Abstract:
A direct-fired generator for an absorption chiller includes an inner shell, in which combustion occurs, and an outer shell. The inner shell supports a tube bundle through which a first portion an absorption solution is conveyed. Combustion products makes a single pass across the tube bundle within the inner shell. Such construction minimizes the number of potential leak paths and facilitates leak testing of the generator at an intermediate stage of assembly. A flow distributor apportions solution flow to the tube bundle and to a second solution flow path which bypasses the tube bundle but which is likewise heated by the combustion occurring within the inner shell. The two solution flow paths converge after the solution flowing therethrough has been heated by the combustion occurring in the inner shell. A vapor separator disentrains solution in liquid form from vaporized solution before the vapor exits the generator.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a generator for an absorption cooling system. More particularly, the present invention relates to fluid flow patterns in a direct-fired generator of an absorption chiller.  
           [0003]    2. Description of Related Art  
           [0004]    Typical absorption chillers have a refrigerant or working fluid consisting of at least a two-part solution, such as a solution of lithium bromide and water or ammonia and water. Varying the solution&#39;s concentration by cyclically vaporizing and reabsorbing of the solution&#39;s two components allows for the use of a pump or multiple pumps to circulate the solution through the chiller to create a cooling effect.  
           [0005]    In operation, one or more so-called generators add heat the solution to raise its absolute pressure and to vaporize one solution part. The vaporized part will be referred to hereinbelow as a weak or less concentrated solution and for a solution of lithium bromide and water, the term “weak solution” refers to pure or nearly pure water which may be found in a liquid or vaporous state downstream of the generator. For systems using a solution of ammonia and water, the weak solution is pure or nearly pure ammonia. The unvaporized portion of the solution in the generator is referred to as a more concentrated or strong solution.  
           [0006]    Weak solution flows from the generator of an absorption chiller to a condenser where it is cooled and condensed to liquid form. From the condenser, the solution flows to and functions as a refrigerant within a relatively lower-pressure evaporator component. The lower pressure found in the evaporator causes the solution to expand. That expansion further lowers the solution&#39;s temperature and permits that solution to be used as a refrigerant to cool still another liquid, most typically water. That cooled liquid is then used as needed, such as to cool rooms or other areas of a building or in an industrial process application.  
           [0007]    After performing its cooling function in the evaporator and vaporizing in the process, the weak solution migrates, in vaporous form, to the absorber component where it is reabsorbed resulting in the creation of a liquid solution of intermediate concentration. That solution is delivered to the generator component to repeat and gain the effect of the solution separation process.  
           [0008]    A generator is referred to as being direct-fired if its source of heat is from direct combustion instead of from steam or waste heat delivered to the chiller from another process and/or location. In direct-fired generators, hot combustion gas is typically directed across the exterior of a tube set through which solution of intermediate concentration flows so as to heat the solution and cause the vaporization of a portion of it.  
           [0009]    The heating of solution in a direct-fired generator often involves multiple passes of combustion gas across the tube set so as to extract as much heat from the combustion gas as possible. While efficient in that regard, multi-pass designs typically add significantly to the cost and complexity of a generator for the reason that such designs generally have more parts including, but not limited to, a turn box which redirects the flow of combustion gas from one pass across the tube set to another.  
           [0010]    In so-called single-pass direct-fired generator designs, combustion gas makes only one pass across the tube set. In such designs, an outer shell often surrounds an inner combustion chamber. Combustion gas heats some of the solution as it travels vertically upward through the tube set and heats the rest of the solution as it travels upward between the inner and outer shells of the generator.  
           [0011]    In practice, it can be very challenging to manufacture shell-within-shell units. Further, once the shells are assembled and welded together, it can be very difficult to find and repair any leaks between the two that might exist. Even a slight leak can dramatically affect an absorption chiller, not only from a performance standpoint, but from a reliability standpoint. In that regard, the leakage of air into an absorption chiller can lead to rapid and extensive corrosion inside the unit.  
           [0012]    Other concerns with existing single-pass generator designs exist. For example, rapid upward flow and discharge of solution from the vertical tubes or from between the sides of inner and outer generator shells in such designs can create a geyser-like effect at the surface of the solution pool which is found just above the combustion chamber. Such disruption of the solution pool surface tends to cause the vaporous solution above that pool surface to entrain and carry liquid out of the generator and into the system condenser, evaporator, and, eventually, absorber. Any such liquid carryover reduces an absorption chiller&#39;s capacity.  
           [0013]    The need continues to exist for a readily manufacturable single-pass direct-fired generator for an absorption chiller wherein the generator can be leak tested before final assembly and in which provision is made to minimize the carryover of liquid entrained in the vapor that flows out of the generator&#39;s interior.  
         SUMMARY OF THE INVENTION  
         [0014]    It is an object of the present invention to provide an absorption chiller with a single-pass, direct-fired generator the inner shell of which can be fabricated and completely leak checked before fabricating the outer shell.  
           [0015]    Yet another object of the present invention is to apportion liquid solution flow within a direct-fired generator between a first path, through the generator&#39;s tube bundle, and a second path, which bypasses the tube bundle, such that most of the heat transfer between combustion gas and solution occurs within the tube bundle.  
           [0016]    A further object of the present invention is to provide a direct-fired generator having an inner shell in which less than half of the shell volume is taken up by a tube bundle.  
           [0017]    A still further object of the present invention is to provide a single-pass, direct-fired generator with a vapor separator situated an appreciable distance away from the location of liquid solution discharged from the generator&#39;s tube bundle.  
           [0018]    Yet another object of the present invention is to provide a vapor separator for a direct-fired generator having a geometry which inhibits the entry of liquid solution into the interior thereof yet out of which any liquid solution that does enter may readily drain.  
           [0019]    Another object of the present invention is to provide a vapor separator for a direct-fired generator having flow deflectors that direct vapor-entrained liquid droplets away from the generator&#39;s vapor outlet and which assist in creating a vapor flow pattern that facilitates liquid disentrainment during the course of vapor flow therethrough.  
           [0020]    Another object of the present invention is to provide a single-pass, direct-fired generator whose combustion gas inlet and vapor outlet are found in a common end plate.  
           [0021]    These and other objects of the present invention are provided by a direct-fired generator for an absorption chiller that includes inner and outer shells having lower, generally U-shaped half-shells welded to inverted, generally U-shaped upper half-shells. The inner shell defines a combustion chamber and supports a tube bundle such that the combustion gas makes a single pass across the tube bundle. The majority of liquid solution flow within the generator is vertically upward through the tube bundle while a lesser liquid portion flows between the shells. A vapor separator is disclosed and is disposed within the generator so as to significantly limit the carryover of liquid solution out of the generator. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIG. 1 is a schematic diagram of an absorption chiller that includes a single-pass, direct-fired generator.  
         [0023]    [0023]FIG. 2 shows the generator of FIG. 1 in a cross-sectional view taken along line  2 - 2  of FIG. 1.  
         [0024]    [0024]FIG. 3 shows the generator of FIG. 1 in a cross-sectional view taken along line  3 - 3  of FIG. 2.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]    Referring initially to FIG. 1, single-pass, direct-fired, high temperature generator  10  of the present invention is shown schematically to illustrate its relationship with other components of an exemplary absorption chiller  12 . In addition to generator  10 , other major components of chiller  12  include a condenser  14 , an evaporator  16 , an absorber  18  and a low temperature generator  20 . It will be appreciated by those skilled in the art that generator  10  can readily be adapted for use in absorption chillers having different configurations, fluid circuiting and component layouts.  
         [0026]    Chiller  12  makes use of a solution  22  which is a solution having at least one constituent that can be separated from and then reabsorbed into a second constituent. While chiller  12  will be described with reference to a solution consisting of water and lithium bromide, other solutions, such as ammonia and water, are also within the scope of the invention.  
         [0027]    The concentration of solution  22  in the preferred embodiment will vary throughout chiller  12  from weak to strong with the weak solution being pure or nearly pure water. The phase of solution  22  will likewise vary from liquid to vapor/gas depending upon its location within the chiller.  
         [0028]    Solution pumps  24 ,  25 ,  26  and  27  circulate solution  22  through the various components of chiller  12 . The number and type of pumps employed by chiller  12  may vary from one chiller design to the next and is not material to the generator of the present invention.  
         [0029]    The purpose of chiller  12  is to cool a liquid, indicated at  28 , which passes through heat exchanger  30  of evaporator  16 . Liquid  28  can be water, glycol, a mixture of water and glycol, or another fluid that is conveyed from chiller  12 , once it has been cooled, to wherever it is needed. For example, liquid  28  can be circulated through a remote heat exchanger (not shown) used in an industrial process or to cool a room or other area within a building. The process by which liquid  28  is chilled will now be explained in the context of the various components of chiller  12 , starting with direct-fired, high temperature generator  10 .  
         [0030]    Generator  10  heats solution  22  which creates within its confines a weak solution  22   a,  consisting primarily of water vapor, and a more concentrated solution  22   b,  consisting of water in the liquid state with a relatively high concentration of lithium bromide. Concentrated solution  22   b  exits generator  10  through a liquid outlet  32  while weak vaporous solution  22   a  passes through a liquid-vapor separator  34  prior to exiting the generator through a vapor outlet  36 .  
         [0031]    Following first the flow of weak vaporous solution  22   a,  from vapor outlet  36  of direct-fired generator  10 , vaporous solution  22   a  passes through a heat exchanger  38 , which is disposed within low temperature generator  20 , in heat exchange contact with solution  22   d.  Solution  22   d  is of intermediate concentration and is distributed onto heat exchanger  38  from reservoir  40  within the low temperature generator as will further be described.  
         [0032]    The heat from solution  22   a  vaporizes solution  22   d  within low temperature generator  20 . This results in the creation of a weak vaporous solution  22   e  within the upper portion thereof and a more concentrated liquid solution  22   b  at the bottom thereof. Weak vaporous solution  22   e  migrates through vapor separator  42  into condenser  14 .  
         [0033]    A heat exchanger  44  exists within condenser  14  through which water flows. That water is often water which has been cooled by a conventional cooling tower. Heat exchange between the water flowing through heat exchanger  44  and vapor  22   e  within the condenser cools vapor  22   e  and causes it to condense. The condensate collects at the bottom of condenser  14  and mixes with weak solution  22   a,  which is received from heat exchanger  38  in the low temperature generator, to form a pool of relatively cool weak liquid solution  22   c  within the condenser.  
         [0034]    Weak solution  22   c  is conveyed by line  43  to the relatively lower pressure evaporator  16 . As this weak solution is fed into the relatively lower pressure evaporator it expands and its temperature drops further. As a result, a pool of weak liquid solution  22   f  of relatively low temperature is created within the evaporator. That solution is circulated upward within evaporator  16  by pump  24 , is fed into reservoir  46  and is directed thereoutof onto heat exchanger  30 . The flow of low temperature solution  22   f  onto heat exchanger  30  cools liquid  28  which it is, once again, the purpose of chiller  12  to cool.  
         [0035]    As a result of the heat exchange process within the evaporator, solution  22   f  absorbs heat from liquid  28 , vaporizes and migrates through a vapor separator  48  into absorber  18 . Pump  26  circulates solution  22   d  of intermediate concentration to distributor  49  within absorber  18  which, in turn, distributes that solution onto heat exchanger  50 . The distributed solution flows downward through heat exchanger  50  and through an atmosphere of vapor  22   g  within the absorber. As a result of this process, solution  22   d  absorbs vapor  22   g  and then collects at the bottom of the absorber.  
         [0036]    Pump  25  then pumps solution  22   d  from the absorber to replenish the supply of more concentrated solution in low temperature generator  20  while pump  27  pumps solution from low temperature generator  20  to direct-fired generator  10  to replenish the supply of more concentrated solution there. As will be noted, as solution is conveyed to low temperature generator  10  and to direct-fired generator  20 , it is preheated within heat exchangers  52  and  54  by the recovery of what otherwise would be waste heat from liquid solution that flows from the generators.  
         [0037]    Referring primarily now to FIGS. 2 and 3, the structure of direct-fired, high temperature generator  10  includes an inner shell  56  surrounded by an outer shell  58 . Inner shell  56  includes a generally U-shaped lower inner shell section  56   a  and an inverted, generally U-shaped upper inner shell section  56   b.  Each of sections  56   a  and  56   b  is preferably a unitary piece which is continuously formed from end to end. That is, the U-shape is preferably not created by a series of individual panels welded or otherwise fastened together though they could be. Sections  56   a  and  56   b  are welded along two substantially parallel lap joints  60 . To avoid or minimize corrosion at joints  60 , lower shell section  56   a  fits inside upper section  56   b  which prevents the creation of a pocket or ledge on which liquid solution  22   b  might otherwise collect. Shell  56 , once assembled, comprises a two-piece fire tube/tube sheet assembly of simple design and manufacture having open rectangular ends.  
         [0038]    A tube bundle  62 , which includes a group of vertical heat transfer tubes through which solution is conveyed upward within generator  10 , extends across the interior of inner shell  56 . The upper and lower tube ends are welded to upper and lower shell sections  56   b  and  56   a  respectively. The welds are made on the solution side of the tube/shell interface to avoid corrosion of the weld by exposure to combustion products. End plates  64  and  66  are then welded to opposite ends of the inner shell.  
         [0039]    End plate  64  includes vapor outlet  36 , as earlier noted, and a combustion inlet  68  to which burner  69  is attached and through which a burning combustion fluid  70  is introduced into the interior of shell  56 , generally upstream of the tube bundle in an area referred to as the fire tube portion of the shell. End plate  66  includes a combustion outlet  72  through which combustion products exit the shell&#39;s interior after making a single pass therethrough.  
         [0040]    Once welded together, inner shell sections  56   a  and  56   b,  tube bundle  62 , and end plates  64  and  66  can be readily leak checked as a unit by attaching leak check covers to the combustion inlet and outlet. If a leak is discovered, all welded joints are readily accessible for repair.  
         [0041]    Similar in construction to inner shell  56 , outer shell  58  includes a generally U-shaped lower outer shell section  58   a  and an inverted, generally U-shaped upper outer shell section  58   b.  Like the sections of inner shell  56 , each of sections  58   a  and  58   b  is preferably a continuously formed piece, as opposed to being created by a series of individual panels, and are welded/joined along two substantially parallel lap joints  74 . To avoid or minimize corrosion due to liquid collection and stagnation at the joint location, upper shell section  58   b  fits inside lower section  58   a.    
         [0042]    Before welding sections  58   a  and  58   b  together, vapor separator assembly  34  which, in the preferred embodiment, includes a V-shaped trough  34   a,  inner deflectors  34   b  and outer deflectors  34   c,  is assembled into upper outer shell section  58   b.  Outer shell sections  58   a  and  58   b  are then welded along lap joints  74  and end plates  64  and  66  are welded thereto.  
         [0043]    In operation, solution  22   d,  of intermediate concentration, enters solution inlet chamber  80 , defined generally at the bottom of generator  10  and within channel  85 , after passing through inlet  82 . A liquid inlet flow distributor  76  can be created by providing lower shell section  58   a  with apertures  84  and enclosing those apertures within channel  85  which is welded to the underside of lower shell section  58   a.  Channel  85  can, but need not, be considered to be an integral part of lower section  58   a  and distributor  76  could be configured so as to be disposed internal of inlet chamber  80 .  
         [0044]    Apertures  84  can vary in size and/or spacing to apportion and restrict, in a controlled manner, the flow of solution into the interior of outer shell  58 . Those of apertures  84  which are located under tube bundle  62  are preferably larger and/or their spacing is closer so as to cause more solution to flow upward and into tube bundle  62  than flows upward between the walls of shells  56  and  58 . For that reason, most of the heat transfer between combustion fluid  70  and solution  22  within generator  10  is at the location of the tube bundle. Regardless of which flow path the solution follows, it makes its way into a outlet chamber  86  which is located within shell  58 , above inner shell  56 .  
         [0045]    The vaporization of solution that occurs within generator  10  as a result of its being heated creates a more concentrated solution  22   b  in the upper region of the generator. That solution readily mixes with and assimilates the incoming, less concentrated solution  22   d  which itself becomes more concentrated in its flow upward through the generator.  
         [0046]    As hot combustion products travels from inlet  68  to outlet  72  within inner shell  56 , they make a single pass across the exterior of tube bundle  62  thereby heating the solution flowing inside the tubes. However, a significant amount of heat also transfers through the walls of inner shell  56  and heats the portion of the solution that flows upward between the walls of the inner and outer shells.  
         [0047]    In the preferred embodiment, tube bundle  62  takes up less than half the interior volume of inner shell  56  which leaves ample space for open-flame combustion upstream of the tube bundle without having to resort to a special, more costly burner that produces a compact flame for purposes of avoiding direct and detrimental flame contact with the exterior of the tubes of the tube set. Generally speaking, most or all of tube bundle  62  is downstream of midpoint  71  of the length of generator  10  in the preferred embodiment.  
         [0048]    Vapor  22   a  travels out of outlet chamber  86  within generator  10 , into and through separator  34  which helps to disentrain any liquid from the vapor  22   a  prior to its exit from the generator interior. Outer deflectors  34   c  operate to initially deflect liquid solution that may spew upward from between the walls of shells  56  and  58  away from trough  34   a  and from vapor outlet  36  which is found therein. One end  88  of trough  34   a  is blocked off while an opposite end  90  is open to vapor outlet  36 . Inlet slits  92  along upper edges of trough  34   a  allow vapor  22   a  to enter the trough&#39;s interior.  
         [0049]    Once inside trough  34   a,  the geometry of the trough and its interior deflectors  34   b  cause the vapor to swirl generally along the length of the trough. That swirling motion slings remaining liquid droplets  22   h  within vapor  22   a  against an interior surface  94  of the trough. Those droplets accumulate along the bottom of the trough until sufficient in amount to drain out of the trough&#39;s open end  90 . The net result of the separator configuration is that vapor  22   a  exits through vapor outlet  36  only after traveling through a tortuous path and after much of its previously entrained liquid is removed.  
         [0050]    Although the generator of the present invention is described with reference to a preferred embodiment, it will be appreciated by those skilled in the art that other variations are well within the scope of the invention. For example, generator  10  can be used in single-stage or multi-stage absorption chillers. Also, the various components of chiller  12  can be rearranged in a variety configurations. The shells of generator  10 , auxiliary generator  20 , condenser  14 , absorber  18 , and evaporator  16  can be individual shells interconnected by piping or various combinations of shells which share a common wall. Therefore, the scope of the invention is to be determined only with reference to the claims, which follow.