Methods and apparatus to generate liquid ambulatory oxygen from an oxygen concentrator

The present invention is directed to a much safer and less expensive way of providing portable oxygen from a gas concentrator for patients who do not want to be tied to a stationary machine or restricted by present oxygen technology. In one preferred embodiment, the present invention splits off some of the excess capacity gas flow from a gas concentrator which is then stored via liquefaction. The stored gas can then be used as a portable supply. A portion of the oxygen gas flow generated by the oxygen concentrator is channeled to a condenser which receives and liquefies the oxygen gas using cryocooler. A storage dewar is used for storing the oxygen liquefied by the condenser. Liquid is then selectively transferred to a smaller portable dewar. A controller can be used for monitoring the parameters of liquefaction, including oxygen concentration, the amount of liquid oxygen in the dewar, and for controlling the parameters of liquid oxygen generation and transfer. In one embodiment, the flow rate into the condenser is chosen to exceed the capacity of the condenser to minimize the liquefaction of argon, nitrogen and trace gases, and to purge the system.

BACKGROUND OF THE INVENTION 
The field of this invention relates to using an oxygen concentrator to 
create a portable supply of supplementary oxygen for ambulatory 
respiratory patients so that they can lead normal and productive lives--as 
the typical primary oxygen sources are too bulky to carry or require 
excessive power to operate. 
There is a burgeoning need for home and ambulatory oxygen. Supplemental 
oxygen is necessary for patients suffering from lung disorders; for 
example, pulmonary fibrosis, sarcoidosis, or occupational lung disease. 
For such patients, oxygen therapy is an increasingly beneficial, 
life-giving development. While not a cure for lung disease, supplemental 
oxygen increases blood oxygenation, which reverses hypoxemia. This therapy 
prevents long-term effects of oxygen deficiency on organ systems--in 
particular, the heart, brain and kidneys. Oxygen treatment is also 
prescribed for Chronic Obstructive Pulmonary Disease (COPD), which 
afflicts about 25 million people in the U.S., and for other ailments that 
weaken the respiratory system, such as heart disease and AIDS. 
Supplemental oxygen therapy is also prescribed for asthma and emphysema. 
The normal prescription for COPD patients requires supplemental oxygen flow 
via nasal cannula or mask twenty four hours per day. The average patient 
prescription is two liters per minute of high concentration oxygen to 
increase the oxygen level of the total air inspired by the patient from 
the normal 21% to about 40%. While the average oxygen flow requirement is 
two liters per minute, the average oxygen concentrator has a capacity of 
four to six liters of oxygen per minute. This extra capacity is 
occasionally necessary for certain patients who have developed more severe 
problems but they are not generally able to leave the home (as ambulatory 
patients) and do not require a portable oxygen supply. 
There are currently three modalities for supplemental medical oxygen: high 
pressure gas cylinders, cryogenic liquid in vacuum insulated containers or 
thermos bottles commonly called "dewars," and oxygen concentrators. Some 
patients require in-home oxygen only while others require in-home as well 
as ambulatory oxygen depending on their prescription. All three modalities 
are used for in-home use, although oxygen concentrators are preferred 
because they do not require dewar refilling or exchange of empty cylinders 
with full ones. 
Only small high pressure gas bottles and small liquid dewars are portable 
enough to be used for ambulatory needs (outside the home). Either modality 
may be used for both in-home and ambulatory use or may be combined with an 
oxygen concentrator which would provide in-home use. 
As we describe below, the above-described current methods and apparatus 
have proven cumbersome and unwieldy and there has been a long-felt need 
for improved means to supply the demand for portable/ambulatory oxygen. 
For people who need to have oxygen but who need to operate away from an 
oxygen-generating or oxygen-storage source such as a stationary oxygen 
system (or even a portable system which cannot be easily carried), the two 
most prescribed options generally available to patients are: (a) to carry 
with them small cylinders typically in a wheeled stroller; and (b) to 
carry portable containers typically on a shoulder sling. Both these 
gaseous oxygen and liquid oxygen options have substantial drawbacks. But 
from a medical view, both have the ability to increase the productive life 
of a patient. 
The major drawback of the gaseous oxygen option is that the small cylinders 
of gaseous oxygen can only provide gas for a short duration. Oxygen 
conserving devices that limit the flow of oxygen to the time of inhalation 
may be used. However, the conserving devices add to the cost of the 
service and providers have been reluctant to add it because there often is 
no health insurance reimbursement. Indeed, the insurance reimbursement for 
medical oxygen treatment appears to be shrinking. 
Another drawback of the gaseous oxygen option is the source of or refill 
requirement for oxygen once the oxygen has been depleted from the 
cylinder. These small gas cylinders must be picked up and refilled by the 
home care provider at a specialized facility. This requires regular visits 
to a patient's home by a provider and a substantial investment in small 
cylinders for the provider because so many are left at the patient's home 
and refilling facility. Although it is technically possible to refill 
these cylinders in the patient's home using a commercial oxygen 
concentrator that extracts oxygen from the air, this task would typically 
require an on-site oxygen compressor to boost the output pressure of the 
concentrator to a high level in order to fill the cylinders. Additionally, 
attempting to compress the oxygen in pressurized canisters in the home is 
dangerous, especially for untrained people. This approach of course 
presents several safety concerns for in-home use. For example, in order to 
put enough of this gas in a portable container, it must typically be 
compressed to high pressure (.about.2000 psi). Compressing oxygen from 5 
psi (the typical output of an oxygen concentrator) to 2000 psi will 
produce a large amount of heat. (Enough to raise the temperature 
165.degree. C. per stage based on three adiabatic compression stages with 
intercooling.) This heat, combined with the oxygen which becomes more 
reactive at higher pressures, sets up a potential combustion hazard in the 
compressor in the patient's home. Thus, utilizing and storing a high 
pressure gas system in the patient's home is dangerous and not a practical 
solution. 
The convenience and safety issues are not the only drawbacks of this 
compressed oxygen approach. Another drawback is that the compressors or 
pressure boosters needed are costly because they require special care and 
materials needed for high pressure oxygen compatibility. For example, a 
Rix Industries, Benicia, Calif., 1/3 hp unit costs about $10,000 while a 
Haskel International, Burbank, Calif., air-powered booster costs about 
$2200 in addition to requiring a compressed air supply to drive it. Litton 
Industries and others also make oxygen pressure boosters. 
Turning now to the liquid oxygen storage option, its main drawback is that 
it requires a base reservoir--a stationary reservoir base unit about the 
size of a standard beer keg--which has to be refilled about once a week. 
The liquid oxygen can then be obtained from a base unit and transferred to 
portable dewars which can be used by ambulatory patients. Also, with the 
liquid oxygen option, there is substantial waste, as a certain amount of 
oxygen is lost during the transfer to the portable containers and from 
evaporation. It is estimated that 20% of the entire contents of the base 
cylinder will be lost in the course of two weeks because of losses in 
transfer and normal evaporation. These units will typically boil dry over 
a period of 30 to 60 days even if no oxygen is withdrawn. 
There are other complications. Typically, supplemental oxygen is supplied 
to the patient by a home care provider, in exchange for which it receives 
a fixed monetary payment from insurance companies or Medicare regardless 
of the modality. Oxygen concentrators for use in the home are preferred 
and are the least expensive option for the home care provider. For outside 
the home use however, only small high pressure gas bottles and small 
liquid dewars are portable enough to be used for ambulatory needs. One of 
these two modalities may be used for both in-home and ambulatory use or 
may be combined with an oxygen concentrator which would provide in-home 
use. In either case, the home care provider must make costly weekly or 
biweekly trips to the patient's home to replenish the oxygen. One of the 
objects of this invention is to eliminate these costly "milk runs." 
Portable oxygen concentrators are commercially available for providing 
patients with gaseous oxygen. These devices are "portable" solely in the 
sense that they can be carried to another point of use such as in an 
automobile or in an airplane. At present, there are no home oxygen 
concentrators commercially available that can provide liquid oxygen. One 
type of medical oxygen concentrator takes in air and passes it through a 
molecular sieve bed, operating on a pressure swing adsorption cycle, which 
strips most of the nitrogen out, producing a stream of .about.90% oxygen, 
for example, as shown in U.S. Pat. Nos. 4,826,510 and 4,971,609 (which are 
incorporated herein by reference). While, as set out in the Information 
Disclosure Statement, complex oxygen liquefaction systems have been 
disclosed for use by the military in jet aircraft, and while large-scale 
commercial plants have been disclosed, this technology has not yet found 
its way into the home to help individual patients and to benefit the 
general public. A truly portable oxygen concentrator has not yet been 
perfected and this event is unlikely, at least in the near future, because 
the power requirements are too large to be provided by a lightweight 
battery pack. 
Since liquid oxygen requires periodic delivery and home oxygen 
concentrators are not commercially available that would create liquid 
oxygen, there has existed a long-felt need for a device or method having 
the capability to concentrate oxygen from the air, liquefy it, and 
transfer it into portable dewars in a home environment, and for a home 
oxygen concentrator unit which allows excess flow capacity from the 
concentrator to be stored by either compression or liquefaction for later 
use. 
SUMMARY OF THE INVENTION 
The present invention presents a much safer and less expensive way of 
providing portable oxygen for patients who do not want to be tied to a 
stationary machine or restricted by present oxygen technology. In one 
preferred embodiment, the present invention splits off some of the excess 
capacity gas flow from a PSA (pressure swing adsorption) or membrane gas 
concentrator which has a relatively stable base load. This small portion 
of the excess flow capacity, about one liter per minute (.about.1 LPM) is 
stored via liquefaction. The stored gas can then be used as a portable 
supply away from the location of the gas concentrator. The daily six hour 
range capacity for a two liter per minute patient can be accumulated by 
liquefying a one liter per minute gas flow for less than 24 hours. 
Therefore, the entire daily requirement for mobility can be produced every 
day if needed. 
A summary of one of the many representative embodiments of the present 
invention is disclosed including a home liquid oxygen ambulatory system 
for supplying a portable supply of oxygen, where a portion of the gaseous 
oxygen output obtained from an oxygen concentrator is condensed into 
liquid oxygen, comprising: (a) an oxygen concentrator which separates 
oxygen gas from the ambient air; (b) an outlet flow line to transfer flow 
of oxygen gas from said oxygen concentrator for patient use; (c) a valve 
placed in the outlet flow line for splitting off a portion of the oxygen 
gas flow generated by the oxygen generator; (d) a generally vertically 
oriented, gravity assisted, condenser for receiving and liquefying the 
split off portion of the oxygen gas flow; (e) a cryocooler associated with 
said condenser; (f) a first storage dewar in fluid communication with said 
condenser for storing the oxygen liquefied by the condenser, the first 
storage dewar having an outlet selectively engageable to and in fluid 
communication with at least one second smaller dewar and a fluid path for 
supplying liquid oxygen from the first dewar to the second dewar; (g) a 
heater for heating said first storage dewar; (h) a controller for 
monitoring (i) oxygen concentration of the oxygen gas flowing from said 
concentrator, and (j) the amount of liquid oxygen in said first dewar, and 
for controlling the parameters of liquid oxygen generation and transfer 
from said first storage dewar. 
Another representative embodiment is the feature where the flow rate into 
the condenser is chosen to exceed the capacity of the condenser. In 
particular, only 20 to 90% of the incoming flow into the condenser is 
condensed to minimize the liquefaction of argon, nitrogen and trace gases, 
and to purge the system. 
Additionally, the controller may control condenser parameters so that the 
condenser temperature varies in the range from approximately 69.2 to 109.7 
K, the condenser pressure varies from approximately 5 to 65 psia, and the 
concentrations of gas into the condenser varies with the oxygen range 
being 80 to 100%, the nitrogen range being 0 to 20%, and the argon range 
being 0 to 7%. 
A unique condenser design is also disclosed where the condenser is in 
thermal contact with a cryocooler for use in liquefying oxygen and 
comprises: (a) an inlet conduit for receiving oxygen; (b) an outer member; 
(c) an inner member; (d) a passage defined by said outer and inner 
members; (e) said inner member having radial slots to passages; (f) means 
for circulating said oxygen in said condenser. 
Also disclosed is a representative method for controlling a home ambulatory 
liquid oxygen system comprised of an oxygen concentrator, a controller 
having a microprocessor, a condenser, a cryocooler and a storage dewar, 
where all or only a portion of the oxygen flow is utilized for 
liquefaction, comprising: (a) providing the microprocessor with a database 
and control functions; (b) sensing the parameters relating to the 
concentration and supply of gaseous oxygen, the level of liquid oxygen in 
the dewar, and the pressure of the condenser; (c) providing the 
microprocessor with these sensed parameters and having the microprocessor 
calculate optimal conditions; (d) controlling servomechanisms to regulate 
the system so that optimal conditions are realized as a function of said 
calculations. 
The feature is also described wherein the liquid dewar will be periodically 
boiled dry to eliminate any small amounts of water and hydrocarbons that 
may pass through the gas concentrator. 
The above-summarized apparatus and methods, more specifically set out in 
the claims, fill long-felt needs without posing any new safety issues to 
the patient in that, for example, there are no potentially dangerous 
canisters of high pressure compressed oxygen. The end result is that 
patients can ultimately use equipment with which they are familiar. For 
example, patients on liquid oxygen currently perform liquid transfers from 
large (30-50 liquid liter) dewars to small (0.5-1.2 liquid 
liter--corresponding to six hours of support) portable dewars. The present 
invention will provide a means of supplying ambulatory oxygen for a lower 
life cycle cost than the conventional method. Unlike industrial or 
military use liquefiers, which can take up whole rooms, the claimed oxygen 
liquefier (not including the oxygen concentrator) weighs less than 60 
pounds and takes up less than six cubic feet of volume. There are 
currently about 700,000 patients in the United States using ambulatory 
oxygen with an average yearly cost of about $1,960 per patient. The 
estimated annual cost of oxygen per patient with the present invention is 
about $540.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A flow chart of the preferred embodiment of the invention is set out in 
FIG. 1. Its main components include an oxygen concentrator 11, a 
cryocooler 12, a condenser 13, and a storage/collection dewar or vacuum 
insulated container 14. In the preferred embodiment, the oxygen 
concentrator 11 operates on a pressure swing adsorption cycle and 
essentially strips all or most of the nitrogen from air along with other 
minor components such as H.sub.2 O, CO.sub.2, CO, NO.sub.x, etc. The 
result is a stream of dry gas with high oxygen concentration (.about.92%) 
flowing in fluid outlet 50. A portion of the gas from this output in fluid 
outlet 50 is routed to a condenser 13 in association with a cryocooler (or 
cryogenic refrigerator) 12 through flow lines 51 and 57. The cryocooler 
provides cooling of the condenser heat exchanger 13 to liquefaction 
temperatures, causing the oxygen in contact therewith to go from the 
gaseous to the liquid phase. The condenser 13 typically must be insulated 
from ambient heating and may in practice even be located inside the dewar 
14. In order to lessen the load on the cryocooler 12, a recuperator 15 may 
be used to pre-cool the incoming stream utilizing the vent flow through 
line 52 out of the dewar as a cooling medium. In practice, this 
recuperator 15 may also be located within the dewar 14 to reduce ambient 
heating. 
Controller 16 may be equipped with a microprocessor, adequate memory, 
software and ancillary equipment comprising a computer which can be used 
to monitor and control the operation of the system. The controller 16 may 
be provided with signals from liquid level sensor 17, oxygen sensor 18, 
pressure transducer 9, and temperature sensor 10 via lines 53, 59, 55 and 
56, respectively. These signals are sensed and processed by the computer, 
with the controller operating valve 19, valve 25, heater 21, and 
cryocooler 12, in accordance with predetermined programs. 
The controller also provides output indicators for the patient. The liquid 
level in the dewar is continuously displayed and the patient is alerted 
when the oxygen concentration is low and when the system is ready for them 
to transfer liquid to a portable dewar. A modem or wireless link may be 
included to enable remote monitoring of the key parameters of the system 
by the home care provider as well as information which is useful for 
repair, maintenance, billing, and statistical studies of patients for the 
medical oxygenation market. Key system parameters of interest include the 
number of liquid transfers performed, the oxygen concentration history, 
number of run hours on the cryocooler, and time of the last boil-dry as 
well as number of boil dries performed. The controller may include a 
computer and/or a microprocessor located either integrally with the 
liquefaction system claimed herein or remotely therefrom but in 
communication therewith using either a modem and telephone lines or with a 
wireless interface. The computer and/or microprocessor may include memory 
having a database, or may be remotely connected to a memory or database 
using a network. An Optimal Liquefaction Schedule for optimal operation of 
the liquefaction system is set out in FIGS. 7-10 and may be stored in said 
controller using the memory and database. The controller can sense optimum 
parameters of the system and optimally control, including by activating 
servomechanisms, liquefaction and transfer of liquid oxygen. 
Dewar 14 is equipped with a dip tube 20 and heater 21. Heater 21 is used to 
build pressure in the dewar in order to expel liquid out the dip tube 20 
when so desired. A quick disconnect valve 22 or other flow control means 
is located on the end of the dip tube. This allows connection of a 
portable LOX dewar 23, which can then be carried by the patient requiring 
a mobile/ambulatory supply of oxygen. 
In another embodiment of this system shown in FIG. 2, the dewar 14 could be 
eliminated and replaced with a portable dewar 23 which is modified 
slightly from those existing today. The new portable dewar would interface 
with the condenser 13, recuperator 15, and controller 16. This embodiment 
requires a slightly different control scheme from that given for the 
preferred embodiment as the transfer and boil-dry modes are eliminated. 
Any small amount of accumulated water and hydrocarbons are eliminated from 
the portable dewar 23 after each use by allowing it to warm to room 
temperature before reuse. 
In operation, in the preferred embodiment of FIG. 1, where a pressure swing 
adsorption ("PSA") system is used, air is drawn into the oxygen 
concentrator 11, where it is compressed by an oilless compressor, cooled, 
and passed through molecular sieve beds operating on the pressure swing 
adsorption cycle, as shown in U.S. Pat. Nos. 5,366,541; 5,112,367; 
5,268,021 and Re. 35,009, which are incorporated herein by reference. This 
PSA system produces a 5-6 liters per minute (LPM) gas stream with high 
oxygen concentration at 3-7 pounds per square inch gauge (psig). The 
composition of this gas stream varies but is typically 90-95% oxygen, 5-6% 
argon, 0-4% nitrogen, &lt;15 parts per million (ppm) water vapor, and &lt;1 ppm 
hydrocarbons. Exhaust from the PSA cycle (80-84% nitrogen, 15-19% oxygen, 
0.6-0.8% argon, and trace amounts of water vapor, carbon dioxide and 
hydrocarbons) is vented into the atmosphere as waste gas. In the preferred 
embodiment, the high concentration oxygen stream in fluid outlet 50 is 
split with 0-4 lpm going through control valve 24, for patient 
consumption, and 0.5-1.5 lpm through line 51 and control valve 19 for 
liquefaction. Oxygen sensor 18, monitors the oxygen concentration produced 
by oxygen concentrator 11. If the oxygen concentration falls below 88%, 
controller 16 will close valve 19 and turn off the cryocooler 12. 
Even though 88% oxygen is adequate as supplemental oxygen therapy, if this 
was liquefied, as will be described below, the initial revaporized stream 
may have a reduced oxygen content because of the close boiling points of 
the components of the mixture. The temperature of the split gas stream 
entering the recuperator 15 is about room temperature. It is cooled to 
about 270 K (or colder) by the vent gas from the dewar flowing through the 
other side of the recuperator via line 52. The recuperator 15 reduces the 
load on the cryocooler by using the cold vent gas to pre-cool the 
oxygen-rich gas stream flowing into the condenser 13. From the recuperator 
15 the high oxygen concentration stream flows through a line 57 to the 
condenser 13, which is cooled to .about.90 K by the cryocooler 12. 
The condenser 13 provides cold surfaces to further cool and condense the 
flow. It is important to note that the gas passing through the condenser 
13 is a mixture of oxygen, argon, and nitrogen. The normal boiling points 
of these components are: 90.18 K, 87.28 K, and 77.36 K respectively. 
Because of the close boiling points of the components of this mixture, 
there was initial skepticism because of the concern that all the nitrogen 
and argon would condense along with the oxygen. If this concern was 
realized, when this liquid mixture was revaporized, the lower boiling 
point components; i.e., nitrogen and argon, would boil off first, 
resulting in flow with high concentrations of nitrogen, argon and a much 
lower oxygen concentration than that which was supplied to the 
condenser--which would make the process of oxygen treatment ineffective or 
a failure. 
This concern is explained in FIGS. 3 and 4 which are temperature 
composition diagrams for binary mixtures of oxygen-argon and 
oxygen-nitrogen. In these diagrams taken from K. D. Timmerhaus and T. M. 
Flynn, Cryogenic Process Engineering, Plenum Press, 1989, pp. 296-297, the 
upper curve at a given pressure defines the dew point and the lower curve 
defines the bubble point. Looking at FIG. 4 for a pressure of 0.10 1 MPa, 
if there is a gas mixture with 10 mole percent nitrogen (point 1), 
condensation will start when the gas has cooled to the dew point curve 
(point 2g) which is at a temperature of about 89.5 K in this case. Because 
oxygen has a higher boiling point than nitrogen, the initial liquid formed 
(point 2f) will have only 7.4 mole percent nitrogen. If the temperature is 
lowered to point 3, the liquid will have the composition of point 3f while 
the remaining vapor will have the composition of point 3g. As the 
temperature is lowered further to point 4f or below, all of the mixture 
liquefies and the composition is 10 mole percent nitrogen, the same as at 
point 1. If this liquid is heated, the nitrogen which has a lower boiling 
point will vaporize first. Thus, the composition of the first vapor formed 
will be that of point 4g or about 30 mole percent. As the remaining liquid 
boils, the mole percent of nitrogen in the vapor drops back to 10 mole 
percent when point 2g is reached. It is believed that the composition 
swings with a ternary mixture of oxygen, argon and nitrogen will be even 
more pronounced than those shown in FIGS. 3 and 4 for binary mixtures. 
Fortunately, this concern was avoided when the system was set so that only 
20 to 90% of the incoming flow to the condenser was actually condensed and 
when the condenser was controlled in accordance with the parameters as 
explained herein. This is believed to work because the excess flow 
operates to purge the vapor with higher impurity concentration from the 
system and avoid the aforementioned problem. Instead, the results realized 
were that a high concentration stream of oxygen could be liquefied and 
stored as set out in the portable ambulatory device claimed herein. 
FIG. 5 shows typical oxygen concentration test data for condensing and 
re-vaporizing part of the product outlet stream from the oxygen 
concentrator 11 in the preferred embodiment. For the first 120 minutes 
after the system was turned on, the system was cooling down without any 
net liquid accumulation in the dewar 14. From this point up to about 500 
minutes, condensation continued with liquid accumulation. During this time 
phase, the inlet stream to the condenser 13 had an oxygen concentration of 
95%, while the vent flow through line 52 had an oxygen concentration of 
only 92-93%. After 500 minutes the inlet stream and condenser cooling were 
stopped. The oxygen concentration of the re-vaporized liquid increased as 
the liquid boiled off due to the lower boiling point components (argon and 
nitrogen) boiling off first. This change in oxygen concentration presents 
no problem for medical ambulatory use because the oxygen concentration 
remains above 85%. 
Because of the aforementioned mixture problem, it is important and even 
critical not to let the amount of argon and nitrogen in the liquid become 
too high or when it is revaporized, the oxygen concentration will 
initially be much lower than that conventionally used in supplemental 
oxygen therapy (&gt;85%). This can be accomplished by selecting the proper 
condenser temperature, which is a function of pressure, and by not 
condensing all of the incoming flow. If only part of the incoming flow 
(20-90%) is liquefied, the remainder of the flow will purge the vapor with 
higher impurity concentration from the system. A condenser temperature of 
about 90 K (for .about.17 psia) minimizes the amount of argon and nitrogen 
liquefied without overly diminishing the yield of oxygen. Hence there will 
be both liquid and vapor leaving the condenser. The liquid will fall into 
the dewar 14 and collect. The vapor which has not condensed is vented to 
the atmosphere through line 52 and the recuperator 15. 
The amount of incoming flow liquefied is controlled by setting the mass 
flow rate relative to the cooling capacity of the cryocooler. The 
parameters of the condenser and/or cryocooler can be stored in the memory 
of the controller and/or computer and the controller regulating the 
incoming flow depending on the parameters stored and/or sensed. Having a 
mass flow rate which exceeds the cooling capacity of the 
cryocooler/condenser combination, prevents the incoming flow from being 
completely liquefied. The mass flow rate is controlled by the amount of 
flow restriction between inlet valve 19 and flow control valve 25. This 
includes the flow losses of the valves themselves as well as those in the 
recuperator, condenser, and all of the interconnecting plumbing. 
The pressure in the dewar 14 is maintained slightly above ambient pressure 
while the cryocooler is operating by valve 25. It is desirable to keep the 
pressure in the condenser as high as possible because this increases the 
condensation temperature (as shown in FIGS. 3 and 4) which eases the 
requirements on the cryocooler. Once again this can be controlled by the 
controller and/or the computer, microprocessor and memory system. 
This pressure regulating function of the solenoid on-off valve 25 is 
accomplished by the pressure transducer 9 and controller 16. Alternately, 
a back pressure regulating valve (such as a Tescom BB-3 series) or a 
suitable servomechanism may be used in lieu of the actively controlled 
solenoid. Liquid keeps accumulating in the dewar 14 until the liquid level 
sensor 17 signals the controller that the dewar is full or until the 
oxygen sensor 18 signals that the oxygen concentration of fluid exiting 
the oxygen concentrator 11 is too low. 
In the best mode, operating parameters for optimal operation of the system 
for the condenser should be that the condenser surface temperature should 
be in the range from 69.2-109.7 K and pressure should be in the range from 
5-65 psia. The gas concentrations into the condenser for medical use 
should have oxygen in the range of 80-100%, nitrogen from 0-20%, and argon 
from 0-7%. 
In order to transfer liquid from the dewar 14; e.g. to fill a portable LOX 
dewar 23, the pressure in the dewar 14 must be increased so that liquid 
can be forced up the dip tube 20. As shown in FIG. 1, heater 21 is used 
for this purpose. Heater 21 may be immersed in the liquid oxygen or 
attached to the outer surface of the inner vessel. The controller 16 
ensures that the cryocooler 12 is turned off and valve 25 is closed before 
the heater 21 is energized. The heater 21 remains turned on until the 
pressure, measured by pressure transducer 9, reaches about 22 psig. 
In order to eliminate accumulation of solid water and hydrocarbons which 
may be supplied in trace amounts from the oxygen concentrator, the dewar 
14 will be warmed to room temperature periodically (preferably after about 
30 fillings of a portable dewar, or every two months). This procedure is 
accomplished most economically when the inventory of liquid in the storage 
dewar is low; e.g. shortly after liquid transfer and a portable dewar has 
been filled. In this "boil-dry" mode, valve 19 will be closed, the 
cryocooler 12 is turned-off, valve 25 is open, and heater 21 is energized. 
The heater will boil-off the remaining liquid in the dewar and with it any 
trace amounts of water and hydrocarbons which are condensed and solidified 
in the liquid oxygen or on the cold surfaces. The heater 21 will remain 
turned on until the dewar temperature, measured by temperature sensor 10, 
has warmed to about 300 K. Any remaining water vapor will be flushed out 
by gaseous oxygen during the subsequent cool-down. 
At initial start-up or after a periodic boil-dry phase, the dewar, 
condenser, recuperator, and all associated hardware are at room 
temperature and must be cooled down. This is accomplished in the 
"start-up" mode, where valve 19 (see FIG. 1) is open, the heater is off, 
the cryocooler is on, and valve 25 is modulated to control the 
pressure/flow rate. It is desired to keep the pressure and hence the 
density of the gas as high as possible while maintaining the flow rate. 
The higher density gas will have better heat transfer with the dewar walls 
and associated hardware. It is noted that higher flow rates will enhance 
the convection heat transfer but may exceed the cooling capacity. Based on 
the cooling characteristics of the cryocooler between room temperature and 
90 K, the flow rate can be changed to minimize the cool-down time. 
The dewar 14 is equipped with at least one relief valve 26 as a safety 
feature. Another relief valve 29 is provided and in communication with the 
inlet gas stream 51, before flowing into the recuperator 15. This serves 
as a back-up for relief valve 26 as well as providing a means to eliminate 
accumulated water from the recuperator 15 during periods when the 
cryocooler 12 is off, if valve 25 is closed. A check valve 27 is also 
provided to prevent backflow into the oxygen concentrator in the event of 
a malfunction. 
FIG. 6 provides a block diagram of the controller 16 control system with 
sensor input value ranges and output states. It also shows interfaces to 
an indicator and a modem or wireless interface. The mode switch 28 may be 
used by the patient to request the system to prepare for a liquid transfer 
to a portable dewar. The indicator then provides a visual signal that the 
system is building pressure in the dewar. Once the pressure has reached 
the desired value, a visual and/or audio signal is given to alert the 
patient that the system is ready to transfer liquid. The controller may 
also be programmed to perform an unattended liquid transfer. The modem, 
telephone line or wireless interface connections are optional hardware 
that may be added to the controller to enable remote monitoring of the 
system by the home care provider (e.g., to assist with maintenance and 
repair) or insurance companies or health providers/administrators (e.g., 
to assess if patients are using enough ambulatory oxygen to justify 
payments, etc.). 
FIGS. 7A-D show a logic flow chart for the controller for the normal 
operation modes. This can also be referred to as the "input/output control 
schedule." The mode switch 28 can also be used by a repair or factory 
technician to put the controller in a calibration mode which serves as a 
method to check and reset the program. As shown in FIG. 6, the indicator 
provides liquid level readout, transfer request status, and low oxygen 
concentration information to the patient. All of the sensors are 
continuously scanned to provide the controller with the latest 
information. FIGS. 8 through 11 provide detailed output states as a 
function of input levels for the normal operating modes (start-up, 
condense, transfer, and boil-dry), which can be referred to as the 
"Optimal Liquefaction Operational Schedule." 
For example, FIG. 8 relates to the start-up mode; i.e., when the system is 
first turned on or after the boil-dry cycle. As shown in FIGS. 6 through 8 
and as depicted in FIG. 1, at start-up mode, the liquid sensor 17 shows 
zero liquid volume in the dewar and, when the oxygen sensor 18 shows an 
oxygen concentration greater than 88%, valve 19 is open, heater 21 is off, 
cryocooler 12 is on, and the indicator or the controller indicates a 
cool-down state. Valve 25 is modulated to control pressure. 
Once the system attains a cool enough temperature, steady state or normal 
operational condense mode is used. As shown in FIG. 9, the input to the 
controller 16 is such that when the oxygen sensor indicates oxygen 
concentration being greater than 88% and when the other criteria in the 
left-hand column of FIG. 9 are achieved, the output states set out in the 
right portion of the chart are attained. For example, when the level of 
the dewar is sensed as being full, the liquid level sensor indicates a 
level of approximately 100%, causing closure of valves 19 and 25, keeping 
the heater off, turning the cryocooler off, and having the indicator 
signal that the dewar is full. 
The transfer mode in FIG. 10 is the stage where one can fill the portable 
thermos bottles or dewars 23 from the main storage dewar 14. The top 
portion of FIG. 10, for example, shows controller readouts where, if the 
liquid sensor indicates a liquid level of less than 20%, then the 
conclusion is computed that there is not enough liquid to transfer into 
the portable dewar from the main dewar as shown. When the operator wants 
to increase the pressure in the storage dewar to force the liquid oxygen 
into the portable dewar, the heater is activated, and when the pressure 
sensor indicates that the pressure exceeds 22 psig, as shown on the last 
line in the left-hand column of FIG. 10, the heater 21 is then turned off 
and the controller readout or indicator shows that the transfer of liquid 
oxygen can be made to the portable dewar. Finally, FIG. 11 indicates the 
boil-dry mode, with valve 25 open to allow the vapor to escape, and the 
various parameters relating thereto. 
FIG. 12 shows a schematic of a pulse tube refrigerator, the preferred 
embodiment of the cryocooler 12 in FIG. 1. Because the cooling load on the 
condenser is small (7-15 W), the pulse tube refrigerator is preferred for 
use in the subject ambulatory oxygen system because of its good efficiency 
with only one moving part, the pressure oscillator. Pulse tube 
refrigerators are shown in U.S. Pat. Nos. 5,488,830; 5,412,952 and 
5,295,355 the disclosure of which are hereby incorporated by reference. 
FIG. 11 depicts a pulse tube refrigerator of the double inlet type. Other 
types of pulse tube refrigerators (PTR) could also be used such as the 
basic PTR or the inertance tube PTR (Zhu et al., 9.sup.th International 
Cryocooler Conference, NH, June 1996). 
The double inlet pulse tube refrigerator as shown in FIG. 12 is comprised 
of a pressure oscillator 30, primary heat rejector 31, regenerator 32, 
heat acceptor 33, pulse tube 34, orifice rejector 35, bypass orifice 36, 
primary orifice 37, and reservoir volume 38. The preferred refrigerant gas 
in the PTR closed and pressurized circuit is helium but various other 
gases such as neon or hydrogen could also be used. In operation, the PTR 
essentially pumps heat accepted at low temperature in the heat acceptor 33 
to the orifice heat rejector 35 where it is rejected at near ambient 
temperature. Although FIG. 12 depicts a "U-tube" configuration of the PTR, 
in-line and coaxial configurations are other possible options. Depicted 
therein is a piston type pressure oscillator, but other types are possible 
such as those utilizing diaphragms or bellows. 
FIG. 13 shows a schematic of another embodiment of the cryocooler. This is 
a vapor compression cycle cryocooler using a mixed gas refrigerant such as 
shown in U.S. Pat. No. 5,579,654; a 1969 German Patent by Fuderer & 
Andrija; British Patent No. 1,336,892. Other types of cryocoolers will 
work as long as they meet the important criteria of small size, 
convenience and low cost. In FIG. 13, the refrigerant is compressed by the 
compressor to high pressure. Then it is cooled by the aftercooler with 
heat Qh being rejected to the environment. Oil is separated in the oil 
separator. Oil flows back to the compressor inlet through a flow 
restriction. The refrigerant gas flows to a heat exchanger where it is 
cooled by the returning cold stream. Some components of the mixture may 
condense in this process. The liquid/gas refrigerant mixture flows through 
a throttle valve where its pressure is reduced and its temperature drops. 
This cold refrigerant enters the evaporator where the heat load Qc is 
absorbed and some liquid is boiled into vapor. This vapor flows up the 
cold side of the heat exchanger absorbing heat from the incoming stream. 
Then it flows back to the compressor. Heat Qc is accepted at cold 
temperature Tc. This is where the condenser would interface with the 
cryocooler. 
It is noted that with this type of cryocooler, it may be possible to remove 
some of the heat from the oxygen stream at a temperature warmer than Tc. 
One possible geometry of the generally vertically oriented, gravity 
assisted condenser 13 in FIG. 1 is shown in FIGS. 14 and 15. The incoming 
gas from the oxygen concentrator flows from conduit 57 to chamber 58 and 
then is distributed through an annular passage 59 between the outer tube 
41 and inner rod 42. The inner rod 42 is made of a high thermal 
conductivity material such as OFHC (Oxygen Free High Conductivity) copper, 
to minimize the temperature gradient between the surface on which the 
oxygen condenses (13) and the cryocooler 12. The cold end of the 
cryocooler is shown by cross-hatched member 61. Due to surface tension, 
the axial slots or grooves 43 will draw in liquid as it condenses. This 
will enhance heat transfer from the incoming gas by preventing a liquid 
film from forming over the entire condenser surface. Condensed liquid will 
drip off the bottom of the rod 42 while non-condensed gases flow out the 
end of the annulus 60. It is possible to liquefy all of the incoming flow 
to the condenser provided the cryocooler has sufficient cooling capacity 
and temperature capability. However, in order to minimize the amount of 
nitrogen and argon condensed, the preferred embodiment only condenses 
between 20-90% of the incoming flow. The incoming flow rate can be 
determined by the appropriate sizing of flow restrictions downstream of 
and by controlling valve 19. As mentioned previously, the mass flow rate 
is chosen to exceed the cooling capacity of the condenser/cryocooler so 
that only part of the incoming flow is liquefied. Also, the pressure in 
the condenser is maintained as high as possible while maintaining the 
desired flow rate. The higher pressure increases the condensation 
temperature which in turn reduces the requirements on the cryocooler. 
FIGS. 16 and 17 show another embodiment of the condenser that allows easier 
integration with the mixed gas refrigerant cryocooler. This configuration 
also allows access to the liquid in the dewar through the center of the 
condenser. The cold end of the cryocooler 45 is in thermal contact with an 
outer tube 46 and an inner tube 47, both of which are made of a high 
thermal conductivity material such as OFHC copper and which utilize 
flanges 62 and 64 to interface with the cryocooler. The inner tube has 
axial slots or grooves 48 cut into its outer surface (see, FIG. 17) to 
increase the surface area and to wick condensed liquid, preventing a 
liquid film from forming over its entire surface. Gas enters the condenser 
through port 63. The liquid and vapor flow down through an annular passage 
49. An isometric view of this embodiment is shown in FIG. 18. 
Thus, an improved home/ambulatory liquid oxygen system is disclosed. While 
the embodiments and applications of this invention have been shown and 
described, and while the best mode contemplated at the present time by the 
inventors has been described, it should be apparent to those skilled in 
the art that many more modifications are possible, including with regard 
to scaled-up industrial applications, without departing from the inventive 
concepts therein. Both product and process claims have been included and 
in the process claims it is understood that the sequence of some of the 
claims can vary and still be within the scope of this invention. The 
invention therefore can be expanded, and is not to be restricted except as 
defined in the appended claims and reasonable equivalence departing 
therefrom.