Apparatus for cooling and freezing of products

Abstract: Apparatus for cooling or freezing food items comprising an entrance module (10) having a top (8), bottom (5) and four walls (2,3,6) comprising an inlet (11) through a first wall (6) for introducing food items in to the entrance module (10); a belt (16) for conveying the food items from the inlet (11) through the entrance module (10); an impeller (32) for circulating vaporised liquid cryogen above and below the belt (16); an impinger (17) capable of directing the circulated vaporised cryogen in to a plurality of impingement jets in the vicinity of the food items; and, a sprayer (25) for providing at least one stream of liquid cryogen in to the impingement jets; and an exit module (40) having a top (8), bottom (5) and four walls comprising a belt (16) for conveying the food items conveyed through the entrance module (10) through the exit module (40); a divider baffle (7) for conveying at least a portion of the vaporised cryogen from the entrance module (10); an impeller (32) for circulating the vaporised cryogen above and below the belt (16); an impinger (17) capable of directing the circulated vaporised cryogen in to a plurality of impingement jets in the vicinity of the food items; and, an outlet (42) capable of allowing cooled or frozen food items to exit the exit module (40).

Description: This invention relates to an apparatus for cooling and freezing food items according to the preamble of claim 1 and to a method using the apparatus. More specifically, this invention relates to device wherein a liquid and gas-phase cryogens are used to cool and/or freeze food items that are moved through the device on a belt. The transfer of heat from the food item to the cryogen is maximised through the use of a process in which liquid cryogen is sprayed in to a stream of gaseous cryogen which is circulated around the food item while also using a novel impingement plate to create a stream of cryogen. A novel design of the device increases the heat transferred from the food items to the cryogen. Commercial freezers as exemplified by US-A-4 783 972 and US-A-5 444 985 typically rely on the transfer of heat from a food product that is to be chilled or frozen by using a fan or blower which is situated near a conveyer upon which the food is being carried. The food product entering the freezer has a boundary layer of air surrounding it which insulates the food product in the surrounding atmosphere. Traditional freezers have employed blowers that generate currents of cooling vapour in many directions so that a significant portion of the vapour does not contact the food product only in a perpendicular direction. Under these conditions the vapour which does contact the food product often does not possess sufficient energy substantially to reduce the boundary layer around the surface of the food product. There is therefore a need to generate directed jets of cooling vapour so as to disturb the boundary layer. US-A-4 479 776 discloses an apparatus using a plurality of vertical tubes to provide a unidirectional air flow toward a food product. US-A-4 626 661 discloses the use of a plurality of nozzles along the pathway of a food product for delivering discrete jets of unidirectional cooling air. The use of tubes or slots to direct air in a cooling or freezing device has met with only limited success due to the build-up of condensation or ice in the tubes or slots which quickly reduces the efficacy of the devices. US-A-5 487 908 discloses a method and device for heating or cooling a food product on a moving substrate in which a continuous channel traversing at least a major portion of the width of the moving substrate converts multi-directional flow in to unidirectional flow. Such a device suffers, however, from having such an increased rate of flow that the food products may become entrained in the flow and controlled processing of the food item through the device becomes difficult. Increasing the velocity of the stream of cryogenic vapour which impinges the food item will increase the average heat transfer coefficient in a linear manner. At a certain point, however, unless the impingement stream is carefully controlled the velocity may also be sufficient to damage the food product or to carry the food product off the conveyor and in to undesirable locations elsewhere in the freezer. Overall heat transfer rates are dependent on local heat transfer coefficients, ie the amount of heat transferred from the food products to the cryogen is dependent on the rate of heat transfer locally between the cryogen and the food item. Local heat transfer rate can be changed by controlling the distance from the source of impingement jets to the food product, the velocity of the impingement jets, the turbulence in the jet and the efficiency of the flow of cryogen. A need remains, therefore, for a device which can rapidly chill and/or freeze a food item while reducing the amount of cryogen needed by extracting the maximum cooling effect from a given amount of cryogen. The device must also be capable of transporting food from an inlet to an outlet without damaging the food product. Additionally, the device must be able to control the throughput of food items and must be resistant to the freezing and plugging of internal components by snow and ice build up. Apparatus and method of the invention generally increases the amount of heat transferred from an item, particularly a food product, to a cryogen by generating impingement jets capable of breaking through the thermal boundary layer of the product, but which are not capable of damaging the product. Furthermore, the invention provides a jet of cryogenic gas to impinge the surface food products without causing the food products to become entrained in the impingement jet. Furthermore, the invention provides an efficient path for re-circulation of cryogenic gases back to the blower means so as to increase the efficiency of the freezer. Another advantage of the invention is to provide a modular design which can be adapted to provide a plurality of solutions to food processing requirements. A further advantage of the invention is that the connection of the modules provides for continual impingement of cryogenic gas on food items from their entrance in to the freezer apparatus until exiting. An additional advantage of the invention is the ability to reduce the dehydration of the food items which is accomplished through the immediate freezing of the exterior of the product upon entry in to the apparatus. A still further advantage of the invention is the ability to effect consistent cooling and or freezing of items across the width of the belt upon which the food items travel. The invention generally provides a modular food chilling and/or freezing apparatus which comprises an entrance module, an exit module and one or more intermediate modules. Each module contains a section of belt upon which food is transported. Each module contains an impinger which enables high velocity jets of cryogenic gas to impinge the upper and lower surfaces of the food items. The impinger may be a plate having a specific configuration of rounded or chamfered holes. In another embodiment a series of channels is used. A sprayer is provided in one or more modules in order to entrain droplets of liquid cryogen in the jets of cryogenic gas. Preferably, the interface between the modules includes a baffle for controlling the pressure differential and transfer of cryogen between modules. In accordance with the invention, there is provided apparatus for cooling or freezing food items comprising the features defined in claim 1. The apparatus advantageously further comprises an intermediate module capable of being connected between the entrance module and the exit module comprising a belt for conveying the food items from the entrance module through the intermediate module to the exit module; a divider baffle for conveying at least a portion of the vaporised cryogen from the entrance modules to the intermediate module; an impeller for circulating the vaporised cryogen above and below the belt; and, an impinger capable of directing the circulated vaporised cryogen in to a plurality of impingement jets in the vicinity of the food items. Preferably, a pneumatically actuated ball valve vibrator is used to remove the build-up of snow and ice from impingement plates. Preferably also, a hydraulic system is used to provide easy access to the interior of the apparatus. For a better understanding of the invention, reference will now be made, by way of exemplification only, to the accompanying drawings, in which: Figure 1 is a longitudinal cross-sectional representation of a freezer according to the invention. Figure 2 is a plan view of the freezer of Figure 1 as seen from the entrance end of Figure 1 with the exterior wall and inlet removed. Figure 3 is an axial cross-sectional representation of the freezer of Figure 1 through line B-B. Figure 4 is a perspective view of an impingement plate and the pneumatically actuated ball valve vibrator. Figure 5 is top plan view of a portion of an impingement plate having holes for use in a freezer according to the invention. Figures 6 and 7 are top and bottom perspective views of a portion of the impingement plate having holes for use in a freezer according to the invention. Figure 8 is a cross-sectional view of a portion of the impingement plate of Figures 6 and 7 through line 6-6. Figure 9 is a cross-sectional view of channel impingement device for use in a freezer according to the invention. Figure 10 is a cross-sectional view of an additional embodiment of an intermediate module of the invention. The invention is directed to an apparatus for cooling and/or freezing food products in which a food item is conveyed on a belt or other substrate in to a chamber in which the food product is cooled or frozen due to its contact with gas-phase cryogens such as nitrogen or carbon dioxide. The heat transfer resulting in the cooling or freezing of the food products results from the impingement of a stream of cryogenic vapour on the food item. Additional heat transfer is also achieved by spraying liquid or solid cryogen in to the impingement jet streams of cryogenic vapour. With reference to Figure 1, an entrance module 10 , intermediate modules 20 and 30 and an exit module 40 define one embodiment of the freezer of the invention. The modularity of the invention enables a set of modules 10, 20, 30 and 40 to be arranged to meet specific freezing requirements for various food types. The entrance module 10 has a liquid cryogen piping 24 and a sprayer 25 which enable a stream of liquid cryogen to be sprayed in to the jet of gaseous cryogen circulating within the module 10 and onto a belt 16 and food items (not shown). The sprayer 25 should preferably comprise a plurality of full cone low flow rate spray nozzles. The use of the sprayer 25 enables a rapid transfer of heat from the exterior of the food item resulting in rapid cooling and/or crust freezing of food items upon entering the series of modules and decreases dehydration of the food items. The sprayer 25 may be included in any other module if additional or continuous crust freezing is necessary or desirable. An impeller 32 of the module 10 is in fluid communication with an intake cone 34 and generates a flow of cryogen out of an exit 33 by which it circulates a flow of cryogenic vapour around the interior of the module 10 in accordance with the flow patterns represented by arrows in Figure 2. The cryogenic vapour flows from the exit 33 through an impinger 17 past the sprayer 25 entraining liquid cryogen in to the stream and then impinging on food items on the belt 16 . A high pressure flow of cryogen enters high pressure plenums 14 and the impinger 17 (which comprises the top of high pressure plenum 14 ) and flows under and through the belt 16 which provides impingement jets on the underside of the food items on the belt 16 . The belt 16 is a standard woven stainless steel belt typically used in food freezers. The flow of cryogenic vapour is returned to the intake cone 34 of the impeller 32 through the low pressure plenum 15 . The impeller 32 can be a 762mm diameter centrifugal fan operating at 283 m 3 /minute at 0.5 kpa static pressure having a 7.45 KW inverter driven motor or other type of blower having similar characteristics. Piping 24 is connected to a supply of liquid cryogen (not shown) providing a conduit for a supply of liquid cryogen for the sprayer 25 and to provide a source of cryogenic vapour for circulation within each freezer module. The entrance module 10 comprises a top, bottom and four walls. In Figure 1, a wall 6 of the module 10 is configured so as to include an inlet 11 . Bottom plate 5 and top plate 8 provide a top and bottom to the entrance module 10 as well as the other modules 20, 30, and 40 . At the right hand side of the entrance module 10 is a divider baffle 7 on the high pressure side of the impinger 17 which can be opened and closed so as to control the amount of cryogenic vapour passing from one module to another as well as to control the amount of internal pressure within modules. The gas flow through the system can also be controlled using a sensor mounted on the plenum. This sensor will note the changes in the pressure of gas in the chamber and will communicate with the impeller to either increase or decrease the impeller speed and regulate gas flow through the system. This will also improve the overall efficiency of the system. In one embodiment the divider baffle 7 comprises one or more sliding doors which are manually opened and closed in order to control the transfer of cryogen between modules. In a further embodiment an electromechanically controlled damper is used as the divider baffle 7 . The final two walls of the entrance module 10 are depicted in Figure 2 as walls 2 and 3 which are sliding door mechanisms contained in framework 12 and 13 respectively and enable access to the interior of the apparatus. The intermediate modules 20 and 30 are similar to the module 10 in construction in that they contain a portion of the belt 16 which conveys food items through the module. The intermediate module 20 is depicted in cross-section whereas the intermediate module 30 is depicted so as to show the exterior doors 18 which can be raised for internal access by use of a counterweight 19. In the intermediate module 20 or 30 an impeller 32 powered by a motor 22 circulates cryogenic vapour according to the arrows shown in Figure 3 which is a cross-section of the intermediate module 20 taken through the line B-B of Figure 1. The bottom plate 5 and the top plate 8 provide a top and bottom to the module. The top plate 8 is also where the impeller 32 is mounted and connected to the drive shaft of the motor 22 . As in the other modules, the impingers 17 provide a means for increasing the velocity of the cryogenic gas to generate impingement jets prior to impingement of the gas on food items on the belt 16. Food items pass from one module through an opening on the low-pressure side of the impingers 17 which permits the belt 16 to pass continuously from one module to another. A series of intermediate modules 20 and 30 can be used to provide a certain length of freezing time depending on the belt speed or throughput of food items required and the amount of time such food items need to be in the cryogenic environment to reach a desired goal. The modularity of the freezer provides for any variation in these parameters. The exit module 40 is similar to the other modules with the exception of the placement of an outlet 42 in the side wall 6 and a series of rollers 43 for return of the belt 16 through the modules. The exit module 40 also has an un-insulated plenum 44 attached to its exterior to catch gas which will fall upon exit from the module. In a further embodiment of the invention, the freezer exhaust is divided in to three sections. These separate sections provide a central exhaust. The primary exhaust will remove about 80% of the total cryogenic gas from the low pressure exit end of the freezer. The secondary exhausts each capture about 10% of the cryogenic gas from the inlet and exit of the freezer. This allows for both a reduction in the infiltration of air but also creates co-current gas flow down the length of the freezer. The cryogenic gas can be exhausted at warmer temperatures which will increase the overall efficiency of the freezer unit. Reduced icing in the exhaust ducts will also result. In addition, smaller diameter exhaust ducts and smaller exhaust blowers can be employed while reducing make-up air usage. The entrance end of the exit module 40 is similar to the entrance end of the intermediate modules 20 and 30 in that a module divider baffle 7 provides for control of the transfer of cryogen between modules. Likewise the impeller 32 circulates a stream of cryogenic gas through the high pressure plenum 14 and impingers 17 and onto the food items. A sprayer 25 and supporting piping 24 could be added to the exit module 40 if additional crust freezing is necessary. An important feature of the invention is the design of the impinger 17 . It could be an impingement plate, a portion of which is depicted in Figure 5, or a series of sheet metal channels as depicted in Figure 9. Depending on the configuration of the module the size of the plate may vary. However, the total open area of the impinger, i.e., the area of the holes should be between approximately 3% and 6% of the total area of the impinger. The most preferred percentage of open area is 4 to 5%. In the preferred embodiment of Figure 5, the axial pitch 51 and lateral pitch 52 are both 1.875 inches when the hole diameter 54 is 0.5 inch. Also, offset or stagger 53 of the centre of the holes should be approximately 0.625 inch in the preferred embodiment. The reason for the offset or stagger is that it has been found to provide an even chill or freeze of product across the width of the belt, thereby reducing or eliminating impingement lines on the food items. The plate version of the impinger 17 is fabricated from 22 gauge steel in preferred embodiments. The holes in plate version of the impinger 17 are radiused at 15%. Figures 6 and 7 show the rounded edges of the holes in the impinger 17 . Figure 8 depicts a cross-sectional view of the impinger 17 through section lines 6-6. This design reduces or prevents ice-build-up inside the hole and produces an impingement jet which has a velocity profile which is more effective in chilling or freezing the food item. Figure 4 depicts an impinger 17 of the plate type as described above. Each impinger 17 advantageously has a vibrator 48 which vibrates impinger 17 which is preferably free-floating in a rigid frame. The vibrator 48 may be of the electrical variety. However, in preferred embodiments vibrator 48 is a ball valve pneumatically actuated by compressed gas (for example nitrogen) supplied through conduits 49 at or about 60 psi from either an external source or from a vaporiser and compressor (not shown) internal to the apparatus. The frequency and time intervals at which the vibrator 48 is used to vibrate the impingers 17 is dependent on process conditions including the moisture content of the food items, the humidity of the ambient air in and around the apparatus and the temperature of the module. Figure 9 depicts the cross section of an impinger 17 which comprises a series of channels fabricated from a sheet of metal. In preferred embodiments, channel width 61 should be approximately 3 inches, channel pitch 62 should be approximately 12 inches, channel depth should be approximately 14 inches and channel opening 63 should be approximately 0.625 inch. The distance from the impinger 17 to the product surface should be approximately 3 inches but could vary from approximately 1 inch to approximately 5 inches. The position and spacing of holes or vortices in the impinger 17 determines the total overall heat transfer rate. Through the connection of a series of modules, total product coverage along the entire length of the freezer (continuous heat transfer area with no breaks) maximises the overall heat transfer rate. Typical freezing temperatures for an apparatus according to the invention are minus 120°C in the entrance module 10 and minus 50°C at the exit module 40 . In preferred embodiments, four modules are employed each being 3.048 m long, 1.753 m wide and 3.150 m tall. The belt width in the preferred embodiment is 0.712 m. Modules of various sizes may be constructed according to the invention. The impellers 32 may also be mounted on the side in order to reduce the height of the apparatus. In a lower profile embodiment, a plurality of impellers 32 are mounted either on the side or on the top. Using a plurality of lower height impellers 32 enables an overall height reduction in the design. Figure 10 is a cross-sectional representation of an additional embodiment of the invention. An intermediate module 50 is shown which is comprised of two portions. A top portion 68 and bottom a portion 66 together create an enclosure. A motor 22 is mounted through the top portion 68 where it connects with and drives an impeller 32 . The module 50 comprises a belt 16 , impingers 17 , a vibrator 48 and conduits 49 . The use of the two piece construction of module 50 enables the use of a hydraulic lift 69 to raise and lower top portion 68 in order to enable cleaning of the internal components of the apparatus and to enable servicing of internal components. The invention is preferably operated with liquid nitrogen. However, other cryogens may also be employed such as synthetic liquid air (SLA) and carbon dioxide. In standard operation of the invention, liquid cryogen is sprayed in to the entrance module of the apparatus upon start-up. As the liquid cryogen impinges on the impingers 17 and the belt 16 a portion will vaporise. The vaporised liquid cryogen is then circulated by the impeller 32 from the low pressure plenum 15 to the high pressure plenum 14 wherein it is forced through the holes or channels in the impinger 17 thereby creating the impingement jets. The impingement jets then continue to entrain additional liquid cryogen which is sprayed from the sprayer 25 . All of the vaporised cryogen will pass from the entrance module to the intermediate or exit module through the low pressure side of the impingers, ie through the hole which permits the belt 16 and food items to pass from one module to another. The majority of the vaporised cryogen, however, is passed from one module to another through the divider baffle 7 which enables the use to control the amount passed.

Claims: Modular apparatus for cooling or freezing food items comprising: an entrance module (10) having a top, bottom and four walls comprising: an inlet (11) through a first wall for introducing food items in to the entrance module; a belt (16) for conveying the food items from the inlet (11) through the entrance module; an impeller (32) for circulating vaporised liquid cryogen above and below the belt (16); and a sprayer (25) for providing at least one stream of liquid cryogen; and an exit module (40) having a top, bottom and four walls comprising: a belt (16) for conveying the food items conveyed through the entrance module through the exit module; a divider baffle (7) for controlling transfer of vaporised cryogen to the exit module (40); an impeller (32) for circulating the vaporised cryogen above and below the belt; and an outlet (42) capable of allowing cooled or frozen food items to exit the exit module (40), characterised in that both said entrance module (10) and said exit module (40) have an impinger (17) capable of directing the circulated vaporised cryogen into a plurality of impingement jets in the vicinity of the food items and the said sprayer (25) is arranged to spray the cryogenic liquid into the impingement jets in the entrance module (10). An apparatus according to Claim 1 further comprising an intermediate module (20, 30) capable of being connected between the entrance module (10) and the exit module (40) comprising: a belt (16) for conveying the food items from the entrance module (10) through the intermediate module to the exit module (40); a divider baffle (7) for controlling transfer of vaporised cryogen from the entrance module (10) to the intermediate module (20, 30); an impeller (32) for circulating the vaporised cryogen above and below the belt (16); and, an impinger (17) capable of directing the circulated vaporised cryogen in to a plurality of impingement jets in the vicinity of the food items. An apparatus according to Claim 2 in which at least one of the intermediate module or the exit module includes a sprayer (25) for spraying one or more streams of liquid cryogen into the impingement jets. An apparatus according to Claim 2 in which the divider baffles (7) both comprise at least one sliding door or an electromechanically controlled damper. Apparatus according to any preceding claim in which each module (10, 20, 30, 40) further comprises at least one high pressure plenum (14) wherein the vaporised cryogen is circulated by said module's impeller (32) prior to entering respective said module's impinger (17). Apparatus according to Claim 5 in which a pressure sensor is mounted on the said high pressure plenum (15) and is in communication with the said impeller (32) to increase or decrease the speed of the said impeller (32). Apparatus according to any preceding claim in which each module (10, 20, 30, 40) further comprises at least one low pressure plenum (15) wherein the vaporised cryogen is collected upon exiting said module's impingers (17) and through which the vaporised cryogen is recirculated to said module's impeller (32). Apparatus according to any preceding claim in which each impinger (17) is a plate having a plurality of holes therethrough and the edges of the holes are chamfered or rounded. An apparatus according to Claim 8 in which the total area of the holes comprises between approximately 3% and approximately 6% of the total area of the plate. Apparatus according to any preceding claim further comprising a vibrator (48) for vibrating each impinger (17) to reduce or remove the accumulation of snow or ice on the impinger. Apparatus according to any preceding claim wherein at least two walls of each module (10, 20, 30, 40) are comprised of at least two portions so as to permit the top portions to be raised and lowered along with the top of the module (10, 20, 30, 40). Apparatus according to any one of the preceding claims, in which the exit module further comprises an exhaust line which is divided in to three sections. A method of cooling or freezing food items comprising use of the apparatus of any one of Claims 1 to 12.

Inventors: MCCORMICK STEPHEN, NEWMAN MICHAEL DUANE, MCCORMICK, STEPHEN, NEWMAN, MICHAEL DUANE
Applicants: BOC GROUP INC, THE BOC GROUP, INC.
Application year: 2001
Main classification: F25D3
IPC codes: A23L3, F25D3