WO2005003657A1

WO2005003657A1 - Contact freezer

Info

Publication number: WO2005003657A1
Application number: PCT/SE2004/000903
Authority: WO
Inventors: Per-Oskar Persson
Original assignee: Frigoscandia Equipment Ab
Priority date: 2003-07-02
Filing date: 2004-06-11
Publication date: 2005-01-13
Also published as: PL1649227T3; DE602004025115D1; SE0301948D0; SE0301948L; EP1649227A1; ES2336210T3; ATE455282T1; SE525486C2; CL2004001652A1; EP1649227B1; DK1649227T3

Abstract

A contact freezer for food products comprises a conveyor belt (3) and a refrigerating unit (2). It has an upper side in contact with part of an underside of the conveyor belt (3) and is adapted to refrigerate food products positioned on the conveyor belt (3) as they pass over the refrigerating unit (2), so that a layer of the food products next to the conveyor belt (3) is transferred to a frozen state. The refrigerating unit (2) comprises an upper metal sheet (34) which is convexly curved in the travelling direction of the conveyor belt (3) and which forms the upper side of the refrigerating unit (2) with a surface in direct sliding contact with the conveyor belt (3) during the movement thereof, a lower metal sheet (35) which is arranged the first metal sheet (34) with a gap (36) thereto and has a plurality of groups of inlets (40) which are arranged alternately with a plurality of groups of outlets (41), sealing means (39) for closing the gap (36) round an area which comprises the groups of inlets (40) and the groups of outlets (41), and means for feeding coolant in a closed loop which comprises the parts of the area which are located between the groups of inlets (40) and the groups of outlets (41), which are arranged alternately with the groups of inlets. The conveyor belt (3) consists of a non-metallic material.

Description

CONTACT FREEZER

Field of the Invention The present invention relates to a contact freezer for food products. More specifically, the invention concerns such a contact freezer for freezing a surface layer of the food products. The final freezing of the food products can then suitably take place in a freezer using convection cooling of cold air.

Background Art Contact freezers for partial or complete freezing of food products are known in various designs. As a rule they use a conveyor belt on which the products are conveyed through a cooling station where the temperature of the belt is lowered enough for the products while passing therethrough to be frozen at least in a surface layer in contact with the conveyor belt. To lower the temperature of the conveyor belt at the cooling station, it is known to use direct cooling, for instance applying brine to the underside of the conveyor belt. Such a design is disclosed in US-A-3, 644, 149. However, this technique may cause problems due to difficulties in removing the brine from the conveyor belt when this leaves the cooling station and also in preventing the brine from contaminating the food products. Alternatively, the conveyor belt can be cooled indirectly by its underside at the cooling station contacting the upper side of a refrigerating unit which in turn is directly cooled in a suitable manner. Here it is important to achieve a good contact in terms of heat conduction between the underside of the conveyor belt and the upper side of the refrigerating unit. To ensure such a good contact, it is, for instance, according to US-2,610,476 known to give the upper side of the refri- gerating unit a curvature in the longitudinal direction of the conveyor belt and to use a lubricating and heat transferring liquid between the underside of the conveyor belt and the upper side of the refrigerating unit. This lubricating liquid, of course, reduces friction, which is otherwise increased due to the curvature, between the conveyor belt and the refrigerating unit, but it also causes the same problems as in the above case with direct cooling of the conveyor belt. Further embodiments of the refrigerating unit for a contact freezer are known from, inter alia, EP 0 759 529 A2, US-A-3,037,366, US-A-3, 528 , 257 , US-A-3, 584 , 471, US-A-4,205,536 and US-A-6, 009, 719. A common feature of the known refrigerating units is a relatively complicated and thus expensive construction which after all exhibits a limited degree of efficiency.

Summary of the Invention The object of the present invention therefore is to provide an effective contact freezer of a simpler construction. The object is achieved by a contact freezer according to claim 1. Preferred embodiments are evident from the dependent claims. Thus, the invention provides a contact freezer for food products of the type that comprises a conveyor belt and a refrigerating unit, which has an upper side in contact with part of an underside of the conveyor belt and is adapted to refrigerate food products positioned on the conveyor belt as they pass over the refrigerating unit, so that a layer of the food products next to the conveyor belt is transferred to a frozen state. This contact freezer according to the invention is charac- terised in that the conveyor belt consists of a flexible, non-metallic material, and the refrigerating unit comprises an upper metal sheet which is convexly curved in the travelling direction of the conveyor belt and which forms the upper side of the refrigerating unit with a surface which preferably consists of stainless steel in direct sliding contact with the conveyor belt during the movement thereof, a lower metal sheet which is arranged under the upper metal sheet with a gap thereto and which has a plurality of groups of inlets which are arranged alternately with a plurality of groups of outlets, sealing means for closing the gap round an area which com- prises the groups of inlets and the groups of outlets, and means for feeding a coolant in a closed loop which comprises the parts of the area which are connected in parallel and located between the groups of inlets and the groups of outlets, which are arranged alternately with the groups of inlets. The invention thus provides a simple refrigerating unit, which by the bending of the upper metal sheet, resulting in surface smoothness, and the use of a flexible, non-metallic conveyor belt gives sufficiently good heat transfer over the boundary layer between them, without requiring the use of some kind of contact-creating liquid. The refrigerating unit also exhibits highly efficient refrigeration through the division of the cooling area into a plurality of partial areas to which the coolant is supplied in parallel, so that the difference between the maximum and minimum temperatures of the coolant in the total cooling area can be kept relatively small . To ensure in an optimal manner the desired surface smoothness of the upper metal sheet, use is advantageously made of a cold-rolled sheet of stainless steel, particularly of austenitic, molybdenum-alloyed steel, such as Sandvik 1200 SA. It is essential for the surface smoothness that the material of the metal sheet should be resilient, which is the case with cold-rolled sheet. It is required that the upper metal sheet have good surface smoothness and also good thermal conductivity. This can also be achieved with a metal sheet of, for instance, aluminium or an aluminium alloy, but also with a laminated metal sheet having a lower layer with excellent thermal conductivity and an upper layer with good wear properties and good sanitary properties. In a preferred embodiment of the contact freezer, each group of inlets and each group of outlets comprise a plurality of openings arranged transversely to the travelling direction of the conveyor belt. This ensures sub- stantially uniform cooling of the carrying surface of the conveyor belt and, thus, of the food products on the conveyor belt over the entire width thereof. However, it is possible to orient the inlets in a different manner relative to the outlets . Spacers are suitably arranged to determine the size of the gap and, in the preferred embodiment, they may comprise elements which are extended in the travelling direction of the conveyor belt and arranged between the upper metal sheet and the lower metal sheet. This ensures a substantially constant gap size over the entire area of the gap, which is essential for good distribution of the coolant flow. In particular, the size of the gap and the thickness of the upper metal sheet can be in the order of 1 mm, while the lower metal sheet can have a thickness of about 2 mm. By forming the inlets with, taken together, a substantially smaller opening area than twice the area of the gap cross-section adjacent to the inlets, and forming the outlets with, taken together, a larger opening area than twice the area of the gap cross-section adjacent to the outlets, the pressure is reduced to atmospheric pressure adjacent to the outlets, and the pressure drop occurs mainly adjacent to the inlets. This gives the advantage that lower demands can be placed on the seal round the gap. In the preferred embodiment, the upper and lower metal sheets also form a lid for a vessel, which is arranged to receive the coolant flowing out of the outlets and to feed this through a bottom outlet. The upper and lower metal sheets are preferably supported by two supports extended in the travelling direc- tion of the belt and having a convex profile, which abut against the lower metal sheet at the lateral edges thereof. The vessel can be supported in a stand, in which a first guide roller for the conveyor belt is mounted at a feed end of the refrigerating unit, and a second guide roller for the conveyor belt is mounted at a discharge end of the refrigerating unit. In the preferred case involving an endless conveyor belt, a third guide roller is mounted under the refrige- rating unit. By this design, the product-carrying surface of the endless conveyor will not come into contact with anything else than the product, which is important from the sanitary point of view. Since condensation may form on the non-product-carrying surface of the conveyor belt after passing the guide roller at the discharge end, a drying device for this surface is also suitably arranged along the return run of the conveyor belt between the* discharge end and the feed end. An exemplary embodiment of a contact freezer accord- ing to the present invention will now be described in more detail with reference to the accompanying drawings.

Brief Description of the Drawings Fig. 1 is a schematic side view of the exemplary embodiment. Fig. 2 is a cross-sectional view along line II-II in Fig. 1 through a refrigerating unit included in the exemplary embodiment. Fig. 3 is a cross-sectional view along line III-III in Fig. 2 through the refrigerating unit. Fig. 4 is a top plan view of a carrier sheet shown in Fig. 3. Fig. 5 shows part of a cross-sectional view on a larger scale along line V-V in Fig. 3 through part of the refrigerating unit. Fig. 6 is a cross-sectional view along line VI-VI in Fig. 3 through part of the refrigerating unit. Fig. 7 is a partial view on a larger scale of the right end of the refrigerating unit shown in Fig. 3. Fig. 8 shows schematically how the cooling of the refrigerating unit can be performed.

Description of Embodiments A contact freezer shown in Fig. 1 comprises a stand

1, a refrigerating unit 2 supported thereby and a conveyor belt 3 following a path determined by three guide rollers 4-6 and the upper side of the refrigerating unit

2. The guide roller 4 is more specifically arranged at a feed end of the contact freezer and constitutes the drive roller for the conveyor belt 3, the guide roller

5 is arranged at a discharge end of the contact freezer, and the guide roller 6 is arranged under the refrigerating unit 2. The motor for the drive roller can be arranged in the same or drive, in a conventional way, by pulleys using a drive belt (not shown) . The guide roller 5 arranged at the discharge end of the contact freezer suitably has a much smaller diameter than the guide roller 4, thus allowing the products on the conveyor belt 3 to easily leave the same at the discharge end. Moreover the guide roller 6 is mounted in arms 7 which in turn are pivotally mounted in the stand 1 in such a manner that the weight of the guide roller

6 will tension the conveyor belt 3 in its path. If necessary, the belt tension of the conveyor belt 3 can be further increased by yet another force, for instance generated by a spring, being applied to the arms 7. Since the guide roller 6 is pivotally mounted in the stand 1, the conveyor belt 3 can easily be removed, for instance to be exchanged, by pivoting the guide roller 6 upwards. The belt tension is then released, thus allowing the conveyor belt 3 to be removed from the guide rollers 4 and 5 by lateral displacement. In some cases, a conveyor belt passed over guide rollers may tend to move in the lateral direction. The guide roller 6 can then be used to guide the conveyor belt 3. Sensor means (not shown) can in this case be arranged to detect any lateral deviation of the conveyor belt. The information is supplied to a compensating means (not shown) which in response to said information is adapted to angle the guide roller to compensate for said deviation. It will be appreciated that it is also possible to arrange one of the guide rollers 4 and 5 to provide said belt guiding. The refrigerating unit 2 is shown in more detail in Figs 2-7. It comprises a vessel 8 and a cooling sheet 9 which forms a lid for the vessel 8, and liquid pipes 10 and 11 for supplying coolant to the refrigerating unit 2 and discharging used coolant from the refrigerating unit 2. The vessel 8 may comprise a varying amount of coolant and thus eliminates the need for a special expansion vessel. The coolant can advantageously be a brine with a temperature of about -50°C for instance, which is circulated in a closed loop between the secondary side of a heat exchanger and the refrigerating unit 2, the primary side of the heat exchanger being cooled in a suitable manner by means of a refrigerator connected thereto, as schematically shown in Fig. 8. It should be noted that a pump 52 used for circulating the coolant is arranged immediately after the refrigerating unit 2 in the closed loop, since this gives the lowest cooling temperature in the refrigerating unit 2 for a given evaporation temperature in the refrigerator. The vessel 8 has double walls 12 and 13, preferably of stainless steel, with an intermediate insulation 14 in the bottom 15 and also in the side walls 16, 17 and in the end walls 18, 19. The bottom 15 slopes towards a centrally arranged opening 20, which connects to the liquid pipe 11 for discharge of used brine back to the heat exchanger. Centrally in the bottom 15 there is also a tight lead-through 21 for the liquid pipe 10 for sup- plying brine from the heat exchanger. The vessel 8 is supported and fixed to the stand 1 by means of four supports 22 on its underside. A rail 23 is fixed by a plurality of brackets 24 and 25 at a distance from the inside of the side wall 16 and extends along the side wall 16 practically all the way to the end walls 18 and 19. As is evident from Figs 3 and 7, it has a convex upper edge 26. A corresponding rail 27 is in the same way fixed by a plurality of brackets 28 and 29 at a distance from the inside of the side wall 17. Additional rails of this type can be arranged between the rails 23 and 27. As a preferred but non-limiting example, the convexity of the rails 23, 27 can correspond to a curvature of 0.5-6% per unit of length, and still more preferred a curvature of 1-3% per unit of length. The cooling sheet 9 rests along its lateral edges on the rails 23, 27 and is kept fixed to them by means of screws 30 which through holes 31 in the cooling sheet 9 are inserted in threaded holes 32 in the brackets 24 and 28 respectively. The screws 30 do not engage the upper side of the cooling sheet 9 directly with their heads, but by the intermediary of a flat strip 33. Furthermore the holes 31 have a larger diameter than the screws 30 so as to allow lateral movement of the cooling sheet in case of changes in temperature. The actual cooling sheet 9 consists of an upper metal sheet 34 and a lower metal sheet 35, between which a gap 36 forms by a plurality of elongate spacers 37 being arranged between the metal sheets 34 and 35. The elongate spacers 37 extend mainly in the travelling direction of the conveyor belt 3 over the cooling sheet 9 and are thus arranged with interspaces transversely to the travelling direction of the conveyor belt 3. As shown in Fig. 7, the upper metal sheet 34 is bent downwards adjacent to the end wall 18 of the vessel 8 and is there fixed and sealed against a flange 38 of the end wall 18. The same applies to the end wall 19. The gap 36 is outwardly laterally defined by a seal 39 which extends in an endless manner along and inside the periphery of the cooling sheet 9, as is evident from Figs 5 and 7, and is pressed together between the metal sheets 34 and 35 when tightening the screws 30. The lower metal sheet 35 has five groups of inlets 40, each group consisting of a plurality of openings arranged in a row transversely to the travelling direction of the conveyor belt 3 and inside the seal 39 over the entire width of the lower metal sheet 35. These five groups of inlets 40 are arranged alternately with six groups of outlets 41, where each group also consists of a plurality of openings arranged in a row transversely to the travelling direction of the conveyor belt 3 and inside the seal 39 over the entire width of the lower metal sheet 35. Thus, a group of outlets 41 is positioned next to the seal 39 adjacent to each end wall 18 and 19 respectively of the vessel 8. Under each group of inlets 40, a collecting pipe 42-46 is formed by a cup which is sealingly fixed to the underside of the lower metal sheet 35, and the collecting pipes 42-46 are interconnected by means of connecting pipes 47-50. The liquid pipe 10 finally opens in the middle collecting pipe 44. Taking into consideration that the contact freezer is intended for foodstuffs, all metal parts, i.e. the parts 1, 7, 10-13, 15-19, 21-25, 27-30, 33-35, 38 and 42-50, are preferably made of stainless steel, and the fixed connections between them are suitably provided by welding. However, other materials with equivalent proper- ties in terms of cooling and food technology are conceivable. As an example of dimensioning, the upper metal sheet 34 and also the gap 36 can have a thickness of about 1 mm, while the thickness of the lower metal sheet 35 can be about 2 mm. Then the spacers 37 may consist of stain- less steel wire with a diameter of 1 mm. The seal may consist of an O-ring wire with a diameter of at least 1.5 mm. It is most important that the fastening of the metal sheets 34, 35 be such that their linear expansion is not prevented in the plane of their own in case of a temperature change of the metal sheets from ambient temperature (usually room temperature) down to operating temperature (for instance -50°C) or vice versa. Furthermore the inlets 40 may have a diameter of about 2 mm, while the outlets may have a greater diameter of about 10 mm. The number of inlets and the number of outlets are then determined so that the pressure of the coolant at the outlets is atmospheric pressure and the greatest pressure drop occurs at the inlets. It goes without saying that this can be achieved by different combinations of the numbers and diameters of the respective openings. The conveyor belt 3 is preferably a plastic-coated matrix, where the matrix can be a fabric reinforcement and the plastic some kind of silicone polymer or PTFE, for instance of the type Teflon^®. Its thickness is pre- ferably in the range of 0.1-0.3 mm. Although the conveyor belt 3 is shown as an endless belt, the invention can also be applied to a belt with a finite length. In the case illustrated involving an endless conveyor belt 3, a drying device 51 for the non-product- carrying surface of the conveyor belt 3 is arranged directly adjacent to the second guide roller 5 at the discharge end, so as to eliminate the condensation that may form on this surface of the conveyor belt 3 when the conveyor belt leaves the refrigerating unit 2. It is, of course, decisive to the function of the contact freezer that the non-product-carrying surface of the conveyor belt 3 be perfectly dry when entering the re rigerating unit 2 at the feed end, since otherwise a layer of ice will form on the upper side of the upper metal sheet 34 and deteriorate the heat conduction from the conveyor belt 3 to the upper metal sheet 34. The drying device 51 may consist of a means for blowing of hot air, an electric plate heater or some other suitable means. In an embodiment which is not shown, the inventive contact freezer comprises a casing for covering the conveyor belt 3 when passed over the upper side of the refrigerating unit. The casing is arranged at a suitable distance above the conveyor belt to make room for products that are being frozen. The casing, which is easy to remove to allow good access and good sanitary conditions, increases the freezing capacity of the contact freezer since the upper side of the re rigerating unit is not freely exposed to the environment. The air enclosed in the casing will be cooled and will thus contribute to cooling of the products. Moreover, the heat load on the upper side of the refrigerating unit will be reduced. Additional modifications of the described embodiment are, of course, feasible within the scope of the invention as defined by the appended claims .

Claims

1. A contact freezer for food products, comprising a conveyor belt (3) and a refrigerating unit (2), which has an upper side in contact with part of an underside of the conveyor belt (3) and is adapted to refrigerate food products positioned on the conveyor belt (3) as they pass over the refrigerating unit (2) , so that a layer of the food products next to the conveyor belt (3) is transferred to a frozen state, ch a r a c t e r i s e d in that the conveyor belt (3) consists of a flexible, non- metallic material, and the refrigerating unit (2) comprises an upper metal sheet (34) which is convexly curved in the travelling direction of the conveyor belt (3) and which forms the upper side of the refrigerating unit (2) with a surface in direct sliding contact with the con- veyor belt (3) during the movement thereof, a lower metal sheet (35) which is arranged under the upper metal sheet (34) with a gap (36) thereto and which has a plurality of groups of inlets (40) which are arranged alternately with a plurality of groups of out- lets (41), sealing means (39) for closing the gap (36) round an area which comprises the groups of inlets (40) and the groups of outlets (41), and means (Fig. 8) for feeding a coolant in a closed loop, which comprises the parts of the area which are connected in parallel and located between the groups of inlets (40) and the groups of outlets (41) , which are arranged alternately with the group of inlets. 2. A contact freezer as claimed in claim 1, wherein the upper metal sheet (34) is a cold-rolled sheet of stainless steel.

3. A contact freezer as claimed in claim 1, wherein the upper metal sheet (34) is made of aluminium or an aluminium alloy. 4. A contact freezer as claimed in any one of claims 1-3, wherein spacers (37) for determining the size of the gap (36) comprise elements which are extended in the travelling direction of the conveyor belt (3) and which are arranged between the upper metal sheet (34) and the lower metal sheet (35) .

5. A contact freezer as claimed in any one of claims 1-4, wherein the size of the gap (36) and the thickness of the upper metal sheet (34) are in the order of 1 mm. β. A contact freezer as claimed in any one of claims 1-5, wherein each group of inlets (40) and each group of outlets (41) comprise a plurality of openings arranged transversely to the travelling direction of the conveyor belt (3) .

7. A contact freezer as claimed in any one of claims 1-6, wherein the inlets (40) taken together have a substantially smaller opening area than twice the area of an adjoining gap cross-section.

8. A contact freezer as claimed in claim 7, wherein the outlets (41) taken together have a larger opening area than twice the area of an adjoining gap cross- section.

9. A contact freezer as claimed in any one of claims 1-8, wherein the upper and lower metal sheets (34, 35) form a lid (9) for a vessel (8) which is arranged to receive the coolant flowing out of the outlets (41) and to feed this through a bottom outlet (20) .

10. A contact freezer as claimed in claim 9, wherein the groups of inlets (40) are each connected to a collecting pipe (42-46), which collecting pipes are connected to a common supply pipe (10) .

11. A contact freezer as claimed in claim 10, wherein the supply pipe (10) and the bottom outlet (20) are connected to the secondary side of a heat exchanger. 12. A contact freezer as claimed in any one of claims 1-11, wherein the upper and lower metal sheets (34, 35) are supported by two supports (23, 27) extended in the travelling direction of the belt and having a convex profile, which abut against the lower metal sheet (35) at the lateral edges thereof.

13. A contact freezer as claimed in any one of claims 9-12, wherein the conveyor belt (3) is endless and the vessel (8) is supported in a stand (1), in which a first guide roller (4) for the conveyor belt (3) is mounted at a feed end of the refrigerating unit (2), a second guide roller (5) for the conveyor belt (3) is mounted at a discharge end of the refrigerating unit (2) , and a third guide roller (6) for the conveyor belt (3) is mounted under the refrigerating unit (2).

14. A contact freezer as claimed in claim 13, wherein a drying device (51) for the non-product-carrying surface of the conveyor belt (3) is arranged along the return run of the conveyor belt (3) from the second guide roller (5) to the first guide roller (4).