrimps, diced meat, and meatballs.
In a contact freezer the product is either in direct contact with the freezing media—immersed—or indirectly by being in contact with a belt or plate containing the freezing media.
The immersion freezers consists of a tank with a cooled freezing medium, such as salt, sugar, or alcohol solution in water or other nontoxic mixtures of water and solutes. The product is immersed in this brine or sprayed while being conveyed through the tank.
This type of equipment has been quite commonly used for surface freezing of turkeys and other poultry on markets where a light color is demanded. Final freezing is accomplished in a separate blast tunnel or during cold storage. The latter, however, may jeopardize quality because of slow core freezing. It is necessary to protect the product from contact with the brine by using high-quality packaging materials with absolutely tight seal. Brine residues on the packages are washed off with water at the freezer exit.
A sodium chloride brine was earlier sometimes used in direct contact with the product in the fishing industry. For freezing tuna fish, for example, it is still used in some places.
The most commonly contact freezer is the plate freezer, where the product is pressed between hollow metal plates that are positioned horizontally or vertically, with a refrigerant circulating through them.
Another type of freezer utilizes two belts with the refrigerant circulating outside the belts or alternatively placing the product on a single belt. All these arrangements provide a very good heat transfer, which is reflected in short freezing times, provided the product itself is a good heat conductor.
The advantage of good heat transfer at the surface is gradually reduced with increasing product thickness. For this reason it is often limited to a maximum of 50-60 mm. It is further important that the packages are well filled and if metal trays are used to carry the packs that these are not distorted.
The pressure from the plates or belts maintain throughout the freezing process practically eliminates what is known as "bulging" and the frozen packs will maintain their regular shape within very close tolerances.
There are two main types of plate freezer: the horizontal and the vertical plate freezer. Either type can be manual or automatic. The typical manual horizontal plate freezer contains 15-20 plates. The product is placed on metal trays or in other systems metal frames and transported to the freezer where they are manually loaded between the plates.
In order to obtain automatic operation of the horizontal plate freezer the whole battery of plates is moved up and down in an elevator system. At the loading conveyor level the plates are separated, and a row of packages that have accumulated on a transport conveyor from the processing and packaging are pushed in between the open plates, simultaneously discharging a row of frozen products at the opposite side. This cycle is repeated until all frozen packages at this level has been replaced. Then the space between the two plates are closed, all plates are indexed up, and the next set of plates are opened for loading and unloading. A typical automatic plate freezer is shown in Figure 14.
The vertical plate freezer is used mainly for freezing products in blocks weighing 10-15 kg and has been specially developed for freezing fish at sea. The freezer consists of a number of vertical freezing plates forming partitions in a container with an open top. The product is simply fed in from the top and the blocks after freezing is discharged either to the side, upwards, or down through the bottom. Usually this mechanism is automized. The discharge of the products is enhanced by a short period of gas defrost at the end of the freezing cycle and the use of compressed air or a hydraulic system to force out the product.
Single-band and double-band freezers are designed to freeze thin product layers. The freezers can either be straight forward bands as shown in Figure 15 or as a drum (Fig. 16).
The band freezer illustrated in Figure 16 is designed to freeze and form liquids and semiliquids into individual pellets as an in-line operation.
Figure 15. Contact band freezer (PELLoFREEZE).
The product is formed and frozen between two endless stainless-steel bands, of which the top band is flat and the lower band is corrugated with flexible seals on each side. The product is supplied on the corrugated band by a spreading device, after which the flat band is brought in contact with the product, thus totally enclosing it. After the freezing-forming zone the two bands are separated. The liquid is now frozen to a mat and passes through the final forming operation. The product finally enters the out-feed conveyor in an IQF form.
A monopropylene glycol-water brine is used as an intermediate freezing medium. The brine is circulated by pumps from a sump below the freezing pump via a brine cooler to the freezing zone, where it passes over the outer surfaces of the band with high velocity.
TVpical products frozen in band freezers are chopped spinach purée, fruits pulps, egg yoke, sauces, and soups.
The drum freezer can be viewed as a more compact band freezer. Distinction is made between both vertical and horizontal drums. This type of freezer is also known as a rotary freezer.
Cryogenic freezers differ from all other types of freezers in one fundamental respect: they are not connected to a refrigeration plant. The heat-transfer medium is nitrogen or carbon dioxide liquefied in large industrial installations and shipped to the food freezing factory in low-temperature well-insulated pressure vessels.
The design of cryogenic freezers has improved significantly in recent years. As for all types of freezers also this type can be found as straight-belt, multitier, and spiral belt as well as immersion designs. In principle the same basic equipment can be used for both gases, but with slight modifications. The size and mobility of cryogenic freezers allow for flexibility in design and redesigning a processing plant.
Key attributes of the equipment are high heat-transfer rates, low investment costs, and rapid installation and startup. Especially interesting applications are those for chilling, firming, or crusting products.
A typical belt liquid nitrogen (LIN) freezer is shown in Figure 17. Liquid nitrogen at — 196°C is sprayed into the freezer in which the atmosphere is circulated by small fans. The freezant or liquid nitrogen partially evaporates immediately leaving the spray nozzles and on contact with the products. The cold gas is circulated by fans toward the infeed end precooling the products entering the freezer and thereafter extracted by an exhaust fan.
Figure 17. Cryogenic freezer (AGA FREEZE).
The freezant thus passes in countercurrent to the movement of products on the belt and giving a high heat-transfer efficiency, which is an advantage in terms of quality for some special products.
However, the quick freezing may also result in cracking of the product surface if sufficient precautions are not taken. The freezant consumption is in the range of 1.2-2.0 kg per kilogram of product. The capacity can vary from 150 to 1000 kg/h, and typical products are meat cuts, fish fillets, seafood, fruit, berries, pies, and pastries.
A recent development utilizing liquid nitrogen as freezing medium is an apparatus designed for quick crust freezing of extremely wet, sticky, or sensitive products that can then be easily handled in a spiral belt freezer or a fluidized-bed freezer for completion of the freezing process without deformation or breakage. The freezers also offer a possibility of freezing products that are difficult to freeze in conventional systems (see Fig. 18).
The products are frozen by means of direct immersion in liquid nitrogen, which gives an almost instantaneous freezing of the surface. The product is fed vertically into an IQF tank with a continuous flow of liquid nitrogen.
In this stream the products are gently received and separated at the same time as a very thin layer of the product surface is frozen. From this first step the products drop down on a belt and are fed into a bath of liquid nitrogen that together with a spray completes the crust freezing. The liquid nitrogen is then separated from the product and collected in a sump from where it is recirculated by means of a specially designed pump.
Liquid carbon dioxide is normally stored under high pressure. At atmospheric pressure it exists only as a solid or a gas. When the liquid is released to the atmosphere, 50% of the liquid becomes dry-ice snow and 50% vapor both at — 79°C. Because of these unusual properties, carbon dioxide freezer designs vary widely.
In a LIN freezer the cold gas phase is used to precool the product before it is exposed to the nitrogen spray. As liquid carbon dioxide forms snow that needs time for sub-
limation, the injection is moved closer to the product in-feed as compared to the LIN freezer.
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