reference will now be made in detail to embodiment ( s ) of the present invention , examples of which is / are illustrated in the accompanying drawings , wherein like reference numerals refer to the like elements throughout . the embodiment ( s ) is / are described below to explain the present invention by referring to the figures . although the following text sets forth a detailed description of at least one embodiment or implementation , it is to be understood that the legal scope of protection of this application is defined by the words of the claims set forth at the end of this disclosure . the detailed description is to be construed as exemplary only and does not describe every possible embodiment since describing every possible embodiment would be impractical , if not impossible . numerous alternative embodiments and / or implementations are both contemplated and possible , using either current technology or technology developed after the filing date of this patent , which would still fall within the scope of the claims . it is to be understood that , unless a term is expressly defined in this application using the sentence “ as used herein , the term ” is hereby defined to mean . . . ” or a similar sentence , there is no intent to limit the meaning of that term , either expressly or by implication , beyond its plain or ordinary meaning , and such term should not be interpreted to be limited in scope based on any statement made in any section of this patent ( other than the language of the claims ). to the extent that any term recited in the claims at the end of this patent is referred to in this patent in a manner consistent with a single meaning , that is done for sake of clarity only so as to not confuse the reader , and it is not intended that such claim term is limited , by implication or otherwise , to that single meaning . finally , unless a claim element is defined by reciting the word “ means ” and a function without the recital of any structure , it is not intended that the scope of any claim element be interpreted based on the application of 35 u . s . c . § 112 , sixth paragraph . as used herein , the term centrifugal force refers to the tendency of a flow following a curved path to be urged away from the center of curvature due to inertia . centrifugal force is referred to as a force for convenience and ease of explanation . this centrifugal force urges the liquid phase of two - phase flow following a curved path to be urged away from the center of curvature due to greater specific gravity than the gaseous phase . referring to fig1 a and 1b , there is shown a non - limiting example of a cryosurgical instrument comprising a heat exchanger that features multiple helical coils , which is consistent with an exemplary embodiment of the present invention . as shown , the cryosurgical instrument 100 includes a shaft 103 surrounding an inlet 102 , a core 116 , a coiled ( i . e . helical ) cryogen delivery tube 101 , and a tip 109 . a rear portion of the instrument 100 includes insulation 104 that separates the inner surface of the shaft 103 from a return flow 113 of cryogen . the portion of the cryosurgical instrument from the tip 109 to the insulation 104 defines a cooling zone 105 . this cooling zone is where heat is exchanged between the cryogen and the cryoprobe . thus , the freezing ( ablation ) of tissue occurs around the cooling zone 105 , when the instrument is in operation and in contact with tissue . the inlet 102 receives cryogen and delivers the received cryogen into the shaft 103 . as explained above , the shaft 103 features insulation 104 that surrounds the shaft 103 and is mounted to the inner surface of the shaft 103 . the insulation 104 is disposed outside of the cooling ( heat exchange ) zone 105 . the presence of the insulation 104 ensures that freezing occurs only where treatment is desired , which is generally around the cooling zone 105 . the core 116 is disposed lengthwise along a longitudinal or lengthwise axis of the cryosurgical instrument . for this reason , it is preferable that the core has a substantially cylindrical shape with a substantially circular cross - section . it is to be understood , that the core may have other cross - sectional shapes . in operation , cryogen enters through the inlet 102 , as indicated by arrows 114 , and is directed to coil 101 . this inflowing of cryogen then enters the coil 101 and flows in the helical coil such that it spirals around the core 116 as it travels toward the tip 109 . this spinning of the inflowing cryogen causes the cryogen to at least partially separate into a liquid phase and a gaseous phase , with the heavier liquid phase tending to displace the gaseous phase along the inner surface of the coil 101 . the return flow , indicated by arrows 115 , flows in the spiral gap created by the outside surface of the coil 101 , the inner surface of the shaft 103 , and the outer surface of the core 116 . the relationship between the coil 101 and the core 116 , and the cooling zone is illustrated in fig1 b . as cryogen exits the coil 101 , it is reflected against a reflective surface 108 of the tip 109 . in the illustrated embodiment , the tip 109 is hollow , and the reflective surface 108 is optionally placed close to the inner surface of tip 109 , as shown . it is to be understood , however , that the tip 109 need not be hollow and / or the reflective surface 108 may be placed distal to the inner surface of the tip 109 ( not shown ). the inflowing cryogen ( indicated by arrows 114 ) boils at the inner side of the reflective surface 108 and in a return flow gap 152 between the coiled tubes of the coil 101 , the core 116 , and the inner surface of the outer shaft 103 . this volume defined by the reflective surface 105 , the distal end of the core 116 and the return flow gap 152 represents an expansion chamber . the boiling that occurs in the expansion chamber cools the cooling zone 105 as heat energy is absorbed by the expanding cryogen . after being reflected by the reflective surface 108 , the exhausted cryogen flows through the gap 152 between the heat exchanger 101 and the shaft 103 , as shown . the curved gap 152 enhances the continued boiling of the liquid phase close to the external wall , as described before . the direction of this return flow is shown by return flow arrows 115 . the cryosurgical instrument 100 may optionally include a solid core 116 at the innermost part of the shaft 103 and in cooling zone 105 , such that neither the coil 101 nor the gap 152 are present within the core 116 . instead , both the coil 101 and the gap 152 are preferably arranged around the core 116 as shown . however , as previously described , heat exchange between the inlet and the outlet flow is negligible in this arrangement , due to the negligible temperature difference between the respective flows . the return flow of exhausted cryogen leaves the cooling zone 105 and continues as the return flow 113 , as shown , through a return plenum 111 and into a return gap 107 . the exhausted cryogen then exits through a cryogen exhaust outlet 153 at an end of the cryosurgical instrument 100 distal from the tip 109 . the return flow is preferably permitted only between the inlet tube of the coil 101 and the inner surface of the shaft 103 at the cooling zone 105 , by providing a core 116 that prevents the return flow from returning through the inner gap of the cryosurgical instrument 100 ( i . e ., the inlet and the return flow are centrifugal ). flow in the center of the helical coil ( defined by the coils of the coil 1014 ) would have been straight . as the foregoing detailed description illustrates , an aspect of the present invention yields a heat exchanger that advantageously uses centrifugal force to separate an exhaust flow of cryogen into two phases by spinning a flow of cryogen . to do so , the exhaust flow is directed away from the expansion chamber in a spiraling manner between the core and the inner surface of the shaft . this spiraling urges the heavier liquid phase of the exhaust cryogen against the inner surface of the shaft , which encourages enhanced cooling in the cooling zone since the liquid phase has a higher heat capacity ( i . e . thermal capacity ) than the gaseous phase . additionally , the incoming flow of cryogen , which is delivered via a helical coil , also spins and tends to separate into liquid and gaseous phases . as the foregoing also illustrates , the spiraling flow of the exhausted cryogen is achieved by the cooperation of a solid core element extending along a longitudinal axis of the heat exchanger and a cryogen delivery tube that spirals around and contacts the solid core element . in particular , the spirals of the cryogen delivery tube are respectively spaced from each other and in fluid tight contact with both ( 1 ) the solid core and ( 2 ) the inner surface of the shaft , so as to form a spiraling cryogen exhaust pathway from the distal end of the shaft to the proximal end of the shaft . still further , it is to be appreciated that the phase separation that occurs may occur during cryogen delivery and / or during cryogen exhaust . it may be partial , substantially complete , or complete . indeed , it may be partial during delivery and completed during exhaust . also , it is to be appreciated that the amount of phase separation during delivery may be the same or different than the amount of phase separation during exhaust . examples of various features / aspects / components / operations have been provided to facilitate understanding of the disclosed embodiments of the present invention . in addition , various preferences have been discussed to facilitate understanding of the disclosed embodiments of the present invention . it is to be understood that all examples and preferences disclosed herein are intended to be non - limiting . although selected embodiments of the present invention have been shown and described individually , it is to be understood that at least aspects of the described embodiments may be combined . although selected embodiments of the present invention have been shown and described , it is to be understood the present invention is not limited to the described embodiments . instead , it is to be appreciated that changes may be made to these embodiments without departing from the principles and spirit of the invention , the scope of which is defined by the claims and the equivalents thereof .
a cryosurgical instrument including : a shaft having a closed distal end defining an expansion chamber and a proximal end that receives an inflow of cryogen and to exhaust a flow of expanded cryogen ; and a heat exchanger with a cryogen delivery tube that spirals around a core disposed along a longitudinal axis thereof . the cryogen delivery tube , where it spirals , is in fluid tight contact with the inner surface of the shaft so as to form spiraling cryogen exhaust pathways that extend along a portion of a length of the cryosurgical instrument from the distal end of the shaft . the cryogen delivery tube forces a two - phase flow of cryogen to undergo phase separation by centrifugal forces and urges the liquid phase against the external walls of the spiral coil and the outer surface of the spiral gap . heat exchanging occurs close to the external wall of the instrument .