GB2394528A

GB2394528A - Cryogenic pressure building device

Info

Publication number: GB2394528A
Application number: GB0224541A
Authority: GB
Inventors: Mark John Robbie; Andrew Aitken
Original assignee: Gore Enterprise Holdings Inc
Current assignee: Gore Enterprise Holdings Inc
Priority date: 2002-10-22
Filing date: 2002-10-22
Publication date: 2004-04-28
Also published as: GB0224541D0; AU2003278319A1; WO2004038280A1

Abstract

Disclosed are a number of pressure building devices which all feature an inner chamber 9 or tube 9 carrying cryogenic fluid and leaching gaseous cryogen into a second outer chamber 15 which is impermeable to the gaseous cryogen. A first device additionally features second chamber 15 gas removal means. A second device additionally features second chamber 15 gas feedback 22 to a cryogen reservoir 1. A third device additionally features second chamber 15 gas sensor means. A fourth device additionally features second chamber 15 gas driving a pressure or flow activated device. The most significant device appears to be the second one with the gas feedback 22. This may be used to build pressure in a cryogenic fluid, such a liquid nitrogen, in a reservoir 1. A natural balance between leaching of the gas out of the tube 9 and feedback pressure to the reservoir 1 may be established so that over-pressure in the reservoir 1 is avoided. The device may permit the portable dispensing of cryogenic fluids, without the need for pumps or compressors and their related systems.

Description

CRYOGENIC PRESSURE-BUILDING DEVICES AND SYSTEMS

BACKGROUND OF THE INVENTION

5 FIELD OF THE INVENTION

The present invention relates to pressure-building devices and systems utilising the flow and evaporation of cryogenic liquids without the use of mechanical pumps or compressors.

DESCRIPTION OF RELATED ART

10 Cryogenic fluids -i.e. fluids which are liquid only below -150 C at atmospheric pressure-have applications in a wide variety of industrial processes and therefore there arises the need for a reliable source of cryogenic fluids which is compact, portable and able to supply liquids or gases on demand.

Cryogenic liquids are usually stored at their boiling points in well insulated 15 reservoirs such as Dewars and, therefore, to effect reliable, on-demand, delivery of the liquid from such reservoirs it is necessary to pressurise the contents of the reservoirs by some form of pressurebuilding system. It is preferable that such pressure-building systems do not comprise mechanical pumps or compressors as mechanical pumps and compressors are cumbersome and detract from the 20 portability of the liquid or gas source. Also systems utilising pumps or compressors require inclusion of pressure sensing devices and control circuits to prevent over-pressurising the systems. Such control circuits add further undesirable complexity to the system.

Various approaches have been proposed to address this need for portable, 25 compact and reliable pressure-building systems.

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In U.S. Patent No. 4,838,034 to Leonard et. al. a portable cryogenic system for powering portable gas driven tools is described, in which cryogen liquid from the insulated container is allowed to pass through external warming coils and subsequently fed back through coils immersed in the liquid in the 5 container to warm the cryogenic liquid therein and cause evaporation of the liquid thereby increasing the pressure within the container.

In U.S. Patent No. 4,947,651 to Neeser et. al. a pressure-building circuit for a double walled cryogenic tank is disclosed in which warming coils are interposed between the walls of the tank. The gas generated is fed to the head 10 space of the tank through a restricting orifice which maintains a pressure gradient between the head-space and the warming coils.

In U.S. Patent No. 5,373,701 to Siefering et. al. a cryogenic station is disclosed in which the pressure-building circuit comprises an evaporation coil which is fed from the bottom of a cryogen storage tank, the evaporated cryogen 15 being fed into the head-space of the storage tank.

U.S. Patent No 5,924,291 to Weller et. al. teaches a high pressure cryogenic fluid system in which the pressure-building system comprises an evaporating coil which feeds the evaporated cryogen to a separate high pressure holding tank which is used to pressurise a stream of cryogen through a high 20 pressure vaporiser to give a supply of high pressure gas.

U.S. Patent 5,937,655 to Weller et. al. discloses a pressure-building device consisting of a tubular enclosure incorporating an electric heater; the enclosure being fitted into a cryogen-containing tank. The electric heater when energised causes a flow of cryogenic liquid and gas to an external heat exchanger where 25 the liquid portion of the mixture is vaporised, the resulting gas being returned to the head-space of the cryogen-containing tank.

see e e e e e e eee e e e e ee e e À e À e I e e e e e e e e e eee À e e One current method used to pressurize the containers in order to speed the delivery of cryogenic fluids involves placing evaporative coils inside the Dewars.

The coils warm the fluid, and as it boils, the pressure in the ullage increases which drives the liquid out of the Dewar. This process inefficiently utilizes liquid 5 cryogen. Alternative methods involve directly pressurizing the ullage via a pump or a compressed gas cylinder but in order to avoid contaminating the liquid cryogen the pressurizing gas must be the same as the liquid, which can present difficulty when using pumps. The use of compressed gas is problematic due to the large 10 quantity of gas that is consumed.

It is an object of embodiments of the present invention to provide a pressure-building system which is more efficient, more compact, and simpler to use than systems of the prior art.

It is a further object of certain embodiments of the invention to provide a 15 system in which the pressure within the system is self-limiting; that is, it requires no instrumentation or feedback circuits to control the pressure within the system.

SUMMARY OF THE INVENTION

20 The present invention is directed to pressure-building devices and systems particularly for use with cryogenic liquids.

The invention features an inner chamber and an outer, pressure-building chamber enclosing the inner chamber. The inner chamber may be connected to a source of liquid cryogen, and comprises a wall structure which restricts the 25 passage of cryogenic fluid in the liquid phase while permitting the passage of cryogenic fluid in the gaseous phase. Liquid cryogen fed into the inner chamber

À À e À À À e À a Àe evaporates (inside the inner chamber and the inner chamber wall) and the resultant gas permeates the wall of the inner chamber and is contained in the space between the inner and outer chambers. It has been found, surprisingly, that for a given ambient temperature, the gas pressure within the outer 5 (pressure-building) chamber stabilises at a maximum value and that this value is considerably higher than the pressure within the liquid cryogen feed line. The particular maximum value of the gas pressure may be at least partially dependent on the construction of the wall(s) of the inner chamber. Additionally, the effectiveness of the present invention can be enhanced by warming the outer 10 chamber by any suitable means, such as by the use of a fan, addition of fins to the outer chamber, or immersion of the device in a water bath.

In one embodiment of the invention the inner chamber is in the form of a tube comprising a wall having a structure that restricts the passage of liquid cryogen while permitting the flow of gaseous cryogen. The tube is connectable 15 to a source of liquid cryogen and is enclosed in an outer chamber, the walls of which are substantially impermeable to gaseous cryogen. Entrance and exit lines to and from the tube allow axial flow of liquid cryogen from the source, such as a Dewar, through the tube. Subsequent evaporation of the liquid cryogen through the wall of the tube results in pressure build-up within the chamber 20 surrounding the tube. For reasons of simplicity in application and compatibility with readily available pipe connectors and control devices, the tube is preferably circular in cross-section. Tubes having other cross-sections such as square, rectangular, or polygonal may also be used.

In a second embodiment of the invention the inner chamber is a closed 25 chamber the wall(s) of which allow the passage of cryogen in the gaseous phase but restrict the passage of cryogen in the liquid phase. The inner chamber is

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À a À. À supplied with liquid cryogen and is enclosed in an outer pressure-building chamber capable of containing gas at the pressure generated by the system. The inner chamber is defined as a "closed" chamber in that whereas the chamber has an entrance port to allow liquid to enter the chamber no exit port or other means 5 of exit for liquid is provided. Fluid can only escape from the chamber by evaporation through the walls of the chamber or by vapour in the chamber flowing back into the supply Dewar. In this embodiment the inner chamber may be a tube of any cross-section as in the first embodiment or, alternatively, the inner chamber may be spherical or spheroidal in shape.

10 In another embodiment of the invention the outer pressure-building chamber is connected to a gas driven device such that a flow of gas from the chamber may be used to drive the device. Suitable devices may include a motor, for example, as in a mechanical tool or meters, sensors, and the like.

In a further embodiment the outer pressure-building chamber is connected 15 to a pressure-respondent device such that the pressure in the outer chamber actuates the device. Suitable devices may include, for example, a pneumatic or electro-pneumatic switch or the like. Such devices may further be used to control other devices.

In yet a further embodiment the outer pressure-building chamber is 20 connected to the pressurizing port in the head-space of a Dewar such that the gas in the outer chamber pressurizes the contents of the Dewar to aid in the delivery of the contents such as, for example, cryogenic liquids. In this embodiment the pressure building system utilizes some of the energy associated with the evaporation of controlled small amounts of liquid cryogen to pressurize 25 the Dewar. Furthermore, additional advantages of efficient energy storage and delivery can achieved with the present invention when the device is connected

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as part of the Dewar delivery line. In this case, the Dewar will be automatically pressurised on demand when the delivery valve is opened thus allowing the cryogen to be stored at low pressure until it is required.

Storage of liquid cryogen in a low pressure reservoir for long periods of 5 time has advantages as it will cause the temperature of the cryogen to reach a saturation temperature corresponding to the low storage pressure. If the reservoir is then pressurized the cryogen as delivered will be below its saturation temperature for a certain period of time. That is, it is in effect, subcooled and subcooled cryogen has advantages in that it allows delivery of cooling in a faster 10 and more efficient way.

Subcooling and pressurizing cryogenic liquid to increase the boiling point are known in the art. Articles of the present invention, however, enable the realization of the benefits of subcooling in an entirely new and surprisingly easy manner. Utilizing pressure building systems as described in this specification can

15 result in dramatic cost savings associated with the decreased waste of liquid cryogens. This embodiment of the invention also has particular value in decanting recently depleted and refilled Dewar flasks. Dewars filled with liquid cryogen become pressurized as the liquid cryogen evaporates in the flask. This evaporation is brought about as heat energy is added to the liquid cryogen both 20 from thermal energy initially contained in the Dewar itself and through continuous heat transfer from the ambient conditions through the Dewar walls.

The increase in pressure associated with evaporation is used to drive fluid through the exit port of the Dewar. Cold Dewars that recently have been filled are already precooled and require more time to deliver liquid due to the increased 25 time required to add heat energy to the Dewar liquid contents by heat transfer alone. The pressure created in the annular space between the inner and outer

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chambers can be used to quickly pressurise the Dewar as well as to provide the means to control the pressure in the Dewar flask. This embodiment therefore may be used to provide an on-demand self-regulating, pressurizing system which is devoid of moving parts. In a still further embodiment the invention may be 5 used as a means for detecting the presence of liquid cryogen in a gaseous cryogen stream. The source of gaseous cryogen is connected to the inner chamber of the invention which in this embodiment is preferably in the form of a tube. When liquid cryogen enters the inner chamber the pressure in the outer pressurebuilding chamber will increase significantly and this increase may be 10 used to actuate a device such as, for example, an alarm, shut-off valve or diverting valve.

The inner chamber of the invention may comprise any suitable material such as metal, ceramic, synthetic polymers, or natural polymers such as cellulosic polymers. Preferred materials are fluoropolymers (e.g., 15 polytetrafluoroethylene) and more preferred is expanded polytetrafluoroethylene.

Most preferred are porous inner chambers made by combining multiple layers of expanded polytetrafluoroethylene membrane as disclosed in International Patent Application W000/75558 A2. Such constructions may also include discontinuous or porous layers of other fluoropolymers to act as bonding or 20 reinforcing elements. Particularly suitable as such agents are co-polymers of tetrafluoroethylene and hexafluoropropene or co- polymers of tetrafluoroethylene and perfluoroalkyl vinyl ethers. Examples of such copolymers are Teflon FEP and Teflon PEA (both Registered Trade Marks of E.l. du Pont de Nemours and Company) The porous inner chamber preferably has a Liquid Nitrogen Leak 25 Pressure (defined hereinafter) of at least 0.002MPa.

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The pressure-building outer chamber surrounding and enclosing the inner chamber may be constructed of any suitable material such as metal, ceramic or polymeric materials. The selection of the material and the wall thickness will preferably take account of the pressure within the chamber, the requirements for 5 maintaining the heat flux into the annular space and the dimensions of the chamber. Adequate safety margins as dictated by good pressure vessel design practice familiar to those skilled in the art should preferably be incorporated. A pressure venting device set in connection with the pressure building chamber to control the maximum pressure developed, as dictated by good practice, may be 10 included in the design.

The cross section of the pressure-building outer chamber may be any shape. For practical applications shapes conforming to good pressure vessel design as known to those skilled in the art will be used.

15 BRIEF DESCRIPTION OF THE DRAWINGS

Details of various embodiments of the invention by means of drawings and examples are given below.

Figure 1 is a diagrammatic representation of one form of test apparatus for determining the Liquid Nitrogen Leak Pressure of the inner chamber for use in 20 embodiments of the present invention.

Figure 2 is a diagrammatic representation of an embodiment of the present invention in which the pressure-building chamber is closed. This embodiment may be used to assess the pressure-building capability and pressure limiting characteristics of various forms of the inner chamber of the invention.

25 Figure 3 is a diagrammatic representation of an embodiment of the present invention wherein the gas under pressure in the chamber is allowed to exit to

Àe r as ! À e À e I À atmosphere or to another device. This embodiment may also be used to assess the gas flow characteristics of various forms of the inner chamber of the invention. Figure 4 is a diagrammatic representation of an embodiment of the present 5 invention in which the pressure of the gas in the pressure-building chamber is used to activate a device such as a pneumatic switch, as in Example 2.

Figure 5 is a diagrammatic representation of an embodiment of the present invention in which the exiting gas from the pressure-building chamber is used to pressurise the ullage of a Dewar containing the liquid cryogen which flows 10 through the inner chamber of the invention.

Figure 6 is a graphical representation of the Dewar discharge characteristics for the embodiment of the invention detailed in Example 3.

TEST METHODS

1 5 Isopropanol Bubble Point, Gurley Air Permeabilitv and Tube Dimension Measurement Testing for the Tubes.

The tubes were mounted to barbed luer fittings and secured with clamps and tested intact. The values of three samples per tube were obtained and 20 averaged for the isopropanol (IPA) bubble point and the thickness measurements. One Gurley air permeability measurement was made per tube.

The isopropanol bubble points (IBP) were tested by first soaking the tubing fixtures in IPA for approximately six hours under vacuum, then removing the tubing from the IPA and connecting the tubing to an air pressure source. Air 25 pressure was then manually increased at a slow rate until the first steady stream of bubbles was detected. The corresponding pressure was recorded as the IBP.

r r r r 8. a a. r I r C À r a, À À À r À I À a. Or The air permeability measurement was determined using a Gurley Densometer (Model 4110, W. & L. E. Gurley, Troy, NY) fitted with an adapter plate that allowed the testing of a length of tubing. The average internal surface area was calculated from the measurements utilising a Ram Optical Instrument 5 (OMIS II 6 x12, Ram Optical Instrumentation Inc., 15192 Triton Lane, Huntington Beach, CA). The Gurley Densometer measures the time it takes for 100 cc of air to pass through the wall of the tube under 4.88 inches (12.40 cm) of water head of pressure. The air permeability value was calculated as the inverse of the product of the Gurley number and the internal surface area of the 10 tube expressed in units of cc/min cm2.

The wall thickness and inner diameter of the tube were measured using the same OMIS II optical system.

Isopropanol Bubble Point and Thickness Testing for Films 15 Bubble point of films was measured according to the procedures of ASTM F31 6-86. The film was wetted with Isopropanol. Film thickness was measured with a snap gauge (Mitutoyo, model 2804-10, Japan).

Liquid Nitrogen Leak Pressure (LNLP) Testing for the Tubes.

20 A liquid nitrogen leak pressure test was used to measure the pressure at which liquid nitrogen permeates through a cryogen tube wall. Liquid nitrogen was added to the lumen of tested tubes and pressurised. The tube was examined to ensure the permeation of gaseous nitrogen through the tube wall.

The pressure at which liquid nitrogen leaks through the walls of the tube was 25 noted and recorded. This pressure corresponds to the pressure at which the mass flow rate of liquid nitrogen flowing through the wall in the radial direction

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À exceeds the mass evaporation rate of the liquid at the outer wall surface. A schematic representation of the test apparatus appears in Figure 1. A 0.5 Litre Dewar flask (Cryo Jem Cryomedical Instruments Ltd. Nottinghamshire UK) 1 was obtained. (A larger flask may be used if desired.) The Dewar flask lid 2 was 5 dried to avoid the outlet valve 3 becoming blocked due to moisture ingress leading to accumulation of ice particles. The Dewar flask 1 was filled with liquid nitrogen and the lid 2 slowly screwed onto the canister allowing excess liquid nitrogen to boil off.

Air pressure was applied to the top of the liquid nitrogen reservoir. The 10 pressure was regulated via a precision regulator (Moore. Model 41-100) 4. A pressure vent tap was included in the line entering the flask for safety reasons.

The Dewar flask 1 inlet pressure was measured with a multi-port pressure transducer (Heise, model PM, Newtown, CT) or gauge 5. Liquid nitrogen was forced out of the flask through a 0.062 inch (1.58 mm) inner diameter stainless 15 steel dip tube 6 that extends from near the bottom of the flask to an opening in the flask lid 2. A lever valve 3 at the head controls the exit flow. The dip tube 6 extends beyond this valve 3, enclosed in a larger plastic conduit 7. Threaded fittings 8 were attached to the larger conduit 7. Another pressure monitoring tap was included in the line in order to measure the inlet pressure to the tested tube 20 (using the same pressure monitor as described above). The fitting 8, comprised a threaded end to fit into conduit, 6, a 1/8" Swagelok side Tbranch to allow pressure monitoring and a '/"Swagelok fitting to allow connection of the tube sample under test.

The first tube 9 to be tested was cut to a length of 320 mm. The test 25 length was about 300 mm since portions of the ends were attached over stainless steel mounting tubes 10, 1 1. One end of the test sample 9 was slipped 1 1

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À over tube 10 and secured by Oetiker stainless steel crimp fittings 12 compressed around the outside of the tube 9. The other end of the test sample 9 was fitted with another stainless steel tube 11 and secured in the same manner. These two mounting tubes, 10 and 11, were cut to a length to facilitate the mounting of 5 the tube in a subsequent test. The outlet of tube 11 was fitted with another in line '/"Swagelok fitting into which a 0.50 inch (25 mm) long stainless steel cylindrical plug 13 is inserted. The plug 13 has a 0.062 inch (1.58 mm) diameter, 0.075 inch (1.90 mm) long hole 14 drilled through its centre, which was counter- bored to 0.125 inch (3.18 mm) diameter for a length of 0.425 inch 10 (10. 8 mm). It was ensured that the outlet orifice diameter was smaller than the dip tube inside diameter. The outlet orifice was the greatest restriction to flow in the line exiting the flask. This choice of outlet orifice 14 enables sufficient test duration before exhausting the liquid nitrogen from the flask. Venting the outlet to atmosphere enhances the flow of liquid nitrogen into the tube to be tested.

15 The tube 9 was positioned horizontally. The test was performed under a hood at ambient conditions: pressure 1003 mbar, room temperature was 23 C, relative humidity was about 37% and the air was essentially still. The nitrogen exiting the end of the tube was directed so as not to disturb the air flow under the hood.

20 The tube 9 was tested in the following manner. The Dewar flask lever valve 3 was opened. The pressure regulator 4 was adjusted until liquid nitrogen exited the orifice 14 at the end of the test sample tube. The discharge of liquid nitrogen was readily confirmed by placing an expanded PTFE membrane in the path of the exiting nitrogen and noting wetting of the membrane. All fittings and 25 connections were examined to ensure that no leaks were present. The tube 9 was then examined for gaseous permeation of nitrogen through its wall, along

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I r I l.. the length of the tube as evidenced by a plume of condensed water vapour in the vicinity of the tube. The applied pressure was adjusted until such a steady plume was observed. A steady plume indicates both gas permeation and that the air was still in the test environment. The plume as described demonstrates that 5 gaseous nitrogen was exiting along the length of the tube 9, which was indicative of distributed evaporative cooling. Note that the pressure increase in the Dewar flask 1 resulting from the evaporation of the nitrogen alone may be sufficient to pressurize the tube 9.

The tube under test was allowed to chill for a period of 30 seconds prior 10 to further pressure adjustment. The pressure was increased until the first droplet of liquid nitrogen appears on the outer surface of the tested tube 9. The pressure regulator 4 was slowly and slightly opened and closed to ensure that this was the pressure corresponding to the formation of the first stable droplet. A stable droplet was one that under constant pressure, remains about the same size 15 during testing for at least 5 seconds, without dripping. By decreasing the pressure the droplet will evaporate. With increasing pressure, the droplet size increases past stability until liquid was first dripping rapidly and then running out of the tube wall. The pressure corresponding to this stable droplet size condition is the Liquid Nitrogen Leak Pressure. This pressure is measured at the entrance 20 to the tested tube 9 and was recorded. Venting the tube 9 to atmosphere via the use of the plug 13 with the 0.062 inch (1.58 mm) orifice 14 was important to achieve the distribution of liquid nitrogen across the length of the tube 9. Tubes in accordance with the preferred embodiments of the present invention allowed permeation of the most gas when liquid cryogen was present on the interior 25 surface.

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r r I I À I À t I J I À À 8 8 It 1 8 Whereas this test was developed specifically for testing tubes, the same principles may be applied to create a test for the examination of the properties of other shapes of materials. The important elements of the test include: controlled application of pressure and ability to measure the pressure required to force a 5 mass of liquid nitrogen sufficient to form a stable drop of liquid on the outside wall of the test article, through the thickness of the article, while the internal surface of the article was in contact with liquid.

Example 1

10 A tube was made using an expanded PTFE film possessing a thickness of 0.0035 inch (0.09 mm), a Gurley number of 39.5 seconds and an isopropanol bubble point of 48.5 psi (0.334 MPa). All measurements were made in accordance with the procedures previously described. The film was slit to provide a width of 0.875-inch (22.2 mm) and was helically applied over a base 15 tube on a 0.25 inch (6.35 mm) stainless steel mandrel. The base tube was made from a thin longitudinally expanded PTFE tube possessing a wall thickness of 0.24 mm, an inner diameter of 0.252 in (6.4 mm), and an IBP of 1.0 psi (0.007 MPa). The film was applied with approximately 11 % overlap to provide about 18 layers of film over the base tube.

20 The ends of the layered film construction were restrained by suitable means to prevent shrinkage in the longitudinal direction of the construction (the longitudinal axis of the mandrel) during subsequent heat treatment.

The restrained construction was placed in a fluidised bed containing glass beads as media set to 350 C and heated for 4 minutes in order to bond the 25 ePTFE layers and impart dimensional stability to the tube. The tube was allowed

see À a'. , À a , À to cool in air for one hour. The restraints were removed and the tube was removed over the end of the mandrel.

The tube was measured for inner diameter, wall thickness, Gurley number, and IBP in accordance with the techniques previously described. The 5 inner diameter was 6.19mm, the wall thickness was 0.0234 inch (0.595 mm) , the Gurley number was 143.8 seconds., and the IBP was 62.0 psi.

Further tests were performed to determine the LNLP of the tube. The tube controlled the passage of gaseous nitrogen and inhibited the passage of liquid nitrogen at an average LNLP of 128 mBar.

10 Evaluation of Pressure-Building Capability Closed annular chamber Two sections, 150 mm and 300 mm in length, were cut from the tube for further testing. Each tube was mounted in the test arrangement diagrammatically represented in Figure 2, which is a modification of the test arrangement represented 15 in Figure 1.

The porous tube 9 under test was enclosed in a sealed pressure-building chamber 15 fitted with a 1/4in. (6.3mm) Swagelok T-piece 16 (South of Scotland Valve and Fitting Co. Ltd., Irvine, Scotland) to allow connection to pressure gauge 5 The upper port of the T-piece was blanked- off. The chamber 15 comprises a 12 '/ 20 in. (318mm.) length of clear PVC pipe 1 7/8in.(47.63mm) diameter, having a bore 1 5/8in. (41.28mm) (United States Plastic Corp. Lima, Ohio) and having end pieces comprising 1in. (25.4mm.) female x male SP 1607 PVC adapter into which is fitted a 1/2in. (12.7mm) x 1in. (25.4mm) BSP male /female PVC adapter (both adapters available from United States Plastic Corp., Lima, Ohio). The stainless steel tubes 10 25 and 11 are sealed into the end pieces through 1/4in. (6. 4mm.) Swagelok x 1/2in.(12.7mm.) BSP tapered nylon fittings (South of Scotland Valve and Fitting Co. Ltd. Irvine, Scotland).

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Liquid nitrogen from Dewar 1 was fed to the tube 9 under test through stainless steel tube 10 by adjusting the pressure regulator 4 and opening valve 3. The complete test apparatus was angled at about 4 from the horizontal to allow the tube under test to fill with liquid nitrogen. Pressure (P1) inside the inner tube, and 5 pressure (P2) in the annularspace, and the flow rate out of the inner tube were measured 3, 4 and 5 minutes after initiation of the flow of liquid nitrogen for both lengths of tubing.

P1 averaged about 117 mbar for the shorter tube and about 111 mbar for the longer tube. The values of P2 were about 820 mbar and 1045 mbar for the 10 short and long tubes, respectively. These results demonstrate the surprising increase in pressure, about seven- to nine-fold, associated with gas permeation through the inner tube wall. The increase in pressure attests to the ability of the inner tube to function in a way to create a gas at high pressure while storing or transporting liquid cryogen.

15 After 3 minutes the pressure in the chamber had become quite stable. This stabilised maximum value of the pressure in the outer tube demonstrates the self-limiting pressure feature of the invention.

Evaluation of flow characteristics Annular chamber open to atmosphere 20 The flow rates into the annular space of both lengths of tube were also measured using the test arrangement diagrammatically represented in Figure 3.

The test arrangement was as in Figure 2 but with the upper port of the Tpiece 16 connected to a flow meter 17 ("Rotameter" from Platon Flow Meters Ltd. Sheffield England) having an integral control valve 18. The exit of the flow meter

25 was open to atmosphere. After the valve 18 was opened P1 was about 2530 mbar and P2 was about 500 mbar for both tubes. The resulting flow rates into

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Example 2

Figure 4 is a diagrammatic representation of an embodiment of the 5 invention in which the pressure generated in the chamber 15 is used to actuate a pneumatic pressure switch 19 (Parker Hannifin spring return control valve). The inlet port of the switch was connected to a pressurised (approx. 60psi (0.41 MPa)) air supply line 21 and the outlet port connected to a pneumatic drill 20.

The control port of the switch was connected to the chamber 15 through T 10 piece 16. In the un-actuated state the switch does not allow airflow from the inlet to the outlet.

When liquid nitrogen from the Dewar 1 was allowed to enter into tube 9 and sufficient pressure had been generated in the chamber 15 the control switch was actuated and the drill was activated.

15 This set-up demonstrates the utility of the present invention to actuate control devices. Similarly, devices could be directly driven by the pressure in the annular space.

Example 3

Another embodiment of the present invention pertains to using the 20 pressure generated in the annular space to pressurise the ullage of a Dewar.

Figure 5 represents the experimental set-up. The tubes of Example 1 were used to generate high pressures in the chamber 15. The outlet of the pressure chamber through T-piece 16 was connected to the pressurizing line of Dewar 1 through a tube 22 and control valve 23. Figure 6 compares the discharge rate of 25 the Dewar before and after opening the control valve 23 and the results for the 150 mm and the 300 mm tubes, respectively. Note that the inflection point of 1 7

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À a À À À both curves corresponds with the opening of valve 23 in the line connecting the chamber and the Dewar at about time equal 3 minutes. Once the valve 23 was opened, the pressure in the chamber charged the Dewar, immediately resulting in greater flow of liquid cryogen through the inner tube.

5 A variant of this embodiment could, for example, be used as a liquid cryogen detector in a system where the presence of liquid cryogen may be undesirable. The presence of liquid cryogen in a gas stream passing through the inner chamber would lead to the generation of pressure in the annular space which could actuate control valves to shut-off the cryogen liquid flow or vent 10 the pressure in the Dewar ullage.

Example 4

A further sample tube was produced from expanded PTFE. The tube was manufactured in the same manner as in Example 1 but this time 30 instead of 18 layers of film were used. Tests were performed to determine the LNLP of the 15 tube. The tube controlled the passage of gaseous nitrogen and inhibited the passage of liquid nitrogen at an average LNLP of 225 mbar.

The tube was mounted in the closed annular chamber arrangement, see Figure 2. The tube was mounted in the test chamber described in Example 1.

Pressure inside the inner tube (P1), pressure in the annular space (P2), and the 20 flow rate out of the inner tube were measured every 30 seconds until the Dewar was empty at 3minutes 30 seconds.

P1 averaged about 225 mbar during the test. The values of P2 at the steady state condition, between 2mins. 30secs. and 3mins. 30secs, averaged 1353 mbar. The increase in pressure in the annular space compared to a tube 25 with a lower LNLP (in a previous example) shows that the pressure generated can be varied through variation in tube physical characteristics.

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While certain embodiments of the invention have been herein shown and described, these are not intended to be limiting as to the scope of the invention.

It will be apparent to those skilled in the art that variations and alternatives to these embodiments may be developed without departing from the spirit of the 5 invention, the scope of which is defined by the following claims.

1 9

Claims

À À CLAIMS

5 We claim:

1. A pressure-building device comprising an outer chamber substantially impermeable to cryogenic fluid in the gaseous phase; and an inner chamber which restricts the passage of cryogenic fluid in the liquid phase while permitting the passage of cryogenic fluid in the gaseous phase; 10 further comprising means for removing cryogenic fluid in the gaseous phase under pressure from the space between the inner and outer chambers. 2. The device of claim 1, wherein the wall(s) of the inner chamber restrict the passage of cryogenic fluid in the liquid phase while permitting the 15 passage of cryogenic fluid in the gaseous phase.

3. The device of claim 1 wherein the means for removing cryogenic fluid in the gaseous phase is connected to a pressure or flow actuated device.

4. The device of claim 1 wherein the means for removing cryogenic fluid in the gaseous phase is connected to a reservoir for containing cryogenic 20 fluid in the liquid phase, such that, in use, the reservoir may be pressurised. 5. The device of claim 1 wherein the wall(s) of the inner chamber are porous. 6. The device of claim 1 wherein the Liquid Nitrogen Leak Pressure of the 25 inner chamber is greater than 0.002 MPa.

7. The device of claim 1 wherein, in use, the gas pressure in the outer chamber is greater than the pressure in the inner chamber.

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. À À 8. The device of claim 7 wherein the pressure in the outer chamber is self limiting. 9. The device of claim 1 wherein the inner chamber comprises a polymer.

10.The device of claim 9 wherein the polymer comprises a fluoropolymer.

5 11.The device of claim 10 wherein the fluoropolymer comprises polytetrafluoroethylene. 1 2.The device of claim 11 wherein the polytetrafluoroethylene comprises expanded polytetrafluoroethylene.

13.The device of claim 10 or 12 wherein the inner chamber comprises a 10 second fluoropolymer.

14.The device of claim 1 3 wherein the second fluoropolymer comprises a copolymer of tetrafluoroethylene and hexafluoropropene.

15.The device of claim 13 wherein the second fluoropolymer comprises a copolymer of tetrafluoroethylene and perfluoroalkyl vinyl ether.

15 16.The device of claim 1 wherein the inner chamber comprises a metal.

1 7.The device of claim 1 wherein the inner chamber comprises a ceramic.

18.The device of claim 1 wherein the inner chamber comprises a tube.

19.The device of claim 1 wherein the inner chamber comprises a sphere or spheroid. 20 20.The device of claim 1 wherein the inner chamber is closedended.

21.A system comprising: a. a reservoir for containing liquid; and b. a pressure-building device attached to said reservoir, said device comprising an outer chamber substantially impermeable to 25 cryogenic fluid in the gaseous phase; and an inner chamber which restricts the passage of cryogenic fluid in the liquid phase while

. ... c À Àc À À À À À À c.À À c À À À. c.. À. c c À À. c À c c Be À c c permitting the passage of cryogenic fluid in the gaseous phase; further comprising means for allowing cryogenic fluid in the gaseous phase under pressure to pass from the space between the inner and outer chambers to the reservoir, thereby pressurising 5 fluid in said reservoir.

22.The system of claim 21 wherein the liquid is liquid cryogen 23.The system of claim 22 wherein, in use, the temperature of the liquid cryogen exiting the reservoir is below the boiling point of the liquid cryogen. 10 24. The system of claim 22 wherein, in use, the temperature of the liquid cryogen in the reservoir is below the boiling point of the liquid cryogen.

25.A cryogenic liquid sensing detector comprising the device of claim 1 wherein an increase in pressure in the space between the inner and outer chambers results from the presence of liquid cryogen in the inner 1 5 chamber.

26.The system of claim 25 wherein the increase in pressure is detected by a pressure-sensing means 27.A cryogenic liquid sensing detector comprising an outer chamber substantially impermeable to cryogenic fluid in the gaseous phase; and an 20 inner chamber which restricts the passage of cryogenic fluid in the liquid phase while permitting the passage of cryogenic fluid in the gaseous phase; further comprising means for sensing pressure of gas in the space between the inner and outer chambers, wherein an increase in pressure in said space is indicative of the presence of liquid cryogen in the inner 25 chamber.

À.. À. . À À À À e À q À À e e e.e e 28. A pressure-building device comprising an outer chamber capable of containing gas under pressure and an inner chamber adapted to contain a cryogenic fluid in the liquid phase and permit said cryogenic fluid, when in the gaseous phase, to pass through the wall(s) of said inner chamber and 5 be contained by said outer chamber and utilised to drive a pressure or flow actuated device.