CO.sub.2 food freezer

A food freezer having a thermally insulated enclosure and an endless conveyor having a loading section, a food freezing section and an unloading section. Blowers circulate gas throughout the interior of the enclosure, and devices for injecting liquid CO.sub.2 to preferably cause CO.sub.2 snow particles to impinge upon the food products on the belt are located at selected positions about the belt and may be at various vertical levels, some of which are preferably at least slightly above the belt. By piping the liquid CO.sub.2 so that it flows only upward and/or horizontally toward the spray nozzles within the thermally insulated enclosure, any solid CO.sub.2 that may accumulate adjacent the upstream side of the spray nozzle orifices is melted by CO.sub.2 vapor which gravitates upward in the vertical piping.

This invention relates to freezers wherein food products or the like are 
cryogenically frozen with in a freezing region, and more particularly to a 
food freezing apparatus wherein food products or the like are continuously 
conveyed along a path and liquid CO.sub.2 is injected to form solid 
CO.sub.2 and CO.sub.2 vapor to provide the low temperature environment 
which freezes the food products. 
BACKGROUND OF THE INVENTION 
There are many advantages to cryogenic freezing of food products which have 
come to light in the past several decades, and in a number of instances, 
carbon dioxide is the cryogen of choice for efficient and economical 
cryogenic freezing applications. Cryogenic carbon dioxide food freezers 
often utilize liquid carbon dioxide under pressure sufficient to maintain 
it in the liquid state and supply it to spray nozzles through which it is 
injected into the interior of a thermally insulated enclosure wherein the 
food products to be frozen are delivered to a freezing region, as by being 
transported on an endless conveyor or the like. In a CO.sub.2 food 
freezer, the low temperatures which can be achieved by the creation of 
solid CO.sub.2, can create a tendency for liquid CO.sub.2 in the lines 
leading to the spray nozzles to freeze, particularly at times when there 
is no flow or only very low flow. Gassing systems have been devised and 
utilized to clear the lines of liquid CO.sub.2 at certain times to prevent 
such freezing. 
Freezing of food products is typically accomplished by heat transfer to the 
colder gas that is being circulated past the food products, although some 
heat may be withdrawn by direct removal to a vaporizing cryogen at the 
surface of such a food product. Accordingly, the movement of the gas and 
its velocity become important in accomplishing efficient freezing of the 
food products, and one or more blowers is generally always provided to 
assure the food products are exposed to and in contact with the 
circulating cold vapor. 
U.S. Pat. No 4,356,707, in FIGS. 10-12, shows cabinet freezers including 
one having a spiral or helical conveyor wherein CO.sub.2 injectors fed 
from an upper liquid CO.sub.2 source are located in corner regions of the 
cabinet to inject CO.sub.2 snow and cold vapor and to induce additional 
vapor flow generally horizontally and in a direction concurrent with the 
movement of the food products along the helical path. U.S. Pat. No. 
4,324,110 shows a cryogenic food freezer wherein liquid CO.sub.2 from an 
upper supply line 38 is injected through nozzles and discharged 
countercurrently into streams of moving gas or vapor from fans to effect 
rapid vaporization of the injected CO.sub.2. U.S. Pat. No. 3,733,848 shows 
a food freezer wherein a header 82 extending along the roof of a freezer 
enclosure supplies spray nozzles that inject CO.sub.2 into discharge 
streams from vertically elongated blowers having vane-carrying squirrel 
cage rotors which rotate about vertical axes. U.S. Pat. No. 4,078,394 
shows a spiral freezer designed for cold gas to flow through the various 
regions of a helical belt by driving a center drum of circular 
cross-section having both its axial ends open and having a varied 
perforation pattern in its sidewall wherein gas sucked from the interior 
of the drum by a motor-driven is fan is discharged past a plurality of 
injectors in the top wall of the freezer where CO.sub.2 vapor is injected 
to effect cooling of the gaseous atmosphere. 
When liquid CO.sub.2 is injected through spray nozzles located within a 
thermally insulated enclosure, it is possible for solid cryogen to begin 
to form in the lines leading to the nozzles as by freezing liquid CO.sub.2 
which is in contact with the cold metal surfaces of the lines within such 
cold environment and/or the nozzles, which can cause the nozzle orifices 
to clog. Moreover, there is always the possibility that small amounts of 
CO.sub.2 snow will also form in the lines through momentary pressure 
drops, and because of the cold environment, such snow will be relatively 
slow to redissolve in liquid CO.sub.2 and will generally be carried along 
to the snow nozzles where buildup and blockage can occur. 
Inefficiencies result from such clogged spray nozzles in a freezer, and 
accordingly, solutions to such problems were sought. 
SUMMARY OF THE INVENTION 
One object of the present invention is to provide an improved CO.sub.2 food 
freezer wherein more efficient overall freezing is achieved by means of 
improved injection of liquid CO.sub.2 using nozzles which are 
self-unclogging. 
It has been found that, by piping a CO.sub.2 food freezer from the bottom 
so that there are no downward extending legs in the piping arrangement 
leading to the spray nozzles but only vertically upward or horizontal or 
diagonally upward legs, a self-unclogging arrangement is achieved. In such 
an arrangement, the small amounts of CO.sub.2 vapor in the liquid line 
will flow by gravity upward in the piping and reach the upstream side of 
the spray nozzle orifices, and as a result, any CO.sub.2 snow accumulating 
in this region that would potentially build up and eventually clog such 
nozzles will be melted to liquid by the warmer vapor. In addition, by 
appropriately jacketing pipes or headers that carry liquid CO.sub.2 within 
such a cryogenic temperature environment, particularly where the pipe in 
question is generally vertically aligned, a natural convection flow of 
ambient air can be created in the jacketed region, so that ambient 
atmosphere provides sufficient warmth immediately adjacent this region of 
the liquid CO.sub.2 piping to prevent freezing

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A food freezer 11 is designed to rapidly and efficiently freeze food 
products or the like which move along a continuous conveyor 13 as they 
travel along a helical path which constitutes the major portion of the 
length of travel within an insulated cabinet or enclosure 15. Illustrated 
in FIGS. 1 through 4 is a freezer which includes a large insulated cabinet 
15 generally in the form of a rectangular parallelepiped having four 
vertical walls 17, a top wall 19 and a bottom wall 21, all of which are 
suitably, thermally insulated to maintain a low-temperature environment 
therewithin. Several doors and, if desired, a window are included to 
provide physical and visual access to the interior of the cabinet wherein 
an endless conveyor belt 23 of highly porous, i.e., open mesh, 
construction is located, as is well known in this art. 
The conveyor includes such an endless belt 23, preferably made of stainless 
steel, which is arranged to have a short straight-line loading section 25 
disposed near the bottom of the cabinet which may protrude a short 
distance through an entrance opening 27 in the cabinet enclosure. It 
should also have a short, straight, unloading or exit section 29 that may 
also similarly extend a short distance through an upper exit opening 31 in 
the insulated wall of the enclosure. The conveyor can be of the type 
generally illustrated in U.S. Pat. No. 4,078,655, issued Mar. 14, 1978, or 
in U.S. Pat. No. 4,878,362, to Lewis Tyree, Jr., issued Nov. 7, 1989, the 
disclosures of which are incorporated herein by reference. The entrance 
and exit openings 27, 31 are generally aligned with each other and are 
adjacent lower and upper corners of opposite walls of the enclosure. A 
main curved section 33 of the endless conveyor belt 23 lies between the 
straight loading and unloading sections wherein the belt travels along a 
helical path in a plurality of flights or tiers about a center cage or 
drum 35 which is drivingly rotated. 
The cage 35 is preferably circular in horizontal cross-section, although it 
might be oval or have some other generally circular shape. The rotatable 
cage is mounted on a composite center shaft assembly 37 which extends 
downward through the bottom wall 21 of the insulated enclosure; the shaft 
carries the usual bearings (not shown) and terminates in a sprocket or 
gear 39 which is preferably driven by a chain 40 that extends laterally to 
a drive motor (not shown) mounted adjacent the wall of the enclosure 
containing the exit opening 31, all as well known in this art. A 
tensioning motor is also preferably located near the exit and provides 
suitable low tension in the belt to assure that it remains in engagement, 
usually frictional, with the exterior surface of the central rotating cage 
35 which provides the primary motive power for the conveyor throughout 
nearly all of its length. A belt take-up arrangement 43 is also preferably 
provided inside the insulated cabinet 15 which compensates for the 
difference in the overall length of the belt 23 when the freezer is at 
ambient conditions and when the belt is in its shorter contracted state 
during operation at temperatures below freezing. 
As can best be seen in FIG. 1, the belt proceeds generally along the bottom 
wall of the enclosure to the entrance opening 27 where it extends outward 
for a short distance to constitute the loading section 25 where food 
products to be frozen are received. At the end of the straight-line 
loading section, the belt undergoes a transition to the curved orientation 
where it engages, usually frictionally, the outer surface of the rotating 
cage and begins its travel along a plurality of tiers, for example, nine 
or ten, as it proceeds gradually upward until finally undergoing a 
transition back to the straight-line unloading section 29. The rotating 
cage 35 is usually supported generally along its upper rim 45 by guide 
rollers or the like (not shown) mounted within the enclosure, and the 
individual tiers of the belt in its helical section 33 are supported by 
arms 49 at each vertical level which extend radially inward from a 
plurality of vertical posts 51 which extend between the top and bottom 
walls of the insulated enclosure 15. 
The circulation of the gas or atmosphere within the insulated enclosure is 
affected by a large centrifugal blower 53, preferably one of those 
commercially referred to as a "Plug Fan", which includes a convolute 
entrance guide 55 that leads to the chamber wherein a vane-carrying wheel 
57 rotates. The blower 53 is mounted coaxially with the rotating cage 35 
at a location vertically thereabove. The entrance baffle 55 to the blower 
is preferably proportioned to provide clearance between it and the 
interior surface of the rotating cage which may have an imperforate sleeve 
58 which depends from the upper rim 45 and prevents short circuit flow 
therethrough in the region of the entrance guide. Alternatively, the 
imperforate sleeve 58 could be provided as a depending portion of an 
overlying baffle that directs the discharge flow from the blower. The 
blower 53 takes its suction from the interior of the cage and discharges 
high pressure gas horizontally outward throughout 360.degree. generally 
along the undersurface of the top insulated wall 19 of the enclosure and 
across a ring-shaped cover or baffle 59 that overlies the helical conveyor 
section. Whereas the upper end of the rotating cage 35 is in open 
communication with the blower, the lower or bottom end of the rotating 
cage is completely closed by a conical bottom wall 60 which is affixed at 
its upper end to the center shaft assembly 37. 
The cage includes an upper rim or ring 45, and a similar lower rim, which 
are interconnected by a plurality of vertical bars 63 that are 
equidistantly spaced apart from one another and constitute the sidewall 
region of the cage. Struts 65 extending between the cage shaft assembly 
and the sidewall at the rim provide overall structural strength to the 
cage structure and stabilize the composite drive shaft assembly 37 
therewithin. Inasmuch as the entire bottom of the cage is closed, the gas 
being sucked from the interior of the cage by the blower 53 comes through 
the spacings between the cage bars 63 in the sidewall, and the conical 
shape of the bottom wall 60 assures an aerodynamically smooth flow pattern 
in the lowermost region of the cage. Thus, this arrangement creates a 
radial inflow of gas for essentially 360.degree. throughout the sidewall 
of the cage and subjects the food products carried by the belt in the 
helical section 33 to heat transfer with such radial inflow of cold gas. 
To complete the overall, generally toroidal circulation pattern, the gas 
being discharged horizontally from the blower travels toward and then 
downward at the four vertical walls 17 of the enclosure, along which it 
flows until beginning its radially inward path through the conveyor. The 
gas flow is assisted in turning downward by a plurality of turning vanes 
66 (FIG. 2). 
Cooling for the freezer is provided by injecting liquid carbon dioxide 
through nozzles directed radially inward so that the injected CO.sub.2 
snow travels concurrently with the flow of gas and impinges upon the food 
products being carried on the helical section 33 of the conveyor. The 
freezer 11 contains five separate banks or sets of arrays 67 of spray 
nozzles 69; however, fewer or additional arrays could be used. As best 
seen in FIG. 3, an array includes seven spring-loaded spray nozzles each 
connected via a vertical tube 71 to a horizontal manifold 73. A vertically 
adjustable bracket assembly 75 is provided which is mechanically 
interconnected, as by welding or the like, with each of the vertical tubes 
71. Each bracket assembly includes four split-collar assemblies 77 which 
are slidably received upon a pair of vertical rods 79 of circular 
cross-section that extend from the bottom wall to the top wall of the 
enclosure; these allow the nozzle array to be adjusted to an appropriate 
vertical level where the radially inward cryogen sprays from the nozzles 
69 will impinge against food products on four different tiers of the 
helical conveyor section. A flexible conduit 81 is employed to 
interconnect each manifold 73 to a slightly lower outlet 83 on a 
permanently mounted vertical pipe 85 extending upward through the 
insulated floor 21 of the enclosure which pipe serves as a header and 
contains a plurality of side outlets 83, as best seen in FIG. 4. As a 
result, each pipe supplies liquid CO.sub.2 upward to at least two arrays 
67 of spray nozzles. 
Liquid CO.sub.2 is supplied via a liquid CO.sub.2 line 86 through a flow 
control means or valve 87 (FIG. 4), located upstream of the entrance of 
the pipe 85 into the enclosure through the bottom wall 21, that controls 
the downstream pressure and the rate of flow therethrough by modulating, 
in response to the demand for refrigeration as discussed hereinafter. To 
inject CO.sub.2 snow, the spray nozzles 69 preferably have spring-loaded 
stem arrangements set to open at a supply pressure of about 125 psig or 
higher and preferably at a pressure of at least about 200 psig. When 
injected into an environment at about atmospheric pressure, liquid 
CO.sub.2 is immediately transformed to a mixture of CO.sub.2 snow and cold 
CO.sub.2 vapor, and the snow impinges against the food products being 
carried on the continuously moving conveyor belt. It is possible that 
liquid CO.sub.2, at pressures between about 125 psig and about 300 psig, 
may freeze to solid CO.sub.2 in a cold environment; it is also possible 
that minor amounts of solid CO.sub.2 will form in a flowing stream of 
liquid CO.sub.2 as a result of momentary pressure drops, which solid 
CO.sub.2 will be carried along to a nozzle orifice and accumulate on the 
upstream side thereof where it can cause clogging. It has been found that, 
by surrounding each of the liquid cryogen pipes 85 with a coaxial riser 
tube 88 that extends upward through the bottom wall of the insulated 
enclosure and is open to ambient conditions below the freezer, a positive 
deterrent to such freezing is provided. Because the annular region between 
the riser tube 88 and the CO.sub.2 feed pipe 85 is open to the atmosphere 
at its bottom, a natural convection flow of ambient air upward and 
downward through this annular region is created which warms the cryogen 
feed pipe sufficiently to prevent any such freezing. 
In addition, the piping is arranged so that the flow path within the cold 
environment from the liquid flow control valve 87 is either horizontal or 
upward to the individual spray nozzles (having no downward oriented 
sections behind which vapor could be trapped), and a connection downstream 
from the control valve 87 is also provided to a vapor line 89, which is 
conveniently made to the bottom end of each CO.sub.2 pipe 85 adjacent its 
connection to the high pressure liquid CO.sub.2 line 86. The vapor 
pressure in the line 89 should be above the triple point pressure of 
CO.sub.2, i.e., 75 psig, and it is preferably at least about 150 psig, so 
as to assure that the pressure in the pipe 85 and in the associated piping 
to the spray nozzles is maintained above the triple point pressure. 
Moreover, it is preferably set just slightly above the pressure at which 
the spring-loaded spray nozzles are set to open so that a slow flow of 
vapor is maintained through the nozzles after the control valve has closed 
and all liquid CO.sub.2 has been purged, e.g. a vapor flow of about 15-25 
standard cubic feet per hour. For example, CO.sub.2 vapor, at about 160 
psig, reduced from the usual storage vessel pressure of 300 psig by a 
pressure regulator 91, may be provided when the spray nozzles are set to 
open at about 155 to 158 psig. 
During normal operation, the reduction in pressure at the flow control 
valve 87 results in the creation of some CO.sub.2 vapor which travels with 
the flowing liquid; CO.sub.2 solids that are formed, as mentioned above, 
and carried to the orifices of the spray nozzles can result in momentary 
clogging. If clogging occurs, CO.sub.2 vapor bubbles will gravitate upward 
through the liquid CO.sub.2 in the vertical feed pipe and in the 
associated array of spray nozzles to the blocked orifice where the vapor 
will melt any solid CO.sub.2 to create liquid CO.sub.2 removing the 
blockage. Whenever the control valve 87 is shut, the connection to the 
line 89 assures a minimum pressure of at least about 160 psig is 
maintained upstream of the spray nozzles. If the freezer is to be shut 
down for a period of time, the remaining liquid CO.sub.2 in the feed pipe 
85 and in the piping downstream of the control valve 87 will slowly 
vaporize at the pressure of the vapor line 89 which exceeds the setting of 
the spring-loaded spray nozzles, so that one or more of them will open, 
slowly venting CO.sub.2 vapor into the freezer until all of the liquid 
CO.sub.2 has vaporized. Even if the supply of vapor to the regulator 91 
should be halted before all of the liquid CO.sub.2 has vaporized, it will 
simply vaporize as the freezer is allowed to warm up. 
In such a cryogenic food freezer, liquid CO.sub.2 is generally fed 
simultaneously to the spray nozzles 69 of all of the banks of arrays in 
the different locations. Control is normally via a control system 93, 
mounted as a part of a panel which also receives a signal from a monitor 
for the temperature within the enclosure 15; by causing the control valve 
87 to modulate, the control system adjusts the flow rate of liquid 
CO.sub.2 being fed to the injectors so as to maintain the temperature in 
the freezer within a desired range. It will, of course, be realized that 
the temperature will vary somewhat, depending upon where it is measured 
within the enclosure, inasmuch as the vapor warms as it passes over the 
food products and, of course, cools when it intermingles with fresh cold 
vapor that is being generated along with the CO.sub.2 snow being injected 
from the spray nozzles. It is generally accepted that a representative 
temperature reading in freezers of this general type is obtained by 
measuring the temperature at one or more locations in a vapor section 
relatively isolated from the injection of cryogen. Accordingly, a pair of 
thermocouples 95 or other suitable temperature-measuring devices are 
provided at appropriate locations within the freezer, for example, along 
one vertical wall 17 at distances downward from the top and upward from 
the bottom, as depicted in FIG. 2, equal to about one-fourth of the height 
of the chamber, which send signals to the control system 93 that are used 
to modulate the injection flow of liquid CO.sub.2 through the spray 
nozzles--usually by altering the downstream pressure of liquid CO.sub.2 
exiting the pressure-regulating valve 87 and thus the rate of flow through 
the valve. Preferably, a plurality of thermocouples or other temperature 
sensors 95 are provided within the freezer, and the signals from these are 
averaged to control the temperature within the freezer. 
As an example of the efficiency of a freezer of this general design, tests 
are run with a freezer having about 230 feet of conveyor belt, which 
includes the loading section 25, the main helical section 33 and the 
unloading section 29. Liquid carbon dioxide is supplied to the arrays of 
spray nozzles so as to impinge CO.sub.2 snow upon chicken nuggets, which 
are discrete, compressed, cooked, composite pieces of chicken about one 
inch in greatest dimension. The belt is about 34 inches wide and is 
operated at a lineal speed of about 16 feet per minute. As such, over a 
dwell period of about 14-16 minutes, the freezer is able to freeze the 
entire output from two nugget-forming machines, which together supply 
about 3400 pounds of chicken nuggets per hour to the freezer. The chicken 
nuggets being supplied are coated with a batter and breading and, 
following immersion frying, are at a temperature of about 180.degree. F. 
when deposited on the loading section of the conveyor. The efficiency of 
the freezer is such that, by maintaining a gas atmosphere temperature of 
about -40.degree. F. within the freezer, it is found that the chicken 
nuggets are acceptably uniformly frozen across the width of the belt. 
Examination of individual nuggets shows that their innermost regions have 
hardened and their outer regions have not been cooled lower than necessary 
upon reaching the unloading section of the conveyor. Calculations show 
that, by delivering about 5690 pounds per hour of liquid CO.sub.2 (at 
about 0.degree. F. and 300 psig) to the freezer, freezing of about 3400 
pounds per hour of chicken nuggets to an equilibrated temperature of about 
10.degree. F. is achieved. 
The liquid control valve 87 modulates both the pressure and the flow rate 
of the liquid CO.sub.2 supplied to the feed pipe, and the use of injectors 
69 having spring-loaded conical stems which function to create orifices of 
variable areas results in the injection of cryogen into the freezer at a 
substantially greater rate when higher liquid CO.sub.2 pressures are 
applied. The control is such that, should a temperature be reached within 
the freezer above the desired temperature range, supply of cryogen through 
the injectors 69 may be momentarily halted; however, during normal 
operations, the controlled flow of CO.sub.2 liquid keeps the temperature 
within the desired range. As mentioned, any blockages at the injectors are 
self-clearing because the design causes CO.sub.2 vapor to migrate to the 
site and melt the solid CO.sub.2 causing the blockage. Although as earlier 
indicated there are particular advantages to a freezer wherein liquid 
CO.sub.2 is directly injected so as to cause impingement upon food 
products being transported along a helical conveyor, there are many 
applications for which straight-line tunnel freezers are adequate or may 
even be preferred. Shown in FIGS. 5 and 6 is a CO.sub.2 tunnel freezer 101 
wherein an insulated freezer enclosure 103 is provided which provides a 
straight-line freezer path wherein an endless conveyor belt 107 driven by 
a standard motor drive 108 travels from a loading station 109, through a 
straight-line freezing region or section 111 to an unloading station 113. 
The freezing of food products on the belt results from heat transfer to 
the cold gas being circulated by blowers 1-5 mounted within the enclosure 
having electric motors which are supported on the upper wall of the tunnel 
and from heat transfer to CO.sub.2 snow sprayed downward against the upper 
surfaces of the moving food products and, optionally, sprayed upward 
against their lower surfaces. 
In this embodiment, the insulated enclosure 103 is extended in a 
longitudinal direction to create a tunnel which extends from an entrance 
to an exit and is defined by a pair of vertical sidewalls 117, a top wall 
119 and a bottom wall 121. CO.sub.2 injectors in the form of spray nozzles 
123 are located in the upper region of the tunnel above the conveyor belt 
107 generally along the undersurface of the top wall of the enclosure, and 
the CO.sub.2 snow from the nozzles is directed against the food products 
moving therebeneath. Optionally, additional spray nozzles 124 are located 
in the central region between the upper and lower runs of the belt 107 so 
as to spray CO.sub.2 snow through the porous belt against the 
undersurfaces of the food products. The blowers or fans 115 are 
appropriately located to circulate vapor throughout the enclosure and 
direct the vapor in the upper region downward onto the food products 
passing therebelow to cool the food products by heat transfer to the vapor 
from the warmer surfaces thereof. 
Arrays 125 of spray nozzles are located to spray liquid CO.sub.2 downward 
toward the food products traveling along the porous conveyor belt, which 
arrays are fed from a manifold 127 that is interconnected to a vertical 
feed pipe 131 via a distribution pipe arrangement 129 having legs which 
are preferably oriented at least slightly above the horizontal. A liquid 
CO.sub.2 supply line 132 supplies liquid CO.sub.2 to the three vertical 
pipes 131, being connected to each at a location outside of its entrance 
into the tunnel through the bottom wall 121. Similar arrays 128 of 
upward-directed nozzles are located below the upper run of the belt 107 
and similarly connected by piping 130 to the vertical feed pipes 131 at 
tee connectors 135. 
As best seen in FIG. 6, each of the manifolds 127 serves as a support for 
seven spring-loaded spray nozzles 123 to which it supplies liquid 
CO.sub.2. A control valve 133 is located conveniently above the tunnel as 
a part of a CO.sub.2 control panel 141. Thus, this valve controls the 
supply of liquid CO.sub.2 to all the spray nozzles. A connection is made 
to a CO.sub.2 vapor supply line 139 at a location downstream of the 
control valve 133 via branch tubing 137 that preferably contains a check 
valve 138. The pressure in the CO.sub.2 liquid line upstream of the valve 
133 and in the CO.sub.2 vapor line 139 may be controlled by suitable 
valving (not shown) located at the CO.sub.2 supply vessel or in the 
control panel, as well known in this art. As previously discussed, the 
pressure within the vapor line 139 is maintained above about 75 psig and 
preferably at a pressure just slightly above the pressure at which the 
spring-loaded spray nozzles 123 and 124 are set to open. The pressure 
within the CO.sub.2 liquid supply line 132 is generally maintained 
somewhere between about 200 and 300 psig. 
The CO.sub.2 liquid pressure supplied to the spring-loaded spray nozzles 
123 is controlled by the control valve 133 which receives a suitable 
signal from within the control panel 141 and is preferably capable of 
regulating the downstream pressure as opposed to merely being an on-off 
valve. Again, a temperature monitoring device, such as thermocouple 143, 
is appropriately located within the tunnel and is electrically connected 
to transmit the signal which it generates to the control panel 141. 
Control circuitry within the panel compares the signal to the temperature 
which it is desired to maintain within the tunnel, and the control valve 
133 is appropriately regulated to achieve a rate of injection of liquid 
CO.sub.2 that will maintain the desired temperature. 
Whenever the control panel 141 signals the valve 133 to open, liquid 
CO.sub.2 flows through the manifolds and is injected through all of the 
snow-making nozzles 123 downward onto the food products being carried 
thereunder on the moving conveyor belt 107 as well as upward from the 
nozzles 124 against the undersurfaces of food products, such as hamburger 
patties. When there is a large amount of food being frozen, a high rate of 
injection of CO.sub.2 may be required, and the control panel 141 will 
cause the valve 133 to open widely to supply liquid CO.sub.2 near the top 
of the pressure range that will result in a fairly high rate of CO.sub.2 
flow through the spray nozzles 123 and 124. Any small amounts of CO.sub.2 
snow formed within the liquid lines, will be carried to the upstream 
surfaces of the nozzles where potential accumulations can occur and cause 
clogging of one or more of the orifices. However, when there is no flow of 
liquid in a line leading to a particular nozzle, vapor which is created 
downstream of the control valve 133 will gravitate to a clogged or closed 
nozzle. The warmer vapor, upon reaching such solid CO.sub.2, will melt the 
solid CO.sub.2 and thus alleviate any blockage or potential blockage. 
Accordingly, the system illustrated in FIGS. 5 and 6 is self-unclogging, 
as was that previously described with regard to the embodiments shown in 
FIGS. 1 through 4, and it is particularly advantageous in that it 
facilitates the optional inclusion of upward directed nozzles that spray 
solid CO.sub.2 against the undersurfaces of the food products. 
Although not illustrated in FIGS. 5 and 6, each liquid CO.sub.2 pipe 131 
protruding upward through the bottom wall 121 is preferably located within 
a surrounding casing, such as that illustrated in FIG. 4, to provide a 
natural convection flow of ambient atmosphere. 
Although it is preferred that spray nozzles be employed that are 
spring-loaded to the closed position, it would be possible to employ spray 
nozzles in the form of relatively small orifices in a manifold of very 
substantially larger diameter to serve as simple spray orifices which 
inject a plurality of sprays of solid CO.sub.2 downward onto an underlying 
conveyor or otherwise into a freezing region where food products are being 
frozen. For example, a manifold having an inner diameter of about 3/8 inch 
could be provided with a series of drilled holes of about 0.045 inch 
diameter that serve as spray orifices. With such an arrangement, a simple 
off-on valve might be used to supply liquid CO.sub.2 at normal tank 
pressure, i.e. about 300 psig, which would flow downstream of such off-on 
valve to the nozzles and expand to form streams or sprays of solid 
CO.sub.2 particles and cold CO.sub.2 vapor upon reaching the atmospheric 
side of the orifices. A modulating valve might be used instead of an 
off-on valve to provide some flow control. A gassing arrangement would be 
interconnected with each line downstream of the control valve, generally 
similar to that depicted in FIG. 4, which would supply CO.sub.2 vapor from 
the storage tank at a pressure reduced to about 150 psig. The overall 
control system might incorporate a suitable timer which would be activated 
each time the closing of the control valve terminates the supply of liquid 
CO.sub.2 to the manifold, and such timer might allow CO.sub.2 vapor to 
flood the piping and the manifold for a sufficient length of time, e.g. 
about 30 seconds, to insure that all of the liquid CO.sub.2 in the line 
had either been expelled through an orifice or had turned to 150 psig 
vapor. In this way, it is assured that the interior of the piping 
downstream of the control valve and the manifold remains clear of solid 
CO.sub.2. 
Although the invention has been described with regard to certain preferred 
embodiments, it should be understood that various changes and 
modifications as would be obvious to one having the ordinary skill in this 
art may be made without departing from the scope of the invention which is 
defined in the claims appended hereto. For example, instead of locating 
spray nozzles vertically above and/or below the conveyor belt in a tunnel 
freezer, some could also be located in the regions alongside the belt and 
directed downward and inward toward the upper surface of the conveyor. 
Instead of using a single control valve as illustrated in a tunnel 
freezer, one or more manually adjustable valves could be incorporated 
upstream of one or more of the individual manifolds to preferentially 
adjust the rate of flow through different manifolds. Individual inducers, 
such as are disclosed in U.S. Pat. No. 4,356,707, may also be provided 
adjacent the spray nozzles to supplement the flow of vapor in certain 
regions where it is desired to create a still higher rate of vapor flow. 
Although it is believed there are particular advantages to incorporating 
the system in a freezer wherein food products are delivered to the 
freezing region on a conveyor belt, the invention may also be employed in 
cabinet freezers such as those described in detail in U.S. Pat. No. 
4,356,707, issued Nov. 2, 1982, the disclosure of which is incorporated 
herein by reference, wherein food products are delivered manually to a 
freezing region through a hinged door. Even though the apparatus is 
expected to find its primary commercial use as a freezer for food products 
and, as such, it has been so referred to hereinbefore, it should be 
understood that it can be employed to significantly lower the temperature 
or chill any products and that the term freezer is used to broadly 
describe apparatus capable of such use and is not intended to be limiting 
of the apparatus. 
Particular features of the invention are emphasized in the claims that 
follow.