Patent Publication Number: US-2013233512-A1

Title: Heat exchanger

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This non-provisional application claims the benefit of provisional application No. 61/601,544 filed on Feb. 21, 2012, entitled “Heat Exchanger”, which application is incorporated herein in its entirety by this reference. 
    
    
     BACKGROUND 
     The present invention relates to a heat exchanger. 
     A heat pipe is a hermetically sealed evacuated tube typically comprising a mesh or sintered powder wick and a working fluid in both the liquid and vapor phase. When one end of the tube is heated the liquid turns to vapor upon absorbing the latent heat of vaporization. The hot vapor subsequently passes to the cooler end of the tube where it condenses and gives out the latent heat to the tube. The condensed liquid then flows back to the hot end of the tube and the vaporization-condensation cycle repeats. Since the latent heat of vaporization is usually very large, considerable quantities of heat can be transported along the tube and a substantially uniform temperature distribution can be achieved along the heat pipe. 
     Heat can both enter and leave heat pipes of a heat exchanger by convection. The amount of heat transferred is dependent upon the convection heat transfer coefficient, which is dominated primarily by the average velocities of the hot gas passing over the heat pipe at the vaporization end and cold gas passing over the heat pipe at the condensation end. The working temperature of the heat pipe, namely the temperature at which the pipe settles during use, adjusts automatically according to the amount of heat absorbed from the hot gas on the vaporization section and the amount of heat given off and thus absorbed by the cold gas, at the condensation section. When there is an unequal volume flow rate between the hot and cold gas at the vaporization and condensation sections respectively, the working temperature of the heat pipes can depart significantly from the desired value. Moreover, it is found that the flow of gas over an array of heat pipes at the condensation and vaporization section, can vary from one heat pipe to the next and so there can be a variation in the working temperature of the heat pipes within a heat exchanger. This can result in the generation of incondensable gases within the pipe due to a decomposition of the working fluid, which can severely restrict the performance of the heat pipe. 
     SUMMARY 
     The main object of the present invention is to provide an improved heat pipe heat exchanger for heat recovery from exhaust flues from boilers or industrial processes and to utilize the heat to preheat air or other kind of gas when there is an unequal volume flow rate between the hot and cold gas at the vaporization and condensation sections, respectively. It is a further object of the present invention to provide an arrangement of heat pipes for a heat exchanger, so that the flow of gas over the arrangement provides a similar rate of heat exchange for each pipe. 
     A further object of the present invention is to provide a heat pipe heat exchanger having a low pressure drop and high overall heat transfer coefficient on a hot side of the heat exchanger. It is a further object to provide a heat exchanger which can be retro-fitted to existing boilers or within industrial processes, which almost always have limited pressure drop available. 
     A further object of the present invention is to provide a heat pipe heat exchanger with an improved overall heat transfer coefficient and low pressure drop on the cold air side of the heat exchanger. 
     A further object of the present invention is to provide a heat pipe heat exchanger where the amount of recovered heat can be controlled. 
     A further object of the present invention is to provide a heat pipe heat exchanger that offers better installation possibilities directly on an exhaust stack of an industrial process, for example, particularly in very limited spaces. 
     A further object of the present invention is to provide a heat pipe heat exchanger where the heat pipes can be easily installed and uninstalled. 
     A further object of the present invention is to provide a heat pipe heat exchanger with better cleaning possibilities of the heat transfer area particularly on the hot side of the heat exchanger. 
     A further object of the present invention is to produce a heat pipe heat exchanger which is easier and cheaper to manufacture. 
     In accordance with a first aspect of the present invention, there is provided a heat exchanger for exchanging heat between a first gas and a second gas, the exchanger comprising a first heat exchanging chamber, a second heat exchanging chamber and an array of heat pipes which are arranged to extend between the first and second heat exchanging chambers; 
     the first heat exchanging chamber comprising an inlet for receiving the first gas into the chamber and an outlet through which the first gas can exit the first chamber, the flow of first gas being substantially along the array of heat pipes; 
     the second heat exchanging chamber comprising an inlet for receiving a second gas into the chamber and an outlet through which the second gas can exit the second chamber, the flow of second gas being substantially across the array of heat pipes; wherein, 
     the flow of second gas is split into two subsidiary flow paths, with each subsidiary flow path comprising a substantially identical array of heat pipes. 
     The heat exchanger of the present invention thus ensures that the flow of second gas is uniformly distributed across each heat pipe to ensure that each heat pipe of the array receives a similar rate of heat exchange for optimal performance. 
     Preferably, the flow of second gas is split into the two subsidiary flow paths by the outlet of the first heat exchanging chamber. The inlet and outlet of the first heat exchanging chamber are preferably disposed on a longitudinal axis of the heat exchanger. 
     The cross-sectional area, through which the subsidiary flows of second gas pass, is reduced by the outlet of the first heat exchanging chamber thereby causing the second gas to flow with an increased average velocity. 
     Preferably, the flow of first gas is arranged to move across the array of heat pipes, in passing along the first chamber, by at least one baffle, which extends across the first chamber substantially transverse to the longitudinal axis of the heat exchanger. The baffle preferably comprises a valve, which can be selectively opened and closed to enable the rate of flow of gas along the first chamber to be controlled. When the valve is closed all of the first gas is arranged to pass across the array of heat pipes to increase the transfer of heat between the first gas and the heat pipes within the first chamber. The amount of heat transferred between the first gas and the heat pipes is preferably controlled by selectively opening the valve to effect the flow of gas across the array of heat pipes. 
     Preferably, the first and second heat exchanging chambers comprise a substantially cylindrical housing. The first heat exchanging chamber preferably further comprises an internal wall arrangement which is arranged to substantially conform to an outer periphery of the array of heat pipes. Preferably, the wall arrangement comprises a first and second pair of walls which extend along the length of the first chamber. Preferably, the walls of each pair separately converge from the housing of the first chamber to an apex positioned proximate a longitudinal axis of the first chamber. The first and second pair of walls are preferably positioned diametrically opposite each other within the first chamber, such that the apex of each wall extend within a common plane which also comprises the longitudinal axis. 
     The housing of the first and/or second chambers preferably comprise at least one panel which is removably coupled to the respective housing. 
     The heat pipes are preferably configured to an array of arcuate rows. Preferably, each arcuate row of the array of heat pipes comprises a radius of curvature that is centered on the longitudinal axis of the heat exchanger. The arcuate disposition of the heat pipes and the first and second pair of walls increase the surface area of the heat pipes encountered by the first flow of gas and further provide for a minimal pressure drop of the first gas between the inlet and outlet of the first chamber. The is because the first gas flows along the first chamber while making one or more passes across the arcuate rows of heat pipes and as such, the gas passes through a large number of heat pipes per arcuate row than the second gas. 
     The second gas flows across the first chamber and thus the heat pipes, and therefore passes across fewer heat pipes per row than the first gas (the flow of second gas in the second chamber is across an approximate linear arrangement of rows). In order to prevent the working temperature of the heat pipes rising above a preferred value, the heat absorbed by the second gas and thus the mass rate of second gas across the second chamber must be greater than the mass rate of gas along the first chamber. This is achieved by splitting the flow of second gas along the two flow paths, namely one either side of the outlet of the first chamber to increase the flow velocity of second gas along each flow path compared to the flow of first gas along the first chamber. 
     The arcuate arrangement of heat pipes further provides for a similar rate of heat exchange for each pipe within a particular row. Preferably, the flow of gas along a particular arcuate row within the first and second chambers is substantially uniform so that each heat pipe in the particular arcuate row provides a similar rate of heat exchange. This is achieved by ensuring the gas flows with the substantially same velocity at all positions along a given row; there will be a different velocity in different rows, but it is necessary for the gas flow velocity to be substantially the same at all positions along a given row. 
     Preferably, the first gas is hotter that the second gas. 
     Preferably, the second heat exchanging chamber comprises a cover that is removably coupled thereto, to provide access to the heat pipes for maintenance and or replacement etc. 
     In accordance with a second aspect of the present invention there is provided a gas processing system, the processing system comprising the heat exchanger of the first aspect. 
     Note that the various features of the present invention described above may be practiced alone or in combination. These and other features of the present invention will be described in more detail below in the detailed description of the invention and in conjunction with the following figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the present invention may be more clearly ascertained, some embodiments will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is a sectional view along the length of the heat exchanger according to an embodiment of the present invention; 
         FIG. 2  is a cross-sectional view across the first chamber of the heat exchanger of  FIG. 1 ; 
         FIG. 3  is a cross-sectional view across the second chamber of the heat exchanger of  FIG. 1 ; 
         FIG. 4  is a sectional view along the length of the heat exchanger of  FIG. 1 , as viewed in the direction of the inlet of the second chamber; 
         FIG. 5  is a cross-sectional view across the first heat exchanging chamber illustrating the removable panels; 
         FIG. 6  is a cross-sectional view across the second heat exchanging chamber illustrating the removable panels; and 
         FIG. 7  is a plan view of the heat exchanger of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     The present invention will now be described in detail with reference to several embodiments thereof as illustrated in the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of embodiments of the present invention. It will be apparent, however, to one skilled in the art, that embodiments may be practiced without some or all of these specific details. In other instances, well known process steps and/or structures have not been described in detail in order to not unnecessarily obscure the present invention. The features and advantages of embodiments may be better understood with reference to the drawings and discussions that follow. 
     Referring to  FIG. 1  of the drawings, there is illustrated a sectional view through a heat exchanger  10  according to an embodiment of the present invention. The heat exchanger  10  comprises a first heat exchanging chamber  11  and a second heat exchanging chamber  12 , each chamber  11 ,  12  comprising a substantially cylindrical housing. The chambers  11 ,  12  comprise substantially the same diameter and are mounted on top of each other so that a longitudinal axis of the first chamber  11  is substantially collinear with a longitudinal axis of the second chamber  12 . 
     The first chamber  11  of the heat exchanger is disposed below the second chamber  12  and comprises an inlet  13 , which is disposed at the underside of the first chamber  11 , for receiving a first gas into the chamber  11 . The first chamber  11  further comprises an outlet  14 , which extends from an upper region of the first chamber, from which gas can leave the chamber  11 . The inlet  13  and outlet  14  are centered on the longitudinal axis of the first chamber  11 , so that the flow of first gas is substantially along the longitudinal axis of the chamber  11 . 
     The second chamber  12  comprises an inlet  15  for receiving a second gas into the second chamber  12 , which extends through the arcuate side wall of the housing, and an outlet  16  also disposed in the arcuate side wall, from which the second gas can leave the chamber  12 . The inlet  15  and outlet  16  of the second chamber  12  are disposed substantially diametrically opposite each other so that the flow of second gas within the chamber  12  is substantially across, namely substantially transverse to the longitudinal axis of the chamber  12 . 
     The first and second chambers are separated from each other by a plate  17 , so that the first and second gas flows remain separated. The outlet of the first chamber  11  however comprises a duct  18  which extends from an upper region of the first chamber  11  through the second chamber  12 . The duct  18  extends substantially along the longitudinal axis of the second chamber and terminates at an upper region of the second chamber  12 . 
     The heat exchanger further comprises a plurality of substantially linear heat pipes  19  which extend from within the first chamber, through the plate  17  and terminate in the second chamber  12  so as to enable heat to be transferred between the chambers  11 ,  12 . The heat pipes  19  extends substantially parallel to the longitudinal axis of the first and second chambers  11 ,  12  and are configured in a substantially arcuate arrangement of rows of heat pipes (as illustrated in  FIGS. 2 and 3 ), the radius of curvature of each arcuate row being centered substantially on the longitudinal axis. In this manner each chamber  11 ,  12  comprises a plurality of arcuate rows of heat pipes  19 , having different radii of curvature. 
     The first chamber  11  of the heat exchanger  10  further comprises a plurality of baffles  20  (only one of which is illustrated in  FIG. 4 ) which extend across the first chamber  11 . Each baffle  20  comprises a plate  20   a  having a valve  21  disposed therein, which can be selectively opened and closed to alter the flow path and thus the rate of flow of first gas along the first chamber  11 . The first chamber  11  further comprises an internal wall arrangement  22  that extends between opposite longitudinal ends of the first chamber  11 . The wall arrangement  22  comprises a first and second pair of walls  22   a,    22   b,  which extend from diametrically opposite positions on the cylindrical housing of the first chamber  11 . The walls of each pair  22   a,    22   b  separately converge toward each other and meet at an apex positioned proximate to the longitudinal axis of the first chamber  11 . The wall arrangement  22  is arranged to substantially conform to the outer periphery of the array of heat pipes  19  so as to remove any pocket regions within the chamber  11  which could otherwise provide a flow path for gas along the chamber  11  without encountering a heat pipe  19 . 
     The housing of each chamber  11 ,  12  of the heat exchanger  10  is supported by a plurality of pillars  23  which extend between opposite longitudinal ends of each chamber  11 ,  12 . The housing of each chamber  11 ,  12  further comprises a series of panels  24  (as illustrated in  FIGS. 5 and 6 ) which may be removably coupled thereto using nuts and bolts (not shown), for example, so that the panels  24  can be removed to provide access to the interior of the respective chamber for cleaning and maintenance of the heat pipes. In addition, the heat exchanger  10  further comprises a removable cover  25  (as illustrated in  FIG. 7 ) which extends over the upper region of the second chamber  12  so that access can be gained to the heat pipes to enable them to be removed and replaced if necessary. 
     In use, hot gas from an industrial process for example, is passed into the inlet  13  of the first chamber  11 . The baffle  20  causes the hot gas to move across the heat pipes  19  in moving along the first chamber  11  to the outlet  14 . As the hot gas passes across the heat pipes  19 , it gives up the heat associated therewith to the heat pipes  19  thereby causing the temperature of the heat pipes  19  to rise. The hot gas thus becomes cooled in moving along the first chamber  11  between the inlet  13  and the outlet  14 . 
     In order to reduce the amount of heat transferred to the heat pipes  19 , the valve  21  on the baffle  20  may be opened, so that the hot gas can also pass along the longitudinal axis of the chamber  11  without passing across the pipes  19 . When fully opened, very little heat transfer to the heat pipes  19  takes place since the hot gas can pass direct from the inlet  13  to the outlet  14 , and so the working temperature of the pipes  19  remains relatively low. However, upon fully closing the valve  21 , all the hot gas is forced to pass across the heat pipes to encourage maximum heat transfer. Accordingly, by controlling the extent of the opening of the valve  21 , the working temperature of the heat pipes  19  can be controlled. 
     The heat pipes  19  subsequently transfer the heat to the second chamber  12  which receives cold air from an air conditioning system, for example, through the inlet  15  disposed on the arcuate housing wall. The cold air flow is split by the duct  18  of the outlet  14  of the first chamber  11  into two flow paths which extend around opposite sides of the duct  18 . Each flow path encounters an identical array of heat pipes  19  so that each flow path provides a uniform exchange of heat with the heat pipes  19 . The cold air absorbs the heat from the heat pipes  19  and thus becomes warmed as it flows between the inlet  15  and the outlet  16  of the second chamber  12 . Each of the two flow paths presents a reduced cross-sectional area to the flow of gas, in view of the obstruction provided by the duct  18  forming the outlet  14  of the first chamber  11 , thereby causing the average velocity of the flow of cold gas along each path to increase. 
     From the foregoing therefore, it is evident that the heat exchanger of the present invention provides for an improved control over the working temperature of the heat pipes and provides an array of heat pipes each of which receives a similar rate of heat exchange. 
     While this invention has been described in terms of several embodiments, there are alterations, modifications, permutations, and substitute equivalents, which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and apparatuses of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, modifications, permutations, and substitute equivalents as fall within the true spirit and scope of the present invention.