Self-pressurizing cryogenic apparatus and method
This application is a continuation of my application for U.S. Letters Patent Ser. No. 936,909, filed Aug. 25, 1978, entitled SELF-PRESSURIZING CRYOGENIC APPARATUS AND METHOD, now abandoned, which application is a division of my application for U.S. Letters Patent. Ser. No. 688,962, filed May 24, 1976, now abandoned. 
    
    
     This invention relates to cryogenic apparatus and more particularly to simply constructed, hand-held means for dispensing a jet of cryogenic coolant onto an area to be necrotized from a self-pressurizing source of heat saturated liquid coolant having a boiling point below -100° C. 
     BACKGROUND OF THE INVENTION 
     Equipment for dispensing a cryogenic liquefied gas onto a surface or object to be sharply cooled has been proposed heretofore. These are designed to utilize liquefied gas such as helium, nitrogen, oxygen, air, carbon dioxide, etc., stored in a suitable container. The liquid coolant is supplied therefrom to a dispensing port or nozzle in various ways including applying heat electrically or otherwise to the coolant to vaporize the liquid. Other designers pressurize the storage container to produce coolant flow by introducing air under pressure into the storage container while other designers provide the storage container with a normally open vent which is closed to initiate coolant flow whereby the vapor pressure which develops within the container produces a flow of coolant to the dispensing nozzle. The pressure build-up is slow and uncertain and varies widely depending on the volume of the vapor space and the quantity of liquid coolant in the storage chamber, the rate of heat leakage into the container and other factors. 
     One category of prior cryogenic devices dispenses a jet of coolant vapor directly onto the area to be cooled whereas those of another category confine the stream of coolant vapor to a flow passage typically provided with a return bend sector formed of excellent heat conductive material and venting to the atmosphere at a remote point. The heat conducting member can be placed close to or in contact with the surface to be cooled without risking direct contact of the gas coolant with that surface. 
     It has been recognized that it would be advantageous to deliver coolant in liquid phase onto or in close proximity to the area to be sharply cooled in order to utilize the latent heat or vaporization of the coolant as it changes to the gas phase. However, this presents numerous serious problems which have not been resolved prior to this invention. Foremost among these problems is control of the liquid coolant delivered and the amount of cooling provided. It is manifestly not feasible to deliver even a minute stream of liquid coolant onto living tissue. Not only is the cooling capacity of liquid coolant very great but, upon entering the atmosphere, becomes rapidly superheated with resultant vigorous boiling and uncontrolled dispersal. 
     Attempts have been made to create a spray of liquefied coolant particles of which can be dispensed directly onto the area to be necrotized. However, the equipment heretofore provided for this purpose is subject to many shortcomings and disadvantages including instability and erratic coolant flow, inability to form a stable, small diameter jet, fluctuating spurts of the coolant jet, time delay and waste involved in establishing a coolant jet, continuing change in the rate of flow, the highly inefficient use of a given charge of liquid coolant, and the need for recharging a hand-held reservoir several times a day. 
     SUMMARY OF THE INVENTION 
     To avoid the numerous shortcomings and disadvantages of prior cryogenic equipment, there is provided by this invention a self-pressurizing, self-stabilizing source of heat saturated liquid coolant. This coolant may be stored either in a stationary large capacity reservoir or a readily portable hand-held dewar flask. Such a flask having a capacity of approximately one pint of liquid is found adequate for seven to eight hours of normal usage without recharging, yet is so small and lightweight to be grasped in the hand and readily manoeuvred to meet operating requirements. The liquid coolant is automatically maintained at a predetermined uniform pressure by a pressure relief valve and, in consequence, the liquid is in continuous readiness for dispensing in a stable heat saturated condition. When open, a coolant flow control valve permits the coolant to flow into an expansion chamber equipped with a relatively long small bore outlet nozzle or orifice and normally open venting orifice cooperating with the outlet or coolant jetting orifice to limit the pressure differential across the flow control valve to a fraction of the pressure in the supply chamber. This pressure differential greatly limits the flashing of coolant into vapor. Additionally, and by properly proportioning the sizes of the outlet and venting orifices and the regulation of the latter, the invention apparatus is instantly self-stabilizing and operable to dispense either a needle-like jet of coolant in a dual phase of fine liquid particles and gas, or a jet of gaseous phase coolant depending upon whether gaseous coolant is vented from the venting orifice. The increased velocity imparted to the jet owing to the flashing of some superheated liquid into gas aids very substantially in increasing the &#34;chill-factor&#34; and thereby the effectiveness and efficiency of the device. For example, when using liquid nitrogen as the coolant, the temperature of this coolant in gas phase when dispensed from the nozzle of this invention is somewhat about 32° F. whereas the temperature of the issuing jet of dual phase coolant containing superheated liquid coolant is at least -300° F. Further lowering of the temperature known as the &#34;chill-factor&#34; results as the rapidly flowing jet of particles of liquid coolant flash into the gaseous phase. 
     If the venting passage is open a portion of the coolant which is flashed into gas is bled from the expansion chamber and the resulting small drop of pressure in the expansion chamber converts the jet discharge substantially instantly from single to dual phase constituency without need for adjusting the flow control valve. Likewise, the jet discharge is converted instantly back to single phase by closing the venting orifice. Partial closing of the venting orifice merely varies the relative proportions of gas and liquid in the jet. 
     The stability of the device is enhanced by insulating the liquid flow tube extending between the coolant supply and the expansion chamber. This avoids re-liquefication of a portion of the coolant vapor in the upper portion of the dewar during a dispensing cycle where a dispensing operation occurs before a new charge of coolant has become fully heat saturated. 
     Accordingly, it is a primary object of this invention to provide a self-pressurizing, self-stabilizing cryogenic device selectively operable to dispense a jet of single-phase or dual phase coolant having a boiling point below -100° C. 
     Another object of the invention is the provision of a unique method and apparatus for dispensing a jet of cryogenic coolant from a heat saturated uniformly pressurized source thereof and which is quickly convertible between single phase gaseous constituency and dual phase constituency. 
     Another object of the invention is to provide a self-contained hand-held cryogenic device which is instantly self-stabilizing to dispense a jet of finely divided liquid coolant. 
     Another object of the invention is the provision of a cryogenic device for selectively dispensing a stabilized jet of coolant in either single gaseous phase at a temperature above freezing or in a dual phase at a temperature at least as low as -300° F. 
     Another object of the invention is the provision of a self-pressurized cryosurgery device which is instantly self-stabilizing and effective to dispense a continuous non-varying coolant jet or a readily varied jet of either single or dual phase coolant. 
     Another object of the invention is the provision of a self-pressurized, self-stabilizing cryogenic device having means for varying the phase consistency of a jet of dispensed coolant by controlling a venting orifice or bleed passage in communication with the space at the entrance end of the jet dispensing orifice. 
     These and other more specific objects will appear upon reading the following specification and claims and upon considering in connection therewith the attached drawing to which they relate. 
    
    
     Referring now to the drawing in which a preferred embodiment of the invention is illustrated: 
     FIG. 1 is an elevational view partly in cross-section of one illustrative embodiment of the invention; 
     FIG. 2 is a fragmentary cross-sectional view on an enlarged scale taken through the upper portion of FIG. 1; 
     FIG. 3 is a fragmentary view partly in cross-section of the control valve for the venting orifice; and 
     FIG. 4 is a graphical representation of the pressure conditions existing in the expansion chamber for various positions of the throttle valve, the uppermost horizontal line representing the constant coolant reservoir pressure, the middle curve representing the pressure when the venting valve is closed, and the lower curve representing the pressure condition when the venting valve is opened. 
    
    
     Referring initially more particularly to FIG. 1, there is shown an exemplary embodiment of the invention cryogenic device, designated generally 10, when utilizing a dewar flask 11 as a source of heat saturated liquid coolant or cryogen 12. A suitably widely used coolant comprises liquefied nitrogen although it will be understood that any of a considerable number of other liquefied gases having a boiling point of -100° C. or lower, are suitable and may be used in practicing the principles of this invention. Dewar 11 is of generally conventional construction including a stainless steel inner container 13 sealed at its charging inlet 14 to a stainless steel outer evacuated container 15. The tubular neck 16 of the outer container is provided with threads 17 mateable with the threads of a heat insulating lining 18 of the dewar cover or cap 19. 
     The dispensing and control components of the cryogenic device will be best understood by reference to FIGS. 2 and 3. These components include a main body fitting 20 having a threaded tubular shank projecting downwardly from its lower side and extending through an opening in the top of the dewar cap 19. A threaded bushing 22 is screwed to the lower end of shank 21 and serves to hold the latter firmly assembled to cap 19 along with a thick-walled elastomeric stopper 24 normally closing the tubular dewar charging inlet or opening 25. As is best shown in FIG. 1, stopper 24 has a shoulder seating against the outer rim edge of this inlet. Clamped between the lower end of main body 20 and the top of cap 19 is a curvilinear guard 27 effective to safeguard the user&#39;s fingers from contact with the very cold components of the device when in use. Guard 27 may be made of poor heat conductive material, such as stainless steel and includes an upturned tang 28 (FIGS. 1, 3) extending along one sidewall of main body 20. 
     Supported on the side of main body 20 is a precision valve 30 having a rotary control knob 31 for controlling the position of a throttle or needle valve 32 normally closed against a seat 33. This valve controls the flow of liquid coolant into a tubular expansion housing 35 projecting laterally from the top of main body 20. The lower end of passage 34 is in communication with a tube 36 which is silver-soldered to main body 20 and to a tube 38 extending into close proximity to the bottom of the coolant container 13. Tube 38 is thin-walled brass or stainless steel to present as small a conductive heat path as is practical into the liquid coolant and to assure that failure or breakage of the cryogen dispensing tubes will occur in tube 38 rather than in the inner end of tube 36. This junction between tubes 36 and 38 may be silver-soldered and is located below the threaded tubular extension 21 to simplify replacement and repair of the relatively weak tube 38 should the latter become damaged when cap 19 is disassembled for charging or otherwise. Each of the tubes 36 and 38 is preferably covered with heat insulating material 39, 40 to aid in stabilizing the operation of the apparatus by preventing the flow of liquid coolant through these tubes to cool and condense portions of the coolant vapor present above the liquid level. This could interfere with maintaining a uniform pressure head on the liquid while valve 32 is open and could cause unstable conditions in the jet of coolant before the coolant becomes fully heat saturated. 
     The dewar is automatically maintained at a predetermined superatmospheric pressure by the vapor pressure of the coolant supply as by a pressure relief valve 43 mounted in the side of main body 20 in communication through passage 44 with the interior of the tubular extension or shank 21. The valve proper 45 of the relief valve is normally held seated against the outlet end of passage 34 by a calibrated spring, not shown, but housed within the valve body. An extension 46 protruding from this valve is readily manipulatable by the operator&#39;s finger to unseat the valve and release the pressure in the dewar whenever this is desirable, as before removing cap 19 to renew the supply of coolant 12. 
     Permanently assembled to the outer end of expansion chamber 35 is a Luer fitting 50 of well known construction providing a readily disconnectable coupling for the shank fitting 51 of a non-pointed or blunt-ended hypodermic needle 52 providing an elongated small bore nozzle or outlet orifice for chamber 35. It will be understood that other outlet orifice constructions can be employed but readily available hypodermic needles having bores ranging in size from 22 to 25 guage have been found highly satisfactory and are quickly and easily substituted for one another to vary the size of the coolant jet dispensed from the expansion chamber. A 22 guage needle has a bore diameter of approximately 17 mils as compared to the 11 mil bore of a guage 25 needle. Needle lengths of 1/4&#34; to 3/4&#34; are found very satisfactory for use with outlet orifices ranging in size from 11 to 17 mils in diameter. The length of the needle varies with the size of the bleeder orifice. A longer needle necessitates bleeding a greater volume of gaseous phase coolant from the expansion chamber. 
     A very important further feature of the invention is the provision of means for venting coolant in gas phase from chamber 35 thereby to vary the pressure in chamber 35 relative to the substantially predetermined pressure in the dewar flask without adjusting the flow control valve 30. As herein shown by way of example, this venting means is provided by tube 54 extending along the interior of chamber 35 with its right-hand end in communication with a passage 55 formed interiorly of body 20. This passage is shown in dot and dash lines and includes a vertical leg and a horizontal leg the latter of which opens to the atmosphere on the left-hand side of the main body 20 (FIG. 3). Normally, the outlet end of passage 55 is closed by a valve 56 mounted on one end of an L-shaped operating lever 57 of poor heat conductive material such as stainless steel. Lever 57 is supported on pivot pins 58 carried by a bracket 59 secured to the side of body 20, and is spring biased by spring 60 to seat the valve 56 against the outlet end of passage 55. 
     To avoid tolerance problems in the manufacture of the venting orifice or passage, it is found convenient to provide this passage with a loose packing or restrictor to limit the flow of coolant gas allowed to escape to the atmosphere. A very satisfactory restrictor comprises a mass of brass wool or the like here shown as located at the junction of the vertical and horizontal legs of passage 55. Access to this mass 63 is provided by a closure cover 64 to the end that the density of this restrictor may be readily adjusted at the time of manufacture. Thereafter, closure plug 64 may be suitably secured in place since no further adjustment is required so long as the same size pressure relief valve 43 and the same range of sizes of outlet orifice needles 52 are employed. 
     Referring now to FIG. 4, there is shown a graph depicting typical operating conditions for the cryogenic device herein described when using liquid nitrogen as the coolant and a pressure relief valve calibrated to maintain a pressure of 10 psig. It will be understood that this pressure is merely representative of a pressure found to provide excellent operating characteristics when using outlet passages of the range of sizes described above. This constant pressure condition is represented by the straight horizontal line 65 in FIG. 4. The next lower curve 66 represents the pressure condition within expansion chamber 35 when a 22 guage needle 52 is employed as the jetting orifice as needle valve 32 is opened from its closed to its open position. Curve 67 shows the slightly lower pressure condition created in the expansion chamber when the control lever 57 is depressed to open valve 56 normally closing the venting passage or orifice 55. 
     Normally the dewar flask is charged with liquid nitrogen and needle valve 32 and venting valve 56 are closed. With the pressure relief valve 43 set, for example, to maintain the pressure within the flask at 10 psig, the coolant will be heat saturated and have a temperature of -312° F. As soon as control knob 31 is operated to open needle valve 32, liquid coolant will rise through conduit 38 and flow past the needle valve whereupon a portion of it will quickly become super-heated as it experiences a pressure drop in flowing past the needle valve into the relatively warm expansion chamber 35. The resulting violent boiling or flashing of the coolant fractionates the liquid into a multiplicity of fine minute particles as other portions convert to the gaseous phase, thereby almost instantly raising the back pressure in the expansion chamber to a value somewhat in excess of one pound less than the internal dewar pressure. Coolant in substantially pure gaseous phase then jets at high velocity from the jetting orifice. A layer of frost also quickly collects on the outer surface of the expansion chamber 35 thereby providing an excellent heat insulating layer for this chamber which greatly minimizes the quantity of coolant going into its gas phase. The cryosurgery device may now be grasped in the operator&#39;s hand and manipulated to direct the jet of gaseous coolant onto an area to be sharply cooled. 
     If the operator wishes to convert the jet to one comprised of finely divided particles of coolant in liquid phase intermixed with gaseous phase coolant, he simply depresses lever 57 to open vent valve 56. Immediately that this takes place the jet converts to a high velocity small diameter jet of dual phase coolant. The conversion occurs substantially instantly, smoothly and in a highly stable manner without need for making any adjustments or change in the position of the needle valve. 
     Opening of the venting valve is observed to allow a small amount of gaseous phase coolant to be bled from the forward end of the expansion chamber and this is accompanied by a slight drop in the expansion chamber pressure as is clearly evident from a comparison of curves 66 and 67 in FIG. 4. Under the operating conditions depicted, this pressure drop is a small fraction of 1 psi for all positions of throttle valve 32 corresponding to half to fully open position. The volume of coolant issuing from the venting passage 55 is relatively small and completely in gas phase. So long as the venting valve 56 remains open the device continues to operate in the same uniform stable manner irrespective of the position in which the device is held or the manner in which it is manipulated. The coolant jet may be directed at will by the operator in either vertical direction or in any intermediate direction between vertical and horizontal without change in the characteristics of the jet. 
     Upon release of the pressure on the operating handle 57 of valve 56, this valve recloses cutting off the discharge of gaseous coolant from the venting orifice 55 whereupon the device instantly resumes jetting of gas phase coolant through the jetting orifice. There is no need to make any adjustment of the needle valve. 
     If the operator wishes to change the size and flow rate of the jet he simply closes valve 32 and detaches needle 51, 52 from the Luer fitting 50 and substitutes a needle of different size. 
     When the coolant dispenser is not in use, valves 32 and 56 remain closed. Relief valve 43 opens only momentarily at intervals as necessary to maintain the internal pressure of the dewar constant with the result that coolant losses are very small between operating cycles. For example, it is found that a dewar having a charge capacity of approximately one pint is adequeate for seven to eight hours of normal usage, or many times longer than prior cryogenic dispensers of similar storage capacity. 
     While the particular self-pressurizing cryogenic apparatus and method herein shown and disclosed in detail is fully capable of attaining the objects and providing the advantages hereinbefore stated, it is to be understood that it is merely illustrative of the presently preferred embodiment of the invention and that no limitations are intended to the detail of construction or design herein shown other than as defined in the appended claims.