Tunnel freezer

Abstract: A tunnel freezer (1) comprises an elongate tunnel (2) which is inclined to the horizontal and which can be rotated about its longitudinal axis (X-X). The inside of the tunnel (2) is provided with a weir (6) which retains a pool (7) of liquid nitrogen between the weir (6) and the upper end (5) of the tunnel (2). Particles (12) to be frozen are introduced into the tunnel (2) on a conveyor (8) and drop into the pool (7) where they freeze substantially without aggregation. Rotation of the tunnel (2) carries the particles out of the liquid nitrogen and through the remainder of the tunnel (2) to the outlet end (13) of the tunnel (2). The frozen particles and the nitrogen vapour from the tunnel (2) pass through a common outlet (15) which is provided with a pivoting plate (16) which restricts the flow through the common outlet (15) so that the tunnel (2) operates at or slightly above atmospheric pressure thereby inhibiting ingress of moist air into the tunnel (2). Movement of particles through the tunnel (2) is facilitated by lifting bars (27, 28, 29, 30) which form part of a liner (33) which can be readily removed and replaced as desired. In a second embodiment the tunnel (2') is provided with a spiral ribbon (6') which forms a multiplicity of pools (7') extending along the tunnel (2'). This embodiment is particularly useful where the supply of frozen particles is variable.

Description: This invention relates to a method of freezing material and to a tunnel freezer. A tunnel freezer usually comprises an elongate tunnel of generally circular cross-section. In use, the tunnel is inclined to the horizontal and rotated about its longitudinal axis. Particles of material to be frozen are introduced into the upper end of the tunnel. The rotation of the tunnel causes the particles to pass along the tunnel to the lower end where they are collected. As the particles pass along the tunnel they are frozen by heat-exchange with a cold medium, for example liquid nitrogen. One of the disadvantages of known tunnel freezers is that, if the particles of material are moist, they tend to freeze in clusters. This disadvantage has limited the use of tunnel freezers to processes where the formation of clusters is not of particular significance, for example for embrittling material for cryogenic grinding. Thus, tunnel freezers have not been successfully used for producing high quality, individually quick-frozen foodstuffs, for example frozen prawns and celery. GB-A-2 131 142 describes a food freezer in which particles to be frozen are first dropped into a bath of liquid nitrogen. The vaporizing nitrogen forms a thin layer around each particle which inhibits the particles sticking together. The particles stay in the liquid for a sufficient time for the surface to freeze and are then withdrawn upwardly from the liquid by a ribbon auger which conveys the particles through a cooling tunnel where they freeze throughout in contact with cold nitrogen vapour. Whilst this arrangement works extremely successfully, the food freezer itself is relatively expensive because of the manufacturing cost of the ribbon auger. EP-A-0 245 708 discloses a tunnel freezer which can be cooled with either nitrogen or carbon dioxide snow. In the case of nitrogen, the nitrogen is introduced at the outlet end of the freezer and flows counter-current to the flow of particles to be frozen. In the case of carbon dioxide snow, the snow is introduced with the particles to be frozen at the inlet to the tunnel. Vanes are provided to move the snow and particles to be frozen along the tunnel. According to the present invention there is provided a method of freezing material, which method is characterized in that it comprises the steps of: (a) forming a pool of cryogenic liquid near the upper inlet end of a downwardly inclined tunnel freezer; and (b) introducing material to be frozen into said pool. Preferably, said method includes the step of conveying material to be frozen into said tunnel freezer on a conveyor and causing the material to fall off the end of said conveyor into said pool. Advantageously, said cryogenic liquid is liquid nitrogen. The present invention also provides a tunnel freezer for carrying out the method in accordance with the invention, which tunnel freezer comprises an elongate tunnel which is inclined downwardly and which can be rotated about its longitudinal axis in use, characterized in that the inside of said tunnel is provided with a weir which, in use, traps a pool of cryogenic liquid near the upper inlet end of said tunnel, and means are provided for introducing material to be frozen into said pool. The tunnel freezer may be provided with a conventional inlet chute through which particles to be frozen can be introduced. More preferably, however, a conveyor is provided which is arranged to drop particles into said pool. Such a conveyor is preferably movable longitudinally with respect to the pool so that the residence time of the particles in the pool can be varied as required. When a conveyor is used, a spray bar is preferably situated in the tunnel above the conveyor so that cryogenic liquid can be sprayed on to the particles while they are on the conveyor. Such a spray bar may be the sole source of cryogenic liquid to the freezer. Alternatively, or in addition, the cryogenic liquid may be introduced directly to the pool. When the tunnel freezer is in use, particles to be frozen are introduced into the cryogenic liquid and their surfaces are frozen with minimal aggregation. The rotation of the tunnel transfers the particles along the tunnel. Whilst this arrangement can work extremely well when the supply of particles to be frozen is relatively uniform, we have found that the particles may not be uniformly frozen when the supply is erratic. This tends to be the case when the particles being frozen are particles of foodstuff. In order to reduce this problem we have found that the tunnel should be provided with two or more weirs. Advantageously, the weirs are formed by a spiral ribbon which is mounted on the inside of the tunnel. In use, the tunnel is rotated so that successive pools of cryogenic liquid progressively travel towards the outlet end of the tunnel. The flow of cryogenic liquid supplied can be adjusted with a view to obtaining minimal aggregation and adequate freezing of the particles. Preferably, lifting bars are provided which may extend between the inlet of the tunnel and the weir closest thereto and/or between adjacent weirs. Such lifting bars may, for example, be provided with a multiplicity of holes to allow cryogenic liquid (but not the particles) to flow back into the associated pool. Alternatively, the lifting bars may be mounted with a small clearance from the inside of the tunnel to allow the cryogenic liquid to flow back into the associated pool. The shape and configuration of the lifting bars may be chosen to facilitate movement of the particles along the tunnel or may be chosen simply to reduce aggregation. When the weirs are formed by a spiral ribbon, lifting bars may be provided along the entire length of the spiral ribbon or along a selected portion only. In a particularly preferred embodiment the spiral ribbon extends over the entire length of the tunnel and the lifting bars extend from the lower end to a position intermediate the inlet and outlet ends. Reference is made to divisional applications EP-A-0 519 578 and EP-A-0 519 579. For a better understanding of the present invention reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a simplified cross-section through one embodiment of a tunnel freezer in accordance with the present invention in use; Figure 2 is a simplified cross-section taken on line II-II of Figure 1; and Figure 3 is a partly cut-away perspective view of another embodiment of a tunnel freezer in accordance with the invention. Referring to Figure 1 of the drawings there is shown a tunnel freezer which is generally identified by reference numeral 1. The tunnel freezer 1 comprises a tunnel 2 which is of circular cross-section. The tunnel 2 comprises a stainless steel tube 3 which is lagged with insulation 4 to reduce heat transfer. The tunnel 2 is inclined to the horizontal and is rotatable about its longitudinal axis X-X. The tunnel 2 is supported on bearings (not shown) and is rotatable by an endless belt (not shown) which passes over a gear wheel fast with the tunnel 2 and driven by a variable speed motor (not shown). Adjacent its upper end 5, the tunnel 2 is provided with a weir 6 which extends around the entire internal diameter of the tunnel 2 and projects radially inwardly. Since the tunnel 2 is inclined to the horizontal a pool 7 is defined between the weir 6 and the upper end 5 of the tunnel 2. A conveyor 8 projects into the upper end 5 of the tunnel 2 and can be moved back and forth in the direction of arrows B-B as desired. A spray bar 9 which is connected to a source of liquid nitrogen via a pipe 10 is disposed directly above the conveyor 8 as shown. A pipe 11, which is also connected to the same source of liquid nitrogen, is also disposed above the pool 7. In use, the tunnel 2 is rotated about its longitudinal axis X-X. Particles 12 to be frozen, for example chopped celery, prawns, diced ham, herbs, minced meat and diced chicken, are introduced into the tunnel 2 on conveyor 8. The particles are first passed through liquid nitrogen being sprayed from spray bar 9 and then drop into the pool 7 which is full of liquid nitrogen. As the particles drop into the liquid nitrogen, they are surrounded by a thin film of boiling nitrogen which inhibits adjacent particles sticking together whilst freezing the surface. As the tunnel 2 rotates the particles tumble over the weir 6 and move towards the outlet end of the tunnel 2. As they pass along the tunnel 2 the particles are further cooled by heat exchange with cold nitrogen vapour from the spray bar 9 and the pool 7. The particles leave the tunnel 2 at the outlet end 13. It will be noted that the arrangement disclosed possesses substantially all the individual freezing benefits of an immersion freezer with the economic benefits of a tunnel freezer. Returning again to Figure 1 it will be seen that the outlet end 13 of the tunnel 2 extends into an enclosure 14 which has a common outlet 15 for both frozen particles and cold nitrogen vapour. A plate 16 extends across the common outlet 15 and is connected to an arm 17 which is pivoted about a pin 18. A counterweight 19 is mounted on the arm 17 and biases the plate 16 into the position shown. It should be noted that the plate 16 is not a tight fit in the common outlet 15. The common outlet 15 is contained within a hood 20 which is provided with insulation and forms part of a duct containing an extractor fan 21. The inlet end 5 of the tunnel 2 is provided with a surround 22 which is provided with an opening 23 to accommodate the conveyor 8. In use, gaseous nitrogen passes along the tunnel 2 and flows downwardly towards the common outlet together with frozen particles. The frozen particles collect on the plate 16 whilst the gaseous nitrogen flows through the gaps between the periphery of the plate 16 and the common outlet 15. Part of the gaseous nitrogen also flows through the opening 23. A temperature sensor 24 is located in the immediate vicinity of the opening 23 monitors the temperature and adjusts the displacement of the fan 21 to maintain the temperature slightly below room temperature. It will be appreciated that this temperature corresponds to a small flow of nitrogen outwardly from the tunnel through the opening 23. As can be seen from Figure 1 the hood 20 vents both nitrogen from the common outlet 15 and some surrounding air. This reduces freezing problems in the hood 20. It will also be noted that seals 25 and 26 are provided to inhibit the ingress of air between the tunnel 2 and the enclosure 14 and between the tunnel 2 and the surround 22 respectively. Finally, it will be noted that the tunnel is provided with four lifting bars 27, 28, 29 and 30 which extend along the tunnel 2 and which project radially inwardly. As shown in Figure 1, the portions of the lifting bars 27, 28, 29 and 30 between the weir 6 and the inlet end 5 of the tunnel 2 are provided with a multiplicity of small holes 31 so that the liquid nitrogen remains in the pool 7 as the drum rotates. The holes 31 are smaller than the particles. As can best be seen from Figure 2, the lifting bars 27, 28, 29 and 30 and the weir 6 are welded to a thin tube 32 of stainless steel. The tube 32 and the lifting bars 27, 28, 29 and 30 together form a liner which is generally identified by reference numeral 33. The liner 33 also includes a flange 34 which is bolted to the tunnel 2 by bolts (not shown). The liner 33 shown in the figures can be replaced simply by removing conveyor 8 and surround 22. The bolts (not shown) securing the flange 34 to the tunnel 2 are removed and the entire liner 33 withdrawn. A new liner can then be inserted into the tunnel 2 and bolted into place. Various modifications to the arrangement described are currently envisaged, for example the liner 33 could comprise a plurality of units which can be fastened together prior to insertion in the tunnel 2. In addition, additional clamps may be provided which project through the tunnel 2 to hold the liner 33 in position. Typically the tunnel 2 will be rotated at from 0 - 8 r.p.m. and will be inclined at from 2 to 6 degrees, preferably 3 to 4 degrees from the horizontal. EXAMPLE 1 : A test tunnel 2 having an internal diameter of 0.4 m and a length of 3 m was inclined at 3.24 degrees to the horizontal and rotated at 3.125 r.p.m. The tunnel 2 successfully froze 180 kg/h chopped celery using 252 kg/h liquid nitrogen. EXAMPLE 2 : A test tunnel 2 having an internal diameter of 0.5 m and a length of 5 m was inclined at 4 degrees to the horizontal and rotated at 7.2 r.p.m. The tunnel 2 successfully froze 66 kg/h parsley using 85.8 kg/h liquid nitrogen. An unexpected benefit of this process was the almost total separation of the leaves of parsley from the stems and other woody material. Turning now to Figure 3, there is shown a tunnel freezer which is generally similar to the tunnel freezer shown in Figures 1 and 2. Parts in Figure 3 which are similar to parts shown in Figures 1 and 2 have been identified by the same reference numerals with the addition of an apostrophe. The tunnel freezer 1' differs from the tunnel freezer 1 in several respects. Firstly, the weir 6 has been replaced by a spiral ribbon 6' which is welded to a thin tube to form a liner 33' which is secured to the stainless steel tube 3'. The spiral ribbon 6' extends the full length of the tunnel 2' and defines a plurality of weirs. A plurality of lifting bars 27' are provided between adjacent weirs from the outlet end 13' of the tunnel freezer 1 to a position approximately one fifth the distance from the inlet end 5' to the outlet end 13' of the tunnel 2'. The outlet end 13' of the tunnel 2' extends into an enclosure 14' which has a downwardly opening outlet 35 for frozen particles and an outlet tube 36 which extends upwardly into an exhaust duct 37 which contains an exhaust fan (not shown). A gap 38 is provided between the outlet tube 36 and the exhaust duct 37 to allow ambient air to enter the exhaust duct 37 and inhibit ice forming on the inside of the exhaust duct 37. The tunnel 2' is inclined downwardly at preferably 1 - 2 degrees to the horizontal.

Claims: A method of freezing material, which method is characterized in that it comprises the steps of: (a) forming a pool of cryogenic liquid near the upper inlet end of a downwardly inclined tunnel freezer; and (b) introducing material to be frozen into said pool. A method according to Claim 1, characterized in that said material to be frozen is dropped into said pool. A method according to Claim 2, characterized in that it includes the step of conveying material to be frozen into said tunnel freezer on a conveyor and causing the material to fall off the end of said conveyor into said pool. A method according to any preceding Claim, characterized in that said cryogenic liquid is liquid nitrogen. A tunnel freezer for carrying out the method according to Claim 1, which tunnel freezer comprises an elongate tunnel (2; 2') which is inclined downwardly and which can be rotated about its longitudinal axis (X-X) in use, characterized in that the inside of said tunnel (2; 2') is provided with a weir (6, 6') which, in use, traps a pool (7; 7') of cryogenic liquid near the upper inlet end (5; 5') of said tunnel (2; 2') and means (8; 8') are provided for introducing material to be frozen into said pool (7; 7'). A tunnel freezer as claimed in Claim 5, characterized in that said means includes a conveyor (8; 8') which is arranged to drop particles to be frozen (12; 12') into said pool (7; 7') when said freezer (1; 1') is in use. A tunnel freezer as claimed in Claim 6, wherein said conveyor (8; 8') is movable longitudinally with respect to said pool (7; 7') so that the residence time of said particles (12; 12') in the pool (7; 7') can be varied. A tunnel freezer as claimed in Claim 6 or 7, including a spray bar (9; 9') situated in said tunnel (1; 1') above said conveyor (8; 8') so that cryogenic liquid can be sprayed on to said particles while they are on said conveyor (8; 8'). A tunnel freezer as claimed in any of Claims 5 to 8, characterized in that it comprises at least two weirs (6'). A tunnel freezer as claimed in Claim 9 characterized in that said weirs (6') are formed by a spiral ribbon (6') which is mounted on the inside of said tunnel (2'). A tunnel freezer as claimed in any of Claims 5 to 10 characterized in that it includes lifting bars (27, 28, 29, 30; 27'). A tunnel freezer as claimed in Claim 11, characterized in that said lifting bars (27, 28, 29, 30) extend between the inlet end (5) and the outlet end (13) of said tunnel (2). A tunnel freezer as claimed in Claim 11 or 12 characterized in that said lifting bars are mounted with a clearance from the inside of said tunnel. A tunnel freezer as claimed in Claim 11, 12 or 13 characterized in that said lifting bars (27, 28, 29, 30) are provided with a multiplicity of holes (31) to allow cryogenic liquid to drain back into the pool (7,7') when said freezer (1) is in use.

Inventors: MILLER JEREMY PAUL, SUMMERS JOHN VINCENT, MILLER, JEREMY PAUL, SUMMERS, JOHN VINCENT
Applicants: AIR PROD & CHEM, AIR PRODUCTS AND CHEMICALS, INC.
Application year: 1989
Main classification: F25D3
IPC codes: A23L3, F25D3