Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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MODULAR APPARATUS FOR COOLING AND FREEZING
OF A FOOD PRODUCT ON A MOVING SUBSTRATE
BACKGROUND OF THE INVENTION
This invention relates to an improved method and device for cooling
and freezing a food or other .item which is carried through the device on a
belt or other moving substrate. 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 maximized through
the use of a process in which liquid cryogen is sprayed into 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 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.
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Traditional freezers have employed blowers that generate currents of
cooling vapor in many directions so that a significant portion of the vapor
does not contact the food product in a perpendicular direction. Under these
conditions the vapor which does contact the food product often does not
possess sufficient energy to substantially reduce the boundary layer around
the surface of the food product. There is, therefore, a need to generate
directed jets of cooling vapor so as to disturb the boundary layer.
United States Patent No. 4,479,776 to Smith discloses an apparatus
using a plurality of vertical tubes to provide a unidirectional air flow
toward
the food product.
United States Patent No. 4,626,661 to Henke 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.
United States Patent No. 5,487,908 to Appolonia et al. 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
2o of the width of the moving substrate converts multi-directional flow into
unidirectional flow. Such a device suffers, however, from having such an
increased rate of flow that the food products 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 vapor 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
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the food product or to carry the food product off the conveyor and into
undesirable locations elsewhere in the freezer.
Overall heat transfer rates are dependent on local heat transfer
coefficients, i.e., 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.
1o 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.
SUMMARY OF THE INVENTION
Accordingly, the present invention 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 present 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.
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Furthermore, the present 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 present invention is to provide a modular
design which can be adapted to provide a plurality of solution to food
processing requirements.
A further advantage of the present invention is that the connection
of the modules provides for continual impingement of cryogenic gas on
food items from their entrance into the freezer apparatus until exiting.
1o An additional advantage of the present invention is the reduction in
the dehydration of the food items which is accomplished through the
immediate freezing of the exterior of the product upon entry into the
apparatus.
A still further advantage of the present invention is the consistent
cooling and or freezing of items across the width of the belt upon which
the food items travel.
In one embodiment of the present invention a modular food chilling
and/or freezing apparatus is provided 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.
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In an embodiment of the present invention the interface between
modules includes a baffle for controlling the pressure differential and
transfer of cryogen between modules.
In a further embodiment a pneumatically actuated ball valve vibrator
is used to remove the build-up of snow and ice from impingement plates.
In a still further embodiment of the present invention a hydraulic
system is used to provide easy access to the interior of the apparatus.
It is to be understood that both the foregoing general description and
the following detailed description are exemplary, and are intended to
provide further explanation of the invention as claimed.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings are included to provide a further
understanding of the invention and are incorporated in and constitute a part
of this specification. The drawings illustrate embodiments of the invention
and, together with the description, serve to explain the principles of the
invention but are not intended to limit the invention as encompassed by the
claims forming part of the application.
Figure 1 is a longitudinal cross-sectional representation of a freezer
according to the present 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.
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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 present 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
present invention.
Figure 8 is a cross-sectional view of a portion of the impingement
plate of Figures 6 and 7 through line 6-6.
1o Figure 9 is a cross-sectional view of channel impingement device for
use in a freezer according to the present invention.
Figure 10 is a cross-sectional view of an additional embodiment of an
intermediate module of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The present 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 into 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 vapor on
the food item. Additional heat transfer may also be achieved by spraying
liquid or solid cryogen into the impingement jet streams of cryogenic vapor.
With reference to Figure 1 entrance module 10, intermediate
modules 20 and 30 and exit module 40 define one embodiment of the
freezer of the present invention. The modularity of the present invention
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enables a set of modules 10, 20, 30 and 40 to be arranged to meet
specific freezing requirements for various food types. Entrance module 10
has liquid cryogen piping 24 and sprayer 25 which enable a stream of liquid
cryogen to be sprayed into the jet of gaseous cryogen circulating within the
module 10 and onto belt 16 and food items (not shown). Sprayer 25
should preferably comprise a plurality of full cone low flow rate spray
nozzles. The use of 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.
Impeller 32 of module 10 is in fluid communication with intake cone
34 and generates a flow of cryogen out exit 33 by which it circulates a
flow of cryogenic vapor around the interior of the module 10 in accordance
with the flow patterns represented by arrows in Figure 2. The cryogenic
vapor flows from exit 33 through impinger 17 past sprayer 25 entraining
liquid cryogen into the stream and then impinging on food items on belt 16.
A high pressure flow of cryogen enters high pressure plenums 14 and an
impinger 17 (which comprises the top of high pressure plenum 14) and
2o flows under and through belt 16 which provides impingement jets on the
underside of the food items on belt 16. Belt 16 is a standard woven
stainless steel belt typically used in food freezers. The flow of cryogenic
vapor is returned to the intake cone 34 of impeller 32 through the low
pressure plenum 15. Impeller 32 can be a 762mm diameter centrifugal fan
operating at 283 cubic meters per minute at .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 sprayer 25 and to
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provide a source of cryogenic vapor for circulation within each freezer
module.
Entrance module 10 is comprised of a top, bottom and four walls. In
Figure 1 wall 6 of module 10 is configured so as to include inlet 11.
Bottom plate 5 and top plate 8 provide a top and bottom to entrance
module 10 as well as the other modules 20, 30, and 40. At the right hand
side of entrance module 10 is a divider baffle 7 on the high pressure side of
impinger 17 which can be open and closed so as to control the amount of
cryogenic vapor passing from one module to another as well as to control
l0 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 entrance module 10 are depicted in
Fig. 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.
Intermediate modules 20 and 30 are similar to module 10 in
construction in that they contain a portion of belt 16 which conveys food
items through the module. Intermediate module 20 is depicted in cross-
section whereas intermediate module 30 is depicted so as to show the
exterior doors 18 which can be raised for internal axis by use of
counterweight 19. In intermediate module 20 or 30 an impeller 32 powered
by motor 22 circulates cryogenic vapor according to the arrows shown in
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Figure 3 which is a cross-section of intermediate module 20 taken through
line B-B of Figure 1. Bottom plate 5 and top plate 8 provide a top and
bottom to the module. Top plate 8 is also where impeller 32 is mounted
and connected to the drive shaft of motor 22. As in the other modules
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 belt 16. Food items pass from one module through an opening on
the low-pressure side of impingers 17 which permits the belt 16 to pass
continuously from one module to another.
1o A series of intermediate modules 20 and 30 can be used to provide
for 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.
Exit module 40 is similar to the other modules with the exception of
the placement of an outlet 42 in side wall 6 and a series of rollers 43 for
return of belt 16 through the modules. 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 present
2o invention, the freezer exhaust is divided into three sections. These
separate sections are a central exhaust. The primary exhaust will remove
80% of the total cryogenic gas from the low pressure exit end of the
freezer. The secondary exhausts each capture 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. Further, smaller
diameter exhaust ducts and smaller exhaust blowers can be employed
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while reducing make-up air usage. The entrance end of exit module 40 is
similar to the entrance end of intermediate modules 20 and 30 in that a
module divider baffle 7 provides for control of the transfer of cryogen
between modules. Likewise 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.
A important feature of the present invention is the design of the
impinger 17 which 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 4-5%. In
the preferred embodiment of Figure 5 the axial pitch 51 and lateral pitch 52
are both 1 7/8 inches when the hole diameter 54 is %2 inch. Also, offset or
stagger 53 of the center of the holes should be approximately 5/8 inch in
the preferred embodiment. The reason for the offset or stagger is that is
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 impinger 17 is fabricated from 22 gauge
steel in the preferred embodiment. The holes in plate version of impinger
17 are radiused at 15%.
Figures 6 and 7 show the rounded edges of the holes in impinger 17.
Figure 8 depicts a cross-sectional view of 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.
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Figure 4 depicts an impinger 17 of the plate type as described above.
Each impinger 17 needs to have a vibrator 48 which vibrates impinger 17
which is preferably free-floating in a rigid frame. Vibrator 48 may be of the
electrical variety, however, in the preferred embodiment vibrator 48 is a
ball valve pneumatically actuated by compressed nitrogen gas supplied
through conduits 49 at or about 60 psi from either an external source or
from a vaporizer and compressor (not shown) internal to the apparatus.
The frequency and time intervals at which vibrator 48 is used to vibrate
impingers 17 is dependent on process conditions including the moisture
1o 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 the
preferred embodiment 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 5/8 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
impinger 17 effects 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) maximizes the overall heat transfer rate. Typical
freezing temperatures for an apparatus according to present invention are -
120C in entrance module 10 and -50C at exit module 40. In the referred
embodiment four modules are employed each being 3.048 meters long,
1.753 meters wide and 3.150 meters tall. The belt width in the preferred
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embodiment is .712 meter. Modules of various sizes may be constructed
according to the present 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 present invention. An intermediate module 50 is shown
1o which is comprised of two portions. Top portion 68 and bottom portion 66
together create an enclosure. Motor 22 is mounted through top portion 68
where it connects with and drives impeller 32. Belt 16, impingers 17,
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 present invention is preferably operated with liquid nitrogen,
however, other cryogens may also be employed such as synthetic liquid air
(SLA) and carbon dioxide.
2o In the standard operation of the present invention liquid cryogen is
sprayed into 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
vaporize. The vaporized 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 impinger 17 thereby creating
the impingement jets. The impingement jets then continue to entrain
additional liquid cryogen which is sprayed from sprayer 25. All of the
vaporized cryogen will pass from the entrance module to the intermediate
or exit module through the low pressure side of the impingers, i.e., through
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the hole which permits the belt 16 and food items to pass from one module
to another. The majority of the vaporized cryogen, however, is passed from
one module to another through the divider baffle 7 which enables the use
to control the amount passed.
While various embodiments of the present invention have been
described in detail, it is apparent that further modifications and adaptations
of the invention will occur to those skilled in the art. However, it is to be
expressly understood that such modifications and adaptations are within
the spirit and scope of the present invention.
