Abstract:
The invention involves a siphon for delivery of a liquid cryogen from a container such as a Dewar flask. The siphon ensures delivery of a liquid cryogen with a lower proportion of the gaseous fraction. The siphon comprises a central feeding conduit, which is largely contained within the Dewar flask. There is an auxiliary conduit surrounding the central feeding conduit; the outer upper section of this auxiliary conduit is provided with an adjustable valve intended to release a gaseous fraction of the cryogen contained in the annular gap between the auxiliary and central feeding conduits. The upper section of the central feeding conduit is provided with an external layer of a porous capillary coating or with a wick; this ensures that the upper section of the central feeding conduit is continuously wetted with the liquid cryogen. This porous capillary coating prevents gasification of the liquid cryogen in the central feeding conduit. Alternatively, the problem of liquid cryogen gasification may be solved through thermal insulation of the central feeding conduit.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to cryogen devices and, in particular, to a Dewar flask siphon which ensures delivery of a high quality of liquid cryogen even for low values of the flow rate. 
       BACKGROUND OF THE INVENTION 
       [0002]    Siphons intended for feeding a liquid cryogen contained in a Dewar flask are known in the art, although all suffer from various drawbacks, particularly as they increase the amount of gas generated from the liquid cryogen as it becomes heated. 
         [0003]    For example, Tsals (U.S. Pat. No. 6,012,453) describes an apparatus that provides for withdrawal of the liquid contents from a closed container independent of the spatial orientation thereof. The liquid withdrawal apparatus includes flexible withdrawal conduits disposed inside the container and in fluid flow communication with external heat exchangers. The heat exchangers serve to transfer heat to the withdrawn liquid to thereby provide a breathable gas mixture. The upstream end of the withdrawal conduits are provided with a weighted pick-up means comprising a wicking material that draws liquid into the interior thereof to ensure contact of the liquid with the conduits, even when the supply of liquid is nearly depleted. A pressure differential between the inside of the container and the external heat exchangers, normally brought about by an inhalation event of the user, provides the motive force for withdrawing the liquid contents from the container through the conduits. Thus, this solution is clearly intended to generate gas not to block the generation thereof. 
         [0004]    James (U.S. Pat. No. 5,417,073) teaches a portable Dewar flask for cooling an object through use of cryogenic fluids comprising a reservoir for holding cryogenic fluid. The reservoir includes a fill port, a wicking material adapted to be in thermal contact with the object to be cooled, a transfer tube connected between and coupling the reservoir and the wicking material to permit transfer of the cryogenic fluid from the reservoir to the wicking material and a venting channel adjacent the reservoir for providing a vent for evaporated cryogenic fluid from the wicking material. The evaporated cryogenic fluid has thermal contact with the reservoir. An outer wall defines a vacuum space circumferentially surrounding the reservoir, venting channel and wicking material. Again, the solution for gas generation is merely to vent the gas from the system. 
         [0005]    Caldwell (U.S. Pat. No. 5,438,837) discloses an apparatus for storing and delivering a liquid cryogen. The apparatus is a Dewar flask having a rotating liquid cryogen intake, a rotating gas supply vent, and a rotating capacitance gauge. Also disclosed are a system and a process employing the system for liquefying a gas to produce a liquid cryogen in the Dewar flask wherein the gas is subcritically cooled and then condensed in the pressure vessel of the Dewar flask. Again, the solution to the problem of gas generation is simply to vent the gas. 
         [0006]    Caldwell (U.S. Pat. No. 5,361,591) describes a portable life support system, comprising: a liquid cooled garment; an orientationally independent Dewar flask for containing liquid cryogen; means for circulating liquid cryogen from the Dewar flask in heat exchange relation with the cooling liquid so as to cool the wearer of the garment and vaporize the liquid cryogen; and means for delivering vaporized cryogen to the wearer of the garment for breathing purposes. This solution is actually intended for gas generation, which is considered to be desirable in this context. 
         [0007]    Cowans (U.S. Pat. No. 3,699,775) describes a liquid processing system, featuring a container including a liquid and a pressurizing gas which is substantially non-reactive with respect to the liquid and which establishes a controlled pressure differential between the interior of the container and its surroundings. A porous conduit, extending between the interior and exterior of the container, is maintained in contact with the liquid. The conduit transports liquid along its length, forming a meniscus of extended surface upon portions of the conduit not submerged in the liquid. The meniscus defines a gas barrier; the conduit nevertheless transports fluid at a selected rate between the container and its surroundings. When employed in a cryogenic system, fluid may be transported in response to heat interchange by the container, the rate depending on the temperature change required. Yet again, this solution depends upon the use and generation of gas from the cryogenic liquid. 
         [0008]    In addition, a Dewar flask siphon described in the book: Verkin B. I. et al. “.” LOW TEMPERATURES IN STOMATOLOGY”, Naukova Dumka, Kiev, 1990, pp.62÷63 
         [0009]    (originally in Russian) should be noted. This book proposes a siphon design, which is based on application of a finned external housing and a jacket surrounding the central feeding conduit The gap between the jacket and the central feeding conduit is filled with liquid-gaseous mixture of cryogen. In addition, this liquid-gaseous mixture of cryogen enters via a set of holes on the internal surface of the finned housing with further evaporation. It causes, in turn, quick elevation of pressure in the internal space of the dewar flask. However, this design does not solve the problem of the low quality of the liquid cryogen supplied from the central feeding conduit for low magnitudes of the supply rate of the liquid cryogen. 
       SUMMARY OF THE INVENTION 
       [0010]    None of the above background art references teaches or describes a design of a Dewar flask siphon which ensures delivery of high quality of liquid cryogen even for low values of the flow rate. Furthermore, none of the above background art references teaches or describes a Dewar flask siphon which reduces the amount of gas generated from the liquid cryogen. 
         [0011]    The present invention overcomes these drawbacks of the background art by providing a siphon system for a container such as a Dewar flask, which ensures delivery of high quality of a liquid cryogen even for low values of the flow rate. 
         [0012]    According to preferred embodiments of the present invention, the siphon ensures delivery of a liquid cryogen with a lower proportion of the gaseous fraction as compared to other siphon/Dewar flask systems which are known in the art. The siphon comprises a central feeding conduit, which is preferably largely positioned within the Dewar flask such that at least about 50% and more preferably at least about 60%, and most preferably at least about 75% of the central feed conduit is positioned within the Dewar flask. Preferably an external (auxiliary) conduit surrounds the central feeding conduit; and, the outer upper section of this auxiliary conduit is preferably provided with a port and an adjustable valve intended to release a gaseous fraction of the cryogen contained in the annular gap between the auxiliary and central feeding conduits. 
         [0013]    The upper section of the central feeding conduit preferably features an external layer of a porous capillary coating or with a wick, or any other type of capillary material, for wetting the upper section of the central feeding conduit with the liquid cryogen. This capillary material wetted with the liquid cryogen prevents gasification of the liquid cryogen in the central feeding conduit. Alternatively, the problem of liquid cryogen gasification may be solved through thermal insulation of the central feeding conduit, as described according to some embodiments of the present invention. 
         [0014]    According to preferred embodiments of the present invention, the siphon system comprises: an external (auxiliary) conduit, its lower section is situated in the Dewar flask and the upper section is located outside the Dewar flask; a sealing unit, preferably in the form of a annular rubber ring, which allows installation of the siphon in the Dewar flask neck and a section of the tubular piece is joined sealingly with the annular rubber ring; and a central feeding conduit, wherein part of this central feeding conduit is positioned in the aforementioned external conduit and its lower end is situated substantially near the bottom of the internal space of the Dewar flask. The upper edge of the external conduit is sealed with the outer section of the central feeding conduit. 
         [0015]    According to some embodiments, a capillary wicking structure is situated at least between the upper sections of the external and central feeding conduits. This capillary wicking structure has such characteristics (length and size of the capillary open pores) that wetting its lower edge with the liquid cryogen ensures wetting the whole capillary wicking structure with liquid cryogen. Preferably, there is also provided a mechanism and/or system for maintenance of a proper level of the liquid cryogen in the annular gap between the external and central feeding conduits, such that the lower section of the capillary wicking structure is wetted by the liquid cryogen in the Dewar flask on one hand, and flooding this annular gap by the liquid cryogen is prevented on the other hand; various non-limiting examples of suitable mechanisms and/or systems are described herein. 
         [0016]    Optionally and preferably the external conduit is surrounded by a jacket, while the upper edge of the jacket is sealed with the external conduit. The jacket preferably is formed as a tubular piece. 
         [0017]    Also optionally and preferably, a shut-off valve is installed on the outer section of the central feeding conduit. Optionally and preferably, safety and relief valves are installed on the outer section of the jacket. More preferably, the outer section of the external conduit is provided with an opening which is provided, in turn, with a duct, which most preferably features an adjustable valve installed thereto. 
         [0018]    According to some embodiments, the jacket is provided with ports; and the device further features a pressure gauge for measuring pressure of the cryogen, a safety valve and a release valve communicating with a respective one of the ports of the jacket for reducing the pressure of the cryogen. Optionally and preferably, the jacket is provided with a port for introducing a suppressed gas into the Dewar flask. Also the siphon preferably features a gap between the jacket and the external conduit for increasing hydraulic resistance of cryogen flow. 
         [0019]    Preferably, a pressure gauge is installed on the outer section of the jacket which serves for measuring pressure in the Dewar flask. 
         [0020]    According to these preferred embodiments of the present invention, the capillary wicking structure provides thermal protection of the upper section of the central feeding conduit of the siphon by evaporation of the liquid cryogen from the external side of this central feeding conduit; this evaporation occurs with a rate which matches the rate of the heat influx from the outside sources at this section. 
         [0021]    A capillary wicking structure may optionally be fabricated as a wick from thin fibers maintained on the outer wall of the central feeding conduit. Alternatively, this capillary wicking structure may optionally comprise a porous coating from a sintered metal powder. 
         [0022]    According to optional but preferred embodiments of the present invention, there is optionally and preferably provided a measuring system for determining a level of the liquid cryogen in the annular gap between the external and central feeding conduits and preferably for ensuring a proper and sufficient level thereto. The measuring system also optionally and preferably comprises a control unit. More preferably, the control unit (according to the measured level) causes the adjustable valve to release evaporated gas from the annular gap between the external and central feeding conduits at a sufficient rate to ensure wetting of the lower edge of capillary wicking structure by the liquid cryogen. Alternatively or additionally and most preferably, the control unit controls the adjustable valve activity in order to prevent or at least alleviate overflowing the gap between the external and central feeding conduits by liquid cryogen. Alternatively or additionally, the adjustable valve may optionally be controlled manually. 
         [0023]    According to one optional embodiment, the measuring system preferably comprises a level gauge, which is positioned in the annular gap between the external and central feeding conduit and indicates a level of the liquid cryogen in this gap. 
         [0024]    According to another optional embodiment, the measuring system preferably comprises a temperature measuring device for measuring the temperature of the gas released from the port of the annular gap, which optionally and preferably measures the temperature of the gaseous-liquid medium released from the space between the central feeding conduit and the external conduit. 
         [0025]    According to yet another optional embodiment, the measuring system preferably comprises a density measuring device for measuring the density of the mist emitted from the port of the annular gap. For example, this device may optionally comprise an optical or ultrasound measuring unit. The optical device measures scattering of light by the mist, and the ultrasound device measures absorption of ultrasound by the mist depending on concentration of droplets in this mist. Optionally and preferably, the siphon features an optical measuring device for measuring density of an exhausted medium, which more preferably measures density of an exhausted medium from the space between the central feeding conduit and the external conduit. Alternatively or additionally, and optionally and preferably, the measuring means comprises an acoustical measuring device for measuring density of an exhausted medium, which more preferably measures density of an exhausted medium from the space between the central feeding conduit and the external conduit. 
         [0026]    The lower edge of the central feeding conduit may optionally be provided with a filter in order to collect mechanical particles contained in the supplied liquid cryogen. 
         [0027]    The lower section of the internal surface of the external jacket can be provided with a divider for dividing the upper and lower internal space of the Dewar flask, with the divider featuring high hydraulic resistance for passage of the gas through it. This prevents the liquid cryogen in the Dewar flask from being forced up and out in the case of opening the relief valve of the siphon. The divider may optionally comprise an internal threading of the external jacket with the internal diameter, which fits the outer diameter of the external conduit. Such an embodiment enables the spiral groove of the threading to present a high hydraulic resistance, which prevents boiling and overflow of the liquid cryogen in the Dewar flask when opening the relief valve. 
         [0028]    In addition, the system may optionally and preferably comprise a check valve with a heat exchanger on the upper section of the central feeding conduit (before or after the shut-off valve), for optionally and preferably providing a pulse-wise supply of liquid cryogen on the expense of fast evaporation of a certain fraction of the liquid cryogen in the heat exchanger. 
         [0029]    If the check valve is installed after the shut-off valve, it is possible to heat pulses of the liquid cryogen provided from the dewar flask in order to enhance pressure of the supplied pulses of the cryogen. In order to achieve this, preferably a low inertia electrical heater is installed immediately after the check valve and a low inertia temperature sensor is installed in the central feeding conduit. Delivery of a portion of the liquid cryogen via the check valve lowers the temperature as measured by the temperature sensor, which preferably sends a signal into a control power unit. This control-power unit preferably generates a pulse of electrical current, which is provided to the low inertia electrical heater and it causes the liquid cryogen to boil with a subsequent sharp elevation of its pressure, preferably through flash boiling. As a result, the check valve is closed and the high pressure portion of the liquid-gaseous cryogen is emitted. 
         [0030]    According to some embodiments, the Dewar flask siphon allows elevation of the pressure of the liquid cryogen supplied from it without application of expensive cryogenic pumps. This improvement is based on compression of the evaporated gas from the annular gap by a compression means with following condensation of this compressed gas in a heat exchanger of the recuperative type. The amount of the evaporated gas to be compressed is chosen in such a manner that the amount of the liquid cryogen supplied from the central feeding conduit is able to condense the evaporated pressurized gas completely. 
         [0031]    The condensed pressurized cryogen may optionally be provided from the heat exchanger in the form of pulses by application of a controllable valve, which is installed on the conduit communicating the compression means with the heat exchanger. This version presents another technical solution of obtaining high pressure pulses of cryogen in contrast with the design of a cryosurgical system described in Levin (U.S. Pat. No. 7,137,978) , wherein it teaches that pulses of the liquid cryogen were obtained by application of a multi-way valve and a balloon with pressurized gas was used as propulsion agent for portions of liquid cryogen, in contrast to the present invention. 
         [0032]    The check valve can be incorporated as well into the distal upper section of the central feeding conduit situated in the Dewar flask, when the upper edge of the aforementioned capillary wicking structure is positioned somewhat lower than the check valve. The upper section of central feeding conduit, which communicates the check and shut-off valves, serves in this case as the aforementioned heat exchanger. 
         [0033]    In addition, the proposed siphon can be provided with an inlet port in its jacket for introducing pressurized gas into the Dewar flask in order to establish a required pressure in it. 
         [0034]    According to other preferred embodiments of the present invention, a gaseous cryogen at low temperature or a gas-liquid cryogenic mixture, which is removed from the annular gap between the external and central feeding conduits, can be used for cooling the interior of a hose, which serves for transportation of the liquid cryogen from the siphon. In this case the hose preferably comprises two conduits with a thermal insulation, which fills the internal space between these conduits and the external shaft of the hose. The main conduit serves for transportation of the liquid cryogen from the central feeding conduits and the auxiliary conduit serves for transportation of the cold cryogenic gas or liquid-gas cryogenic mixture from the annular gap between the external and central feeding conduits with resulting cooling the interior of the hose. Optionally and preferably, the hose transporting liquid cryogen from the Dewar flask comprises an envelope and a main conduit in flow communication with said central feeding conduit. More preferably, the hose further comprises an internal auxiliary conduit intended for the exhausted gaseous-liquid mixture from the space between the central feeding conduit and the external conduit; the distal end of the internal auxiliary conduit being in flow communication with an outer auxiliary conduit for releasing the cryogen into the atmosphere. 
         [0035]    The main and auxiliary conduits can be positioned in the hose in parallel side by side or coaxially. 
         [0036]    According to other embodiments of the present invention, gasification of the liquid cryogen in the upper section of the central feeding conduit of the siphon may optionally be based on application of thermal insulation of the upper section of this central feeding conduit, such as for example a vacuum induced insulation of the upper section. For this embodiment, the aforementioned check valve is optionally and preferably installed on the central feeding conduit in the vicinity of the upper edge of the thermal insulation. 
         [0037]    According to preferred embodiments of the present invention, thermal insulation is preferably provided around the upper section of the central feeding conduit, which more preferably comprises a vacuum insulation. 
         [0038]    According to preferred embodiments of the present invention, a check valve is installed after the shut-off valve in the direction of flow, wherein the device further comprises a low inertia electrical heater installed immediately after the check valve in the direction of flow; a low inertia temperature sensor installed in the central feeding conduit; and a control power unit receiving signals from the low inertia temperature sensor and generating pulses of electrical current provided to the low inertia electrical heater. Most preferably, the low inertia temperature sensor is a low inertia thermocouple. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0039]      FIG. 1   a  and  FIG. 1   b  show an axial cross-sectional view of a Dewar flask with a siphon installed in its neck and an enlarged axial cross-sectional view of the upper section of the Dewar flask and the siphon. 
           [0040]      FIG. 2  shows an axial cross-sectional view of a siphon with a capillary wick in its annular gap between the external and central feeding conduits. 
           [0041]      FIG. 3   a  and  FIG. 3   b  show an axial cross-sectional view of a siphon with a level gauge in its annular gap between the external and central feeding conduits, and an enlarged axial cross-sectional view of the upper section of the siphon. 
           [0042]      FIG. 4   a  and  FIG. 4   b  show an axial cross-sectional view of a siphon with a control unit, which is functioning on the base of measuring temperature of the gaseous-liquid mixture released from the annular gap between the external and central feeding conduits, and an enlarged axial cross-sectional view of the upper section of the siphon. 
           [0043]      FIG. 5   a  and  FIG. 5   b  show an axial cross-sectional view of a Dewar flask with a siphon installed in its neck and a hose with main and auxiliary conduits and an enlarged axial cross-sectional view of the upper section of the siphon, and the Dewar neck. 
           [0044]      FIG. 6   a  and  6   b  show radial cross-sectional views of two possible constructions of the hose with the main and auxiliary conduits positioned in its internal space. 
           [0045]      FIG. 7   a  and  FIG. 7   b  demonstrate an axial cross-sectional view of a Dewar flask with a siphon; in addition there are a compression means, a valve means and a heat exchange means intended to provide high pressure pulses of the liquid cryogen, and an enlarged axial cross-sectional view of the upper section of the siphon, and the Dewar neck. 
           [0046]      FIG. 8  shows an axial cross-sectional view of a siphon with a thermal insulation of the upper internal section of the central feeding conduit. 
           [0047]      FIG. 9   a  and  FIG. 9   b  show an axial cross-section of a Dewar flask with a siphon installed in its neck ( FIG. 9   a ) and an enlarged axial cross-sectional view of the upper section of the siphon ( FIG. 9   b ); a central feeding conduit of the siphon is provided with a vacuum evacuated jacket and a check valve. 
           [0048]      FIG. 10   a  and  FIG. 10   b  show an axial cross-sectional view of a siphon with a control unit, which measures a density of the mist emitted from the port of the annular gap between the external and central feeding conduits, and an enlarged axial cross-sectional view of the upper section of the siphon. 
           [0049]      FIG. 11   a  and  FIG. 11   b  show an axial cross-section of a Dewar flask with a siphon installed in its neck ( FIG. 11   a ) and an enlarged axial cross-sectional view of the upper section of the siphon ( FIG. 11   b ), with a low inertia temperature sensor and an electrical heater. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0050]      FIG. 1   a  and  FIG. 1   b  show an axial cross-sectional view of an exemplary Dewar flask with a siphon installed in its neck according to preferred embodiments of the present invention, and an enlarged axial cross-sectional view of the upper section of the Dewar flask and the siphon.  FIG. 1A  shows a Dewar flask  101  with neck  102 , which is intended to be filled with a liquid cryogen to be supplied by the siphon  121 ;  FIG. 1B  shows an expanded view of neck  102  and the upper siphon sections  120 . Siphon  120  comprises an external conduit  103  and jacket  104  surrounding the external conduit  103  with gap  117  formed between them. The upper edge of jacket  104  is sealed with the external conduit  103  as shown. Siphon  121  also features an central feeding conduit  106  with gap  118  between the central feeding conduit  106  and the external conduit  103 ; this central feeding conduit serves for supply of the liquid cryogen to a target place. There is also a seal for sealing jacket  104  to the Dewar flask, an there is an annular rubber ring  105  installed on jacket  104  and inserted partially into neck  102 , for holding siphon  121  in Dewar flask  101 . The upper section  119  of the outer surface of the central feeding conduit  106  is preferably covered with a cryogen absorbing or wettable material, preferably a capillary material  107 , which may optionally and preferably be a capillary coating. As shown, in the preferred embodiment, the capillary material is situated between the upper sections of said internal and external conduits. The upper edge of the external conduit  103  is sealed with the outer section of the central feeding conduit  106  as shown. Also, preferably a shut-off valve  108  is installed on the outer section of the central feeding conduit  106 . The shut-off valve  108  ensures control of the supply of the liquid cryogen. Additionally, an outer section of the external conduit  103  includes at least one port and at least one corresponding valve  113  for releasing a gaseous-liquid cryogenic mixture from a space between the central feeding conduit  106  and the external conduit  103 . This provides a required level of elevation of the liquid cryogen in gap  118 , which provides wetting the capillary material  107 . 
         [0051]    In the preferred embodiment, preferably safety and relief valves  109  and  110  are installed on ports of the outer section of jacket  104  for this purpose. Jacket  104  also preferably features a pressure gauge  114  which is installed on the outer section of the external conduit  103  for measuring internal pressure in the Dewar flask  101 . The outer section of the external conduit  103  is preferably provided with port  111  which is preferably provided in turn with duct  112 , more preferably featuring the adjustable valve  113  for controlling wetting of the capillary material  107 . 
         [0052]    The lower section of the internal surface of jacket  104  is provided with an internal threading  115  with an internal diameter, which fits the outer diameter of the external conduit  103 . 
         [0053]    The lower end of the central feeding conduit  106  is provided with filter  116  in order to prevent ingress of solid particles into it. 
         [0054]    With opening the adjustable valve  113 , the level of liquid cryogen in the gap between the external conduit  103  and the central feeding conduit  106  rises which wets the capillary material  107 . As a result, the temperature of the upper section of the central feeding conduit is lowered to the temperature of liquid cryogen and, after opening the shut-off valve  108 , liquid cryogen of high quality is supplied into the central feeding conduit  106 . By “high quality” it is meant that the liquid cryogen has relatively low amounts of gas present. 
         [0055]      FIG. 2  shows an axial cross-sectional view of a siphon with a capillary wick in the gap between the external and central feeding conduits. As shown, this preferred embodiment of the present invention features an external conduit  201  and jacket  202  surrounding the external conduit  201 . The upper edge of jacket  202  is sealed with the external conduit  201 . An annular rubber ring  203  is preferably installed on jacket  202  as for  FIG. 1   a  and  FIG. 1   b . The external conduit  201  surrounds the section of the central feeding conduit  204 , preferably covered (at least at the upper section  214  of its outer surface) with a liquid cryogen absorbing or wettable material which is preferably a capillary material  214 . The upper edge of the external conduit  201  is sealed with the outer section of the central feeding conduit  204 . 
         [0056]    A shut-off valve  205  is preferably installed on the outer section of the central feeding conduit  204 , while safety and relief valves  206  and  207  are preferably installed on ports  208  and  209  of the outer section of jacket  202 . Also, the outer section of the external conduit  201  is preferably provided with duct  210  which is provided in turn with a duct  211 , more preferably featuring an adjustable valve  212 . A pressure gauge  213  is preferably installed on the outer section of jacket  202 , which more preferably serves for measuring pressure in a Dewar flask. These components preferably function as described for  FIG. 1 . 
         [0057]    This exemplary illustrative embodiment of a siphon in combination with a dewar flask filled with a liquid cryogen preferably functions as follows. 
         [0058]    Upon opening the adjustable valve  212 , the level of liquid cryogen in the gap between the external conduit  201  and the central feeding conduit  204  is elevating, with wetting the capillary material  214 . As a result, the temperature of the upper section of the central feeding conduit  204  is reduced to the temperature of the liquid cryogen and, after opening the shut-off valve  205 , liquid cryogen of high quality is supplied into the outer section of the central feeding conduit  204 . The level of the liquid nitrogen in the gap between the external conduit  201  and the central feeding conduit  204  is maintained by manually adjusting the adjustable valve  212 , for example according to the visual characteristics of the liquid-gaseous mixture of the cryogen emitted from the adjustable valve  212 . 
         [0059]      FIGS. 3A and 3B  show an axial cross-sectional view of a siphon according to other preferred embodiments of the present invention with a level gauge in the gap between the external and central feeding conduits, and an enlarged axial cross-sectional view of the upper section of the siphon. 
         [0060]    Siphon  300  preferably comprises an external conduit  301  and jacket  302  surrounding the external conduit  301 . The upper edge of jacket  302  is sealed with the external conduit  301 . An annular rubber ring  303  is preferably installed on jacket  302  as for  FIG. 1   a  and  FIG. 1   b . The upper section  319  of the outer surface of the central feeding conduit  304  is preferably covered with a liquid cryogen absorbing or wettable material, preferably a capillary material  318 , which may optionally be a capillary coating. The upper edge of the external conduit  301  is sealed with the outer section of the central feeding conduit  304 . 
         [0061]    A shut-off valve  305  is preferably installed on the outer section of the central feeding conduit  304 , while safety and relief valves  306  and  307  are preferably installed on ports  308  and  309  of the outer section of jacket  302 . The outer section of the external conduit  301  is preferably provided with port  310  which is provided in turn with duct  311 , more preferably featuring an adjustable valve  312 . A pressure gauge  313  is preferably installed on the outer section of jacket  302  for measuring the internal pressure in the Dewar flask. These components preferably operate as described for  FIGS. 1   a  and  2 . 
         [0062]    The lower section of the internal surface of jacket  302  is preferably provided with an internal threading  320  with an internal diameter which fits the outer diameter of the external conduit  301 . The lower end of the central feeding conduit  304  is preferably provided with a protecting grid  321  in order to prevent penetration of solid particles. 
         [0063]    A level gauge  314  preferably interacts with an induction coil  316 , more preferably through magnet  315  (which is optionally and more preferably an annular magnet). The induction coil  316  sends, in turn, a signal to a control unit  317  via cables  322  for regulating the activity of the adjustable valve  312 . The adjustable valve  312  is controlled according to signals sent through cables  323  in order to achieve a desirable level of liquid cryogen in the annular gap between the external conduit  301  and the central feeding conduit  304 ; this level enables the capillary material  318  to be wetted without flooding the gap. 
         [0064]    The preferred embodiment of the siphon in combination with a Dewar flask filled with a liquid cryogen preferably functions as follows. After opening the adjustable valve  312 , the level of the liquid cryogen in the gap between the external conduit  301  and the central feeding conduit  304  is elevated such that the capillary material  318  is wetted. Once sufficient cryogen has entered, the level gauge  314  is elevated to a certain level. The level of the liquid cryogen in the gap between the external conduit  301  and the central feeding conduit  304  is maintained by the control unit  317 , which closes and opens the adjustable valve  312  according to the signal provided by the induction coil  316  according to the level measured by the level gauge  314 . 
         [0065]    The temperature of the upper section of the central feeding conduit  304  is lowering to the temperature of the liquid cryogen and, after opening the shut-off valve  305 , liquid cryogen of high quality is supplied into the outer section of the central feeding conduit  304 . 
         [0066]      FIG. 4   a  and  FIG. 4   b  show an axial cross-sectional view of a preferred embodiment of a siphon with a control unit, which operates on the basis of the temperature of the gaseous-liquid mixture released from the annular gap between the external and central feeding conduits ( FIG. 4A ), and an enlarged axial cross-sectional view of the upper section of the siphon ( FIG. 4B ). 
         [0067]    This embodiment includes an external conduit  401 ; jacket  402  surrounding the upper section of the external conduit  401 , wherein the upper edge of jacket  402  is sealed with the external conduit  401 ; an annular rubber ring  403 ; a central feeding conduit  404 , wherein the upper section of its outer surface is coated with a capillary coating  416  and the upper edge of the external conduit  401  is sealed with the outer section of the central feeding conduit  404 ; a shut-off valve  405 , which is installed on the outer section of the central feeding conduit  404 ; and safety and relief valves  406  and  407 , which are installed on ports  408  and  409  of the outer section of jacket  402 . The outer section of the external conduit  401  is provided with port  410  which is provided in turn with duct  411 . There is an adjustable valve  412  installed on this duct. A pressure gauge  413 , which is installed on the outer section of jacket  402 , serves for measuring the pressure in the Dewar flask. These components correspond to similar components described with regard to  FIGS. 1-3 . 
         [0068]    The siphon in combination with a dewar flask filled with a liquid cryogen preferably operates as follows. After opening the adjustable valve  412 , the liquid cryogen in the gap between the external conduit  401  and the central feeding conduit  404  is elevated to a level which wets the capillary material  416 . The temperature of the upper section of the central feeding conduit  404  is reduced to the temperature of the liquid cryogen and, after opening the shut-off valve  405 , liquid cryogen of high quality is supplied into the outer section of the central feeding conduit  404 . The level of the liquid cryogen in the gap between the external conduit  401  and the central feeding conduit  404  is maintained by the control unit  415  through cables  418 , which closes and opens the adjustable valve  412  according to the signal provided by the temperature sensor  414  (measuring device) installed on duct  411 ; this signal is supplied to the control unit  415  through cables  417 . 
         [0069]      FIG. 5   a  and  FIG. 5   b  show an axial cross-sectional view of preferred embodiments of a system according to the present invention, featuring a Dewar flask with a siphon installed in its neck and its associated siphon hose and an enlarged axial cross-sectional view of the upper section of the siphon, and the Dewar neck. 
         [0070]    System  500  includes a Dewar flask  501  with neck  502 , further comprising an external conduit  503  and jacket  504  surrounding the upper section of the external conduit  503 . The upper edge of jacket  504  is sealed with the external conduit  503 . An annular rubber ring  505  is installed on the outer surface of jacket  504  and is partially inserted into neck  502  for sealing thereto. There is a central feeding conduit  504 ; a large fraction of the central feeding conduit is surrounded by the external conduit  503 . The upper section  520  of the outer surface of central feeding conduit  506  is preferably covered with a liquid cryogen absorbing or wettable capillary material  524 , which may optionally be a capillary coating. The upper edge of the external conduit  503  is sealed with the outer section of the central feeding conduit  506 . 
         [0071]    A shut-off valve  508  is preferably installed on the outer section of the central feeding conduit  506 , while safety and relief valves  509  and  510  are preferably installed on ports  521  and  522  of the outer section of jacket  504 . The outer section of the external conduit  503  is preferably provided with opening  511  which is provided in turn with a duct  512 , more preferably featuring an adjustable valve  513 . 
         [0072]    A pressure gauge  514  is preferably installed on the outer section of jacket  504  for measuring the internal pressure in a Dewar flask  501 . The above components are similar in function to those described above. 
         [0073]    According to preferred embodiments of the present invention, hose  523  is provided for transporting liquid cryogen from the Dewar flask  501 . 
         [0074]    Hose  523  preferably comprises: envelope  515 ; a main conduit  516 , which is in flow communication with the central feeding conduit  506 ; and an internal auxiliary conduit  517 , which is in flow communication with duct  512 . The distal end of the internal auxiliary conduit  517  is in flow communication with an outer auxiliary conduit  518 , which serves for release of the gas phase of the cryogen into the external atmosphere. The internal space of envelope  515  of hose  523  (between the other components of hose  523  as shown herein) is preferably filled with a thermo-insulating filler  519 . 
         [0075]    Upon opening the adjustable valve  513 , the level of liquid cryogen in the gap between the external conduit  503  and the central feeding conduit  506  is elevated and thereby wets the capillary material  524 . As a result, the temperature of the upper section of the central feeding conduit is reduced to the temperature of liquid cryogen and, after opening the shut-off valve  508 , liquid cryogen of high quality is supplied into the outer section of the central feeding conduit  506 . The liquid gaseous mixture of the cryogen from duct  512  enters hose  523  through the internal auxiliary conduit  517  and the outer auxiliary conduit  518 , and the gas phase is exhausted into the external atmosphere. Regulation of the adjustable valve  513  is performed manually, for example according to visual characteristics of the liquid-gaseous mixture released from the outer auxiliary conduit  518 . The main conduit  516  enables delivery of the high-quality nitrogen to a target location. 
         [0076]      FIG. 6   a  and  FIG. 6   b  show radial cross-sectional views of two exemplary illustrative implementations for the main and internal auxiliary conduits in the envelope of the hose. 
         [0077]    In a first exemplary embodiment shown in  FIG. 6   a , the main conduit  601  is preferably situated next to the internal auxiliary conduit  602  in envelope  603  and the internal space of envelope  603  is preferably filled with a thermo-insulating filler  604 . 
         [0078]    In a second exemplary embodiment shown in  FIG. 6   b , the main conduit  601  is preferably situated coaxially with respect to the internal auxiliary conduit  602  in envelope  603  and the internal space of envelope  603  is preferably filled with the thermo-insulating filler  604 . 
         [0079]      FIG. 7   a  provides an exemplary, illustrative implementation of a Dewar flask with a siphon according to the present invention, preferably featuring a compression means, a valve means and a heat exchange means intended to provide high pressure pulses of the liquid cryogen. In addition,  FIG. 7   b  shows an enlarged axial cross-sectional view of the upper section of the siphon and the Dewar neck. 
         [0080]    This exemplary embodiment comprises: a Dewar flask  701  with neck  702 . A siphon comprises an external conduit  703 ; and a jacket  704  surrounding the upper section of the external conduit  703 . The upper edge of jacket  704  is sealed with the external conduit  703 . An annular rubber ring  705  is preferably installed on the outer surface of jacket  704  for sealing with neck  702 . There is a central feeding conduit  706 . A main part of the central feeding conduit  706  is surrounded by the external conduit  703 . This central feeding conduit  706  preferably comprises an upper section  719  having an outer surface covered with an absorbent or wettable material, preferably a capillary material  707 , more preferably a capillary coating. The upper edge of the external conduit  703  is sealed with the outer section of the central feeding conduit  706 . 
         [0081]    A shut-off valve  708  is preferably installed on the outer section of the central feeding conduit  706 , while safety and relief valves  709  and  710  are preferably installed on ports of the outer section of jacket  704 . The outer section of the external conduit  703  is preferably provided with opening  711  which is provided in turn with duct  712 , more preferably featuring an adjustable valve  713 . A pressure gauge  714  is optionally and preferably installed on the outer section of jacket  704 , for measuring the internal pressure in the Dewar flask  701 . 
         [0082]    The gaseous-liquid cryogenic medium, which flows from duct  712  through pipeline  720 , is preferably pressurized by at least one and more preferably a plurality of compressors  716  and  717  arranged in sequence with pipeline  721  communicating between them. The compressed medium then preferably enters through pipeline  723  to a heat exchanger  718  of the recuperative type as it is known in the art, preferably through a controllable valve  715  and more preferably in the form of high pressure pulses. The liquid cryogen at relatively low pressure also preferably enters the heat exchanger  718  through pipeline  724 . As the result, the gaseous medium is condensing in the heat exchanger  718 , and high pressure pulses of the liquid cryogen are supplied from the output of the heat exchanger  718  through pipeline  722 . 
         [0083]      FIG. 8  shows an axial cross-sectional view of another exemplary, illustrative embodiment of a siphon according to the present invention, with thermal insulation of the upper internal section of the central feeding conduit. 
         [0084]    The siphon  800  preferably includes a central feeding conduit  801  and jacket  802  surrounding the upper section of the central feeding conduit  801 . The upper edge of jacket  802  is sealed with the central feeding conduit  801 . An annular rubber ring  803  is preferably present on the outer surface of jacket  802 . The upper edge of jacket  802  is sealed with the outer section of the central feeding conduit  801 . 
         [0085]    A shut-off valve  805  is preferably installed on the outer section of the central feeding conduit  801 , while safety and relief valves  806  and  807  are preferably installed on ports  808  and  809  of the outer section of jacket  802 . A thermal insulation  804  is installed on the outer surface of the central feeding conduit  801 . These components operate as described above. 
         [0086]      FIG. 9  shows an axial cross-sectional view of some embodiments of a siphon system according to the present invention, comprising a Dewar flask with a siphon installed in its neck; a central feeding conduit of the siphon provided with a vacuum evacuated jacket and a check valve for providing liquid cryogen of a high quality (with a minimal proportion of gas) in the form of pulses. 
         [0087]    The siphon system  900  includes: a Dewar flask  901  comprising neck  902  and a central feeding conduit  903 . Jacket  904  preferably surrounds the upper section the central feeding conduit  903 , while the upper edge of jacket  904  is sealed to the central feeding conduit  903 . Optionally and preferably, an annular rubber ring  905  is present on the outer surface of jacket  904 . 
         [0088]    Optionally and preferably, a shut-off valve  906  is installed on the outer section of the central feeding conduit  903 . Also optionally and preferably, safety and relief valves  907  and  908  are installed on ports  912  and  913  of the outer section of jacket  904 . Optionally and more preferably, a pressure gauge  909  is installed on the outer section of jacket  904 , for measuring the internal pressure in the Dewar flask  901 . 
         [0089]    The upper section of the central feeding conduit  903  is preferably provided with jacket  910  comprising an internal vacuum. Preferably, a check valve  911  is installed on the upper section of the central feeding conduit  903  in the immediate vicinity to the distal edge of jacket  910 . 
         [0090]    The system preferably operates as follows: the liquid cryogen enters through the open check valve  911  into the upper section of the central feeding conduit  903 . As the result of heat exchange with jacket  904 , the liquid cryogen starts to boil, causing an elevation of its pressure and closing the check valve  911 . This closing of the check valve  911  causes further elevation of the pressure in the upper section of the central feeding conduit  903  and accelerated propulsion of the liquid cryogen portion outwards. 
         [0091]      FIG. 10   a  and  FIG. 10   b  show an axial cross-sectional view of a siphon with a control unit, which is functioning on the base of measuring a density of the mist emitted from the port of the annular gap of the gaseous-liquid mixture released from the annular gap between the external and central feeding conduits ( FIG. 10   a ), and an enlarged axial cross-sectional view of the upper section of the siphon ( FIG. 10   b ). 
         [0092]    This embodiment includes an external conduit  1001 ; jacket  1002  surrounding the upper section of the external conduit  401 , wherein the upper edge of jacket  1002  is sealed with the external conduit  1001 ; an annular rubber ring  1003 ; a central feeding conduit  1004 , wherein the upper section of its outer surface is coated with a capillary coating  1016  and the upper edge of the external conduit  1001  is sealed with the outer section of the central feeding conduit  1004 ; a shut-off valve  1005 , which is installed on the outer section of the central feeding conduit  1004 ; and safety and relief valves  1006  and  1007 , which are installed on ports  1008  and  1009  of the outer section of jacket  1002 . The outer section of the external conduit  1001  is provided with port  1010  which is provided in turn with duct  1011 . There is an adjustable valve  1012  installed on this duct. A pressure gauge  1013 , which is installed on the outer section of jacket  1002 , serves for measuring the pressure in the Dewar flask. These components correspond to similar components described with regard to  FIGS. 1-3 . 
         [0093]    The siphon in combination with a dewar flask filled with a liquid cryogen preferably operates as follows. After opening the adjustable valve  1012 , the liquid cryogen in the gap between the external conduit  1001  and the central feeding conduit  1004  is elevated to a level which wets the capillary material  1016 . The temperature of the upper section of the central feeding conduit  1004  is reduced to the temperature of the liquid cryogen and, after opening the shut-off valve  1005 , liquid cryogen of high quality is supplied into the outer section of the central feeding conduit  1004 . The level of the liquid cryogen in the gap between the external conduit  1001  and the central feeding conduit  1004  is maintained by the control unit  1015  through cables  1018 , which closes and opens the adjustable valve  1012  according to the signal provided by a density sensor  1014  (measuring device) installed on duct  1011 ; this signal is supplied to the control unit  1015  through cables  1017 . 
         [0094]      FIG. 11   a  and  FIG. 11   b  show an axial cross-section of another optional embodiment of a Dewar flask with a siphon installed in its neck ( FIG. 11   a ) and an enlarged axial cross-sectional view of the upper section of the siphon ( FIG. 11   b ), featuring a low inertia temperature sensor, an electrical heater installed in the central feeding conduit and a control-power unit, which generates pulses of electrical current. 
         [0095]    The siphon system  1100  includes: a Dewar flask  1101  comprising neck  1102  and a central feeding conduit  1103 . Jacket  1104  preferably surrounds the upper section the central feeding conduit  1103 , while the upper edge of jacket  1104  is sealed to the central feeding conduit  1103 . Optionally and preferably, an annular rubber ring  1105  is present on the outer surface of jacket  1104 . 
         [0096]    Optionally and preferably, a shut-off valve  1106  is installed on the outer section of the central feeding conduit  1103 . Also optionally and preferably, safety and relief valves  1107  and  1108  are installed on ports  1112  and  1113  of the outer section of jacket  1104 . Optionally and more preferably, a pressure gauge  1109  is installed on the outer section of jacket  1104 , for measuring the internal pressure in the Dewar flask  1101 . 
         [0097]    The upper section of the central feeding conduit  1103  is preferably provided with jacket  1110  comprising an internal vacuum. Preferably, a check valve  1111  is installed on the upper section of the central feeding conduit  1103  in the immediately after the check valve  1111 . There is a low inertia electrical heater  1115  installed immediately after the check valve  1111 . A low inertia temperature sensor  1114  is preferably installed in the central feeding conduit  1103 . Delivery of a portion of the liquid cryogen via the check valve  1111  lowers the temperature measured by low inertia thermocouple  1114  (as an example of a temperature measuring device), which sends a signal via cables  1118  into a control-power unit  1116 . This control-power unit  1116  preferably generates a pulse of electrical current, which is provided via cable  1117  to the low inertia electrical heater  1115 , thereby causing the liquid cryogen to boil, preferably through flash boiling, followed by a sharp elevation of its pressure. As a result, the check valve  1111  closes and the high pressure portion of the liquid-gaseous cryogen is emitted. 
         [0098]    While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made and still be within the spirit and scope of the invention.