Method and system for cryogenic refrigeration using air

A method and system for cooling air to cryogenic temperatures [e.g., below - 100.degree. F. (- 730.degree. C.)] for use as a refrigerant medium for direct contact cooling of articles such as foodstuffs for fast freezing.

FIELD OF THE INVENTION 
The present invention relates to a method and system for cooling air to 
cryogenic temperatures, the cooled air to be used for, inter alia, 
introduction into a freezer for quick freezing articles such as 
foodstuffs. 
BACKGROUND OF THE INVENTION 
U.S. Pat. Nos. 4,315,409 and 4,317,665 disclose and claim improvements to 
cryogenic freezing systems utilizing air at cryogenic temperatures such as 
disclosed in U.S. Pat. Nos. 3,733,848 and 3,868,827. In the systems of the 
foregoing patents, air taken from that surrounding the apparatus to be 
cool ed, e. g., food freezer, is cooled to temperature below -180.degree. 
F. so that when introduced into the freezer at this temperature quick 
freezing of articles in the freezer can take place. Such freezers find use 
in the food industry for quick freezing foods for preservation and 
shipping of the foods. 
The prior art systems rely upon the recirculation of the atmosphere from 
the freezing compartment after extracting some of the refrigeration by 
recompression and expansion to achieve the very low temperatures. Problems 
with the recycle system center on the fact that the federal government 
requires thorough cleaning and sanitation of this type of equipment. A 
recycle system embodied in a heavy piece of equipment such as a system 
including compressors and the like to take air from ambient temperature to 
below -180.degree. F. are generally not easily opened up for cleaning. 
Thus such systems are susceptible to frost buildup and the recycle of 
bacteria particles and frost particles since the atmosphere is constantly 
reused. 
SUMMARY OF THE INVENTION 
The present invention pertains to the use of a cryogenic air refrigeration 
cycle whereby very cold air in gaseous form is produced by a series of 
intercooled stages of a compressor and a turbo expander. The cold gas is 
supplied to an insulated enclosure to accomplish quick freezing of 
articles contained inside of the insulated enclosure. Once such insulated 
enclosure is a conventional cryogenic food freezer, wherein the food to be 
frozen is contacted by air at temperatures of below approximately 
-200.degree. F. (-129.degree. C.). Air withdrawn or exiting from the 
insulated compartment is integrated into the system and is used after heat 
exchange with air to be cooled for injection into the insulated 
compartment prior to expansion. The withdrawn air is warmed to an elevated 
temperature to regenerate systems for moisture and gaseous contaminant 
removal from the compressed air stream prior to cooling and expansion. A 
portion of withdrawn air is subjected to sterilization prior to being used 
for regeneration and then is vented to the atmosphere. Thus, the method 
and apparatus of the invention rely on non-recirculating air to avoid the 
problems of the prior art systems.

DETAILED DESCRIPTION 
One of the significant problems in using mechanical refrigerators to freeze 
foodstuffs is that at temperatures produced by mechanical refrigerators 
utilizing chlorofluorocarbons or ammonia as a refrigerant, the product 
being frozen, especially foodstuffs, are subject to severe dehydration and 
loss of flavor and quality when used by the ultimate consumer. Mechanical 
refrigerators can produce cold air at temperatures approximately 
-35.degree. F. (-37.degree. C.). Cryogenic food freezers utilizing liquid 
nitrogen are well known and will serve to prevent excessive dehydration. 
However, cryogenic food freezers utilizing a cryogen other than air, e.g., 
nitrogen or carbon dioxide, are expensive and do have the problem of 
safely venting vaporized cryogen in and around the freezing apparatus. 
According to the present invention the method and system permit the use of 
air to achieve all of the efficiency and product enhancement using 
cryogenic freezing of prior art devices with the additional benefits of 
reduced freezer frost build-up, reduced maintenance time and costs, and 
improved sanitation due to the fact that the air is used only once in a 
true open cycle configuration. 
Referring to the drawing, the system 10 includes an insulated enclosed 
space 14. Insulated enclosed space 14 represents, among other things, a 
conventional food freezer of the spiral, impingement, or tunnel type such 
as are well known in the art. Insulated enclosed space represented by 14 
is cooled by taking a stream of air 16 passing the stream of air 16 
through a particulate air filter 20 of the type that will filter out over 
98% of particulate matter having a size greater than 20 microns average 
diameter. The filtered air is conducted via a conduit 22 to a multi-stage 
compressor 24, the inlet air having a temperature in the range of 
approximately 20.degree. F. (-6.7.degree. C.) to 105.degree. F. 
(40.5.degree. C.) and a pressure of 14.1 psia (97.21 Kpa). Compressor 24 
is a multi-stage (e.g. four-stage) compressor with intercooling so that 
the air in conduit 26 exiting the compressor 24 is at approximately 198 
psia (1365.01 Kpa) and approximately 200.degree. F. (93.degree. C.). 
Conduit 26 conducts the compressed and heated air to an aftercooler 28 
where the compressed air stream is cooled without loss of pressure to 
within plus or minus 10.degree. F. of ambient and conducted via conduit 30 
to a separator 32 where water is removed from the compressed air stream. 
Water from separator 32 can be removed via conduit 34 for disposal as is 
well known in the art. The compressed air stream is conducted from 
separator 32 via conduit 36 to a dryer/particulate removal arrangement, 
the components being outlined in box 38 which includes at least two 
vessels 39 and 40 containing material, e.g. molecular sieves for moisture 
and gaseous contaminant removal. Depending upon the type of material in 
the vessels 39, 40 in addition to removal of final amounts of water vapor, 
gaseous contaminants such as carbon dioxide can also be removed. The 
system 38 includes the necessary switching valves 42, 44 so that the 
vessels 39 and 40 can be onstream and/or regenerated as is well known in 
the art. Also included in the dryer/particulate removal arrangement 38 is 
a particulate trap 46 to remove any carryover sieve material or other 
particulates in the compressed air stream. The compressed air stream is 
conducted from trap 46 via conduit 48 to a heat exchanger 50 where the 
compressed air stream is cooled to a temperature of approximately 
-90.degree. F. (-68.degree. C.) without loss of more than a negligible 
amount of pressure. The cooled compressed air stream is conducted from 
heat exchanger 50 via conduit 52 through a particulate strainer 54 into 
conduit 56 for introduction into a turbo expander 58. Particulate strainer 
54 is included to protect the turbo expander 58. The cooled gas stream 
exits turbo expander 58 via conduit 60 at approximately -250.degree. F. 
(-157.degree. C.) and 15.2 psia (104.79 Kpa) where it is injected into the 
insulated space 14 for producing a cooled refrigerated space for cooling 
or freezing articles contained therein. As in all balanced flow 
refrigeration system, air that has given up its all or part of its 
refrigeration capacity is withdrawn from the insulated space via conduit 
62 and is passed through an ice and particle filter 64 to conduit 66 
through heat exchanger 50 where the air entering heat exchanger at 
approximately -100.degree. F. (-73.degree. C.) and 14.7 psia (97.21 Kpa) 
exits the heat exchanger 50 in conduit 68 at approximately 13.3 psia 
(91.69 Kpa) and 90.degree. F. (32.2.degree. C.). The warmed withdrawn gas 
stream in conduit 68 is introduced to a blower 70, exits blower 70 through 
conduit 72 is introduced into a sterilizer 74 such as a ultraviolet light 
sterilizer, exits sterilizer 74 through conduit 76 and then can be 
introduced into the system 38 for regenerating the vessels 39, 40 and then 
exits the system through conduit 78. Alternatively the withdrawn air can 
be discharged from the system via conduit 78. The withdrawn air is never 
recycled into the system but is used only for regenerating the adsorbers 
in system 38, thus there is no contamination of the incoming air since the 
withdrawn air has been sterilized and there is no ice buildup in the 
recycled air because it has been passed through the ice and particulate 
filter 64. 
The compressor 24 and expander 58 are joined by providing an additional 
pinion in the compressor for mounting of the expander. The compressor can 
be run by a double shafted 1,500 horsepower induction motor which can also 
be used to drive the vacuum blower 70. The entire system except for the 
insulated container 14 can be mounted on a skid for ease in installation 
into an existing plant utilizing other types of refrigeration systems. The 
aftercooler 28 can be a closed loop glycol radiator system which can be 
used to provide cooling for the interstages of the main air compressor 24 
as well as providing cooling of the discharge from the main air 
compressor. The insulated container 14 can be a freezer such as a spiral 
type food freezer. 
From the foregoing it can be seen that air can be used to produce cryogenic 
temperatures for cooling an insulating container or for effecting food 
freezing with minimum dehydration and product deterioration during the 
freezing process. The system of the present invention achieves the 
elimination of recycling bacteria and frost particles, minimizing freezer 
frost buildup and thus reducing the maintenance costs and improving the 
sanitation of the system.