APPARATUS AND METHOD FOR FREEZING FOOD PRODUCTS

Abstract: Apparatus and method for freezing food products including a first freezer section (14) utilizing a liquid or solid cryogen and a second freezer section (16) which utilizes cryogenic vapor from the first freezer section to complete the freezing operation, the first and second freezer sections being in a vertical arrangement to minimize floor space and improve cooling efficiency.

Description: APPARATUS AND METHOD FOR FREEZING FOOD PRODUCTS Related Application This is a continuation-in-part application of pending U.S. Serial No. 08/41 2,238 filed on March 28, 1 995. Technical Field The present invention is directed to an apparatus and method for freezing food products employing the combination of a liquid or solid cryogenic freezer section and a second freezer section employing cryogenic vapor from the first freezer section wherein the two sections are arranged vertically so as to minimize construction cost, floor space and cryogenic vapor loss. Background of the Prior Art The commercial freezing of food products, especially soft food products such as shrimp, scallops, vegetables, meat and the like is often carried out rapidly to minimize water loss and damage to the product. Quick freezing may be carried out using cryogenic substances such as liquid nitrogen and solid carbon dioxide. Among the more common cryogenic quick freezing techniques are those employing either a freezing tunnel or an immersion bath. Freezing tunnels are elongated chambers which may contain spaced apart nozzles that are used to spray the cryogenic substance on top of the food product as it passes through the tunnel on a conveyor. Heat exchange between the food product and the cryogenic substance produces cryogenic vapor which is circulated by fans to improve cooling efficiency. The liquid cryogen immersion bath system provides for a vessel containing a liquid cryogen (e.g. liquid nitrogen) and a conveyor passing therethrough to thereby immerse the food product in the liquid cryogen. The immersion bath system is advantageous over tunnel systems because freezing is faster. However, the efficiency of the immersion bath system is generally no better than most tunnel systems, typically because the cryogenic vapor generated in the immersion bath is not effectively utilized. In an effort to improve the efficiency of quick freezing, systems have been developed that utilize the combination of a liquid immersion bath and a freezer tunnel. In such systems the unfrozen product is introduced into the liquid immersion bath and moved along the bath on a conveyor. The immersion bath provides the first stage of cooling. A cryogenic vapor is produced via heat exchange between the food product and the liquid cryogen. The food product which emerges from the bath is passed into a freezing tunnel which is typically positioned downstream of the immersion bath. The cryogenic vapor is drawn into the tunnel by fans to complete the freezing process. Such systems are disclosed, for example, in R.C. Webster et al. , U.S. Patent No. 3,485,055; I. Rasovich, U.S. Patent No. 4,403,479; L. Tyree, Jr. et al., U.S. Patent No. 4,783,972; P. Gibson, U.S. Patent No. 4,843,840; A. Acharya et al., U.S. Patent No. 4,852,358; I. Rasovich, U.S. Patent No. 4,858,445; E. Kiczek et al., U.S. Patent No. 5, 1 68,723; E. Kiczek et al., U.S. Patent No. 5,220,802; E. Kiczek et al., U.S. Patent No. 5,220,803; and R. Howells, U.S. Patent No. 5,267,490. The combination of a liquid immersion bath and a downstream freezer tunnel, particularly those using circulated cryogenic vapor provide improved cooling efficiency over systems using the immersion bath alone or the freezer tunnel alone. However, the combined system suffers from a number of disadvantages. Since the freezer tunnel extends horizontally from the immersion bath, the combined system occupies a large amount of floor space and is relatively expensive to construct. Furthermore, high energy fans are needed to draw the cryogenic vapor from the immersion bath into and through the elongated freezer tunnel, thereby increasing the cost of operating the combined system. In addition, the combined systems of the prior art typically have an entrance leading into the immersion bath and an exhaust outlet at the remote end of the freezer tunnel. In this type of arrangement it is difficult to control the flow of cryogenic vapor within the freezing apparatus and also difficult to prevent loss of the same. It would therefore be a significant advance in the art of freezing food products if a combined freezer system, utilizing both an immersion bath and a freezer tunnel could be made more compact, less costly and operate more efficiently than present combined freezer systems. Summary of the Invention The present invention is directed to an apparatus and method for freezing food products in a compact, and energy efficient manner. The freezer system of the present invention employs a first freezing operation utilizing a liquid or solid cryogen and a second freezing operation utilizing cryogenic vapor generated from heat exchange of the food product with the liquid or solid cryogen of the first freezing operation. In accordance with the present invention, the first freezing operation is positioned immediately above the second freezing operation. The entire freezing apparatus is contained within a housing which occupies significantly less floor space than conventional combined freezer systems. More specifically, the present invention is directed to an apparatus and method for freezing a food product comprising: a) a housing having an entrance for receiving unfrozen food product and an exit; b) a first freezing means for receiving the unfrozen food product from the entrance and for contacting said unfrozen food product with a liquid or solid cryogenic substance to at least partially freeze the food product and produce a cryogenic vapor; c) a second freezing means positioned below the first freezing means for receiving the partially frozen food product and completing the freezing of the food product while directing the food product towards the exit of the housing; and d) circulation means for circulating the cryogenic vapor from the first freezing means to the partially frozen food product contained in the second freezing means to thereby complete the freezing operation. In a preferred form of the present invention, multiple vessels containing a liquid cryogen are provided for the first freezing means. The food product travels from one immersion bath to another by conveyors traveling in opposite directions so that the vessels may be arranged essentially in a single column with the second freezing means positioned below the lowermost vessel. The present invention may also be provided with an exhaust control system which minimizes cryogenic vapor loss, maintains a safe level of cryogenic vapor in the freezer and provides exceptional cooling efficiency. More specifically, the exhaust control system comprises: a) a temperature sensing means positioned within the entrance of the housing for determining the temperature within the entrance of the freezing apparatus and for transmitting a first signal corresponding to the sensed temperature; b) temperature comparing means for comparing the first signal to a predetermined temperature and for generating a second signal corresponding to the difference between the first signal and the predetermined temperature; and c) exhaust control means comprising a motor operatively connected to a fan for drawing exhaust gas from the freezing apparatus and second signal detection means for detecting the second signal from the temperature comparing means and adjusting the speed of the fan in response to the second signal. In a further aspect of the present invention, there is provided a cleaning system for removing debris, such as residual food product and/or ice from the second freezer means while the freezing apparatus is operational. Conventional cleaning systems can be used for this purpose. It is preferred however, to employ a unique cleaning system in which movement of a conveyor belt used for transporting the frozen food product provides the force for directing an impact device against the conveyor belt in a manner sufficient to dislodge debris thereon. Brief Description of the Drawings The following drawings in which like reference characters indicate like parts are illustrative of embodiments of the invention and are not intended to limit the invention as encompassed by the claims forming part of the application. Figure 1 is a schematic cross-sectional view of an embodiment of the invention employing a single vessel containing a liquid cryogen in the first freezer section; Figure 2 is a schematic cross-sectional view of an embodiment of the invention employing two vessels each containing a liquid cryogen in the first freezer section; Figure 3 is a schematic cross-sectional view of an embodiment of the invention employing a single conveyor in the first freezer section and a source of a solid cryogenic substance; Figure 4 is a schematic cross-sectional view of an embodiment of the invention employing two conveyors in the first freezer section and a source of a solid cryogenic substance. Figure 5 is a cross-sectional view of an embodiment of the invention utilizing a downwardly sloping trough configuration containing a liquid cryogen for use in the first freezer section; Figure 6 is a schematic cross-sectional view of an embodiment of the invention similar to Figure 1 with a system for removing exhaust gas in a controlled manner from the freezer; and Figure 7 is a perspective view of an embodiment of a cleaning assembly used to dislodge debris from the conveyor belt in the second freezer section. Detailed Description of the Invention The present invention is directed to an apparatus for freezing food products in which a first freezer section and a second freezer section are vertically arranged to reduce cost and floor space and provide more efficient cooling. Referring first to Figure 1 there is shown an embodiment of the invention in which the first freezer section contains a single vessel containing a liquid cryogen. As shown specifically in Figure 1 , a freezing apparatus 2 includes a housing 4 having an entrance 6 for receiving an unfrozen food product 8 and an exit 10 for discharging the food product 1 2 in the desired state of freezing. In accordance with the present invention, the freezing apparatus 2 includes a first freezer section 14 and a second freezer section 16. The first and second freezer sections 14 and 1 6 are vertically arranged so that the food product moves generally downwardly within the freezing apparatus 2 from the entrance 6 at the top of the housing 4 to the exit 10. As shown in Figure 1 , the first freezer section 14 includes a single immersion bath 1 8 comprised of a vessel 20 containing a liquid cryogen 22. As used herein the term "liquid cryogen" shall include all cryogenic substances in liquid form including nitrogen, helium, argon and the like. Nitrogen is the preferreα liquid cryogen because of its ready availability and low cost. Passing through the bath of liquid cryogen is a conveyor belt 24 which carries the food product 8 entering the freezing apparatus 2 from one end 26 of the bath 1 8 to the opposite end 28 thereof. The conveyor belt 24 travels through a circuitous path over opposed rollers 30, 32 to provide continuous transportation of food product entering the first freezer section 14. The food product 8 entering the immersion bath 1 8 is partially frozen by heat exchange with the liquid cryogen 22. Cryogenic vapor is thereby produced and is drawn downwardly into the second freezer section 1 6 as explained hereinafter. Exiting the immersion bath 1 8 over the roller 32 is a partially frozen food product 34 which falls downwardly into the second freezer section 1 6. The second freezer section is comprised of a conveyor belt 36 passing in a circuitous route over respective upper rollers 38a, 40a and lower rollers 38b, 40b so that the food product 1 2 travels from one end 42 to an opposed end 44 of the conveyor belt 36 which lies in proximity to the exit 1 0. The partially frozen food product 34 tumbles onto the conveyor belt 36 where it is subjected to further chilling by the cryogenic vapor from the first freezer section 1 6. The cryogenic vapor is drawn downwardly from the first freezer section by the presence of an exhaust vent 45 having associated therewith a fan (not shown). The vapor pressure within the freezing apparatus, owing to the generation of the cryogenic vapor, generally exceeds atmospheric pressure. As a result, the cryogenic vapor is drawn toward the exhaust vent 45. As shown specifically in Figure 1 , a single fan 46 is placed below the conveyor belt 36. The fan 46 serves to redirect the flow of the cryogenic vapor upwardly through the conveyor belt 36 to contact the food product 1 2 being transported thereon. The fan 46 also recirculates the cryogenic vapor generated in the first freezer section 14 to provide continuous chilling of the food product 1 2 in the second freezer section 1 6. The velocity of flow of the cryogenic vapor upwardly through the conveyor belt 36 may be adjusted by adjusting the speed of the motor 48. If a sufficient velocity is provided, the food products on the conveyor belt 36 are caused .o tumble thereby creating a fluidized bed which minimizes adherence of the individual pieces of food product to each other. In certain instances, it may be desirable to transport the food product over the conveyor belt without tumbling. In this embodiment, the second freezer section 16 operates in a fixed bed mode. This embodiment can be instituted by limiting the force of the cryogenic vapor flowing through the conveyor belt 36 to a level below that required for fluidization. The cryogenic vapor directed to the second freezer section 16 completes the freezing operation. The frozen food product 12 then leaves the freezing apparatus through the exit 10. A flexible curtain 50 may be provided at the exit 10 to minimize entry of ambient air into the freezing apparatus and to minimize the loss of the cryogenic vapor through the exit 10. The present invention may be provided with more than one immersion bath in the first freezer section. Referring specifically to Figure 2, there is shown an embodiment of the invention utilizing two immersion baths 18a and 18b. The uppermost bath 18a is positioned off center of the lowermost bath 18b. As a result, the food product 34 which leaves the uppermost bath 18a falls downwardly into the bottommost bath 18b, and then travels in the opposite direction where it leaves the bottommost bath and falls downwardly onto the conveyor belt 36 of the second freezer section 16. In operation, the unfrozen food product 8 enters the food apparatus 2 through the entrance 6 where it falls into the immersion bath 18a containing liquid cryogen at the end 28a of the vessel 20a. The conveyor belt 24a transports the food product through the vessel 20a to the end 26a thereof and over the roller 30a where the partially frozen food product 34 falls into the immersion bath 18b, because the same extends beyond the edge of the immersion bath 18a. The partially frozen food product, while immersed in the immersion bath 18b within vessel 20b, is transported by the conveyor belt 24b traveling in the opposite direction of movement of the conveyor belt 24a. The food product is eventually transported to the end 28b of the immersion bath 1 8b and over the roller 32b where it falls into the second freezer section 1 6. The movement of the respective conveyor belts 24a and 24b in opposite directions enables the respective vessels 20a and 20b to be arranged vertically and thereby minimize the length of the freezing apparatus. The travel path of the food product from the entrance 6 to the exit 1 0 in the present freezing apparatus is forward and backwards as the food product progresses through the vessels 20a and 20b and downwardly as the food product moves between the vessels. In this arrangement a plurality of immersion baths and the combination of first and second freezer sections as defined herein may be employed which occupy substantially the same length of floor space as a single immersion bath system or a single freezer tunnel system. The embodiment of Figure 2 contains a second freezer section 1 6 substantially as described in connection with Figure 1 . The food product which has fallen from the second vessel 20b lands on the conveyor belt 36 and is transported in the direction opposite to the direction of movement of the conveyor belt 24b. While in transit, the food product is further chilled with cryogenic vapor drawn from the first freezer section 1 4 and redirected upwardly by the fan 46 and motor 48. The cryogenic vapor is preferably driven upwardly through the conveyor belt 36 with sufficient force to create a fluidized bed to insure that the frozen food particles are completely separated during the final freezing operation. Alternatively, the force of the flow of cryogenic vapor may be reduced so that the frozen food particles are chilled in a fluidized bed mode as previously described in connection with the embodiment of Figure 1 . Because air can infiltrate through the entrance and exit of the freezing apparatus, the conveyor belt in the second freezer section may become laden with ice particles. In addition, the conveyor belt can retain particles of the frozen food products as they pass through the second freezer. There may therefore be a need to clean the conveyor belt to remove the ice particles and particles of food. One such cleaning system is disclosed in Ron C. Lee et al., U.S. Patent No. 5,349,828, incorporated herein by reference. In this system a liquid cryogen conduit conveys a stream of liquid cryogen into the freezing chamber. The stream is then vaporized by a vaporizer to form cryogenic vapor which is passed through a nozzle or the like to form a jet of cryogenic vapor directed against the conveyor belt. The stream has sufficient force so as to be able to dislodge the ice and food particles attached to the conveyor belt. Referring to Figure 2 there is shown a similar cleaning device for removing ice and food particles and the like. The cleaning device 90 generally includes a vaporizer, shown schematically and identified by reference numeral 92 and a conduit 94 connecting the vaporizer 92 to at least one nozzle 96. The vaporizer 92 itself is connected to a source of liquid cryogen (not shown) which may be the same source which supplies liquid cryogen to the vessels 20a and 20b. The vaporized liquid cryogen is ejected out of the nozzle 96 onto the conveyor belt 36. It is preferred to use multiple nozzles 96 spaced apart across the width of the conveyor belt 36 to insure adequate cleaning. As shown specifically in Figure 2, the nozzle(s) 96 are preferably directed at the conveyor belt 36 at a position between the rollers 38a and 38b. Removal of the ice and food particles in this location insures that the conveyor belt 36 will be clean as it receives the partially frozen food product 1 2 arriving from the first freezer section 1 4. The flow of cryogenic vapor out of the nozzles may be continuous, intermittent or the flow of cryogenic vapor may be discontinued for significant periods of time depending on the rate of buildup of ice particles and/or the extent to which food particles adhere to the conveyor belt 36. The flow of cryogenic vapor through the nozzles may also be used to control the temperature of the second freezer section. The temperature within the second freezer section 1 6 is preferably maintained at a constant level. There are numerous methods of maintaining the temperature constant including adjusting the flow of cryogen into the freezer, varying the residence time of the food product within the cryogen and the like. One such temperature control system employs a temperature control loop for maintaining the second freezer section 1 6 at a predetermined temperature. The cryogenic vapor generated in the first freezer section, through heat exchange between the food product and the liquid cryogen, may be insufficient to provide sufficient cooling for the second freezer section. It may therefore be necessary to add cryogen directly into the second freezer section. Referring to Figure 2 the cryogen may be delivered to the vaporizer 92 and passed into the second freezer section 1 6 through the same nozzles 96 used to clean the conveyor belt 36. For example, the temperature in the second freezer section 1 6 is sensed by a temperature sensor 1 20. If the sensed temperature is higher that the predetermined set temperature established in the temperature controller 122, a signal corresponding to the difference in temperature is generated. The signal operates to increase the opening of a control valve 1 24 to allow additional cryogen to enter the vaporizer 92 to be ejected out of the nozzle(s) 96. Additional cryogen can be added until the actual temperature sensed by the temperature sensor equals the predetermined temperature. Conversely, if the temperature is too low, less cryogen must be added. Accordingly, the signal sent to the control valve 1 24 will decrease the opening and thereby lower the flow rate of the cryogen. Another method of controlling the temperature within the second freezer section 1 6 is to control the rate at which cryogenic vapor is generated in first freezer section. By varying the rate of movement of the conveyor belt 24, the residence time of the food product in contact with the cryogen will vary as well. The longer the food product is in contact with the cryogen the greater will be the production of cryogenic vapor. For example, if the temperature in the second freezer section 16 is too high, then the rate of travel of the conveyor belt 24 in the first freezer section 14 can be reduced to thereby increase the residence time of the food product within the liquid cryogen containing immersion bath 18. The increased residence results in the generation of more cryogenic vapor which is drawn downwardly into the second freezer section 16 as previously described to thereby lower the temperature within the second freezer section 16. Conversely, if the temperature in the second freezer section 16 is too low, the speed of the conveyor belt 24 can be increased to reduce the residence time of the food product within the immersion bath 18. As a result, less cryogenic vapor is produced and therefore less cryogenic vapor is available to cool the second freezer section 16. A preferred cleaning system of the present invention is shown in Figure 7. The conveyor belt used to convey the food product in the second freezer section provides the energy needed to drive an impact bar for periodically contacting the conveyor belt at a force sufficient to dislodge debris thereon such as residual food product and ice. Referring specifically to Figure 7, the cleaning assembly 200 includes a rotatable drive wheel assembly 202 which is operatively connected to and driven by the conveyor belt 36. The drive wheel assembly 202 includes a first bar 204 having opposed gears 206a and 206b each having respective teeth 208 for engaging corresponding holes 210 in the conveyor belt 36. Forward movement of the conveyor belt 36 causes the first bar 202 to rotate in a counterclockwise direction. The rotatable drive assembly 202 includes a second bar 212 which is spaced apart from and downstream of the first bar 204. Attached to the - 14 - second bar 212 is a cam device 214 for providing periodic movement to the impact bar as described hereinafter. The cam device 214 includes a plurality of spaced apart projections 216 defining corresponding valleys 218 therebetween for receiving the impact device 220. As shown in Figure 7, the impact device 220 includes an impact bar 222 having a head 224 for contacting the conveyor belt 36 at force sufficient to dislodge debris thereon. The impact bar 222 is connected to a bar 226 through a bar of opposed shafts 228a and 228b. The bar 226 engages the cam device 214 by traveling over the projections 216 and into the respective valleys 218. The bar 226 is maintained within its travel path over the cam device 214 through opposed shafts 230a and 230b, respective stoppers 232a and 232b and springs 234a and 234b. Movement of the bar 226 over the cam device is provided by chain drive 236 which is provided rotational movement from the conveyor belt 36 through the bar 204 and gears 206a and 206b. The chain drive 236 thereby rotates the bar 212 causing the cam device 214 to rotate in a like direction. As the cam device 214 rotates, the bar 226 moves along a valley 218 where it is engaged by one of the projections 216. This movement causes the bar 226, and thereby the impact bar 224, to be pulled away from the conveyor belt 36. As the bar 226 is pulled away from the conveyor belt 36, tension is built up owing to the compression of the springs 234a and 234b. When the bar 226 rises above the projection 216 and is thereby released, the springs 234a and 234b are likewise released thereby enabling the impact bar to contact the conveyor belt 36 with a force sufficient to dislodge any debris that may be contained on the conveyor belt 36. Movement of the impact bar 222 from engagement by the cam device 214 thereby generates the periodic movement of the impact device 220 which results in the efficient removal of debris from the conveyor belt 36. The force necessary to remove debris from the conveyor belt 36 can be varied by varying the distance of travel of the impact bar 222 and/or the compression of the springs 234a and 234b. In general, the force applied to the impact bar 222 can be increased by increasing the distance between the impact bar and the conveyor belt 36 and/or the compression of the springs 234a and 234b. In a further embodiment of the invention, a solid cryogenic substance (e.g. carbon dioxide snow) may be used in the first freezer section. The use of a solid cryogen eliminates the need for a vessel 20 as used in the embodiments of Figures 1 and 2. Referring to Figure 3, the first freezer section 14 is comprised of a conveyor belt 52 which extends from one end 54 to an opposed end 56 about respective rollers 58 and 60. Mounted in vicinity of the end 54 is a conventional solid cryogen generating device 62 which generates a sufficient amount of solid cryogen (e.g. carbon dioxide snow) so as to at least partially freeze the food product being transported on the conveyor belt 52. Therefore as unfrozen food product 8 enters through the entrance 6 it falls onto the conveyor belt 52 which transports the food product from the end 54 to the end 56. While on the conveyor belt 52 the unfrozen food product is contacted with a sufficient amount of solid cryogen to become at least partially frozen. During this operation heat exchange between the food product and the solid cryogen results in the production of cryogenic vapor which is drawn into the second freezer section 1 6 as previously described. The partially frozen food product 34 falls onto the conveyor belt 36 of the second freezer section. The direction of movement of the conveyor belt 36 is opposite to the direction of movement of the conveyor belt 52. As a result, the first freezer section 14 and the second freezer section 1 6 may be constructed in a vertical arrangement to minimize the length of the freezing apparatus. While being transported on the conveyor belt 36 toward the exit 1 0, the partially frozen food product is contacted with cryogenic vapor from the first freezer section 14 in the same manner described above in connection with the embodiments of Figures 1 and 2. Solid cryogen can be delivered to the first freezer section while the food product is transported on multiple conveyors. Referring to Figure 4, there is shown a solid cryogen delivery system using dual conveyor belts arranged in a manner similar to the liquid cryogen embodiment shown in Figure 2. The first freezer section 14 contains a pair of conveyor belts 52a and 52b. The uppermost belt 52a is positioned off center of the lowermost belt 52b. As a result, the food product 34 which leaves the uppermost belt 52a falls downwardly onto the bottommost belt 52b, and then travels in the opposite direction where it leaves the bottommost bath and falls downwardly onto the conveyor belt 36 of the second freezer section 1 6. During its travel on the belts 52a and 52b, the food product is subjected to solid cryogen (e.g. carbon dioxide snow) from a conventional solid cryogen generating device 62a and 62b. In operation, the unfrozen food product 8 enters the food apparatus 2 through the entrance 6 where it falls on the conveyor belt 52a while being contacted with solid cryogen through the solid cryogen generating device 62a. The conveyor belt 52a transports the food product to the end 26a thereof and over the roller 30a where the partially frozen food product 34 falls onto the second belt 52b, because the same extends beyond the edge of the first belt 52a. The partially frozen food product, while in contact with the solid cryogen from the solid cryogen generating device 62b, is transported by the conveyor belt 52b traveling in the opposite direction of movement from the conveyor belt 52a. The food product is eventually transported to the end 28b of the belt 52b and over the roller 32b where it falls into the second freezer section 1 6. As shown in Figure 4, the movement of the respective conveyor belts 52a and 52b in opposite directions enables the first freezer section 14 to be arranged directly above the second freezer section 1 6 and thereby minimize the length of the freezing apparatus. The operation of the second freezer section 1 6 is the same as previously described in connection with the embodiments of Figures 1 - 3. The first freezer section 14 may be provided with a liquid cryogen which flows downwardly through at least one angled trough, preferably under turbulent flow conditions. In this embodiment, the first freezer section 1 4 can employ the principles of construction and operation disclosed in Jack Appolonia, U.S. Patent Application Serial No. 08/282,057 filed July 28, 1 994, incorporated herein by reference. The troughs are adapted to house a flow of liquid cryogen and the food product as the food product at least partially freezes and to transport the same to the second freezer section 16. The extent to which the individual pieces of food are frozen is dependent on the residence time of the food product in the liquid cryogen, the depth of the liquid cryogen and the temperature of the food entering the freezer. The residence time of the food product within the liquid cryogen is dependent on the length of the trough and its angle of inclination θ. The longer the trough, the greater the time the food product will be in contact with the liquid cryogen and, therefore, the greater the extent of freezing. Conversely, as the angle of inclination θ increases, the time the food product is in contact with the liquid cryogen decreases resulting in a lesser degree of freezing. The preferred angle of inclination is from about 0.5 to 5.0° . The depth of the liquid cryogen is also a factor in the freezing of the food product. The depth of the liquid cryogen should be sufficient to allow all sides of the food product to be in contact with the liquid refrigerant for a time sufficient to freeze at least the outer surface thereof. It is desirable to maintain the depth of the liquid cryogen within the range of from 0.5D to 2.0D, preferably 0.5D to 1 .5D, where D is the maximum thickness of the food product. Referring to Figure 5 there is shown a first freezer section 1 4 containing two angled troughs 100 and 102, respectfully, each angled downwardly by an angle θ of preferably about 0.5 to 5 ° to create a flow of liquid cryogen and food product from the rear end 104 of the trough 1 00 to the front end 1 06 thereof. The liquid cryogen and the food product fall into the second trough 1 02 which is angled downwardly by an angle θ away from the trough 1 00 from a rear end 108 to a front end 1 1 0. There is also provided a conveyor belt 1 1 2 which receives the food product from the lower trough 102 and passes the same over a roller 1 14 so that the food product may fall into the second freezer section 1 6 where the food product is further chilled as described in connection with Figures 1 - 4. The freezing apparatus of the present invention may be provided with an exhaust system for controlling the rate of exhaust gas removal. Referring to Figure 6, a temperature sensor 70 such as a thermocouple, infrared sensor or the like senses the temperature within the entrance 6 of the freezing apparatus 2 and translates the temperature into a signal. The signal is transmitted to a temperature controller 72 which compares the signal to a predetermined setpoint temperature and generates a signal corresponding to the difference between the actual temperature within the entrance 6 and the predetermined setpoint temperature. The signal from the temperature controller 72 is sent to a motor controller 74. The motor controller varies the speed of a gas motive device 76, such as an electric motor which is coupled to a fan 78 located external to the housing 4 of the freezing apparatus 2. The fan 78 is operatively connected to the interior of the housing 4 through a duct 80. The exhaust control system varies the amount of cryogenic vapor drawn out of the freezing apparatus and thereby the amount of air being drawn into the freezing apparatus through the entrance. If the temperature sensed by the temperature sensor 70 within the entrance to the freezing apparatus rises above the setpoint established in the temperature controller 72, it indicates that too much air is being drawn into the system from the outside of the freezing apparatus. The controller 72 then transmits a signal corresponding to this condition causing the fan 78 to operate at a reduced speed until the actual temperature in the entrance 6 equals the setpoint. Conversely, if the temperature sensed is below the predetermined desired temperature, the controller 72 will transmit a corresponding signal to increase the speed of the fan. The fan will then draw in additional ambient air thereby raising the temperature at the entrance of the freezing apparatus until it reaches the predetermined setpoint. In practice the predetermined setpoint is from 0° to -40°F for most applications. This temperature range is generally between the operating temperature of the freezing apparatus (e.g. -40° to - 1 50°F) and the temperature outside of the freezing apparatus (e.g. 50° to 1 00°F). The present freezing apparatus supplies food through an entrance in the top of the housing. The predetermined temperature can be set between the temperature of the freezing apparatus and ambient because there is a significant difference in density between the cold, heavy gas within the freezing apparatus and the relatively warmer, lighter gas outside the freezing apparatus. Because of the significant difference in gas density, a setpoint between these two extremes can be selected which can differentiate between the cold cryogen vapor and the warm air vapor. The amount of cryogenic vapor in the freezing apparatus may be determined in other ways, not just by sensing the temperature at the entrance. For example, an optical sensor may be used to measure the degree of cloudiness in the freezing apparatus which is directly related to the amount of cryogenic vapor therein. Other ways of measuring the amount of cryogenic vapor would be apparent to those skilled in the art. The present invention may be modified to accommodate additional freezing operations. For example, a solid or liquid cryogen source may be positioned in proximity to the falling food product to provide additional cooling. Such a source could be positioned between the vessels 20a and 20b (as shown in Figure 2), between the vessel 20b and the conveyor belt 36, or between the vessel 20 or conveyor belt 52 and the conveyor belt 36 in the embodiments, for example, shown in Figures 1 and 3, respectively. EXAMPLE Utilizing an embodiment of the invention as shown in Figure 2, five hundred pounds per hour of steam cooked and water chilled seafood product are fed through the top of a 4x5x6 foot tall insulated enclosure. The articles land in a bath of liquid nitrogen. A conveyor belt removes the articles from the bath and deposits them into a second immersion bath. A second conveyor removes the articles from the second bath and deposits them on the conveyor belt of the second freezer section. The immersion baths generally contain sufficient liquid so as to substantially cover the food articles. Each immersion bath is approximately 8" wide and 20" long. Typical residence times in the liquid nitrogen bath are between 6 and 10 seconds. Actual residence times will depend on the temperature of the incoming food products. The immersion process removes roughly 25% of the total internal energy to form the product (U,,.,,^-, = 0 btu/lb/°F@-10°F). This removes sufficient heat from the surface of the food products to make them firm and practically dry. The ensuing tumbling action caused by transfer from the first immersion vessel to the second vessel aids in breaking up clumps of food articles. The second freezer section of the freezing apparatus is approximately 1 0" wide by 4' long. Cold nitrogen gas developed during the immersion process is drawn into the second freezer section. The refrigeration available is used to continue the freezing process. In this instance, the final heat removal process is carried out in a fixed bed mode of operation. Operating temperatures of approximately -80°F to -1 50°F are used in this stage. The actual temperatures depend on production rate. The cooked and chilled seafood emerges from the fluidized bed freezer and is packed for storage and later processing.

Claims: CLAIMS We Claim: 1 . Apparatus for freezing a food product comprising: (a) a housing having an entrance for receiving unfrozen food product and an exit; (b) a first freezing means for receiving an unfrozen food product from the entrance and for contacting said food product with a liquid or solid cryogen to at least partially freeze the food product and produce a cryogenic vapor; (c) a second freezing means positioned below the first freezing means for further freezing the partially frozen food product and for directing the food product toward an exit; and (d) circulation means for circulating the cryogenic vapor from the first freezing means to the partially frozen food product contained in the second freezing means to thereby complete the freezing operation. 2. The apparatus of claim 1 wherein the first freezing means comprises a vessel containing a liquid cryogen and a conveyor belt traveling in a first direction through the liquid cryogen, said second freezing means including a conveyor belt traveling in a second direction toward the exit opposite to the first direction. 3. The apparatus of claim 1 wherein the first freezing means comprises a plurality of vessels arranged in a column each containing a liquid cryogen, each vessel having a conveyor belt passing therethrough in a manner such that the direction of movement of each conveyor belt is opposite to the direction of movement of each conveyor belt in a preceding vessel. - 23 - 4. The apparatus of claim 3 wherein the plurality of vessels are arranged off center so that the end of one vessel extends beyond the corresponding end of the vessel immediately below or above it wherein food product discharged from the end of an upper vessel falls into a lower vessel immediately below said upper vessel. 5. The apparatus of claim 4 comprising a top vessel and a bottom vessel, each containing a conveyor belt, said top vessel having a conveyor belt for receiving the unfrozen food product and for passing the unfrozen food product in a first direction through the liquid cryogen and over an end of the top vessel, said bottom vessel having a conveyor belt for receiving the food product from the end of the top vessel and for passing the food product through the liquid cryogen in a second direction and over an end of the second vessel onto the second freezing means. 6. The apparatus of claim 1 wherein the liquid cryogen is liquid nitrogen. 7. The apparatus of claim 1 wherein the first freezing means comprises a conveyor belt traveling in a first direction for receiving the unfrozen food product and a solid cryogen generating means for generating a solid cryogen and for contacting the unfrozen food product with the solid cryogen, said second freezing means including a conveyor belt traveling in a second direction toward the exit opposite to the first direction. 8. The apparatus of claim 1 wherein the first freezing means comprises a plurality of conveyor belts arranged in a column, each conveyor belt moving in a direction opposite to the direction of r .ovement of a preceding conveyor belt. 9. The apparatus of claim 8 wherein the plurality of conveyor belts are arranged off center so that the end of one conveyor belt extends beyond the corresponding end of the conveyor belt immediately below or above it wherein food product discharged from the end of an upper conveyor belt falls onto a lower conveyor belt immediately below said upper conveyor belt. 10. The apparatus of claim 9 comprising a top conveyor belt and a bottom conveyor belt, said top conveyor belt for receiving the unfrozen food product and moving in a first direction over an end of the top conveyor belt, said bottom conveyor belt for receiving the food product from the end of the top conveyor belt and for moving the food product in a second direction and over an end of the second conveyor belt onto the second freezing means. 1 1 . The apparatus of claim 8 wherein the first freezing means comprises at least one solid cryogen generating device for generating a solid cryogen and for contacting the food product traveling on at least one of the conveyor belts with a solid cryogen. 12. The apparatus of claims 1 1 comprising a plurality of solid cryogen generating devices. 1 3. The apparatus of claim 1 2 wherein each solid cryogen generating device is positioned in proximity to a single conveyor belt so that the food product on each conveyor belt is contacted with solid cryogen from a separate solid cryogen generating device. 1 4. The apparatus of claim 1 wherein the first freezing means comprises at least one trough having a pair of opposed walls and a base defining at least one channel for housing the food product while in contact with the liquid cryogen, said trough being positioned above and angled downwardly from the entrance to the second freezing means. 15. The apparatus of claim 14 wherein the base has at least two spaced apart ridges traversing the base and extending between the opposed walls. 16. The apparatus of claim 14 comprising a plurality of inclined troughs, each trough having a rear end and a forward end, the forward end of an upper trough lying above the rear end of the next lowest trough, wherein the liquid cryogen and food product travel along the upper trough toward and out the forward end onto the rear end of the next lowest trough until the flow of liquid cryogen reaches the lowest trough. 17. The apparatus of claim 1 wherein the second freezing means comprises a conveyor belt for receiving the food product from the first freezing means and transporting the food product to the exit while the food product is contacted by the cryogenic vapor. 18. The apparatus of claim 17 further comprising cleaning means for cleaning the conveyor belt in the second freezing means. 19. The apparatus of claim 18 wherein the cleaning means comprises, a source of liquid cryogen, a vaporizer for converting the liquid cryogen to cryogenic vapor and at least one nozzle directed at the conveyor belt for converting the cryogenic vapor from the vaporizer into a stream of cryogenic vapor having sufficient force to clean the conveyor belt. 20. The apparatus of claim 19 comprising a plurality of nozzles arranged along the width of the conveyor belt. 21 . The apparatus of claim 19 wherein at least one nozzle is positioned in proximity of the conveyor belt at a location before the conveyor belt receives food product from the first freezing means. 22. The apparatus of claim 21 further comprising means for controlling the flow rate of the cryogenic vapor through the nozzle. 23. The apparatus of claim 18 wherein the cleaning means comprises impact means operatively connected to the conveyor belt for periodically impacting the conveyor belt to thereby apply a periodic force to the conveyor belt sufficient to dislodge debris thereon. 24. The apparatus of claim 23 wherein the impact means comprises: (a) a rotatable drive wheel assembly operatively connected to the conveyor belt; (b) cam means engaged to the drive wheel assembly and adapted for periodically engaging and releasing an impact bar; and (c) an impact bar for periodically contacting the conveyor belt at a force sufficient to dislodge the debris. 25. The apparatus of claim 24 wherein the rotatable drive wheel assembly comprises: (a) a first bar operatively engaged to the conveyor belt and rotatable thereby; (b) a second bar operatively engaged to the cam means, and rotatable in the same direction as the first bar; and (c) drive means operatively engaged to the first and second bars for rotating the second bar. 26. The apparatus of claim 25 wherein the drive means comprises a chain drive. - 27 - 27. The apparatus of claim 24 further comprising spring means operatively engaged to the cam means for providing a force to the impact bar when it is released from the cam means. 28. The apparatus of claim 1 further comprising exhaust control means for controlling the amount of cryogenic vapor within the freezing apparatus. 29. The apparatus of claim 28 wherein the exhaust control means comprises: (a) temperature sensing means within the entrance of the housing for determining the temperature within the entrance of the freezing apparatus and for transmitting a first signal corresponding to the sensed temperature; (b) temperature comparing means for comparing the first signal to a predetermined temperature and for generating a second signal corresponding to the difference between the first signal and the predetermined temperature; and (c) exhaust control means comprising a motor operatively connected to a fan for drawing exhaust gas from the freezing apparatus and second signal detection means for detecting the second signal from the temperature comparing means and adjusting the speed of the fan in response to the second signal. 30. The apparatus of claim 29 further comprising temperature control means for controlling the temperature within the freezing apparatus. 31 . The apparatus of claim 30 wherein the temperature control means comprises: - 28 - (a) temperature sensing means within the freezing apparatus for determining the temperature therein and for transmitting a first signal corresponding to the sensed temperature; (b) temperature comparing means for comparing the first signal to a predetermined temperature and for generating a second signal corresponding to the difference between the first signal and the predetermined temperature; and (c) cryogen dispensing means comprising a control valve responsive to the second signal for delivering a cryogenic vapor to the freezing apparatus in a manner such that the temperature in the freezing apparatus equals the predetermined temperature. 32. The apparatus of claim 31 wherein the cryogen dispensing means further comprises at least one nozzle for delivering the cryogenic vapor to the freezing apparatus. 33. A method for freezing a food product comprising: (a) passing an unfrozen food product into the entrance of a freezing apparatus having a first freezing section and a second freezing section below the first freezing section; (b) contacting the unfrozen food product with a liquid or solid cryogen in the first freezing section; (c) vaporizing a portion of the liquid or solid cryogen in the first freezer section and circulating the cryogenic vapor to the second freezer section; and (d) transporting the food product from the first to the second freezer section and freezing the food product contained therein with the cryogenic vapor obtained from the first freezing section.

Applicants: BOC GROUP INC, THE BOC GROUP, INC.
Application year: 1996
Main classification: A23L3
IPC codes: A23L3, F25D3, F25D29