WO2005073649A1

WO2005073649A1 - Apparatus and method for refrigerating products in a fluidised bed

Info

Publication number: WO2005073649A1
Application number: PCT/SE2005/000079
Authority: WO
Inventors: Joakim Fajerson; Branko Pancer
Original assignee: Frigoscandia Equipment AB
Current assignee: John Bean Technologies AB
Priority date: 2004-01-30
Filing date: 2005-01-26
Publication date: 2005-08-11
Anticipated expiration: 2006-07-30
Also published as: SE0400167D0; SE527465C2; TR200603896T1; DK178194B1; DK200601001A; SE0400167L

Abstract

The invention relates to an apparatus (1) for freezing products in a fluidised bed. The apparatus (1) comprises an air-permeable conveyor belt (6, 61) which along part of its length forms a bottom surface (4) of a trough (2, 21), which is adapted to receive the products that are to be frozen. An air-directing means (10', 10 10') is arranged along said part of the conveyor belt (6, 61) for directing a flow of air (F', F') which is supplied to the trough via its bottom surface (4). The air directing means (10', 10') is arranged to direct the flow of air (F', F') so that a bed formed of said products in the trough (2, 21) has a first fluidisation in a precooling zone (A', A') of the trough (2, 21) and a second fluidisation in a freezing zone (B', B') of the trough (2, 21) downstream of the precooling zone (A', A'). The second fluidisation is more vigorous than the first and so vigorous that freezing together of adjoining products is counteracted. The invention also relates to a method for performing said freezing.

Description

APPARATUS AND METHOD FOR REFRIGERATING PRODUCTS IN A FLUIDISED BED

FIELD OF THE INVENTION The present invention relates to an apparatus for refrigerating or freezing products in a fluidised bed, comprising an air-permeable conveyor belt which along part of its length forms a bottom surface of a trough, which is adapted to receive the products that are to be frozen; an air-directing means which is arranged along said part of the conveyor belt for directing a flow of air supplied to said trough via said bottom surface, the air-directing means being arranged to direct said flow of air so that a bed formed of said products in the trough has a first fluidisation in a precooling zone of the trough and a second fluidisation in a freezing zone of the trough downstream of the precooling zone. The inven- tion also relates to a method for freezing products in a fluidised bed.

BACKGROUND ART In the food industry, products such as vegetables, fruits, berries and shellfish are usually frozen in an individually separate state. This can occur, for instance, by the product being supplied to a trough which from its bottom is supplied with a flow of refrigerated air. As a result, the products will form a fluidis- ed bed while being frozen. This technique is disclosed, for instance, in US 3,169,381, SE 204,095, SE 217,470, SE 216,963, SE 336,362, US 4,281,521 and US 3,886,762. The main object of this technique is to provide very quick freezing to a temperature just below 0°C, for instance -2 to -3°C, after which the frozen products can be conveyed to another apparatus for deep-freezing to a temperature below, for instance, -18°C. Such a trough has an inlet end and an outlet end. Fresh products are supplied at the inlet end and discharged in a frozen state at the outlet end. The trough further comprises a bottom surface in the form of an end- less air-permeable conveyor belt. Under the conveyor belt there is an air-directing means for directing a flow of air up through the conveyor belt and into the trough. To provide a required flow of air, but also a required speed of the air, in the trough to form a fluidised product bed, the air-directing means comprises some kind of throttling. Traditionally, throttling has always been arranged so that the flow of air will be at its maximum in connection with the inlet end of the trough where the products are supplied so as then gradually to decrease towards the outlet end of the trough, see for instance US 3,169,381. This was believed, and is still believed, in the trade to be necessary since products supplied to the trough are considered at the inlet end to require a higher flow of air initially. A high flow of air is considered to result in effective disruption of the insulating layer of air surrounding the product and, thus, quick tempering. A high initial flow of air is also necessary for the transport of products if the apparatus has no conveyor belt, like some of the prior-art apparatus. This distribution of the airflow is, however, less advantageous for products that have a surface structure and consistency that is susceptible to shock or high flows of air. For instance, freezing of fresh raspberries can be mentioned. Raspberries in a fresh state are very soft and easily damaged while at the same time they have a high juice ^• content and an irregular surface. When these berries are supplied to the trough and meet the high flow of air and are fluidised, they will initially, before surface freezing has occurred, collide with each other and be deformed, thereby emitting juice. This juice is collected in the irregular surface structure where it freezes and can be said to glaze the berry. The juice also tends to clog the conveyor belt, causing undesirable throttling of the flow of air to the trough. The juice may also cause freezing of products on the side walls of the trough, which requires frequent and expensive production stoppages for cleaning. Furthermore raspberries have "hairs" on their surface, which are broken, when a raspberry, especially in a frozen condition, collides with other raspberries. Another factor that can contribute to the hairs being broken is a high speed of the air. It is not unusual that raspberries after such freezing have lost their hairs . Berries which in this way have lost their original appearance and quality bring a lower price when sold and cannot be used, for instance, for decoration of pastry. In connection with freezing of products using such an apparatus, a considerable amount of "snow", i.e. ice crystals, forms in and around the evaporators that are used to precool the air. This is a problem especially in products that emit a large amount of liquid. Owing to the snow, the effect of the evaporators is gradually reduced, thus necessitating at regular intervals removal of the snow from the evaporators and the space in which the evaporators are accommodated. This must be carried out during costly production stoppages. In continuous production, this may be necessary one or more times a day.

OBJECTS OF THE PRESENT INVENTION The object of the present invention thus is to provide an apparatus and a method for performing smoother individual freezing of products, in which emission of liquid and surface deformations of the products are reduced, and in which the products are not smashed. A further object of the invention is to provide an apparatus and a method in which the amount of snow formation is reduced. It is thus an object to provide an improvement of an apparatus and a method according to prior-art technique for individual freezing of products.

SUMMARY OF THE INVENTION To achieve at least one of the above objects and also other objects that will appear from the following description, an apparatus and a method having the features defined in claims 1 and 13 are provided according to the present invention. Preferred embodiments are defined in claims 2-12 and 14-17, respectively. More specifically, the invention relates to an apparatus for freezing products in a fluidised bed, comprising an air-permeable conveyor belt which along part of its length forms a bottom surface of a trough, which is adapted to receive the products that are to be frozen, and an air-directing means which is arranged along said part of the conveyor belt for directing a flow of air supplied to said trough via said bottom surface, the a±r- directing means being arranged to direct said flow of air so that a bed formed of said products in the trough has a first fluidisation in a precooling zone of the trough and a second fluidisation in a freezing zone of the trough downstream of the precooling zone. The apparatus is char- acterised in that the air-directing means comprises a throttled first portion which is arranged in connection with the precooling zone, and a throttled second portion which is arranged in connection with the freezing zone, which second portion has a smaller throttling than said throttled first portion, and that said second fluidisation is more vigorous than the first and so vigorous that freezing together of adjoining products is counteracted. By changing the flow of air through the trough in the precooling zone and the freezing zone, respectively, and arranging a more vigorous fluidisation of the products in the freezing zone than in the precooling zone, the very surprising effect is achieved that also the most fragile products can be individually frozen in a very smooth manner. Experiments have demonstrated that raspberries can be frozen by means of the inventive apparatus with a result where the berries are practically unaffected by the mutual collisions that are inevitable in fluidisation. By ensuring a more vigorous fluidisation in the freezing zone, any tendency of adjoining products freezing to each other, which may occur in and around the transition between the two zones, will be eliminated since the more vigorous fluidisation and, thus, the mixing are capable of separating products that may have started to freeze to each other. Of course, it will be appreciated that the fluidisation and the construction of the trough imply that there is no distinct transition between said zones. In spite of the changed flow of air, the required retention time of the products in the apparatus and its trough is not significantly influenced. By the damage to the products being reduced, also the emission of juice and, thus, the amount of liquid that on the one hand may cause clogging of the conveyor belt and getting stuck by freezing on the walls of the trough and, on the other hand, may be converted into snow, will be reduced. This results in fewer production interruptions, which in large-scale production yields economic gains. Also the quality of the products will be improved. The difference in throttling between the two por- tions means that the flow of air and, thus, the fluidisation in the trough will be more vigorous in the freezing zone. By thus using different throttlings for distributing the flow of air, a fan or a plurality of fans with the same control can be used in the apparatus, which makes the apparatus simple and inexpensive. The air-directing means may comprise a non-throttled second portion which is arranged in connection with the freezing zone. The air-directing means can be arranged in one piece. However, it is preferred for the air-directing means to be divided into segments in a direction transversely to the feeding direction of the conveyor belt, which segments comprise different degrees of throttling. Owing to the division into segments, the air-directing means will be very easy to handle, for example, in cleaning and maintenance, compared with an air-directing means which is arranged in one piece. Furthermore, the individual segments can easily be exchanged or shifted to adjust the flow of air through the trough to the type of product and the product quality which is optimal for the purpose . If desired, the trough can also have a deep-freezing zone. This means that the apparatus can be used for complete treatment of the products from precooling to deep- freezing, thus making it possible for the apparatus to be succeeded by, for example, a device for packaging. If the trough has a deep-freezing zone, the air- directing means should comprise a throttled third portion which is arranged in connection with the deep-freezing zone, which third portion has a greater throttling than said second portion. As a result, the flow of air in the deep-freezing zone will be lower than in the freezing zone. Since the products have already been frozen, the further freezing process is more or less independent of a vigorous flow of air, which may therefore be reduced once more. Moreover, some products can in a frozen state be more fragile than in a non-frozen state, and therefore it may be very advantageous to reduce the flow of air in the deep-freezing zone in order to be careful with the pro- ducts. This may apply to, for instance, shrimps, whose "feelers" are very sensitive when frozen and, hence, rigid. Preferably the flow of air in the precooling zone has a speed of 2-4 m/s, the flow of air in the freezing zone a speed of 5-7 m/s and the flow of air in the deepfreezing zone a speed of 2-4 m/s. This means that the products will be handled most carefully when they are the most sensitive to external influence, i.e. in the precooling zone and in a deep-freezing zone, if any. In addition, the mixing is most vigorous in the freezing zone where the products otherwise run the risk of freez- ing to each other. For a continuous transport of the products through the trough from its inlet end to its outlet end, the air- permeable conveyor belt is advantageously endless. The inventive apparatus may comprise a second air- permeable conveyor belt which along part of its length forms a bottom surface of a second trough which is adapted to receive and deep-freeze the products frozen in the first trough. Such a second conveyor belt allows easy control of the retention time of the products in the second trough and, thus, the time for deep-freezing of the products. This also means that the first and second troughs respectively are allowed to have different depths of the product bed. A low depth of the product bed in the first trough is important since it results in quicker freezing, whereas the depth is less important, or not at all important, in the subsequent deep-freezing. Said first trough can be accommodated in a first compartment and said second trough can be arranged in a second compartment succeeding the first compartment. According to another aspect, the invention relates to a method for freezing products in a fluidised bed, comprising fluidising a bed of products contained in a trough, so that the bed has a first fluidisation in a precooling zone of the trough, and a second fluidisation, which is more vigorous than said first fluidisation, in a freezing zone of the trough, said first and second fluidisations respectively being provided by the trough being supplied with a more vigorous flow of air in the freezing zone than in the precooling zone, which flow of air is controlled by an air-directing means provided with throttlings, said air-directing means having a greater throttling in said precooling zone than in said freezing zone, said second fluidisation being given such an extent that freezing together of adjoining products is counteracted.

DESCRIPTION OF DRAWINGS In the following the invention will be described in more detail by way of example with reference to the accompanying drawings, which illustrate prior-art technique and currently preferred embodiments of the inven- tive apparatus and method. Fig. 1 is a schematic cross-section of an apparatus according to prior art seen transversely to the conveyor belt. Fig. 2 is a schematic cross-section of an apparatus according to prior-art seen along the conveyor belt. Fig. 3 is a schematic airflow diagram and temperature diagram, respectively, of a prior-art apparatus. Fig. 4 is a schematic cross-section of a first embodiment of an apparatus according to the invention seen along the conveyor belt. Fig. 5 is a schematic airflow diagram and temperature diagram, respectively, of the apparatus according to Fig. 4. Fig. 6 illustrates a second embodiment of the inven- tive apparatus, in which precooling and freezing occur in a first trough and deep-freezing occurs in a second trough. Fig. 7 is a schematic airflow diagram and temperature diagram, respectively, of the apparatus according to Fig. 6. TECHNICAL DESCRIPTION For better understanding of the invention, an apparatus 1 according to prior-art technique is described by way of introduction, with reference to Figs 1 and 2. The apparatus 1 is intended for individual precooling and freezing of products, mainly food products such as fruits, vegetables, berries or shellfish. The terms precooling, freezing and deep-freezing are used throughout the text. The term precooling refers to tempering of the products from the initial temperature to a temperature exceeding 0°C. The term freezing refers to tempering of the products beyond the freezing point, i.e. from slightly above 0°C to about -5°C, preferably to -2 to -3°C. Finally, the term deep-freezing refers to tempering of the products from the final freezing temperature to a temperature which preferably, but not necessarily, is below -18°C. The apparatus 1 comprises an elongate trough 2 which is accommodated in a compartment 3. The trough 2 has a bottom surface 4 and side walls 5. The side walls 5 can, as shown, consist of separate walls or, alternatively, consist of the walls of the compartment 3. In the embodiment shown, an endless air-permeable conveyor belt 6 forms along part of its length a bottom surface 4 of the trough 2. The conveyor belt 6 can have different extensions. In the shown embodiment, the conveyor belt 6 is in its entirety arranged in the compartment 3 and extends along the major part of the length thereof. It can also be arranged to extend outside the compartment and out through the end walls 7 thereof in such a manner that the conveyor belt 6 has loading and unloading zones which are arranged outside the compartment 3. Regardless of the extension of the conveyor belt, the compartment 3 has in its end walls 7 an inlet 8 through which the products to be tempered are supplied and received in the trough 2, and an outlet 9 through which the ready-tempered products are discharged. The products are advantageously supplied to the compartment 3 and its trough 2 through the inlet 8 by means of a shaking feeder (not shown) . The discharge from the compartment 3 and the trough 2 is advantageously carried out by the products being discharged through the outlet 9 by a combination of the movement of the conveyor belt 6 and the fluidisation. Under the upper part, i.e. the conveying surface, of the conveyor belt 6, which forms the bottom surface 4 of the trough 2, there is an air-directing means 10. The air-directing means 10 consists in its simplest form of a metal sheet extending along the entire, or the major part of, the width and length of the bottom surface 4. Owing to the length of the trough 2, such an air-directing means 10 can be excessively heavy and unwieldy in maintenance and cleaning and may then instead be divided into a number of separate segments. Such segments are advantageously arranged side by side in a direction transversely to the length of the conveyor belt 6. The air-directing means 10 comprises a hole pattern 11 which forms throttlings 12 of the airflow F which is distributed into the trough for tempering of the products. The airflow F, the hole pattern 11 and the throttling 12 will be described below. Parallel to the compartment 3 there is a space 13 which communicates with said compartment 3 through openings 14 in a partition 15 between the space 13 and the compartment 3. The space 13 accommodates one or more evaporators 16 for precooling of the air that is to be cir- culated in the apparatus 1. The space 13 also accommodates one or more fans 17. The fans 17 are advantageously arranged in said openings 14 in the partition 15. The fans 17 and the evaporators 16 are arranged in said space 13 along the length of the trough 2. The fans 17 are arranged to circulate the refrigerated air, see arrows F, from said space 13 through the openings 14 to the compartment 3, through the air-directing means 10, up through the conveyor belt 6 and into the trough 2 where products received therein are fluidised, and finally back to the space 13 where the air passes through the evaporators 16 to be cooled again. The air circulated through the apparatus is preferably tempered to a temperature in the range of -10 to -75°C, and most preferred in the range of -20 to -35°C. A first propelling force for feeding the products from the inlet 8 of the trough 2 to its outlet 9 thus consists of the products in their fluidised state pressing each other forwards. A second propelling force consists of the feeding of the conveyor belt 6 towards the outlet 9, see arrow P in Figs 2, 4 and 6. Any products coming into contact with the conveyor belt 6 are thus pressed forwards by the belt. Which propelling force is the greatest depends on the degree of fluidisation and may vary from case to case. When the products are supplied to the trough 2 through the inlet 8, they will initially be cooled, which takes place in a zone referred to as the precooling zone A next to the inlet 8 of the trough 2. As the temperature of the products passes 0°C, freezing of the products occurs to a temperature of about -2 to -5°C, preferably to -2 to -3°C, which occurs in a zone referred to as the freezing zone B which is arranged downstream of the precooling zone A. Depending on the length of the trough 2 and the retention time of the products in the trough, the freezing zone B can pass into a deep-freezing zone C, arranged downstream of the freezing zone B, in which the products are frozen to a final temperature which preferably is below -18°C. Of course, it will be appreciated that it is not possible to state exact limits or temperature values indicating where one zone ends and the other begins, since the products during fluidisation are random tumbled in the trough 2 with successive feeding in the feeding direction of the conveyor belt 6. The apparatus 1 may comprise only a precooling zone A and a freezing zone B, in which case deep-freezing is carried out in a separate compartment. Deep-freezing may even be carried out in an apparatus of a completely dif- ferent type . The air-directing means 10 of an apparatus 1 of the prior-art type as described above will now be described with reference to Figs 2 and 3. As mentioned above, prior-art technique uses, with- out exception, the most vigorous airflow F and, hence, the most vigorous fluidisation in the precooling zone A and especially at the end thereof next to the inlet 8 of the trough 2. The reason for this is that, as mentioned above, it is traditionally believed that fresh products require a higher airflow F at the inlet end of the trough and, hence, more vigorous fluidisation in order to quickly decrease their temperature. To provide this higher airflow F and, hence, more vigorous fluidisation, the air-directing means 10 exhi- bits a hole pattern 11 which has a smaller throttling 12 and, thus, a higher airflow F next to the inlet 8 of the trough 2 and a throttling 12 which subsequently gradually increases to provide a lower airflow F in a downstream direction. Consequently the airflow F is highest in the precooling zone A so as then to decrease and be practically constant in the freezing zone B and in a deepfreezing zone C, if any. This is schematically shown in the diagram in Fig. 3. The same diagram also schematically shows how the product temperature Temp gradually decreases as the products are being conveyed towards the outlet 9 of the trough 2. In the present invention, however, it has been realised that this traditional distribution of the airflow causes unnecessarily vigorous mixing of the products in the precooling zone A, i.e. at a stage when the products are the most fragile. Moreover nothing is gained by having the most vigorous mixing at the inlet 8 of the trough 2 since the products do not reach the freezing temperature until in the freezing zone B which in fact is the position where they must be separated to provide individual freezing. The solution to this problem, and thus the invention, will now be described with reference to Figs 4 and 5 which are a cross-section of the apparatus 1 according to the invention and, respectively, a diagram with an airflow F' and a product temperature Temp' seen along the length of the trough. The inventive apparatus 1 has the same construction as the one described above and will therefore not be described once more. The apparatus 1 according to the invention comprises an air-directing means 10' which is arranged as follows. In the precooling zone A', the air- flow F' is moderate and practically constant through a great, or even sharp, throttling 12' in the air-directing means 10'. The throttling 12' in the air-directing means 10' decreases in the transition to the freezing zone B', and the airflow F' and, hence, the fluidisation will be more vigorous. The airflow F' thus is at its maximum, i.e. the throttling 12' is at its minimum, in the freezing zone B' where the products start freezing and thus risk freezing to each other. Any tendencies towards freezing to each other are eliminated by the locally more vigorous fluidisation which separates products that are about to freeze to each other. If the trough comprises a deep-freezing zone C, this is arranged downstream of the freezing zone B'. In, and in connection with, such a deep-freezing zone C, the air-directing means 10' has a throttling 12' which is greater than in the freezing zone B', i.e. the airflow F' and the fluidisation in the deep-freezing zone C are lower than in the freezing zone B'. By way of example, the air speed in the trough 2 can be 2-4 m/s in the precooling zone A', 5-7 m/s in the freezing zone B' and 2-4 m/s in a deep-freezing zone C, if any. In the embodiment described above, the fans (not shown) are arranged in a common space (not shown) parallel to the compartment. It is thus difficult to use the fans to control the airflow through the trough in order to obtain the different zones and their different degrees of fluidisation of the bed of products. The use of an air-directing means with different degrees of throttling is therefore a simple, inexpensive and reliable solution which also is easy to vary according to, for instance, the type of product and its individual requirements. The reduced flow of air in a deep-freezing zone C, if any, is made possible since the magnitude of the airflow, with regard to freezing to each other, is less important once the products have started freezing. The air-directing means 10' according to the invention is advantageously divided into segments (not shown) which are oriented transversely to the feeding direction of the conveyor belt 6. The air-directing means 10' preferably consists of one or more sheets with a hole pat- tern 11' which, for example, is punched or cut in a suitable pattern. The hole pattern 11' may consist of, for instance, circular or oval through holes or slots extending along or transversely to the air-directing means 10' which is arranged in one piece or in segments. The geo- metric shape of the hole pattern 11' is insignificant. What is important for the invention is that the hole pattern is designed so that the throttling 12' is at its minimum in, and in connection with, the freezing zone B' where the products are the most fragile. The throttling 12' can advantageously be gradually decreasing or increasing in, and in connection with, the transition between two zones. For example, the throttling 12' of the air-directing means 10' can be arranged to be gradually decreasing in the precooling zone A', towards the freezing zone B'. Correspondingly, its throttling 12' in the freezing zone B', towards a deep-freezing zone C, if any, may be gradually increasing. The air-directing means 10' is advantageously arranged so as to be hingedly attached along the length of the trough 2. As a result, the air-directing means 10' can easily be pivoted to allow cleaning. Moreover, the air-directing means 10' should be easily detachable and exchangeable to allow the hole pattern 11', and thus the airflow F' through the trough 2, to be adjusted to the type of products to be treated in the apparatus 1. A shrimp requires, for example, a different airflow and fluidisation than, for example, a pea in the different zones to reach the necessary temperature and product quality during its retention time in the trough. With reference to Figs 6 and 7, a second embodiment of the inventive apparatus 1 will be described. Fig. 6 is a cross-section of the apparatus 1 while Fig. 7 is a diagram with an airflow F" and a product temperature Temp", respectively, seen along the length of the apparatus . The apparatus 1 is divided into two compartments 31, 32, which are constructed in essentially the same way as the previously described compartment 3, and therefore only the features that are unique for this embodiment will be described. The trough 21 in the first compartment 31 constitutes a precooling zone A" and a freezing zone B", while the trough 22 in the second compartment 32 constitutes a deep-freezing zone C". The first compartment 31 comprises an air-directing means 10" which in a first portion next to the inlet 8 has a great, or even sharp, throttling 12" to create said precooling zone A". The airflow F' and, thus, the fluidisation in this precooling zone A" is moderate or low and practically constant. In the transition to, and in, the freezing zone B", the throttling 12" of the air- directing means 10" decreases, thus making the airflow F" and the fluidisation in the freezing zone B" more vigorous than in the precooling zone A". By way of example, the airflow F" in the precooling zone A" can have a speed of 2-4 m/s, and the airflow F" in the freezing zone B" a speed of 5-7 m/s. The first trough 21 has such a length that the products reach a suitable freezing temperature during their retention time therein. By a suitable freezing temperature is meant, like before, a temperature in the range of 0°C to -5°C, and more preferred -2 to -3°C. In the space (not shown) parallel to the first compartment 31 there are one or more fans which generate the airflow which is later distributed by the air-directing means to achieve the desired airflow through the trough 21 and its precooling zone A" and freezing zone B". When the products have reached the desired freezing temperature, they leave the first compartment 31 and are supplied to a second compartment 32 succeeding the first compartment. In the second compartment 32, the products are received in a second trough 22, in which the products are deep-frozen. Thus the second compartment 32 is a deep-freezing zone C" where the products are fluidised to reach a temperature of preferably at least -18°C. The second compartment 32 lacks air-directing means. The second compartment 32 adjoins a second space (not shown) , which comprises one or more fans which generate the airflow F" that is directed up through the second trough 22 for the desired degree of fluidisation of products received therein. Since the purpose of the second compartment 32 is to deep-freeze the products, a uniform airflow F" is only necessary through the second trough 22. The airflow F" in the second compartment 32 can be kept at a low level, for instance have a speed of 2-4 m/s, i.e. considerably lower than in the freezing zone B" in the first compartment 31. It is in fact preferred for the products, during deep-freezing, to be subjected to a less vigorous fluidisation than during freezing, on the one hand to prevent damage due to collision, and on the other hand because the degree of fluidisation does not affect the speed at which the products are deep-frozen once the products have reached a temperature below 0°C. The retention time of the products in the second trough 22 and thus the time for reaching the desired deep-freezing temperature are controlled by the feeding speed of the conveyor belt 62. Since the time for freezing of products is shorter than the time for deep-freezing, the conveyor belt 62 in the second compartment 32 can have a considerably lower feeding speed than the conveyor belt 61 in the first compartment 31. The two compartments 31, 32 are advantageously arranged in immediate succession and with a relative difference in height, whereby products discharged from the first compartment 31 fall directly into the second compartment 32 by gravity. The two compartments 31, 32 are arranged physically separate from each other but, as is evident from Fig. 6, their low-pressure zones, i.e. the zones above the troughs 21, 22, can communicate with each other. The selected material and the surface quality of the compartment 3, 31, 32, the space 13 and the equipment arranged therein should be adapted for optimum food hygiene. This means that the material should if possible be stainless steel, and that unnecessary corners, edges and joints should be eliminated or be made accessible for cleaning. The compartment 3, 31, 32 and the space 13 should be easily accessible for cleaning and maintenance. The compartment 3, 31, 32 and the space 13 advantageously have doors or covers (not shown) through which members of the staff can enter for maintenance and cleaning. Moreover the air-directing means 10, 10' and the conveyor belts 6, 61, 62 should be accessible for cleaning. The air-directing means 10', 10" is suitably provided with a hinge mechanism (not shown) , thus allowing it to be pivoted from its operating position, where it is preferably oriented in the horizontal plane, to a maintenance position, where its surfaces and hole pattern are easy of access and easy to exchange. The air-directing means 10', 10" has been described above to comprise a hole pattern 11' with throttlings 12', 12" to obtain the desired airflow F', F" . This hole pattern 11' has further been described to comprise holes that are cut or formed in some other manner. It will, of course, be appreciated that the hole pattern 11' can also be arranged in the form of shutters or valves. Such shutters or valves can be controlled to obtain the desired degree of throttling along the length of the air- directing means 10', 10" depending on which product is treated in the apparatus at the moment. It will be appreciated that the present invention is not limited to the shown embodiments. Several modifications and variants are thus conceivable within the scope of the invention which consequently is exclusively defined by the appended claims.

Claims

1. An apparatus (1) for freezing products in a fluidised bed, comprising an air-permeable conveyor belt (6, 61) which along part of its length forms a bottom surface (4) of a trough (2, 21) , which is adapted to receive the products that are to be frozen, and an air-directing means (10', 10") which is arranged along said part of the conveyor belt (6, 61) for directing a flow of air (F', F") supplied to said trough (2, 21) via said bottom surface (4), the air-directing means (10', 10") being arranged to direct said flow of air (F', F") so that a bed formed of said products in the trough (2, 21) has a first fluidisation in a precooling zone (A', A") of the trough (2, 21) and a second fluidisation in a freezing zone (B', B") of the trough (2, 21) downstream of the precooling zone (A', A"), c h a r a c t e r i s e d in that the air-directing means (10', 10") comprises a throttled first portion which is arranged in connection with the precooling zone (A', A"), and a throttled second portion which is arranged in connection with the freezing zone (B', B"), which second portion has a smaller throttling (12', 12") than said throttled first portion, and that said second fluidisation is more vigorous than the first and so vigorous that freezing together of adjoining products is counteracted.

2. An apparatus as claimed in claim 1, in which the air-directing means (10', 10") comprises a non-throttled second portion which is arranged in connection with the freezing zone (B', B").

3. An apparatus as claimed in claim 1 or 2, in which the air-directing means (10', 10") is arranged in one piece.

4. An apparatus as claimed in claim 1 or 2, in which the air-directing means (10', 10") is divided into segments in a direction transversely to the feeding direction (P) of the conveyor belt (6, 61), said segments having different degrees of throttling (12', 12").

5. An apparatus as claimed in any one of claims 1-4, in which said trough (2, 21) has a deep-freezing zone (C, C") .

6. An apparatus as claimed in claim 1 or 2, in which the air-directing means (10') comprises a throttled third portion which is arranged in connection with said deepfreezing zone (C), said third portion having a greater throttling (12') than said second portion.

7. An apparatus as claimed in any one of claims 1-6, in which the flow of air (F', F") in said precooling zone (A¹, A") has a speed of 2-4 m/s.

8. An apparatus as claimed in any one of claims 1-6, in which the flow of air (F', F") in said freezing zone (B', B") has a speed of 5-7 m/s.

9. An apparatus as claimed in claim 5, in which the flow of air (F', F") in said deep-freezing zone (C, C") has a speed of 2-4 m/s.

10. An apparatus as claimed in any one of claims 1-9, in which the air-permeable conveyor belt (6, 61) is endless.

11. An apparatus as claimed in any one of claims 1-9, comprising a second air-permeable conveyor belt (62) which along part of its length forms a bottom surface (4) of a second trough (22) which is adapted to receive and deep-freeze the products frozen in the first trough (2, 21) .

12. An apparatus as claimed in claim 11, in which said first trough (2, 21) is accommodated in a first com- partment (3, 31) and said second trough (22) is arranged in a second compartment (32) succeeding the first compartment .

13. A method for freezing products in a fluidised bed, comprising fluidising a bed of products contained in a trough (2, 21), so that the bed has a first fluidisation in a precooling zone (8, 8') of the trough (2, 21), and a second fluidisation, which is more vigorous than said first fluidisation, in a freezing zone (B', B") of the trough (2, 21), said first and second fluidisations respectively being provided by the trough (2, 21) being supplied with a more vigorous flow of air (F', F") in the freezing zone (B', B") than in the precooling zone (A¹, A"), which flow of air (F', F") is controlled by an air- directing means (10', 10") provided with throttlings (12', 12"), said air-directing means having a greater throttling (12', 12") in said precooling zone (A', A") than in said freezing zone (B', B"), said second fluidisation being given such an extent that freezing together of adjoining products is counteracted.

14. A method as claimed in claim 13, comprising the feature that the fluidisation occurs in such a manner that the bed has a third fluidisation in a deep-freezing zone (C, C") , which third fluidisation is less vigorous than said second fluidisation.

15. A method as claimed in claim 14, in which said deep-freezing zone (C, C") is arranged in a second trough (22) .

16. A method as claimed in claim 15, in which the first trough (21) is arranged in a first compartment (31) and the second trough (22) is arranged in a second compartment (32) succeeding the first compartment.

17. A method as claimed in any one of claims 13-16, in which the bed during its fluidisation is transported through the respective troughs (2, 21, 22) carried on a bottom-forming, air-permeable conveyor belt (6, 61, 62) arranged therein.