Abstract:
The invention relates to an improved thermosiphon and to a method for transferring heat. The thermosiphon has a higher efficiency than existing thermosiphons because it does not rely on a pool-boiling evaporator but rather uses a forced-convection boiling evaporator. The inlet of the evaporator is located in its upper portion and is in fluid communication with a condenser. The fluid in its liquid phase enters the evaporator from its inlet in its upper portion and, by gravity, flows down the piping network of the evaporator, clinging on the inner surface of the tubes of the piping network. As the liquid flows down, it evaporates such that the fluid at the bottom of the evaporator is predominantly in a gaseous phase. The fluid in gaseous phase is then returned to the condenser.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention generally relates to the field of heat exchangers. More particularly, the invention relates to an improved thermosiphon.  
       BACKGROUND OF THE INVENTION  
       [0002]     A thermosiphon is a type of heat exchanger. The thermosiphon induces movement in a fluid by creating density gradients in the fluid through the exchange of heat. Currents are then generated by gravity as denser regions in the fluid tend to fall and the lighter ones tend to rise.  
         [0003]     A closed-loop thermosiphon is one that forms a closed circuit within which the fluid circulates. The fluid is heated in one part of the circuit. If the density of the fluid varies inversely with temperature—which is generally the case—the heating causes it to rise to another part of the circuit that is cooled. The heated fluid releases its heat in this cooled part of the circuit. The circulating fluid then falls back down to the heated part of the circuit to start the cycle once again.  
         [0004]     The advantage of a two-phase thermosiphon is that it permits the fluid to circulate faster in the circuit since gas immediately rises with little impediment, and a steady stream of condensate flows easily under the force of gravity. Furthermore, the latent heat that is liberated by the flow is much greater than its sensible counterpart. In a two-phase thermosiphon, the heated part of the circuit is henceforth referred to as the “evaporator” and the cooled part as the “condenser”. Thermosiphons are very well known in the art and are documented in literature such as in the “2004 ASHRAE Handbook on HVAC Systems and Equipment,” published by the American Society of Heating, Refrigeration and Air-Conditioning Engineers.  
         [0005]     If they were effective enough, two-phase closed-loop thermosiphons could be particularly appealing in energy-saving strategies in buildings since they may be used to extract heat from the warm air being evacuated from the building. The condenser would be installed in the stream of cold air entering the building and the evaporator in the stream of warm air being evacuated from the building. The fluid within the condenser would release its heat to the cold air, thus warming it up. Interestingly, thermosiphons need no pumps or compressors to operate since gravity may be used to circulate the fluid. Hence, thermosiphons necessitate minimal maintenance, further contributing to their cost-effectiveness.  
         [0006]     Although a very interesting product, existing thermosiphons are often overlooked for many applications because the heating they produced is not important enough to justify their use. This is due to their inherent design limitations.  
         [0007]     A typical closed-loop two-phase thermosiphon is illustrated and described in  FIG. 1 . Existing thermosiphons use evaporators operating on the principle of “pool boiling”. The evaporators contain a stationary pool of liquid that boils slowly by the formation of bubbles at the liquid-solid interface. Unfortunately, for an application such as transferring heat from air evacuated from a building, pool boiling is not very effective. For this application, the heat transfer coefficients obtained from pool boiling are relatively small. Consequently, the rate of evaporation is relatively low, impairing the efficiency of the thermosiphon. In contrast, the vertical tubes of the condenser permit condensation to operate effectively. However, if either of the boiling or condensation processes is restricted or reduced, the other will be restricted or reduced as well. Therefore, ineffective boiling in the evaporator prevents the condenser from functioning at full capacity.  
         [0008]     Because the resulting heat transfer produced by thermosiphons is relatively low, their manufacturing and installation costs for heat recovery of evacuated air in buildings are not justified. In fact, the closed-loop two-phase thermosiphons for this application are virtually non-existent on the market. What are found on the market are more expensive alternatives such as heat pipes in which the flow is not driven by gravity but by capillary action.  
         [0009]     There is therefore a longstanding need to provide thermosiphons having an improved efficiency that could justify their use in building applications.  
       SUMMARY OF THE INVENTION  
       [0010]     In accordance with a first aspect of the present invention, there is provided a method for transferring heat comprising the steps of providing a condenser having a condenser lower portion, an evaporator having a downwardly extending piping network, the piping network itself having an upper end and a lower end, and providing a fluid in the evaporator. A temperature difference is then provided between said condenser and said evaporator. The fluid in gaseous phase is circulated from the evaporator to the condenser and the fluid in liquid phase is circulated through a conduit from the condenser lower portion to the upper end of the piping network. The fluid in liquid phase is allowed to flow by gravity inside the piping network from the upper end of the piping network towards the lower end of the piping network, while the same fluid in a liquid phase is caused to cling to at least a portion of an inside surface along the piping network, the fluid co-existing in both liquid phase and gaseous phase at a cross-section of the piping network. Along the evaporator piping network, heat is transferred from the outside of the evaporator piping network to the inside surface in order to evaporate the fluid clinging to the inside surface. The fluid in gaseous phase is condensed in the condenser.  
         [0011]     Preferably, the condenser lower portion is placed at a higher position than the upper end of the piping network. More preferably, the conduit is routed such that the fluid in a liquid phase flows by gravity from the condenser lower portion to the upper end of the piping network. Most preferably, the lower end of the piping network is closed.  
         [0012]     Optionally, the fluid in liquid phase is distributed on the inside surface of the piping network by providing a wick adjacent the inside surface of the piping network.  
         [0013]     Preferably, the fluid in gaseous phase is circulated from the evaporator to the condenser inside the conduit.  
         [0014]     Alternatively, the fluid in gaseous phase is circulated from the lower end of the piping network to the upper portion of the condenser.  
         [0015]     Optionally, the fluid in liquid phase is pumped from the condenser lower portion to the upper end of the piping network.  
         [0016]     In a second broad aspect of the invention, there is provided a thermosiphon comprising a condenser having an upper portion and a lower portion and an evaporator having. a downwardly extending piping network. The piping network has an upper end, a lower end and at least one pipe while the lower portion of the condenser is provided with a condenser outlet. The upper end of the piping network is connected to the condenser outlet by a conduit. The evaporator is adapted to receive a fluid in liquid phase and have it flow downwardly by gravity in contact with the inner surface of the pipe of the piping network.  
         [0017]     Preferably, the inner surface of the pipe is textured to increase its surface roughness. Optionally, the piping network further comprises a wick located in contact with the inner surface of the pipe.  
         [0018]     In a preferred embodiment of the present invention, the pipe is formed in the shape of a coil.  
         [0019]     In another embodiment of the present invention, the condenser has an inlet located at its upper portion, the inlet being connected to the lower end of the piping network. Alternatively, the condenser inlet may still be located at its upper portion but is connected to the upper end of the piping network. Most preferably, however, the lower end of the piping network is closed.  
         [0020]     Optionally a sight glass is located at the lower end of the piping network.  
         [0021]     Preferably, the thermosiphon comprises a fluid in both a gaseous phase and a liquid phase. The fluid in a liquid phase is operative to flow from the condenser outlet to the upper end of the piping network. More preferably, the fluid in liquid phase is further operative to flow downwardly by gravity from the upper end of the piping network to the lower end of the piping network. Most preferably, the fluid is still further operative to cling on the inner surface of the at least one pipe of the piping network.  
         [0022]     In the most preferred embodiment of the present invention, the fluid in gaseous phase circulates in the conduit from the upper end of the piping network towards the condenser outlet and into the lower portion of the condenser. Preferably then, the lower end of the piping network is closed.  
         [0023]     In another embodiment of the present invention, the fluid in gaseous phase circulates from the lower end of the piping network to the upper portion of the condenser. Alternatively, the fluid in gaseous phase may circulate from the upper end of the piping network to the upper portion of the condenser.  
         [0024]     Optionally, the thermosiphon further comprises a pump operative to circulate the fluid in liquid phase from the lower portion of the condenser to the upper end of the piping network.  
         [0025]     In yet another preferred embodiment of the present invention, the fluid is predominantly in a gaseous phase at the lower end of the piping network.  
         [0026]     The present invention increases the heat transfer obtained by the thermosiphon to such a level that it becomes a viable alternative to costlier heat pipes, thermal wheels and direct air heat exchangers. Furthermore, the thermosiphon does not require geometrical constraints that may be inconvenient such as with heat pipes and thermal wheels that require the exhaust and intake ducts to be side-by-side. Since it operates with gravity, the thermosiphon only requires the exhaust duct to be at a lower elevation than the intake duct allowing for much greater liberty in the layout of a system of ducts. When that is not possible, a pump may be provided in the thermosiphon to move the fluid. The thermosiphon of the present invention may thus be used effectively in heat recovery schemes, considerably reducing the amount of heat required from the main heating systems. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]     A detailed description of an example of implementation of the invention is provided hereinbelow in which reference is made to the appended drawings in which:  
         [0028]      FIG. 1  is a schematic view of a prior art thermosiphon.  
         [0029]      FIG. 2  is a schematic view of a thermosiphon in accordance with an embodiment of the present invention.  
         [0030]      FIG. 3  is a cross-sectional view of a conduit taken along lines A-A of the thermosiphon of  FIG. 2 .  
         [0031]      FIG. 4  is an isometric view of an embodiment of the evaporator of the thermosiphon of  FIG. 2 ;  
         [0032]      FIG. 5  is a schematic view of a thermosiphon in accordance with another embodiment of the present invention.  
         [0033]      FIG. 6  is a schematic view of a thermosiphon in accordance with yet another embodiment of the present invention.  
         [0034]      FIG. 7  is a partial cross-sectional front view taken along the longitudinal axis of the conduit between the condenser and the evaporator of the thermosiphon of  FIG. 6 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0035]      FIG. 2  shows the thermosiphon  1  of the present invention. The thermosiphon  1  contains a fluid  70 , which is best seen in  FIG. 3 , under two different phases: liquid  70   a  and gaseous  70   b . When the thermosiphon is not functioning, the liquid fluid  70   a  rests at the bottom of an evaporator  20 . When functioning, the evaporator  20  exchanges heat by convection with the surrounding warm fluid that moves through it. As the liquid fluid  70   a  starts to evaporate and becomes gaseous, it exits evaporator outlet  24  and rises via the conduit  50  to the condenser  10  through its inlet  14 . Since condenser  10  also exchanges heat by convection with the surrounding cold fluid that moves through it, the gaseous fluid  70   b  condensates on the walls of the condenser  10 . By gravity, the liquid fluid  70   a  falls to the bottom of the condenser  10  where it exits through the condenser outlet  12 . Normally in a thermosiphon, to further save energy by not having to use a pump, the condenser outlet  12  is located higher than the evaporator inlet  22  such that the liquid fluid  70   a  is transported from the condenser to the evaporator by gravity. It is important to ensure that conduit  60  has a descending slope from the condenser outlet  12  to the evaporator inlet  22 , such that no liquid fluid  70   a  may accumulate in the conduit  60 . A stream of liquid fluid  70   a  enters the evaporator  20  through the evaporator inlet  22 . As the constant stream of liquid fluid  70   a  flows to the bottom of the evaporator  20 , it gets heated by the warm fluid surrounding the evaporator and slowly evaporates. The gaseous fluid  70   b  exits the evaporator  20  through the evaporator outlet  24 , rises to the condenser  10  and the cycle starts again.  
         [0036]     Contrary to thermosiphons of the prior art, the thermosiphon  1  of the present invention does not operate with a pool-boiling evaporator. The evaporator  20  is designed to operate on the principle of “forced-convection boiling”. The heat transfer that results is comparable to that obtained in heat pipes of corresponding size, but the thermosiphons can be manufactured at much lower costs. The forced convection boiling process is characterized by bubble formation in a heated tube  25  where the liquid fluid  70   a  is flowing. Hydrodynamic effects differ considerably from those corresponding to pool boiling where there is negligible flow in the liquid fluid. Forced convection boiling is also characterized by high heat transfer coefficients. The evaporator  20  was designed to permit the movement of a thin stream of liquid fluid  70   a  through its tube . 25  and this results in relatively high rates of evaporation. Indeed, contrary to usual practice in thermosiphons, the evaporator inlet  22  is located in the upper portion of the evaporator rather than at its bottom. Preferably, the evaporator inlet  22  is located at the top of the evaporator  20 , close or at the upper end of the tube  25  of the evaporator  20 , such as to generate the continuous stream of liquid fluid  70   a  down the length of the tube  25 . This is an important difference as connecting the evaporator inlet at the bottom of an evaporator, as per the conventional scheme of connection, would create a pool of liquid fluid at the bottom of the evaporator. One of the results of connecting the evaporator inlet  22  at the upper portion of the evaporator  20  rather than at its bottom portion as with prior art thermosiphons is that the fluid  70  is mostly in the gaseous phase at the bottom portion of the evaporator  20  in the present invention whereas it was predominantly in the liquid phase in the prior art thermosiphons.  
         [0037]     It has been found that both the condenser  10  and the evaporator  20  perform very well when the stream of liquid fluid  70   a  is relatively small. For the condenser  10  and the evaporator  20  to perform properly, the stream of liquid fluid  70   a  should occupy only a small fraction of the cross-section of the tube  25  it flows through, as show in  FIG. 3 . Since it has a relatively small thermal inertia, the liquid fluid  70   a  evaporates most readily when that is the case. Consequently, the entire thermosiphon  1  is charged mostly with fluid  70  in the gas phase. This is a departure from the common practice in the thermosiphons of the prior art where the evaporator was essentially flooded with fluid, or at least in its bottom portion. The evaporator  20  of the present invention would simply not work if flooded.  
         [0038]     It has been found that properly determining the correct charge of liquid and gaseous fluid  70  is very important. A sufficient quantity of liquid fluid  70   a  must reside in the evaporator  20  for the following reason: Once the thermosiphon  1  begins operating, gaseous fluid  70   b  begins condensing into liquid in the condenser  10 . Because of the loss of gaseous fluid  70   b  through this condensation, the pressure of the gaseous fluid  70   b  reduces and condensation slows down and eventually stops until the liquid fluid  70   a  reaches the evaporator  20  where it can evaporate and increase the pressure again in the system. This decrease in pressure caused by condensation may halt the processes in the thermosiphon  1 . Hence, to prevent an interruption in the processes, which could have adverse effects on the performance of the thermosiphon  1 , a minimum quantity of liquid fluid  70   a  must be kept in the thermosiphon  1  such that there is always enough to evaporate to compensate for the condensation of the gaseous fluid  70   b . On the other hand, any pool of liquid fluid  70   a  that would accumulate in the evaporator  20  would adversely affect its performance. Hence, it is really important to determine precisely the right amount of liquid fluid  70   a  in the thermosiphon  1 . At this time, this amount of liquid fluid  70   a  needs to be determined experimentally based on the thermosiphon size, configuration, installation and on the particular application. A sight glass  40  is provided to monitor the level of liquid fluid  70   a  in the thermosiphon  1 .  
         [0039]     A valve  30  is placed in conduit  60 , between the condenser outlet  12  and the evaporator inlet  22 , to close the circuit when operation of the thermosiphon is not needed. Once the valve  30  is closed, no more liquid fluid  70   a  flows to the evaporator and the cycle is stopped. Preferably, a full-port valve is used.  
         [0040]     The choice of fluid:  70  is also important. The fluid  70  is chosen with respect to its appropriate latent heat, to its viscosity, which needs to be low enough to permit its flow in the liquid phase to be driven by gravity, and to its low enough saturation pressure to permit an inexpensive thermosiphon design. Preferably, Freon  134 A is used.  
         [0041]     Both the evaporator  20  and the condenser  10  may take different forms, as long as their designs do not allow the accumulation of liquid fluid  70   a  anywhere, except for a small accumulation at the bottom of the evaporator  20  as previously discussed. For example, the evaporator  20  takes the form of a downwardly oriented tube  25 . The tube  25  may be of any cross-section, particularly round or rectangular. Furthermore, the tube  25  may use a wick (not shown) of the type used in heat pipes to draw the liquid on the complete internal surface of the tube. Preferably, the wick is slotted to provide a canal for the fluid to flow unrestricted except for the sides of the stream that would contact the wicking. For practical reasons, the tube  25  may be bent on itself such that it does not take too much space, as shown in  FIG. 4 . Preferably, the tube  25  is coiled. The surrounding fluid goes through the coiled evaporator  10  in its longitudinal direction. Preferably, the tube  25  is made of copper because of its high conductivity.  
         [0042]     The condenser preferably uses straight tubes  15  running vertically. Heat tubes  15 , having a wick may be used, but are not necessary. In fact, for economical considerations, it is preferred just to use straight tubes  15  having no wick.  
         [0043]     It will of course be appreciated that many modifications and alternative embodiments are possible within the broad scope of the present invention. For example,  FIG. 5  depicts a further embodiment of the invention where the evaporator  20  is closed at its bottom. Because only a stream of liquid fluid  70   a  constantly flows down the tube of the evaporator  20 , there is space left for the evaporated fluid  70  under its gaseous phase to flow back up the same tube of evaporator  20 . At the top of the evaporator  20 , the gaseous fluid  70   b  may enter either the evaporator outlet  24  or the evaporator inlet  22  and flow up to the top of the condenser  10  either through its inlet  14  or its outlet  12 .  
         [0044]      FIG. 6  shows the preferred embodiment of the invention where once again the evaporator is closed at its bottom and the evaporator inlet  22  combines both functions of inlet and outlet. Indeed, since the conduit  60  and tubes  15  and  25  and conduit  60  are never filled completely with liquid fluid  70   a , as shown in  FIG. 3 , it is possible to have the fluid  70 . in both phases, liquid and gaseous, circulate in opposite directions within the same conduits, as shown in  FIG. 7 . Indeed, because of gravity, the denser liquid fluid  70   a  tends to flow down. Once vaporized in the evaporator  10 , the gaseous fluid  70   b  tends to rise towards the top of the condenser  10 , running counter-flow to the liquid fluid  70   a.    
         [0045]     A further embodiment may include adding a pump to the thermosiphon  1  for applications where the condenser outlet  12  may not be located higher than the evaporator inlet  22 . Although not as energy efficient, it still permits the use of the thermosiphon.  
         [0046]     The invention is not limited in its application to the details of the arrangement of components illustrated in the accompanying drawings, or the description of the steps referred to above, but is defined by the claims that follow.