U.S. patent number 3,807,396 [Application Number 04/623,616] was granted by the patent office on 1974-04-30 for life support system and method.
This patent grant is currently assigned to E & M Laboratories. Invention is credited to Halbert Fischel.
United States Patent |
3,807,396 |
Fischel |
April 30, 1974 |
LIFE SUPPORT SYSTEM AND METHOD
Abstract
Life support is provided by a cryogenic closed circuit system
for either hyperbaric, isobaric or hypobaric respiratory
environments. Oxygen content in a gas mixture is maintained in a
selected partial pressure range by establishing thermodynamic
equilibrium between a liquid oxygen supply, which is maintained at
an appropriate temperature, and a gas mixture within an enclosed
volume communicating with the liquid oxygen. Incoming gases are
passed into the gas-containing volume and adjusted to an
appropriate partial pressure of oxygen solely by virtue of
saturation equilibrium, and independently of ambient pressure and
gas mixture variations. The useful life of a given oxygen supply
volume is greatly extended because of the high compaction ratio of
liquid oxygen with respect to oxygen gas at standard temperature
and pressure, and because of full physiological use of the oxygen.
The temperature of the liquid oxygen may be controlled through the
use of a cryogenic reservoir, a cryogenic refrigerator or other
means. A heat exchanger may be deployed in the closed circuit
system to cool incoming contaminated gases to cryogenic levels and
to heat outgoing breathable gases to acceptable levels, and to
provide more efficient use of the cryogen. The closed circuit
system may further include means for compensating for ambient
pressure variations by the injection of an inert gas or gases and
for cleansing of incoming gases by removal of water vapor and
carbon dioxide.
Inventors: |
Fischel; Halbert (Los Angeles,
CA) |
Assignee: |
E & M Laboratories (Van
Nuys, CA)
|
Family
ID: |
24498766 |
Appl.
No.: |
04/623,616 |
Filed: |
March 16, 1967 |
Current U.S.
Class: |
128/201.21;
62/48.3; 62/46.1; 165/66 |
Current CPC
Class: |
A61M
16/1045 (20130101); F17C 9/02 (20130101); A62B
7/06 (20130101); F17C 9/00 (20130101); B63C
11/24 (20130101); F25J 3/04981 (20130101); A61M
2202/0007 (20130101); F17C 2270/02 (20130101); A61M
2202/0208 (20130101); A61M 16/22 (20130101); A61M
2202/0208 (20130101); F17C 2265/025 (20130101); F25J
2215/02 (20130101); F25J 2270/904 (20130101); F25J
2210/42 (20130101); F25J 2215/40 (20130101); A61M
2202/03 (20130101); A61M 16/10 (20130101) |
Current International
Class: |
A61M
16/10 (20060101); B63C 11/24 (20060101); F25J
3/08 (20060101); B63C 11/02 (20060101); F17C
9/00 (20060101); A62B 7/00 (20060101); A62B
7/06 (20060101); F17C 9/02 (20060101); A62b
007/06 () |
Field of
Search: |
;128/140,142-142.7,145,203 ;62/45,50,223 ;55/267,268,269 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
623,816 |
|
Jul 1961 |
|
CA |
|
1,268,044 |
|
Jun 1961 |
|
FR |
|
816,874 |
|
Jul 1959 |
|
GB |
|
875,651 |
|
Aug 1961 |
|
GB |
|
642,034 |
|
Jul 1962 |
|
IT |
|
Primary Examiner: Howell; Kyle L.
Attorney, Agent or Firm: Fraser and Bogucki
Claims
What is claimed is:
1. A life support system providing a safe respirable breathing
mixture in an otherwise hostile environment comprising:
a closed circuit gas circulating system operatively responsive to
the pressure of the environment of the life support system and
including a supply of liquid oxygen;
means for maintaining said liquid oxygen supply within a selected
temperature region, including temperature sensing means responsive
to the temperature of the liquid oxygen; and,
means coupled to the gas circulating system for passing the gases
into communication with said liquid oxygen and to the life support
system as a safe respirable breathing mixture, including at least
one heat exchanger.
2. The invention as set forth in claim 1 above, wherein the system
in addition includes means responsive to the pressure of the
environment of the life support system for injecting an inert gas
to provide the respirable breathing mixture at a pressure within a
selected pressure range.
3. A life support system providing a safe respirable breathing
mixture in an otherwise hostile environment comprising:
a closed circuit gas circulating system operatively responsive to
the pressure of the environment of the life support system and
including a supply of liquid oxygen;
means for maintaining said liquid oxygen supply within a selected
temperature region, including cryogenic refrigerator means
receiving at least a portion of the gases;
means coupled to the gas circulating system for passing the gases
into communication with said liquid oxygen into the life support
system as a safe respirable breathing mixture; and
means responsive to the liquid oxygen temperature for operating
said cryogenic refrigerator means to control the temperature of the
liquid oxygen supply.
4. A system for providing oxygen content control for an oxygen
demand system comprising:
a substantially enclosed vessel containing liquid oxygen;
a cryogenic reservoir encompassing said vessel; means supplying
inert gas disposed within said cryogenic reservoir, and;
means coupled to said oxygen demand system for passing gases
requiring oxygen content control through said substantially
enclosed vessel in communication with said liquid oxygen, said
means including means for selectively varying the pressure of the
gases.
5. A system for providing oxygen content control for an oxygen
demand system comprising:
a substantially enclosed vessel containing liquid oxygen that only
partially fills the vessel to define a gas containment volume;
a cryogenic reservoir encompassing said vessel;
means coupled to said oxygen demand system for passing gases
requiring oxygen content control through said substantially
enclosed vessel in communication with said liquid oxygen; and,
conduit means coupled to said means for passing gases through said
substantially enclosed vessel for confining said gases in a
tortuous path in heat exchange relationship within said cryogenic
reservoir prior to entering said vessel.
6. The invention as set forth in claim 5 above, wherein in addition
said conduit means comprises a pair of conduits passing in
generally arcuate paths through said cryogenic reservoir and
terminating within said vessel in oppositely disposed ports both
normally within the gas containment volume.
7. A system for providing oxygen content control for an oxygen
demand system comprising:
a substantially enclosed vessel containing liquid oxygen;
a cryogenic reservoir encompassing said vessel;
means coupled to said oxygen demand system for passing gases
requiring oxygen content control through said substantially
enclosed vessel in communication with said liquid oxygen; and,
means coupled to said cryogenic reservoir for regulating the rate
of cryogen boil-off therefrom in response to the temperature of the
liquid oxygen.
8. The invention as set forth in claim 7 above, wherein in addition
said system includes cryogen boil-off conduit means coupled into
said cryogenic reservoir, said conduit means comprising a plurality
of individual conduits having terminal ports disposed at different
spaced regions within said cryogenic reservoir.
9. The invention as set forth in claim 8 above, wherein said
conduit means comprises four individual conduits, and said conduits
have extended tortuous lengths disposed in mutually exclusive
paths, each of said conduits extending in two mutually orthogonal
directions through spans that are more than half of the
corresponding dimension of said cryogenic reservoir, and wherein
said system additionally includes manifold means coupling said four
conduits together at a common region.
10. A system for providing efficient oxygen utilization from a
liquid oxygen supply used for breathing purposes, and including
means providing expiratory and inspiratory flows, comprising:
a heat exchanger coupled to receive expiratory flows and to provide
inspiratory flows;
a substantially enclosed vessel containing liquid oxygen;
a cryogenic reservoir encompassing said vessel;
means coupling expiratory flows of gases from said heat exchanger
to said substantially enclosed vessel; and,
means coupling inspiratory flows of gases from said substantially
enclosed vessel to said heat exchanger.
11. The invention as set forth in claim 10 above, wherein said heat
exchanger has a heat transfer efficiency in excess of 90 percent
and wherein expiratory and inspiratory gases are passed in
counter-current relation.
12. The invention as set forth in claim 10 above, wherein said
system further includes means coupled to said cryogenic reservoir
for regulating the cryogen boil off, and wherein the cryogen boil
off is passed in heat transfer relation to said heat exchanger.
13. A life support system comprising:
a conduit system for receiving contaminated gases along an inlet
conduit and for providing a breathable mixture along an outlet
conduit, said gases being subject to ambient pressure
variations;
means responsive to said ambient pressure variations for injecting
inert breathing gas into said inlet conduit to equalize pressure to
the ambient level;
liquid oxygen supply means including an oxygen chamber providing a
liquid oxygen-gas mixture interface;
cryogenic means in heat transfer relationship to said liquid oxygen
supply means, and including means for maintaining the liquid oxygen
in a selected temperature range; and,
additional conduit means including heat exchanger means coupling
said inlet conduit means through said oxygen chamber to said outlet
conduit means, said heat exchanger means being coupled to pass
incoming gases in counter-current relation to outlet gases, and
said heat exchanger means further including CO.sub.2 precipitator
means in the coupling between said inlet conduit means and said
oxygen chamber.
14. A life support system for an hyperbaric environment
comprising:
a conduit system for receiving contaminated expiratory gases along
an inlet conduit and for providing a breathable inspiratory mixture
along an outlet conduit, the hyperbaric environment providing
pressure variations affecting such gases;
means responsive to said pressure variations for injecting inert
breathing gas into said inlet conduit to equalize pressure to the
hyperbaric environment level;
liquid oxygen supply means including an interface chamber providing
an interface region between the liquid oxygen and a gas
mixture;
cryogenic means in heat transfer relationship to said liquid oxygen
supply means for maintaining the liquid oxygen in a temperature
range from approximately 77.degree. K. to approximately 90.degree.
K.;
heat exchanger means coupling said inlet conduit means through said
interface chamber to said outlet conduit means, said heat exchanger
means having a heat exchange efficiency in excess of approximately
90 percent, and passing expiratory gases in countercurrent
relationship to inspiratory gases, said heat exchanger means
cooling expiratory gases to below the precipitation temperature of
CO.sub.2, and further including CO.sub.2 precipitate collector
means; and,
means disposed along the inlet conduit of said conduit system for
removing water vapor from expiratory gases prior to said heat
exchanger means.
15. The invention as set forth in claim 14 above, wherein said
cryogenic means comprises a supply of a cryogen substantially
encompassing said liquid oxygen supply means, and means for
regulating the temperature of said cryogen, and wherein said means
for removing water vapor comprises a desiccant chamber.
16. The invention as set forth in claim 15 above, wherein in
addition means are disposed within said interface chamber in
contact with the liquid oxygen for providing an increased liquid
oxygen surface area; and,
wherein in addition isothermal conduit means are coupled to pass
expiratory gases between said heat exchanger means and the
interface chamber, said isothermal conduit means passing in heat
transfer relationship through said cryogen.
17. A self-contained life support system comprising:
a substantially enclosed cryogen vessel;
a liquid oxygen vessel disposed within said cryogen vessel; said
liquid oxygen vessel being less than full of liquid to define a
substantially enclosed gas containment volume in communication with
the liquid oxygen;
a high efficiency counter-current flow heat exchanger for gases
mechanically coupled to said cryogen vessel, said heat exchanger
having a passageway system for expiratory gases and a passageway
system for inspiratory gases each having an inlet and outlet end,
said heat exchanger further cooling expiratory gases below the
CO.sub.2 precipitation temperature;
a respiratory mechanism for system users;
a first conduit coupling said respiratory mechanism to the inlet
end of said heat exchanger passageway system for expiratory
gases;
a second conduit coupling the outlet end of said heat exchanger
passageway system for expiratory gases to the gas containment
volume within said liquid oxygen vessel;
a third conduit coupling the gas containment volume within said
liquid oxygen vessel to the inlet end of said heat exchanger
passageway system for inspiratory gases;
a fourth conduit coupling the outlet end of said heat exchanger
passageway system for inspiratory gases to said respiratory
mechanism;
a pressure operable control valve system coupled to vent gas from
said cryogen vessel; and,
a pressure generating control device in communication with the
liquid oxygen in the liquid oxygen vessel and responsive to the
temperature thereof for exerting variable pressure thereon to vent
gas from said cryogen to maintain the temperature of the liquid
oxygen substantially within a selected range.
18. The invention as set forth in claim 17 above, and including in
addition a vessel for a pressurized inert gas disposed within said
cryogen vessel;
a pressure regulator coupled to receive the inert gas and to inject
the inert gas into said first conduit in response to environmental
pressure changes;
a water removal chamber coupled into said first conduit; and,
a CO.sub.2 precipitation chamber coupled to the heat exchanger
passageway system for expiratory gases at the region at which the
expiratory gases are approximately at precipitation
temperature.
19. The invention as set forth in claim 18 above, wherein said
pressurized inert gas is helium, wherein said cryogen is nitrogen,
wherein the selected range is approximately 77.degree. K. to
approximately 90.degree. K., and wherein the water removal chamber
comprises a desiccant chamber.
20. The invention as set forth in claim 18 above, wherein the
CO.sub.2 precipitation chamber is disposed at approximately
three-fourths of the length of the heat exchanger system from the
inlet end for expiratory gases.
21. The invention as set forth in claim 20 above, wherein said
boil-off conduit comprises a manifold and a plurality of individual
inlet lines each following an extended path within said cryogen
vessel from inlet openings at different regions within said vessel
and each coupled to the manifold.
22. The invention as set forth in claim 21 above, wherein said
cryogen vessel comprises a double-walled insulated vessel of
generally cylindrical shape with hemispherical ends, and wherein
said individual inlet lines for said boil-off conduit extend in
reentrant paths in at least two orthogonal directions within said
cryogen vessel, the limits of said paths spanning more than half of
the corresponding dimension of the cryogen vessel to block passage
of liquid therein.
23. The invention as set forth in claim 18 above, wherein said
system further includes a boil-off conduit coupling the cryogen to
said control valve, said conduit passing in heat exchange
relationship to said heat exchanger; and,
wherein said second conduit passes in an extended path through said
cryogen in said cryogen vessel and terminates in oppositely
disposed outlets normally within the gas containment volume within
said liquid oxygen vessel.
24. The invention as set forth in claim 18 above, wherein said
system further includes a high surface area wicking member disposed
within the liquid oxygen vessl and extending into the liquid oxygen
to facilitate thermodynamic equilibrium and attainment of oxygen
vapor saturation.
25. The invention as set forth in claim 18 above, wherein said
system further includes a boil-off gas receiver coupled to said
pressure operable control valve, and wherein said control valve
includes adjustable means for varying the pressure response
thereof.
26. A system for controlling the partial pressure of a selected gas
comprising:
at least one reservoir maintaining a supply of the liquid of the
selected gas and defining a gas containment volume in communication
with the liquid;
conduit means having a first closed end in heat exchange relation
with the liquid within the reservoir means, said conduit means
including an enclosed gas-liquid interface of a second selected
gas, the liquid being adjacent the closed end thereof; and,
means responsive to pressure variations in the gas in the conduit
means for controlling the temperature of the liquid in said
reservoir means.
27. A system for controlling the partial pressure of a selected gas
comprising:
at least one source of the selected gas in the liquid phase;
at least one enclosed reservoir disposed adjacent said source and
containing said selected gas, the gas in the reservoir being in
communication and thermodynamic equilibrium with said liquid
phase;
means responsive to the temperature of said source for controlling
the temperature of said source, thereby to control the partial
pressure of the selected gas; and,
means for passing foreign gases into and adjusted gases out of said
at least one enclosed reservoir for the selected gas without
altering the partial pressure of said selected gas.
28. A system for providing gas at a selected partial pressure,
irrespective of ambient and gas mixture variations, comprising:
at least one storage vessel maintaining a liquid supply of the
selected gas, and including an enclosed reservoir for maintaining a
gas mixture containing the selected gas in communication and
thermodynamic equilibrium with the liquid;
a liquid reservoir encompassing said storage vessel and in heat
exchange relation therewith;
sensing means disposed in the liquid supply of selected gas and
responsive to the temperature thereof; and,
control means coupled to said liquid reservoir and responsive to
said sensing means for controlling the back pressure within said
liquid reservoir to tend to maintain the temperature of the liquids
in a selected range.
29. The invention as set forth in claim 28 above, wherein said
sensing means comprises conduit means having an enclosed end in
heat exchange relation with the liquid supply in said storage means
and containing a gas-liquid interface of a selected constituent,
with the liquid being disposed in heat exchange relation with said
liquid supply.
30. The method of adjusting the constituents of a gaseous mixture
for life support purposes comprising the steps of substantially
cleansing the gaseous mixture of water vapor, toxic hydrocarbons
and carbon dioxide, cooling the cleansed gases to a cryogenic
level, establishing thermodynamic equilibrium between the cooled
gases and liquid oxygen maintained in a selected temperature range,
with the oxygen vapor at saturation, and reheating the gases to
breathable temperature.
31. The method of life support in a positive pressure environment
comprising the steps of circulating contaminated gases through a
cryogenic environment containing liquid oxygen to adjust at least
the gaseous oxygen partial pressure by establishing thermodynamic
equilibrium of the gaseous mixture containing oxygen with the
liquid oxygen, with the gaseous oxygen at saturation, and returning
the adjusted gases to the life support system.
32. The method of adjusting the constituents of a gaseous mixture
for life support purposes in a positive pressure environment
comprising the steps of cooling breathed gases to precipitate
CO.sub.2, saturating the cooled gases with oxygen vapor in
association with liquid oxygen, maintaining the liquid oxygen in a
selected temperature range and subsequently heating the gases back
to a breathable temperature.
33. The method of adjusting the partial pressure of one selected
gas in a gas mixture comprising the steps of bringing the gas
mixture into communication with the liquid of the selected gas,
maintaining the liquid in a selected temperature range to provide
thermodynamic equilibrium between the gas mixture and the liquid,
with the vapor of the liquid at saturation, and substantially
increasing the temperature of the liquid of the selected gas by
diluting the liquid with at least a second liquid.
34. The method of adjusting the partial pressure of one selected
gas in a gas mixture comprising the steps of bringing the gas
mixture in communication with the liquid of the selected gas,
maintaining the liquid in a selected temperature range to provide
thermodynamic equilibrium between the gas mixture and the liquid,
with the vapor of the liquid at saturation, and compensating for
ambient pressure variations by diluting the liquid with at least
one other liquid while maintaining the liquid mixture in a
different temperature range.
35. A system for maintaining a selected gas in a chosen partial
pressure range within an enclosed operative volume maintained at a
positive pressure relative to a superatmospheric or space
environment, comprising:
a storage vessel providing a liquid source of the selected gas,
said storage vessel also defining a substantially enclosed
gas-containment chamber in communication with the liquid, such as
to establish thermodynamic equilibrium between the gas and liquid
forms;
conduit means circulating gas from and to the gas containment
chamber;
means operatively coupled to the liquid in the storage vessel for
sensing the temperature thereof; and,
refrigerator means coupled in the conduit means and responsive to
the sensed temperatures for cooling the circulating gas to tend to
maintain the gas and liquid temperatures within said storage vessel
within a selected range.
36. The invention as set forth in claim 35 above, wherein said
selected gas is oxygen, and the chosen partial pressure range
comprises approximately 0.2 to 1 atmospheres and wherein said
refrigerator means at least partially comprises means in heat
exchange relationship with the environment.
37. The invention as set forth in claim 36 above, wherein said
system further includes heat exchanger means coupled into said
conduit means adjoining said storage vessel for heating and cooling
the gases between breathable and cryogenic gases.
38. A system for maintaining a breathable oxygen mixture containing
a selected proportion of an inert gas within an enclosed operative
volume comprising:
a source of liquid oxygen, including means defining a substantially
enclosed gas containment chamber in communication with the liquid
oxygen;
means for maintaining the liquid oxygen in a selected temperature
range to provide a chosen range of oxygen vapor partial
pressures;
conduit means circulating gas from and to the enclosed operative
volume through the gas chamber; and,
means providing a regulated source of the inert gas for
establishing a desired total pressure within the enclosed operative
volume.
39. A system for separating CO.sub.2 from a gas mixture
comprising:
a first source of gases within a temperature range below the
precipitation temperature of CO.sub.2 ;
a second source of gases containing CO.sub.2 and within a
temperature range above the precipitation temperatures of CO.sub.2
;
at least one heat exchanger coupled to said first and second
sources of gases for passing said gases in heat exchange relation
with one another, said at least one heat exchanger providing
substantially linear gas flow with low pressure gradient and having
sufficient length and area to cool gases from said second source to
below the precipitation temperature of CO.sub.2 through passing
said gases in said heat exchange relation with one another;
and at least one CO.sub.2 collector having an enclosed volume open
to gases from said second source passing along said at least one
heat exchanger, said enclosed volume communicating with the region
along said at least one heat exchanger at which CO.sub.2
precipitates.
40. The invention as set forth in claim 39 above, wherein said at
least one heat exchanger is divided into two sections, and wherein
said CO.sub.2 collector means is disposed between said sections,
and includes an outlet conduit and an inlet conduit within an
enclosed chamber, with the outlet and inlet conduits being disposed
in overlapping relation, and further including screen means
covering said outlet conduit.
41. A system for life support comprising:
a cryogenic system for adjusting the oxygen partial pressure in a
breatheable gas mixture;
at least one cryogen reservoir for liquid temperature control
cryogen coupled to said cryogenic system in heat exchange relation
therewith for stabilizing the temperature thereof;
means coupled to said at least one cryogen reservoir for reacting
exothermically with gases boiled off therefrom due to heat
absorption from said cryogenic system; and
heat transmitting means for providing heat for life support
purposes from said means for reacting exothermically.
42. A system for life support of a respiring user in a hyperbaric
environment comprising:
a cryogenic system for adjusting the oxygen partial pressure in a
gas mixture for breathing;
a reservoir containing a liquid cryogen at cryogenic temperature in
heat exchange relation with at least part of said cryogenic system
for stabilizing the temperature thereof;
receiving means coupled to said cryogenic reservoir for receiving
cryogen evaporated due to heat exchange with said cryogenic system
and containing exothermic means for reacting exothermically with
said evaporated cryogen, said receiving means isolating the liquid
cryogen from the hyperbaric environment; and
means in heat transfer relation with said receiving means for
supplying heat to said user for life support.
43. The invention as set forth in claim 42 wherein said system
further includes heat conduit means for transferring heat to said
respiring user from said means for supplying heat, said heat
conduit means being in heat transfer relation to said means for
supplying heat.
44. A simulation system for extracting oxygen from a separate
system operating to tend to maintain a chosen breathable gas
mixture comprising:
means including a liquid oxygen supply maintained in a selected
temperature region within a substantially enclosed volume;
means receiving the gas mixture from the separate system for
passing the gas mixture in communication with the vapor of the
oxygen supply;
means for returning the gas mixture to the separate system;
wherein the selected temperature range provides an oxygen vapor
partial pressure below the range of a breathable mixture;
means coupled to the means for returning for adding gaseous
contaminants to the gas mixture; and,
heat exchanger means for passing the received and returning gas
mixtures in counter-current relationship.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to systems for adjusting the gas
constituents in a gaseous environment and particularly to closed
circuit systems providing breathable gas mixtures in hostile
environments and in self-contained life support systems.
2. Description of the Prior Art
As underwater explorations and other operations in hostile
environments for scientific and industrial purposes have increased,
concentrated attention has been directed to the development of life
support systems. To the present, such systems have taken a limited
number of basic forms. One widely employed underwater life support
system is based upon the use of an externally serviced diving
helmet or suit, sealed partially or entirely about a diver and
providing a constant pressurized supply of fresh gases while
withdrawing contaminated gases. Self-contained underwater breathing
apparatus, or SCUBA gear, is most widely used in its open circuit
version. That is, gas tanks carried by a diver provide fresh gases
in appropriate mixture, and contaminated gases are expelled from
the system.
The amount of gas taken in by users in a single breath increases
with pressure. Because the duration of use of a given breathable
gas supply is limited by volume and weight to that which can be
manipulated by a diver or supported by an underwater system, work
has also been directed toward semi-closed circuit systems by virtue
of their more efficient utilization of the gas supply. In such
systems, at least in theory, the contaminating gases (principally
CO.sub.2, hydrocarbons, and water vapor) are eliminated from the
system, and a portion of the oxygen supply is recirculated so that
its use rate is limited, in part, to that at which it is
physiologically converted by users and that which is lost from the
system where the latter is ventilated with fresh oxygen.
More recently, a number of underwater life support systems have
been devised in which diving bells or tanks maintain preselected
gas mixtures, either by self-contained systems or by couplings to
surface sources. These systems have the advantage of providing easy
ingress and egress to the substantial depths in which they are
contained, but generally rely upon umbilical connections or open
circuit arrangements. Some systems use low temperature (cryogenic)
reservoirs for supplying the needed gases to a semi-closed circuit
arrangement in which mixtures are continuously sensed and proper
valving adjustments are made for the injection of effluent oxygen
gas from the reservoir. Such systems are both large and complicated
in their present form.
NATURE OF THE PROBLEM
A number of considerations greatly complicate the problems involved
in providing life support in hostile environments such as are
encountered under positive pressure conditions. Regardless of the
pressure at which a diver is working, he must have available the
proper amount of oxygen, and this amount, in terms of oxygen
partial pressure, is in the range of 0.2 to approximately 1
atmosphere. Thus in working at substantial depths (200-400 feet),
divers are required to breath a mixture in which the oxygen is a
very low percentage, e.g. 5 to 2 percent respectively, of the
complete mixture. Unless one is to be confined to a given depth or
required to make constant adjustments oxygen percentage variations
must be made automatically because either an excess or deficiency
will be fatal within a short time, the former from oxygen poisoning
and the latter from anoxia. Maintenance of precise ratios is also
of great importance in reducing the possibilities of non-fatal
injury and in reducing the time needed for decompression.
The term "positive pressure" is here intended to refer to an
hyperbaric environment, or one in which the environmental pressure
exceeds that of a normal human environment. If the pressure is high
enough it is intrinsically hostile to human life, just as is an
hypobaric environment if its pressure is sufficiently low. Taking
the normal atmospheric range as a reference, the hypobaric
environment may also be referred to as a negative pressure
environment. Even an isobaric environment may be hostile to human
existence and may require a life support system, as if temperature
extremes or excessive pollutants are encountered.
The proper breathing mixture for positive pressure environments is
not comparable to that of normal atmospheres. There is of course
the requirement that the exhaled carbon dioxide be removed from the
system if the gas is to be recirculated. If the system is fully or
semi-closed circuit, it is further desirable that there should be
no substantial variation in the breathing pressure required as
CO.sub.2 accumulates, as is the case when a back pressure exists.
The narcotic effects of inert gases under pressure must be
suppressed or eliminated. For example, if nitrogen is present in
the breathing mixture it should be limited to a partial pressure of
no greater than approximately 50 psi. For these and other reasons,
breathable mixtures for underwater work primarily use helium or
some other stabilizing inert gas or mixture of inert gases and
oxygen in combination.
Other critical requirements imposed on life support systems stem
from the necessity of deriving the longest possible useful life
from given supplies of oxygen and supportive gases, and from the
safety factors inherently required for all such apparatus. The
volume and mass which can be manipulated by a diver, for example,
represents one controlling limitation on an underwater system.
Within such limitations, the longer a given supply of oxygen and
supportive insert gas can be used and the greater will be the work
capacity of the user or users. The work capacity increases
disproportionately with oxygen supply duration because of the time
consumed in preliminary preparations, travel to and from a work
site, and subsequent decompression. Even if the starting and
returning point is an underwater station, and if depths are so
great that the energy capacity of the diver is the most significant
limitation, the safety factors inherent in a greater supply of
breathable gas are self evident. Safety factors generally are of
great concern at present, because modern systems rely on mechanical
and pressure responsive valves whose failure, catastrophic or
intermittent, is intolerable.
OTHER ENVIRONMENTS AND FUNCTIONAL REQUIREMENTS
As systems have been more widely used, other factors than safety
have also become more significant. For example, when breathing a
mixture having high helium content, body heat losses are extreme.
Thus, means must be provided for heating underwater devices or
stations in order to maintain reasonable working or living
conditions. The expenditure of heat is generally unrelated to the
performance of other functions within a life support system, but if
it could be effectively combined with the life support function,
the entire installation would not only be more compact and less
costly but available energy sources would be more efficiently
utilized.
Systems of this nature are required to perform a variety of
different functions, including the cleansing from contaminated
gases of water vapor, toxic hydrocarbons, and carbon dioxide, the
use of efficient heat transfer arrangements, and the exercise of
necessary control functions for regulating temperatures, and, where
necessary, pressures. The present invention is concerned will all
such aspects, because although they are corollary to the princpal
problem of controlling the partial pressure of a selected gas
within the gas mixture, they are fundamental to the principal
objectives of life support systems, and additionally have a
substantial number of independent uses. Many of these functional
requirements are satisfied by systems in accordance with the
invention in particularly advantageous and unique fashion, as is
separately pointed out hereafter.
Other life support systems may confront wholly different
environments but nonetheless must provide a breathable mixture in
the proper pressure range. In superatmospheric and space systems
hypobaric environments are encountered and it is often preferred to
use pure oxygen at a selected subatmospheric pressure. While
pressure regulation is relatively simple, failures can still occur.
Perhaps greater dangers are introduced by the flammability of a
pure oxygen breathing medium, and these dangers may be greatly
reduced by admixing even a minor proportion of inert gas. With
existing systems, the extraction of contaminants from pure oxygen
maintained at a given pressure involves relatively simple
regulatory problems. When, however, the oxygen is to be diluted in
a given proportion with a suitable inert gas, substantial
additional equipment must be utilized with present techniques. The
constituents of the gas mixture must be determined with relation to
the total pressure in the system, and the needed corrections must
be made by appropriate combinations of injections of pure gas
constitutents and withdrawal of the gas mixture.
SUMMARY OF THE INVENTION
The purposes and objectives of the present invention are achieved
for life support systems by automatic adjustment of oxygen partial
pressure by passing a gas mixture containing oxygen through a
temperature controlled gas-liquid-oxygen interface. The liquid
oxygen is in communication with an enclosed gas-containment volume
and is maintained at a preselected temperature or within a selected
temperature range, establishing a partial pressure of oxygen vapor
in the enclosed volume that is independent of the partial pressures
of other gases and ambient pressure variations. This form of
control may be employed for other gases, and whether the selected
gas must be added to or extracted from the system. By bringing the
vapor content to saturation quickly, the partial pressure of the
selected constituent in a mixture may be continually corrected.
Closed circuit systems have particular advantages for life support
applications, but an aspect of the invention is that regulation in
this manner may be effected with other systems and with a variety
of further modifications. In the hypobaric (negative pressure)
environments, such form of regulation may provide the basis for
maintaining selected proportionalities between oxygen and an inert
gas constituent in the mixture. The pressure of the selected gas
may be maintained within a selected broad or narrow range. A
substantial variation may be introduced in the temperature of the
liquid reservoir in communication with the vapor without changing
the partial pressure range of the corresponding vapor by controlled
dilution of the liquid in the reservoir. Thus, the invention
encompasses a number of methods, as well as a number of apparatus
concepts dealing with both system and subsystem aspects.
In accordance with several of the principal aspects of the
invention, a fully closed circuit system may employ a gas-liquid
interface volume in a highly interrelated fashion with other
functional subsystems to provide a complete life support system for
environments hostile to man. A chamber containing the gas-liquid
interface volume is, in one specific example, disposed within a
cryogenic reservoir and incoming and outgoing gases are passed
through the enclosed volume for adjustment of oxygen contact. The
gases are preliminarily and subsequently passed through a high
efficiency heat exchanger, to cool the incoming contaminated gases
substantially, and to reheat the outgoing properly compensated gas
mixture to a breathable temperature level. During cooling, CO.sub.2
is precipitated from the gases. In conjunction with the same
system, gaseous helium may also be more efficiently stored in a
separate container within the cryogenic reservoir, and injected
into the incoming gases to compensate for ambient pressure
variations.
A benefit of this usage of liquid oxygen in a closed circuit
arrangement is that dramatic increases in operative cycle time for
the system are achieved because of the high compaction ration of
liquid oxygen, and because efficient use is made of heat exchange
relationships, and because oxygen is not dissipated. The system
operates automatically over a wide dynamic range and with the
needed precision. The primary requirements as to absolute and
relative pressure variations are fully met.
Closed circuit systems in accordance with the invention for use in
superatmospheric and space environments may use a cryogenic
refrigerator for adjusting the gas-liquid-oxygen interface
temperature. The cryogenic refrigerator may be a power operated
unit or comprise a passive unit giving up heat to the environment.
Dependent upon the oxygen temperature adjustment needed, controlled
cooling of the circulating gas may be effected as needed by varying
rates or proportions of flow, or the efficiency of the cryogenic
refrigerator.
A significant aspect of the invention relates to control of the
partial pressure of the oxygen in the enclosed volume. The
temperature of the oxygen is sensed and used to control boil off
from the cryogenic reservoir in order to regulate the back pressure
therein. A feature of this system is an arrangement for automatic
and reliable temperature control. The vapor pressure of the single
constituent within the cryogenic reservoir represents the total
pressure in this essentially closed container, and the vapor
pressure is therefore stable for a given temperature of the cryogen
and varies isomorphically with the cryogen temperature. The back
pressure of the cryogen is regulated to adjust its temperature,
thus to adjust the temperature of the liquid oxygen which is
maintained in thermal equilibrium with the cryogen in which it is
immersed. The oxygen partial pressure is therefore maintained
within selected limits, irrespective of the oxygen supply,
variations in system demand, and variations in the breathable
mixture.
Another feature of the invention is a temperature control
arrangement that is analog in nature, completely self contained,
and highly accurate and reliable. A hollow sensing tube has a
sealed end containing a liquid disposed within the liquid oxygen
supply, so that the liquid sealed in the tube assumes the
temperature of the liquid oxygen. The tube itself contains an
entrapped gas-liquid interface within an invariant volume, so that
the pressure within the tube is essentially the vapor pressure of
the liquid, and thus varies with the liquid oxygen temperature. At
the temperature range of the liquid oxygen, a substantial pressure
is generated within the tube and this pressure varies within a wide
dynamic range. The pressure regulates the back pressure of the
cryogen by operating a diaphragm mechanically coupled to control
the venting of the boil-off gases from the cryogenic reservoir.
This portion of the system comprises a closed loop servo that
maintains the liquid oxygen temperature within any predetermined
range as long as there are adequate cryogenic sources
available.
Systems in accordance with the invention for control of the partial
pressure of a gas are reciprocal in nature, in that they may also
extract a gas to reduce an excess in a gas mixture. Accordingly,
systems and methods are also provided by which a selected gas, such
as oxygen, may be condensed from a given mixture. Test systems are
provided, in one specific example, which the operation of a life
support system is tested under conditions corresponding to actual
operation. The test system continually simulates the breathing of
users by extracting oxygen from a breathable mixture, and returns
an oxygen deficient mixture to which contaminants may be added.
Another feature of systems in accordance with the invention is a
heat exchanger providing extremely efficient heat transfer between
the incoming and outgoing gases. The heat exchanger passes the
incoming and outgoing gases in counter-current fashion along
adjacent passageways on opposite sides of thin heat exchanger
membranes. Substantially direct heat transfer over large surface
areas is effected, with thin sheets of gases moving linearly to
provide wide temperature differentials over short length with
relatively little pressure drop. The oxygen and cryogen are more
efficiently utilized, and the proper temperatures are maintained
for both the breathable and cryogenic mixtures. The heat exchanger
entities may be built up in successive laminations to achieve
desired cross-sectional passageway areas. Furthermore, standardized
heat exchanger elements may be added together in series or parallel
fashion to increase heat exchange surface area. The heat exchanger
entities may be disposed in opposed and facing relation, and
arranged to receive other conduit systems in selected heat exchange
relation to provide either a heat source or heat sink effect. In a
particular system in accordance with the invention, boil-off gases
from a cryogenic reservoir may be passed through the heat exchanger
to extract heat from the gases and to further increase the
efficiency of the system by providing a tertiary heat transfer
effect.
The precipitation point of CO.sub.2 is reached within a very short
length as the incoming contaminated gases are cooled, and these
factors are used to advantage in novel systems and methods for
cleansing CO.sub.2 from the gases. At the appropriate point along
the length of the heat exchanger, at which CO.sub.2 precipitates,
the gases are directed into an enclosed volume, but without
substantial change of pressure. CO.sub.2 precipitate is accumulated
in a receiving chamber, while the cleansed gases are diverted into
the succeeding section of the heat exchanger. Because the CO.sub.2
particles are uncompacted and travel in unrestricted fashion along
linear paths without encountering substantial pressure drops, the
CO.sub.2 mass remains loose and permeable and does not clog the
passageways or otherwise impede normal gas flow.
Further in accordance with the invention, means are provided for
insuring rapid and stable attainment of equilibrium conditions in
control of the breathable mixture. Incoming expiratory gases are
not only passed through the heat exchanger, but are additionally
passed in isothermal relationship with the cryogen by flowing along
a length of conduit within the cyrogenic reservoir immediately
prior to entering the vessel containing liquid oxygen. Thus, in
accordance with this feature, the system reduces the thermal
exchange occurring within the vessel containing the liquid oxygen.
Furthermore, the internal arrangement within the liquid oxygen
vessel facilitates establishment of the desired equilibrium
relationships between gas and liquid. A wicking member extending
from the liquid oxygen into the gas-containing volume is
substantially fully wetted by the liquid oxygen and greatly
increases the surface area of liquid oxygen in communication with
the gas mixture. The oxygen level in the mixture is quickly brought
to saturation for the given liquid oxygen temperature while the gas
mixture is concurrently brought into thermodynamic equilibrium.
A number of additional features of use in cryogenic systems have
separate utility and importance. The liquid oxygen does not pass
through the conduits containing the incoming expiratory gases
because these conduits are disposed such that the liquid tends
ultimately to return irrespective of temporary attitude changes. A
bifurcated conduit system having oppositely facing ports normally
above the liquid oxygen insures that the line is always open to
pass expiratory gases. A tortuous conduit system is also used to
assure free passage of boil-off gases from the cryogen. A set of
four conduits are used, in one specific example, each extending
from a common manifold within the cryogen reservoir, and each
following at least two orthogonally disposed reentrant paths whose
individual limits are more than half the corresponding internal
dimension of the reservoir.
In accordance with other aspects of the invention, gas cleansing
and needed pressure adjustments may also be effected within the
system. A variable volume chamber may be coupled into the inlet
portion of the system, to expand and contract to compensate for
lung volume changes. Incoming gas may be passed through a water
removal device. Inert gas injected prior to the heat exchanger may
be used to flush the entire system when required.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic representation of a system in accordance with
the invention for life support for positive pressure environments,
and including an alternative simulator system;
FIG. 2 is a graphical representation of phase equilibrium
relationships for oxygen and nitrogen plotted against temperature
and pressure;
FIG. 3 is a perspective view, partially broken away, of an example
of a detailed system in accordance with the invention for use in
positive pressure environments;
FIG. 4 is a side sectional view of the system of FIG. 2;
FIG. 5 is an idealized and simplified view of a heat exchanger
system for use in the system of FIGS. 3 and 4.
FIG. 6 is a perspective, partially broken away and greatly
enlarged, detailed view of a portion of the heat exchanger of FIG.
5;
FIG. 7 is a combined block diagram and partially broken away
perspective of a temperature control system for use in the system
of FIGS. 3 and 4; and,
FIG. 8 is a block diagram representation of a different form of
life support system in accordance with the invention useful in
negative pressure environments.
DETAILED DESCRIPTION OF THE INVENTION
1. Life Support System for Positive Pressure Environments
The diagram of FIG. 1 illustrates a complete system in idealized
form, and also a basic modification that accomplishes a
complementary purpose. It should be expressly understood, however,
that the various aspects of the present invention can be applied to
many different needs in different ways. The system of FIG. 1 is
highly integrated and performs the various functions needed for a
life support system for hostile environments, particularly
hyperbaric environments. Depending upon system requirements and
particular environments, however, only certain of the aspects need
be employed. Also, as shown in greater detail hereafter, individual
features, and variations and modifications of the features can be
utilized in various combinations. For a given life support system,
the most suitable combination will be dependent on such factors as
whether the system is intended for individual or multiple use,
whether it is to be a fixed or mobile system, whether it is to be
used in a positive pressure or negative pressure environment, and
whether the user is to be isolated from or exposed directly to the
environment.
The closed circuit system of FIG. 1 incorporates an inlet outlet
device 10, such as a diver's mouthpiece for an individual mobile
unit or appropriate couplings to a volume containing breathable gas
in a self-contained diving system. The volume to which the purified
and properly adjusted gas mixture is fed and from which
contaminated gases are drawn forms a part of the closed circuit
system. In most positive pressure systems the gases in the volume
are at pressures of several atmospheres or more. The flow path for
gases in the system is established by a set of unidirectional
valves 12, 13, 14 which insure circulation of gases in the proper
direction in the system. As is conventional with many positive
pressure systems for individual use, an expansion bag 16 is coupled
in the flow path for the incoming contaminated gases and outgoing
purified gases. Thus an individual breathes into the expansion bag
16 and then withdraws gases through the system via the expansion
bag 16, so that the bag 16 eases the work of breathing by
compensating for changes in lung volume. The bag 16 further acts as
a condensation collector for some of the water vapor in the
gas.
Further water vapor present in the incoming gases is removed in a
conventional water removal chamber 18, such as a desiccant unit.
Compensation for changes in ambient pressure is achieved by
injecting pressurized gaseous helium through a controllable
pressure regulator 20 of any suitable commercially available type.
The pressure regulator 20 may include a control valve actuator 22,
referred to as a purge button, which may be manually actuated to
fully open the helium line for momentarily pressurizing the system.
The incoming gases are passed from the water removal chamber 18
into an enclosed liquid oxygen vessel 24 having a gas-containing
volume separate from but communicating with liquid oxygen 25. Gases
are extracted from the gas-containing volume and returned through
the unidirectional valve 14 to the inlet-outlet mechanism 10. The
incoming gases, cleansed of water vapor, may have carbon dioxide
removed by any conventional means, and may be lowered to an
appropriate temperature level by any conventional cooling means,
such as a cryogenic reservoir or refrigerator. Although a heat
exchanger 26 is shown and has particular advantages, its use is not
required in delivering appropriately conditioned gases to the
gas-containing region of the liquid oxygen reservoir 24. Similarly,
the gases withdrawn from the liquid oxygen reservoir 24 may be
heated to a breathable temperature by suitable mechanisms added to
or separately utilized in the installation.
It should be appreciated that important aspects and advantages of
the present invention derive from the combination thus far
described. Proper adjustment of the proportion of oxygen in the
cleansed gas mixture is necessary to provide a breathable
inspiratory mixture. This result is effected by the closed circuit
system components described above. As pointed out previously, the
partial pressure of the oxygen is required to be between
approximately 0.2 and 1 atmospheres, in order to avoid the
injurious deficiency and excess conditions. In accordance with the
present invention no gas mixing or control valve arrangement need
be employed for injection or proportionment of oxygen nor need
sensing or detecting equipment for direct measurement of oxygen
pressure or percentage be used. The temperature of the liquid
oxygen in the vessel 24 is maintained in a selected range, e.g.,
approximately 77.degree. K-90.degree. K establishing a
predeterminedd oxygen gas partial pressure in the gas-containing
volume communicating with the liquid oxygen. The oxygen partial
pressure is solely dependent upon equilibrium conditions in the
gas-liquid-oxygen interface. The equilibrium conditions are
achieved when the oxygen vapor is at saturation level while the gas
mixture is in thermodynamic equilibrium with the liquid.
Equilibrium for oxygen at saturation is depicted in the pressure
versus temperature phase equilibria curves for oxygen and nitrogen
in FIG. 2. These equilibrium relationships between multiple phase
partial pressures and temperatures are not significantly affected
by the partial pressure of other constituent gases, or by the
absolute pressure level of the mixed gases. Consequently, when the
liquid oxygen temperature is kept between selected limits, the
oxygen partial pressure is also controlled, so that the oxygen
content in the breathable mixture is related directly to the needs
of the user, and not to the characteristics of the respirator gas
mixture itself. In effect, physiological needs are supplied by
nascent gas from the liquid oxygen 25. Again, it should be
explicitly noted that although superior temperature control can be
achieved by utilizing a cryogenic reservoir 28 encompassing, wholly
or partially, the liquid oxygen vessel 24, the controlling
criterion is the maintenance of a selected gas-liquid interface
temperature range, and not the means by which the range is
obtained. For example, an adequately large liquid oxygen reservoir
maintained in an insulated environment may have adequate life for
many uses, even though no adjustable control is used to maintain
the liquid oxygen temperature within the selected range.
Particular advantages derive, however, from the system of FIG. 1,
especially when used with individual and mobile equipment. A heat
exchanger 26, of the counter-current type, passes the incoming
contaminated gases from an inlet side 30 to an outlet side 31 of
the heat exchanger in a flow direction opposed to the breathable
gas mixture passing from its inlet 33 to its outlet 34. Thus, the
incoming expiratory gases moving between the points 30 and 31 are
brought from near the breathing temperature level down to the near
cryogenic level, while the outgoing inspiratory mixture is raised
in temperature approximately through the same range. The drop in
temperature of the incoming gases precipitates the contaminated
carbon dioxide, which may therefore be removed in a solid CO.sub.2
collector 36 coupled into a bypass conduit 38 in the conduit for
the incoming gases.
The cryogenic reservoir 28 is preferably insulated, and contains
liquid nitrogen or another suitable cryogen or cryogenic mixture
having a boiling point comparable to that of liquid oxygen,
provided that the critical temperature, the upper bound of the
vapor-pressure equilibrium curve, of the cryogen is not below the
operating temperature of the liquid oxygen. The cryogenic reservoir
28 may also encompass a gaseous helium containment vessel 42
coupled by a conduit 44 to the controllable pressure regulator 20.
Storage of the gaseous helium vessel 42 at low temperature in this
fashion provides an extremely large gas volume for use in the
breathable mixture. Other gases, such as neon or argon, may,
however, be used for pressure adjustment of the breathable
mixture.
The cryogenic reservoir 28 is maintained in the selected
temperature range desired for the liquid oxygen. To this end, a
boil-off intake 46 is coupled to a conduit passing through a
pressure control 50 to an exhaust receiver 52. While the boil-off
gases may be employed for heat exchange purposes as well, the
primary use of this arrangement is to regulate the temperature of
the cryogen through control of the back pressure of its vapor, as
is described in greater detail below. The pressure control 50 may
comprise simply a mechanically biased valve, or may utilize other
expedients set forth hereafter. The exhaust receiver 52 may take
any of a variety of forms, and is employed to isolate the
environment from the nitrogen pressure control 50 by supplying the
energy needed (if any) to exhaust the boil-off gases into the
environment. Typically, a pump will be suitable for this purpose.
Where feasible, however, a vacuum reservoir or a separate
utilization device may be used as the receiver.
To summarize, therefore, the vital functions of a life support
system are accomplished within this system. Not only is the oxygen
partial pressure regulated, but appropriate compensation is made
for pressure changes in the hyperbaric environment, as reflected
within the closed circuit system. Contaminants are removed and the
appropriate breathable mixture is continuously supplied. This
system provides a high degree of protection against catastrophic
failure, inasmuch as no sensitive operative components are employed
that are critical to the functioning of the system. The oxygen
partial pressure control has, in effect, a long time constant
inasmuch as the temperature of the cryogenic reservoir and the
liquid oxygen cannot shift suddenly. Therefore in the event that
temperature control becomes inoperative, adequate time is available
for the user to observe and adjust system conditions.
This closed circuit system also provides a highly interrelated
usage of temperature relationships through the disposition of the
various component parts of the system. These relationships may,
however, be considerably revised. If a suitably large cryogenic
reservoir 28 may be employed, the heat exchanger 26 may be of
relatively small size or low efficiency, or may in fact be
completely eliminated. Where the heat exchanger 26 is of a highly
efficient type, the useful duration of a given supply of cryogen is
greatly increased and therefore the volume of cryogen may be
reduced. For example, in one practical system in accordance with
the invention, a heat exchanger having approximately 98 percent
efficiency at a uniform counter-current heat transfer rate of 500
BTU/min. but relatively small size is used in conjunction with a
compact cryogenic reservoir 28 within which a liquid oxygen vessel
24 and a gaseous helium vessel 42 are maintained. This system
provides an operative life in excess of 5 hours at ambient
pressures in excess of 500 psi with about one-half cur. ft. of
fluid storage. The unit is not only highly efficient but may be
compact and light in weight.
The feasibility of use of liquid oxygen in this closed circuit
system will be further evident from the following considerations.
The heat capacity of 1 mole of helium gas, taking helium as the
inert gas, at constant pressure is about 5 calories per degree
Kelvin. The average breathing rate for one man is approximately
one-half to 1.0 moles per minute at atmospheric pressure going from
relaxed activity to moderate work. As a comparative reference, at
50 atmospheres of ambient pressure, corresponding to 1,620 feet of
ocean water depth, the moderate work respiratory rate goes to 50
moles per minute. If expiratory gas, entering the system, is at a
nominal temperature of 285.degree. Kelvin (.degree.K) and the
cryogenic control temperature is at 85.degree. K. then a heat
exchanger efficienty of 97.5 percent causes a temperature
differential of approximately 5.degree. Centigrade (.degree.C) over
the 200.degree. K. range, substantially throughout the length of
the counter-current heat exchanger. Thus, 5.degree. C. of
equivalent heat will be transferred in the thermal exchange from
the expiratory gas to the cryogen before entering the liquid oxygen
container. The corresponding heat dump at 50 atmospheres of
pressure is 250 gram-calories per minute per degree Kelvin of
temperature differential for a respiratory minute volume comparable
to 1.0 mole of gas at standard conditions of 1.0 atmosphere at
4.degree. C. For 5.degree. C. of temperature drop to the cryogenic
fluid temperature, the heat transferred is 1,250 calories per
minute. This is approximately the heat of vaporization of 1.0 mole
of liquid nitrogen or oxygen at the temperatures employed. Thus,
the boil-off rate of these fluids, used as jacketing cryogens is
about 1 mole per minute at an ocean depth of 1,620 feet. This is a
fluid consumption rate of about 2.0 liters per hour of duration.
The boil-off rate is about 0.8 standard cubic feet per minute.
2 Life Support Test System
Automatic adjustment of oxygen content in an incoming gas can be,
it is to be noted, reciprocal in direction or opposite in purpose
from a life support system, or both. In the generalized system of
FIG. 1 flow in the direction reverse to that shown is not
contemplated. As to that part of the system concerned with control
of oxygen partial pressure reverse and different operation are
fully feasible. FIG. 1 does additionally illustrate an arrangement
serving to provide the converse function to a life support system.
Specifically that portion of FIG. 1 within the dotted line box 43
labeled simulator and including the additional functional units
coupled into the heat exchanger 26 and cryogenic reservoir system
by dotted line connections serves as a simulator or test system
having characteristics corresponding to a human user. In this
system, the interface to any conventional life support system (not
shown) is represented at the left margin of the dotted line box 43.
The life support system under test provides a gas mixture adjusted
for oxygen content into the test system, and receives a gas mixture
deficient in oxygen content. Therefore the test system may be
called a humanoid as well as a simulator or other test system for
evaluation of life support systems under arbitrary pressure
conditions.
In this variation of the system of FIG. 1, water in the incoming
gas mixture may conveniently be removed by a condenser 45 preceding
the final water removal or desiccant device 18. The CO.sub.2
collector 36 is bypassed, as shown by the dotted coupling within
the heat exchanger 26 inasmuch as the system under test is to
provide the necessary removal of CO.sub.2 and toxic hydrocarbons.
Water removal is however employed to prevent clogging of the heat
exchanger lines.
The source of inert gas 42 is not employed, this constituent again
being supplied by the system being tested. A substantially constant
or intermittent flow of gaseous carbon dioxide into the system may
be injected into the outgoing gas mixture from a source 47 through
a regulator 48. Finally, water vapor corresponding to the
approximate amount present in an expiratory mixture is added by
passing the outgoing gas through an evaporator 51.
Heretofore, appropriate testing of a life support system has
demanded either human participation or the use of a complex test
system. In either event adequate simulation of human usage under
hyperbaric or other pressure conditions has not been satisfactorily
achieved. The test system of FIG. 1 may be placed in a test
environment at any pressure desired for operation of a life support
system and still provide the desired humanoid simulation. The back
pressure control 50 for the cryogen is set to maintain the liquid
oxygen temperature such that the outgoing gas contains less than a
breathable oxygen mixture. The system under test is thus required
to make up the deficiency in returning the gas mixture to the
simulator unit 43. Further, the water vapor introduced by the
evaporator 51 and the CO.sub.2 from the source 47 are eliminated
during operation as a part of the test. As the life support system
purifies the outgoing gases from the simulator and adds oxygen the
simulator 43 depresses the oxygen content to impose a continued
demand for oxygen. The liquid oxygen temperature is held, for pure
oxygen, below a temperature level at which the oxygen partial
pressure is less than 0.2 atmospheres, for example. Both systems
may be cycled through various pressure and temperature ranges and
the effective simulation of human demand is thus accomplished.
3. Detailed System for Positve Pressure Environments
A detailed example of a system in accordance with the invention is
illustrated in FIGS. 3 and 4, to which reference may now be made.
This detailed example comprises a self-contained system which not
only provides a closed circuit life support arrangement for an
individual, but additionally makes available heat by-products for
use in the system.
In the arrangement of FIGS. 3 and 4, a cryogenic system is used
comprising a cylindrical storage vessel 60 for a suitable cryogen,
here liquid nitrogen. The storage vessel 60 is provided with
cryogenic insulation, not shown in detail, such as a double wall
structure with an intermediate spacing maintained at vacuum.
Alternatively, the wall of the storage vessel may be a composite
including cryogenic insulation of the type having multiple layers
of heat reflective material, e.g, thin aluminum sheet between which
layers are interspersed glass or other fibrous mats. A liquid
oxygen tank 62 is disposed within the principal storage vessel 60,
the tank being of spherical form and only partially filled with
liquid oxygen 63, so that an enclosed volume is maintained above
the liquid oxygen 63 for containment of gases. The liquid oxygen
less than half fills the spherical container so that gas outlet
port 64 which reaches to the center of the sphere does not extend
into the liquid in any orientation. The liquid oxygen may be fed in
by an inlet valve coupled to the conduit leading to the port 64. A
wicking member 65 here having the general form of a hollow surface
of revolution is disposed within the liquid oxygen tank 62, and
extending into the liquid oxygen 63. The wicking member 65 may
comprise any woven screen or fibrous material, such as asbestos,
that is capable of being wetted by the liquid oxygen and thus
greatly increasing the area of the exposed surface. The gas-liquid
interface region is increased to assure total saturation of
incoming gases with oxygen vapor and rapid attainment of
thermodynamic equilibrium at a given temperature between the oxygen
in the liquid and gas states.
A helium tank 67 is also disposed within the storage vessel 60, and
an outlet conduit 68 is extended into the interior of the helium
tank 67 for injection of inert gas into the system as is described
in greater detail below.
An advantageous arrangement of a heat exchanger system 72 for
incoming and outgoing gases disposes the conduits for the gases in
heat exchange relationship within a second storage vessel 70 here
mounted adjacent and in parallel relation to the storage vessel 60
for the cryogen. Through advantageous use of a heat exchanger 72
comprising formed membranes defining multiple parallel passages and
arrayed to form a generally rectangular cross-section, as described
in greater detail in FIGS. 5 and 6 below, a heat exchange system of
minimal volume and high effective heat transfer surface area is
provided. For each of manipulation of the life support system by an
individual diver the second storage vessel 70 may be side-by-side
relation to the cryogen storage vessel 60. The heat exchanger 72
may, as illustrated, comprise a principal elongated section at one
end, separated by a CO.sub.2 trap and filter section 73 from a
relatively shorter section at the other end. The principal heat
exchange section may, for purposes of fabrication and assembly, be
divided into a group of standardized, series coupled shorter
sections. Reference should be made to the detailed views of FIGS. 5
and 6 and the accompanying description for better understanding of
the internal arrangement of the heat exchanger elements.
In general terms, however, incoming expiratory contaminated gases
(indicated by solid line arrows) enter a terminal header 74,
providing an inlet to selected heat exchanger passageways. After
passage through the heat exchanger and cooling to a suitable
temperature, these gases are collected in an outlet terminal header
75. The heat exchanger 72 is arranged such that the cleansed and
properly compensated gases follow flow paths (illustrated by the
dotted lines arrows as directed upwardly in these Figures) opposite
to the adjacent but distinctly separated passageways containing
contaminated gases, in counter-current relation. An inlet side
header 77 feeds the compensated gases from the tank 62 to
particular passageways from which they are extracted from the heat
exchanger 72 at an outlet side header 78. Interposed between the
ends of the heat exchanger 72, and shunting the CO.sub.2 filter 73,
the compensated gases are passed through a bypass conduit 79.
The CO.sub.2 filter 73 may take any of a number of forms, any of
which comprise a means for removing the flocculent solid CO.sub.2
precipitate from the gas flow within the heat exchanger 72. In the
arrangement shown, the incoming gases are cooled to a level at
which the gaseous carbon dioxide begins to precipitate out of
solution in the form of a solid residue, this being at
approximately three-quarters of the length of the heat exchanger 72
in one practical system. In the CO.sub.2 filter 73, an outlet 80
from the principal length of the heat exchanger 72 is disposed
spaced apart from and slightly overlapping an inlet 82 for the
remaining part of the heat exchanger 72. The inlet 82 is covered
with a fine screen 83 acting to insure separation of the solidified
CO.sub.2 from the gas stream taken into the remainder of the
system.
Significant advantages are derived from this combination of the
CO.sub.2 filter 73 and heat exchanger 72. As is described in
greater detail below, the heat exchanger 72 provides substantially
linear passageways having no substantial transitions. This together
with the fact that there is a substantial cross-sectional area
available for gas flow in a given direction during heat transfer
results in a low pressure gradient and little velocity change along
the length of flow. The high efficiency of the heat exchanger
precipitates the CO.sub.2 within a relatively short length, this
length being less than 4 inches in the practical system referred
to. The substantially enclosed chamber defined by the CO.sub.2
filter is fully open to the solid CO.sub.2 particles. Along this
relatively short length of passage, the CO.sub.2 particles are not
acted on by any substantial mechanical or physical forces, and a
mass of umpacked particles is collected in the bottom (as seen in
FIG. 3) receiving chamber portion of the CO.sub.2 filter 73. The
gas stream is diverted in an arcuate path through the screen 83 and
into the inlet 82. The overlapping relation of the outlet 80 and
the inlet 82 facilitates segregation of the flocculent solid
CO.sub.2, but inasmuch as this material does not tend to adhere to
other elements such as the screen 83, no substantial back pressure
is introduced into the system.
For better insulation, the second storage vessel 70 may be sealed
and its interior maintained at vacuum, or alternatively it may be
filled with foam or a conventional cryogenic insulative
material.
The flow path of incoming gases into the system originates at the
mouthpiece 85 of the breathing apparatus and passes through the
conduit system including the expansion bag 86 (shown only
generally) into an inlet portion of the water removal device, here
comprising a desiccant chamber 88. Coupled into the conduit at the
outlet portion of the desiccant chamber 88 is a regulator valve 90
responsive to the pressure of the hyperbaric environment. The
regulator valve 90 opens into a conduit 92 connected to the outlet
conduit 68 having access to the helium tank 67. A regulator 93 in
the helium line may be used to introduce a desired pressure drop
into the gas from the high pressure helium tank 67. If desired, the
helium line may be passed in heat exchange relation to a warmer
body so as to be brought closer to a breathable level. The
regulator valve 90 includes a purge button for pressurizing the
system with helium to cleanse lines and insure free flow. A unique
safety capability is thus added to the system by virtue of the
closed circuit arrangement in combination with inert gas injection
at high pressure. If clogging does occur in the heat exchanger
passageways, or if liquid should be introduced into conduits
forming part of the system, these may be virtually instantaneously
reopened for use by the injection of high pressure helium. This
flushing of the system does not affect the oxygen balance in the
system when the purging is completed, because the proper oxygen
proportion is substantially immediately restored.
For simplicity of illustration, a harness for the diver operating
the unit has not been shown inasmuch as any conventional backpack
or harness structure may be utilized. Similarly, the unidirectional
control valves have not been illustrated in FIGS. 3 and 4.
The incoming gases flow from the outlet terminal header 75 of the
heat exchanger 72 into the storage vessel 60. Within the storage
vessel 60, the gas stream is further cooled by being passed in heat
exchange relation to the cryogen in coils 95 wound within the
storage vessel 60. The coils 95 provide an isothermal heat exchange
between the cryogen and the incoming gas stream, and the resultant
cooling of the gas stream is sufficient to insure that the heat
balance within the liquid oxygen tank 62 is not substantially
disrupted by the incoming gases. In other words, expiratory gases
do not tend to heat the liquid oxygen excessively.
To insure continued flow, the gas line containing the incoming
gases is divided into two separate lines forming the coils 95, each
curving in an opposite sense and terminating within the liquid
oxygen tank 62 in facing and opposed inlet ports 96 that are above
the level of the liquid oxygen. In the normal position of operation
of this system, the attitude of the tanks is generally as shown in
FIG. 3, i.e., at least somewhat vertical. When the tanks are in
this attitude, the inlet ports 96 lie above the liquid oxygen
level. A constant attitude cannot of course be assumed under all
conditions of operation. Therefore, the use of two ports 96 insures
that irrespective of tilting in one direction or another at least
one of the ports 96 will be open to the incoming gas stream.
Furthermore, even though the remaining port 96 may temporarily be
filled, liquid oxygen within it returns to the main portion of the
tank 62 as soon as the attitude of the system is again
approximately normal.
The gas outlet port 64, centrally disposed within the wicking
member 65, provides a means for withdrawal of the compensated gases
from the gas containment volume within the liquid oxygen tank 62
irrespective of the orientation of the latter. These gases are
passed out of the storage vessel 60 into the inlet side header 77
of the heat exchanger and returned through the conduit system via
the outlet side header 78 to the mouthpiece 85.
It has previously been stated that a number of means, including a
large cryogen mass, may be utilized for maintaining the partial
pressure of the oxygen within a selected range. The arrangement
shown in FIGS. 3 and 4 employs the removal of boil-off gases from
the cryogen at an appropriate rate determined directly by the
temperature within the liquid oxygen tank 62. The arrangement is
only generally described here, inasmuch as further details as to
the heat exchange relationships and control of venting are
described in detail in conjunction with FIGS. 5 to 7 below.
Boil-off gas line 97 extends directly into the cryogen storage
vessel and feeds the boil-off gases through a supply manifold to
interdigitated and convoluted paths within the heat exchanger 72.
During filling of the vessel 60 the cryogen may be fed in via the
gas line 97. The boil-off gases extract heat from the heat
exchanger 72 in passage through a manifold system to a venting
control valve 100. No liquid valving arrangement is needed because
the pressure of the boil-off gases in the line 97 is arranged block
flow of the liquid within the boil-off line 97. Confluent cryogen
boil-off uptake lines 98A, 98B, 98C and 98, with ends open and
extending into the cryogen storage volume, are disposed within the
cryogenic storage vessel in a manner which precludes the escape of
liquid by gravity feed. The confluent lines 98 A-D are joined in a
common header 99 to boil-off line 97 within the cryogenic storage
vessel. The arcuate paths of the several gas uptake lines 98 A-D
are designed to provide a liquid trap for virtually any orientation
of the system although there is always at least one clear path for
gas flow. The common uptake manifold header 99 is disposed within
the cryogen containing vessel to provide an opportunity for fluids
momentarily trapped in the uptake lines due to rapid movements, to
evaporate within the confines of the vessel for the purpose of
exerting the full influence of their related cooling capacity. Each
of the gas uptake lines 98 A-D terminates in a distinct separate
region within the cryogenic reservoir 60. Each follows a pair of
mutually orthogonal reentrant paths within the reservoir 60, the
limits of each reentrant path being spaced more than half the
corresponding dimension of the reservoir 60.
The venting control valve 100 is a regulator operating in response
to a controlling mechanical force to pass the gases into a boil-off
receiver system 102. A control knob 103 may be adjusted, as
described in more detail below, to select an appropriate operating
level. The desired controlling mechanical force for regulating the
boil-off rate passing into receiver system 102 is exerted from a
thermal sensor tube arrangement 104 described in detail in
conjunction with the description of FIG. 7. At this point it
suffices to state that the thermal sensor tube 104 extends into the
interior of the liquid oxygen tank 62 and in response to the
temperature therein generates a mechanical force action on the
venting control valve 100. Consequently, when the temperature of
the gas-liquid interface in the liquid oxygen tank 62 rises above a
predetermined point, the pressure in the sensor tube 104 opens the
venting control valve 100, to pass the boil-off cryogen gases into
the receiver system 102. The cryogen pressure, and therefore
temperature, are correspondingly adjusted, and the liquid oxygen
temperature is maintained within the desired range.
A positive action receiver system 102 is required to extract the
boil-off gases only when a low pressure receiver is not available.
In hypobaric environments, the environments themselves can comprise
an infinite receiver. In fixed and other self-contained systems,
there may be a receiver available of a different type, such as a
gas or liquid container whose principal contents are gradually
utilized for other purposes and which thus effectively provides a
low pressure enclosure of increasing volume as the system operates,
or those principal contents (solid, liquid or gas) absorb or
chemically combine with the boil-off gases to form a residue (solid
or liquid) resulting in a pressure that is lower than the pressure
of the boil-off gases at the entrance to the receiver system
102.
The receiver system 102 of the mechanism, as shown in FIGS. 3 and
4, however, is of particular utility for hyperbaric, individual
diver, systems. The vented gases are withdrawn by a compressor 105
driven by a battery 106. An outlet vent on the compressor 105
simply injects the gases into the environment, although a separate
collector tank could be used to limit bubbling of gases into the
environment. The heat generated within the battery 106 and the
compressor 105 is also usefully employed in the system, however,
inasmuch as a cooling fluid is circulated in a closed conduit path
(not illustrated in detail to simplify the representation) through
heat transfer conduits 108 in the diver's suit. Most receiver
systems are exothermic in character, giving off heat as they
operate, and this heat, as shown, is employable for the benefit of
a system user, whether an individual wearing a suit or a group of
individuals within a selfcontained system.
The presence of these different subsystems, however, is
illustrative of the many significant aspects of the invention which
are corollary to the fundamental aspect of adjustment of the
breathable gas mixture. Apart from straightforward and reliable
flow direction controls and a compensating regulator for ambient
pressure, it will be observed that a breathable oxygen mixture is
provided without any mechanisms for performing sensing, computing
or regulating functions. The inlet gases are brought to a suitable
temperature range within the heat exchanger 72, being purified
through the action of the expansion bag 86, desiccant or condensing
and freezing chamber 88 and carbon dioxide filter 73. Upon entering
the gas-containing volume within the liquid oxygen tank 62, the
gases are in an environment whose characteristics, in terms of
partial pressure of oxygen, are determined solely by the saturation
and thermodynamic equilibrium relations between the liquid oxygen,
and the gas mixture including gaseous oxygen in communication with
it, as previously described. When the temperature in the liquid
oxygen tank 62 is maintained in a selected range, the partial
pressure of oxygen is automatically kept within the approximately
0.2 to 1 atmosphere range that is required for the breathable
mixture.
Inasmuch as the basic source of replenishment oxygen is in liquid
form, a large breathable oxygen supply is available, greatly in
excess of amounts which can be manipulated and used by most present
day self-contained underwater breathing apparatus. Furthermore, the
system efficiency is extremely high because of the closed circuit
arrangement, which insures that only replenishment oxygen need be
added during operation of the system.
The large cryogen reservoir comprising the storage vessel 60
represents a stable cryogenic source. Most effective use is made of
this stable source, however, by utilizing an efficient
counter-current heat exchanger 72 to limit heat gains in lowering
the temperature of the expired mixture to the cryogenic level,
while raising the adjusted mixture to a respirable level.
Furthermore, the heat exchanger 72 is used to extract solidified
CO.sub.2 during the cooling of the incoming gases, and provides
another advantageous aspect of the system.
Injection of helium to compensate for pressure differentials
between system pressure and the hyperbaric environment, and the use
of a helium tank 67 within the cryogen have additional advantages.
A large volume of helium may be maintained at the low cryogen
temperature. At least several times the normal useful quantity of
inert gas is made available in this fashion. Although the helium
provides a high percentage of the gas mixture at substantial
pressures, it is not utilized physiologically and therefore a
substantially lesser amount is needed. In practical systems in
accordance with the invention, the useful work cycle for a diver is
extended appreciably. An operating period of five hours can readily
be achieved without the employment of masses or sizes of equipment
so large as to limit a diver's work product.
In addition, use is made of the heat generated in the operation of
the receiver system 102 to provide a part of the heat supply
inevitably needed in underwater environments. The presence of large
amounts of helium in a breathable mixture greatly increases the
rate at which heat is lost by an individual. Thus the utilization
of generated heat to supply the conduits 108 in the diver's suit
supplies a basic need for such environments and increases the
overall efficiency of the present system.
4. Variation of the Liquid Oxygen Purity
The useful range of partial pressure for oxygen, for purposes of
human life support, has been stated as approximately 0.2 to 1.0
atmosphere. As the total respired medium is brought into thermal
vapor pressure equilibrium with liquid oxygen in order to achieve
the desired oxygen partial pressure, it follows that the
temperature range for the pure oxygen liquid, corresponding to the
safe oxygen levels cited in accordance with the data of FIG. 2, is
approximately 77.degree. K. to 90.degree. K. The same temperature
control range therefore applies to the surrounding cryogen. If, on
the other hand, the liquid oxygen is diluted with another cryogenic
fluid, the former's vapor pressure is correspondingly diluted, all
else remaining equal. Specifically, the partial pressure of a gas
in thermal equilibrium with its liquid phase, irrespective of the
ambient pressure of foreign gases in direct combination with the
liquid, is proportional to the product of the subject gas partial
pressure (obtaining for the pure liquid) and the fractional molar
concentration of the substance in question in the liquid state.
Thus, for example, whereas the partial pressure of oxygen gas in
equilibrium with pure liquid oxygen at 90.degree. K. is 0.98
atmospheres, the partial pressure for a 50 percent molar
concentration of liquid oxygen (diluted, e.g., with 50 percent
molar liquid nitrogen) at the same temperature of 90.degree. K. is
reduced by one-half, to 0.49 atmospheres. So long as the dilution
remains fixed the respective partial pressures of the liquid
constitutents are invariant.
This physical principle is applicable with advantage to the
cryogenically controlled system. Thus, the liquid oxygen may be
diluted with another cryogenic fluid provided the vapor pressure of
the diluting fluid which results thereby is tolerable for human
breathing. The operating temperature of the cryogenic fluids is
consequently collectively raised from that which would be employed
with pure liquid oxygen, in order to provide the same level of
oxygen partial pressure in the breathing medium. If the dilution is
used, consideration must be given to the progressive dilution of
oxygen liquid as the latter is selectively extracted from the
solution by evaporation. If allowed to continue, the level of
oxygen partial pressure would be altered even if temperature were
held fixed. The effects of dilution may be suppressed by carrying a
larger supply of liquid, so that the rate of oxygen consumption,
when compared with the period of use, is not sufficient to alter
the oxygen partial pressure level beyond tolerable limits. Also
adjustments may be made periodically as oxygen liquid is consumed,
to raise the operating temperature and offset the oxygen level
decrement. The principal advantage of oxygen liquid dilution, if
only for abbreviated periods of use, is the higher operating
temperature of the surrounding cryogen which allows the latter a
correspondingly higher operating pressure. In those cases where the
cryogen pressure exceeds that of the ambient environment, system
use in a positive pressure environment, without the need of an
exhaust compressor or other type of receiver system, save for the
environment itself may be permitted.
5. Heat Exchanger System
FIGS. 5 and 6 illustrate the general organization and particular
details of a heat exchanger system having high efficiency and
extreme compactness but using elements that can be readily
fabricated and easily assembled. The heat exchanger is of the
counter-current type, and solves the problem of headering, i.e.,
directing the gases into separated passageways without a complex
maze of interconnections.
As best seen in FIG. 6, which is a greatly enlarged view of a
segment of the heat exchanger, in which relative dimensions are not
to scale in order to show the elements more clearly, the basic heat
exchanger elements comprise thin corrugated membranes 110, with the
corrugations running parallel and lengthwise along the membrane. Of
the order of 50 corrugations per inch may be employed for use in
the present system, and the peak-to-valley dimension may be of the
order of approximately 30 mils, with a membrane of the order of
0.002 inches to thickness. The membrane 110 itself is preferably of
a thermosetting plastic of one of the types conveniently used in
thermoforming such as the material sold under the trademark
"Lexan." The corrugated configuration can be achieved by shaping
the plastic between dies after bringing it above the plastic flow
temperature. The waviness or repetitive deviation of the membrane
110 from its median plane to define gas passageways need not follow
the generally sinusoidal form that is associated with the term
"corrugation." Instead, the membrane 110 may deviate to define
grooves, or peaks and valleys, in any desired periodic or even
aperiodic fashion.
As to this basic heat exchanger element, a hot gas that is to be
cooled is passed on one side of the membrane 110, and cold gas that
is to be heated is passed on the other side of the membrane 110 in
the opposite direction. Heat conduction then occurs through the
thickness of the membrane. While the passage of oppositely flowing
gases on different sides of an intervening separator is known for
liquid and other systems, the present system is unique in at least
several respects. The passageways defined by the grooves are small,
and the membrane 110 is a poor heat conductor but very thin.
Thermal energy is therefore readily transferable between the gases
in the adjacent passageways through the membrane thickness. An
advantage accrues due to the use of the corrugated membrane as the
periodic surface of separation between the counter flowing gases
rather than as heat conducting fins, characteristic of earlier
designs. The heat transfer efficiency and amount of heat transfer
surface per unit volume are thereby greatly increased. In fact, the
primary limitation on the interchange of heat energy is not the
minute insulative effect interposed by the membrane but the heat
energy transfer within the gas itself. At the same time, however,
the relatively low thermal conductivity of the membrane 110 insures
that heat will not be conducted along its length parallel to the
gas flow directions. Unlike prior art heat exchange systems,
therefore, the opposite hot and cold ends are not interconnected by
a highly conductive medium comprising the heat exchange member
itself and acting in the nature of a heat sink tending to reside at
a median temperature along its entire length and thereby reducing
the efficiency of the heat exchanger. Because of the insulative
properties of the heat exchanger material, it is possible to employ
a large cross-sectional area to length ratio without significant
loss of efficiency. The pneumatic impedance of the passageway
system is low, providing a low pressure differential and permitting
greater ease of breathing.
For separation of the gases, and for headering purposes, the peaks
and valleys on opposite sides of the membrane 110 are affixed
respectively to an intermediate thin lamination 112 and a
relatively thicker spacer 114. As may be seen in both FIGS. 5 and
6, the intermediate laminations are disposed between a pair of
membranes 110, and run the full length of the heat exchange
structure. The spacers 114, however, are discontinuous along the
length of the heat exchanger, and the spacers 114 and intervening
open volumes are utilized for several purposes.
It is convenient for purposes of illustration and description to
regard the laminated structure comprising a pair of membranes 110
between a pair of thicker spacers 114 as a heat exchanger entity.
This entity is then bounded by the thicker spacers 114, and
includes an adjacent and coextensive pair of membranes 110 between
which the intermediate thin lamination 112 is interposed for the
full length of the heat exchanger. The laminations 112 and the
spacer 114 may, as with the membranes 110, be of a suitable plastic
material. Regarding the thin lamination 112 as the center of the
structure, the interior adjacent passageways whose sides are
bounded by the lamination 112 and the two adjacent membranes 110
provide flow paths from one end of the heat exchanger to the other.
What may be termed the exterior passageways within the entity are
the passageways defined between the opposite sides of the membranes
110 and the outer spacers 114. A first gas or mixture passing in
one direction along the length of the heat exchanger within the
interior passageways is therefore completely separated from a
second gas or mixture passing along the exterior passageways. If
leakage occurs due to improper bonding between the membrane and the
intermediate lamination 112, there is neither a substantial
temperature differential between the gases nor a mixing of unlike
gases. The open volumes between the separate thicker spacers 114
provide access to all of the exterior passageways of an entity from
a side of the heat exchanger. These outer open volumes communicate
with the outer passageways on the lower side (as seen in FIG. 5) of
the upper entity, and the upper side of the next lower heat
exchanger entity. All of these open volumes communicate with common
side manifolds or headers positioned at two or more regions along
the heat exchanger. The necessary separation between the gas
mixtures is provided by sealing surfaces 120 closing off the
exterior passageways at the ends of the system, and by sealing
membranes 121 closing off the interior passageways at the side
header regions.
As may be seen in both FIGS. 5 and 6, therefore, gases in the
interior passageways moving in a first direction (from right to
left in FIGS. 5 and 6) pass from one end header 118 to the opposite
header 116. The second gas mixture passing in the opposite flow
direction is fed from one side heater 122 through the heat
exchanger, to the opposite side heater 124.
The greatly simplified and idealized representation of FIG. 5
therefore shows a complete heat exchange structure 72 built up of
successive laminations of the basic heat exchange entities until
the desired cross-sectional areas for flow of the two gas mixtures
are attained. It will be appreciated that the continued lamination
of additional elements does not in any way change or further
complicate the headering arrangement and that many hundreds of
entities may be utilized. In a practical example, a heat exchanger
having external dimensions of approximately 6 inches by 6 inches by
2 feet provides the needed heat transfer capacity for an individual
life support system. This system operates between the breathable
temperature range and at approximately the cryogenic range,
transferring approximately 500 B.T.U. per minute with approximately
98 percent actual efficiency, and with a total heat exchanger
volume of the order of one-half cubic foot. The gas passageways are
linear, and as noted, a substantial cross-sectional area is made
available for gas flow, together with an extremely large heat
transfer area. Thus, a low pressure differential exists within the
system. This arrangement is further characterized by the fact that
standardized heat exchange sections may be assembled of selected
length and cross-sectional areas. For greater capacity or
efficiency, these may be series or parallel connected simply by
appropriate interconnection between the heat exchange headers.
The heat exchanger structure is also arranged to provide an
additional heat transfer function, extracting heat from the
principal counter-current gases with high efficiency. It will be
appreciated that a temperature drop or rise may be augmented simply
by insertion of an available high or low temperature source in
close contact with the heat transfer entities, in view of the
unrestricted interior volumes between the entities and headers.
Apart from this obvious expedient, however, it is desirable in the
present example to provide heat extraction from the counter flowing
gas mixtures without substantially impeding gas flow and by
utilizing a substantial heat exchange area. To this end, low
temperature gases from a boil-off manifold 125 are passed through a
tortuous conduit 126 interposed between the facing sets of exterior
passageways in adjacent heat exchanger entities, and between the
side headers 122, 124. The gases in the exterior passageways are
kept separated by thin interior laminations 127. A tertiary heat
exchange is thus effected, utilizing the boil-off gases as a heat
sink that acts substantially upon both counter-flowing gases
through that gas flowing in the second direction within the heat
exchanger, between the side headers 122, 124.
6. Pressure and Temperature Control
Because the cryogenic reservoir, containing the gas-liquid
interface volume, is not itself invested with the breathing medium,
the partial pressure which exists therein by virtue of the
temperature which also obtains is the total pressure in the
cryogenic container. In view of the fundamentally isomorphic
relationship which exists between temperature and partial pressure
for any fluid in equilibrium with its vapor in and enclosed volume
it is possible to control the temperature of the cryogenic fluid by
mechanically regulating the back pressure of the cryogenic
container. The partial pressure of oxygen vapor is obscured, in the
mechanical sense, by the presence of the supportive breathing
medium, consisting of appropriately proportioned inert gases,
precluding, thereby, direct mechanical control of oxygen pressure.
The latter is, nevertheless, indirectly regulated by equilibrium
with its related liquid, the temperature of which, is identified
with that of the cryogenic fluid and may be controlled as a
consequence of control of the cryogenic fluid.
The arrangement of FIG. 7 illustrates one suitable arrangement of a
receiver for boil-off gases and a control system for maintaining
the oxygen partial pressure constant or within a selected range in
the system of FIGS. 3 and 4. Portions of the system may be
conventional and accordingly have been illustrated in block diagram
or idealized form. Other portions of the system are shown in FIGS.
3 and 4 and accordingly are omitted herein.
The controlling mechanical force for regulating the boil-off
exhaust rate to the receiver system 102 is generated by the
pressure of a gas within a thermal sensor tube 130. The thermal
sensor tube 130 has a sealed end 131 disposed within the liquid
oxygen 63 in the liquid-oxygen tank 62, with a sufficient pressure
of gas, e.g., nitrogen initially being contained within the sealed
tubing to insure that a portion of the internal gas is liquefied
within the coiled length 131 of tubing immersed in the liquid
oxygen bath. The immersed length is appropriately coiled or
otherwise formed to maintain contact with the liquid oxygen 63 in
all orientations. The tubing extends outside the liquid oxygen tank
62 into communication with a venting control valve 100. Gas
pressure within the tube is in communication with a diaphragm 134
bearing against an axially slidable piston 136 having a radial vent
138 which moves as the piston 136 slides. The radial vent 138 opens
and closes an exhaust conduit 139 for the compressor 105. A spring
140 normally opposes opening of the radial vent 138, with a force
dependent upon the setting of an adjustable control knob 103.
The thermal sensor tube 130 generates a mechanical force that is
substantially completely determined solely by the liquid oxygen
temperature. The sealed end 131 of tubing maintained within the
liquid oxygen bath insures that the liquefied nitrogen within the
sealed tube assumes the liquid oxygen temperature. Therefore, the
nitrogen partial pressure in the enclosed volume constituting the
remainder of the tube represents the total gas pressure within the
tube, and is directly dependent upon thermodynamic equilibrium
between the gas and the liquid in the interface region. Relatively
small variations in the absolute level of the liquid oxygen
temperature nevertheless represent large relative pressure
variations, and the pressure within the tube therefore covers a
wide dynamic range, providing accurate and substantially linear
control forces to operate the diaphragm 134. Because substantially
perfect communication of pressure within the confined gas may be
assumed and because that pressure is dominated by the liquefaction
equilibrium condition at one end the desired force may be
transmitted through a long tube, and the tube may pass through
widely varying temperature zones, such as the cryogenic reservoir
and the associated positive pressure environment. This closed loop
temperature control system is not only extremely simple and free
from reliance upon components or functional units subject to
failure, but also effects the necessary temperature sensing and
transduction to an amplified mechanical force at very low cost.
7. System Operating in Negative Pressure Environments
A different form of closed circuit life support system is
illustrated in FIG. 8, this form having particular applicability to
enclosed environments for life support in superatmospheric or space
conditions. A life containment structure 150 may comprise one or
several individual pressure suits, or an enclosed volume. In either
event, the life containment structure 150 forms part of a closed
circuit system for satisfying physiological needs for oxygen. A
supply of liquid oxygen is maintained in an enclosed and insulated
tank 152 that defines an enclosed volume within which gaseous
oxygen is maintained at saturation and in thermodynamic equilibrium
with the liquid oxygen. Gases are circulated through the life
containment structure 150 via an inlet-outlet system including
outlet terminal 153 and an inlet terminal 154 with the inlet
terminal communicating with the enclosed volume within the liquid
oxygen tank 152.
This system advantageously employs what may be regarded as the
infinite heat sink capacity of a superatmospheric or outer space
environment, and does not require a receiver system or a cryogen or
other means for controlling the gas mixture. Instead, a pump 156
circulates the gas through a cleansing system 157 and the remainder
of the system. A sensor 158, which may be of the sensor tube type
or a different form of thermally responsive sensor, operates a
controllable by-pass valve 159 to circulate gases through the
system so as to tend to maintain a selected range of oxygen partial
pressure. The circulation path from the pump 156 includes a
cryogenic refrigerator 161 operating in conjunction with
counter-current heat exchanger 160 and a bypass path, both coupled
into the liquid oxygen tank 152. In space applications, an extended
length of conduit and fins providing an adequate area of heat
exchange surface with the environment is sufficient as a cold
source, if insulated from radiant energy, because the environment
is at approximately 4.degree. K. and therefore represents an
infinitely great negative heat capacity factor. Otherwise, a
conventional power operated cryogenic refrigerator may be used. The
heat exchanger 160 receives gases from the enclosed volume within
the tank 152 for effecting heat transfer with the incoming
gases.
When the life containment structure 150 operates without
substantial physiological demand for oxygen, the only change in the
temperature of the liquid oxygen in the tank 152 results from heat
losses to or intake from the surrounding environment. As the oxygen
in the structure 150 is lost through leakage and physiological use,
without regard to variations in circulation flow rate, however, the
valve 159 is operated to by-pass the cryogenic refrigerator 161 to
the extent that the liquid oxygen temperature is maintained in the
selected range.
The cleansing system for the contaminated gases may conveniently be
incorporated in the cryogenic refrigerator 161 or heat exchanger
160 with collected water vapor and carbon dioxide being ejected
into the environment. The use of a by-pass system is merely one
expedient that may be employed. Variable cooling may also be
achieved by using variable flow rates, or by varying the efficiency
of the cryogenic refrigerator, as by varying the length of flow
within a conduit system. Alternatively flow may be divided in
controlled fashion between heating and cooling paths.
It is common to utilize a 100 per cent oxygen supply and a pressure
of the order of 0.35 atmospheres. The flammable nature of pure
oxygen, however, indicates the need for an inert component for
quenching purposes in the gas mixture. The inert gas may be
provided from a source 162 through a pressure regulator 164 open to
the life containment structure 150. Thus, with an oxygen partial
pressure of approximately 0.35 atmospheres, a total pressure of
0.50 atmospheres (or any other selected value) may be achieved by
injection of the needed additional gas via the pressure regulator
164. In any such system the quenching gas should be such that it
does not liquefy at the range of partial pressures utilized.
Nitrogen and helium are satisfactory for the example given.
It will also be recognized, moreover, that the partial pressure of
the inert gas may itself be controlled directly, with oxygen
pressure being adjusted by a pressure regulator, to provide the
converse of the system of FIG. 8.
Control of the partial pressure of oxygen can moreover be utilized
in an open circuit system, even though such a system is wasteful of
a supply of breathable gas mixture. Furthermore, the added
efficiency achieved by the use of heat exchanger mechanisms can
either be realized in other ways, or need not be utilized in the
system.
Although a number of alternative forms and modifications of systems
in accordance with the invention have been described, it will be
appreciated that the invention is not limited thereto but
encompasses all variations and modifications falling within the
scope of the appended claims.
* * * * *