Abstract:
Systems and methods for heat exchange in accordance with the invention define adequately long-interchange distances for two fluids by wrapping a tube containing a first fluid about the wall of an inner cylindrical tank, within a gap formed with a second concentric tank. A second fluid is transmitted in the space defined between the turns of the tube and the two walls, providing effective short length thermal interchange through the tube walls. The tube is in the line contact with both tank walls and the fluids can flow rapidly over an adequately long length, so that high efficiency is provided in a low cost system.

Description:
PRIORITY  
       [0001]     This invention relies for priority on a previously filed provisional patent application Ser. No. 60/576,706 filed Jun. 2, 2004 by Kenneth W. Cowans, William W. Cowans and Glenn W. Zubillaga. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates to heat exchanger systems and to temperature control units which use such systems, with the objective of providing more efficient, compact, economical and versatile heat exchange functions.  
       BACKGROUND OF THE INVENTION  
       [0003]     The tremendous variety of heat exchangers that is currently available is actually insufficient to satisfy the needs and goals of current developments and technology. The heat exchangers available include many metal, plastic and other configurations in which thermal energy is transferred between different liquids, between gases and liquids, between liquid/vapor fluids and liquids, and between other combinations of media. Such heat exchangers are used for cooling or heating or both purposes.  
         [0004]     As the art has developed, however, increasing demands have been made on the heat exchangers and the temperature control units that utilize them, in terms of efficiency, size and particularly cost. For example, in semiconductor fabrication facilities controlling the temperature of a cluster tool may require that different temperature levels be established and closely maintained in different modes of operation. Since floor space in such installations is very expensive, the footprint of the temperature control unit should be as small as feasible. In addition, the unit should operate reliably for long periods so as not to impede or interrupt tool or overall system operation. The emphasis on lowering cost applies not only to labor and materials but to fabrication techniques. The design should also permit the alternative incorporation of a pump or heater. The present system has been devised as a radically different approach in hardware and method having many potential applications not only in this context, but also in a variety of other applications.  
       SUMMARY OF THE INVENTION  
       [0005]     Heat exchanger units in accordance with the invention transport a temperature controlled gas or liquid, or mixture thereof, at substantial velocity in intimate and uninterrupted relation with respect to a moving thermal transfer fluid that is to be used for temperature control, as in a semiconductor fabrication facility. To this end, thermal transfer fluid is directed in a confined but unrestricted helical path at a radius about a central axis, while a variable temperature fluid or gas, such as a refrigerant, is flowed coextensively and continuously in an adjacent helical path in either a parallel or a counter-flow direction. The cross-sectional areas of both flows are small but the fluids may flow at substantial velocities over paths which are arbitrarily long. The heat transfer distances between the fluids in contrast can be very short through the tubing walls, affording high efficiency operation.  
         [0006]     The flow paths are preferably established between two concentric tanks spaced apart by a small distance, within which refrigerant tubing is disposed in a helical geometry about the central axis. Thermal transfer fluid flows in the spaces between the turns of the tubing and the cross-sectional areas of the two flows are small. Thus there is intimate thermal interchange between the two fluids throughout long path lengths and minimal if any thermal losses along the paths. The interior of the tanks provides a volume which may include an impeller for driving thermal transfer fluid and a heater element for energy additive or corrective purposes. Refrigerant flow is driven by pressure from a compressor in a conventional vapor-cycle system but minimal flow impedance is introduced.  
         [0007]     This system may use a liquid refrigerant after compression and condensation, or the refrigerant as a pressurized hot gas after compression but without condensation. Pressurized refrigerant after condensation will be in a liquid/vapor mix in which the temperature is controlled by an expansion valve. Modern molding techniques and assembly techniques can be used in manufacture of the tanks, so the containers can be of low cost materials and precisely reproducible in quantity. Whether cooling or heating, the thermal transfer fluid can regulate temperature efficiently and precisely, and if cooling and heating are both used, a wide temperature range can be established with an electronic controller system.  
         [0008]     In a more particular example of this versatile heat exchanger, a helical ridge about the periphery of the inner tank provides a guide and support structure for the encircling refrigerant tubing that intimately engages both of the opposite walls in line contact. The refrigerant flow exits from the double cylinder configuration via a vertical tubing parallel to the central axis, for return to the associated refrigeration system. The thermal transfer fluid that is fed into the helical path between the cylinders also fills the interior volume, advantageously through a flow-control pipe extending up from the bottom which encompasses an electrical heater element depending from the top of the tank unit insuring that the heater is immersed in fluid. A multi-stage impeller extending down into the interior of the inner tank from a drive motor on the tank top wall is advantageously employed to drive the thermal transfer fluid. The thermal transfer fluid flows from an input at the side, through the helical path within the gap between the tank walls, then through the interior tank volume to the pump inlet and thence to an outlet port in the top wall about the pump axis. An orifice is included at a point along the thermal transfer fluid loop in a position to release any air in the fluid.  
         [0009]     Methods in accordance with the invention have a number of different aspects. The long and confined but unrestricted flow of fluid at substantial velocities assures that effective thermal exchange occurs over an adequately long path length and large surface area. This occurs without leakage of fluid or thermal energy between the turns along the flow paths. Thermal energy is transferred over short distances between the two fluid flows, which are of small cross-sectional areas. Assembly of the tanks relative to the helical refrigerant tubing uses the flexibility of the unstressed tubing to position the tubing against the helical ridge on the outside of the inner tank. The refrigerant tubing is also tensioned into firm and precise position between the two tank walls, maximizing thermal exchange efficiency without the need for high precision machining. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     A better understanding of the invention may be had by reference to the following description, taken in conjunction with the accompanying drawings in which:  
         [0011]      FIG. 1  is a perspective view, in exploded form, of a heat exchanger system in accordance with the invention;  
         [0012]      FIG. 2  is a top view of the heat exchanger of  FIG. 1  showing section lines A-A and C-C for reference;  
         [0013]      FIG. 3  is a simplified cross-sectional view of the system of  FIG. 1 , taken along the section line A-A in  FIG. 2 ;  
         [0014]      FIG. 4  is a simplified and fragmentary view of a small enlarged segment of the system showing of the relative flow paths of refrigerant and thermal transfer fluid in the system of  FIGS. 1-3 ;  
         [0015]      FIG. 5  is an exploded perspective view of the system, showing the double tank arrangement and conduits in further detail;  
         [0016]      FIG. 6  is a different cross-sectional fragmentary view showing the elements of the system as viewed along the line C-C in  FIG. 2 , looking the direction of the appended arrows;  
         [0017]      FIG. 7  is an enlarged fragmentary view of a portion of the system as seen in  FIG. 6 , showing some of the inlet details of the inlet for thermal transfer fluid;  
         [0018]      FIG. 8  is a fragmentary view of a portion of the system as seen in  FIG. 6 , showing details of how the tanks are attached together and to the top plate;  
         [0019]      FIG. 9  is a perspective fragmentary view of the top plate, inner tank, helical tubing and threaded studs for coupling the elements together;  
         [0020]      FIG. 10  is a flow chart of the steps of a method of assembling the helical refrigerant tubing and double cylinder combination of the system of  FIGS. 1-9 , and  
         [0021]      FIG. 11  is a block diagram of a thermal control unit for a process tool employing a heater exchanger as shown in  FIGS. 1-9 .  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]      FIGS. 1-9  are perspective and cross-sectional views of a heat exchanger  10  in accordance with the invention, utilizing a volumetric arrangement including an outer tank  12  of generally cylindrical form. The outer tank  12  has a closed bottom wall and a top edge with a circumferential rim enclosed by a top plate  13 . A radial space of predetermined size, established generally by the diametral dimensions of a refrigerant tubing to be inserted between them, is established between the inner wall of the outer tank  12  and the outer wall of an inner tank  14  which is concentric therewith and nested therein. The radial gap is slightly greater than ¾″ in this example. The tanks  12 ,  14  are generally concentric about a central axis (shown vertical in the Figures), and the unit rests on a number of hollow feet  15  in the bottom walls. The feet  15  may be filled with foam or otherwise internally filled.  
         [0023]     The outer surface of the inner tank  14  includes a helical peripheral ridge  16  that extends continuously from approximately the top to bottom about the tank  14 . The ridge  16  is shaped as a rounded triangular form in cross-section and has a pitch p ( FIGS. 5 and 7 ) between successive turns that also defines the vertical spacing between turns of the refrigerant tubing  20  helix, as described below. The top surface of the ridge  16  throughout its length defines a seating surface for a small arc of the outer surface of the adjacent helical tubing  20  segment. In position on the ridge  16 , the tubing  20  thus angles gradually downwardly in a helical path from an inlet port where a stiffened inlet section  21  ( FIG. 3 ) of the tubing enters through the top plate  13 . The inlet section  21  and port also provide a circumferential positional reference for the somewhat compliant tubing turns when assembling the tubing  20  between the two tanks  12 ,  14  in accordance with the method described in relation to  FIG. 10  below. Referring to  FIGS. 1, 3  and  5 , particularly, the helical tubing  20  descends about the inner tank  14  to a lower-most turn, at which a transition section  22  ( FIGS. 1 and 5 ) leads radially inwardly to the bottom of a vertical output line  23  that extends up through the top plate  13  and out the heat exchanger system.  
         [0024]     The tanks  12 ,  14  and the refrigerant tubing  20  are sized so that the tubing  20  when properly tightened wedges between the outer tank wall and against the upper surface of the helical ridge  16  throughout the vertical span along the tanks. The tubing  20  firmly contacts and seats against both these generally opposing surfaces with line contact. In this example, the tubing  20  has an outer diameter of 0.75″ and the pitch (p) is about 1.75″.  
         [0025]     As seen in  FIG. 4 , the successive turns of the helical tubing  20  and the facing sides of the tanks  12 ,  14  define a four sided cross-sectional area for the thermal transfer fluid, with two sides flat (the tank walls) and two sides concave (defined by the convex tubing exteriors). This cross-sectional area is greater than the internal cross-sectional area of the tubing  20 , but both are small. For the configuration shown, the flow area is being less than 0.50 in 2  for the tubing  20  and less than 1.00 in 2  for the space between the tubing turns and their walls. The lengthwise flow paths, on the other hand, will be more than 20 feet long for each of the fluids, and can be of almost arbitrary length.  
         [0026]     The thermal transfer fluid typically has both a high boiling point and a very low freezing point. It is very common for these applications to use a proprietary fluid named “Galden”, which has the needed boiling and freezing properties and a flowable viscosity throughout its temperature range. Mixtures which have similar properties, such as ethylene glycol (a typical antifreeze) and distilled water, may also be used. The particular thermal transfer fluid that is chosen is a matter of choice for the particular installation. For many less demanding heat exchange applications a specialized thermal transfer fluid may not be needed.  
         [0027]      FIGS. 1, 3 ,  4  and  6  show the thermal transfer fluid path and flow direction, starting with an inlet port  36  ( FIGS. 1 and 7 ) in the top plate  13  which leads into the gap between the tank walls and the tubing  20  turns. The thermal transfer fluid also flows helically between the tubing  20  turns at an angle downwardly to the bottom level within the outer tank  12 . The fluid flow at the bottom first enters the open base of a vertical flow tube  25  ( FIG. 6 ) which is offset from the central axis and forms a separate chamber that is also spaced apart from top plate  13  at its upper end, so that fluid can spill over into the main interior volume. When the fluid level fills up the flow tube  25  to the top, the fluid spills outwardly through the upper gap between the flow tube  25  and the top plate  13  and pours into the main cavity of the inner tank  14 . It next fills the inner tank  14  interior, including the volume below an axial pump motor  28  that is mounted on the top plate  13 . The pump system includes multiple stages of pumping impeller elements  27  which extend down into the interior of the inner tank  14 . The pump impeller  27  and motor  28  may advantageously be of a type of multistage centrifugal pump that is made by Grundfos of Germany. This pump impeller  27  may, for example, have 12 stages, each stage driving the fluid to a successively higher level until the ultimate output stage level is reached at the top position and the fluid exits via a radial output port  35  (best seen in  FIGS. 1 and 2 ). The bottom of the outer tank  12  includes a pair of drain ports  29 ,  30  in which removable plugs are threaded to allow draining of liquid from within the tanks  12 ,  14 .  
         [0028]     Referring to  FIG. 2  the core  32  of an optional electrical heater  31  is also advantageously mounted (though a heater may not be required) on the top plate  13 . The heater core element  32  extends down into the flow tube  25 . The core  32  is assuredly immersed once circulation of thermal transfer fluid begins. When the core  32  is energized it heats the surrounding fluid with high efficiency. The heater  31  is selected to provide sufficient power, e.g. 1250 watts, to heat the fluid to a predetermined maximum temperature level, when in the heating mode. The heater may also be used to adjust output temperature more precisely if the associated process tool is below a desired level. The flow tube  25  isolates the heater  31  from the stages of the axial pump impeller  27  as well as insuring that the heater element is encompassed by fluid.  
         [0029]     Flow-paths in the top plate  13  about and concentric with the pump axis  27  lead into the radial output port  35  ( FIGS. 1 and 3 ) just above the top plate  13 . The input conduit  36  for the thermal transfer fluid feeds into the gap between the two tanks  12 ,  14  at one circumferential position, here spaced apart from the inlet tube  21  for the refrigerant.  
         [0030]     Consequently, assuming here that the heater element  31  and the pump impeller  27  are both energized, operation commences by input of the thermal transfer fluid into the gap between the tanks  12 ,  14  to flow helically around the gap between the turns of the refrigerant tubing  20 . Concurrently refrigerant is fed into the input line  21  to the tubing  20  leading through the top plate  13  and flows helically in parallel paths adjacent to the thermal transfer fluid flow paths. Since the thermal transfer fluid moves helically within the gap defined by adjacent tubing  20  turns and the opposing tank walls there is only a short, heat conductive, tubing wall between the two fluids. Efficient thermal interchange through the short path of the tubing  20  wall heats or chills the thermal transfer fluid with the refrigerant in accordance with the temperature setting for the system. No meaningful leakage path exists between the tubing  20  and the inner tank  14  on one side and the tubing  20  and the inner wall of the outer tank  12  on the other, because the diametral size of the refrigerant tubing  20  fits closely to the gap, and the assembly method used tightens the tube  20  against both inner and outer surfaces. Cross-leakage of thermal transfer fluid between the turns therefore does not introduce significant heat energy losses.  
         [0031]     Thermal energy interchange and efficiency are facilitated by the substantial velocities of the two fluids. In the tubing  20 , the refrigerant is in a liquid-vapor state, and transported at a mass flow rate, in one practical example, of 100 g/sec. The thermal transfer fluid is, in the same example, transported at about 100 cm/sec. The example is based on use of a 3 HP compressor and a thermal transfer fluid flow of 2-15 gal/min. The flow rates are sufficient to ensure flow turbulence, enhancing thermal interchange.  
         [0032]     The preferred arrangement for filling the inner tank  14  is to pour thermal transfer fluid in via an upstanding fill port  43  that extends down, into the interior volume. The fluid level may be observed at a sight gauge (not shown) or measured by the signal from a level indicator  45  located extending into the interior from the top plate  13 . The fill port  43  is then closed off during circulation of the thermal transfer fluid.  
         [0033]     Alternatively, the tank  14  can be filled by normal input flow so that when the thermal transfer fluid reaches the bottom level of the tanks  12 ,  14 , within the outer tank, it first fills the flow tube  25 , then spills over the top of the flow tube  25 , pouring into the major portion of the interior volume. With some fluid at least partially filling the inner volume, the heater  31  can be turned on, and then the pump  27  can drive thermal transfer fluid upwardly toward the outlet apertures  42  in the top wall  13  and the outlet port  43 . Alternatively, if the tanks fill sufficiently rapidly, the pump  27  can be turned on at the outset and delay can be tolerated without overheating the pump, or the pump chosen can be of a design which does not require cooling.  
         [0034]     A bleed hole  47  ( FIG. 1 ) in the uppermost part of the top wall  13  of the inner tank  14  allows air in the thermal transfer fluid to escape into the main volume as the system fills, and precludes air entrapment in the space between the inner and outer walls. Orifices for this purpose may be placed elsewhere to eliminate an air entrapment condition.  
         [0035]     In contrast to the thermal transfer fluid, the refrigerant need not be separately pumped because the pressurization provided by the compressor in the system drives the refrigerant, via the inlet  21 , down through the helical tubing  20 . The flow continues through the turns of tubing until the exit section  22  at the bottom leads to the vertical outlet riser  23  forming the exit path along one circumferential side of the inner tank  14 , from where the refrigerant flows outwardly to return to the compressor in the system.  
         [0036]     This system thus efficiently heats or chills thermal transfer fluid with virtually maximum efficiency. Both the thermal transfer fluid and the refrigerant circulating in the tubing within it are closely interspersed and both move at whatever velocity is desired, without restriction.  FIGS. 4, 6  and  7  show in the enlarged views particularly how the turns of the tube  20  have wedged firmly with line contact against the upper surfaces of the ridges  16  on the outside of the inner tank  14 .  FIGS. 4 and 7  also show that on the opposite side there is line contact between the tube  20  and the inner wall of the outer tank  12 . This result is achieved without ultra-precise machining or selection of matching parts.  
         [0037]     The method of assembly of this system so as to precisely fit the helical tubing  20  within the double walls of the volumetric housing  10  is illustrated in the steps of  FIG. 10 , and can better be visualized with the aid of the perspective views of  FIG. 1  and  FIG. 5 . A tubing (e.g. copper tubing) of selected outer diameter, e.g. ¾″ having some flexibility when unstressed is disposed in coil form concentric with a central axis. The partially loose coil is fitted over the inner tank  14  and seated loosely on the helical ridge  16 . The top wall of plate  13  is then attached, with the inlet section  21  of the tube  20  fitted into an aperture in the plate  13  which fixes its circumferential position. Threaded studs  39  are vertically inserted into the plate  13 , engaging the top turn of the tubing  20  and forcing it down onto the ridge  16 . The tubing  20  having been circumferentially secured by the stiffened inlet section  21 , the coil of tubing  20  is drawn downwardly, which radially compresses the coiled tubing  20  against the ridge  16 . The inner tank  14 , with the tubing  20  in position, is fed into the outer tank  12  concentric with the central axis, nesting into the volume of the outer tank  12  as the tanks  12 ,  14  are coaxially positioned. Then the tubing  20  is tensioned circumferentially, by exerting torque on the exit post  23  against the counteracting force from the fixed input end. This allows the outer tank to  12  to slide easily over the inner tank  14  and tubing  20 . After this assembly, the coiled turns of tubing  28  are expanded by depressing the center tube  23  to force the tubing  20  to assume the predetermined pitch (p) spacing between the ridges  16  ( FIGS. 4 and 7 ) on the surface of the tank  14 .  
         [0038]     The top rim of the outer tank  12  periphery may then be bonded to the outer tank  12 , in the position seen in  7 . The threaded studs  39  are tightened down onto the top tubing turn, holding the tubing in a reference position. The tubing system is held in the position shown in  FIGS. 5, 6  and  7 , and the assembly of major parts is thus concluded.  
         [0039]     The flow of thermal transfer fluid through the system and the flow of refrigerant through the system may be reversed for specific applications. The pump for thermal transfer fluid may comprise any of a number of pumps although the Grundfos-type gradient pump is advantageous for its size and form factor. The heater element, as mentioned, need not be employed, but the flow tube provides an advantageous operating factor in assuring that the thermal transfer fluid fills the interior cavity of the heat exchanger between outer tank  14  and outer wall of center tank  12 .  
         [0040]     A thermal control unit that takes advantage of some of the potential of this heat exchanger is depicted in block diagram form in  FIG. 11 . As in a typical refrigeration mode control system, a compressor  50  cycles a refrigerant (say R507) in one loop while a thermal transfer fluid (say Galden) is directed internally to control the temperature of a process tool  52  after being heated (or cooled) by the refrigerant in a heat exchanger  54 . In this instance the pump  54 ′ and heater  54 ″ shown in block form only in  FIG. 11  are incorporated within the body of the heat exchanger  54 , as previously described in conjunction with  FIG. 1 . The conventional refrigeration loop includes a condenser  56  cooled by an ambient fluid and a thermal expansion valve (TXV)  58 . The valve  58  then feeds a temperature variable liquid/vapor mix, at a temperature as set by a controller  59  or operator, to determine the temperature desired for the process tool  52 . In this mode the heat exchanger  54  may function as an evaporator, taking up heat to chill the thermal transfer fluid to a controlled level in accordance with the degree of vaporization and the pressure of the refrigerant.  
         [0041]     In this configuration, in which the pump  54 ′ feeds the process tool  52  after the thermal transfer fluid is chilled, some minor amount of refrigeration (or heating) capacity is lost in the fluid line. The small added increment of chilling power that is needed is more than compensated economically by the cost-advantages of the exchanger  54 . Moreover a differently placed pump can always be used.  
         [0042]     In a heating mode, the compressed hot gas from the compressor  50  bypasses the condenser  56  to a hot gas valve  57  as the TXV  58  is shut down and a shunt solenoid expansion valve (SXV)  60  is opened with a varying duty cycle to supply the hot gas to the heat exchanger  54  for temperature control. This proportional control greatly increases the temperature range at which the system can operate.  
         [0043]     The controller  59  receives a signal (T 1 ) from a sensor  65  coupled to the output line from the process tool  52 , and may receive pressure and temperature signals from other sensors (not shown) in the system, in conventional fashion. A bleed orifice  66  may be included to permit the release of air, if any, in the thermal transfer fluid as it circulates, but may alternatively be placed at other points. A bypass orifice can be included to allow some flow between input and output to insure pump cooling. As is well known, the controller  59  can operate in any one or more of a number of control modes, responsive to inputs from these or other transducers and sensors.  
         [0044]     In the double tank system of  FIGS. 1-9  low cost, readily replicable materials can be utilized, such as industrial plastics. These also have the advantage of low thermal conductivity, and allow the ridges  16  on the inner tank  14  to be made integral with the molded body. Metal materials, such as stainless steel, have also proven to be satisfactory.  
         [0045]     Although various alternatives and expedients have been described, the invention is not limited thereto but includes all forms and variations within the scope of the appended claims.