Patent Publication Number: US-2005133202-A1

Title: Heat exchanger, combination with heat exchanger and method of manufacturing the heat exchanger

Description:
The present invention relates to a heat exchanger for heat exchange between a first fluid and a second fluid and comprising a generally cylindrical casing with a first inlet and a first outlet for allowing said first fluid to flow through said casing in a generally axial direction, and at least one helical coil of a finned or corrugated tube arranged inside said casing.  
      Heat exchangers of this type are known where the first fluid is forced to flow from inside the coil or coils outwards or vice versa. The transfer of heat from the first fluid to the second fluid in the coils is not very well controlled and therefore not as efficient as possible.  
      It is an object of the invention to provide a heat exchanger of the type in reference where the flow of first fluid, for instance exhaust gas from a natural gas fired turbine, an internal combustion engine, an incinerator, a furnace, a burner or the like, takes place around the finned tube in a well controlled and efficient manner affording a highly efficient heat transfer from the first fluid to the second fluid, for instance water.  
      According to the invention this object is obtained by providing a generally cylindrical fluid conduit inside said casing generally coaxial therewith so that an axially extending first tubular space is defined between said conduit and said casing, said conduit having a second inlet and a second outlet for allowing said first fluid to flow through said conduit in a generally axial direction, and said first tubular space having a third inlet and a third outlet for allowing said first fluid to flow through said first tubular space in a generally axial direction, the at least one helical coil of a finned or corrugated tube being arranged inside said first tubular space generally coaxial therewith and having a fourth inlet and a fourth outlet for allowing said second fluid to flow through said finned tube.  
      Hereby, the first fluid is forced to flow around the finned tube coil windings in a very efficient manner for heat exchange. This also reduces the space requirements for the heat exchanger, the so-called “footprint”.  
      According to the invention, the heat exchanger further comprises first adjustable throttle means for adjustably throttling said flow of said first fluid through said conduit and/or second adjustable throttle means for adjustably throttling said flow of said first fluid through said first tubular space.  
      Hereby, the flow of first fluid may by-pass the finned tube coils so that the heat transfer to the second fluid may be reduced according to the demand for heated second fluid. Furthermore, the pressure loss from the inlet of the casing to the outlet thereof may be reduced by by-passing the finned tube coils which is desirable during start up of for instance a gas fired turbine generating the first fluid in the form of exhaust gas from the gas combustion.  
      In one of the currently preferred embodiments of the heat exchanger according to the invention, said first throttle means comprise a first butterfly valve, preferably arranged adjacent said second inlet or said second outlet, and said second throttle means comprise a second butterfly valve, preferably arranged adjacent said third inlet or said third outlet. Hereby, rather simple mechanisms that are simple to adjust and regulate are provided.  
      In an alternative embodiment, said first throttle means comprise a first butterfly valve, preferably arranged adjacent said second inlet or said second outlet, and said second throttle means comprise a ring having planar dimensions corresponding to the cross section of said first tubular space and being arranged for being displaced from a heating position wherein said flow of first fluid through said tubular space is substantially unhindered to a bypass position wherein said flow is substantially obstructed. In this embodiment, the space requirements are reduced.  
      In another currently preferred embodiment, said first and second throttle means comprise a fixedly arranged stationary plate with first and second apertures provided therein arranged such that said second and third inlets or outlets are obstructed by said plate such that the flow of first fluid through said conduit and said first tubular space takes place through said first and second apertures, respectively, in said stationary plate, and one or two movable plates with third and fourth apertures provided therein and arranged displaceable, preferably rotatably displaceable, from a bypass position overlying said stationary plate, wherein said third apertures coincide with said first apertures and said fourth apertures do not coincide with said second apertures, to a heating position overlying said stationary plate wherein said fourth apertures coincide with said second apertures and said third apertures do not coincide with said first apertures. This embodiment of the first and second throttle means requires a minimum of space and is particularly well suited for precise adjustment of the by-pass flow through the conduit relative to the heat transfer flow through the tubular space.  
      In the currently preferred embodiment, the heat exchanger according to the invention further comprises preferably motorized actuating means for adjusting the throttling effect of said first and second throttle means, and said throttling means and said actuating means are preferably adapted such that substantially any rate of flow between a maximum and minimum rate of flow of said first fluid through said second inlet and said third inlet may be obtained. Said minimum rate is substantially equal to zero. Hereby, any distribution of fluid flow between the by-pass conduit and the tubular space may obtained which allows simple and precise regulation of the output of the heat exchanger according to the requirements for heat transfer from the first fluid. By allowing substantially total by-pass, no heat is transferred to the second fluid which is advantageous in case no heat transfer is needed to means exterior of the heat exchanger and therefore no temperature increase with consequent steam formation will take place in the finned coil or coils.  
      The currently preferred embodiment of a heat exchanger according to the invention comprises two or more of said helical coils arranged concentrically and such that mutually adjacent coils are radially spaced such that an axially extending second tubular space is provided between said mutually adjacent coils, and the outer surface of said conduit is spaced radially from the coil adjacent said surface such that an axially extending third tubular space is provided between said surface and said adjacent coil, the radial dimensions of said second and third tubular spaces being adapted so as to achieve a certain pressure loss for a given rate of flow of said first fluid through said first tubular space.  
      Hereby, the pressure loss from said first inlet to said first outlet for a given flow of first fluid may be kept at a minimum while not substantially reducing the efficiency of the heat exchanger. This is particularly of importance in connection with gas fired turbines being the origin of said first fluid because gas turbines are particularly sensitive to the back pressure at the exhaust outlet thereof.  
      Preferably, the mutually adjacent individual windings of a coil are mutually axially spaced such that a helically extending space is provided between said adjacent windings. Hereby any differential thermal expansion or contraction of the casing and/or conduit relative to the coils in the axial direction will be taken up by said helically extending space.  
      In a currently preferred embodiment of a heat exchanger according to the invention and comprising three or more of said helical coils arranged concentrically and the interior diameter of the finned tubes constituting the coils preferably being the same, third throttling means are provided in the tubes constituting the coils located radially inwards of the outermost coil for increasing the pressure loss through the tubes of said inner coils so as to compensate for the shorter length of said tubes relative to the length of the tubes of the outermost coil such that the rate of flow of said second fluid through the tubes of all the coils is substantially the same for a given uniform pressure in said second fluid at said fourth inlets. Hereby, the heat transfer efficiency of each coil will be substantially the same without having to vary the diameter of the tubes of each coil to achieve this effect.  
      In the currently preferred embodiment of a heat exchanger according to the invention said third throttling means are constituted by a reduction of the cross sectional area of the flow of said second fluid relative to the internal cross sectional area of said tubes, the heat exchanger preferably further comprising an inlet header tube and an outlet header tube in fluid communication with said fourth inlets and fourth outlets, respectively, of all said tubes through corresponding communication apertures in said header tubes, said reduction of flow cross sectional area being constituted by reduced size of said communication apertures in said inlet header tube and/or in said outlet header tube. This is a particularly simple and inexpensive way of compensating for the different lengths of the different coils.  
      In case the heat exchanger according to the invention is to be used for generating steam, then according to the invention each helical coil may advantageously comprise two or more helically wound finned tubes extending adjacent one another with the same pitch. Hereby the number of flow paths is increased which is advantageous in connection with the large volume expansion of the water in the coil tubes resulting from the steam generation.  
      In another aspect, the present invention relates to a combination of a heat exchanger according to the invention and an exhaust gas generating combustion means such as a natural gas fired turbine, an internal combustion engine based on gasoline, diesel oil or natural gas, a furnace, a burner, an incinerator and the like, the combination comprising interconnection means for interconnecting an exhaust gas outlet of the combustion means with said second and third inlets of the heat exchanger such that said exhaust gas constitutes said first fluid.  
      The currently preferred embodiment of the combination according to the invention may further comprise heat exchanging means for heat exchange between said second fluid and a third fluid and/or the surroundings of said heat exchanging means, said heat exchanging means being in fluid communication with said fourth outlet, measuring means for measuring the rate of heat exchange of said heat exchanging means, signal output means for emitting a signal representing the result of a measurement carried out by said measuring means, and first control means for controlling the adjustment of said first and second throttle means and adapted for receiving said signal.  
      Preferably, the combination according to the invention further comprises second control means for controlling the adjustment of said first throttle means such that the throttling effect thereof is at a minimum during the start up phase of the combustion means.  
      In yet another aspect, the present invention relates to a method of manufacturing a heat exchanger according to the invention and comprising the steps of providing a first length of finned or corrugated tube, providing a body having a substantially circular cylindrical surface, providing rotating means for causing relative rotation of said tube and said surface, arranging a lead portion of said tube abutting against said surface, and causing relative rotation of said surface and said lead portion such that said first length of tube is helically wound on said surface to form a first helical coil. Hereby, a particularly simple, precise and inexpensive method of manufacturing a heat exchanger according to the invention is achieved.  
      In connection with heat exchangers according to the invention having two or more concentric coils, the method according to the invention preferably comprises the further steps of providing spacing means, attaching said spacing means to said first helical coil, providing a second length of finned or corrugated tube, arranging a lead portion of said second length of finned tube abutting against said spacing means, causing relative rotation of first helical coil and said lead portion of said second length of tube such that said second length of tube is helically wound on said spacing means to form a second helical coil radially spaced from said first helical coil.  
      So as to avoid inaccuracies in the diameter of the coils and other disadvantages, the method according to the invention preferably comprises the further steps of fixating said helical coil relative to said body, and subjecting said body and said coil to annealing heat treatment and/or fixating said second helical coil relative to said body and/or said first helical coil, and subjecting said body and said first and second coils to annealing heat treatment. Hereby the diameter alteration of the coils because of elasticity and stresses in the steel of the coil tubes is avoided in a simple and cost efficient manner. 
    
    
      In the following the invention will be explained more in detail with reference to different embodiments thereof shown, solely by way of example, in the accompanying drawings where:  
       FIG. 1  is an elevational partly sectional diagrammatic view of a first currently preferred embodiment of a heat exchanger according to the invention,  
       FIGS. 2-3  are schematic plan views illustrating a fin configuration of the finned tubes according to the invention,  
       FIG. 4  is a schematic bottom view of the embodiment of  FIG. 1 ,  
       FIG. 5  is an elevational partly sectional diagrammatic view of a second currently preferred embodiment of a heat exchanger according to the invention,  
       FIG. 6  is a schematic view of an inlet header tube for an embodiment of the heat exchanger according to the invention provided with four concentrically arranged finned tube coils,  
       FIG. 7  is a broken away elevational view of a third embodiment of a heat exchanger according to the invention,  
       FIG. 8  is a diagrammatic enlarged scale view of a portion of the embodiment in  FIG. 1  illustrating the spacing of the finned tubes of the coils,  
       FIG. 9  is a schematic elevational, broken away, partly sectional view of the top of the embodiment shown if  FIG. 5  illustrating a first embodiment of throttle means according to the invention,  
       FIG. 10  is a schematic elevational, broken away, partly sectional view of the top of a fourth embodiment of a heat exchanger according to the invention illustrating a second embodiment of throttle means according to the invention,  
       FIG. 11  is a schematic top view of the embodiment of  FIG. 10 ,  
       FIG. 12  is a schematic elevational, cut away view illustrating a third embodiment of throttle means according to the invention,  
       FIG. 13  is a schematic top view illustrating a fourth embodiment of throttle means according to the invention,  
       FIG. 14  is a schematic, partly sectional, perspectival, enlarged scale view of the top header tube and fastening means for the coils of the embodiment of  FIG. 1 ,  
       FIG. 15  is a schematic top view illustrating the method according to the invention of manufacturing the embodiment of  FIG. 5 , and  
       FIG. 16  is a diagram illustrating one embodiment of control means according to the invention for adjusting the throttle means according to the invention. 
    
    
      Referring first to  FIGS. 1 and 4 , a heat exchanger  1  according to the invention comprises an outer cylindrical casing  2  provided with a flanged inlet  3  and a flanged outlet  4 . An interior cylindrical casing or conduit  5  having an inlet  6  and an outlet  7  is arranged coaxially with the outer casing  2  thereby defining a tubular space  8  wherein two coils  9  and  10  of finned tubing are arranged. The finned tubing consists of a tube  11  provided with fins  12  arranged generally transversely to the axis of the tube  11 .  
      The finned tube coils  9  and  10  are arranged mutually concentric and coaxially with the outer and inner casings  2  and  5 . A flanged outlet header tube  13  and a flanged inlet header tube  14  communicate with the interior of the tubes  11  of the coils  9  and  10  through apertures  15  and  16 , respectively.  
      A butterfly valve  17  (by-pass valve) is pivotably arranged on a shaft  18  at the outlet  7  of the conduit  5 , a position wherein the valve  17  is in an intermediate position between fully closing the outlet  7  and fully opening said outlet  7  being shown with dotted lines at  17   a . Semicircular rings  19  and  20  are arranged for abutting the rim of the butterfly valve  17  in the closed position thereof thereby ensuring a good closing function of the valve  17 .  
      The heat exchanger  1  is primarily intended for use in combination with a natural gas fired turbine for recuperating and utilizing the heat of the exhaust gases thereof, but may in principle be used in combination with any means producing a heated gas such as internal combustion engines, furnaces, burners, incinerators and the like.  
      During maximum output operation of the heat exchanger  1 , the butterfly by-pass valve  17  is in the closed position shown in full lines in  FIG. 1  whereby all the exhaust gas from the gas turbine introduced into the inlet  3  flows through the tubular space  8  past the finned tube coils  9  and  10  as indicated by the full line arrows. Water to be heated is introduced into the coils  9  and  10  through inlet header  14  and is discharged through apertures  15  and  16  and outlet header  4  after having been heated by heat transmission from the exhaust gas through fins  12  to the tubes  11  and thereby to the water in said tubes.  
      The heated water is transported to not shown exterior heat exchange means for transmitting some of the heat of the water to some other fluid or to the surroundings, typically radiators in a building heating system or a district heating system.  
      Either during start up of the gas turbine (when the pressure loss through the heat exchanger  1  should be at a minimum to facilitate the turbine start up) or when the exterior heat exchange means do not require the full heating capacity of the heat exchanger  1 , then the butterfly valve  17  is pivoted on shaft  18  so as to allow some or all the exhaust gas from the gas turbine to flow through the internal conduit  5  as indicated with dotted arrows thereby by-passing the tubular space  8  and the coils  9  and  10 .  
      Hereby, the pressure loss through the heat exchanger  1  is decreased and the heat transmission to the water in the tubes  9  and  10  is decreased. The butterfly valve  7  can also be described as a throttling means and may be substituted by other throttling means as described in the following in connection with  FIGS. 9-13 .  
      Referring now to  FIGS. 2 and 3 , a strip  12   a  of carbon steel is laterally cut to form tabs or fingers  12   b  that are bent transverse to the plane of the strip in alternating directions and thereafter welded onto the surface of the tube  11  in a spiral configuration by means of welding seam  12   c . Hereby a very effective heat transfers from the hot exhaust gas to the fingers  12   b  and thereby to the tube  11  may be achieved. Other configurations with circular plate shaped fins or corrugations may also be employed instead of the serrated spirally wound fins shown in  FIGS. 2 and 3 .  
      Referring now to  FIG. 14 , the tubes  11  are welded to the upper header tube  14  around the apertures  15  and  16  thereof whereby the interior of the tubes  11  communicates with the interior of the header tube  14 , The coils  9  and  10  are attached to and suspended from the outer casing  2  and the inner conduit  5  by means of a beam  22  welded to said casing and conduit. The beam  22  is welded to two rings  23  and  24  fitting tightly around the fins  12   b  of the coils  9  and  10 . A similar attachment is carried out at the bottom of the heat exchanger adjacent the inlet header tube  13 .  
      Referring now to  FIG. 8 , the position of the coils  9  and  10  relative to one another and relative to the outer casing  2  and the inner conduit  5  as well as the spacing between the windings of each coil is illustrated.  
      The innermost coil  10  is spaced from the outermost coil  9  by a tubular space having a thickness or radial dimension t 1 , while the innermost coil  10  is spaced from the outer surface of the conduit  5  by a tubular space having a thickness or radial dimension t 3 . The outermost coil  9  is not spaced from the inner surface of the outer casing  2 , i.e. the coil  9  abuts the casing  2 .  
      The spacings t 1  and t 3  are chosen such that the loss of pressure through the tubular space  8  is maintained at a level acceptable for the optimal operation of the gas turbine (or other hot exhaust gas generating means) delivering exhaust gas to the heat exchanger  1 . The heat exchange efficiency of the heat exchanger  1  is not substantially affected by the spacings t 1  and t 3 . On the other hand, operational tests show that if a spacing were present between the outer casing  2  and the outermost coil  9 , then the efficiency of the heat exchanger  1  would be considerably reduced. These two phenomena are at least to a certain degree owing to, on one hand, turbulent flow between the coils  9  and  10  and between the conduit  5  and the coil  10  and, on the other hand, laminar flow in a tubular space between the outer coil  9  and the casing  2 .  
      There are several parameters determining the spacings ti and t 3  between the coils and between the innermost coil and the conduit  5 . The two most important considerations or parameters are:  
      Exhaust Gas Pressure Drop  
      The exhaust gas pressure drop or loss is very dependent on the exhaust gas velocity and the geometry of the heating surface of the coil windings. The velocity is dependent on the free gas flow cross sectional area (total area for the gas flow between the tubes and fins in a cross section). 
          Δp=ξ·½·ρ·w2, where     Δp: exhaust gas pressure drop [Pa]    ξ: pressure drop coefficient, dependent on geometry (fin shapes, tube diameter, inline/staggered configuration, number of windings etc),     ρ: Density of the gas at mean temperature between inlet  3  and outlet  4  [kg/m 3]      w: exhaust gas velocity [m/s]       

      In most cases, the allowable exhaust gas pressure drop in heat exchangers and boilers after gas turbines (and engines as well) is quite limited. For gas turbines it is extremely important to minimize the exhaust gas pressure drop as the power production on the turbine (and thus the efficiency of the turbine) is very dependent on the back pressure. In connection with the heat exchanger according to the invention the allowable exhaust gas pressure drop is preferably limited to be below 500 Pa (50 mm water column), giving very low exhaust gas velocities and thus large distance between the coils (alternatively more coils giving larger gas cross section area and larger diameter of the unit).  
      Heat Transfer Coefficient  
      In general the heat transfer coefficient should be as high as possible to minimize the heating surface area. The heat transfer coefficient is increased with higher exhaust gas velocities and more turbulent flow. For the heating surface chosen (serrated spiral wound fin tubes) the turbulence of the flow is very good, in general giving high heat transfer coefficient even for low exhaust gas velocities.  
      Designing the heat exchanger according to the invention with the spacings t 1  and t 3  is also advantageous from a production point of view because it allows the coils to be inserted in the casing individually as compared with coils designed to abut one another or to be nested in one another that must be handled and inserted as a unit comprising several coils.  
      Still referring to  FIG. 8 , a helically extending space is provided between adjacent windings of each coil  9  and  10 , the thickness or axial dimension of said space being t 2 . This spacing t 2  of the windings allows the casing  2  and/or the conduit  5  to thermally expand and contract axially relative to the coils  9  and  10  without causing unacceptable stresses as any differences in such expansion or contraction is taken up by variations of the spacing t 2  between the windings of the coils.  
     EXAMPLE  
      In the following, the basic technical specifications for a combination according to the invention of a two coil heat exchanger according to the invention and a gas fired turbine are listed as a non-limiting example:  
      Dimensions of the Heat Exchanger  
                                          Height excl. inlet:   1550   mm       Diameter excl. insulation:   633   mm                     Insulation:   100 mm covered with galvanized steel plate       Flue gas outlet flange:   DN 450, DIN 86044       Water inlet/outlet connections:   Carbon steel pipe, OD 60.5 × 3.6 mm, 2 “RGW                         Thickness of casing (inner 5 and outer 2):   5   mm       Weight of heat exchanger excl. water:   475   kg       Weight of heat exchanger incl. water:   500   kg       Outside diameter of tubes 11:   38   mm       Tube material thickness:   3.6   mm                     Fin type:   Serrated spiral wound fins                         Height of fins:   15   mm       Fin density:   250   pcs/m       Thickness of fins:   1   mm                     Material, tube and fins:   Carbon steel       Tube configuration:   Inline                         Number of coaxial and concentric coils:   2           Number of windings:   10       Tube pitch in gas direction:   70   mm       Free spacing t2 between fins on coil windings in gas direction:   2   mm       Diameter of by-pass channel (inner casing 5):   323.9   mm       Length of inner casing 5, incl. by-pass valve:   860   mm       Centre diameter of inner coil 10:   401   mm       Centre diameter of outer coil 9:   555   mm       Free space t3 between inner casing 5 and fins on inner coil 10:   4.5   mm       Free space t1 between fins on the two coils:   9   mm       Free space between fins on outer coil and inside of outer casing 2:   0   mm       Size of holes 15, 16 in header 13 for coil connection (both coils)   30.8   mm                  
 
      Process Data  
                                      Micro Gas turbine type:   HONEYWELL           Parallon 75       Max. electric output power from gas turbine:   75 kW(e)       Exhaust gas flow:   0.68 kg/s       Exhaust gas inlet temperature to heat exchanger:   246° C.       Exhaust gas outlet temperature from heat exchanger:    90° C.       Exhaust gas pressure loss across heating surface:   300 Pa       Heating capacity of heat exchanger:   120 kW       Water inlet temperature:    50° C.       Water outlet temperature:    70° C.       Water flow, approx:   1.44 kg/s       Pressure drop, water side:   0.2 bar                  
 
      Referring now to  FIG. 5 , an embodiment of a heat exchanger  31  according to the invention having three concentric coils  32 - 34  is shown with the same reference numbers being utilized for elements similar to elements in  FIG. 1 . The main difference between the  FIG. 1  and  FIG. 5  embodiments, apart from the number of coils, is that inlet apertures  35 ,  36  and  36  of the outlet header tube  38  are different sizes so as to compensate for the difference in coil length between the coils  32 - 34  as explained below.  
      As the length of the tubes  11  of the different coils  32 - 34  are different and all the coils are interconnected at the outlet header  38  and at the inlet header  39 , a flow distribution will be established in the coils giving the same pressure loss through each of the coils.  
      Thus, the inner coils  33  and  34  [where the second fluid (typically water) have shorter flow paths than in the outermost coil  32 ] can transport more water than the outer coil  32 . The water in the coils  33 ,  34  and  35  will then not be heated to the same temperature and will result in a skewed and reduced recuperation of the heat contained in the first fluid (for instance exhaust gas from a gas fired turbine).  
      It is therefore desirable that the flow rate through the coils be regulated so that best possible heat recuperation is obtained with best possible temperature distribution both in the water and in the exhaust gas. This is achieved by creating an extra pressure loss in the inner coils  33  and  34  relative to the outer coil  35  and each other.  
      This can be achieved in two manners: 
          By providing the tubes of the coils with different diameters. From a practical point of view this is not desirable except in case a large number of concentric coils are involved in which case 2-3 different tube diameters may be acceptable.     By installing throttle or baffle means in the tubes or at the inlet or outlet thereof. As can be seen in  FIG. 5  and  FIG. 6 , this can be achieved by providing the outlet header tube  38  with apertures  35 - 37  and  40 - 43 , respectively, having different diameters. The diameters of the individual apertures are determined based on the tube diameter and tube length in the individual coils of a heat exchanger. As an example of diameters for the four apertures shown in  FIG. 6  for an inner diameter of 56 mm of all four coil tubes, aperture  40  is 56 mm, aperture  41  is 13 mm, aperture  42  is 11 mm and aperture  43  is 9 mm. Other throttle or baffle means well known in the art may also be used to achieve the different pressure loss coefficients for the individual coil tubes.        

      Referring now to  FIG. 7 , the inner coil adjacent the inner conduit  5  comprises two parallel wound finned tubes  50  and  51  establishing two parallel flow paths for the second fluid (typically water) indicated by the arrows R 1  and R 2 .  
      This embodiment is intended for use for steam generation where it is necessary to take into consideration the large volume expansion of the mass inside the tubes (at the transition from liquid to vapour, water to steam) with corresponding increase in flow rate and velocity as well as pressure loss at the inner surface of the tubes. So as to provide sufficient inner flow cross sectional area in the tubes it will therefore often be necessary to use a larger tube diameter, a larger number of coils or provide for a larger number of flow paths in other ways.  
      Apart from providing many coils, more flow paths may be obtained by having several parallel extending windings in the same coil as shown in  FIG. 7 , i.e. several coils with the same coil diameter and large pitch “screwed” into each other. Hereby it is obtained that a larger inner cross section area is achieved without having a large number of coils with a large diameter of the outermost coil and therefore the outer casing  2  (large footprint). This smaller footprint or outer diameter of the heat exchanger entails important advantages both for the end user and during manufacture, erection and transport. The axial length or height of the heat exchanger for a given output will of course be larger, but this does normally not represent a substantial problem during manufacture or for the end user.  
      Referring now to  FIGS. 9-13 , various embodiments of throttle means for throttling the flow of first fluid (typically exhaust gas) through the conduit  5  and the tubular space  8 , respectively, are shown.  
      In the embodiment of  FIG. 9 , the butterfly valve  17  cooperates with a ring  52  that is suspended in three steel wires  53  attached at equidistant points along the ring  52 . The wires  53  extend over pulleys  54  to the shaft  18 .  
      In the situation shown with full lines, the butterfly valve  17  is closed and does not allow any exhaust gas to flow through conduit  5  while the ring  52  is in its highest position in which it does not throttle the flow of exhaust gas through the tubular space  8 .  
      In the situation shown with dotted lines, the butterfly valve  17   a  functions as a by-pass valve and allows unthrottled flow of exhaust gas through the conduit  5  while the ring  52   a  is in its full throttle position supported on tightening rings  55  thereby preventing flow of exhaust gas through the tubular space  8 .  
      The shaft  18  may be actuated manually, by an electric motor or a pneumatic or hydraulic mechanism. In the simplest version, the wires are wound onto and off not shown pulleys arranged on the shaft  18  such that rotation of the shaft  18  for opening of the butterfly valve  17  automatically entails lowering of the ring  52  and vice versa.  
      When no heat is required by the external heat consumption means connected to the heat exchanger  31 , then the butterfly valve  17  is in its fully open position ( 17   a ) and the ring  52  is in its lowered fully closed position ( 52   a ) so that all the exhaust gas is by-passed through the conduit  5 . Hereby, the water in the finned tube coils is not heated so that external cooling means to avoid overheating of this water are not necessary.  
      A very simple means for regulating the heat output of the heat exchanger  31  is thus provided. A temperature sensor and transmitter (not shown) may be provided in the outlet header  38  for transmitting a signal to the not shown actuator (electric motor) for the shaft  18  so that if the temperature measured at the outlet header does not conform to the required temperature, then the shaft rotates in the corresponding direction to either open or shut the by-pass valve  17 . Many different regulating circuits are conceivable depending on the requirements of the end user and the configuration of the external heat consumption devices connected to the heat exchanger  31 .  
      Referring now to  FIGS. 10 and 11  showing an elevational and top view, respectively, of a second embodiment of the first and second throttling means for the flow of exhaust gas through the internal conduit and the tubular space, respectively, the internal conduit  5  is connected to a further conduit  56  having an outlet  57  in which a butterfly valve  58  is rotatably mounted on a shaft  59 . The outlet  57  communicates with the outlet  4  of the casing  5 . The butterfly valve  58  may rotate with the shaft  59  from the shown closed position wherein the outlet  57  is totally obstructed and an open position wherein flow of exhaust gas through outlet  57  is unhindered.  
      The tubular space  8  communicates with a space  60  defined by an extension of the outer casing  2 , said space communicating with the outlet  4  through an aperture  61  in a plate  62 . A butterfly valve  63  is mounted in said aperture  61  on the shaft  59  such that rotation of the shaft  59  rotates the butterfly valve  63  from the shown fully open position in which flow of exhaust gas from the tubular space  8  through the space  60  and through the aperture  61  is unhindered to a fully closed position in which flow of exhaust gas through the aperture  61  is totally obstructed. The shaft  59  is connected to an electric motor  64  for being rotated in opposite directions so as to rotate the valves  58  and  63  between the two positions described above and to any intermediate position.  
      Referring now to  FIG. 12 , in this embodiment the butterfly valves  58  and  63  of the embodiment in  FIGS. 10 and 11  have been substituted by a butterfly valve  65  and two butterfly valves  66 , respectively, the operation of the valves  65  and  66  and the shaft  59  being the same as described in connection with the embodiment of  FIGS. 10 and 11 .  
      Referring now to  FIG. 13 , a stationary circular plate  70  is arranged horizontally in an embodiment of the heat exchanger similar to the one shown in  FIG. 1  or  FIG. 4  (without the butterfly valve  17 ) over the outlet of the conduit  5  and the annular outlet of the tubular space  8 . The plate  70  is provided with apertures  71  communicating with the interior of conduit  5  and apertures  72  communicating with the tubular space  8 .  
      A rotatably arranged circular plate  73  is arranged on top of plate  70  on a pivot  74 . The plate  73  is provided with apertures  75  identical in shape and distribution to apertures  72  in plate  70  and with apertures  76  identical in shape and distribution to apertures  71  in plate  70 . An electrical motor  77  is arranged for rotating the rotatable plate  73  in both directions.  
      In the position of the rotatable plate shown in  FIG. 13  the apertures  71  and  76  coincide or overly each other so that exhaust gas can flow practically unhindered through the conduit  5  underlying these coinciding apertures while flow through the tubular space  8  is obstructed because the apertures  72  and  75  do not communicate with each other at all. By rotating the plate  73  by means of the motor  77 , a position thereof may be attained where flow through the tubular space is relatively unhindered because the apertures  72  and  75  coincide and flow through the conduit  5  is totally obstructed because the apertures  71  and  76  do not communicate with each other at all.  
      The lower plate  70  may instead be the rotatable one whereby the pressure from the exhaust gas will press the plate  70  against the stationary plate  73  and enhance the sealing effect of abutment of the plates  70  and  73  against each other. Sealing between the plates may also be achieved in many other ways obvious to those skilled in the art.  
      Referring now to  FIG. 15 , a method according to the invention of manufacturing a heat exchanger (the embodiment of  FIG. 5 ) according to the invention is illustrated.  
      A cylindrical body  80  is constituted by a steel plate  81  with a thickness of  10  mm, a steel rod  82  inserted between the free axially extending edges of the plate  81 , and not shown circumferentially extending tightening straps or wires for holding the plate  81  and rod  82  in the shown cylindrical configuration.  
      The inner coil  34  is wound helically around the body or core  80 , the leading end (not shown) and the trailing end  11   a  of the pipe  11  of the coil  34  being attached to the body by means of brackets or rods  83  welded to said ends and to the body  80 . The tightening straps mentioned above are located outside the area of the body  80  to be covered by the coil  34 .  
      Four cylindrical plates  84  having a quarter circle cross section and a thickness equal to the required radial dimension t 1  (see  FIG. 8 ) are arranged on the outer surface of coil  33  with rods  85  arranged between mutually adjacent axially extending edges of the plates  84 . The plates  84  and rods  85  are held in place by not shown circumferentially extending tightening straps or wires. The rods  85  have an oval or elliptical cross section and are placed between the plates  84  such that the major cross sectional dimension of the oval or ellipse is tangential to the circular circumference of the plates  84 .  
      The next coil  33  is wound helically around the plates  84  and rods  85  with the leading and trailing ends of the coil  33  being welded to one of the plates  84  by means of brackets or rods  83  in a manner very similar to the winding and attachment of the inner coil  34 .  
      The process is repeated for the next coil  32 . If more coils than three are required, the process described above may be repeated for any such further coils.  
      The unit comprising the body  80 , the coils  32 - 34  and the plates  84  with rods  85  is thereafter subjected to annealing heat treatment for avoiding elastic diameter expansion of the coils and to remove potentially damaging stresses.  
      After annealing, the attachments by means of brackets  83  are removed, the rods  85  are rotated such that the major dimension of the oval or elliptical cross section is oriented radially whereby the coils  32 - 34  are forced to expand slightly such that the cylindrical plates  84  may be removed. Finally rod  82  is removed such that the body  80  may be removed. The coils  32 - 34  are now ready for being inserted in a casing  2  with a conduit  5  placed inside the inner coil  34 .  
      A heat exchanger according to the invention is particularly well suited for use in a combination or system comprising a gas fired turbine (or an internal combustion engine utilizing natural gas as fuel). Such a system is furthermore particularly well suited for (but not in any way limited to) for use in a system for small scale combined production of electricity and heat, for instance for large buildings, hospitals, small district heating systems and the like.  
      Referring now to  FIG. 16 , a system or combination according to the invention including a heat exchanger according to the invention, a gas fired turbine and external heat consuming devices is shown with the following characteristics:  
                                       Item   Component   Description                  101   Heat exchanger according to the               invention.       102   Exhaust gas by-pass damper or   Can be regulated manually or as shown           valve (butterfly valve for instance)   here: regulated by an actuator (electrical               motor), item 11       103   Exhaust gas stack       104   Gas fired turbine   Could be another kind of component               producing exhaust gas       105   Circulation pump   A forced circulation system, to circulate the               required water/fluid flow. The pressure drop               on water side in the heat exchanger, valves               and piping system to be taken into account               when calculating the delivery head of the               pump.       106   Expansion tank   To take the expansion/contraction of the               fluid in the system, when the temperature               varies.       107   External end user   Or other heat consumption device. Can be           Heat exchanger   for heating use in buildings, in               greenhouses, district heating systems etc.       108   Safety valve   To be opened if the pressure in the system               becomes too high       109   Stop valves   Normally open. Possible to close in case of               repair.       110   Air venting valve       111   Electrical motor   Electrical motor, automatically controlled by               signals from the temperature transmitter               113. Opens the by-pass valve 102 a little, if               the water temperature becomes too high,               closes the valve 102 a little if the water               temperature becomes too low. Set points to               be decided by the end user.       112   Drain valve   To drain the water/fluid from the system       113   Temperature transmitter   Sends signal to electrical motor if               temperature measured is too high or too               low                  
 
      The system can comprise other exhaust gas generating devices, and the regulation of the heat exchanger&#39;s by-pass valve (and the heat exchanger&#39;s throttle valve for throttling fluid flow through the tubular space containing the finned tube coils) can be provided for in many different manners depending on the configuration of the end user&#39;s heat consuming devices.