Abstract:
A thermal transfer system for a pressure vessel adapted for use in an underwater environment includes a housing having a bore adapted to receive a heat pipe. Embodiments of the housing include a mounting flange for mounting the housing to the pressure vessel, a radially extending profile to enhance thermal transfer between the housing and the underwater environment, and an aperture in fluidic communication with the bore. A method of providing thermal transfer between an interior of the pressure vessel and the underwater environment includes inserting the heat pipe into the bore, sealing a distal end of the bore, and mounting the housing to the pressure vessel.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to and the benefit of, and incorporates herein by reference in its entirety, U.S. Provisional Patent Application No. 61/750,892, which was filed on Jan. 10, 2013. 
     
    
     TECHNICAL FIELD 
       [0002]    In various embodiments, the invention relates to heat transfer systems and, more particularly, to devices and methods for cooling pressure vessels in underwater environments. 
       BACKGROUND 
       [0003]    Oceanographic systems for use in underwater environments often include electronic components, housed within pressure vessels, that generate heat and may cause the temperature inside the pressure vessels to become excessive. Cooling devices may be needed for removing heat from the pressure vessels, to extend longevity and efficiency of the electronic components and/or prevent undesired drifts in instrumentation. 
         [0004]    The need for cooling of electronic components in oceanographic systems is particularly significant for large, all-electric, remotely operated vehicles (ROVs) and cabled sea-floor observatories, which may utilize high power and telemetry electronics. Although power-conversion devices have become smaller and more efficient, the overall heat load per unit volume within pressure vessels has increased dramatically. From a heat transfer standpoint, the challenge is to remove a sufficient amount of heat from the pressure vessels despite the limited heat transfer surface area available within the pressure vessels. 
         [0005]    Existing devices and methods for transferring heat from pressure vessels in underwater environments include dry contact methods, forced water cooling, and oil immersion techniques. Unfortunately, each of these existing approaches has its drawbacks. For example, in many instances, the surface area available for heat transfer within the vessel may be inadequate for dry contact and oil immersion methods. Further, forced water cooling methods require moving parts (e.g., a pump) and are ineffective when the moving parts fail and/or when an energy source for the moving parts is not available. 
         [0006]    There is a need for improved heat transfer systems for pressure vessels in underwater environments. In particular, needs exist for cooling systems that occupy minimal space within the pressure vessels, operate passively (e.g., no mechanical moving parts), and achieve sufficient heat transfer rates to meet current and future demands. 
       SUMMARY OF THE INVENTION 
       [0007]    In general, embodiments of the present invention feature devices and methods for removing heat from pressure vessels in underwater environments. The devices and methods achieve high rates of heat transfer from the pressure vessels while occupying minimal space within the pressure vessels. The devices and methods also operate passively, without the use of mechanical moving parts, making the devices less susceptible to failure than previous devices, with little or no need for maintenance. Passive operation of the devices also eliminates the need for a separate energy source (e.g., a battery) for operating the device. The devices and methods may instead be driven entirely by a temperature difference between the inside of the pressure vessel and the underwater environment outside of the pressure vessel. 
         [0008]    In one aspect, embodiments of the invention relate to a thermal transfer system for a pressure vessel adapted for use in an underwater environment. The thermal transfer system includes a housing (e.g., an elongate housing) forming a bore adapted to receive a heat pipe (e.g., in a close sliding fit, a braze fit, and/or a shrink fit). The housing includes: a mounting flange or connection member disposed at a proximal end of the housing for mounting the housing to and aligning the bore with an aperture formed in the pressure vessel; a radially extending profile disposed at least partially along an intermediate portion of the housing, the profile adapted to enhance thermal transfer between the housing and the underwater environment; and an aperture formed in a distal end of the housing in fluidic communication with the bore. 
         [0009]    In certain embodiments, the pressure vessel forms at least a portion of an underwater vehicle and/or a moored component. The heat pipe may include, for example, a sealed housing including a working fluid disposed therein. In one embodiment, the mounting flange or connection member forms a series of apertures defining a bolt hole pattern for receiving fasteners therethrough for affixing the flange to the pressure vessel. The mounting flange may also form a circumferential groove on a radial face thereof adapted to receive a gland seal for sealing the flange to an exterior surface of the pressure vessel. A gland seal may be disposed in the groove. 
         [0010]    In some embodiments, the proximal end of the housing includes an extension adapted to be received in the pressure vessel aperture. The extension may form a circumferential groove on an outer surface thereof adapted to receive a gland seal for sealing the extension in the pressure vessel aperture. A gland seal may be disposed in the groove. The proximal end of the housing (e.g., the connection member) may include a threaded portion (e.g., for securing the housing in a threaded opening). In one embodiment, the radially extending profile includes a plurality of fins, which may be disposed from the mounting flange or connection member to the distal end of the housing. In various embodiments, the bore includes the distal end aperture. The thermal transfer system may also include a faceplate removably affixed to the distal end of the housing to seal the distal end aperture. In one embodiment, the faceplate forms a series of apertures defining a bolt hole pattern for receiving fasteners therethrough for affixing the cap or faceplate to the distal end of the housing. 
         [0011]    In certain embodiments, the faceplate and/or the distal end of the housing form a circumferential groove on a respective radial face thereof adapted to receive a gland seal for sealing the cap or faceplate to the distal end of the housing. A gland seal may be disposed in the groove. In various embodiments, the thermal transfer system also includes the heat pipe, which may include at least one bend. The thermal transfer system may also include a component (e.g., a heated component and/or a heat generating component) in thermal communication with the heat pipe. In some embodiments, the thermal transfer system includes a heat transfer device in thermal communication with the heat pipe and including at least one fin. A fan may be in fluidic communication with the heat transfer device (e.g., by blowing air on the at least one fin). In one embodiment, the thermal transfer system includes the pressure vessel. 
         [0012]    In another aspect, embodiments of the invention relate to a method of providing thermal transfer between an interior of a pressure vessel adapted for use in an underwater environment and the underwater environment. The method includes the steps of: inserting a heat pipe into a bore formed in a housing (e.g., an elongate housing), the bore extending from a proximal end to a distal end of the housing; thereafter, sealing the distal end of the bore; and mounting the proximal end of the housing to the pressure vessel so that an exposed portion of the heat pipe extends into the interior of the pressure vessel. 
         [0013]    In certain embodiments, the pressure vessel forms at least a portion of an underwater vehicle and a moored component. The housing bore is preferably sized to receive the heat pipe in a close sliding fit, a braze fit, and/or a shrink fit. The method may also include the step of inserting a thermal compound into the bore to enhance thermal transfer between the heat pipe and the housing when there exists the close sliding fit. The heat pipe may include or consist essentially of a sealed housing including a working fluid disposed therein. In one embodiment, the mounting step includes bolting the housing to the pressure vessel. 
         [0014]    In various embodiments, the method also includes the step of sealing the housing to the pressure vessel. The distal end of the housing may include a thermal transfer system or radially extending profile, which may include a plurality of fins. The sealing step may include attaching a faceplate to the distal end of the housing. For example, attaching the faceplate may include bolting the faceplate to the distal end of the housing. The faceplate may also be sealed to the distal end of the housing. In one embodiment, the method also includes the step of mounting a component (e.g., a heated component and/or a heat generating component) in thermal communication with the exposed portion of the heat pipe. 
         [0015]    The method may also include the step of mounting a heat transfer device to the exposed portion of the heat pipe, wherein the heat transfer device includes at least one fin. In one implementation, the method includes blowing an internal pressure vessel fluid (e.g., air) on the heat transfer device to achieve convective heat transfer between the internal pressure vessel fluid and the heat transfer device. At least one bend may be introduced to the heat pipe. 
         [0016]    These and other objects, along with advantages and features of embodiments of the present invention herein disclosed, will become more apparent through reference to the following description, the figures, and the claims. Furthermore, it is to be understood that the features of the various embodiments described herein are not mutually exclusive and can exist in various combinations and permutations. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which: 
           [0018]      FIG. 1  is a schematic, cross-sectional view of a pressure vessel and a cooling device, in accordance with an illustrative embodiment of the invention; 
           [0019]      FIG. 2  is a schematic, exploded view of a cooling device and heat generating components, in accordance with an illustrative embodiment of the invention; 
           [0020]      FIG. 3  is a schematic, cross-sectional view of a cooling device having a heat pipe, in accordance with an illustrative embodiment of the invention; 
           [0021]      FIG. 4  is a schematic, cross-sectional view of a cooling device having two heat pipes, in accordance with an illustrative embodiment of the invention; 
           [0022]      FIG. 5  is a schematic, perspective view of four cooling devices mounted to an endcap for a pressure vessel, in accordance with an illustrative embodiment of the invention; 
           [0023]      FIG. 6  is a schematic, side view of a cooling device having an internal flow path for a cooling fluid, in accordance with an illustrative embodiment of the invention; 
           [0024]      FIG. 7  is a schematic, side view of a cooling device with a bent heat pipe installed within a pressure vessel, in accordance with an illustrative embodiment of the invention; 
           [0025]      FIG. 8  is a schematic, perspective view of the cooling device and the pressure vessel of  FIG. 7 , in accordance with an illustrative embodiment of the invention; and 
           [0026]      FIG. 9  is a schematic, perspective view of a heat transfer device thermally coupled to a cooling device, in accordance with an illustrative embodiment of the invention. 
       
    
    
     DESCRIPTION 
       [0027]    Referring to  FIG. 1 , in various embodiments, a thermal transfer system or cooling device  10  is provided for removing heat from an interior portion of a pressure vessel  12 . In general, the pressure vessel  12  includes or contains heat generating components  14  (e.g., power converters or processors) that generate heat within the pressure vessel  12 , and the cooling device  10  transfers this heat to a fluid (e.g., water) outside of the pressure vessel  12 . By removing heat from the pressure vessel  12 , the cooling device  10  may prevent the temperature within the vessel  12  from becoming excessive, which may cause the heat generating components  14  to fail or otherwise perform improperly. 
         [0028]    In the depicted embodiment, the pressure vessel  12  is substantially cylindrical and includes a central tube portion  16  and two endcaps  18  mounted to axial ends of the tube portion  16 . Alternatively, the pressure vessel  12  may be or include any other shape, such as spherical. The pressure vessel  12  may be made of any material(s) having the desired thermal and mechanical properties. For deep sea applications, the pressure vessel  12  is preferably made of one or more materials that are capable of withstanding extremely high pressures and are corrosion resistant. A preferred material for the pressure vessel  12  is titanium (e.g., grade  5 ), due to its high strength to weight ratio. In one example, the endcaps  18  are made of grade  5  titanium, with each endcap  18  having a diameter of about 10 inches and a thickness of about 2 inches. 
         [0029]    Still referring to the embodiment in  FIG. 1 , the cooling device  10  includes a housing  20  and a heat pipe  22 . The housing  20  is attached to one of the endcaps  18  of the pressure vessel  12  and resides substantially outside of the pressure vessel  12 , within the surrounding fluid. The heat pipe  22  includes a warm end  24  and a cool end  26  and passes through an opening  28  in the pressure vessel  12 . The warm end  24  of the heat pipe  22  is in thermal communication with the one or more heat generating components  14  within the pressure vessel  12 . The cool end  26  of the heat pipe  22  is disposed in a bore or cavity  30  within the housing  20 . During operation of the cooling device  10 , the heat pipe  22  transfers heat from the heat generating components  14  to the housing  20 , which then transfers the heat to the surrounding fluid. Like the pressure vessel  12 , the housing  20  is preferably made from titanium (e.g., grade  5 ), although any other material (e.g., aluminum, copper, and/or stainless steel) that provides the desired structural and thermal properties may be utilized. 
         [0030]    The housing  20  may be secured to the endcap  18  using any suitable attachment method or connection member. For example, as depicted, the housing  20  may include a threaded end  31  that engages with threads in the opening  28  of the endcap  18 . Alternatively or additionally, the housing  20  may include a flange portion  32  that is secured to the endcap  18  using, for example, one or more screws or other fasteners. In general, the housing  20  and/or the pressure vessel  12  include one or more seals  33  (e.g., gland seals, pressure resistant seals, or the like disposed in grooves) to provide a sealed connection and prevent materials (e.g., water) from entering or exiting the pressure vessel  12 . 
         [0031]    The heat pipe  22  is typically a sealed, thermally conductive pipe or tube (e.g., a copper tube) that contains a working fluid or phase-change material. In its liquid form, the phase change material evaporates when it comes into contact with the warm end  24  of the tube. The vapor then travels to the cool end  26  of the tube where the phase change material condenses back to a liquid, releasing latent energy. The liquid returns to the warm end  24  of the tube (e.g., with the aid of capillary action along the tube wall) where the liquid evaporates again and the cycle is repeated. Advantageously, the heat pipe  22  contains no moving mechanical parts and generally requires little or no maintenance. Compared to conventional heat sinks, the heat pipe  22  is generally smaller and lighter (i.e., less thermal inertial) and has a faster response time. The small size of the heat pipe  22  occupies minimal space within the pressure vessel  12 , and allows the packaging geometry within the pressure vessel  12  to accommodate other design requirements. A suitable heat pipe for the cooling devices described herein is a THERMAL PIN™, manufactured by NOREN products, Inc., of Menlo Park, Calif., or a heat pipe manufactured by Thermacore, Inc., of Lancaster, Pa. A cylindrically shaped heat pipe with water or water and alcohol as the phase change material may work well in many applications. 
         [0032]    In general, the heat pipe  22  may have any size and/or shape. For example, a diameter or cross-dimension of the heat pipe  22  may be from about 0.1 inches to about 1 inch, or preferably about 0.25 inches, about 0.375 inches, or about 0.5 inches. An axial length of the heat pipe  22  may be, for example, from about 4 inches to about 40 inches, or preferably about 20 inches. Actual diameters and/or axial lengths may vary from application to application. 
         [0033]    Although the cooling device  10  generally includes a heat pipe  22  that is a separate component from the housing  20 , in alternative embodiments, the housing  20  and the heat pipe  22  may be the same component. For example, the housing  20  may include an integrated heat pipe that is fabricated directly into the housing  20 . 
         [0034]    In general, to achieve optimal heat transfer between the heat generating components  14  and the cooling device  10 , it is preferable to position the hottest components  14  closest to the housing  20 . Such positioning minimizes the distance heat must travel through the heat pipe  22  before being transferred to the housing  20  and the surrounding environment. Heat transfer between the components  14  and the heat pipe  22  is also improved by keeping any intermediate layers (e.g., metal layers) as thin and thermally conductive as possible (e.g., by using thin layers of copper or aluminum), and by avoiding any air gaps between the components  14  and the heat pipe  22 . Flat surfaces are generally preferable to improve contact between the heat pipe  22  and the components  14 , and to avoid air gaps. Any air gaps may be filled with suitable heat conductive pastes and/or other heat conductive materials, such as settable polymers and/or glues. 
         [0035]      FIG. 2  is an exploded view of the housing  20 , the heat pipe  22 , and the heat generating components  14 , in accordance with certain embodiments of the invention. As depicted, the heat generating components  14  are attached to a mounting plate  34  having a bore  36  for receiving the warm end  24  of the heat pipe  22 . The heat generating components  14  may be attached to the mounting plate  34  using any acceptable attachment device, such as screws and/or adhesive. In the depicted embodiment, the mounting plate  34  includes two separate plates that together form the bore  36  for receiving the heat pipe  22 . A thermally conductive grease, paste (e.g., a silver-based thermal compound), or self-setting heat conducting putty paste or glue, may be included within the bore  36 , to improve heat transfer between the bore  36  and the heat pipe  22 . 
         [0036]    Referring to  FIG. 3 , in certain embodiments, a distal end of the housing  20  includes a face plate  40  which may be removed to facilitate insertion of the cool end  26  of the heat pipe  22  into the cavity  30  (e.g., to allow gases to escape). The face plate  40  may be attached to the remainder of the housing  20  using, for example, screws or other fasteners. One or more seals  42  (e.g., gland seals disposed within grooves) may be included to provide a sealed connection. In various embodiments, the cavity  30  is sized and toleranced to achieve a small gap (e.g., about 0.001 inches) and a close sliding fit between the heat pipe  22  and the cavity  30 , around the outer surface of the heat pipe  22 . A thermally conductive grease or paste (e.g., a silver-based thermal compound) may be disposed within the gap to eliminate air pockets and improve heat transfer between the cavity  30  and the heat pipe  22 . Due to the small gap and the grease filling the gap, a vacuum lock may be achieved within the cavity  30  that maintains the heat pipe  22  in a fixed position within the cavity  30 . In alternative applications, the heat pipe  22  may be secured to the housing  20  with a braze fit and/or a shrink fit. 
         [0037]    In certain instances, when the heat pipe  22  is pressed into the housing  20 , a vapor lock prevents the heat pipe  22  from fully seating in the housing  20 , due to an inability of gas to escape from the cavity  30 . By removing the face plate  40  during insertion of the heat pipe  22 , however, any gases trapped behind the heat pipe  22  can freely escape from the housing  20 , which allows the heat pipe  22  to be fully seated within the housing  20 . After the heat pipe  22  is properly seated, the face plate  40  may be reinstalled to seal or plug the cavity  30 . 
         [0038]    In some implementations, the face plate  40  is replaced with a plug or other suitable device for sealing the cavity. For example, a threaded plug may be inserted into a threaded end of the cavity  30  to seal the cavity  30 . The face plate  40  or other sealing device is preferably removable to facilitate replacement, insertion, or removal of the heat pipe  22 , as needed. 
         [0039]    In general, the outer surface of the housing  20  is designed to promote heat transfer with and/or resist the hydrostatic pressure of the surrounding fluid. For example, in the depicted embodiment, the housing  20  includes one or more fins  44  that extend radially from the housing  20 , to increase an outer surface area of the housing  20 . The fins  44  may have any shape and orientation, but generally are shaped in a way to optimize heat transfer and prevent housing collapse due to hydrostatic pressure. For example, the fins  44  may be aligned with a circumferential direction (as shown) and/or an axial direction of the housing  20 . Other surface features, such as textures, roughness, or protrusions may likewise be utilized to improve the heat transfer from the housing  20 . In certain embodiments, the fins  44  extend in a radial direction from the housing  20  by a distance from about 0.1 inches to about 4 inches, from about 0.5 inches to about 2 inches, or about 1 inch. Alternatively, the housing  20  may not include the fins  44 . 
         [0040]    In certain embodiments, the fins  44  on the housing  20  are shaped and/or configured to promote convective heat transfer with the surrounding fluid. For example, when the housing  20  is attached to or forms part of a moving underwater vehicle, the fins  44  may be arranged in a corkscrew pattern or include channels that direct or funnel the fluid over the fins  44 . Such fin arrangements may be used to increase fluid velocities over the fins  44 , thereby increasing heat transfer coefficients and heat transfer rates between the fins  44  and the surrounding fluid. 
         [0041]    In various embodiments, the cooling devices described herein include more than one heat pipe per cooling device. For example, referring to  FIG. 4 , a housing  50  may include two cavities  30  for receiving two heat pipes  22 . With more than one heat pipe  22 , the housing  50  is generally capable of removing more heat from within the pressure vessel. In general, any number of heat pipes  22  may be included within a single cooling device. As depicted, the housing  50  may include a flange portion  52  with openings  54  for receiving one or more fasteners (e.g., screws) to secure the housing  50  to the pressure vessel. 
         [0042]    Likewise, in some embodiments, the endcap may include more than one opening for receiving more than one cooling device. For example, referring to  FIG. 5 , an endcap  60  includes four openings for receiving four cooling devices  10 . Each cooling device  10  includes the housing  20  and the heat pipe  22  inserted within the housing  20 . Inside the pressure vessel, heat generating components  14  are attached to mounting plates  64 , which include bores for receiving the heat pipes  22 . As depicted, the mounting plates  64  may be attached to endplates  66  that may help stabilize or position the mounting plates  64 . One or both endplates  66  may be secured to the endcap  60  using, for example, spacers  68  and/or screws. In general, using multiple cooling devices  10  per endcap increases the available rate of heat transfer from the pressure vessel. While the cooling devices described herein are generally intended for use in underwater environments, in some instances, a cooling device is used to cool a pressure vessel that is above water (e.g., on the deck of a ship). Referring to  FIG. 6 , to provide heat removal in above-water applications, a housing  70  may include an internal passage or flow path  72  through which a cooling liquid (e.g., water) is pumped. A fitting may be attached at each end of the flow path  72  to connect the flow path  72  to tubing and/or a pump. As the cooling liquid is pumped through the housing  70 , the cooling device transfers heat from the heat generating components to the cooling liquid. The flowrate of the cooling liquid through the housing  70  can be adjusted depending on the temperature of the cooling water, the material used for the housing and the amount of heat to be removed. 
         [0043]    Alternatively, heat removal in above-water applications may be provided by contacting an outer surface of a housing with the cooling liquid. For example, a sleeve, tube, or hose may be fitted over a finned surface of the housing, and the cooling liquid may be pumped through the sleeve, tube, or hose. The finned surface may include a corkscrew or other fin pattern that directs the cooling liquid over and around the housing to achieve improved convective heat transfer between the housing and the cooling liquid. After passing through the sleeve, tube, or hose, the cooling liquid may drain away from the unit and/or return to the cooling liquid supply, which may be a tank or other body of water (e.g., an ocean, lake, or river). 
         [0044]    Referring to  FIGS. 7 and 8 , in certain embodiments, a heat pipe  76  includes one or more bends  78  that allow the heat pipe  76  to occupy a desired position within a pressure vessel  80 . For example, the pressure vessel  80  may include electrical components  82  or other objects disposed within a center of the pressure vessel, such that a warm end  84  of the heat pipe  76  cannot occupy the center of the pressure vessel  80 . In such instances, the heat pipe  76  may not extend in a straight line from the housing  20  but may, instead, be bent to avoid the center of the pressure vessel  80 . In the depicted embodiment, two heat pipes  76  are bent to avoid the center of the pressure vessel  80  and position the warm ends  84  of the heat pipes  76  closer to an interior wall  86  of the pressure vessel  80 . The bent shape also allows the warm end  84  of the heat pipe  76  to be elevated above the housing  20 , which may improve heat transfer efficiency within the heat pipe  76 . For example, by elevating the warm end  84  of the heat pipe  76 , condensate within the heat pipe  76  may be able to collect more easily, due to gravity, at a lower, cool end of the heat pipe  76 , such that the condensate is more easily wicked back up to the warm end  84 . 
         [0045]    Referring to  FIG. 9 , in some implementations, a warm end  90  of a heat pipe  92  is attached to or otherwise in thermal communication with a heat transfer device  94 . The heat transfer device  94  may include a bore  96  for receiving the heat pipe  92 , e.g., in a close sliding fit, a braze fit, or a shrink fit. The heat transfer device  94  may also include one or more fins  98  for increasing heat transfer rates to a fluid (e.g., air) within the pressure vessel. A fan  100  may be used to blow the fluid within the pressure vessel across the heat transfer device  94 , to achieve forced convection between the heat transfer device  94  and the fluid. Advantageously, the heat transfer device  94  allows the temperature of the fluid within the pressure vessel to be controlled, which may also help control the temperature of remote components within the pressure vessel (e.g., components that are not directly attached or thermally coupled to the heat pipe  92 ). In the depicted embodiment, heating generating components  102  (e.g., dc-dc converters) are attached to the heat transfer device  94 . Heat from the heat generating components  102  may be transferred by conduction through the heat transfer device  94  and into the heat pipe  92 , which transfers the heat to a housing  104 . 
         [0046]    In certain embodiments, the objects or components to be cooled need not be housed or contained within a pressure vessel. For example, the objects or components to be cooled may be thermally isolated from the surrounding environment but be exposed to the same pressure as the surrounding environment. 
         [0047]    In underwater applications, the pressure vessel  12  may be a component of an underwater vehicle, such as an ROV or an Autonomous Underwater Vehicle (AUV). For example, an ROV  80  may include one or more pressure vessels  12 , and each pressure vessel  12  may include any number (e.g., 1, 2, or 4) of cooling devices. In alternative applications, the pressure vessel  12  may be moored, for example, as a component in a sea-floor observatory. 
         [0048]    Various techniques may be used to manufacture the cooling devices described herein. A brief list of acceptable machining and fabrication techniques includes: wire electrodynamic machining (wire EDM), ram EDM, abrasive water jet machining, electroplating and electroforming, modern bonding methods, spin welding, friction stir welding, vacuum furnace brazing, and hydrogen furnace brazing. Water-jet and EDM methods lend themselves to working refractory metals and other hard-to-machine metals such as titanium and copper. EDM is appropriate when tight tolerances are required. Wire EDM is most often used with plate stock and ram EDM is most often employed when blind holes are needed in refractory metal. Water jet also is excellent for hard-to-machine plate stock, but should be utilized when designs are not predicated on higher tolerances and exactness of fit that conventional lathe and mill operation are capable of performing. In instances where limited cross-sectional areas are available in conductive interfaces, it may be important to maximize the effective contact area. While being mindful of intermediate assembly requirements, modern bonding methods should be considered to ensure effective contact. 
         [0049]    Advantageously, the cooling devices and methods described herein may be used to transfer heat from any objects or components that generate or otherwise store heat. The objects or components may be, for example, electronics, solid state electronics, power converters, processors, data storage devices, motors, friction points, winches, batteries, power sources, fluids (e.g., hot water or air), and lights. 
       EXPERIMENTS 
       [0050]    For the cooling devices described herein, a heat transfer rate q may be expressed as q=hA(T in −T out ), where h is a heat transfer coefficient, A is a heat transfer area (e.g., an outer surface area of the housing), T in  is a temperature inside the vessel, and T out  is a temperature outside the vessel. When the cooling device includes a single heat pipe, the heat transfer rate q may be, for example, from about 5 W to about 500 W, or from about 25 W to about 100 W. For example, with a single half-inch heat pipe (i.e., a heat pipe with a diameter of 0.5 inches), the heat transfer rate q achieved by the cooling device may be from about 50 W to about 70 W. When the cooling device includes more than one heat pipe, the heat transfer rate q may increase accordingly, though not necessarily in a linear fashion. For example, in one experiment, the heat transfer rate was measured to be about 100 W for a cooling device that included two half-inch heat pipes. In one instance, four cooling devices, each having two half-inch heat pipes, were measured to achieve a heat transfer rate of about 400 W. 
         [0051]    In general, the heat transfer rate q for the cooling devices is proportional to a difference between the temperature inside the vessel and the temperature outside the vessel T out . For example, when the outside temperature T out  is reduced, the heat transfer rate from the vessel may be correspondingly increased. In one experiment, the temperatures inside and outside the vessel (i.e., T in , and T out , respectively) were measured during steady state operation of a cooling device having two heat pipes. The test indicated that the cooling device was capable of maintaining a temperature difference of 30° C. between these two locations. 
         [0052]    In general, the heat transfer rate q achieved by the cooling devices described herein increases as the difference between T in , and T out  increases. The cooling devices may remove heat from the pressure vessel when T in  is greater than T out . 
         [0053]    In various embodiments, the heat transfer coefficient h for a cooling device with a single heat pipe may be, for example, from about 100 W/m 2 -K to about 300 W/m 2 -K. In one instance, the heat transfer coefficient h was estimated to be about 180 W/m 2 -K, through experimentation and numerical analysis. Exemplary system parameters for the cooling devices and methods described herein are presented in Table 1. 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Exemplary system parameters. 
               
             
          
           
               
                 Parameter 
                 Min. 
                 Typical 
                 Max. 
               
               
                   
               
             
          
           
               
                 Number of heat pipes per housing 
                 1 
                 1 
                 4 
               
               
                 Number of cooling devices per endcap 
                 1 
                 2 
                 4 
               
               
                 Heat transfer for cooling devices (W) 
                 5 
                 50 
                 500 
               
               
                 Heat pipe length (inches) 
                 4 
                 20 
                 40 
               
               
                 Heat pipe diameter (inches) 
                 0.1 
                 0.5 
                 1 
               
               
                 Radial height of fins on housing (inches) 
                 0 
                 1 
                 4 
               
               
                   
               
             
          
         
       
     
         [0054]    Each numerical value presented herein, for example, in a table, a chart, or a graph, is contemplated to represent a minimum value or a maximum value in a range for a corresponding parameter. Accordingly, when added to the claims, the numerical value provides express support for claiming the range, which may lie above or below the numerical value, in accordance with the teachings herein. Absent inclusion in the claims, each numerical value presented herein is not to be considered limiting in any regard. 
         [0055]    The terms and expressions employed herein are used as terms and expressions of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof. In addition, having described certain embodiments of the invention, it will be apparent to those of ordinary skill in the art that other embodiments incorporating the concepts disclosed herein may be used without departing from the spirit and scope of the invention. The features and functions of the various embodiments may be arranged in various combinations and permutations, and all are considered to be within the scope of the disclosed invention. Accordingly, the described embodiments are to be considered in all respects as only illustrative and not restrictive. Furthermore, the configurations, materials, and dimensions described herein are intended as illustrative and in no way limiting. Similarly, although physical explanations have been provided for explanatory purposes, there is no intent to be bound by any particular theory or mechanism, or to limit the claims in accordance therewith.