Process and system for producing high-density pellets from a gaseous medium

A process and system for producing pellets of high density carbon dioxide or other gases utilize a chamber containing a plurality of cell-like freezing compartments within which ice is to be formed. A gas desired to be frozen into ice is introduced into the chamber while the internal pressure of the chamber is maintained at a level which is below the equilibrium triple pressure of the gas. The temperature of the freezing compartments is lowered to a temperature which is below the equilibrium vapor pressure temperature of the gas at the chamber pressure so that the gas condenses into ice within the compartments. The temperature of the freezing compartments is thereafter raised so that the ice is thereby released from and falls out of the compartments as pellets for collection.

BACKGROUND OF THE INVENTION 
This invention relates generally to the production of ice and relates, more 
particularly, to the production of ice from a medium, such as carbon 
dioxide, which is gaseous when at room temperature and atmospheric 
pressure. 
Ice, such as dry ice, is commonly used for the refrigeration of food 
products, for shipping of chilled packages and for use, when combined with 
compressed air or turbine wheel accelerators, as a blast media for 
cleaning surfaces. 
Prior art processes used to form dry ice pellets of relatively high density 
commonly involve the flashing of liquid carbon dioxide into solid carbon 
dioxide snow which is subsequently compacted and formed into pellets via 
presses, by ram extrusion or by rotary extrusion. The density of the 
formed pellets formed by these processes is largely dependent upon 
parameters attending the extrusion step and can approach, but rarely 
achieves, the maximum density of solid carbon dioxide. Furthermore, the 
size of the pellets formed in such processes is normally determined by the 
sizes of the openings provided in plates associated with the extrusion 
step. 
A limitation associated with prior art pellet-forming processes, such as 
the flashing/pressing technique described above, relates to the fact that 
only about forty percent of the liquid carbon dioxide is converted to 
solid while about sixty percent is converted into gas. Consequently, 
methods have been employed to convert a higher percentage of the liquid 
carbon dioxide into solid during pellet-forming processes. For example, 
one method described in U.S. Pat. No. 5,257,503 reclaims the lost carbon 
dioxide by capturing, compressing and re-liquefying the flash gas. Another 
method, addressed in each of U.S. Pat. Nos. 1,863,377, 2,138,758 and 
3,901,044 involves a refrigeration unit which cools and directly freezes 
liquid carbon dioxide into solid form in molds, thereby eliminating the 
flashing step. The freezing of liquid in molds, however, requires the use 
of relatively complicated mechanisms to release the pellets from the 
molds. It would be desirable to provide a method and system for producing 
dry ice pellets in a manner which circumvents any need for or use of 
liquid carbon dioxide. 
Accordingly, it is an object of the present invention to provide a new and 
improved process and system for producing ice from a process medium which 
is normally gaseous at room temperature and atmospheric pressure, such as 
gaseous carbon dioxide. 
Another object of the present invention is to provide such a process and 
system which is capable of converting a relatively high percentage of 
available gas, such as gaseous carbon dioxide, into solid form, i.e. ice, 
wherein the solid form possesses a very high density. 
Still another object of the present invention is to provide such a process 
and system which circumvents the need or use of a process medium when in a 
liquid phase. 
Yet another object of the present invention is to provide such a process 
and system which requires no moving parts which must directly contact the 
ice forms, such as pellets, during production. 
A further object of the present invention is to provide such a method or 
system which is well-suited for use in a process requiring gas pumping or 
in some other vacuum pump application. 
A still further object of the present invention is to provide such a 
process and system which is capable of making ice and/or ice pellets of a 
predetermined shape and size. 
SUMMARY OF THE INVENTION 
This invention resides in a process and system for freezing a gas into ice. 
The process of the invention includes the steps of providing a chamber 
containing a freezing compartment having at least one surface against 
which ice is to be formed and introducing a gas to be formed into ice into 
the chamber while maintaining the internal pressure of the chamber at a 
level which is below the equilibrium triple pressure of the gas. While the 
internal pressure of the chamber is maintained as aforesaid, the 
temperature of the at least one surface of the compartment is lowered to a 
temperature which is below the equilibrium vapor pressure temperature of 
the gas at the chamber pressure so that the gas condenses into ice within 
the compartment and against the at least one surface thereof. Thereafter, 
the temperature of the at least one surface of the compartment is raised 
so that the ice is thereby released from the compartment in solid form for 
collection or use. 
The system of the invention includes components for carrying out the 
process of the invention. For example, the system includes a chamber 
having a freezing compartment having at least one surface against which 
the ice is to be formed and means for introducing a gas to be formed into 
ice into the chamber while maintaining the internal pressure of the 
chamber at a level which is below the equilibrium triple pressure of the 
gas. Means are provided for lowering the temperature of the at least one 
surface of the compartment to a temperature which is below the equilibrium 
vapor pressure temperature of the gas at the chamber pressure so that the 
gas condenses into ice within the compartment and against the at least one 
surface thereof, and means are also provided for raising the temperature 
of the at least one surface of the compartment following the production of 
ice against the surface of the compartment so that the ice is thereby 
released from the compartment for collection or use. 
The theory of operation attending the instant invention is not commonly 
employed to produce ice. For example, common methods for forming ice 
shapes involve the freezing of a liquid medium into a solid form, such as 
is routinely performed in conjunction with the making of ice cubes from 
water, the making of candles from wax, and the casting of metal. In 
contrast, the present invention produces ice forms by freezing the gas 
phase of the medium in refrigerated molds at a pressure and temperature 
below the triple point of the medium. 
Briefly, the triple point of a substance is the condition, i.e. a function 
of temperature and pressure, at which the gas phase, liquid phase and 
solid phase of a substance can all be in equilibrium. At temperatures and 
pressures above the triple point, the solid phase will melt into liquid. 
At temperatures and pressures below the triple point, the liquid phase 
does not normally exist, and gases condense and evaporate directly to and 
from the solid phase. The evaporation of a solid below the triple pressure 
is referred to as sublimation. The reverse process of freezing a gas 
directly to a solid is referred to herein as reverse sublimation. For many 
substances, such as nitrogen and hydrogen, the triple point is below 
atmospheric pressure and room temperature. To freeze these materials 
directly from the gas phase requires operating in a vacuum chamber at 
temperatures well below room temperature. Some materials, such as carbon 
dioxide (CO.sub.2) and UF.sub.6 have triple points which are well above 
atmospheric pressure but below room temperature. Ice forms of these 
materials will sublime when introduced to ambient pressure and 
temperatures. Alternatively, these materials can be frozen directly to 
solid when the gas comes into contact with a surface which is colder than 
the equilibrium vapor pressure temperature at pressures below the triple 
point. 
When refrigerating surfaces are at temperatures well below the equilibrium 
vapor pressure temperatures of the gas in a chamber, the gas freezes 
against the refrigerating surfaces into a solid in the form of a frost. 
This is because the condensing atoms on the refrigerating surfaces are at 
such low temperature that they cannot migrate to preferred crystal sites 
before other gas atoms condense thereupon. Consequently, this condensation 
results in a random solid form with voids. However, at temperatures below 
but close to the equilibrium vapor temperature, simultaneous evaporation 
and condensation of the gases occurs at the surfaces which results in the 
accumulation, or build up, of a clear solid ice on the surface wherein the 
ice is substantially free of voids and crystalline imperfections. This 
invention typically operates in this temperature/pressure regime for 
producing high quality dense ice forms. 
The rate at which gases will reverse sublime on a surface, thereby forming 
a solid layer, is determined by the thermal conductivity of the solid ice, 
the ice layer thickness, and the temperature difference between the 
freezing surface and the temperature at the growing surface of the ice. 
Typically, the surface temperature of the ice rises to a value very close 
to the equilibrium vapor pressure temperature of the gas in the chamber. 
The growth rate of ice is proportional to the rate at which the heat of 
sublimation of the condensing gas is carried to the refrigerating surface, 
which is the product of the temperature difference times the solid thermal 
conductivity of the ice divided by the ice thickness. 
Gases can be directly frozen in a growing chamber to a solid form at any 
pressure below the triple pressure. The triple pressure for carbon dioxide 
is 75 psia, so that it can be reverse sublimated at atmospheric pressure. 
Since the equilibrium temperature of carbon dioxide at atmospheric 
pressure is -110.degree. F., a temperature lower than -110.degree. F. 
would be required for reverse sublimation. For example, a temperature of 
120.degree. F. would provide a temperature differential of 10.degree. F. 
between the ice surface and the freezing surface. By raising the pressure 
in the growing chamber to a pressure closer to the triple pressure, e.g. 
about 65 psia, the gas forms into a solid at temperatures below about 
-70.degree. F., which would require a temperature of -80.degree. F. for a 
temperature differential of 10.degree. F. Since a refrigeration system 
operating at a freezing temperature of -80.degree. F. is normally smaller 
and is more efficient than such a system operating at -120.degree. F., the 
utilization of a pressurized chamber of the present invention is 
advantageous in this respect. 
A chamber is typically required to grow ice of pure substances so as to 
contain the desired gas and to exclude air and water vapor from 
contaminating the solid and fouling the freezing panels with water. 
The same general principles of operation used to produce dry ice pellets 
govern the production of hydrogen ice pellets except that the triple point 
pressure is 54 Torr (7,200 Pascals) and the triple temperature is 
14.degree. K. so that the growing chamber is a vacuum chamber and the 
freezing surfaces are refrigerated to very low temperatures, typically 
10.degree. K. 
The process described herein of freezing gas directly into ice produces ice 
which is optically transparent, similar to glass, which indicates that the 
solid is devoid of and does not contain unformed pockets and imperfections 
which would scatter light. This is in contrast to the dry ice formed by 
compressing snow, which is typically an opaque white solid form of 
CO.sub.2. The clear pellets formed by this invention are thereby denser 
and harder than pellets produced by prior art techniques, as can be 
appreciated by the difference in hardness of sleet, which is a dense form 
of water ice, to snow balls, which are a compressed form of water snow. 
The harder pellets, when used for example for blasting of surfaces to 
remove unwanted coatings, are more aggressive and efficient at removing 
adherent coatings. 
In addition, the process of freezing gas directly into ice with this 
invention converts a high percentage of feed gas to pellets. The 
conversion efficiency for this process utilizing CO.sub.2 is above eighty 
percent and can approach a theoretical value of over ninety percent. This 
is in contrast to the machines which flash liquid CO.sub.2 to produce snow 
and then compress the snow into pellets, which converts less than forty 
percent of the CO.sub.2 into pellets. Prior art processes involving a 
flashing process can increase their efficiency by employing gas recovery 
systems, but these gas recovery systems are typically effective only in 
very large installations. The advantages of the high gas-to-pellet 
conversion efficiency obtained by the process of the present invention is 
that it reduces cost by reducing the quantity of CO.sub.2 required to 
produce the pellets and that it reduces the release of CO.sub.2, which is 
a greenhouse gas, to the atmosphere. 
Still another advantage of the invention is that it requires few moving 
components in order to produce the frozen pellets. In this connection, the 
compartments within which the ice pellets are grown in the system of the 
invention are arranged so that following release of the pellets formed 
therein, the pellets simply fall from the compartments for collection. In 
contrast, high pressure extrusion mechanisms associated with prior art 
techniques for compressing dry ice snow into more compact forms employ 
many moving parts and are prone to mechanical failure. Furthermore, the 
prior art technique of freezing liquid CO.sub.2 with molds into ice forms 
commonly require that the molds to be periodically tipped upside down or 
made to separate in order in order that the ice formed therein be 
released. 
Yet another advantage of the process of this invention relates to the fact 
that, in addition to use of the invention described herein for the 
production of frozen pellets from gas, the invention can also be used as a 
cryogenic process pump or a cryogenic vacuum pump. Cryogenic vacuum pumps 
(cryopumps) operate by freezing gases into solids on refrigerated surfaces 
maintained below the equilibrium vapor pressure temperature of the gases 
being pumped. Typically, conventional cryopumps must be shut down 
periodically to either defrost or regenerate the pumps. In this 
connection, the cryopumps are taken out of service, usually by closing an 
inlet valve, and subsequently warmed so as to evaporate ice which 
accumulates within the pump during the pumping cycle. As will be apparent 
herein, the principles of the present invention can be used to cryopump 
gases into pellets which can be continuously released and ejected from the 
pump during normal operation, thereby eliminating the need to shut down 
the pump for regeneration. Accordingly, the principles of the present 
invention can be variously applied. 
Further objects and advantages of my invention will become apparent from a 
consideration of the drawings and ensuing descriptions.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
Turning now to the drawings in greater detail, there is illustrated in 
FIGS. 1 and 2 a freezing panel assembly 9 having a series of freezing 
compartments, each of which is generally indicated 8, within which ice is 
formed in accordance with one embodiment of the process of the invention. 
As will be described herein, each compartment 8 is disposed within a 
chamber into which a gas, such as carbon dioxide (CO.sub.2) is introduced, 
and the internal conditions of the chamber are controlled so that the gas 
reverse sublimes against the interior surfaces of the compartments 8 and 
thereby begins to form ice within the compartments 8. The formed ice 
continues to accumulate, or build up, on top of itself within the 
compartments 8 until a desired thickness is achieved, at which time the 
compartments 8 are defrosted in a manner which releases the formed ice 
therefrom. It is a feature of the compartments 8 that they are shaped and 
arranged so that upon release of the ice formed therein, the ice 
gravitationally falls from the compartments 8 for collection. 
In the depicted embodiment of FIGS. 1 and 2, one surface of the 
compartments 8 are provided by the lower surface of a substantially planar 
panel 10, and the compartments 8 are designed to produce dry ice pellets 
having a diameter of about 0.125 inches. The panel 10 can be provided by a 
round copper plate 10 having a thickness of about 0.0625 inches, a 
diameter of about seventeen inches and a center hole of about 2.0 inches 
in diameter. For purposes of controlling the temperature of the lower 
surface of the panel 10, copper refrigeration coils 11 having an inner 
diameter of about 0.25 inches are attached in heat transfer relationship 
to the upper surface of the panel by means of 96-4 tin silver solder 12. 
Covering the top side of panel 10 and coils 11 is a thermally insulating 
coating 16 of epoxy resin filled with glass microballoons. As will be 
apparent herein, refrigerant is directed through the coils 11 by way a 
suitable refrigerating system for controlling the temperature of the panel 
10, and the insulating coating 16 retards the growth of ice against the 
upper surface of the panel assembly 9. 
The sides of the compartments 8 are provided by a honeycomb 13 which is 
attached to the underside of the plate 10 by means of 96-4 tin silver 
solder 15. The honeycomb 13 is constructed of spot-welded 304 stainless 
steel foil having a cell size of 0.125 inches, a foil thickness of 0.004 
inches and a cell depth of 0.5 inches. A honeycomb having the 
aforedescribed dimensions is available from Kentucky Metals Inc. of New 
Albany, Ind. Prior to soldering the honeycomb 13 to the panel 10, the 
honeycomb 13 is plated with copper plating 14 so that the copper plating 
14 covers at least about the upper 0.125 inches (as viewed in FIG. 1) of 
the surfaces of the honeycomb 13 which are ultimately soldered to the 
panel 10. 
It follows that the each compartment 8 is provided by a portion of the 
lower surface of the panel 10 and the interior surfaces of the walls of a 
corresponding cell of the honeycomb 13. During the growth of an ice pellet 
within a honeycomb cell, carbon dioxide preferentially deposits upon the 
copper surfaces of the honeycomb, rather than the uncoated (exposed) 
surfaces of the stainless steel honeycomb due to the higher rate of heat 
transfer through the copper than through the stainless steel. In other 
words, with the aforedescribed design, the CO.sub.2 ice is likely to form 
more quickly on the copper surfaces than on the uncoated (exposed) surface 
of the stainless steel to reduce that likelihood that ice will bridge 
across the wall of adjacent honeycomb cells before the ice pellets achieve 
a desired size within a cell. 
The panel assembly 9 of FIGS. 1 and 2 is utilized with panel assemblies of 
like construction in a pellet-making and handling system 160, 
schematically shown in FIG. 3 used for making dry ice pellets from carbon 
dioxide. To this end, the system 160 includes three cylindrical chambers 
18A, 18B and 18C which are each about 18.0 inches in diameter, about 32.0 
inches high, and constructed of a welded ten gauge 304 stainless steel 
tube with ten gauge flanged and dished header domes welded to the top and 
bottom of the tube. Each chamber 18A, 18B or 18C has a pressure rating of 
95 psig and is equipped with a relief valve 19 which is set to open at 75 
psig. 
As depicted by the chamber 18A of FIG. 4, each chamber 18A includes a 
plurality of panel assemblies 9 supported in stacked relationship therein, 
and each panel assembly 9 is supported within its corresponding chamber so 
that its freezing compartments 8 open downwardly. As will be apparent 
herein, this downward opening of the compartments 8 permits gravity to 
facilitate the release of the ice pellets from the compartments 8. In 
addition, a catch funnel 17 is positioned beneath each freezing panel 
assembly 9 for directing the ice pellets which are eventually released 
from the panel assemblies 9 to a central opening 17A provided in the 
funnel 17 for collection. Each catch funnel 17 is constructed of polished 
twenty-six gauge 304 stainless steel and is separated from the honeycomb 
of its corresponding panel assembly 9 by a spacing of at least 0.5 inches. 
Preferably, as many panel assemblies 9 are positioned within the chambers 
18A, 18B and 18C as is as practically possible to take full advantage of 
the internal volume of the chambers 18A, 18B and 18C. 
With reference again to FIG. 4, there is attached to each panel assembly 9 
at its central opening a relatively short sleeve 20 which depends 
downwardly from the panel 10. These sleeves 20 collectively provide a 
vertical funnel stack which funnels dry ice pellets delivered thereto by 
the catch funnels 17 downwardly toward the bottom of the chambers 18A, 18B 
and 18C. With reference again to FIG. 3, a pipe 21A, 21B or 21C is joined 
between the bottom of each chamber 18A, 18B and 18C and the top of a 
pellet collection hopper 43 so that pellets which fall to the bottom of 
each chamber are gravitationally directed into the collection hopper 43. 
Accordingly, the hopper 43 is disposed below the elevation of the bottom 
of the chambers 18A, 18B and 18C. Each pipe can be provided by a copper 
tube having an internal diameter of 0.8 inches, and its opposite ends are 
joined, respectively, to the bottom of a chamber 18A, 18B and 18C and the 
collection hopper 43 by way of suitable couplings. 
In addition, each chamber 18A, 18B and 18C is provided with a gas exhaust 
port in its top, and 0.25 inch ID couplings 44A, 44B and 44C are joined to 
these exhaust ports. One chamber 18B is provided with a pressure switch 
25. Further still, a pneumatic piston vibrator 37 is attached to the 
bottom of each chamber 18A, 18B and 18C. During the production of dry ice 
pellets, the vibrators 37 enhance the flow of pellets toward and out of 
the bottom of the chambers. 
Associated with the system 160 is a gas supply tank 47 for supplying carbon 
dioxide, under pressure, to the chambers 18A, 18B, and 18C and a supply 
piping network for delivery of the carbon dioxide to the chambers 18A, 18B 
and 18C. This supply piping network includes a 0.25 inch ID supply pipe 22 
leading from the tank 47 to inlet pipe segments 22A, 22B and 22C (0.25 ID) 
which join the supply pipe 22 to the bottom of a corresponding chamber 
18A, 18B and 18C by suitable couplings. A gas feed solenoid valve 24 is 
connected in-line with the supply pipe 22. 
The gas supply tank 47 is equipped with a relief valve 38 set at 150 psig, 
a high pressure switch 26 set at 90 psig and low pressure switch 27 set at 
70 psig. By way of example, the tank 47 can be constructed of Schedule 10 
304 stainless steel having Schedule 10 pipe caps welded to each end and 
measure 44.0 inches in length and 6.0 inches in diameter. 
Preferably, the supply tank 47 and the chambers 18A, 18B and 18C are 
positioned within a insulated box 40 (depicted in dotted lines in FIG. 3) 
constructed of a 304 SS angle frame which is enclosed on all sides by 
twenty gauge stainless steel panels and rigid urethane foam insulation 
having a thickness of 2.0 inches. 
The pellet collection hopper 43 positioned below the chambers 18A, 18B and 
18C is situated outside of the insulated box 40 and is constructed, for 
example, of a 10.0 inch diameter 304 SS F&D top tank head welded to a 10.0 
inch diameter, twelve gauge cylinder. This cylinder is about 6.0 inches 
long and is welded to a twelve gauge, 45.degree. sidewall funnel having a 
1.0 inch pipe outlet 50. The hopper 43 is insulated with thick rubber foam 
having a thickness of about 0.75 inches and is connected to the bottom 
outlet 45 of the chambers 18A, 18B and 18C by way of the aforementioned 
tubes 21A, 21B and 21C. Each tube 21A, 21B and 21C can consist of 0.875 
inch OD copper tubing with flare-to-pipe adaptor couplings at its ends. 
Preferably, each tube 21A, 21B and 21C is insulated with rubber foam 
having a thickness of about 0.5 inches. 
With reference still to FIG. 3, there is positioned below the collection 
hopper 43 a pellet release hopper 46 which has the same construction and 
dimensions as the collection hopper 43 except that the hopper 46 has a 
single 1.0 inch ID central pipe inlet 49 instead of three. In addition, a 
1.0 inch ID pipe pellet discharge outlet 51 is joined to the bottom of 
hopper 46 through which pellets are dispensed from the release hopper 46. 
Connected between the bottom outlet 50 of hopper 43 and the inlet 49 of 
hopper 46 is a ball transfer valve 28 which is a 1.0 inch full-ported 
pneumatically operated valve with a four-way pneumatic control valve 29. 
Attached to the outlet 51 of the release hopper 46 is a conduit 51A having 
a 1.0 inch ID pneumatic ball pellet discharge valve 30 with a four-way 
solenoid control valve 31 mounted therein. For receiving the pellets which 
are discharged from the hopper 46 by way of the conduit 51A, an insulated 
storage chest 41 is positioned beneath the release hopper 44. If desired, 
a vibrator 37 can be mounted on the bottom of the hoppers 43 and 46 to 
facilitate the removal of pellets therefrom. 
Associated with the hopper 46 is a pressure relief valve 39 mounted in the 
top thereof, and the valve 39 is preset to open at 75 psig. Also connected 
to the top of hopper 46 is a needle throttle valve 32 which is connected 
to a gas solenoid valve 33. The gas solenoid valve 33 is, in turn, 
connected to a gas vent solenoid valve 34. One end of a pipe 36 which is 
connected in flow communication to each of the exhaust ports 44A, 44B and 
44C is connected in the pipe 162 extending between valves 33 and 34 by way 
of a Tee-fitting 164. Mounted in-line with the pipe 36 is a chamber 
discharge valve 35. 
For purposes of controlling the temperature of the freezing panel 
assemblies 9 within the chambers 18A, 18B and 18C and with reference to 
FIG. 5, the system 160 includes a cascade refrigeration system, generally 
indicated 166, whose coldest-operating refrigeration coils include the 
coils 11 of the pellet freezing panels 9A, 9B, 9C. As will be apparent 
herein, the refrigerant pumped through the coils 11 is responsible for the 
growth of the pellets within the compartments 8 during one (i.e. a growth) 
phase of the pellet-producing operation and is also responsible for the 
release of the grown pellets from the compartments 8 during another (i.e. 
a release) phase of the pellet-producing operation. 
The construction and operation of a cascade refrigeration system like that 
of the refrigeration system 166 (capable of production of temperatures as 
low as about -110.degree. F.) are known so that a detailed description of 
the system 166 is not believed to be necessary. Briefly, however, the 
system 166 includes three refrigeration loops, or subsystems 168, 170 and 
172, wherein the heat of solidification of CO.sub.2 absorbed (in the 
chambers) by the low-end of the loop is removed by the high-end loop and 
intermediate-temperature loop. One subsystem 168 (i.e. the high-end loop) 
uses R404A as a refrigerant and a five-horsepower scroll compressor 60 
suitable for pumping the R404A refrigerant. The outlet of compressor 60 is 
connected to an air-cooled heat exchanger 64, which in turn is connected 
to a refrigerant receiver 63. The receiver 63 is connected to thermostatic 
expansion valve 62 which is connected to an R404A/R508B plate/plate heat 
exchanger 61. The outlet of 61 is connected to the compressor 60 suction 
inlet completing the high end refrigeration loop. 
A second subsystem 170 (i.e. the intermediate loop) of the cascade system 
166 is an open loop arrangement which uses liquid CO.sub.2 as a 
refrigerant and produces the CO.sub.2 gas for freezing into ice within the 
chambers 18A, 18B and 18C. Associated with this subsystem 170 is a liquid 
CO.sub.2 supply tank 79 provided with a pressure relief valve 82 set at 
400 psig and having a tank outlet which is connected to a liquid CO.sub.2 
solenoid valve 80. The valve 80 is connected to a downstream pressure 
regulating valve 83 which is set at 90 psig and which is connected to a 
CO.sub.2 /R508B plate/plate heat exchanger 67. The CO.sub.2 outlet of heat 
exchanger 67 is connected to the CO.sub.2 gas supply tank 47 (see also 
FIG. 3). 
The remaining subsystem 172 (i.e. the low-temperature loop) of the cascade 
system 166 uses R508B (DuPont SUVA 95) as a refrigerant and a 
four-horsepower scroll compressor 65 capable of pumping the R508B 
refrigerant. The outlet of the compressor 65 is connected to a 
normally-open bypass solenoid valve 68 (which is closed during operation), 
which in turn is connected to the suction inlet of the compressor 65. The 
outlet of compressor 65 is also connected to an oil separation filter 66. 
The oil filter 66 has an oil return outlet which is connected to the oil 
return inlet of compressor 65 and has a gas outlet which is connected to 
the R508B/R404A heat exchanger 61. The heat exchanger 61 is connected to 
the R508B/CO.sub.2 heat exchanger 67 for flow of the R508B refrigerant 
thereto, and a piping network is joined between the heat exchanger 67 and 
the chambers 18A, 18B and 18C for flow of the R508B refrigerant from the 
heat exchanger 67 to the chambers 18A, 18B and 18C. Refrigerant solenoid 
valves 71A, 71B or 71C, one for each freezing chamber 18A, 18B, 18C, are 
disposed within this piping network for control of the refrigerant to the 
chambers. 
Each of the refrigerant solenoid valves 71A, 71B or 71C is connected to an 
R508B TEV (thermostatic expansion valve) 72A, 72B, 72C, and each TEV valve 
72A, 72B or 72C is connected to a corresponding liquid distributor 73A, 
73B or 73C. Each distributor 73 is connected in-line with each of ten 
panel refrigeration coils 11 (corresponding to the number of freezing 
panel assemblies mounted in each chamber 18A, 18B or 18C) by way of 
capillary tubes 75 (FIGS. 4 and 5) so that refrigerant routed through the 
distributor 73A, 73B or 73C is delivered to every coil 11 of a 
corresponding chamber. Each coil 11 is connected to a corresponding 
suction line 76A, 76B or 76C which, in turn, is connected to a suction 
solenoid valve 77A, 77B or 77C. Each suction valve 77A, 77B or 77C has an 
outlet which is connected to inlets of an R508B gas expansion tank 78, and 
the tank 78 has an outlet which is connected to the suction inlet of the 
R508B compressor 65 thereby completing the R508B refrigeration loop. 
The subsystem 172 further includes defrost cycle components includes a 
two-Kilowatt electric gas heater 69 to which the gas outlet of oil filter 
66 is connected, and the outlet of gas heater 69 is connected to a hot gas 
solenoid valves 70A, 70B and 70C. Each hot gas valve 70A, 70B or 70C is 
connected to the gas inlet of a corresponding distributor 73A, 73B or 73C. 
In addition, a capillary tube 81AC is joined between a location in the 
loop downstream of the suction line 76A and a location disposed between 
valves 71C and 72C; a capillary tube 81BA is joined between a location in 
the loop downstream of the suction line 76B and a location disposed 
between valves 71A and 72A; and a capillary tube 81CB is joined between a 
location in the loop downstream of the suction line 76C and a location 
disposed between valves 71B and 72B. A flow check valve 74A, 74B or 74C is 
connected in-line with each capillary tube 81AC, 81BA and 81BC. 
Preferably, each capillary tube 75, as well as the suction manifold 76A, 
76B and 76C is coated with a suitable insulating layer to reduce the 
likelihood of growth of ice upon the surfaces of these components. 
During a pellet-making operation within a chamber 18A, 18B or 18C, the 
valve 70A, 70B or 70C is closed and the corresponding valve 71A, 71B or 
71C and suction valves 77A, 77B and 77C are opened so that the R508B 
refrigerant of the subsystem 172 is directed through the coils 11 of the 
freezing panel assemblies 9 so that ice is formed in the compartments 8 
thereof. To subsequently release the grown pellets from the compartments 8 
of a chamber 18A, 18B or 18C, the valve 71A, 71B or 71C and suction valves 
77A, 77B and 77C are closed and the corresponding valve 70A, 70B or 70C is 
opened so that heated R508B refrigerant from the defrost loop of the 
subsystem 172 is directed into and condenses in the coils 11 of the 
freezing panel assemblies 9 to raise the temperature of the panel 
assemblies 9 to a condition at which the ice sublimes at the surfaces of 
the compartments 8 and thereby releases the pellets from the compartments 
8. As will be apparent herein, by controlling the opening and closing of 
the valves 70A, 70B, 70C, 71A, 71B, 71C, 77A, 77B and 77C, two of the 
chambers can be made to undergo a pellet-production phase at any one 
period of time while the remaining chamber can be made to undergo a 
pellet-release phase so that pellets are continually produced by the 
system 160. 
With reference to FIG. 6, there is depicted the growth of an ice pellet, 
indicated 7, within a freezing compartment 8 over a period of time. For 
example, soon after the pellet has begun to grow within the compartment 8, 
its thickness is quite small and its lowermost surface corresponds with a 
level, designated 7A, which is relatively close to the lower surface of 
the panel 10. When the ice pellet is about one-half grown, its thickness 
is increased so that its lowermost surface corresponds with a level, 
designated 7B, which is further from the panel 10 than is the level 7A; 
and when the pellet is grown to its desired thickness, its lowermost 
surface corresponds with a level, designated 7C, which is appreciably 
spaced from the panel 10. It will be understood that as the pellet 7 is 
grown within the compartment 8, its surfaces cling to the interior 
surfaces of the compartment 8 so that the pellet 7 remains affixed therein 
until released in a manner described herein. 
During the startup of the machine 160 from room temperature, a startup 
cycle is used to purge the chambers 18A, 18B and 18C of air and to aid the 
startup and cooldown of the cascade refrigeration system 166. During this 
startup cycle, gas valve 24 is cycled with exhaust valve 35 and vent valve 
34 to cyclically pressurize and de-pressurize the chambers. The purging of 
the chambers is aided in part by the placement of the gas supply ports (to 
which the pipe segments 22A, 22B and 22C are connected) at the bottom of 
the chamber and the exhaust ports 44 at the top of the chambers since the 
CO.sub.2 (being heavier than air) which is introduced through the supply 
ports displaces the air in the chambers. 
In addition, the refrigeration compressor 60 of the subsystem 168 is 
started during this start-up phase to allows the R508B refrigerant to 
pre-condense in the heat exchanger 61 by passing through the bypass valve 
68. Once the pressure of the R508B refrigerant is lowered below 150 psig, 
the compressor 65 is then turned on and the bypass valve 68 is closed, 
thereby initiating the cool-down of panel assemblies 9 within the 
chambers. During cool-down of the panel assemblies 9, refrigerant valves 
71A, 71B or 71C are turned on and off with a duty cycle sufficient to 
prevent an over-pressure trip of the compressor 65. When the panel 
assemblies 9 reach a temperature of approximately -50.degree. F., the 
normal refrigeration/defrost (i.e. pellet-producing/pellet-release) cycles 
are started. This startup and cool-down process takes approximately twenty 
minutes. 
Gas is delivered to each chamber 18A, 18B and 18C from the supply tank 47 
(which contains about one cubic foot of CO.sub.2 gas at 90 psig) by way of 
the solenoid valve 24. Operation of the solenoid valve 24 is controlled by 
the adjustable pressure switch 25, which is set to the desired operating 
pressure of chambers 18A, 18B and 18C. A typical chamber operating 
pressure of the chambers is 45 psig, which is well below the triple 
pressure of CO.sub.2, which is 60 psig. From the gas feed valve 24, the 
gas is fed into chambers where it freezes within the compartments 8 in a 
condensation or reverse-sublimation process. 
At a given time during the pellet-production phase of the operation, the 
ice forming in the compartments 8 of the panel assemblies 9 of any single 
chamber will be at approximately the same stage of growth. Early during 
the ice-growing stages, the ice forms primarily against the underside of 
the panel 10 and on the lower parts of the copper plating 14. Later in 
time, the ice continue to grow in thickness as additional CO.sub.2 freezes 
against the ice already formed in the compartments 8. When the ice has 
accumulated within each compartment 8 to thereby form a pellet of desired 
size, the panel assemblies 9 are switched to a defrost mode to initiate 
the release of the pellets from the compartments 8. Pellets are considered 
to be fully grown when the thickness thereof corresponds to the length of 
the copper plating 14 along the sides of the honeycomb 13. This way, the 
ice pellet which grows in one compartment 8 does not connect or bridge to 
the pellet which grows in an adjacent compartment 8 before it is released 
therefrom. 
Because the CO.sub.2 gas freezes in the compartments 8 directly from the 
gas phase to a solid phase within the chambers, the downwardly-opening 
orientation of the compartments 8 do not adversely affect the growth of 
the ice pellets therein. If the pressure of the growing chamber was above 
the triple pressure, liquid CO.sub.2 would condense on the panel 
assemblies 9 and run and drip out of the compartments 8. By comparison, to 
prevent liquid carbon dioxide from running out of the freezing molds, the 
liquid-to-solid systems of the prior art utilized upright molds which 
required mechanisms to either invert or separate the molds to facilitate 
the release of the ice forms. 
To initiate the defrosting of the panel assemblies 9 in a selected chamber, 
such as chamber 18A, the flow of refrigerant from the heat exchanger 67 
through the coils 11 of the chamber 18A (designated 11A in FIG. 5) is shut 
off (by closing the valve 71A) and the flow of heated refrigerant from the 
heater 69 to the coils 11A is permitted (by opening the corresponding 
valve 70A) and the flow of gas from the suction manifold 76A is shut off 
by closing the suction valve 77A. The R508B refrigerant entering the coils 
11A of the chamber 18A condenses therein thereby raising the temperature 
of the panel assemblies 9 (of the chamber 18A) to a temperature which is 
above the equilibrium vapor temperature of the CO.sub.2 gas in the 
chamber. The flow of the condensed refrigerant R508B proceeds through the 
capillary tube 81AC to check valve 74C through TEV 72C distributor 73C 
capillary tubes 75 and is evaporated in the coils 11C thereby 
refrigerating coils 11C in a chamber which is in the refrigeration, or 
ice-producing, phase. 
As the temperature of panel assemblies 9 rises above the vapor pressure 
equilibrium temperature, the ice at the interface of panel 10 is 
evaporated, causing the ice to be both physically and thermally detached 
from the panel 10. Heat from panel 10 can then flow down the plating 14 
and eventually onto the steel, unplated surface of the honeycomb 13 
thereby separating the rest of the ice from the compartment 8. When the 
pellet is fully separated from the surfaces of the compartment 8, it falls 
from the compartment 8 and onto the funnel surface 17 (FIG. 4) before 
being guided toward the bottom of the chamber for collection. Upon exiting 
the chamber bottom, the pellets move through the transfer tubes 21A, 21B 
and 21C into the hopper 43. The flow of pellets from the chambers can be 
aided by the operation of the vibrators 37. 
Once the defrost cycle is completed, the refrigeration of panel 10 is 
turned back on by closing the valve 70A, 70B or 70C and opening the valve 
71A, 71B or 71C and valves 77A, 77B, 77C so that the temperature of the 
panel assemblies 9 is returned to a temperature (e.g. -85.degree. F.) 
which is below the equilibrium vapor temperature of the CO.sub.2 gas 
within the chamber, to begin growth of a new batch of pellets therein. 
By providing an appropriate control scheme for controlling the sequential 
opening and closing of the valves 70A, 70B, 70C, 71A, 71B, 71C, 77A, 77B 
and 77C, pellets can be continually produced by the three chambers 18A, 
18B and 18C. In this connection, the ratio of time necessary to make a 
pellet of desired size to the time necessary to defrost (and thereby 
release) the pellets is about two to one. Consequently, the operation of 
the valves 70A, 70B, 70C, 71A, 71B, 71C, 77A, 77B and 77C can be 
controlled to maintain two of the chambers in a pellet-producing mode 
while the third chamber is in a defrost mode. At the end of the defrost 
mode, the third chamber is returned to a pellet-producing mode while a 
defrost phase is initiated in one of the other two chambers. It follows 
that while one of the chambers is in a defrost mode for releasing pellets 
for collection, the remaining chambers are in a pellet-producing mode. For 
purposes of accurately controlling the actuation of the valves 70A, 70B, 
70C, 71A, 71B, 71C, 77A, 77B and 77C and thereby controlling the 
sequencing of the pellet-producing and defrost modes of the chambers (as 
well as the operation of the cascade refrigeration system and 
pellet-handling operations), a control computer 400 (FIG. 5) having a 
microprocessor based programmable logic controller (PLC) is provided 
within the system 160 and operatively connected to the solenoid valves and 
operating components of the system for receiving command signals generated 
therefrom. To this end, the PLC has provisions for up to twenty-four 110 V 
AC relay outputs which are connected to each of the solenoid valves and 
motor-contactors in the system. 
Gas is created by sublimation during the release of the pellets from the 
freezing panels 9 during the defrost cycle, and this released gas can flow 
with the pellets from one chamber through a transfer tube 21A, 21B or 21C 
into the hopper 43. However, upon reaching the hopper 43, this gas is 
permitted to return to the interior of the other chambers (which are 
undergoing a pellet-producing phase) by way of the other transfer tubes 
21B, 21C or 21A. Thus, the CO.sub.2 gas which is evaporated within one 
chamber during a defrost cycle, is permitted to be re-frozen in the other 
chambers undergoing a pellet-producing cycle. 
The pellets collected in hopper 43 are periodically discharged into and out 
of the discharge hopper 46 by way of the transfer valve 28 and discharge 
valve 30 combined with valves 32, 33, 34, and 35. This pellet discharge 
operation can be controlled independently of the pellet-production cycle 
and functions as follows. With the valve 28 closed, pellets collect in the 
hopper 43. When the hopper 43 is filled to a desirable level, the pellets 
are thereafter transferred to the hopper 46. With discharge valve 30 
closed, the transfer is accomplished by first opening valves 35 and 33 
thereby allowing the hopper 46 to fill with CO.sub.2 gas and equilibrate 
in pressure with the chambers 18A, 18B and 18C and the hopper 43. The 
valve 28 is then opened, by activating solenoid valve 29, for a time 
sufficient to empty hopper 43. The flow of the pellets in hopper 43 
through valve 28 can be aided by operation of the associated vibrator 37. 
Valve 28 is then closed, thereby trapping the pellets in the hopper 46. 
Valve 35 is then closed, and valves 33 and 34 are opened thereby allows 
CO.sub.2 gas to vent out of hopper 46 at a rate controlled by needle valve 
32. 
The hopper 46 is preferably de-pressurized slowly since during this pellet 
transfer operation, the pellets drop in temperature from the growing 
equilibrium temperature of -75.degree. F. (before de-pressurization) to 
the atmospheric pressure equilibrium temperature of -110.degree. F. During 
this process, approximately 5% of the mass of the pellets in hopper 46 is 
lost by sublimation. If the hopper 46 is rapidly de-pressurized, the 
pellets tend to crack due to a temperature differential created between 
the interior of the pellets and the pellet surface during rapid 
de-pressurization which because of thermal contraction forces, fractures 
the ice. However, by de-pressurizing the hopper 46 relatively slowly, this 
fracturing of the pellets does not occur. If, of course, it is desirable 
to induce the pellets to crack into smaller pieces, the valving could be 
adjusted to effect rapid de-pressurization and thereby produce cracked 
ice. 
Once the hopper 46 is de-pressurized to the desired pressure level, the 
discharge valve 30 is opened by the control solenoid valve 31 and the 
pellets are emptied from the hopper 46 through the valve 30 into the 
storage chest 41. While the valve 30 is opened, the hopper 46 is purged 
with CO.sub.2 bleed gas to prevent air from entering the hopper 46. This 
is done by closing the vent valve 34 and opening valves 33 and valve 35, 
thus allowing CO.sub.2 from the chambers to flow through the hopper 46 at 
a rate controlled by the needle valve 32. 
Once the pellets are emptied from the hopper 46, the release valve 30 is 
closed while valve 33 and 35 remain open, thus re-pressurizing hopper 46, 
and thereby completing the pellet discharge cycle. 
The hopper 46 is equipped with a pressure relief valve 39 set at 75 psig to 
prevent over-pressurization. The pneumatic ball valves 28 and 30 are 
operated by using CO.sub.2 gas from the gas supply tank 47 since operation 
of the valve actuators below 32.degree. F. would freeze out moisture 
present in a compressed air system. The pressure switch 26 associated with 
the gas tank 47 is used to control the liquid CO.sub.2 supply valve 80 (of 
the intermediate subsystem 170) to thereby control the pressure in the 
tank 47. Meanwhile, the low pressure switch 27 of the tank 47 (set at 70 
psig) is used to ensure that the pressure in the tank 47 and the heat 
exchanger 67 is maintained above the triple pressure so that the liquid 
CO.sub.2 does not freeze up within the subsystem 170. The low pressure 
switch 27 is connected with the control gas supply valve 24, so that if 
the tank 47 pressure falls to the set point of switch 27, the valve 24 is 
prevented from opening. 
The aforedescribed cascade refrigeration system 166 (FIG. 5) possesses the 
capacity necessary to remove the heat of solidification of the CO.sub.2 
ice growing in the compartments 8 of the freezing panel assemblies 9, 
while maintaining the temperature of the freezing panel assemblies 9 below 
the vapor equilibrium temperature of the CO.sub.2 gas in the chambers 18A, 
18B and 18C. To this end, the cascade system 166 is designed to provide 
approximately 24,000 BTU/h of refrigeration at -80.degree. F. More 
specifically, the high temperature loop 168 has a capacity of 
approximately 30,000 BTU/h at -10.degree. F. for cooling and partially 
condensing the R508B refrigerant exhausting from the low temperature 
system compressor 65. The R404A subsystem 170 is similar to the 
refrigeration systems used in commercial freezers in that it utilizes a 5 
HP scroll compressor 60 with an air cooled condenser 64, and the 404A 
refrigerant is stored in a receiver 63. In operation, liquid refrigerant 
is evaporated in the R508B/R404A plate/plate heat exchanger 61, and the 
flow of the R404A refrigerant flow is controlled by the TEV valve 62. 
The intermediate temperature subsystem 170 is used to complete the 
condensation of the R508B refrigerant and sub-cool the R508B refrigerant 
to approximately -40.degree. F. In addition and as mentioned earlier, this 
subsystem 170 also provides a dual use of evaporating liquid CO.sub.2 to 
provide gas to the supply tank 47 for the eventual production of pellets 
within the chambers. The utilization of liquid CO.sub.2 as a feedstock for 
the system 160, rather than CO.sub.2 gas, is advantageous because of the 
fact that liquid CO.sub.2 is a readily available commodity. However, if it 
is desired to produce a pelletizing system which utilized gaseous CO.sub.2 
as a feedstock, a cascade refrigeration system could be designed without 
the intermediate liquid CO.sub.2 stage by increasing the refrigeration 
capacity of the 404A stage. 
With reference again to the depicted system 160, the liquid CO.sub.2 
refrigeration subsystem 170 withdraws liquid CO.sub.2 from the storage 
tank 79 wherein the liquid CO.sub.2 is typically stored at pressures of 
between 200 to 300 psig. The withdrawn liquid CO.sub.2 flows through the 
solenoid valve 80 which is controlled by pressure switches 26 to maintain 
the CO.sub.2 pressure in tank 47 at 90 psig. The liquid CO.sub.2 is 
reduced in pressure by the control valve 83 and is evaporated in the heat 
exchanger 67 by the condensing R508B refrigerant. The liquid CO.sub.2 
refrigerant flow is controlled by the gas supply valve 24 at a rate 
determined primarily by the rate at which CO.sub.2 is frozen into pellets 
within the chambers 18. The pressure relief valve 82, set at 400 psig, 
protects the inlet line from over-pressurization, the relief valve 38, set 
at 150 psig, protects the tank 47 from over-pressurization. The low 
pressure switch 27 prevents gas supply valve 24 from opening if the 
pressure in the tank 47 starts to drop to the triple pressure thus 
preventing the liquid CO.sub.2 in the system 160 from freezing up. 
Within the low-temperature subsystem 172, the R508B refrigerant is 
compressed by the compressor 65 and is subsequently liquified and 
subcooled in the heat exchangers 61 and 67. The heat exchanger 67 also 
serves as a refrigerant receiver, as the R508B charge is sufficient to 
fill approximately one-half of the heat exchanger 67 with liquid R508B. 
The refrigeration of the freezing panels 9A in chamber 18A is accomplished 
by opening liquid refrigerant valve 71A, thus allowing the refrigerant to 
flow through TEV 71A and into the corresponding distributor 73. The 
distributor 73, in turn, splits the refrigerant flow (through capillary 
tubes 75) into ten approximately equal parallel flows corresponding to the 
ten freezing coils 11 mounted in the chamber 18A. The refrigerant is 
evaporated in coils 11 within the chamber 18A thereby absorbing the heat 
of solidification of the CO.sub.2 freezing on the corresponding panel 
assemblies 9. The evaporated R508B from each coil 11 flows into suction 
manifold 76A, and then flows through the open suction valve 77A to 
expansion tank 78 and is returned to the suction inlet of compressor 65, 
thereby completing the low temperature refrigeration loop. As mentioned 
herein, two of the chambers 18A, 18B or 18C are in a refrigeration (i.e. 
pellet-producing) mode at a given time. 
As discussed above, the defrost cycle of the R508B loop is used to heat the 
panel assemblies 9 to cause the CO.sub.2 ice to release from the freezing 
compartments 8 and thereby drop in pellet form toward the bottom of the 
chamber. During the defrost cycle, the compressed R508B which leaves the 
oil filter 66 flows through the electric heater 69 which raises the 
temperature of the R508B from the compressor 65 discharge temperature of 
approximately 130.degree. F. to 250.degree. F. The electric heater 69 
consists of a 2 KW cartridge heater inserted into a copper pipe which has 
a coil of 0.25 inches of copper refrigeration tubing soldered to its 
exterior and a thermostatic switch attached to control the temperature. 
Upon exiting the hot gas valve 70A, 70B or 70C, the hot gas enters a side 
port in the corresponding distributor 73A, 73B or 73C and flows (through 
capillary tubes 75) to the ten coils 11 of the panel assemblies 9. The 
pressure in the coils is raised above the condensation point because the 
corresponding suction valve 77A, 77B or 77C is closed, causing the R508B 
to condense in the coils 11 releasing heat and warming the plate 10 of the 
panel assemblies 9. The condensed refrigerant thereafter flows through the 
coils 11 into the exhaust manifold 76A, 76B or 76C. At the bottom of the 
exhaust manifold (and upstream of the closed suction valve 77A, 77B and 
77C), a capillary tube 81AC, 81BA or 81CB carries the condensed 
refrigerant through check valve 74A, 74B or 74C and back to the 
distributor 73 of a chamber undergoing a pellet-producing phase so that 
the condensed refrigerant is evaporated within the coils 11 of the 
pellet-producing chamber. The evaporated R508B then passes through the 
exhaust manifold of the pellet-producing chamber, through the open suction 
valve, into the expansion tank 78 and eventually back to compressor 65. 
The expansion tank 78 of the system 160 is designed to provide sufficient 
volume to contain the expanded R508B refrigerant at pressures below 400 
psig during periods when the system 160 is at room temperature. In 
addition, the bypass valve 68 is connected between the outlet and inlet of 
the compressor 65 to allow the system pressure to equilibrate at the end 
of operation. The valve 68 is closed during normal operation. 
Typical refrigeration/defrost cycles associated with the system 160 require 
about thirty minutes of refrigeration followed by fifteen minutes of 
defrost. Consequently, the control computer 400 can be programmed so that 
for a first fifteen minute period, two chambers 18A and 18B undergo a 
pellet-producing phase while chamber 18C undergoes a defrost phase, for a 
second fifteen minute period, two chambers 18B and 18C undergo a 
pellet-producing phase while chamber 18A undergoes a defrost phase, and a 
third fifteen minute period, two chambers 18A and 18C undergo a 
pellet-producing phase while chamber 18B undergoes a defrost phase. These 
first, second and third fifteen minute periods are thereafter repeated to 
continually produce pellets from the chambers. Under these circumstances, 
the pellets can be released from the hopper 46 every fifteen minutes. 
The aforedescribed dry ice pellet-making system 160 is adapted to produce 
clear, dry ice pellets having 0.125 inch diameter at the rate of about 
forty pounds per hour with a gas use efficiency of approximately eighty 
percent. The pellets produced by this system 160 are particularly 
well-suited for use in a dry ice pellet blasting machine, although other 
uses of the pellets can be found. The machine uses bulk liquid CO.sub.2 as 
a source of CO.sub.2 since liquid CO.sub.2 is a readily available 
commodity. 
Systems similar in construction to the aforedescribed system 160 could be 
designed with alternative panel geometries, compressor configurations and 
chamber designs. For example, instead of positioning the storage hopper 
outside of a chamber, a storage hopper could be incorporated within the 
bottom of a chamber. Still further, a freezing panel assembly for 
installation within a chamber could be designed for the production of 
larger ice pellets than those produced with the panel assembly 9 of the 
system 160. For example, there is shown in FIGS. 7 and 8 a panel assembly, 
generally indicated 176, which has been designed to produce larger 
pellets, i.e. 3/8 inches to 3/4 inches in diameter, than those produced by 
the earlier-described panel assembly 9. The panel assembly 176 of FIGS. 7 
and 8 is constructed similar to a tube-bank heat exchanger and includes an 
array of pellet-growing tubes 90 which are somewhat frusto-conical in 
shape having side surfaces which are tapered from a larger, open bottom 
end toward the smaller upper end at the rate of about 1/4 inch per linear 
foot, i.e. the diameter of the bottom end of the tube is larger than the 
diameter of the other upper end by 1/4 inch per foot of length. The taper 
of the tubes 90 in this manner facilitates the release of the ice pellet 
during a defrost phase of the operation. 
The tubes 90 of the depicted panel assembly 176 are of a length equal to 
the desired pellet length plus an additional length on each end to allow 
for thermal insulation of the end of the tube from the surrounding 
refrigerated section. The tubes 90 are constructed of stainless steel 
which is brazed or welded into a set of four stainless steel plates with 
holes stamped to accept the tubes 90 fitted therein. The assembly 176 also 
includes a top plate 89, a bottom plate 93, an upper refrigerant enclosure 
plate 91 and a lower refrigerant enclosure plate 92. The spacing between 
the refrigerant plates 91 and 92 is approximately the desired length of 
the ice pellet to be formed. The spacing between top plate 89 and 
enclosure plate 91 is sufficient to provide for a relatively low thermal 
conduction path along tubes 90 between the top plate 89 and the 
refrigeration plate 91, which in this case is approximately one tube 90 
diameter. Likewise, the spacing between plates 92 and 93 is sufficient to 
provide for a relatively low thermal conduction path along the tubes 90 
between the bottom plate 93 and the lower refrigeration plate 92, which in 
this case is approximately one tube 90 diameter. 
The refrigeration plates 91 and 92 are brazed or welded to refrigeration 
side plates 99, 102, 103, 104 which enclose all sides, thus forming a 
closed interior refrigeration space 101 between plates 91, 92, 99, 102, 
103, 104, and the tubes 90 for the passage of refrigerant over the 
exterior of tubes 90 between plates 91 and 92. A refrigerant entrance tube 
94 permits refrigerant to enter the space 101, and the tube 94 is welded 
into plate 99. An exhaust tube 97 is welded into plate 102 and extends out 
of space 101. Top plate 89 and bottom plate 93 are enclosed on all sides 
by side plates 100, 88, 106, and 107, thus forming an insulation interior 
space 105 between the refrigeration plates 91, 92, 99, 102, 103, 104, 
outer plates 89, 93, 100, 89, 106, 107 and tubes 90 wherein the insulation 
interior space 105 completely surrounds refrigeration interior space 101. 
Refrigeration entrance tube 94 passes through and is welded to plate 100, 
and exhaust tube 96 passes through and is welded to plate 102. 
A defrost gas entrance tube 95 is connected to the plate 100, and the 
defrost exhaust tube 97 is connected to the lower end of plate 88 and to 
exhaust tube 96 so as to allow condensed refrigerant to drain from space 
105 into exhaust tube 96. Sufficient baffling plates (not shown) are 
provided within refrigeration space 101 to provide for a uniform flow of 
refrigerant over tubes 90. 
The operation of the panel assembly 176 provides for the freezing and 
release of tapered cylindrical pellets formed within the tapered tubes 90. 
To this end and in the freezing (pellet-producing) cycle, refrigerant 
enters tube 94 through a refrigeration valve 110 and passes over the 
exterior of tubes 90 within space 101, flowing out exhaust tube 96 through 
an open suction valve 108. In addition, the defrost tube 95 is closed by a 
defrost valve 111 during this freezing cycle. The insulation space 105 is 
connected to the exhaust tube 96 through the tube 97 which is filled with 
stagnant refrigeration gas at the suction pressure. Since refrigeration 
gases have relatively poor thermal conductivity, the insulation space 105 
reduces heat transfer from the outer plates 89, 93 to the inner 
refrigeration plates 91, 92. The refrigeration of the space 101 causes ice 
to grow directly from gas in the interior of the tubes 90. During the 
growth of the ice pellets, each pellet grows from the sidewalls of its 
tube 90 toward the center thereof but is prevented from growing to either 
of the ends of the tubes 90 by the thermal insulation provided by the 
insulation space 105 about the ends of the tubes 90. 
When the pellets are grown to the desired size within the tubes 90, the 
panel assembly 176 is defrosted by closing the suction valve 108 and 
refrigeration valve 110 and opening the hot gas defrost valves 111 and 
112. The pressure within the spaces 101 and 105 rises and the refrigerant 
condenses on all the cold surfaces, including those in space 105, thereby 
warming the tube surfaces and causing the ice forms to release and fall 
out of tubes 90. Any liquid which condenses in spaces 101 and 105 flows 
out through tubes 96 and 97 and through capillary tube 109 to refrigerate 
another panel assembly 176 undergoing an ice- growing phase. A design 
variation of the panel assembly 176 would be to have tubes 90 terminate at 
refrigeration panel 91 rather than passing through the upper plate 89. 
However, an advantage provided by the connection of tubes 90 to the plate 
89 is that plate 89 can be of thinner material and still withstand the 
refrigerant pressure since tubes 90 aid in the support of the plate 89. 
With reference to FIG. 9, there is illustrated an arrangement of freezing 
panel assemblies, generally indicated 200, capable of being used for 
producing pellets of frozen hydrogen within a system 202 depicted in FIG. 
10. Each panel assembly 212 includes a deep drawn 305 stainless closed 
tube 212 having an outer diameter of 0.25 inches, a wall of 0.012 inches 
thickness, and a length of 1.5 inches. In addition, each panel assembly 
200 includes one-hundred and twenty-seven downwardly-opening tubes 212 
arranged in a hexagonal array at the top of a pellet growing chamber 282 
and wherein adjacent tubes are spaced 0.4 inches apart. Each tube 212 has 
a closed end portion 214 which is plated across its ends and along a short 
distance along the sides thereof with a copper cap 216 having a thickness 
of 0.006 inches. Each tube 212 also has an open end which is brazed into a 
304 stainless steel manifold plate 218 having a thickness of 0.25 inches 
and a diameter of 6.0 inches. The plate 218 is provided with holes for 
accepting the tubes 212 positioned therein. Brazed to the top of the 
manifold plate 218 is a copper plate 220 having a thickness of 0.125 
inches and which is also drilled to accept the tubes 212 inserted 
therethrough. Soldered upon each copper cap 216 is a thermal conduction 
strip 224 which is made from high purity copper foil of dimensions 
measuring 0.020 inches in thickness, 0.125 inches in width and 2.0 inches 
in thickness. The other end of each strips 224 is soldered to a cold 
copper plate 226 having a thickness of 0.75 inches. 
Attached to each conductor strip 224 directly over each copper cap 216 is a 
small thick-film resistance heater 228. Furthermore, there is attached to 
each strip 224, also over each cap 216 and adjacent to heater 228, is a 
diode thermometer 230. The thermometer 230 and heater 228 are both 
attached to the strips 224 with a filled epoxy resin (Stycast 2850). The 
cold plate 226 is bolted to a second stage head 232 (FIG. 10) of a 
Gifford/McMahon type refrigerator 234 which can produce temperature below 
8.degree. K. In addition, a cold plate 220 is bolted to a first stage of a 
head 236 of the refrigerator 234 capable of producing temperatures down to 
30.degree. K. The manifold plate 218 is welded to a tube 240 which is, in 
turn, connected to vacuum flange 242. This flange 242 is bolted to a base 
plate 244 provided with a hole for accepting the flange 242. Directly 
opposite the flange 242 and welded to the base plate 244 is a vacuum Tee 
246. A gas entrance side port 248 provided in the Tee 246 is closed with a 
gas feed flange 250. Gas feed tube 252 is attached to a flange 250. The 
bottom port of Tee 246 is closed by a reducing flange 256, the outlet port 
258 of which is 0.75 inches in diameter. Connected to the port 258 is a 
gate valve 260 which is connected to a transfer chamber tube 262. 
Furthermore, a release valve 264 is connected to the bottom end of tube 
262, and a mounting flange 266 of the refrigerator 234 is bolted to the 
base plate 244 which is, in turn, bored to allow the refrigerator 234 to 
pass through it. A metal bell jar 268 covers and is bolted to the base 
plate 244 thereby enclosing the cold heads 232 and 236 and the freezing 
panel assembly 200. The volume within the bell jar 268 is evacuated by a 
pump (not shown) to produce vacuum thermal insulation of the components 
positioned within the bell jar 268. To further insulate the cold heads 232 
and 236 and the freezing panel assembly 200, the components positioned 
within the bell jar 268 are wrapped with a blanket (not shown) of 
aluminized mylar film superinsulation (not shown). For purposes of 
directing frozen hydrogen pellets into the port 258, a funnel 276 is 
placed above the port 258 and beneath the pellet assembly 200. To prevent 
pellets from bouncing out of the gas entrance port 248, a screen 278 is 
placed over the port 248 wherein the mesh of the screen is smaller than 
the pellets being produced. Each heater 228 and each thermometer 230 is 
connected to electronic multiplexer circuits (not shown) designed to 
address and actuate each heater 228 and measure each thermometer 230 from 
a computer control station 280. 
In preparation of the system 202 for operation, the hydrogen chamber 240 
and the bell jar 268 are both pre-evacuated with an external pump (not 
shown). The refrigerator 234 is then turned on thereby starting the 
cooldown of the panel assemblies 200. Following a cooldown period of about 
two hours, one cold head 232 reaches a temperature at or below 8.degree. 
K. and the other cold head 236 reaches a temperature of 30.degree. K. 
Hydrogen gas is supplied continuously through gas supply tube 252 at a 
desired rate, for example 20 TorrLiters/second, thereby raising the 
pressure in the chamber 282 to a value above 0.1 Torr which is the vapor 
equilibrium temperature at hydrogen at 8.degree. K. The hydrogen begins 
freezing in the freezing tubes 212 in region directly beneath the caps 
216. As the ice grows within the tubes 212, the pressure within the 
chamber 282 increases to a value near 10 Torr, at which the vapor 
equilibrium temperature of hydrogen is 11.5.degree. K. The heat of 
solidification of the hydrogen ice is carried to the cold head 232 through 
the copper strips 224. After a period of time of approximately ten 
minutes, the pellets in the freezing tubes 212 are grown to desired size. 
At this time, sequential release of the pellets is initiated wherein one 
pellet 284 (FIG. 10) is released from the tubes 212 about every seven 
seconds upon application of a voltage pulse to its associated heater 228. 
The heat from the heater 228 raises the temperature of the cap 216 to a 
temperature of approximately 20.degree. K. so that the pellet 284 is 
released from the tube 212. The strip 224 is designed to limit the thermal 
conduction therethrough so that during the heat pulse of a single heater 
228, the cold head 232 and thereby the rest of the freezing tubes 212 are 
increased in temperature by only approximately 0.1.degree. K. 
The manifold 218 is maintained near 30.degree. K. by the cold plate 220 
which is connected to the first stage 236 of the refrigerator 234. 
Therefore, hydrogen gas which enters the freezing tubes 212 is cooled when 
it comes into contact with the manifold 218. A pellet 284 which has been 
released from a tube 212 falls down and is guided by a funnel 276 through 
an open valve 260. Pellets can then be transferred to a storage chamber 
(not shown) operating at a substantially different pressure by sequencing 
load lock valves 260 and 264. Alternatively, valves 260 and 264 can remain 
open to drop the pellets directly into the loading mechanism of a 
centrifugal hydrogen pellet accelerator (not shown) operating in a chamber 
at the same pressure as chamber 240 and used, for example, to inject 
frozen pellets into a fusion reactor. To prevent wayward pellets from 
bouncing into the gas inlet port 248, the port 248 is preferably covered 
with the screen 278. 
Following the release of a pellet 284 from a tube 212, the cap 216 is 
re-cooled by conduction through strip 224 so that a new pellet begins to 
grow. By the time the system 200 sequences through (i.e. releases the 
pellets from) the one-hundred and twenty-seven tube cells, another pellet 
has been fully grown and is ready for release. In this manner, hydrogen 
continuously flows into chamber 240 as a gas, is manufactured into ice 
pellets by reverse sublimation in tubes 212 and out of chamber 240 as 
pellets through valves 260 and 264. 
With reference to FIG. 11, there is schematically shown an alternative 
hydrogen pellet fabrication machine, generally indicated 300, designed to 
use liquid helium for controlling the temperature of the freezing 
compartment surfaces. System 300 is further designed to pump hydrogen and 
the hydrogen isotopes deuterium and tritium from a process such as a 
fusion reactor and to manufacture hydrogen isotope pellets for refueling 
the reactor chamber. In this FIG. 11 system 300, there is provided a 
substantially-enclosed chamber 296 within which hydrogen is introduced 
through pump entrance flange 299 from a process chamber 287. Enclosing the 
top of chamber 296 is a plate 336 into which a plurality of freezing 
cells, or tubes 302, are brazed. These tubes 302 are constructed of deep 
drawn stainless steel and are tapered in shape so as to provide a 
downwardly-directed open end which is wider at the bottom of the tube 302 
than at the top thereof so as to facilitate the release of hydrogen 
pellets 301 during a defrost phase of the system operation. In this 
connection, the tubes 302 are each 0.42 inches in diameter at the closed 
end, are 0.46 inches in diameter at the open end, are 1.75 inches long and 
have a 0.012 inch wall thickness. The open end of each tube is brazed into 
holes in the plate 336. Around the top or closed end of each tube 302 is 
wrapped and brazed about five turns of a copper capillary tube 304 which 
is 0.12 inches in diameter with a 0.028 inch wall, thus forming a surface 
305 inside each tube 302 under copper tube 304 which extends approximately 
0.6 inches down the side from the closed end of each tube 302 so that the 
surface 305 is in good thermal contact with the tube 304. Each end of 
capillary tube 304 extends to an inlet manifold plate 306 and an outlet 
manifold plate 308. 
At the inlet manifold 306, each capillary tube 304 is brazed to a 304 SS 
(stainless steel) tube 310 having a 3/16 inch outer diameter, a length of 
3.0 inches, and a 0.144 inch inner diameter. Each tube 310 is, in turn, 
brazed to the manifold plate 306. Inserted into each tube 310 (and as best 
shown in FIG. 12) is a cartridge heater 312 having an outer diameter of 
0.1235 inches and a total length of three inches and an unheated section 
of one inch. Each heater 312 passes through an orifice hole 313 bored in 
manifold plate 306 which is 0.125 inches in adiameter. The gap formed 
between the orifice hole 313 and the unheated section of heater 312 
functions to meter the flow of liquid helium refrigerant through each 
capillary tube 304. The manifold plate 306 is welded to a 3.0 inch 
diameter 304 SS tube which is supplied with liquid helium from a storage 
dewar 314 through a constant flow transfer tube 316. The other end of 
capillary tubes 304 are each brazed to tubes 318 which are brazed to the 
exhaust manifold plate 308. The tubes 318 are of the same design as tubes 
310, and the manifold plate 308 is of the same design as the manifold 
plate 306. The manifold 308 is connected to an exhaust plenum tube 320 
which is, in turn, connected to exhaust tube 322. 
Operation of the liquid helium refrigeration and heating cycle of hydrogen 
pellet-fabrication system 300 is described as follows. Liquid helium is 
supplied to the reservoir 324 by pressurization of dewar 314 causing 
helium to flow at a rate controlled by the constant flow transfer tube 316 
into the helium supply reservoir 324. Liquid helium from reservoir 324 is 
split into seventy-three substantially equal flows through orifice 313, 
into each tube 310 flowing over heater 312 and into each capillary tube 
304. The liquid helium is evaporated in the capillary tube coil 304 
thereby absorbing the heat of solidification of the hydrogen ice growing 
on freezing surfaces 305 in tubes 302. The helium gas is then exhausted 
through tubes 318 into the plenum 320 and out through the exhaust tube 
322. The release of a ice pellet 301 from a single tube 302 is 
accomplished by raising the temperature of surface 305 to above 20.degree. 
K. by applying a voltage to its associated heater 312, thereby evaporating 
the liquid helium flowing through the tube 310 and raising the temperature 
of the helium gas flowing therethrough to a temperature substantially 
above 20.degree. K. The heated helium gas then flows through capillary 
coil 304 raising the temperature of surface 305 to 20.degree. K. causing 
ice growing within each tube to be released as a pellet 301 and fall out 
of the tube 302. 
The transport of the hydrogen through hydrogen pellet-fabrication system 
300 is described as follows. Hydrogen gas which is to be pumped from the 
process chamber 287 at an operating pressure below the triple pressure, 
enters pump chamber 296 through the inlet flange 299 which is a six inch 
ID copper seal flange. The hydrogen gas entering chamber 296 flows over 
and through baffle funnels 326 (along paths exemplified by the flow arrows 
203) which are maintained at a temperature of approximately 20.degree. K. 
by a helium gas cooling circuit (not shown). The baffles 326 thus pre-cool 
the hydrogen gas and cryogenically condense impurity gases present in the 
hydrogen gas stream onto the surfaces of baffles 326 thus purifying the 
hydrogen gas stream. The hydrogen gas stream then flows into tubes 302 and 
reverse sublimates onto freezing surfaces 305. After a time sufficient to 
form the desired thickness of ice on surfaces 305, heaters 312 are 
periodically activated in sequence releasing the pellets 301 from the 
surface 305. The pellets 301 fall out of the tubes 302 and are guided by 
funnels 326 through the exhaust port 328 and valves 328 and 330. Valve 330 
can, for example, be connected to a pellet accelerator (not shown) for 
re-injection of the hydrogen pellets into the process chamber 287. In this 
manner, the hydrogen gas from the chamber 287 continuously flows into pump 
system 300, is purified on baffles 326, is formed into ice pellets 301 on 
surfaces 305, which are released and re-injected as solid hydrogen pellets 
into process chamber 287. 
It will be understood that numerous modifications and substitutions can be 
had to the aforedescribed embodiment without departing from the spirit of 
the invention. For example, although the aforedescribed embodiments have 
been shown and described for use in producing pellets of carbon dioxide or 
hydrogen, systems which embody the principles of the present invention can 
be used for the production of pellets of deuterium, tritium or neon. For 
example, the table below sets forth a list of gases capable of being 
converted to pellet form for use in selected applications, and each gas of 
this list is capable of being used as the gas medium for conversion 
directly from the gas form to a solid, pellet form by way of a system and 
process in accordance with the present invention. 
______________________________________ 
triple point 
triple point pressure 
Material Formula temp. Kelvin 
kilo-Pascals 
______________________________________ 
argon A 83.78 68.75 
carbon dioxide 
CO.sub.2 216.5 518.0 
chlorine Cl 172.2 2 
hydrogen H.sub.2 14.0 7.2 
deuterium D.sub.2 18.7 17.2 
tritium T.sub.2 20.6 21.6 
neon Ne 24.5 46 
methane CH.sub.4 90.7 11.7 
silane SiH.sub.4 
88.1 0.13 
xenon Xe 161.2 82 
nitrogen N.sub.2 63.1 12.5 
oxygen O.sub.2 54.4 0.15 
ammonia NH.sub.4 195.4 6.07 
silicon fluoride 
SiF.sub.4 
183 241 
hydrogen chloride 
HCl 158.9 13.4 
ammonia chloride 
NH.sub.4 Cl 
611 100 
uranium 
hexafluoride 
UF.sub.6 337 150 
arsine AsH.sub.3 
156.2 3.0 
______________________________________ 
Accordingly, the aforedescribed embodiments are intended for the purpose of 
illustration and not as limitation.