Abstract:
A high efficiency shell and tube heat exchanger coupled to a hydraulic thermal engine includes an insulated cylindrical shell having first and second ends for conveying a coolant and a plurality of tubes passing through at least one of the ends for conveying a working fluid. A plurality of spaced-apart generally-transverse baffles are disposed in the shell. Each baffle is truncated along one edge defining a slot for passage of coolant along the shell inner wall and past the baffle. Successive of the baffles are rotated with respect to the longitudinal axis of the shell to cause the coolant flowing therethrough to follow a non-axial path. Multiple coolant inlets and outlets may be provided. The heat exchanger is optimized for a use wherein the second working fluid is liquid CO 2  in a supercritical fluid state. The heat exchanger tubes have an inside diameter of 0.26 inches or less.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to heat exchangers; more particularly, to a shell and tube heat exchanger coupled to a hydraulic thermal engine; and most particularly, to a shell and tube heat exchanger having an improved heat-exchanging rate such that the hydraulic thermal engine may be powered using readily available low level heat energy (180° F.), such as for example, solar energy. 
       BACKGROUND OF THE INVENTION 
       [0002]    U.S. Pat. No. 5,899,067, the relevant disclosure of which is incorporated herein by reference, discloses a hydraulic thermal engine (herein also referred to as an “HT engine”) powered by introduction and removal of heat from a working fluid that changes volume with changes in temperature. A container houses the working fluid, and a cylinder secured to the frame includes an interior space. The cylinder also includes a passage for introducing the working fluid into the interior space. A piston is housed within the interior space of the cylinder. The working fluid container, the interior space of the cylinder, and the piston define a closed space filled by the working fluid. The HT engine also includes means for transmitting heat to and removing heat from the working fluid, thereby alternately causing the working fluid to expand and contract without undergoing a phase change. The piston moves in response to the expansion and contraction of the working fluid. 
         [0003]    Shell and tube heat exchangers (also referred to hereinbelow generically as “heat exchangers”) are known in the prior art as well. Such a device typically comprises a cylindrical container (shell), for conveying a cooling or heating fluid (referred to herein as “coolant”) through the shell, and a plurality of tubes passing longitudinally through the shell and immersed in the coolant for conveying a working fluid to be heated or chilled by energy exchange with the coolant through the walls of the tubes. 
         [0004]    Efficiency of a heat exchanger is limited by, among other parameters, the rate at which heat can be transferred through the walls of the tubes. 
         [0005]    It is known in the art to increase heat transfer efficiency by increasing the total transfer surface area by increasing the number of tubes per unit cross-sectional area; by forming the tubes of material having higher heat transfer coefficient; and by making the walls of the tubes thinner. 
         [0006]    It is also known to baffle the flow of coolant through the shell to try to minimize the thickness and residence time of a boundary layer on the outer walls of the tubes and the inner surface of the shell. 
         [0007]    It is also known to minimize the thickness of the tube walls to minimize the tidal heat energy in the system and speed of response during alternating heating and cooling cycles. 
         [0008]    It is also known that the rate of heat transfer is a function of the absolute temperature difference between the coolant and the working fluid. 
         [0009]    It will be appreciated that the capability of the disclosed engine for doing work is dependent directly upon the efficiency of the associated heat exchanger. 
         [0010]    What is needed in the art is an improved high efficiency shell and tube thermal heat exchanger. 
         [0011]    It is a principal object of the present invention to increase the work output of an HT engine. 
         [0012]    It is a further object of the present invention to increase the output of mechanical devices driven by an HT engine. 
         [0013]    It is a still further object of the present invention to improve the efficiency and output of an HT engine through the use of a low level, readily available heat source to power the HT engine. 
       SUMMARY OF THE INVENTION 
       [0014]    Briefly described, an improved high efficiency shell and tube heat exchanger in accordance with the present invention comprises an insulated cylindrical shell having first and second ends for conveying a coolant and a plurality of tubes passing through at least one of the ends for conveying a working fluid. A plurality of spaced-apart generally-transverse baffles are disposed in the shell, wherein one or more (preferably all) of the tubes passes through the plurality of baffles. Each baffle is truncated along one edge defining a slot for passage of coolant along the shell inner wall and past the baffle. Successive baffles may be rotated with respect to the longitudinal axis of the shell to cause the coolant flowing therethrough to follow a non-axial path, preferably sinusoidal or helical. Multiple coolant inlets and outlets may be provided. In one aspect of the invention, the shell is divided by at least one transverse dam into a plurality of shell spaces, each of which is provided with an inlet and an outlet for the coolant (the tubes being continuous through the transverse dam). 
         [0015]    It will be appreciated that the heat transfer efficiency per unit length of the heat exchanger is dependent of the overall length. Thus, the length of the heat exchanger may be selected according to the volume of working fluid a particular application requires. 
         [0016]    In an exemplary embodiment, the heat exchanger is optimized for a use wherein the second working fluid is liquid CO 2  in a supercritical fluid state, and the coolant is water. The heat exchanger is dimensionally optimized to maximize heat exchange rate; provide for capability to handle alternate flow of hot and cold water in a quick cycle; minimize water flow rates; withstand design pressures up to 3600 psi; provide sufficient CO 2  volume to drive associated piston movement; and keep the CO 2  volume low enough to maintain a high pressure differential across the tube walls. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0018]      FIG. 1  is a schematic drawing showing an HT engine operationally connected to exemplary first and second heat exchangers formed in accordance with the present invention; 
           [0019]      FIG. 2  is an isometric view of the exterior of an exemplary heat exchanger in accordance with the present invention; 
           [0020]      FIG. 3  is an isometric view similar to the view shown in  FIG. 2  but with the heat exchanger shell removed; 
           [0021]      FIG. 4  is a plan view of an intermediate transverse dam within the heat exchanger shown in  FIGS. 2 and 3 ; 
           [0022]      FIG. 5  is a plan view of an end plate within the heat exchanger; 
           [0023]      FIG. 6  is a plan view of a baffle within the heat exchanger; and 
           [0024]      FIG. 7  is temperature/pressure phase diagram for carbon dioxide. 
       
    
    
       [0025]    Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner. 
       DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0026]    An HT engine, in its simplest form, comprises a piston slidably disposed in a cylinder. For performing work the piston may be exposed on a first side to a crankshaft through a connecting rod or, as exemplified herein, a first working fluid in communication with a hydraulically driven apparatus, e.g., an air conditioner or a desalination unit. The slidable piston is exposed on its opposite side to and in direct fluid communication with a second working fluid contained in a closed chamber and having a high coefficient of thermal expansion. The closed chamber comprises that portion of the cylinder occupied by the second working fluid and by a heat exchanger. The heat exchanger is a shell and tube type and may be integrated with that portion of the cylinder or, as exemplified herein, disposed distinct from the cylinder portion but in hydraulic communication therewith. 
         [0027]    In operation, hot and cold coolants are alternately pumped through the shell of the heat exchanger, enveloping the tubes in the heat exchanger occupied by the second working fluid. When the coolant is higher in temperature than the second working fluid, heat is absorbed by the second working fluid and the second working fluid is caused to expand, thereby increasing the overall volume of the second working fluid not only in the tubes but in the cylinder adjacent the piston as well, creating a force on the piston to move the piston in a first direction away from the expanding fluid. When subsequently a coolant lower in temperature is pumped through the heat exchanger shell, heat is absorbed by the coolant and the second working fluid in the tubes and in the cylinder is caused to contract, thereby decreasing the overall volume of the second working fluid. The volume of the second working fluid creates a force imbalance on the piston causing the piston to move in a second and opposite direction of the first direction toward the contracting fluid. With appropriate valving of the first working fluid, and the sequential applications of cool and hot coolant, the engine can be useful as a positive displacement pump. 
         [0028]    It will be seen that, on the side opposite the working fluid, the piston may be connected to a crank via an articulated connecting rod, as disclosed in the incorporated reference patent or, as in the example disclosed, by a fixed connecting rod through an intermediate cylinder space to a second oppositely-acting HT engine disposed collinearly and face-to-face with the first HT engine and operated by a second heat exchanger. The first working fluid is disposed between opposing HT engines and may be connected by appropriate valving to a hydraulically powered apparatus in known fashion. The two HT engines may be made to work in tandem by programming the heating and cooling cycles of their respective second working fluids in the respective first and second heat exchangers to be out of phase with each other. 
         [0029]    It will be seen that this arrangement is incapable of doing net work, as the volume of the first HT engine cylinder that is swept by the first piston is exactly equal and opposite to the volume of the second HT engine cylinder that is swept in the opposing portion of the operating cycle by the second piston. Therefore, a third piston is disposed on the rigid connecting rod in the intermediate cylinder space midway between the first and second HT engine pistons. The third piston, which may be thought of as a compression piston, serves to divide the intermediate cylinder (with appropriate valving) into two distinct compression chambers capable of pumping the first working fluid between the compression chambers via an attached hydraulic apparatus. Note that, to optimize the force relationships in the system, the intermediate cylinder need not be of the same diameter as the two HT engine cylinders. 
         [0030]    Regarding the heat exchangers, it has been found that a shell and tube heat exchanger produces surprising engine efficiencies and is well adapted to the present use. In conventional double-ended flow-through shell and tube heat exchangers, although it is desirable to increase the total heat exchange area by decreasing the diameter of the tubes, there is a lower limit to tube diameter because of the reduction of volume of the working fluid and its viscous drag, which increases exponentially with decrease in tube diameter. For practical reasons, then, the prior art teaches to limit tube inside diameters (ID) to values well above 0.25 inches and teaches away from smaller diameter tubes of 0.25 inches or less. 
         [0031]    However, in the present invention, the heat exchanger is a special case because there is no net flow through the tubes, only a tidal volume of a working fluid that expands and contracts. Further, viscous losses decrease with distance from the open end into the tubes because there is progressively less fluid passing through each tube during expansion and contraction; indeed, at the closed ends of the tubes, there is zero fluid flow at any time. Recognizing this, the inventors have found surprisingly that ID values smaller than 0.25 inch are not only practical but are operationally desirable for maximizing the effectiveness of the HT engine and of the heat exchanger for the particular working fluid selected, which is supercritical carbon dioxide. 
         [0032]    Referring to  FIG. 1 , a schematic of a three-piston HT engine system  10  is shown. Engine system  10  comprises a central cylinder  12  containing a central compression piston  14 . A first HT engine cylinder  16  having a first end wall  17  and a second HT engine cylinder  20  having a second end wall  21  interface with central cylinder  12  at intermediate walls  23 , and  25 , respectively. First and second HT engine cylinders  16 , 20  are co-linear with central cylinder  12 . HT engine cylinders  16  and  20  contain respective first and second HT pistons  24 , 26 . The three pistons  14 , 24 , 26  may be rigidly connected by a linear connecting rod  28 , as shown. Connecting rod  28  passes through intermediate walls  23  and  25  through openings  27 , 29 . Rod seals  31  are disposed around openings  27 , 29  between rod  28  and the openings. The volume of cylinder  12  between piston  14  and intermediate wall  23  on one side and between piston  14  and intermediate wall  25  on the other side is occupied by a first working fluid  33 . 
         [0033]    First HT engine cylinder  16  between piston  24  and first end wall  17  contains an amount of a second working fluid  18 ; and second HT engine cylinder  20  between piston  26  and second end wall  21  contains an amount of the second working fluid  22 . 
         [0034]    First working fluid  33  may be in fluid communication with a hydraulically driven apparatus such as, for example, an AC compressor or a desalinization pump (not shown), as disclosed hereinabove, via first and second outlet ports and valves  30 , 32 . 
         [0035]    First HT engine cylinder  16  is in fluid communication via line  34  with a first shell and tube heat exchanger  38  also containing an amount of second working fluid  18  in accordance with the present invention. Second HT engine cylinder  20  is in fluid communication via line  40  with a second shell and tube heat exchanger  42  also containing an amount of second working fluid  18  in accordance with the present invention. 
         [0036]    A first hot water supply  44  and a first cold water supply  46  are connected via a three-way valve  48  to an inlet  50   a,  in the shell of first exchanger  38 . A first hot water return  52  and a first cold water return  54  are connected via a three-way valve  56  to an outlet  58   a  in the shell of first exchanger  38 . 
         [0037]    A second hot water supply  60  and a second cold water supply  62  are connected via a three-way valve  64  to an inlet  66   a  in the shell of second exchanger  42 . A second hot water return  68  and a second cold water return  70  are connected via a three-way valve  72  to an outlet  74   a  in the shell of second exchanger  34 . 
         [0038]    In one aspect of the invention, multiple inlets and outlets may be used along the length of the heat exchangers to optimize the heat transferred to and away from second working fluids  18 , 22 . For example, in first heat exchanger  38 , first hot water supply  44  and a first cold water supply  46  may be connected via three-way valve  48  to a plurality of inlets  50   a , 50   b,  exemplarily shown as two, on opposite sides of a transverse dam  51  in the first exchanger&#39;s shell. Further, first hot water return  52  and first cold water return  54  may be connected via three-way valve  56  to a plurality of outlets  58   a , 58   b , exemplarily shown as two, on opposite sides of dam  51  in the first exchanger&#39;s shell. 
         [0039]    Similarly, in second heat exchanger  42 , second hot water supply  60  and second cold water supply  62  may be connected via three-way valve  64  to a plurality of inlets  66   a , 66   b,  exemplarily shown as two, on opposite sides of a transverse dam  53  in the second exchanger&#39;s shell. Further, second hot water return  68  and second cold water return  70  may be connected via three-way valve  72  to a plurality of outlets  74   a , 74   b , exemplarily shown as two, on opposite sides of dam  53  in the second exchanger&#39;s shell. 
         [0040]    As can be seen in  FIG. 1 , because of the placement of central dam  51 , 53  between sets of inlets/outlets, a first flow circuit isolated from a second flow circuit is formed. 
         [0041]    The following operating description is provided for first heat exchanger  38 , although it should be recognized that the operation (not described) of second heat exchanger  42  is identical but 180° out of phase with that of first heat exchanger  38 . 
         [0042]    In operation, when three-way valves  48 , 56  are in a first position, hot water from supply  44  enters first heat exchanger  38  via inlet  50   a  (or  50   a , 50   b ), passes through the shell as described below, and exits heat exchanger  38  via outlets  58   a  (or  58   a , 58   b ) and hot water return  52 , causing working fluid in the tubes within heat exchanger  38  to expand. The hot water from hot water supply  44  may be heated, for example, by solar energy or by surrounding waste heat. Expanded working fluid flows from heat exchanger  38  through line  34  into cylinder  16 , urging piston  24  to the right in  FIG. 1 . Air in the chamber between piston  24  and intermediate wall  23 , that would otherwise be trapped, may be exhausted through port  36 . When three-way valves  48 , 56  are in a second position, cold water from supply  46  enters first heat exchanger  38  via inlet  50   a  (or  50   a , 50   b ), passes through the shell and exits heat exchanger  38  via outlet  58   a  (or  58   a , 58   b ) and cold water return  54 , causing working fluid in the heat exchanger to contract. When piston  24  is drawn toward the contracting working fluid in cylinder  16 , air may be drawn back into the chamber between piston  24  and intermediate wall  23  through port  36 . Contracted working fluid flows through line  34  from cylinder  16  into heat exchanger  38 , urging piston  24  to the left in  FIG. 1 . 
         [0043]    A great advantage of the present invention is that hot water sources  44  of the exemplary heat exchanger may be derived from a solar hot water heater (not shown) as is known in the art, thus making the present invention highly useful for driving processes such as desalination without requiring fossil fuels and internal combustion engines. Moreover, depending on the rate of thermal expansion of the second working fluid, the volume of the second working fluid and the diameter of piston  24  the temperature difference between the hot water supply  44  and the cold water supply  46  may be surprisingly small, allowing the use of cheap and readily available waste heat, as for example, a working fluid primarily heated by solar energy, to drive the engine. 
         [0044]    Referring now to  FIGS. 2 through 6 , heat exchanger  38  comprises an insulated shell  71  closed at a first end  73  and having first and second coolant inlets  50   a , 50   b  and first and second coolant outlets  58   a , 58   b  as described above. At the open end  75 , a shell flange  76  is mounted to shell  71 . A cap  78  is connected to shell flange  76  to seal the end of shell  71  and withstand high pressures within the heat exchanger. Cap  78  includes a central opening  80  for receiving a connecting fitting (not shown) for attaching line  34 . 
         [0045]    Within shell  71  is a structure comprising a plurality of longitudinal stringer rods  82  passing through and connecting a plurality of perforated baffles  84  having truncated edges  85  (see  FIG. 6 ) and central dam  51 . A plurality of longitudinal heat exchange tubes  86  (only one shown in  FIG. 3 ), each tube  86  being closed at shell end  73 , also passes through baffles  84  and dam  51 . Tubes  86  are open at their opposite ends and terminate in a distribution chamber (not shown) within header flange  78 . Working fluid entering and leaving through opening  80  is distributed to, and collected from, tubes  86 . 
         [0046]    Referring to  FIGS. 4 and 6 , tubes  86  pass through baffles  84  (and optionally through central dam  51 ) in a preferably rectangular pattern. Central dam  51  may be sealed around its periphery to the interior wall of shell  71 . Further, tubes  86  are sealed to dam  51  to prevent the passage of water across dam  51 . 
         [0047]    Referring to  FIG. 5 , open ends of tubes  86  are in fluid communication with the distribution chamber and terminate in openings  88  in a spacer  90  also sealed around its periphery to the interior wall of shell  71 . 
         [0048]    Referring to  FIG. 7 , a currently-preferred first working fluid  18 , 22  is superfluid carbon dioxide (CO 2 ) maintained within an operating temperature and pressure range  92 . In currently-preferred operating conditions, the carbon dioxide is maintained at pressures between about 1150 psi and about 2400 psi, and at temperatures between about 120° F. and about 140° F., using alternating coolant cooling and heating temperatures of about 80° F. and about 180° F. 
         [0049]    In an exemplary embodiment of the apparatus shown in  FIG. 1 , central cylinder  12  and piston  14  are 12.0 inches in diameter. First and second HT cylinders  16 , 20  and HT pistons  24 , 26  are 6.0 inches in diameter. The stroke of pistons  14 , 24 , 26  is 2.5 feet. Each heat exchanger  38 , 42  is 9.5 feet in overall length, has a shell inner diameter of 9.0 inches, and contains  568  tubes arranged at a pitch of 2.0, the pitch being the ratio of the tube OD to the distance between centers of adjacent tubes. The ID of each tube is between about 0.05 inches and about less than 0.26 inches, preferably about 0.118 inches. Baffles  84  are spaced at about 1 foot intervals along the inner wall of shell  71 . Preferably, adjacent baffles are rotated through a central angle between 0° and 90° to create a nonlinear flow path for coolant flowing through shell  71  between the inlets and outlets. “Hot” water temperatures for the second working fluid may be about 182° F. at the shell inlets and about 162° F. at the shell outlets. Average CO 2  temperature within the heat exchanger is about 120° F. at maximum contraction and about 140° F. at maximum expansion. This embodiment is operated optimally at a stroke time of 9.60 seconds and a water flow rate of 74.8 gpm, sufficient to pump 12.27 cfm of first working fluid  27  through outlets/inlets  30 , 32 . 
         [0050]    While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.