Abstract:
In a freezer having an internal space therein in which a blower and an impingement plate are disposed and through which a conveyor for a food product passes, an impingement apparatus, includes a hood disposed at the internal space for coacting with the blower and the impingement plate, the hood including a sidewall defining a sub-chamber in which the blower is received; and a pipe having an end opening into the sub-chamber for introducing a pulse of cryogen to the sub-chamber for increasing a pressure in the sub-chamber and contacting the impingement plate. A related method is also provided.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present embodiments relate to impingement freezing in cryogenic food freezing tunnels and in particular, to heat transfer which occurs with pulsed impingement apparatus in the tunnels. 
         [0002]    Known cryogenic food freezers, such as a food freezing tunnel, have restricted capacity to process food products due to overall, limited heat transfer coefficients. That is, many known food freezing tunnels rely upon increasing air flow velocity across the food product in order to provide a commensurate increase in heat transfer rate at the products. There are, however, practical and economic limitations when increasing heat transfer with these known processes. 
         [0003]    It is also known to be necessary to remove snow and ice accumulation from the impingement plates used with various food freezing tunnels. To date, pneumatically powered mechanical vibrators coact with the impingement plates to remove any accumulated snow and ice from the holes in the plates. However, such mechanical vibrating devices require increased maintenance and can fail under cryogenic temperatures during the freezing applications, especially when such devices are subjected to excessive humidity. These aspects of the devices can result in compromising the freezing process efficiency for the food products. 
       SUMMARY OF THE INVENTION 
       [0004]    The present embodiments provide increases in overall heat transfer rates which permit smaller food freezing tunnels to be fabricated and used, or permit production rates to be increased with existing tunnels. 
         [0005]    The present embodiments provide pulsing impingement jets in an impingement freezing tunnel to increase the overall heat transfer rate of same. 
         [0006]    The present embodiments obviate the need for using known pneumatically powered mechanical vibrators with impingement plates and therefore, substantially reduce if not eliminate the chance that the food processing line will be compromised if such vibrators fail during exposure to the cryogenic temperatures and high humidity conditions. 
         [0007]    Therefore, an apparatus embodiment for generation of a cryogen pulsed flow for impingement hoods in freezers includes a hood constructed and arranged to coact with an impingement plate and a blower of a freezer to provide a sub-chamber in the freezer atmosphere in which pressure waves are generated to contact the impingement plate and increase velocity of impingement jets from the plate. 
         [0008]    A method embodiment is also provided for providing pulsed flows for impingement hoods in freezers which includes constructing and arranging the impingement hood for providing a sub-chamber within the freezer proximate an impingement plate of the freezer, generating a pressure wave of a cryogen substance, introducing the pressure wave into the sub-chamber, and contacting the impingement plate for clearing snow and ice from said plate and increasing a velocity of impingement jets from the plate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    For a more complete understanding of the present invention, reference may be had to the following description of exemplary embodiments considered in connection with the accompanying drawing Figures, of which: 
           [0010]      FIG. 1  shows a side plan view in cross-section of a freezer tunnel with an apparatus for generating pulsed flow for impingement hoods according to the present embodiments; 
           [0011]      FIG. 2  shows a side view of another embodiment of the pulse flow apparatus of the present invention; and 
           [0012]      FIG. 3  shows a top plan view of the apparatus shown in  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    Before explaining the inventive embodiments in detail, it is to be understood that the invention is not limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawings, if any, since the invention is capable of other embodiments and being practiced or carried out in various ways. Also, it is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. 
         [0014]    In the following description, terms such as a horizontal, upright, vertical, above, below, beneath and the like, are to be used solely for the purpose of clarity illustrating the invention and should not be taken as words of limitation. The drawings are for the purpose of illustrating the invention and are not intended to be to scale. 
         [0015]    Basically, an impingement freezer apparatus of the present embodiments includes at least one and for certain applications a plurality of separate and discreet internal impingement hoods, each of which is fluidly connected to a source of high pressure nitrogen gas (N 2 ) controlled by a solenoid valve between the nitrogen source and the respective hood. The nitrogen is introduced under a pressure greater than that under the hood to provide an pressure pulse from the hood onto the underlying impingement plate(s) which provides a pressure wave to contact the plates and clear snow and/or ice from the plates (and holes disposed therein) positioned between the hood and the underlying food product being conveyed for freezing, and to provide a pulse to the nitrogen flow through impingement holes in the plate onto the underlying food product. 
         [0016]    Impingement pulses are most effective when generated as close as possible to the heat transfer surface, which in this case are food products, for example. It is also more practical to generate the pulses in an enclosed volume of space. This is because as the volume of the cavity or space around the heat transfer surface becomes enlarged, a dampening effect is created which minimizes the degree of pulsation which can be achieved. 
         [0017]    In the present embodiments, one or a plurality of separate and discrete impingement hoods are positioned in a freezer for generating a plurality of pulsed impingement jets. The reduced volume or sub-chamber defined by the hood is a more suitable environment to facilitate generating effective, heat transfer pulses. The pressure in the atmosphere within each one of the hoods where impingement jets are generated is at 2-3 inches of water column. 
         [0018]    Pressure pulses are generated by introducing high pressure, small volumes of nitrogen gas into the hood spaces or sub-chambers. See  FIG. 1  which shows a plurality or an array of three impingement hoods, by way of example only. A high pressure cryogen gas pipe is connected to the inside of each impingement hood. By way of example, nitrogen (N 2 ) gas is delivered through the pipe. A high frequency solenoid valve is placed within the high pressure nitrogen pipeline. A high pressure nitrogen gas manifold extends along a length of the freezer to a high pressure nitrogen gas storage tank. Gas pressure in this tank can be held in excess of 200 psig. 
         [0019]    The individual solenoid valves open and close at a rate which corresponds to a desirable pulsed impingement flow to the respective hoods. As the solenoid valve opens a high pressure volume of nitrogen gas is introduced into the sub-chamber defined by the hood. The solenoid valve is then closed and a pulsed pressure wave is created in the hood. The pressure wave serves two purposes: first, it provides a slight increase in overall hood pressure which results in an impingement jet pulse from the hood, and second, the pressure wave assists with clearing snow and ice from the impingement plates. 
         [0020]    The high pressure gas connections for the nitrogen to the hood should be arranged symmetrically with respect to the hood, as shown for example in  FIGS. 2-3 , so that an even distribution of a pressure wave within the hood is provided to the impingement plate disposed at an enclosed portion of the hood above the underlying food product being transported on the conveyor. The greater a number of gas connections used will permit customizing or optimizing of the pulse rate (volume of nitrogen injected vs. time) of the nitrogen gas pressure waves to the impingement hoods and as a result the pulsed rate of impingement jets. 
         [0021]    Referring in particular to  FIGS. 1-3 , a freezer apparatus embodiment of the present invention is shown generally at  10  which includes a housing  12  with sidewalls  14 , a top  16 , and a bottom  18 , all of which define an interior chamber  20  or internal space of the housing. The housing  12  includes an inlet  22  at one end thereof, and an outlet  24  at another end thereof, the inlet and outlet being in fluid communication with the internal space  20 . A conveyor belt  26  is arranged for movement from the inlet  22  through the internal space  20  and to exit out the outlet  24 . The conveyor belt  26  can be of the type used with cryogen freezer tunnels, such as stainless steel mesh-type belt. A plurality of access holes  28  are arranged in the top  16  of the housing  12  for a purpose to be described hereinafter. 
         [0022]    As shown in  FIG. 1 , at least one and for certain applications a plurality of pulsed flow apparatus embodiments (pulse apparatus) are shown generally at  30 . Each one of the pulse apparatus  30  includes an impingement hood  32  of a rectangular, circular or any other cross-sectional shape, which defines a sub-chamber  34  with a sub-atmosphere within confines of the hood. The impingement hood  32  includes an upper end with an upper opening  36  and a lower end with a lower opening  38 . 
         [0023]    The upper opening  36  is sized and shaped to receive a shaft  40  for a blower  42  disposed in the sub-chamber  34 . The shaft  40  is connected to a motor  44  mounted external to the housing  12  at, for example, the top  16  of the housing. The shaft  40  extends through one of the access holes  28  in the top  16  to be mechanically connected to the motor  44 . 
         [0024]    The upper opening  36  is also of a sufficient diameter to provide clearance between the shaft  40  and an edge of the upper opening so that gas flow  46  circulating in the internal space  20  can be drawn through the upper opening and thereafter into the sub-chamber  34 . 
         [0025]    The lower opening  38  has at is lower most edge a lip  48  circumscribing the lower opening upon which is supported at impingement plate  50 . The impingement plate  50  is formed with a plurality of holes  52  through which streams or impingements jets  54  are directed to the underlying conveyor belt  26 . The impingement plate  50  rests on the lip  48  to be supported in position above the underlying conveyor belt  26 . Each one of the pulse apparatus  30  includes a sidewall  31  having formed therein a port  33  or hole in fluid communication with a cryogen gas pipe  56  which similarly extends through an access hole  28  at the top  16  of the housing to be connected to a pipe manifold  58  external to the housing. A solenoid valve  60  is disposed in the cryogen gas pipe  56  downstream of the pipe manifold  58 . The pipe manifold  58  delivers cryogen gas under pressure, such as gaseous nitrogen, from a nitrogen gas storage tank  62  disposed at a remote location. 
         [0026]    Food product  64  is transported by the conveyor belt  26  from the inlet  22  through the internal space  20  to the outlet  24  for chilling and/or freezing, depending upon the type of food product being processed. The food product  64  can include, but is not limited to, hamburger patties, chicken breasts, shrimp, fish, bakery products or other individual quick frozen (IQF) products. 
         [0027]    Referring to  FIGS. 2-3 , an alternate embodiment of the pulse flow apparatus is shown generally at  70 . The pulse flow apparatus  70  includes many of the same components as the pulse apparatus  30 , except for the following. The embodiment  70  of the impingement hood  32  includes a plurality of the ports  33  or holes, only two of which are shown in  FIG. 2  due to the perspective shown in the view of this figure. The pipe manifold  58  is in fluid communication with the cryogen gas pipe  56  and the solenoid valve  60  is disposed to interconnect the manifold  58  and the pipe  56 . The cryogen gas pipe  56  is in fluid communication with a universal joint connection  72  which is branched into a plurality of distribution pipes  74 , each one of which extends through a corresponding one of the ports  33  in the sidewall  31  of the hood  32 , resulting in a plurality of pulses being introduced into the sub-chamber  34  as will be described below. 
         [0028]    In operation and referring to the embodiment of  FIG. 1 , the freezer  10  is cooled down to operating conditions (usually approximately −100° C.). The solenoid valve  60  is in the closed position. The main impingement blowers  42  inside the impingement hoods  32  are brought up to operating speed. Nitrogen gas from the internal space  20  is drawn into the hoods from upper opening  36  and through the blowers  42 . A back pressure is generated upstream of the impingement plates  50  and thus, an operating pressure inside the hoods  32  is maintained (2-3 inches of water column). The now established differential pressure across the impingement plates  50  allows for high velocity flow to be generated through the Impingement holes  52 . At this steady state operating condition, the pulsing effect can be introduced into the sub-chamber  34 . Accordingly, solenoid valve  60  is opened for a predetermined period of time (from approximately 0.5-2 seconds) to allow a volume of high pressure nitrogen gas (200 psig) from tank  62  to enter the impingement hood  32  sub-chamber  34  as a pressure pulse  66  or a wave. The pressure inside the impingement hood  32  is immediately increased which results in an increased impingement jet velocity through the holes  52  of the impingement plate  50 . The pressure pulse  66  serves two purposes: i) it clears the plate  50  and the holes  52  of frozen concentrate, snow and/or ice and ii) it increases the velocity of the impingement jets  54  impacting the food product  64  to increase heat transfer at the food product. The solenoid valve  60  is then closed, resulting in a rapid drop of pressure within the impingement hood  32  at the sub-chamber  34 . The resulting impingement jet velocity decreases. The pressure pulse  66  process by opening and closing the solenoid value  60  continues with the net result being rapidly changing impingement jet velocities (ie, pulses) discharged from the impingement plate  50  through the holes  52  onto the surface of the food product  64 . A pulse rate of the wave must is adjusted so that the impingement jets  54  are never fully developed to be in an unwanted laminar flow. The pulsing action results in increased convective turbulence at the food product  64  surface which results in increased convective heat transfer, and clears the plate  50  and the holes  52  of any frozen condensate, ice and snow. 
         [0029]    In operation and referring to  FIGS. 2-3 , the alternate embodiment  70  includes a plurality of distribution pipes  74  connected to a universal joint connection  72  so that the nitrogen gas can be evenly distributed during introduction of same into the sub-chamber  34  of the impingement hood  32 . With the pulse flow apparatus  70 , pressure pulses  76  or waves are introduced from a plurality of the distribution pipes  74  and therefore from a plurality of different directions for contacting the impingement plate  50 . The pulses  76  can be introduced uniformly and simultaneously into the sub-chamber  34 . The pressure pulses  76  contact a greater amount of the surface area of the plate  50  in a more uniform manner than the embodiment of  FIG. 1  for dislodging and removing any snow or ice which may have accumulated in the holes  52  of the impingement plate  50 . The plurality of pulses  76  can be of uniform pressure which results in an even amount of pressure being exerted on the plate  50 . In addition, the jets exiting the holes  52  onto the underlying food product  64  are more uniform in intensity and distribution to the food product for increased heat transfer at the product. 
         [0030]    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.