Patent Publication Number: US-2005120726-A1

Title: Deep freezer and method of freezing products

Description:
BACKGROUND AND SUMMARY OF THE INVENTION  
      This application claims the priority of 103 51 246.2 filed in Germany on Nov. 3, 2003, the disclosure of which is expressly incorporated by reference herein.  
      The invention relates to a device for the cooling of products, having a conveying device which conveys the products through a cooling zone in which the products are exposed to an atmosphere which, by means of low-temperature gases, is brought to a temperature required for cooling the products. The invention also relates to a method of cooling the products.  
      The term “cooling”, on the one hand, describes the cooling to temperatures above the freezing point, for example, the cooling of heat-treated food products from +70° C. to +2° C. However, “cooling” comprises particularly also freezing and deep freezing. “Deep freezing” means that the food products are subjected to an atmosphere of a sufficiently low temperature for a sufficient period of time, so that, following the freezing process, a core temperature exists which is clearly below the freezing point of water; as a rule, below a temperature of −18° C.  
      This atmosphere or environment can be brought to the desired temperature, for example, by low-temperature gas (liquefied or gaseous) or by cooled air. The heat exchange essentially takes place by way of convective processes. When liquefied gas is used, it also takes place by way of heat conduction by direct contact of the cooling medium with the product to be cooled.  
      Preferably, the gas is therefore introduced into the cooling device in a liquefied state and is vaporized by a thermal transfer from the product to be cooled to the cooling medium. The then warmer and gaseous cooling medium is discharged from the cooling device. The discharge should normally not take place into environment of the facility because some of the suitable gases displace the air and thereby would endanger people in the surroundings of the facility. Thus, for reasons of safety, a removal of the gas has to be ensured from the working environment of such a cooling device.  
      Devices for cooling (deep freezers) are known as passage facilities, for example, as tunnel-type deep freezers and as cabinet-type deep freezers. In the case of tunnel-type deep freezers, immersion deep freezers are also customary for freezing products to be frozen. These immersion deep freezers have a bath of a cryogenic liquid through which a conveying belt travels which conveys the products to be frozen out of the bath. The products to be frozen are either dropped directly into the bath or are placed on a belt which conveys the products to be frozen into the bath. An immersion deep freezer of this type is described, for example, in  Gas aktuell , Messer Griesheim GmbH, Volume 29, June 1985 Edition, Page 10.  
      Problems may occur in the case of such immersion deep freezers when the density of the products to be frozen is such that the products to be frozen float on the surface. In this case, the products to be frozen cannot be conveyed through the bath during a predetermined period of time. Problems also occur in the case of those products which tend to accumulate material. For example, fats or sugars may become glued to one another or to the conveying belt so that, also in this case, no defined dwell time of the products to be frozen can be observed in the bath, or a lumpy undefined product is created.  
      Furthermore, tunnel-type deep freezers are known, in the case of which only the unloaded belt immerses into an immersion bath of low-temperature liquefied gas. However, this creates a non-homogeneous temperature distribution in the device.  
      It is an object of the present invention to provide a device of the initially mentioned type which has a simple construction and permits an effective cooling of the products and an efficient utilization of the used cooling medium, as well as to indicate a method which achieves the above-mentioned object.  
      On the device side, with reference to the single drawing figure, this object is achieved according to certain preferred embodiments of the invention, in that the conveying belt ( 10 ) leads partially through an immersion bath ( 2 ) consisting of liquefied low-temperature gas, and in that at least one ventilator ( 6 ) is mounted below the conveying belt ( 10 ) such that it causes the temperature distribution in the cooling zone to be more uniform, and injection devices ( 1 ,  3 ,  4 ) for liquefied low-temperature gas are provided, the injection devices ( 3 ) being arranged in the proximity of the placing of the product (P) onto the conveying belt ( 10 ) below the part of the conveying belt ( 10 ) loaded with the product, with a spraying direction aimed onto the conveying belt ( 10 ). In certain preferred embodiments, the ventilator further operates to (i) to refrigerate the conveying belt (steel band) which absorbs heat from the product while traveling through the freezing zone, and (ii) to precool the conveying belt on its return to the immersion bath in order to minimize the temperature difference of the immersion bath resulting in energy savings and avoidance of the Leiden frost effect.  
      According to certain preferred embodiments of the invention, injection devices ( 3 ) are arranged such below the part of the conveying belt ( 10 ) loaded with the product (P) that the liquefied low-temperature gas is in a heat-conducting contact with the underside of the conveying belt ( 10 ).  
      Particularly preferably, at least one ventilator ( 6 ) is constructed as an axial-flow ventilator, according to certain preferred embodiments of the invention.  
      Particularly advantageously, according to certain preferred embodiments of the invention, at least one ventilator ( 6 ) has an axis situated in a plane parallel to the conveying belt ( 10 ).  
      According to certain preferred embodiments of the invention, at least one ventilator ( 6 ) advantageously has an axis which is oriented essentially perpendicularly to the conveying direction of the products on the conveying belt ( 10 ). The orientation of the ventilator ( 6 ) permits the temperature in the cooling zone to be more uniform and allows an improved utilization of the gaseous cold gas.  
      According to certain preferred embodiments of the invention, as a result of two axial-flow ventilators ( 6 ) and a partition wall between the two axial-flow ventilators ( 6 ), a further improvement is achieved of the temperature distribution of the atmosphere in the cooling zone.  
      Particularly advantageously, according to certain preferred embodiments of the invention, for example, for the food products to be cooled, the conveying belt ( 10 ) is essentially made of special steel.  
      On the method side, the above-mentioned object is achieved in that, by the immersion into low-temperature liquefied gas and by the injection of low-temperature gases into the device and by the cold atmosphere circulated by at least one ventilator ( 6 ,  8 ), the conveying belt is brought to a temperature by which the products are cooled.  
      Advantageously according to certain preferred embodiments of the invention, low-temperature liquefied gas is sprayed from below onto the conveying belt ( 10 ) and arrives on the underside of the conveying belt ( 10 ), so that a heat-conducting contact is established between the low-temperature liquefied gas and the underside of the conveying belt ( 10 ).  
      Preferred embodiments of the invention offer one or more of the following number of advantages: 
          Not only vaporization heat but also the residual gas heat is utilized.     The underside of the product is not cooled directly but by way of a special-steel band.     The heat transfer from below takes place as heat conduction (significantly better than convection).     The facility does not have to be disassembled for cleaning and drying.     The special-steel belt is cooled from below along the entire length by means of ventilators and injection.     Before the product is placed on it, the special-steel belt is cooled in liquid nitrogen.        

      For example, a hinge plate belt with lateral chains is provided as the conveying system according to certain preferred embodiments of the invention. This conveying system is distinguished by a homogeneous surface. The hinge plate belt is advantageously equipped with additional webs which increase the surface for the heat transfer from below.  
      The invention as well as additional details of the invention will be explained in detail in the following by means of the embodiment illustrated in the single drawing figure.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The single drawing figure schematically illustrates a freezing tunnel with the arrangement of the injection devices and the ventilators according to preferred embodiments of the invention. 
    
    
     FURTHER DETAILED DESCRIPTION OF THE DRAWING  
      The injection of the low-temperature liquefied gas (such as liquid nitrogen “LIN”) takes place by way of a special-steel vat  2 , controlled by a solenoid valve or a ball valve for cooling the conveying belt  10 . As a result of the flow, there is no Leiden frost effect. The injection is only switched off during the operation of the freezing tunnel when the LIN collecting vat becomes too full.  
      The LIN collecting vat  2  is used for precooling the belt  10  and collecting the nitrogen of the actual belt cooling. When the freezing tunnel is opened, this vat  2  folds away downward. For cleaning purposes, a heater can advantageously be installed for evaporating the nitrogen. The vat  2  is equipped with a level monitoring device.  
      The bottom belt injection  3  for the conveying belt  10  is used for the first aftercooling after the product has been placed on the belt. Possibly dripping-down nitrogen will drip into the LIN collecting vat  2 . The control takes place by way of a control valve. The resulting temperature is measured by means of a temperature measuring device  7 .  
      The top belt injection  4  is used for the surface-type heat removal from the product. The control also takes place by way of a control valve. The resulting temperature is measured by means of the temperature measuring device  5 , for example, by means of a Pt-100 thermoelement  5  over the belt  10 , which determines the temperature of the atmosphere above the conveying belt  10 . The ventilators  6 , which below the belt  10 , provide an increased heat transfer onto the belt, are used for the effective utilization of the cold nitrogen gas. The PT-100 thermoelement  7  is used for measuring the atmospheric temperature below the belt  10  (in the case of the ventilators  6 ) is used for apportioning the injection quantity of the injection  3 .  
      The ventilators  8  which, above the belt  10 , provide an increased heat transfer onto the product, permit an effective utilization of the cold nitrogen gas. An exhaust air system sucks off the expanding cold nitrogen and mixes it with the ambient air.  
      In the illustrated embodiment, the immersion bath vat  2  is filled using drip/ooze injection nozzle  1 , whereby the liquid nitrogen drips down onto the inside of the belt  10  and then spills into the bath vat  2 .  
      In preferred embodiments, the bath vat  2  level of liquid nitrogen is preferably monitored by means of a leveling control arrangement, including a bath level detector operable to give a signal to a control device which controls the drip injection nozzle  1 .  
      According to alternative preferred embodiments of the invention, normal flap or center body spray injection nozzles may be used in place of the drip injection nozzles  1 . This arrangement would lead to a direct spraying upon the band with a very high heat transfer. With this arrangement, the immersion bath vat  2  does not normally get filled up, but only serves as a collection pan.  
      By utilizing an appropriate control of the nozzles and the ventilators in conjunction with the movement of the belt  10 , the various functions can be separately controlled and dependence on the desired operating freezing products as desired.  
      In preferred embodiments, the freezer generally operates following the parallel flow principle.  
      In preferred embodiments, the immersion bath operation will automatically be stopped as the tunnel is opened so as to avoid deposits during the cleaning process.  
      It is preferred to not allow for lifting of the freezer as long as there is liquid nitrogen present in the immersion bath vat  2 .  
      In certain preferred embodiments, an electrical heating means is provided in the immersion bath so as to allow for vaporization of the liquid nitrogen.  
      The illustrated preferred embodiment of the invention is a cooling device which mainly cools the belt  10  on which the product travels through the freezing zone. By means of the precooling with liquid nitrogen in the vat  2  and the aftercooling by gaseous cold nitrogen, by means of the axial-flow ventilators  6  below the belt, a heat transfer is generated which leads to a considerable cooling of the product.  
      The deep freezer according to preferred embodiments of the invention is preferably used as a preliminary deep freezer for the initial hardening of the underside or of the entire surface of the product. A deep freezer according to preferred embodiments of the invention is also suitable to be used as a stand-alone unit in which the product is frozen through in one step, so that also the core temperature of the product is cooled to below zero degrees centigrade.  
      The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.