U.S. patent number 3,892,104 [Application Number 05/399,769] was granted by the patent office on 1975-07-01 for cryogenic freezer with variable speed gas control system.
Invention is credited to Roger A. Howells, David J. Klee.
United States Patent |
3,892,104 |
Klee , et al. |
July 1, 1975 |
Cryogenic freezer with variable speed gas control system
Abstract
A cryogenic tunnel-type freezer is disclosed comprising a
cryogenic liquid spray zone, a plurality of gaseous precooling
zones having non-enclosed recirculation fans, and at least one
enclosed, variable speed gas control blower for producing and
varying the net forward flow of cold gas from the liquid spray
zone, in which the gas is generated, toward the inlet end of the
freezer through each of the precooling zones in countercurrent heat
exchange with the product to be cooled and/or frozen; the speed of
the gas control blower being directly responsive to changes in the
amount of cryogenic liquid being supplied to the spray zone.
Inventors: |
Klee; David J. (Emmaus, PA),
Howells; Roger A. (Orefield, PA) |
Family
ID: |
23580886 |
Appl.
No.: |
05/399,769 |
Filed: |
September 20, 1973 |
Current U.S.
Class: |
62/186; 62/216;
62/380; 62/374 |
Current CPC
Class: |
F25D
3/11 (20130101) |
Current International
Class: |
F25D
3/11 (20060101); F25D 3/10 (20060101); F25d
017/04 () |
Field of
Search: |
;62/177,178,179,180,186,216,222,373,374,375,376,380,63 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
2904254 |
September 1959 |
Bahnson et al. |
|
Primary Examiner: O'Dea; William F.
Assistant Examiner: Capossela; Ronald C.
Attorney, Agent or Firm: Sherer; Ronald B. Moyerman;
Barry
Claims
What is claimed is:
1. A cryogenic freezer for freezing a product comprising an
elongated insulated tunnel including a conveyor belt, a cryogenic
liquid injection system and a plurality of gaseous cooling zones
through which the product is passed in heat exchange relationship
with the gas resulting from vaporization of the injected cyrogenic
liquid, each of said plurality of cooling zones including
recirculation fan means for recirculating said gas in contact with
the product, gas control fan means for controlling the net forward
flow of gas through said cooling zones, variable speed motor means
for driving said gas control fan means at various speeds, and a
control system operatively connected to said variable speed motor
means and to said cryogenic liquid injection system for
automatically varying the speed of said gas control fan means as a
function of the amount of cryogenic liquid being injected into said
freezer.
2. The cryogenic freezer as claimed in claim 1 wherein said gas
control fan means is located along the length of the tunnel at a
position at which the temperature of the gas in the tunnel is
colder than minus 50.degree.F.
3. The cryogenic freezer as claimed in claim 1 further including
manual electrical control means for manually varying the speed of
said gas control fan means, and mode selector switch means for
operating said control system in either an automatic or manual
mode.
4. The cryogenic freezer as claimed in claim 1 wherein said
cryogenic liquid injection system includes injection means for
injecting cryogenic liquid into at least one of said gaseous
cooling zones for vaporization in contact with the gas
recirculating therein.
5. A cryogenic freezer as claimed in claim 1 including an
intermediate drive unit connected between said variable speed motor
means and said gas control fan means.
6. The cryogenic freezer as claimed in claim 1 wherein said control
system includes pressure sensing means connected to said cryogenic
liquid injection system, and transducer means for converting the
sensed pressure of the cryogenic liquid into an electrical control
signal.
7. The cryogenic feeezer as claimed in claim 6 further including
pressure smoothing means connected between said cryogenic liquid
injection system and said transducer means for suppressing spurious
pressure fluctuations.
8. The cryogenic freezer as claimed in claim 1 wherein said gas
control fan means comprise a blower mounted inside said insulated
tunnel including an outlet positioned so as to discharge the gas
from said blower in a direction toward the product inlet end of the
freezer.
9. The cryogenic freezer as claimed in claim 8 further including a
discharge duct connected to said blower outlet for directing the
gas toward the product inlet end of the freezer.
10. The cryogenic freezer as claimed in claim 1 wherein said
variable speed motor comprises a D.C. motor, and said control
system includes a D.C. power source having a variable D.C. output
connected to said variable speed D.C. motor.
11. The cryogenic freezer as claimed in claim 10 including a
maximum speed limiting potentiometer connected to said D.C. power
supply and having at least three turns for accurately setting the
maximum RPM of the variable speed motor.
12. The cryogenic freezer as claimed in claim 1 wherein said
cryogenic liquid injection system includes spray header means for
directly contacting the product with cryogenic liquid and thereby
vaporizing the liquid to a gas, and wherein said plurality of
gaseous cooling zones comprise precooling zones positioned between
the product inlet end of the tunnel and said spray header
means.
13. The cryogenic freezer as claimed in claim 12 wherein said gas
control fan means is located between two of said gaseous precooling
zones.
Description
BACKGROUND OF THE INVENTION
May previous cryogenic freezers have been designed for cooling
and/or freezing articles such as food products with liquid nitrogen
(LIN), or with other cryogenic liquids such as liquid CO.sub.2,
liquid air, and various inert halogenated hydrocarbons. In such
freezers, the cryogenic liquid is contacted with the product in a
liquid bath or spray zone wherein the liquid is vaporized into its
gaseous state, and the resulting gas is then passed through a
plurality of precooling zones in countercurrent heat exchange with
the product to cool and/or partially freeze the product prior to
the product being contacted with the cryogenic liquid. However,
there has long been a substantial problem in accurately controlling
the amount and velocity of the net gas flow through the tunnel
toward the product inlet end while, at the same time, not
permitting ambient air to enter the open ends of the freezer, nor
permitting excessive amounts of the very cold gas to escape from
the product discharge end of the freezer; all of which conditions
are critical in order to achieve high thermal efficiency of the
freezer. This critical problem of controlling the gas flow so as to
continuously maintain the above-indicated conditions is further
complicated by the fact that the amount of gas generated per unit
time in the spray zone varies over an extremely wide range. That
is, the pounds of gas generated per hour in the spray zone varies
widely as a result of many factors such as, for example, changes in
the type of product, changes in the initial temperature of the
product, changes in the desired final temperature of the product,
and frequent variations in the production rate including such wide
variations as between a maximum production rate and a zero
production rate whenever the freezer is suddenly put into a
temporary idling condition for any one of many reasons.
Many prior attempts have been made to solve this gas control
problem such as by the use of gas control baffles or dampers as
disclosed in U.S. Pat. Nos. 3,345,828; 3,403,527; 3,600,901 and in
co-pending application Ser. No. 292,869 now U.S. Pat. No.
3,813,895. However, all of these damper and baffle type systems for
controlling the gas flow have required relatively complex
electro-mechanical systems, and they have not been highly accurate
primarily because the amount of gas flow through a variable
position baffle or damper is not a linear function of the change in
the baffle or damper position throughout the range of movement
thereof.
SUMMARY OF THE INVENTION
The present invention provides a simplified and substantially more
accurate gas control system for precisely controlling the forward
net gas flow toward the product inlet end of the freezer so as to
automatically vary the net gas flow in a linear manner in direct
response to any change in the amount of cryogenic liquid being
introduced into the spray zone which, of course, is directly
related to any changes in initial or final temperature of the
product, the type of product, and the production rate. Briefly
summarized, the present gas control system includes a gas control
blower driven by a variable speed D.C. motor the speed of which is
varied by an electrical control circuit which is directly
responsive to the pressure of the cryogenic liquid in the spray
header system; the pressure in the spray header system being a
direct and linear function of the amount of cryogenic liquid being
supplied as will be more fully described hereinafter.
It is therefore a principal object of the present invention to
provide a cryogenic freezer having a simplified and substantially
improved control system for more accurately controlling the net gas
flow through the precooling zones of the freezer in heat exchange
with the product.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side elevational view having portions cut away to more
clearly illustrate some of the internal components;
FIG. 2 is an enlarged sectional view taken along view lines 2--2 of
FIG. 1, including an illustration in phantom line showing the
movable top and bottom sections of the tunnel in their open
position;
FIG. 3 is a fragmentary perspective view of the interior of one of
the stationary tunnel sections illustrating the details of the
conveyor belt supports which are mounted solely on the stationary
sections;
FIG. 4 is a simplified cross-sectional view of several zones of the
freezer including the variable speed gas control blower and the
associated electrical circuitry whereby the speed of the gas
control blower is varied in direct and linear response to changes
in the pressure of the cryogenic liquid in the spray header
system.
DETAILED DESCRIPTION
While the gas control system of the present invention may be
employed in freezers of widely varying design, the gas control
system will be described in connection with the particular freezer
design comprising the subject matter of copending application Ser.
No. 292,869 now U.S. Pat. No. 3,813,895 . Referring to FIGS. 1 and
2, the overall freezer is generally designated by numeral 10 and
includes a frame 12 having legs 13, longitudinally extending box
beams 14, 15 and lateral supports 16 extending between the box
beams 14, 15.
The frame 12 supports an elongated tunnel 17 defined by a series of
alternate movable sections 18 and fixed stationary sections 19 the
latter of which are of short length to facilitate cleaning. The
stationary sections 19 are each supported by a pair of vertical
support posts 20, 21 which extend upwardly from the box beams 14,
15, respectively. The bottom portions 49 of the movable sections 18
are supported by angle frame assemblies 22 which extend beneath the
movable bottom portions 49 and are, in turn, supported by lifting
screws 23 of conventional jacks 24 which are mounted on the lateral
supports 16. Each angle frame assembly 22 may extend beneath from
two to five movable bottom portions and cooperates with four of the
lifting screws 23 and jacks 24 to vertically move the bottom
portions of the movable sections 18 so as to enable thorough
cleaning and inspection.
The freezer 10 also includes a mesh conveyor belt 25 which extends
through the tunnel 17 and has upper and lower reaches 26, 27,
respectively. The belt 25 extends over pulleys 28, 29, 30 at the
inlet end of the tunnel 17 (left side as viewed in FIG. 1), and
over a pulley 31 at the outlet end of the tunnel 17 (right side as
viewed in FIG. 1). The belt 25 is driven by means of a motor 32
through a drive chain and sprocket arrangement (not shown). Food or
like articles intended to be conveyed through the tunnel 17 for
processing are placed on the conveyor belt 25 at a loading table
portion thereof 33.
A cryogenic fluid such as liquid nitrogen is admitted to the
interior of the tunnel 17 through a supply line 34 which
communicates with a spray header 35 mounted in a stationary section
19 disposed near the exit end of the tunnel 17. The liquid nitrogen
may be emitted through spray nozzles 36 directly on the articles
moving through the tunnel 17 whereupon it will vaporize and become
a gas. The gas is forced to move in a plurality of recirculating
paths by the effect of radial fans 37 which are mounted in the
stationary sections 19. The structure and unique operation of the
radial flow fans 37 are more fully explained in the above-noted
U.S. Pat. No. 3,403,527 the complete specification of which is
hereby incorporated by reference. However, the present invention
may be used in conjunction with any type of recirculation fans
including, for example, conventional axial flow fans. The fans 37
are driven by fixed speed A.C. motors 38 also mounted on the
stationary sections 19. Transverse zone baffles 39 depend
downwardly from the upper portions of the movable sections 18 on
each side of the fans 37 to define cooling zones therebetween while
permitting some of the gas to pass therebelow and flow into the
adjacent upstream zone in cascading fashion. Anti-swirl vanes 40
may be provided for inhibiting swirling action of the gas around
the fans 37. Rather than having the gas spill into the room the gas
is preferably removed from the tunnel 17 by means of an exhaust
duct 41, mounted in the first stationary section 19, and a remote
blower 42 which has little or no effect on the actual movement of
the gas through the tunnel 17. Thus, the blower 41 is primarily
used for safety reasons, e.g., to preclude the build-up of non-life
supporting nitrogen gas in the room in which the freezer and
operating personnel are located.
Referring now particularly to FIG. 2, there are illustrated details
of a movable section 18. Each movable section 18 includes a pair of
hinged top covers 43, 44 having upper portions 45 and depending
side portions 46. The side portions 46 terminate in inclined
surfaces 47 which sealingly mesh with correspondingly inclined
surfaces 48 of a bottom portion 49 in the closed condition of the
movable section 18. The bottom portion 49 is supported by and
fastened to the angle frame assembly 22 whereby it may be moved
downwardly away from the top covers 43, 44 by means of the jacks 24
as seen in phantom in FIG. 2. A vertically disposed slide pin 50
extends downwardly from the angle frame assembly 22 and slidingly
cooperates with a slide bushing 51 carried in one of the lateral
supports 16 for preventing lateral movement of the bottom portion
49 during vertical movement thereof.
The jacks 24 are of conventional design and may be manually,
electrically or pneumatically operated. A typical model jack may be
the Duff-Norton Miniature Jactuator No. M-2555 or M-2625.
The top covers 43,44 are each carried by sleeves 52 which are keyed
to torsion bars 53 which are, in turn, mounted in flanged support
blocks 54 mounted on the stationary sections 19. The covers 43, 44
may be swung open approximately 80 degrees and counterbalanced in
the open position by torsion bars 53. The top covers 43, 44 further
include handles 55 which facilitate manual opening and closing
thereof. Toggle latch means (not shown) may be provided for
securing the top covers 43, 44 in the closed position.
As seen most clearly in FIGS. 2 and 3, the upper reach 26 of the
conveyor belt 25 is supported on plastic wear strips 57 mounted on
lateral support bars 58 which, in turn, are carried by
longitudinally extending side rails 59 supported by clips or
brackets 60 mounted in the interior side walls of the stationary
sections 19. The wear strips 57 can be formed of any suitable high
molecular weight material capable of withstanding the low
temperatures and providing the necessary anti-friction function.
For example, polytetrafluoroethylene (e.g., Teflon) can be
satisfactorily employed. The lower reach 27 of the conveyor belt 25
is, on the other hand, supported on stainless steel bars 61 mounted
on vertical support pins 62 extending upwardly through inclined
sloping surfaces 63 of the bottom of the stationary member 19.
From the foregoing description it will be apparent that the
structure thus far described permits the movable sections 18 to be
fully opened so as to gain complete access to all of the internal
portions of the tunnel, including the interior of the short
stationary sections 19, so as to enable thorough cleaning and full
visual inspection of the entire tunnel.
As further illustrated in FIG. 1, the gas control system more fully
described in co-pending application Ser. No. 292,869 utilized a
movable, arcuate baffle 64 to variably obstruct the flow of gas
from radial flow fan 37 toward the spray zone, and thereby, created
a net forward flow of the gas toward the inlet end of the freezer;
the forward net flow being variable by changing the position of the
pivoted baffle 64. While this type of control system has been
proven to be very effective in certain sizes of freezers such as,
for example, a freezer having a 4 foot wide conveyor belt and
having an overall length in the order of 36 feet, difficulties have
been encountered in using this type of movable baffle system for
freezers of substantially greater length and/or having a
substantially narrower cross section such as, for example, a
freezer having a 2 foot wide conveyor belt and having a length of
50 or more feet. Thus, the present invention provides an improved
gas control system which is very effective in all sizes of
freezers, and which is not subject to the previously mentioned
nonlinear characteristics of pivoted baffles and dampers. In
addition, the present gas control system responds very rapidly to
any change in the amount of cryogenic liquid supplied to the spray
zone, whereas, prior systems using temperature sensing devices
inherently result in a significant time lag between the change in
the amount of cryogenic liquid being supplied and the required gas
control correction.
As shown in FIG. 4, the present invention provides a separate
variable speed gas control blower 70 in addition to the radial flow
fans 37. This gas control blower 70 includes a scroll casing 71
which is secured to the internal surface of the upper portion of
one of the fixed sections 19. The scroll casing 71 encloses a
conventional centrifugal fan 72 driven by a shaft 73 extending
through the upper portion of the fixed tunnel section 19 and
connected to a variable speed D.C. motor 74 which is also mounted
on the same fixed tunnel section 19. It will also be apparent that
instead of shaft 73 being directly connected to motor 74, an
intermediate belt or gear drive unit could be provided such as to
operate the fan 72 at a different speed than that of the motor 74.
For example, the intermediate drive unit could provide either a
fixed or variable speed ratio by using a belt, chain or gear drive
unit.
In order to direct the cold nitrogen vapor forwardly toward the
product inlet end of the freezer, one of a pair of relatively short
horizontal partitions 75 are secured to the sides of each of the
cover portions 43-44 and, similarly, one of a pair of flow
deflectors 76 are mounted to the underneath sides of each of the
top portions 43-44. Thus, when the cover portions are in their
closed position, the pairs of partitions 75 and deflectors 76
combine to define a short duct extending the full width of the
freezer through which the cold gas is forced by centrifugal fan 72
so as to be discharged in a downwardly and forwardly directed
manner.
In the embodiment illustrated in FIG. 4, it will be noted that the
variable speed blower 70 is located between two of the radial flow
fan zones such that a portion of the gas recirculating in the
radial fan zone on the right side of the gas control blower 70 is
drawn into the center inlet of the gas control blower and
discharged therefrom toward the radial flow gas zone located on the
left side of the gas control blower so as to create the net forward
flow of gas through the non-directional radial flow fans zones
which recirculate the gas. Alternatively, it will be understood
that the gas control blower 70 may be located immediately adjacent
the spray zone, or in any one or more of the fixed tunnel sections
19 between the spray zone and the product inlet end of the freezer.
However, it has been determined that the optimum location of the
enclosed gas control blower or blowers should be at one or more
locations in the tunnel at which the temperature of the gas is
between minus 50 degrees and minus 70 degrees F. within which the
temperature range the gas will be cold enough so as not to create a
frost accumulation problem within the scroll casing or discharge
duct, while at the same time, not being so cold and therefore so
dense as to require substantially greater amounts of horsepower to
operate the blower within the preferred speed ranges. Of course, in
addition to the use of a plurality of gas control blowers 70 spaced
along the length of long tunnels, pairs of gas control blowers 70
may be utilized in the same fixed tunnel section. That is, two or
more gas control blowers 70 may be provided side by side as, for
example, in the case of a freezer having a conveyor belt width of 4
to 8 feet.
As previously stated, the speed of the gas control blower 70 is
precisely controlled as a linear function of the amount of liquid
nitrogen being supplied to the freezer through spray header system
75. In the preferred embodiment, this is accomplished by connecting
a small pressure sensing line 77 to the spray header system 35
downstream of the main liquid control valve 78 which is regulated
by an actuator 80. Valve actuator 80 is preferably controlled in a
completely automatic manner by the liquid nitrogen control system
as described in U.S. Pat. No. 3,613,386 the complete specification
of which is hereby incorporated by reference. However, regardless
of how liquid nitrogen valve 78 is controlled, including manual
operation or any automatic system, the pressure of the liquid
nitrogen in the spray header system downstream of liquid control
valve 78 within the range of 0 to 10 p.s.i.g. is a direct and
linear function of the amount of cryogenic liquid being supplied to
the freezer. Thus, whenever the flowrate of liquid nitrogen is
varied for any reason such as, for example, whenever the production
rate and/or the initial or final product temperature is changed,
the pressure of the liquid between fixed nozzles 36 and valve 78
accurately reflects any such change.
Pressure sensing line 77 is connected through a small, restricted
orifice device 82 which functions to smooth out and eliminate
spurious fluctuations in the pressure of the liquid nitrogen in the
spray header system. The pressure signal is then transmitted
through line 84 to a conventional transducer 86 which, solely for
purposes of example, is shown as comprising a pressure responsive
bellows 87 which actuates the movable contactor of a potentiometer
88. Thus, the sensed changes in the pressure of the liquid in the
spray header system vary the output voltage signal of transducer 86
which is connected by lines 99 to a conventional, D.C. power supply
100 preferably of the silicon controlled rectifier type. D.C. power
supply 100 receives an A.C. input voltage through lines 103 and
supplies a constant D.C. voltage through lines 101 to the motor
field, while supplying a variable D.C. voltage through lines 102 to
the motor armature so as to vary the speed of motor 74. As a
result, the speed of gas control blower 70 is continuously and
automatically varied so that its pumping capacity always matches
the precise amount of gas generated from the amount of liquid
supplied to the liquid nitrogen spray zone. Thus, the net forward
flow of gas through the freezer is always maintained equal to the
rate of gas being generated in the spray zone so that all of the
critical gas flow conditions previously described are always
maintained regardless of any of the above-indicated changes in the
operation of the freezer.
In addition to the completely automated control system just
described, a back-up manual control is provided in the form of a
manually variable potentiometer 104 which is connected by lines 106
having switches 108 to lines 99 having switches 110. Switches 108
and 110 are preferably of the relay type actuated by a solenoid 111
which is connected to a power source 112 through a mode selector
switch 113, and this relay circuit preferably includes an indicator
light 114. Thus, closure of mode selector switch 113 energizes the
solenoid 111 which closes switches 110 and opens switches 108 so
that the automatic control system is in operation, and this
automatic mode of operation is indicated by indicator light 114.
However, if mode selector switch 113 is opened, or if any
malfunction should occur in the relay circuit such that solenoid
111 becomes deenergized, switches 110 drop to their failsafe open
position, and switches 108 drop to their failsafe closed position,
whereby gas control blower 70 may continue to be varied by manual
operation of potentiometer 104 which then controls the speed of
motor 74, and hence the speed of gas control blower 70 through the
D.C. power supply 100.
The present invention also provides a particularly sensitive
maximum speed potentiometer 116 which adjustably limits the maximum
speed of the gas control blower 70. This potentiometer has at least
3, and preferably, 5 to 10 turns so that it can accurately adjust
the maximum operating speed of the gas control blower 70 which
establishes the relationship between the liquid nitrogen pressure
in the spray header system 75 and the proper RPM of the gas control
blower corresponding to that pressure.
While the foregoing description has illustrated the liquid nitrogen
spray system 75 as producing direct contact of the product with
liquid nitrogen, the present invention is equally applicable where
the liquid nitrogen is introduced and vaporized in any one or more
of the gas zones. That is, either in place of, or in addition to,
the previously described spray system 75, liquid nitrogen may be
introduced through a spray system such as that shown in phantom
line, and generally designated by numeral 75A, which includes a
supply line 34A, a liquid nitrogen control valve 78A, and one or
more nozzles 36A which spray or otherwise inject the liquid
nitrogen directly into the relatively warmer gaseous nitrogen
wherein it is vaporized without direct contact with the product.
The principal advantage of this type of liquid nitrogen injection
system is that it can be used with any type of product including
certain types of delicate products which may be damaged by a direct
spray. When this type of cryogenic liquid injection system is used,
the pressure of the liquid being injected is sensed by a sensing
line 77A, which is identical to sensing line 77, and all of the
other components of the gas control system remain structurally and
functionally identical to that previously described.
From the foregoing description of one preferred embodiment of the
invention, it will be apparent that the present invention provides
a mechanically simple and reliable variable speed gas control
blower which can be accurately and automatically regulated by a
simple and highly reliable electrical control system so as to
always maintain all of the necessary and critical gas flow
conditions regardless of wide changes in the various operating
conditions of the freezer. In addition, the pressure in the storage
tank (not shown) often varies during filling of the tank, and also
varies as the hydrostatic head of the liquid in the storage tank
drops during operation of the freezer. These pressure changes in
the storage tank change the rate of flow of liquid through control
valves 78 and/or 78A. However, the pressure differences producing
these changes of flowrate are also sensed by lines 77 and/or 77A,
and the control system automatically adjusts the speed of gas
control blower 70 in accordance with any such change in the amount
of cryogenic liquid being injected.
Since numerous modifications and changes will be readily apparent
to those skilled in the art, it is to be understood that the
foregoing description is merely illustrative of one preferred
embodiment of the invention, and that the true scope of the
invention is not to be limited other than as expressly defined in
the following claims including all equivalents thereof.
* * * * *