Note: Descriptions are shown in the official language in which they were submitted.
2 ~ a~
. CRYOGENIC FREEZING SYSTEM
The present invention relates to systems for
freezing articles in a freezer using a circulation
system of air at cryogenic temperatures. It is particu-
larly concerned with a more efficient circulating air
refrigeration system for large scale food freezing
applications.
BACKGROUND O~ THE INVENTION
Refrigeration systems em;ploying air at cxyogenic
temperatures for freezing food are commercially available,
for example, see U.S. Patent Nos. 3,733,848 and 3,868,827.
In the latter patent, air is compressed in a first
8tage compressor, cooled in an intercooler, further
compressed in a ~econd stage compressor, cooled in
another intercooler, further cooled by countercurrPnt
exchange with cold air leaving the freezer and finally
expanded in an expansion turbine mechanically coupled
to the second stage compressor where the gas is reduced
to about -180F before being directed into the ~ree~er.
This prior art freezer has been used in combination
with a vortex separator for removing particles of ice
in excess of 5 microns in diameter from the air leaving
~he food free~er. However, despite this separation ice
has been found to build up in the regenerative heat
exchanger and to resul-t in an intolerable pressure drop
across the main heat exchanger which greatly reduces
the efficiency of this system.
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SI~MMARY OF THE INVENTION
_
To overcome the disadvantages of the prior art
cryogenic freezers, the present novel system affords
greater efficiency by replacing the vortex separator
with a pulse bag filter to maintain the temperature
of the air leaving the freeze~ at temperatures of
below -80F. Additional efficiency is achieved by
supplying dry air to maintain an air curtain at the
inlet and outlet o:E the freezer to prevent entry of
wa~m, moist atmospheric air into the freezer and sub-
sequent re~rigeration loss. Finally, air is introduced
into the bottom of the freezer adjacent to the freezer
inlet to promote more rapid freezing of the food articles,
thus limiting the dehydration of the food products and
the ice formed therefrom.
In accordance with one embodiment of the present
invention, but not restricted thereto, the novel system
which is used for refrigeration, particularly in the
rapid freezing of food producl;s comprises a freezer
having an inlet for admitting the articles to be frozen,
an outlet for permitting the frozen articles to leave
'` said freezer and a conveyor fc)x transporting the articles
thxough the freezer rom the inlet to the outlet thereof;
a refrigerant supply main connected to the freezer for
introducing refrigerant air to the freezer; a return
main connected to the free2er for receiving the warmed
air from the freezer; refrige.ration means connected to
the refrigerant supply main for supplying the supply
main with air at cryogenic temperatures; a main heat
exchanger having a high pressure side connected to the
ref~igeration means and a low pressure side connected
betwe~n the refrigeration means and the return main for
exchanging the refrigerant value of the warmed air from
the return main in the low pressure side with the aix
from the refrigeration means in the high pressure side;
and bag filter means connected between the return main
and the main exchanger for removing ice particles from
the ~armed air in said xetu.rn main prior to exchanging
_ _. _ _ _ . _ . . , .. . . , .,,, .. . , _ _
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its refrigeration value in the main heat exchanger.
The filter means comprises at least one bag which is
periodically pulsed to remove ice collected on the
outside thereof through the pulsing action of a portion
S of the air leaving the high pressure side of the main
heat exchanger prior to entering an expansion portion
of the refrigeration means.
The refrigeration means comprises three stages of
compre6sion and one expansion turbine. The air in the
system is progressively compressed by the three compressor
stages, is cooled in the main heat exchanger and is
expanded in the expander of the refrigeration means
before being introduced into the freezer.
The air that is withdrawn from the freezer is
passed through the bags of the pulse bag filter to
remove ice particles in excess of approximately 1
micron in diameter, is warmed in the main heat exchanger
and is recycled through the reErigeration means.
The present apparatus and method thereof enable
one to recover air from the ex:it of the freezer at
temperatures colder than -80F.
BRIEF DESCRIPTION OF THE DRAWING
. . .
FIG. 1 is a schematic flow diagram illustrating a
preferred embodiment o the present invention; and
FIG. 2 is a more detailed flow diagram illustrating
the preferred embodiment of the present invention.
DESCRIPTION OE' PREFERRED EMBODIMENT
Referring now to the FIGURES, FIG. 1 illustrates
the combination of freezer 1, refrigeration unit 2,
main heat exchanger 3, make-up air system 4 and filter
5 comprising bag 6 and rotary valve 7. The details of
the operation of ilter 5 are described below.
In FIG. 2, reezer 1 is shown having feed station
8, discharge station 9 and continuous belt conveyor 10
for transporting the articles to be frozen through the
Il ~1 6~26~
freezer. One suitable freezer for use in combination
with the present invention is described in pending can-
Pat. ~ppln. No. 386,873 filed September 29, 1981, assigned
to Air Products and Chemicals, Inc., the assignee of
the present applicati~n.
Air at approximately 1 atmosphere and -205F is
introduced from refrigerant supply main 12 through line
13 into freezer 1. The flow through line 13 is controlled
by temperature indicating controller 15 which controls
valve 17. Air at approximately ~100F exits freezer 1
through line 20 connected to return main 22. The flow
of the air from freezer 1 is controlled through valve
24 by means of pressure indicating controller 26.
Air in return main 22 is passed to bag filter 5
via line 28 where the particles of ice formed from the
food products being frozen are separated from the air
and is designed to extract 99.9% of all the particles
having a diameter of 1 micron or greater through valve
7. It is understood that for particles that are not
substantially spherical, part:icles having their maximum
dimension equal to 1 micron or greater are removed from
the air in filter 5.
The substantially ice-frt~e air from filter 5 is
then passed through line 30 to main heat exchanger 3.
Specifically the air in line 30 passes through low
pressure side ~1 of main heat exchanger 3 where it is
heated to about 95F at a pressure of about 10.7 psia.
Air from main heat exchanger 3 flows through line 34 to
refrigeration unit 2. Although the apparatus of this
invention comprises the use of any combination of 3
stages of compression and an expander, one ~orm of
refrigeration unit 2 that is preferrably used in this
embodiment illustrated in FIG. 2 is commercially available
under the designation of TA-100 3-Sta~e Centrifugal
Compressor from Joy Manufacturing Company, which compres-
sor has an expansion turbine mounted on the open end of
its second post thereof. This refrigeration unit
1 1 6:~6~
., 5
comprises motor 38 which rotates gear 40 which in turn
rotates first post 42 and second post 43. First compres
~or stage 46 is mounted on one end of first post 42 and
second compressor stage 47 is mo~mted on the other end
thereof. Third compressor stag~ 49 is mounted on one
end o~ second post 43 and expander 50 is mounted on the
other end thereof. Air is compressed in first compressor
stage 46, passed through line 51, cooled in intercooler
52 and compressed in second compressor stage 47. The
air is then passed throllgh line 53, cooled in intercooler
54 1 and compressed in third compressor stage 49 to 78
psia. The air is passed through line 55 and cooled to
about 100F in aftercooler 56. The compressed air from
thè compressor stage of refrigeration unit 2 is passed
through line 57 and cooled to about -95F in the high
pressure side 58 of main heat exchanger 3. The major
portion of the cold air from high pressure side 58 is
passed through line 59 and expanded in expander 50
before being passed into refr:igerant supply main 12 via
line 60. From main 12, the refrigerant air is introduced
into freezer 1 at a temperatu:re of about -205F and 15
psia via line 13.
The flow of air through main heat exchanger 3 is
equalized by means of valve 62 and pressure indicating
~5 controller 63.
Referring to FIG. 1, air in line 28 enters filter
5 through inlet 64 and is deflected by deflector 65.
The largest particles of ice fall directly into the
conical bottom or hopper of filter 5 for removal through
solids discharge spout 66 and rotary valve 7. The air
stream 67 flows upward through at least one bag 6
mounted on clamps 68 and supported by cage 69. The ice
particles remaining in the air having a diameter or
largest dimension of at least 1 micron are collected on
the outside of bag 6. The ice-free air passes through
outlet 70 and through line 3Q to main heat exchanger 3.
Periodically bag 6 is pulsed by means of a small side
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stre~n, approximately 1 to 5% by weight, of high pressure
air (about 75 psig) which is directed through line 71
to a position directly above venturi nozzle 72. The
pulse of air stops the flow of substantially ice-free
air, i.e. the air contains no more than 0.1% by weight
of ice after filtration, and the pulse causes a shock
wave to travel down bag 6. This wave forces bag 6 to
momentarily depart from wire cage 69 to position 73
(shown in phantom), to snap back in place and to dislodge
the ice built up on the outside of bag 6 into the
hoppar of filter 5. The composition of the bag can be
of any suitable material, for example, a polyester felt
bag coated with Teflon~ polymer. A suitable bag filter
for this embodiment is commercially available as P-1-
120 Pulse Dust Collector, which is a single width unitcontaini.ng 1~0 bags; see Bulletin AP-750 entitled
"Buffalo AEROTURN~ Pulse Dust Collector Type P" from
Buffalo Forge Company, Buffalo, New York, September
1977 for further details of this device.
The temperature of the cold air leaving high
pressure side 58 is about -95F or within a few degrees
of the air entering filter 5 ~rom line 28. The latter
is combined with the small side stream used in the
pulsing action described above to form a stream at
about -100F entering low pressure side 31 o heat
exchanger 3. All of the exterior surfaces of filter 5
are provided with suitable insulation 74 to prevent
` heat loss of this stream in filter 5.
Referring again to FIG. 2, ingress of moist air
into freezer 1 is inhibited and heat loss is prevented
by àir curtains 76 and 77 which are positioned above
inlet 8 and outlet 9, respectively. One acceptable
version of an air curtain is commercially available
a5 Transvector~ Air Flow Amplifier. Alternatively, a
venturi system powered by the small compressed air flow
from make-up air system 4 through lines 78 and 79 can
be utilized for this purpose.
7 1 6~2~
Dry make up air for the cryogenic refrigeration
system and for the air curtains is provided by air in
line 80, compressed in compressor 81 and dried in drier
82 containing a suitable dessicant such as alumina or
5A molecular sieves. Th~ dried compressed air is
combined via line 84 with air in line 57 to high pressure
side 70 of main heat exchanger 3.
It is obvious from FIG. 2 that supply main 12 and
return main 22 extend in both directions such that
additional freezers beyond the single freezer 1 shown
may be used provided that sufficient refrigeration
capacity is available from unit 2. Alternatively,
additional refrigeration units identical to unit 2 may
be connected to the supply and return mains.
In order for refrigeration unit 2 to operate
efficiently, the compressed air leaving aftercooler 56
must be cooled as effectively as possible in main heat
exchanger 3. This is achieved by inhibiting the build
up of ice in main heat exchangex 3 by: (1) the provision
of air curtains 76 and 77, (2) hag filter 5 and (3)
maintaining the air leaving freezer 1 colder than
-80F. Most of the ice is like:Ly to form on high
pressure side 58 of main heat exchanger 3. Using the
process of the present invention the rate of icing is
reduced by a factor of roughly 5, if the temperature of
air leaviny freezer 1 is -lOO~F, when compared with the
rate of icing in cryogenic freezers of the t~pe described
above in connection with a discussion of the prior art.
