Abstract:
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. The present invention involves 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. The system includes an oxygen concentrator which separates oxygen gas from the ambient air, a condenser in communication with the oxygen concentrator for receiving and liquefying the oxygen gas flow, a cryocooler associated with the condenser, and a first storage dewar in fluid communication with the condenser and adapted to store the oxygen liquefied by the condenser, the first storage dewar including means for transferring liquid oxygen from the first dewar to a second dewar for storing a quantity of oxygen suitable for moveable oxygen treatment.

Description:
PRIOR APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 08/876,970, filed Jun. 16, 1997 now U.S. Pat. No. 5,979,440. 
    
    
     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&#39;s home by a provider and a substantial investment in small cylinders for the provider because so many are left at the patient&#39;s home and refilling facility. Although it is technically possible to refill these cylinders in the patient&#39;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 (˜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° 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&#39;s home. Thus, utilizing and storing a high pressure gas system in the patient&#39;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., ⅓ 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&#39;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 ˜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 
     An aspect of the present invention involves 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. The system includes an oxygen concentrator which separates oxygen gas from the ambient air, a condenser in communication with the oxygen concentrator for receiving and liquefying the oxygen gas flow, a cryocooler associated with the condenser, and a first storage dewar in fluid communication with the condenser and adapted to store the oxygen liquefied by the condenser, the first storage dewar including means for transferring liquid oxygen from the first dewar to a second dewar for storing a quantity of oxygen suitable for moveable oxygen treatment. 
     In an embodiment of the above aspect of the invention, the liquid oxygen transferring means is adapted to increase the pressure in the first dewar. 
     In a further embodiment of the above aspect of the invention, the liquid transferring means includes a heater immersed within the liquid oxygen in the first dewar. 
     In a still further embodiment of the above aspect of the invention, the first dewar includes an inner vessel in which the liquid oxygen reside, and liquid transferring means includes a heater attached to the outer surface of inner vessel. 
     In another embodiment of the above aspect of the invention, the condenser is in communication with the concentrator through a line, and the liquid transferring means includes a compressor located in the line between the condenser and the concentrator. 
     In an additional embodiment of the above aspect of the invention, the liquid transferring means includes a high-pressure compressor in communication with the concentrator for delivering high-pressure air thereto. 
     In another embodiment of the above aspect of the invention, the liquid transferring means includes a vaporizer loop associated with the first dewar. 
     In a further embodiment of the above aspect of the invention, the liquid transferring means includes a controllable heat leak associated with the first dewar. 
     In a still further embodiment of the above aspect of the invention, the liquid transferring means includes a gravity-assisted dispensing mechanism. 
     In an additional embodiment of the above aspect of the invention, the system further includes the second storage dewar and the second storage dewar is adapted to be filled at a pressure below 20 psig. 
     An additional aspect of the invention involves 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. The system includes an oxygen concentrator which separates oxygen gas from the ambient air, a condenser in communication with the oxygen concentrator for receiving and liquefying the oxygen gas flow, a cryocooler associated with the condenser, and a portable dewar adapted to interface with the condenser and adapted to store the oxygen liquefied by the condenser. 
     Another aspect of the present invention involves a method for generating liquid oxygen in a home from a home liquid oxygen ambulatory system having an oxygen concentrator, a condenser, and cryocooler, a storage dewar and means for transferring liquid oxygen from the first dewar to a second dewar. The method includes generating a gaseous supply of oxygen using the oxygen concentrator; splitting off at least a portion of the gaseous supply to be liquefied; cooling the supply of oxygen using the condenser and cryocooler to transform the gaseous oxygen to liquid oxygen; storing the liquid oxygen in the storage dewar; and transferring the liquid oxygen in the storage dewar with the liquid oxygen transferring means to a second dewar for storing a quantity of liquid oxygen from which smaller quantities can be transferred for moveable oxygen treatment. 
     In an embodiment of the above aspect of the invention, transferring the liquid oxygen includes increasing the pressure in the first dewar. 
     In an additional embodiment of the above aspect of the invention, the liquid transferring means includes a heater immersed within the liquid oxygen in the first dewar and transferring the liquid oxygen includes heating the liquid oxygen in the first dewar so that the pressure is increased in the first dewar. 
     In another embodiment of the above aspect of the invention, the first dewar includes an inner vessel in which the liquid oxygen reside, the liquid transferring means includes a heater attached to the outer surface of inner vessel, and transferring the liquid oxygen includes heating the liquid oxygen in the first dewar so that the pressure is increased in the first dewar. 
     In a further embodiment of the above aspect of the invention, the condenser is in communication with the concentrator through a line, and the liquid transferring means includes a compressor located in the line between the condenser and the concentrator, and transferring the liquid oxygen includes increasing the pressure of gaseous oxygen entering the condenser and the dewar with the compressor. 
     In a still further embodiment of the above aspect of the invention, the liquid transferring means includes a high-pressure compressor in communication with the concentrator for delivering high-pressure air thereto, and transferring the liquid oxygen includes increasing the pressure of gaseous oxygen entering the condenser and the dewar with the compressor. 
     In an additional embodiment of the above aspect of the invention, the liquid transferring means includes a vaporizer loop associated with the first dewar, and transferring the liquid oxygen includes heating the liquid oxygen in the first dewar with the vaporizer loop so that the pressure is increased in the first dewar. 
     In another embodiment of the above aspect of the invention, the liquid transferring means includes a controllable heat leak associated with the first dewar, and transferring the liquid oxygen includes heating the liquid oxygen in the first dewar so that the pressure is increased in the first dewar. 
     In a further embodiment of the above aspect of the invention, the liquid transferring means includes a gravity-assisted dispensing mechanism. 
     In a still further embodiment of the above aspect of the invention, the system further includes the second storage dewar, the second storage dewar is adapted to filled at a pressure below 20 psig. 
     Another aspect of the present invention involves a liquefier for a home liquid oxygen ambulatory system that is resistant to plugging. The home liquid oxygen ambulatory system includes an oxygen concentrator for delivering gaseous flow to the liquefier and a storage dewar having an inner vessel for storing liquid oxygen produced by the liquefier. The liquefier includes a condenser, a refrigerating device associated with the condenser, means for communicating incoming gaseous flow from the oxygen concentrator to the condenser, the communicating means having an inner surface with a dimension D, means for venting gaseous flow not condensed from the inner vessel, the venting means having an outer surface with an dimension d, and whereby the dimension D of the inner surface of the communicating means is significantly larger than the dimension d of the outer surface of the venting means to allow for the build-up of solid contaminants on the outer surface of the venting means without plugging up the communicating means. 
     In an embodiment of the above aspect of the invention, the venting means includes a recuperator comprised of a helical coil of tubing, the tubing having the outer surface with a diameter of the dimension d, whereby the incoming gas stream flows over the outer surface of the helical coil of tubing and a vent stream flows inside the helical coil of tubing. 
     In another embodiment of the above aspect of the invention, the outer surface of the helical coil of tubing has a cold surface to freeze out trace impurities of solid contaminants such as H2O, CO2 and hydrocarbons. 
     In a further embodiment of the above aspect of the invention, the communicating means is comprised of a neck tube having the inner surface with a diameter of the dimension D. 
     In a still further embodiment of the above aspect of the invention, the liquefier further includes a liquid withdrawal tube located central to the refrigerating device, recuperator and condenser for removing liquid oxygen from the storage dewar. 
     In an additional embodiment of the above aspect of the invention, the refrigerating device is integral with the condenser. 
     In another embodiment of the above aspect of the invention, the refrigerating device, condenser and recuperator are integral with the storage dewar. 
     Another aspect of the invention involves a method for generating liquid oxygen in a home from a home liquid oxygen ambulatory system having an oxygen concentrator, a condenser, a cryocooler, a recuperator and a storage dewar. The method includes generating a gaseous supply of oxygen, which includes some trace impurities, using the oxygen concentrator; splitting off at least a portion of the gaseous supply to be liquefied; cooling the supply of oxygen using the condenser and cryocooler to transform the gaseous oxygen to liquid oxygen; condensing less than all of the gaseous oxygen supply flowing into the condenser; freezing out the trace impurities of the gaseous supply of oxygen and venting the non-condensed nitrogen, argon and oxygen with the recuperator; storing the liquid oxygen in the storage dewar; and periodically removing accumulated frozen impurities on the recuperator by boiling-off any stored liquid oxygen and then flow purging the system until the system has reached room temperature. 
     Another aspect of the invention involves a generally vertically oriented, gravity assisted condenser for use with a refrigerating device to liquefy gaseous oxygen in a home liquid oxygen ambulatory system. The condenser includes a generally vertically oriented tubular member adapted to conduct heat axially to the refrigerating device, the tubular member having outer and inner surfaces, at least one of the outer and inner surfaces having a plurality of generally vertically oriented flutes and convex fins adapted to increase the condensation rate per unit area by thinning the liquid film and drain the condensate to keep the condensate from flooding the condensation surfaces. 
     In an embodiment of the above aspect of the invention, the fins have a hyperbolic cosine profile. 
     In an additional embodiment of the above aspect of the invention, the flutes have a profile selected from the group consisting of concave, generally V-shaped, generally rectilinear. 
     In another embodiment of the above aspect of the invention, the plurality of generally vertically oriented flutes and convex fins are located on both the outer and inner surfaces. 
     Another aspect of the invention involves a generally vertically oriented, gravity assisted condenser for use with a refrigerating device to liquefy gaseous oxygen in a home liquid oxygen ambulatory system. The condenser includes a generally vertically oriented tubular member adapted to conduct heat axially to the refrigerating device, the tubular member having outer and inner surfaces, at least one of the outer and inner surfaces includes means for enhancing the condensation rate per unit area by maintaining a small liquid film thickness on the condensation surfaces. 
     In an embodiment of the above aspect of the invention, the condensation enhancing means includes a plurality of generally vertically oriented flutes and convex fins. 
     In an additional embodiment of the above aspect of the invention, the fins have a hyperbolic cosine profile. 
     In a further embodiment of the above aspect of the invention, the flutes have a profile selected from the group consisting of concave, generally V-shaped, generally rectilinear. 
     In a still further embodiment of the above aspect of the invention, the plurality of generally vertically oriented flutes and convex fins are located on both the outer and inner surfaces. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the invention in the medical oxygen preferred embodiment where gaseous oxygen is split off and liquefied for storage in a stationary dewar or container. 
     FIG. 2 shows another preferred embodiment of the invention for ambulatory supplemental oxygen using a portable LOX dewar. 
     FIG. 3 shows a typical temperature-composition diagram for oxygen-argon mixtures at typical dewar pressures. 
     FIG. 4 shows a typical temperature-composition diagram for nitrogen-oxygen mixtures at typical dewar pressures. 
     FIG. 5 shows typical oxygen concentration test data for gas streams into the condenser and out of the dewar during liquefaction, and out of the dewar as the liquid is re-vaporized. (Oxygen source is a PSA oxygen concentrator.) 
     FIG. 6 shows the controller block diagram for operation of the system. 
     FIGS. 7A through 7D are flow charts showing the controller logic. 
     FIG. 8 shows the controller input levels and output states for the start-up mode of the preferred embodiment. 
     FIG. 9 shows the controller input levels and output states for the condense mode of the preferred embodiment. 
     FIG. 10 shows the controller input levels and output states for the transfer mode of the preferred embodiment. 
     FIG. 11 shows the controller input levels and output state for the boil dry mode of the preferred embodiment. 
     FIG. 12 illustrates basic components of a pulse tube refrigerator. 
     FIG. 13 illustrates another embodiment of a cryocooler which may be used in the subject invention. 
     FIG. 14 is a side cross-sectional view of the preferred embodiment of the condenser. 
     FIG. 15 is an end cross-sectional view of the preferred embodiment of the condenser corresponding to FIG.  14 . 
     FIG. 16 shows a side cross-sectional view of another preferred embodiment of the condenser. 
     FIG. 17 shows an end cross-sectional view of another preferred embodiment of the condenser corresponding to FIG.  16 . 
     FIG. 18 is an isometric view of the embodiment of the condenser shown in FIGS. 16 and 17. 
     FIG. 19 is a cross-sectional view of an embodiment of a dewar having a liquefier with an integral vapor compression cycle coldhead, condenser, and recuperator, which is resistant to plugging and performance degradation caused by trace impurities in the gas stream such as water vapor, carbon dioxide and hydrocarbons. 
     FIG. 20 is a cross-sectional view of an alternative embodiment of a dewar having a liquefier with an integral pulse tube or Sterling cycle coldhead, condenser, and recuperator, which is resistant to plugging and performance degradation caused by trace impurities in the gas stream such as water vapor, carbon dioxide and hydrocarbons. 
     FIG. 21 is an end cross-sectional view, similar to FIG. 17, of an embodiment of a condenser having shaped fins to reduce the liquid film thickness. 
     FIG. 22 is a block diagram of the invention in the medical oxygen preferred embodiment where gaseous oxygen is split off and liquefied for storage in a stationary dewar, similar to FIG. 1, and shows numerous means for transferring liquid oxygen from the storage dewar. 
    
    
     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 2 O, CO 2 , CO, NO x , etc. The result is a stream of dry gas with high oxygen concentration (˜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 the 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 ˜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 0.101 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  2   g ) 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  2   f ) will have only 7.4 mole percent nitrogen. If the temperature is lowered to point  3 , the liquid will have the composition of point  3   f  while the remaining vapor will have the composition of point  3   g . As the temperature is lowered further to point  4   f  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  4   g  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  2   g  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 99% 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 (20-99%) 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 ˜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. 
     With reference to FIG. 22, other alternative means for transferring or dispensing liquid from the dewar  14  will now be described. For simplicity, the alternative liquid transferring means are described with respect to this one Figure. It will be readily apparent to those skilled in the art that any, any combination, or all of the disclosed liquid transferring means discussed below could be used in a single home liquid oxygen ambulatory system. 
     An alternative means for transferring liquid by raising the pressure in the dewar  14  includes adding a compressor  300  between the oxygen concentrator  11  and the condenser  13 . The compressor  300  is preferably added in line  51 , either before or after valve  19 . The compressor  300  increases the pressure in the storage dewar  14  so that when the portable dewar  23  is engaged, liquid is forced up the dip tube  20  and into the portable dewar  23 . An additional benefit of adding a compressor  300  at this location is that it increases the pressure during liquefication in the dewar  14 , which increases the saturation temperature. An increased saturation temperature eases the cooling requirements on the cryocooler  12 . 
     A further means for transferring liquid by raising the pressure in the dewar  14  includes using a high-pressure compressor  302  within the oxygen concentrator  11  instead of the typical low-pressure compressor. The high-pressure compressor  302  has the effect of increasing the pressure in the storage dewar  14  so that when the portable dewar  23  is engaged, liquid is forced up the dip tube  20 . In addition to easing the cooling requirements on the cryocooler  12 , a compressor  302  at this location slightly enhances the PSA cycle. 
     A still further means for transferring liquid by raising the pressure in the dewar  14  includes using a vaporizer loop  304 . In this embodiment, the dewar  14  preferably remains at low pressure while liquid is being produced. When transfer of liquid out of the dewar  14  is desired, a valve  306  is opened to allow some liquid to flow into a coil  308  to be vaporized. This would increase the pressure in the dewar  14  so that liquid could be transferred to the portable dewar  23 . 
     Another means for transferring liquid by raising the pressure in the dewar  14  includes a controllable heat leak such as a conductive strap  310  between ambient and the inner vessel of the dewar  14 . When transfer of liquid out of the dewar  14  is desired, the heat leak is controlled so that heat from the ambient is transferred to the liquid, causing it to vaporize. This would increase the pressure in the dewar  14  so that liquid could be transferred to the portable dewar  23 . 
     Another means for transferring liquid by raising the pressure in the dewar  14  includes a controllable pump  312  that is actuated when transfer of liquid out of the dewar  14  is desired. 
     An additional means for transferring liquid without raising the pressure in the dewar  14  includes incorporating a gravity-assisted dispensing mechanism  314  such as a controllable spigot (analogous to those used to dispense liquids from a large insulated cooler) near the bottom of the dewar  14 . Unlike the alternative means for transferring liquid from the storage dewar  14  described above, which expel liquid out of the dip tube  20 , the gravity-assisted dispensing mechanism eliminates the need for the dip tube  20 . The gravity-assisted dispensing mechanism  314  preferably includes a quick disconnect valve  316  or other flow control means, similar to disconnect valve  22  described above, located on the end of the mechanism  314  to allow for connection of a portable dewar  23 . 
     An additional means for transferring liquid without raising the pressure in the dewar  14  includes incorporating a portable dewar  23  adapted to be filled from a pressure less than 20 psig, which is the standard for currently available home stationary liquid dewars. For example, the portable dewar  23  may be adapted to be filled from a pressure such as 5 psig. 
     A further means for transferring liquid without raising the pressure in the dewar  14  involves replacing the storage dewar  14  with a specially designed portable dewar  23  such as that described above with respect to FIG. 2, which interfaces with the rest of the home liquid oxygen ambulatory system. 
     In order to eliminate accumulation of solid water and hydrocarbons which may be supplied in trace amounts from the oxygen concentrator, the dewar  14 , recuperator  15 , and condenser  13  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 until all the liquid has boiled-off as evidenced by, for example, the temperature sensor  10  being above 125 K. The heater will boil-off the remaining liquid in the dewar  14  and with it any trace amounts of water and hydrocarbons which are condensed and solidified in the liquid oxygen or on the cold surfaces. Once valve  19  is re-opened, the flow of concentrated oxygen gas purges and removes most of the water vapor and hydrocarbons from the liquefier. 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. 
     The dewpoint/frostpoint of the gas stream provided by the oxygen concentrator is below −55° C. Although the mass of water flowing into the liquefier is quite small, the ice/frost formed at such a cold temperature has a very low density and hence, can take a appreciable volume of space that can lead to plugging of the liquefier. Therefore, the design of the recuperator  15  and/or condenser  13  must be able to allow for accumulation of frost without plugging. 
     With reference to FIG. 19, an embodiment of a liquefier  70 , which is resistant to plugging, will now be described. Elements similar to those previously described are referred to with the reference numerals and are not described in further detail. The liquefier  70  includes an integral coldhead  72 , condenser  74  and a recuperator  76  located generally within a dewar  14 . Incoming gaseous flow enters a neck tube  78  via an inlet  80  as shown by the arrows. The gaseous flow includes mostly oxygen gas along with a small amount of water vapor and other trace impurities. The gaseous flow is pre-cooled as it passes the helical recuperator  76 . Water vapor in the gaseous flow freezes out on surfaces at temperature below its frostpoint such as outer surface  82  of the recuperator  76  and inner surface  84  of the neck tube  78 . The neck tube  78  has an inner diameter or dimension D and the helical recuperator  76  is constructed of a tube having an outer diameter or dimension d. By making inner diameter D of the neck tube  78  significantly larger than the outer diameter d of the helical recuperator tube  76  at locations along the neck tube  78  where incoming gaseous flow is present, the liquefier  70  allows for accumulation of frost on the inner surface  84  of the neck tube  78  or outer surface  82  of the recuperator  76  without plugging. This extends the period of time required between boil-dries without causing plugging in the liquefier  70 . 
     Oxygen from the gaseous flow condenses into liquid oxygen at the condenser  74 . The condenser  74  is shown in conjunction with a vapor compression cycle cryocooler  86  (evaporator  88 , tube-in-tube heat exchanger  90 , compressor  92 ) as its associated refrigerating mechanism. It will be readily understood by those skilled in the art that other refrigerating mechanisms may be used in conjunction with the condenser such as, but not by way of limitation, pulse tube, Stirling, etc. For example, with reference to FIG. 20, in an alternative embodiment of the invention, a condenser  94  may be used in conjunction with a coldhead  96  from a pulse tube cryocooler  97  to liquefy the incoming flow. The cryocooler  97  includes copper tubes  98 ,  100  that provide strain relief via respective coils  102 ,  104 . Copper tube  98  connects the coldhead  96  to a compressor  106  and copper tube  100  connects the coldhead  96  to a reservoir volume  108 . A flexible copper braid  110  provides a heat conduction path if needed to reject heat from the coldhead  96 . 
     Excess gaseous flow not condensed becomes vent gas that is removed from the liquefier  70  via the recuperator  76 . Vent gas enters the recuperator  76  through inlet  112 , as shown by the arrows, flows through the helical recuperator  76  (providing the aforementioned pre-cooling) and preferably exits to the atmosphere through outlet  114 . 
     The dewar  14  may include a central liquid withdrawal tube  116  for withdrawing liquid oxygen from the dewar  14 . The central liquid withdrawal tube  116  may include an integral liquid level sensor  118  for monitoring the level of the liquid oxygen in the dewar  14 . A heater  21  may be attached to the outer surface of the inner vessel of the dewar  14  to assist in transferring liquid oxygen from the dewar  14 . 
     A getter cup  120  such as those used in commercial cryogenic dewars may be attached to the inner vessel of the dewar  14  to maintain a high vacuum in the dewar  14 . 
     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 (7-15W), 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 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 rejecter  31 , regenerator  32 , heat acceptor  33 , pulse tube  34 , orifice rejecter  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 rejecter  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 &amp; 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 20-99% 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 . 
     FIG. 21 is a cross-sectional view, similar to the views in FIGS. 15 and 17, of another embodiment of a generally vertically oriented gravity assisted condenser  110 . The condenser  110  includes fins  112  and corresponding flutes  114  that extend vertically the longitudinal length of the condenser  110  on both an exterior side  116  and an interior side  118  of the condenser  110 , generally parallel with a vertical longitudinal axis L of the condenser  110 . The vertical longitudinally elongated fins  112  and flutes  114  enhance condensation by using surface tension forces to maintain a small liquid film thickness on the condensation surface. A small liquid film thickness, δ, allows for better condensation performance because the heat transfer coefficient (h) is equal to k/δ(h=k/δ). The fins  112  preferably have a convex surface with a hyperbolic cosine profile to provide a uniform liquid film thickness and maximize condensation; however, other simpler convex curved surfaces that are more easily manufactured can be used with only slightly less performance. A convex surface for the fins  112  produces a small liquid film thickness and a concave surface for the flute  114  supports or receives a larger liquid film thickness. Thus, the convex surfaces of the fins  112  can be drained by concave surfaces of the flutes  114 . Thus, the flutes  114  serve as drainage channels to keep the condensate from flooding the condensation surfaces. By making a condenser  110  with both vertical longitudinally elongated convex and concave portions, the overall condensation rates can be increased many fold even though the surface area is only increased a little. 
     In an alternative embodiment of the invention, the flutes  114  may have a profile that is other than convex such as, but not by way of limitation, generally rectilinear or generally V-shaped. 
     In a further embodiment of the invention, the fins  112  and flutes  114  may exist on only the exterior side  116  or interior side  118  of the condenser  110 . Alternatively, the condenser  110  may have fins  112  and flutes  114  on the interior and/or the exterior and the condenser  110  is used in conjunction with another condensing device such as another condenser located within the interior  118  and/or around the exterior  116  of the condenser  110 . 
     Prior art (U.S. Pat. Nos. 4,253,519, 4,216,819) has been limited to horizontal externally fluted tubes with purely radial conduction through the tube wall. In contrast, the condenser  110  of the present invention may include fins  112  and flutes  114  on both sides  116 ,  118 . Also, heat is conducted axially in the condenser  110  of the present invention. 
     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.