Abstract:
An impingement plate atomizer apparatus includes an electrostatically charged longitudinal member having an upper surface sized and shaped to receive a liquid cryogen thereon, a lower surface opposite to the upper surface, and at least one hole extending through the longitudinal member; an ultrasonic transducer in contact with the longitudinal member for providing ultrasonic energy thereto for atomizing the liquid cryogen into an electrostatically charged cryogen fog. A method is also provided for providing an electrostatically charged cryogen fog.

Description:
BACKGROUND 
       [0001]    The present embodiments relate to freezer apparatus used to cause heat transfer to objects, such as for example food products, by the application of a cryogen to same. 
         [0002]    In a cryogenic food freezing system, nitrogen liquid, for example, is sprayed into the freezing chamber to provide refrigeration for the process. It is desirable to spray the liquid nitrogen onto the warm surface of the incoming food product so that a phase change (heat of vaporization) occurs on the surface of the food product. This evaporative cooling effect creates extremely high heat transfer coefficients. Until now, it has been very difficult if not impossible to direct a high portion of the liquid spray onto the food product, as the spray is injected through nozzles that are positioned in a freezing chamber above a belt upon which the products travel for processing. Although a portion of the liquid nitrogen is deposited onto the surface of the product, another portion of the nitrogen travels to and through the belt without contacting the product. This results in an inefficient use of the liquid cryogen. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]    For a more complete understanding of the present inventive embodiments, reference may be had to the following drawing figures taken in conjunction with the description of the embodiments, of which: 
           [0004]      FIG. 1  shows an electrostatic impingement plate atomizer embodiment of the present invention; and 
           [0005]      FIG. 2  shows an enlarged portion of the embodiment in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0006]    As discussed below with respect to the present invention, when liquid nitrogen (N 2 ) is atomized, surfaces of the atomized droplets are charged by exposure to an electrostatic field. The product, such as for example a food product to be chilled or frozen, can be grounded or provided with an opposite charge which will attract the charged, atomized nitrogen droplets, allowing for a much greater proportion of the nitrogen spray being deposited onto the food product, thereby maximizing overall evaporative heat transfer at the product and use of the nitrogen. 
         [0007]    The liquid nitrogen is disposed on an impingement plate and then atomized by an ultrasonic transducer. The impingement plate is charged such that it provides a charge directly to the atomized nitrogen gas, the charged atomized liquid nitrogen droplets are entrained directly into the flow of nitrogen gas as a fog passing through the impingement plate. This process provides more even distribution of the charged nitrogen fog and can therefore effectively replace cryogen nozzles in a freezing process. 
         [0008]    Referring to  FIGS. 1 and 2 , the inventive embodiment  10  includes an electrostatically charged impingement plate  12  positioned above a conveyor  11  for transporting a food product  14  to be frozen. The plate  12  can be electrostatically charged by supplying an electrical current to the plate. The charged impingement plate  12  is constructed to hold or retain a quantity of liquid cryogen, such as for example liquid nitrogen  16 . This is accomplished by the plate  12  having an upper surface  18  and an edge  20  extending upward therefrom to retain a select amount of the liquid nitrogen  16  on the upper surface  18 . As shown in  FIG. 1 , the edge  20  extends along an outer perimeter or periphery of the plate  12 , but can also be constructed to extend along the plate at another area thereof where necessary to retain the liquid nitrogen. 
         [0009]    The plate  12  is formed with at least one or a plurality of holes  22  or apertures therethrough, each one of the holes  22  having an edge  24  turned downward as shown at  25  toward the product  14  to facilitate the flow of gas as discussed below. A raised or elevated surface area  27  extends around each one of the holes  22  to prevent the liquid nitrogen  16  from pouring or seeping through the holes  22  before the nitrogen has been electrostatically charged and atomized. The raised surface area  27  functions as a dam and can be formed on the upper surface  18  by for example either pressing the plate  12  to have the upper surface  18  between the raised surface areas  27  in relief as compared to the areas  27  with same extending above the upper surface  18  as shown in  FIG. 2 , or by providing a wall of material to extend upward around the edges  24  of the holes  22 . The raised surface area  27  may be formed integral with the plate  12 . Although the holes  22  are shown having a circular shape, other shapes for the holes may be used. A nitrogen injection system (not shown) may also be provided to maintain a constant level of nitrogen on the surface  18  of the plate  12 . 
         [0010]    At least one ultrasonic transducer  26  is mounted to the upper surface  18  of the impingement plate  12 . The ultrasonic transducer  26  vibrates the charged impingement plate  12  at high frequencies. The transducer  26  can also be mounted to a lower surface  34  of the impingement plate  12 , as shown for example in  FIG. 2 . Therefore, the transducers  26  can be provided at both the upper surface  18  and/or the lower surface  34  of the impingement plate  12  to provide ultrasonic energy to the plate. 
         [0011]    The high frequency vibration will cause the liquid nitrogen  16  disposed on the plate  12  to break-up into small atomized droplets  28 . In addition, there are high velocity gas jets  30  created by a flow of nitrogen gas through the holes  22  of the impingement plate  12  by internal fans (not shown). The nitrogen droplets  28  are entrained in the gas jets  30  and forced through the holes  22 . The droplets  28  become charged by direct contact with the impingement plate  12 . 
         [0012]    The droplets  28  are discharged from the impingement holes  22  and seek the closest grounded or oppositely charged object, which will be the food product  14 . The food product  14  is positioned closest to the discharge at the holes  22  by being transported upon the conveyor  11 , such as a belt, as shown in  FIG. 2 . A large portion of the charged atomized nitrogen particles adhere to the food product. The ultrasonic energy transferred to the impingement plate  12  creates the droplets  28  to be of such a small size that the entrainment of the droplets  28  in the gas jets  30  provides for a charged atomized nitrogen fog  32  to be deposited upon the food product transported on the conveyor  11 . The majority of the fog  32  is deposited on the exposed surface of the food product  14 , instead of being deposited on the conveyor  11  or passing through the conveyor. 
         [0013]    The embodiment  10  can be constructed and arranged in a new freezer system, or retrofit to an existing freezer system. 
         [0014]    It will be understood that the embodiments described herein are merely exemplary, and that one skilled in the art may make variations and modifications without departing from the spirit and scope of the invention. All such variations and modifications are intended to be included within the scope of the invention as described and claimed herein. Further, all embodiments disclosed are not necessarily in the alternative, as various embodiments of the invention may be combined to provide the desired result.