Abstract:
The device and methods described herein relate to the isothermal heat transport through an intermittent liquid supply to an evaporator device, thereby enabling high evaporative heat transfer coefficients. A liquid and vapor mixture flows through miniature and micro-channels in an evaporator and addresses flow instabilities encountered in these channels as bubbles rapidly expand. Additionally, a high percentage of the fins are exposed to vapor and limit the required charge of refrigerant within the system due to effective condensate removal in the condenser.

Description:
PRIORITY STATEMENT UNDER 35 U.S.C. §119 &amp; 37 C.F.R. §1.78 
       [0001]    This non-provisional application claims priority based upon prior U.S. Provisional Patent Application Ser. No. 62/118,144 filed Feb. 19, 2015 in the name of Jeremy Rice entitled “Intermittent Thermosyphon,” the disclosure of which is incorporated herein in its entirety by reference as if fully set forth herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Passive heat transfer devices, such as heat pipes, are of much interest in applications such as electronics cooling. Heat pipes are a liquid and vapor device in which liquid is pumped through capillarity from the condenser to the evaporator. The pumping effect in this device requires a wick, which produces a high pressure loss and limits the maximum heat transport distance and or power that can be supported before dry-out occurs. 
         [0003]    Another technology node that is useful is a thermosyphon as shown in  FIG. 1 . In operation, liquid  104  is vaporized in an evaporator  101 . The vapor then travels through a tube  102  to the condenser  100 . Heat is removed from the condenser  100  causing the liquid  104  to accumulate at the bottom. The accumulated liquid  104  in the condenser is driven by gravity through a liquid line  103  back to the evaporator  101 . The evaporators in these devices are typically pool boiling devices with an enhanced surface  105  that may consist of fins, a porous layer or even an etched surface. The maximum boiling heat transfer coefficient can be limited in this device because there are a finite amount of nucleation sites, and therefore a limited length of solid/liquid/vapor contact, where the heat transfer rate is the highest. 
         [0004]    In conventional thermosyphon design, a flow pattern that enters one side of the evaporator and leaves the other side, through a series of channels is typically not used. While this general concept is widely used in most heat transfer products, the implementation in thermosyphon design for electronics is generally prohibited by the limited pressure head provided by gravity to drive the flow and flow instabilities encountered with vapor expansion in a confined channel as shown in  FIG. 2 . As a channel size  201  decreases to the same size of a vapor bubble  202 , the expansion of the vapor causes liquid  203  to flow outwards  204 , irrespective of the desired flow rate. This phenomena poses a few problems. One problem is that the pressure drop associated with high liquid velocities in a channel are quite high, especially relative to the small available pressure head in a thermosyphon device. A second problem that this phenomena can cause is that the middle of the channel is left dry and can increase in temperature, since the vapor has limited heat capacitance. 
       SUMMARY 
       [0005]    This invention is directed toward thermosyphon technology. Certain embodiments are intended for use in electronics cooling applications, wherein a looped flow pattern through channels is formed by fins in the evaporator as well as in the condenser, while allowing for low pressure loss through these channels, thereby enabling this configuration to be applied in low profile systems where the gravitationally-induced liquid pressure head is limited. 
         [0006]    The liquid supplied to the evaporator is intermittent, and passively regulated by the back flow of vapor bubbles. The passively regulated liquid supply enables enhanced solid/liquid/vapor contact, which yields high heat transfer rates on the channels within the evaporator. This characteristic is a solution to the limitations associated with pool boiling in an evaporator flooded with liquid. 
         [0007]    Additionally, the problem of flow instabilities of expanding vapor bubbles in confined channels is addressed through a series of minor vapor and liquid distribution channels cutting across the major channels on the surface. These channels help enable the liquid and vapor to be stratified in a confined space, which provides a free path for vapor to escape the evaporator with minimum impedance of the liquid phase. Additionally, the liquid distribution allows for the bottom of the fins to maintain a wetted region, and maintain stable performance. 
         [0008]    In various embodiments of the condenser, the vapor flow helps drag liquid along with it from the vapor intake orifices to the liquid exit orifice. The liquid exit orifice is located at the bottom of the fins, which helps minimize the required refrigerant charge as well as keeps the fins free from collected liquid, which can block the condensation process. 
         [0009]    The foregoing has outlined rather broadly certain aspects of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures or processes for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
           [0011]      FIG. 1  is a schematic of thermosyphon design in accordance with prior art; 
           [0012]      FIG. 2  is a representation of the vapor expansion process in a miniature channel during boiling; 
           [0013]      FIG. 3  is a schematic of one embodiment of the thermosyphon of the present invention; 
           [0014]      FIG. 4  is a cross-sectional view of one embodiment of the vapor tube of the present invention and a representation of the flow pattern in this tube; 
           [0015]      FIG. 5  is a cross-sectional view of one embodiment of the liquid tube of the present invention and a representation of the flow pattern in this tube; 
           [0016]      FIG. 6  is a cross-sectional view of one embodiment of the evaporator of the present invention and a representation of the liquid and vapor distribution in this device; 
           [0017]      FIG. 7  is a perspective view of one embodiment of a single fin inside of one embodiment of the evaporator of the present invention; 
           [0018]      FIG. 8  is a cross-sectional view of one embodiment of the condenser of the present invention and a representation of the flow pattern inside; 
           [0019]      FIG. 9  is a perspective view of a single fin inside one embodiment of the foregoing condenser; 
           [0020]      FIG. 10  is an isometric view of another embodiment of the thermosiphon of the present invention; 
           [0021]      FIG. 11  is an isometric view of the evaporator with a transparent cover in the foregoing embodiment of the present invention; 
           [0022]      FIG. 12  is a view of a vapor blocking fin inside the foregoing evaporator; 
           [0023]      FIG. 13  is an isometric view of another embodiment of the thermosiphon of the present invention; 
           [0024]      FIG. 14  is a cross-sectional view of the condenser of the foregoing embodiment of the present invention; 
           [0025]      FIG. 15  is a cross-sectional view of the evaporator of the foregoing embodiment of the present invention; 
           [0026]      FIG. 16  is an isometric view of another embodiment of the thermosyphon of the present invention; 
           [0027]      FIG. 17  is a cross-sectional view of the evaporator/condenser of the foregoing embodiment; and 
           [0028]      FIG. 18  is a view of the flow control fin inside the evaporator/condenser of the foregoing embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0029]    The present invention is directed to an improved intermittent thermosyphon. The configuration and use of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of contexts other than an intermittent thermosyphon. Accordingly, the specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention. In addition, the following terms shall have the associated meaning when used herein: 
         [0030]    One embodiment of the present invention is presented in  FIG. 3 . It includes a condenser  100 , two evaporators  101 , a vapor tube  102  connecting the evaporator  101  to the condenser  100  primarily transferring vapor, a liquid tube  103  connecting the condenser  100  to the evaporator  102  primarily transferring liquid, and an access valve  106 , to pull a vacuum, charge and recapture working fluid at production as well as at end of life. The condenser  100  has fins  107  that allow for heat to be rejected to the air passing through. The bottom of the evaporators  101  will contact a heat generating electronics component, such as a central processing unit, through a thermal interface material. The contact surface will require force to be applied through an additional part, which is not detailed, so that adequate pressure may be obtained between the evaporator  101  and the heat generating component. This embodiment is described in detail, however, there may be variants, such as a system with a single evaporator  101 , and three or more evaporators  101 . In these scenarios, the implementation may require a separate vapor tube  102  and liquid tube  103  to each evaporator  101  in a parallel flow scheme or there is the possibility of using a serial flow scheme. 
         [0031]    A cross-section of this embodiment through the vapor tube  102  is represented in  FIG. 4 . The evaporator  101  has fins  201  extending from the bottom surface to the top surface, creating a series of channels, and the fins  201  are partially submerged in liquid  301 . The evaporator fins  201  act to increase the heat transfer area as well as provide structural strength to withstand high internal pressures. Vapor  300  exits the evaporator  101  through an orifice  210  and enters the vapor tube  102 . Vapor  300 , travels through the tube  102  from the evaporator  101  to the condenser  100  in the direction represented by the arrows  302 . The axis of the vapor tube  102  generally parallels a horizontal axis. Vapor  300  enters the condenser  100  through two orifices  206  in the bottom of the condenser  100 . The condenser  100  also has fins  200  extending from the bottom surface to the top surface, creating a series of channels. The condenser fins  200  also act as a means to increase the heat transfer area as well as provide structural support. When height is limited, as is the case for the embodiment represented, the vapor entry orifices  206  in the condenser  100  may be located on the bottom side. In cases where there is additional space, these orifices  206  may also be located on the top side. 
         [0032]    A cross-section of this embodiment through the liquid tube  103  is represented in  FIG. 5 . The center line of the liquid tube  103  parallels a horizontal axis. The liquid  301  primarily fills up the tube  103 . It leaves the condenser  100  through an orifice  205  located on the bottom of the condenser  100 . Since gravity forces the liquid  301  to stratify on the bottom half of the condenser  100 , allowing for liquid  301  to leave through the bottom of the condenser  100  limits the build-up of liquid  301  inside the condenser  100 , both reducing the required refrigerant charge as well as maximizing the exposure of the condenser fins  200  to vapor  300 . Liquid  301  travels along the liquid tube  103  and enters the evaporator  101  through an orifice  209 , and then distributes onto the floor of the evaporator  101 . The flow path of the liquid  301  is depicted by arrows  303 . Since the liquid  301  enters the evaporator  101  through an orifice  209  located at the top of the evaporator  101 , it competes to allow vapor bubbles  304  to escape the evaporator  101  through this same orifice  209 . The vapor bubbles  304  accumulate into larger plugs in the liquid tube  103  and flow back to the condenser  100 , and through the liquid orifice  205  in the condenser  100 , where the vapor  300  also competes to enter the condenser  100 , as liquid  301  exits. Since vapor  300  is accumulated in this tube  103 , it is necessary that any tube bends do not prevent significant vapor accumulation, where the vapor plugs may block liquid  301  from returning to the evaporator  101  entirely and cause a dry-out condition. 
         [0033]    The flow pattern that is produced by the competing flow of the vapor  300  and liquid  301  in liquid tube  103  is intermittent, meaning that liquid  301  is supplied to the evaporator  101  as a series of slugs. This flow pattern is the same behavior that can be observed when turning over a soda bottle and observing the intermittent liquid flow leaving the bottle. Between liquid slugs supplied, there is a liquid starvation period, which must be overcome, which is discussed in a subsequent portion of this section. The liquid starvation period is the duration of time that no liquid is supplied to the evaporator  101 . The benefit of the unsteady liquid supply is that the evaporator fins  201  are only partially submerged in liquid  301 , allowing maximum solid/liquid/vapor contact and high evaporation heat transfer coefficients. A cross-sectional view showing the liquid  301  stratification in the evaporator  101  is depicted in  FIG. 6 . Liquid  301  primarily enters the evaporator  101  through an orifice  209  at one end and vapor  300  primarily leaves an orifice  210  at the other end after passing along channels created by fins  201 . The backflow of a vapor bubble  304  into the liquid tube  103  is represented as well, since vapor  300  is present on the top half of the evaporator  101 . 
         [0034]    Since liquid  301  and vapor  300  both enter and exit an orifice  209  that is smaller than the width of the evaporator  101 , there is a need to allow for liquid  301  to distribute along the base and vapor  300  to collect along the top of the evaporator  101 . A close up of an evaporator fin  201  is represented in  FIG. 7 . This fin  201  has liquid channels  202  that allow liquid  301  to distribute across the fins  201 , so that every fin  201  is wet, to allow for evaporation. These channels  202  are repeated along the fins  201 , so that liquid  301  can easily distribute throughout the evaporator  101 , and help allow liquid  301  to easily flow to parts of the evaporator  101  experiencing a high heat flux. The evaporator fins  201  also have larger channels  203  near the top of the fin  201  to allow for vapor  300  to distribute along the fins  201  and easily flow to the orifice  210 . These vapor channels  203  allow for the fin density to increase, while reducing or eliminating the situation where a flow instability may occur due to the rapid expansion of a vapor bubble in a confined space (refer back to  FIG. 2  and the explanation in the background section). The combination of the liquid  301  and vapor  300  distribution allow for a steady supply of liquid  301  to the fins  201  as well as a steady removal of vapor  300 . 
         [0035]    The evaporator may also have vertical ribs  204  imprinted into the fins  201  to form a corner in which liquid  301  may be pulled up by capillarity. As liquid  301  is pulled up, the length of the solid/liquid/vapor contact will increase and provide additional ability to vaporize liquid at low fin temperature elevation over the saturation temperature of the liquid  301  and vapor  300  mixture. 
         [0036]    The aforementioned “steady” supply of liquid to the evaporator can be achieved if there is a large enough amount of liquid stored in the evaporator to overcome the unsteady delivery of liquid. The mass, m storage , of the liquid stored in the evaporator should be greater than the mass of liquid that is vaporized during the starvation period, τ starvation  as depicted in EQ 1, where the latent heat of vaporization is h fg . The higher the maximum heat load, Q, the greater the liquid reservoir that is required. 
         [0000]    
       
         
           
             
               
                 
                   
                     m 
                     storage 
                   
                   &gt; 
                   
                     
                       Q 
                       
                         h 
                         fg 
                       
                     
                      
                     
                       τ 
                       starvation 
                     
                   
                 
               
               
                 
                   EQ 
                    
                   
                       
                   
                    
                   1 
                 
               
             
           
         
       
     
         [0037]    The concept of liquid storage in the evaporator is very important in many applications, including electronics applications, since the internal volume inside the evaporator is small and the power can be relatively high. There are situations where all the liquid in the evaporator can be vaporized in less than a single second. If the required liquid storage is not properly accounted for, the evaporator can dry-out and lose its functionality. 
         [0038]    While evaporator performance is improved by balancing liquid delivery without flooding or starving the evaporator with liquid, condenser performance is improved by keeping as much of the fins exposed to vapor as possible. A cross-sectional view of the condenser  100  is presented in  FIG. 8 , in which vapor enters orifices  206  flows outward  302  along the fins  200 , cuts through openings  211  (not shown in  FIG. 8 , but described in detail below) created in the fins  200  and then flows inward  305  to the liquid exiting orifice  205 . The vapor helps to push liquid along with it, and prevent too much accumulation of liquid. The outward vapor flow  302  and inward vapor flow  305  are separated by a single fin  207  with openings only located at the far left and far right, as depicted in  FIG. 8 , forcing vapor to flow as depicted. 
         [0039]    The vapor flow pattern within the condenser  100  may be varied, depending on vapor and tube routing requirements, allowable condenser depth and heat source location. For instance, vapor can simply flow from left to right, or even as a “Z” pattern. 
         [0040]    The aforementioned openings  211  in the condenser fin  200  are depicted in  FIG. 9 . These openings  211  allow vapor to pass through while maintaining structural strength to withstand high internal pressures. At the inlet and outlet orifices, the fin  200  can have a cutout  208  allowing unobstructed vapor distribution (at the inlet) and liquid collection (at the outlet). Additionally, these fins  200  have dimples  212  which provide a means to reduce the thickness of the film of liquid created as vapor condenses on the surface and travels down the fin  200 . The dimple  212  creates a convex surface at its peak. The liquid&#39;s surface tension, in conjunction with the dimpled surface creates a relatively high capillary pressure. As the dimple  212  gradually merges into the flat surface of the fin  200 , the curvature continuously changes from a convex surface to a concave surface to a flat surface. In the regions where the curvature is changing, the capillary pressure changes, causing a pressure gradient in the liquid film. This pressure gradient drives the liquid from the relative high pressure to the relative low pressures and acts as a thinning agent. As the film thickness decreases, so does the temperature difference between the saturation temperature of the liquid and vapor mixture to the cooler fin temperature. 
         [0041]    While determining sizing of the internal tube diameters, and maximum supported power, one can use the height difference from the bottom of the condenser to the top of the evaporator as the maximum pumping head potential of the system. The hydrodynamic losses along the tubes, condenser and evaporator may be estimated by determining the velocity of the fluids passing through. Since the flow pattern is transient, an experimental determination of the operating characteristics, such as maximum supported power before liquid cannot return to the evaporator is likely required. The details of the embodiment presented allow for the use of a higher pressure working refrigerant, such as R134a, R1234yf, R1234ze, R410a, or R290, at operating conditions of approximately −10 C to 85 C, which is the approximate range required for most electronics devices. The benefit of higher pressure refrigerants is that the vapor densities are greater, leading to lower vapor velocities and smaller tube diameters. Additionally, the volume of non-condensable gas within the system is compressed and takes up less volume, thereby limiting any adverse effects it may cause. Finally, leaks tend to go outward, and the use of valves may be considered, since the permeation of air through an elastomer O-ring is of minimal concern. 
         [0042]    Another embodiment of the present invention is presented in  FIG. 10 . This embodiment has a condenser  100 , and two evaporators  101  on the same side of the condenser  100 . The evaporators  100  are fluidly coupled to the condenser with a vapor tube  102  and a liquid tube  103 . Integrated into each evaporator  101  are mounting hardware  108 , consisting of springs and screws, to couple the evaporator  101  to a heat generating device. 
         [0043]    An isometric view of the evaporator with a transparent top lid  214  is presented in  FIG. 11 . The lid  214  has two orifices  210  near the center of the lid  214  which allow vapor to enter the vapor tube  102 . At the front and rear end of the lid  214  are two additional orifices  209  which allow liquid to enter the evaporator  101  from the liquid tube  103 . The use of multiple orifices ( 209  &amp;  210 ) reduces pressure loss, which allows more power to be supported with limited liquid gravitational pressure head to drive the flow. In the evaporator  101  is a fin stack  201 , creating rectangular channels inside the evaporator with cross-cuts allowing vapor and liquid to flow freely between the channels. 
         [0044]    One challenge to this embodiment, in which the two evaporators  101  are serially connected on a single side of the condenser  100 , is an increased sensitivity to vapor backflow through the liquid tube  103 . This vapor backflow, while in some situations is desired, can impede liquid from reaching the evaporator  101 , causing a dry-out situation. To limit the degree in which vapor is allowed to backflow through the liquid tube  102 , a vapor blocking fin  213  may be added to the fin stack. A view of the vapor-blocking fin  213  is presented in  FIG. 12 . Similar to the other evaporator fins  201 , the vapor blocking fin  213  has liquid cut-outs  202 , allowing liquid to freely pass through. The vapor blocking fin  213  removes the vapor cut-outs  203 , limiting or preventing vapor to freely flow past this fin  213 . In the space between the two vapor blocking fins  213 , the liquid and vapor will be stratified, as vapor tends to stay on the top. In order to better prevent vapor from crossing the vapor blocking fin  213 , the height of the liquid cut-outs  202  should be lower than the liquid height inside the evaporator  101 . 
         [0045]    For a specific application, the design of the vapor blocking fin  213  may be tuned for a specific power range, by partially blocking the vapor cut-outs  203 . Another design consideration is the location of the liquid orifices  209  in the evaporator, relative to the vapor orifices  210 . 
         [0046]    Yet another embodiment of the present invention is presented in  FIG. 13 , consisting of an evaporator  101  and a condenser  100  located above the evaporator  101 , a vapor channel  102  connecting the evaporator  101  to the condenser  100  and a liquid channel  103  connecting the condenser  100  to the evaporator  101 . In some embodiments, the liquid channel  102  and vapor channel  103  generally travel along a horizontal axis. However, in this embodiment, the liquid channel  102  and vapor channel  103  have vertical axes. 
         [0047]    A cross section of the condenser  100  of the foregoing embodiment is presented in  FIG. 14 . This cross-section is located towards the bottom of the condenser fins  200 , exposing the cut-outs  208  adjacent to the liquid orifice  205  and vapor orifice  206  in the condenser  100 . The fluid flow  306  path inside the condenser  100  travels in a mirrored circular flow pattern. There is a dividing fin  207  that has no cut-outs through the center portion, separating flow that goes in opposite directions. Additionally, there is another added barrier  215  located between the liquid orifice  205  and vapor orifice  206 , preventing short-circuiting of the flow inside the condenser  100 . 
         [0048]    A cross-sectional view of the evaporator  101  of the foregoing embodiment is presented in  FIG. 15 . In this embodiment, the liquid entry orifice  209  and vapor exit orifice  210  are located along the same channels formed by the evaporator fins  201 . The vapor backflow through the liquid orifice  209  is controlled by a solid barrier  215 . This barrier  215  blocks the top portion of the channels, but allows the bottom portion of the channels to be open. When the bottom portion of this barrier  215  is below the stratified liquid level inside the evaporator  101 , it can limit or prevent vapor backflow. The barrier  215  may extend across all of the channels, or just some of the channels, depending upon the permissible amount of vapor backflow. 
         [0049]    Another embodiment of the thermosiphon of the present invention is presented in  FIG. 16 . In this embodiment, the evaporator and condenser are combined into a single evaporator/condenser  109  module. Fins  107  are attached to the evaporator/condenser  109  and allow air to pass through to remove heat. The core of the evaporator/condenser consists of a top piece, a bottom piece and internal fins  216  (not shown in  FIG. 16 , but described in detail below). The internal fins  216  are bonded to the top and bottom piece, and create internal channels. The internal fins  216  have several cross-cuts allowing liquid and vapor to flow across the channels. Heat is applied through the bottom piece, and removed through the top piece of this embodiment. 
         [0050]    A cross-section of the evaporator/condenser  109  is presented in  FIG. 17 . This cross-section cuts through the internal fins  216 . The vapor and liquid flow in the same counter-rotating flow paths  306 . In this embodiment, heat is applied to the central region  218  of the bottom piece. The vapor flow  306  starts from this central region  218 , as liquid vaporizes as a result of the heat input. Since heat is removed from the entire region, condensation occurs along each and every flow channel. The flow pattern is driven by a flow control fin  217 . In the region adjacent to the central region  218 , liquid is allowed to flow  307  through the flow control fin  217  through liquid cut-outs  202  while vapor is not. The difference of liquid height on either side of this fin provides the gravitational pressure head needed to circulate the refrigerant flow  306 . 
         [0051]    The flow control fin  217  may be divided up into several regions, which can be designed to dictate how the refrigerant will flow inside the evaporator/condenser  109 . A front view of this fin is presented in  FIG. 18 . The flow control fin  217  is made up in three distinct section types. The liquid cross section  308 , has liquid cut-outs  202 , but no vapor cut-outs  203 , thus only allowing liquid to pass through, since the vapor is stratified towards the top portion of the fin. The second portion is the flow separation region  309 . There are no vapor  203  nor liquid cut-outs  202  in this region. The flow separation region  309  allows isolation of countering flow currents. The third region is a flow crossing region  310 , which allows both vapor and liquid to pass through their respective cut-outs ( 202 ,  203 ). This region may be utilized to allow the refrigerant flow to change directions. 
         [0052]    It is possible to design an evaporator/condenser  109  without a flow control fin  217 , however the channel height typically needs to be higher, since liquid and vapor will flow counter to each other, which requires a larger gravitational pressure head to overcome the fluid flow losses. 
         [0053]    While the present system and method has been disclosed according to the preferred embodiment of the invention, those of ordinary skill in the art will understand that other embodiments have also been enabled. Even though the foregoing discussion has focused on particular embodiments, it is understood that other configurations are contemplated. In particular, even though the expressions “in one embodiment” or “in another embodiment” are used herein, these phrases are meant to generally reference embodiment possibilities and are not intended to limit the invention to those particular embodiment configurations. These terms may reference the same or different embodiments, and unless indicated otherwise, are combinable into aggregate embodiments. The terms “a”, “an” and “the” mean “one or more” unless expressly specified otherwise. The term “connected” means “communicatively connected” unless otherwise defined. 
         [0054]    When a single embodiment is described herein, it will be readily apparent that more than one embodiment may be used in place of a single embodiment. Similarly, where more than one embodiment is described herein, it will be readily apparent that a single embodiment may be substituted for that one device. 
         [0055]    In light of the wide variety of methods for an intermittent thermosyphon known in the art, the detailed embodiments are intended to be illustrative only and should not be taken as limiting the scope of the invention. Rather, what is claimed as the invention is all such modifications as may come within the spirit and scope of the following claims and equivalents thereto. 
         [0056]    None of the description in this specification should be read as implying that any particular element, step or function is an essential element which must be included in the claim scope. The scope of the patented subject matter is defined only by the allowed claims and their equivalents. Unless explicitly recited, other aspects of the present invention as described in this specification do not limit the scope of the claims.