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 17a. 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 12a of carbon steel is laterally cut to form tabs or fingers 12b 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 12c. Hereby a very effective heat transfers from the hot exhaust gas to the fingers 12b 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 12b 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 t1, 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 t3. 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 t1 and t3 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 t1 and t3. 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 t1 and t3 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).

Ap = --p'w2, where Ap : exhaust gas pressure drop [Pa] pressure drop coefficient, dependent on geometry (fin shapes, tube diameter, inline/staggered configuration, number of windings etc), p : Density of the gas at mean temperature between inlet 3 and outlet 4 [kg/m3] 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 t1 and t3 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 t2. This spacing t2 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 t2 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 Heightexcl. 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 x 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 <BR> <BR> <BR> '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 tl 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 R1 and R2.

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 17a functions as a by-pass valve and allows unthrottled flow of exhaust gas through the conduit 5 while the ring 52a 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 (17a) and the ring 52 is in its lowered fully closed position (52a) 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 t1 (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 Could be another kind of component Gas fired turbine producing exhaust gas 105 A forced circulation system, to circulate the Circulation pump 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's by-pass valve (and the heat exchanger'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's heat consuming devices.