Tunnel freezer

Abstract: A tunnel freezer (1) comprises an elongate tunnel (2) which is provided with a removable liner (3) which includes a weir (6) and a multiplicity of lifting bars (27, 28, 29, 30). The internal geometry of the tunnel freezer (1) can be changed by simply removing the liner (3) and replacing it with another liner having a different arrangement of weirs and/or lifting bars.

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. In order to facilitate movement of the particles through the tunnel, the wall of the tunnel is normally provided with fittings, for example a spiral ribbon (EP-A-0 245 708) or a plurality of longitudinally extending lifting bars which project towards the longitudinal axis of the tunnel. Heretofore, the fittings have been either permanently welded in position in the tunnel or bolted to lugs which are themselves permanently welded or otherwise retained on the tunnel. Recent work which we have carried out strongly suggests that the operation of tunnel freezers can be enhanced by using an arrangement of fittings which is particularly adapted for the particles being frozen. However, in order to change the internal geometry it is necessary to dismantle the entire tunnel which is a time-consuming and expensive operation. In order to meet this problem the present invention provides a tunnel freezer comprising a tunnel which can be rotated about its longitudinal axis, said tunnel having a spiral ribbon and/or a multiplicity of lifting bars for facilitating the passage of particles of material along said tunnel, characterized in that said spiral ribbon and/or multiplicity of lifting bars form part of a liner which can be inserted and withdrawn from said tunnel. Advantageously, said tunnel can be inclined to the horizontal. Preferably, said liner is provided with at least one weir. Preferably, the liner is provided with a flange which can be bolted to the inlet or the outlet of said tunnel. If desired, additional means may be provided to secure said liner to said tunnel. Advantageously, liners are made in discrete lengths which can be bolted or otherwise secured together to make a liner of appropriate overall length to fit in a tunnel. Preferably, the liners are fabricated from stainless steel. It will be appreciated that in order to change the internal shape and configuration of a tunnel the liner simply has to be withdrawn and replaced with a new liner. This operation takes a fraction of the time and expense previously involved. Reference is made to parent application EP-A-0 372 354 and divisional application EP-A-0 519 578. 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. Although the tunnel 2' is preferably inclined downwardly at 1 - 2 degrees to the horizontal to facilitate water drainage during cleaning it will be appreciated that it could be disposed level or even inclined slightly upwardly.

Claims: A tunnel freezer (1, 1') comprising a tunnel (2, 2') which can be rotated about its longitudinal axis (X-X), said tunnel having a spiral ribbon (6') and/or a multiplicity of lifting bars (27,28,29,30,27') for facilitating the passage of particles of material along said tunnel (2; 2'), characterized in that said spiral ribbon (6') and/or multiplicity of lifting bars (27, 28, 29 ,30; 27') form part of a liner (33; 33') which can be inserted and withdrawn from said tunnel (2; 2'). A tunnel freezer as claimed in Claim 1, characterized in that said tunnel (2, 2') can be inclined to the horizontal. A tunnel freezer as claimed in Claim 2, characterized in that said liner (33,33') is provided with at least one weir (6). A tunnel freezer as claimed in Claim 1, 2 or 3, characterized in that said liner (33; 33') is provided with a flange which can be bolted to the inlet (5; 5') or the outlet (13; 13') of said tunnel (1, 1'). A tunnel freezer as claimed in Claim 4, characterized in that additional means are provided to secure said liner (33; 33') to said tunnel. A tunnel freezer as claimed in any preceding Claim, characterized in that said liner (33; 33') comprises discrete lengths which are secured together. A tunnel freezer as claimed in any preceding Claim, characterized in that said liner (33; 33') is fabricated from stainless steel.

Inventors: MILLER JEREMY PAUL CAXTON HOUS, SUMMERS JOHN VINCENT C O AIR P, MILLER, JEREMY PAUL, CAXTON HOUSE, SUMMERS, JOHN VINCENT, Summers, John Vincent, c/o Air Products Plc
Applicants: AIR PROD & CHEM, AIR PRODUCTS AND CHEMICALS, INC.
Application year: 1992
Main classification: F25D3
IPC codes: A23L3, F25D3