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.

Description: This invention relates 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 or solid carbon dioxide as shown in US-A-3 213 634. One of the disadvantages of known tunnel freezers is that in use, a certain amount of moist atmospheric air enters the tunnel. The moisture freezes out on the wall of the tunnel and forms a layer of ice which reduces the effective diameter of the tunnel and the throughput. In extreme cases, particularly where the material is itself moist, the particles can freeze in the ice to totally block the tunnel. The formation of ice means that the tunnel freezer has to be stopped, cleaned and defrosted at relatively frequent intervals to maintain throughput. When addressing this problem it should first be recalled that venting equipment is usually provided with all tunnel freezers to prevent the build-up of asphyxiating cryogenic vapour in the area around the freezer. As shown in US-A-3 992 899 the venting equipment is normally disposed in direct contact with the tunnel and, in practice, the pressure inside the tunnel is usually fractionally below atmospheric. Because of this pressure differential air is sucked into the system and moisture condensed on the inside of the tunnel. The present invention provides a tunnel freezer comprising an elongate tunnel which can be inclined to the horizontal and which can be rotated about its longitudinal axis in use, said tunnel having an inlet end for receiving particles to be frozen and an outlet end rotatably mounted within an enclosure, characterized in that said inclosure has a common outlet for frozen particles and cryogenic vapour and means are provided for restricting the flow through said common outlet. In use, the required flow of cryogenic liquid to the freezer is first established. The means are then adjusted so that the tunnel itself is at atmospheric pressure or, more preferably, at a very small positive pressure with respect to atmospheric pressure thereby inhibiting the ingress of atmospheric air into the tunnel. Care should be taken to minimize any possible sources of air ingress into the tunnel and, advantageously, at least one seal is provided between said tunnel and said enclosure for this purpose. The means for restricting the flow through the common outlet may comprise, for example a plate which can be moved to restrict the flow through the common outlet. A particularly suitable means comprises a pivotally mounted plate which is biased to partially close the common outlet, for example by a counterweight. In use, frozen particles accumulate on the plate until they reach a certain weight. The plate then pivots downwardly and the particles slide off. The lighter plate then pivots back to its initial position. Care must also be taken to inhibit air entering the tunnel via the inlet end. In the case where particles are delivered from a conveyor which projects into the inlet end of the tunnel care should be taken to ensure that a minimal volume of air enters the tunnel. However, care should also be taken to ensure that excess nitrogen does not leave the tunnel via the inlet end. Preferably, an exhaust duct having an extractor fan is arranged to exhaust gas leaving the common outlet. The exhaust fan is preferably controlled in response to a sensor monitoring, for example the temperature, or the composition of the gas, immediately adjacent the inlet to the tunnel. The exhaust duct may be connected directly to the common outlet. However, it is preferably arranged in the vicinity of the common outlet so that a mixture of cryogenic vapour and air is drawn into the exhaust. Reference is made to related patent publications EP-A-0 372 354 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 drawing, in which: Figure 1 is a simplified cross-section through one embodiment of a tunnel freezer in accordance with the present invention in use; and Figure 2 is a simplified cross-section taken on line II-II of Figure 1. Referring to Figure 1 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.

Claims: A tunnel freezer (1) comprising an elongate tunnel (2) which can be inclined to the horizontal and which can be rotated about its longitudinal axis (X-X) in use, said tunnel (1) having an inlet end (5) for receiving particles to be frozen and an outlet end (13) rotatably mounted within an enclosure (14), characterized in that said enclosure (14) has a common outlet (15) for frozen particles and cryogenic vapour and means (16) are provided for restricting the flow through said common outlet (15). A tunnel freezer as claimed in Claim 1, characterized in that at least one seal (25) is provided between said tunnel (1) and said enclosure (14). A tunnel freezer as claimed in Claim 1 or 2, characterized in that said means (16) for restricting the flow through said common outlet (15) comprises a pivotably mounted plate (16) which is biased to partially close said common outlet (15). A tunnel freezer as claimed in Claim 1, 2 or 3, including an exhaust duct (20) having an exhaust fan (21) arranged to exhaust gas leaving said common outlet (15). A tunnel freezer as claimed in Claim 4, characterized in that said exhaust fan (21) is controlled, in use, in response to a sensor (24) disposed immediately adjacent the inlet to the tunnel (2). A tunnel freezer as claimed in Claim 4 or 5, wherein said duct (20) is disposed in the vicinity of the common outlet (15) so that, in use, a mixture of cryogenic vapour and air is drawn into said exhaust duct (20).

Inventors: MILLER JEREMY PAUL, SUMMERS JOHN VINCENT C O AIR P, MILLER, JEREMY PAUL, SUMMERS, JOHN VINCENT, Summers, John Vincent, c/o Air Products Plc
Applicants: AIR PROD & CHEM, AIR PRODUCTS AND CHEMICALS, INC.
Application year: 1989
Main classification: F25D3
IPC codes: A23L3, F25D3