WO2012134293A1

WO2012134293A1 - Device and method for mixing two fluids

Info

Publication number: WO2012134293A1
Application number: PCT/NL2012/050212
Authority: WO
Inventors: Ronald POSTMA; Robert Sakko; Hans Constant Dikhoff; Erik Jan VAN DEN BERG; Bart Jan Van Den Berg
Original assignee: Heatmatrix Group B.V.
Priority date: 2011-04-01
Filing date: 2012-03-30
Publication date: 2012-10-04
Also published as: NL2006526C2

Abstract

A mixing device (10) for mixing a first fluid and a second fluid comprises a housing (12) which is provided with a first inlet (20) for the first fluid, a second inlet (27) for the second fluid, a mixing chamber (42) for mixing the first fluid and the second fluid into a mixture of the first fluid and the second fluid, and an outlet (26) for the mixture of the first fluid and the second fluid. The housing (12) is provided with a plurality of heat exchanger tubes (36) having tube walls (37) which are situated next to each other and extend substantially parallel to each other. The interiors of the tube walls (37) define elongate first channels (38). Each elongate first channel (38) comprises a first supply opening (65) and a first discharge opening (66,81). The first supply openings (65) are in fluid communication with the first inlet (20) for the first fluid. At least a number of the first discharge openings (66,81) are in fluid communication with the mixing chamber (42). The tube walls (37) of adjacent heat exchanger tubes (36) are connected to each other so as to define elongate second channels (58) on the exterior of the tube walls (37). Each elongate second channel (58) comprises a second supply opening (75) and a second discharge opening (76). The elongate second channels (58) are in fluid communication with the second inlet (27) for the second fluid and with the mixing chamber (42) by means of the second openings (75,76).

Description

Title: Device and method for mixing two fluids

The invention relates to a mixing device for mixing a first fluid and a second fluid, the mixing device comprising a housing which is provided with:

- a first inlet for the first fluid,

- a second inlet for the second fluid,

- a mixing chamber for mixing the first fluid and the second fluid into a mixture of the first fluid and the second fluid,

- at least one outlet for the mixture of the first fluid and the second fluid.

US6427671 discloses an exhaust gas recirculation mixer apparatus and method.

An exhaust gas recirculation (EGR) system reduces the generation of undesirable pollutant gases and particulate matter resulting from the combustion process in an internal combustion engine. The EGR system recirculates the exhaust gas into the intake air supply of the internal combustion engine. The exhaust gas has a relatively low oxygen

concentration. Thus, the reintroduction of the exhaust gas to the internal combustion engine lowers the concentration of oxygen. By reducing the oxygen concentration, peak

temperatures in the combustion chamber are lowered. This decreases the formation of nitrous oxides (NO_x).

This known EGR system comprises a shielded conduit which is in fluid

communication with an exhaust conduit which in turn is connected to the exhaust manifold of an internal combustion engine. The shielded conduit is adapted for diverting a flow of exhaust gas from the exhaust conduit to a position proximate the intake manifold of the engine. The amount of exhaust gas passed through the shielded conduit is determined by an EGR diversion valve. An EGR cooler is provided to cool the recirculated exhaust gas that is being passed into the intake manifold of the engine.

Intake air is mixed with the exhaust gas that is recirculated through the shielded conduit with a fluid mixer assembly. The fluid mixer assembly includes the shielded conduit partially positioned within an inlet conduit which leads to the intake manifold of the engine. The shielded conduit forms a fluid diverting portion within the fluid mixer assembly. Intake air is diverted by the fluid diverting portion into two separate streams which are re-combined at a point downstream of the shielded conduit with the exhaust gas. With this known fluid mixer assembly the exhaust gas is not evenly distributed over the cross sectional area of the inlet conduit. If the degree of mixing of the exhaust gas and the intake air is insufficient, this may lead to unstable combustion or increased NO_x-levels.

An object of the invention is to provide an improved mixing device.

This object is achieved according to the invention by a mixing device for mixing a first fluid and a second fluid, the mixing device comprising a housing which is provided with:

- a first inlet for the first fluid,

- a second inlet for the second fluid,

- an outlet for the mixture of the first fluid and the second fluid,

in which the housing is provided with a plurality of heat exchanger tubes having tube walls which are situated next to each other and extend substantially parallel to each other, in which the interiors of the tube walls define elongate first channels, in which each elongate first channel comprises a first supply opening and a first discharge opening, in which the first supply openings are in fluid communication with the first inlet for the first fluid, and in which at least a number of the first discharge openings are in fluid communication with the mixing chamber, and in which the tube walls of adjacent heat exchanger tubes are connected to each other so as to define elongate second channels on the exterior of the tube walls, in which each elongate second channel comprises a second supply opening and a second discharge opening, in which the elongate second channels are in fluid communication with the second inlet for the second fluid and with the mixing chamber by means of the second openings.

The uniformity of the mixture of the first and second fluid, for example exhaust gas or flue gas and combustion air, depends on a number of factors. For example, the degree of mixing is influenced by the velocity of the two fluids, the distribution of components within the two fluids and also the differences in physical properties between the two fluids, such as viscosity, density and temperature. If the two fluids have significantly deviating physical properties, they will have a tendency to mix slowly. The mixing device according to the invention is configured to provide both mixing and heat transfer. A mixer and a heat exchanger are integrated into one apparatus according to the invention.

If the two fluids have different temperatures, for example in the case of hot exhaust gas and cold combustion air, the mixing device according to the invention allows for the mixing of the two fluids to be carried out after the temperature difference has been reduced by heat transfer between the two fluids. This leads to relatively fast and efficient mixing. In particular for chemical reactions it is advantageous to establish relatively fast mixing at substantially equalized temperatures in order to prevent undesired side reactions. Thus, the mixing device according to the invention is improved.

A mixing device according to the invention can be used for recirculation of a fraction of the exhaust gas into the combustion air in order to reduce the formation of NO_x, while simultaneously establishing heat recovery from the fraction of hot exhaust gas that is not mixed into the combustion air. However, the mixing device according to the invention can also be used for other applications.

The mixing device according to the invention may be operated in a co-current or counter-current configuration.

In an embodiment, the second supply openings of the elongate second channels are in fluid communication with the second inlet for the second fluid, in which the second discharge openings of the elongate second channels are in fluid communication with the mixing chamber. Thus, the first and second fluids flowing through the elongate first and second channels, respectively, are in co-current with each other.

It is also possible for the mixing chamber to be provided with the second inlet for the second fluid, in which the second supply openings of the elongate second channels are in fluid communication with the mixing chamber, and in which the second discharge openings of the elongate second channels are in fluid communication with the outlet for the mixture of the first fluid and the second fluid. In this case, the first fluid flowing through the elongate first channels is in counter-current with the mixture of the first and second fluids traversing the elongate second channels.

In an embodiment, the tube walls of the heat exchanger tubes are configured to separate the elongate first channels on the interior of the tube walls from the elongate second channels on the exterior of the tube walls. In the zone of the elongate first and second channels, the tube walls constitute heat transfer areas for heat transfer between the first fluid flowing through the interior of the tube walls of the heat exchanger tubes and the fluid surrounding the tube walls of the heat exchanger tubes. Any direct contact, i.e. mixing, between said two fluids is prevented as long as the fluids are flowing through the elongate first and second channels.

It is not necessary for the tube walls to be completely impermeable for the fluids.

Optionally, small amounts of first fluid may be allowed to leak through the tube walls. For example, small holes may be provided in the tube walls so as to allow relatively small amounts of first fluid to escape from the elongate first channels on the interior of the tube walls to the elongate second channels on the exterior of the tube walls. In this case, such small amounts of first fluid can mix and exchange heat by direct contact with the fluid surrounding the tube walls. In an embodiment, each tube wall comprises a main portion which is provided with a plurality of connecting members and a first end portion which extends from the main portion, in which the tube walls of adjacent heat exchanger tubes are connected to each other by means of the connecting members, and in which the elongated second channels on the exterior of the tube walls are separated from each other by means of the connecting members. For example, the connecting members protrude radially from the tube wall and extend along a longitudinal tube axis. In this case, each elongate second channel provides a separate flow path for the fluid flowing on the exterior of the tube walls. Thus, the fluid flowing through the elongate second channels flows in a strictly co-current or counter-current configuration with respect to the first fluid flowing through the elongate first channels. In other words, any cross-flow of the fluid flowing through the elongate second channels is prevented, which is advantageous for heat transfer efficiency. The elongated second channels on the exterior of the tube walls may be separated from each other by means of the connecting members in a sealing manner. However, it is not necessary for the elongated second channels to be sealed from each other, because there is not a pressure gradient in the transverse direction inducing crossflow between the elongated second channels.

In an embodiment, the first supply opening and the first discharge opening of each elongate first channel are situated opposite to each other. Likewise, the second supply opening and the second discharge opening of the elongate second channels are located opposite to each other. The first supply openings of the elongate first channels are constituted by the front openings of the tube walls. The first fluid enters the elongate first channels through the first supply openings. The rear openings of the tube walls form the first discharge openings of the elongate first channels. Depending on co-current or counter- current operation of the mixing device, the elongate second channels may be fed with the second fluid or a mixture of the first and second fluid at the second supply openings. This is discharged from the elongate second channels at the second discharge openings.

In an embodiment, the housing comprises a first distributing chamber which is provided with the first inlet for the first fluid, in which the first distributing chamber is delimited by a first panel having a plurality of feed openings which are connected to the first supply openings of the elongate first channels. In this case, the first panel may be a tube sheet, in which the heat exchanger tubes are received in the feed openings of the tube sheet.

It is possible that the housing comprises a second distributing chamber which is separated from the first distributing chamber by the first panel, in which the second distributing chamber is situated adjacent to the first distributing chamber, in which the second distributing chamber is provided with the second inlet for the second fluid, and in which the second supply openings of the elongate second channels are in fluid communication with the second distributing chamber, and in which at least a number of the first discharge openings of the elongate first channels and the second discharge openings of the elongate second channels open into the mixing chamber, and in which the mixing chamber is provided with the outlet for the mixture of the first fluid and the second fluid.

In this case, the first fluid flows through the elongate first channels formed by the interior of the tube walls of the heat exchanger tubes. The second fluid flows through the elongate second channels in co-current with the first fluid. When traversing the elongate first and second channels, the first and second fluids exchange heat by heat transfer through the tube walls. Then, the first and second fluids are discharged from the elongate first and second channels into the mixing chamber, in which they are mixed together. This leads to a short mixing time as result of multiple injection points for efficient distribution of the first fluid into the second fluid. The two fluids quickly mix together due to dynamic turbulent forces. The mixture of the first and second fluids exits the mixing chamber via the outlet.

The elongate first and second channels of the mixing device may have a relatively short length if temperature equalization is not a critical factor. Where substantially deviating fluid temperatures have a negative effect on mixing and side reactions, a longer length for the elongate first and second channels will substantially equalize temperature differences as a result of the convective heat transfer between the fluids through the tube walls. A reduction in temperature difference will shorten the mixing time once the fluids are discharged into the mixing chamber.

In an alternative embodiment, the mixing chamber is provided with the second inlet for the second fluid, in which the housing comprises a first collecting chamber which is separated from the first distributing chamber by the first panel, and in which the first collecting chamber is situated adjacent to the first distributing chamber, in which the first collecting chamber is provided with the outlet for the mixture of the first fluid and the second fluid, and in which the second supply openings of the elongate second channels extend from the mixing chamber, and in which the second discharge openings of the elongate second channels open into the first collecting chamber, and in which at least a number of the first discharge openings of the elongate first channels open into the mixing chamber.

In this case, the first fluid also flows through the elongate first channels formed by the interior of the tube walls of the heat exchanger tubes. However, the second fluid enters the housing at the mixing chamber, where the second fluid is mixed with the first fluid. The mixture of the first and second fluids enters the elongate second channels and flows therethrough in counter-current with the first fluid. When traversing the elongate first and second channels, the first fluid and the mixture of the first and second fluids exchange heat by heat transfer through the tube walls. When the mixture of the first and second fluids exits the elongate second channels, it is collected in the first collecting chamber. From there, the mixture of the first and second fluids is discharged through the outlet. Thus, the first fluid is mixed with the second fluid in the mixing chamber and then directly returned into the elongate second channels. Intensive mixing occurs due to the turbulence inside the small elongate second channels, which leads to very fast and efficient mixing.

The mixing device can be constructed in different ways to allow partial mixing of the first and second fluids. In an embodiment, the housing comprises a second collecting chamber which is provided with a discharge for the first fluid, in which the second collecting chamber is separated from the mixing chamber by a second panel. In this case, it is possible that the second panel comprises a plurality of feed openings, in which the tube walls comprise a first group which extend to the feed openings of the second panel, and in which the first discharge openings of the elongate first channels of the first group of tube walls are connected to the feed openings of the second panel, and in which the tube walls comprise a second group, and in which the first discharge openings of the elongate first channels of the second group of tube walls are in fluid communication with the mixing chamber.

As a result, an amount of the first fluid is injected into the second collecting chamber without being mixed with the second fluid, while another amount of the first fluid is discharged into the mixing chamber, where it is mixed with the second fluid. In other words, only a fraction of the first fluid is allowed to mix with the second fluid. This leads to partial mixing of the first and second fluids.

The fraction of the first fluid which is not allowed to mix is discharged via the separate second collecting chamber and the discharge for the first fluid provided therein. The proportion of the fraction which is allowed to mix to the fraction which is not allowed to mix is determined by the dimensions of the opening or openings in the tube walls between the first and the second fluid and the pressure difference between the fluids.

A partial mixing of the first and second fluids can be achieved in other ways. For example, each elongate first channel may comprise at least two discharge openings, in which a first group of the discharge openings of the elongate first channels are connected to the feed openings of the second panel, and in which a second group of the discharge openings of the elongate first channels open into the mixing chamber. The second group of discharge openings of the elongate first channels constitute leak openings.

The invention in particular relates to a mixing device for mixing a first fluid and a second fluid, the mixing device comprising a housing which is provided with:

- a first inlet for the first fluid,

- a second inlet for the second fluid,

- an outlet for the mixture of the first fluid and the second fluid, in which the housing is provided with a plurality of heat exchanger tubes having tube walls which are situated next to each other and extend substantially parallel to each other, in which the interiors of the tube walls define elongate first channels, in which each elongate first channel comprises a first supply opening and a first discharge opening, in which the first supply openings are in fluid communication with the first inlet for the first fluid, and in which at least a number of the first discharge openings are in fluid communication with the mixing chamber, and in which the tube walls of adjacent heat exchanger tubes are connected to each other so as to define elongate second channels on the exterior of the tube walls, in which each elongate second channel comprises a second supply opening and a second discharge opening, in which the elongate second channels are in fluid communication with the second inlet for the second fluid and with the mixing chamber by means of the second openings, and

in which the housing comprises a first distributing chamber which is provided with the first inlet for the first fluid, in which the first distributing chamber is delimited by a first panel having a plurality of feed openings which are connected to the first supply openings of the elongate first channels, and in which

• the housing comprises a second distributing chamber which is separated from the first distributing chamber by the first panel, and in which the second distributing chamber is situated adjacent to the first distributing chamber, in which the second distributing chamber is provided with the second inlet for the second fluid, and in which the second supply openings of the elongate second channels are in fluid communication with the second distributing chamber, and in which at least a number of the first discharge openings of the elongate first channels and the second discharge openings of the elongate second channels are in fluid communication with or debouch/open into the mixing chamber, and in which the mixing chamber is provided with the outlet for the mixture of the first fluid and the second fluid, or

• the mixing chamber is provided with the second inlet for the second fluid, and in

which the housing comprises a first collecting chamber which is separated from the first distributing chamber by the first panel, and in which the first collecting chamber is situated adjacent to the first distributing chamber, in which the first collecting chamber is provided with the outlet for the mixture of the first fluid and the second fluid, and in which the second supply openings of the elongate second channels extend from the mixing chamber, and in which the second discharge openings of the elongate second channels debouch/open into the first collecting chamber, and in which at least a number of the first discharge openings of the elongate first channels are in fluid communication with or debouch/open into the mixing chamber, and in which the housing comprises a second collecting chamber which is provided with a discharge for the first fluid, in which the second collecting chamber is separated from the mixing chamber by a second panel.

In an embodiment, the tube walls are made from plastic material. The connecting members can also be made from plastic material. As a result, the mixing device is able to withstand fouling and/or corrosive fluids. In addition, it is possible for the connecting members between the tube walls of adjacent tubes to be configured to be connected to each other by means of a snap-fit arrangement. As a result, the heat exchanger tubes constitute a rigid stack of tubes.

The invention also relates to an exhaust gas recirculation (EGR) system, comprising:

- a combustion chamber having a fuel inlet, a combustion air inlet, and a exhaust gas outlet,

- a mixing device as described above, in which the first inlet for the first fluid is connected to the exhaust gas outlet of the combustion chamber, and the second inlet for the second fluid is connected to a source of ambient air, and the outlet for the mixture of the first fluid and the second fluid is connected to the combustion air inlet of the combustion chamber.

For example, the exhaust gas recirculation (EGR) system comprises a steam boiler having a number of fire tubes, a firebox, a water feed opening and a steam discharge opening, in which the firebox is provided with the combustion chamber.

Furthermore, the invention relates to a method for mixing a first fluid and a second fluid in a mixing device as described above, in which the method comprises:

- supplying the first fluid to the first inlet of the housing, the first fluid having a first temperature,

- supplying the second fluid to the second inlet of the housing, the second fluid having a second temperature which is lower than the first temperature,

- lowering the temperature of the first fluid by means of heat transfer through the tube walls from the first fluid flowing through the elongate first channels to the fluid flowing through the elongate second channels,

- mixing the first fluid and the second fluid in the mixing chamber after the

temperature of the first fluid has been lowered by means of the heat transfer.

It is possible for the second fluid to flow through the elongate second channels in co-current with the first fluid which flows through the elongate first channels. Alternatively, the mixture of the first fluid and the second fluid may flow through the elongate second channels in counter-current with the first fluid which flows through the elongate first channels. The invention will now be explained in more detail with reference to exemplary embodiments shown in the figures.

Figure 1 is a schematic representation of a steam boiler having an exhaust gas recirculation (EGR) system.

Figure 2 shows a first embodiment of a mixing device according to the invention, in which the first and second fluids are completely mixed together.

Figure 3 shows a stack of heat exchanger tubes of the mixing device shown in figure 2.

Figure 4 shows a second embodiment of a mixing device according to the invention, in which only a portion of the first fluid is mixed with the second fluid.

Figure 5 shows a stack of heat exchanger tubes of the mixing device shown in figure 4.

Figure 6 shows a third embodiment of a mixing device according to the invention, in which only a portion of the first fluid is mixed with the second fluid.

Figures 7a, 7b show flow control members which can be used for the mixing device shown in figure 6.

Figure 8 shows an alternative embodiment of a mixing device according to the invention, in which only a portion of the first fluid is mixed with the second fluid.

Figure 9 schematically shows the flow directions of the heat exchanging first and second fluids in the mixing device shown in figures 2-8.

Figures 10, 1 1 , 12 show examples of snap-fit connections for connecting the heat exchanger tubes of a mixing device according to the invention.

Figure 13 shows an embodiment of an extension of a heat exchanger tube of a mixing device according to the invention.

A steam boiler 2 comprises a first inlet 3 for a fuel, such as natural gas or any other hydrocarbons, and also a second inlet 4 for combustion air (see figure 1). Furthermore, the steam boiler comprises a feed opening 8 for feeding water into the steam boiler 2. The steam boiler 2 includes a firebox or furnace 15 within the interior 5 of the steam boiler 2 in order to burn the fuel and generate heat. The hot combustion gases transfer heat to the water in the steam boiler 2 to make steam. For example, the hot combustion gases flow through fire tubes extending inside the steam boiler 2 (not shown). Thereafter, the combustion gases are removed from the steam boiler 2 via the outlet 6. The produced steam is discharged from the steam boiler 2 via the discharge opening 9. The produced steam can then be used, for example, to produce power via a turbine.

The steam boiler 2 is provided with an exhaust gas recirculation (EGR) system 1.

The EGR system 1 comprises a mixing device 10 for mixing a portion of the combustion gases from the outlet 6 with ambient air. The mixing device 10 comprises a first inlet 20 for supplying the combustion gases (first fluid) and a port or second inlet 27 for supplying the ambient air (second fluid). The mixing device 10 comprises a heat exchanger and mixer which are integrated into one single device. With the mixing device 10 according to the invention, the hot combustion gases and the colder ambient air are subjected to heat transfer before a portion of the combustion gases is mixed together with the ambient air. The mixing device 10 comprises a first chamber forming a mixing chamber 42 in which the first and second fluids are mixed. Thus, the temperature difference between the

temperatures of the combustion gases and the ambient air is decreased by heat transfer before mixing. This leads to a relatively fast and efficient mixing process and heat recovery leading to improved boiler efficiency.

The mixing device 10 comprises a discharge 28 for the portion of the combustion gases which is not mixed with the ambient air in the mixing device 10. The discharge 28 may, for example, be connected to a chimney which opens into atmosphere (not shown). The mixing device 10 also has a port or outlet 26 for discharging/removing the mixture of the combustion gases and the ambient air from the mixing device 10. In this exemplary embodiment, the mixture of the combustion gases and the ambient air is transported from the outlet 26 of the mixing device 10 to the steam boiler 2 through a return line 1 1 which is provided with a fan 7. Thus, the combustion air which is introduced into the steam boiler 2 via the inlet 4 is formed by said mixture of the combustion gases and the ambient air.

By mixing a portion of the combustion gases having a low oxygen concentration with ambient air, the concentration of oxygen in the combustion air is lowered with respect to ambient air. As a result, the combustion gases produced in the firebox will have a lower peak temperature. This reduces the formation of NO_x in the combustion gases and thus decreases NO_x emissions.

In the exemplary embodiment shown in figure 1 , only a portion of the combustion gases is mixed with the heated ambient air. However, the mixing device according to the invention may also be configured to mix the total volume of the combustion gases being removed from the outlet 6 of the steam boiler 2 with the ambient air which is fed to the mixing device 10 via the second inlet 27. In this latter case, the mixing device 10 does not include a discharge for the combustion gases. The total volume of the combustion gases is mixed, and the mixture of the combustion gases and the ambient air flowing out of the outlet 26 of the mixing device 10 flows into the return line 1 1. In this case, the return line 11 has a discharge which opens into atmosphere for discharging a portion of the mixture of the combustion gases and the ambient air. Said discharge is situated downstream of the fan 7 and prevents accumulation of the mixture (not shown).

The mixing device 10 according to the invention can be constructed in various ways, as will now be described. Figure 2 shows a first embodiment of the mixing device 10 according to the invention. The mixing device 10 comprises a housing 12 having end walls 14, 16 and a circumferential wall 18. In this exemplary embodiment, the circumferential wall 18 comprises four side walls defining a substantially rectangular cross-section. The first inlet 20 for supplying the combustion gases (first fluid) is provided in the first end wall 14 at a first end 22 of the housing 12. The port or second inlet 27 for supplying the ambient air (second fluid) is provided in the circumferential wall 18 near the first end 22. The second end wall 16 of the housing 12, which is located at a second end 24 opposite to the first end 22 of the housing 12, comprises the port or outlet 26 for discharging a mixture of the first fluid and the second fluid.

The first inlet 20 is connected to a second chamber 32 which forms a first distributing chamber 32 within the housing 12. The first distributing chamber 32 is delimited by the first end wall 14, an end part of the circumferential wall 18 adjacent to the first end wall 14 and a first panel or distributor panel 34. The housing 12 comprises a plurality of heat exchanger tubes 36 which are arranged within the housing 12 on the side of the distributor panel 34 facing away from the first distributing chamber 32. Each heat exchanger tube 36 comprises a tube wall 37. Each tube wall 37 has an interior which defines an elongate first channel 38 having a supply opening 65 and a discharge opening 66 (see figure 3).

The distributor panel 34 is provided with a plurality of feed openings 46. The feed openings 46 of the distributor panel 34 are connected to the supply openings of the heat exchanger tubes 36 in a sealing manner. Thus, the elongate first channels 38 inside the tube walls 37 are in fluid communication with the first inlet 20 of the housing 12. The combustion gases enter the first distributing chamber 32 via the first inlet 20. From there, the combustion gases are introduced into the elongate first channels 38 within the tube walls 37. The elongate first channels 38 inside the tube walls 37 define a flow path for the combustion gases (first flow path).

The tube walls 37 of the heat exchanger tubes 36 are situated at a distance next to each other and extend substantially parallel to each other. Each tube wall 37 comprise a first end portion 67 and a main portion 68 (see figure 3). The main portions 68 of the tube walls 37 of adjacent heat exchanger tubes 36 are fixed to each other by means of connecting members 50. The connecting members 50 of adjacent heat exchanger tubes 36 are interconnected so as to provide a rigid stack of heat exchanger tubes 36. For example, the connecting members 50 of adjacent heat exchanger tubes 36 are connected to each other by means of snap-fit arrangements. Fig. 3 shows a stack of heat exchanger tubes 36 in a 5x5 matrix. Of course, the stack of heat exchanger tubes 36 may comprise more or less heat exchanger tubes 36. In this exemplary embodiment, each tube wall 37 has a circular cross-section and each tube wall 37 is provided with four connecting members 50 which are spaced apart circumferentially by 90°. The connecting members 50 may be constructed as strips which extend in the direction of the tube axis and protrude radially from the tube walls 37. The connecting members 50 comprise male connecting members 50' and female connecting members 50" (see figure 9). The male connecting members 50' comprise male connectors which may be formed as longitudinal beads 64. The female connecting member 50" comprise female connectors which may be constructed as longitudinal grooves 54 being configured to receive the beads 64 in a snap-fit manner.

The main portions 68 of the tube walls 37 of adjacent heat exchanger tubes 36 and the connecting members 50 between said main portions 68 of the tube walls 37 define elongate second channels 58 on the exterior of said tube walls 37. The connecting members 50 are connected to each other so as to allow co-current flow or countercurrent flow of the fluids in the elongate first and second channels 38,58. The elongate second channels 58 may be separated from each other by the connecting members 50. In addition, the heat exchanger tubes 36 and the connecting members 50 provide a rigid structure.

Each elongate second channel 58 has a first opening and a second opening, which form a supply opening 75 and a discharge opening 76, respectively (see figure 3). The elongate second channels 58 outside the tube walls 37 define a flow path for the ambient air (second flow path). The tube walls 37 and the connecting members 50 are configured to separate the elongated first channels 38 and the elongate second channels 58 from each other along the main portions 68 of the tube walls 37.

The first end portions 67 of the tube walls 37 are defined by the portions of the tube walls 37 between the first supply openings 65 of the elongate first channels 38 inside the tube walls 37 and the second supply openings 75 of the elongate second channels 58 delimited by the connecting members 50 on the exterior of the tube walls 37. In other words, the first end portions 67 of the tube walls 37 are the end portions in which the connecting members 50 are omitted. As the tube walls 37 are situated at a distance from each other, the first end portions 67 of the tube walls 37 define a third chamber 52 which forms a second distributing chamber 52.

The second distributing chamber 52 is separated from the first distributing chamber 32 by the distributor panel 34. The second distributing chamber 52 extends from the distributor panel 34 up to the second supply openings 75 of the elongate second channels 58. The second inlet 27 for the ambient air opens into the second distributing chamber 52. The ambient air which is introduced into the second distributing chamber 52 is distributed by means of cross-flow between the first end portions 67 of the tube walls 37. Then, the ambient air flows into the elongate second channels 58 at their supply openings 75. The ambient air is in co-current with the combustion gases flowing through the elongate first channels 38.

The hot combustion gases traversing the elongate first channels 38 transfer heat through the tube walls 37 to the colder ambient air flowing through the elongate second channels 58. Such indirect heat transfer takes place along the main portions 68 of the tube walls 37. The heat transfer is efficient due to the fact that the elongate first channels 38 and the elongate second channels 58 run substantially parallel to each other. As a result, the temperature of the hot combustion gases is lowered and the temperature of the ambient air increases. Depending on the length of the main portions 68 of the tube walls 37, the temperatures of the hot combustion gases and the ambient air may be substantially equalized.

The combustion gases flow out of the tube walls 37 at the discharge openings 66 of the elongate first channels 38 which debouch/open into the mixing chamber 42. Likewise, the ambient air is discharged from the discharge openings 76 of the elongate second channels 58 and ends up in the mixing chamber 42. The mixing chamber 42 is delimited by the second end wall 16, an end part of the circumferential wall 18 adjacent to the second end wall 16 and the discharge openings 66, 76 of the elongate first and second channels 38, 58. Thus, the combustion gases and the ambient air are mixed in the mixing chamber 42 into a mixture of the combustion gases and the ambient air. As the mixing process is only carried out after heat transfer between the combustion gases and the ambient air, a uniform mixture is achieved in a fast and efficient manner. The mixture of the combustion gases and the ambient air is discharged from the mixing chamber 42 via the outlet 26.

In this exemplary embodiment, the tube walls 37 and the connecting members 50 of the heat exchanger tubes 36 are made from a plastic material, such as polyethylene, polypropylene, polystyrene, polyvinylchloride and poly(meth)acrylaat, fluor containing polymers like PTFE_X and biopolymers. Other plastic materials allowing higher operating temperatures may also be used, for example polycarbonate and polysulfon for temperatures in the range from 100°C to about 120°C, or polyvinylene oxides, polyetherimides, polyethersulfons and especially fluor containing polymers or polyetheretherketon (PEEK) for even higher operating temperatures. Such plastic materials are able to withstand aggressive flue gases.

The mixing device according to the first embodiment shown in figures 2 and 3 may also be operated in a counter-current configuration. In this case, the outlet 26 shown in figure 2 forms the second inlet for the ambient air (second fluid). The combustion gases and the ambient air are mixed in the mixing chamber 42, after which the mixture of the combustion gases and the ambient air flows into the supply openings of the elongate second channels 58 which are located adjacent to the mixing chamber 42 (i.e. discharge openings 66 for co-current configuration). The mixture flows through the elongate second channels 58 in counter-current with the combustion gases traversing the elongate first channels 38.

The counter-current relationship guarantees effective heat transfer through the tube walls 37 of the heat exchanger tubes 36. In addition, turbulent flow of the mixture in the relatively small elongate second channels 58 leads to a very intensive mixing process. The mixture is discharged from the elongate second channels 58 at their discharge openings (corresponding to supply openings 65 for co-current configuration). In this case, the second distributing chamber 52 shown in figure 2 forms a first collecting chamber. The counter- current flow and the discharge of the mixture into the first collecting chamber is shown in more detail in figure 9. In the counter-current configuration, the second inlet 27 shown in figure 2 constitutes the outlet for the mixture of the combustion gases and the ambient air.

Figure 4 shows a second embodiment of a mixing device according to the invention. The same or similar parts are designated by the same reference numerals. The parts which are also present in the first embodiment have the same or similar function and the above description thereof also applies to the second embodiment.

The mixing device 10 shown in figure 4 is configured to mix only a portion of the combustion gases introduced by the first inlet 20 with the ambient air supplied to the second inlet 27. The mixing device 10 comprises a fourth chamber 40 which forms a second collecting chamber 40. The second end wall 16, an end part of the circumferential wall 18 adjacent to the second end wall 16 and a second panel or collector panel 44 delimit the second collecting chamber 40. The second collecting chamber 40 comprises a discharge 28 for the portion of the combustion gases (first fluid) which is not mixed with the ambient air. The collector panel 44 is installed within the housing 12 at a distance from the second end wall 16. The collector panel 44 is provided with a plurality of feed openings 46.

A first group of the tube walls 37 of the heat exchanger tubes 36 extend to the feed openings 46 of the collector panel 44. The discharge openings of the first group of tube walls 37 are denoted by reference numeral 69. The discharge openings 69 of the first group of tube walls 37 are connected to the feed openings 46 of the collector panel 44 in a sealing manner. Thus, the elongate first channels 38 inside the first group of tube walls 37 are in fluid communication with the second collecting chamber 40. A second group of the tube walls 37 of the heat exchanger tubes 36 terminate at a distance from the collector panel 44. The discharge openings of the second group of tube walls 37 are designated by reference numeral 63.

Each tube wall 37 of the first group comprises a second end portion 77 in addition to the first end portion 67 and the main portion 68 (see figure 5). The second end portions 77 of the first group of tube walls 37 are defined by the portions of said tube walls 37 between the discharge openings 76 of the elongate second channels 58 delimited by the connecting members 50 on the exterior of the tube walls 37 and the discharge openings 69. In other words, the second end portions 77 of the first group of tube walls 37 are the end portions in which the connecting members 50 are omitted.

As the tube walls 37 of the first group are situated at a distance from each other, the second end portions 77 of the first group of tube walls 37 define the mixing chamber 42. The mixing chamber 42 is delimited by the collector panel 44, a part of the circumferential wall 18 adjacent to the collector panel 44 on the side facing away from the second collecting chamber 40, and the discharge openings 63 of the second group of tube walls 37 and the discharge openings 76 of the elongate second channels 58. The second end portions 77 of the first group of tube walls 37 run through the mixing chamber 42. The mixing chamber 42 is separated from the second collecting chamber 40 by the collector panel 44.

The discharge openings 63 of the second group of tube walls 37 open into the mixing chamber 42. The combustion gases flowing on the inside of the second group of tube walls 37 are discharged via the discharge openings 63 into the mixing chamber 42. Also, the ambient air flows out of the discharge openings 76 of the elongate second channels 58 on the exterior of the tubes walls 37 into the mixing chamber 42. The combustion gases and the ambient air which are discharged into the mixing chamber 42 are mixed by means of cross- flow between the second end portions 77 of the tube walls 37. The outlet 26 for the mixture of the combustion gases and the ambient air is installed in the part of the circumferential wall 18 associated with the mixing chamber 42.

The mixing device according to the second embodiment shown in figures 4 and 5 may also be operated in a counter-current configuration. In this case, the outlet 26 shown in figure 4 forms the second inlet for the ambient air (second fluid). The combustion gases and the ambient air are mixed in the mixing chamber 42, after which the mixture of the

combustion gases and the ambient air flows into the supply openings of the elongate second channels 58 which are located adjacent to the mixing chamber 42 (i.e. discharge openings 76 for co-current configuration). The mixture flows through the elongate second channels 58 in counter-current with the combustion gases traversing the elongate first channels 38.

The counter-current relationship guarantees effective heat transfer through the tube walls 37 of the heat exchanger tubes 36. The mixture is discharged from the elongate second channels 58 at their discharge openings (corresponding to supply openings 75 for co-current configuration). In this case, the second distributing chamber 52 shown in figure 4 forms the first collecting chamber. In the counter-current configuration, the second inlet 27 shown in figure 4 constitutes the outlet for the mixture of the combustion gases and the ambient air. Figure 6 shows a third embodiment of a mixing device according to the invention. The mixing device 10 shown in figure 6 is also configured to mix only a portion of the combustion gases introduced by the first inlet 20 with the ambient air supplied to the second inlet 27. The same or similar parts are designated by the same reference numerals. The parts which are also present in the second embodiment have the same or similar function and the above description thereof also applies to the third embodiment.

In this case, all tube walls 37 of the heat exchanger tubes 36 are connected to feed openings 46 in the collector panel 44 in a sealing manner. Thus, the elongate first channels 38 inside the tube walls 37 are in fluid communication with the second collecting chamber 40 via the feed openings 46. Each tube wall 37 comprises a second end portion 77 in addition to the first end portion 67 and the main portion 68 (see figure 6). The second end portions 77 of the tube walls 37 extend between the collector panel 44 and the discharge openings 76 of the elongate second channels 58 delimited by the connecting members 50 on the exterior of the tube walls 37 and the discharge openings 69.

The second end portions 77 of the tube walls 37 each comprise a discharge opening 81. The interior of each tube wall 37 is in fluid communication with the mixing chamber 42 via the discharge openings 81. The flow area of the discharge openings 81 influences the mass flow of the combustion gases from the inside of the tube walls 37 into the mixing chamber 42, i.e. the portion of the combustion gases which is mixed with the ambient air.

The mixing device according to the third embodiment shown in figure 6 may be operated in a co-current or counter-current configuration.

In order to control the mass flow of the combustion gases from the inside of the tube walls 37 into the mixing chamber 42, the heat exchanger tubes 36 may be provided with flow control members (see figure 7a). For example, as shown in figure 7a, the discharge openings 81 may be provided with a valve body 83. The valve body 83 may be constructed as an insert which is received within the second end portion 77 of the tube wall 37 in a sliding manner. The insert 83 may be pushed in so as to substantially seal the discharge opening 81. From this position, the insert 83 may be retracted out of the second end portion 77 of the tube wall 37 so as to uncover the discharge opening 81. The flow area of the discharge opening 81 depends on the position of the insert 83.

Figure 7b shows that the second end portion 77 of the tube wall 37 may be provided with a stopper 85. The stopper 85 seals off the interior of the tube wall 37. Thus, the stopper 85 prevents the combustion gases inside the tube wall 37 from being

debouching into the second collecting chamber 40. The entire volume of the combustion gases traversing the tube wall 37 is forced into the mixing chamber 42. The valve body 83 shown in figure 7a and the stopper shown in figure 7b may be combined with the mixing device shown in figure 6. Thus, any number of the feed openings 46 of the collector panel 44 in figure 6 may be provided with a valve body 83 or a stopper 85. The valve bodies 83 and/or stoppers 85 can be designed to adjust the mass flow of combustion gases which is mixed with the ambient air to a desired level.

Figure 8 shows an alternative embodiment of the mixing device shown in figure 6. The same or similar parts are designated by the same reference numerals.

The mixing device 10 shown in figure 8 is also configured to mix only a portion of the combustion gases with the ambient air. In this case, there are no discharge openings 81 in the second end portions 77 of the tube walls 37. Instead, the collector panel 44 is provided with passage openings 86. The total volume of combustion gases is discharged from the tube walls 37 into the second collecting chamber 40. From there, a portion of the combustion gases flows back through the passage openings 86 into the mixing chamber 42.

Thus, the elongate first channels 38 inside the tube walls 37 are in fluid

communication with the mixing chamber 42 via the passage openings 86, whereas the elongate second channels 58 are also in fluid communication with the mixing chamber 42. The portion of the combustion gases which flows back through the passage openings 86 is mixed in the mixing chamber 42 with the ambient air. The portion of the combustion gases which does not flow back through the passage openings 86 is removed from the mixing device 10 via the discharge 28.

In a countercurrent configuration, reference numeral 26 is the second inlet for the ambient air, through which the ambient air flows into the mixing chamber 42. The ambient air is mixed with the portion of the combustion gases which flows back through the passage openings 86. Then, the mixture of the combustion gases and the ambient air is traversed through the elongate second channels and subsequently said mixture is discharged through the outlet indicated by reference numeral 27 in figure 8.

In a co-current operation of the embodiment shown in figure 8, the ambient air is introduced through the second inlet for the ambient air indicated by 27. Then, the ambient air flows out of the elongate second channels 59 into the mixing chamber 42, where the ambient air is mixed with the portion of the combustion gases which flows back through the passage openings 86. In this case, the resulting mixture is discharged through the outlet 26.

Figures 10-12 show additional examples of suitable male connecting members 50' and female connecting members 50" for interconnecting the heat exchanger tubes 36, in particular in a snap-fit manner. As shown in figure 10, each male connecting member 50' may be a flat rib 62 extending radially and in the longitudinal direction of the heat exchanger tube 36. Each female connecting members 50" comprises a pair of parallel ribs which are spaced apart. The width between the space apart ribs of the female connecting members 50" corresponds to the thickness of the rib 62 of the male connecting members 50'. Figure 1 1 shows a rib 62 having a protrusion 64 as a male connecting member 50', while the ribs 70 of the female connecting members 50" define a recess 72 in the rib surfaces 74 facing each other for receiving the protrusion 64. Figure 12 shows a sawtooth configuration. Other suitable connecting members may include slide fit and zip connections.

In figure 13 an extension comprising a tube section 80 having a rejuvenated end 82 is inserted in the open end 84 of a longitudinal tube 36, while the other open end of the tube section 80 extends through a feed opening 46 of the panel 34, 44. An O-ring 92 seales off the tube section 80.

While the invention has been illustrated and described in detail in the drawings and the above description, the same is to be considered as illustrative and not restricted in character. It should be understood that the invention is not limited to the exemplary embodiments described and shown above and that the skilled person may apply many changes and modifications without departing from the scope of the invention.

The invention can also be described by the following clauses:

1. A mixing device (10) for mixing a first fluid and a second fluid, the mixing device (10) comprising a housing (12) which is provided with:

- an inlet (20) for supplying the first fluid,

- a port (27) for supplying the second fluid,

- a first chamber (42), which first chamber forms a mixing chamber (42) for mixing the first fluid and the second fluid into a mixture of the first fluid and the second fluid,

- a port (26) for discharging the mixture of the first fluid and the second fluid, in which the housing (12) is provided with a plurality of heat exchanger tubes (36) having tube walls (37) which are situated next to each other and extend substantially parallel to each other,

in which the interiors of the tube walls (37) define elongate first channels (38), in which each elongate first channel (38) comprises a supply opening (65) and a discharge opening (66, 81), in which preferably the supply opening (65) is situated at a first end of said elongate first channel (38), and preferably the discharge opening (66, 81) is situated at a second end of said elongate first channel (38) remote from said first end, in which the supply openings (65) of the elongate first channels (38) are in fluid communication with the inlet (20) for supplying the first fluid, and in which at least a number of the discharge openings (66, 81) of the elongate first channels (38) are in fluid communication with the mixing chamber (42), and in which

the tube walls (37) of adjacent heat exchanger tubes (36) are connected to each other, preferably by connecting members (50), so as to define elongate second channels (58) on the exterior of the tube walls (37), in which each elongate second channel (58) comprises a first opening (75) and a second opening (76), in which preferably the first opening (75) is situated at a first end of said elongate second channel (58), and preferably the second opening (76) is situated at a second end of said elongate second channel (58) remote from said first end, in which the elongate second channels (58) are in fluid communication with the port (27) for supplying the second fluid and with the mixing chamber (42) by means of said first and second openings (75, 76) of the elongate second channels (58), and

in which the housing (12) comprises a second chamber (32) which is provided with the inlet (20) for supplying the first fluid, which second chamber (32) forms a distributing chamber, and in which the distributing chamber (32) is delimited by a first panel (34) having a plurality of feed openings (46) which are connected to the supply openings (65) of the elongate first channels (38), preferably in a substantially sealing or fluid-tight manner, and in which the housing comprises a third chamber (52) which is separated from the distributing chamber (32) by the first panel (34), and in which the third chamber (52) is situated adjacent to the distributing chamber (32), in which the third chamber (52) is provided with one of the ports (26, 27), i.e. with the port (27) for supplying the second fluid or with the port (26) for discharging the mixture of the first fluid and the second fluid, and in which the third chamber (52) is in fluid communication with one of the openings (75, 76) of each elongate second channel (58), and in which the other openings (75, 76) of said elongate second channels (58) and at least a number of the discharge openings (66, 81) of the elongate first channels (38) are in fluid communication with or debouch into the mixing chamber (42), and in which the mixing chamber (42) is provided with the other port (26, 27),

1. e. with the port (26) for discharging the mixture of the first fluid and the second fluid or with the port (27) for supplying the second fluid, and

in which the housing (12) comprises a fourth chamber (40) which is provided with a discharge (28) for discharging the first fluid, in which the fourth chamber (40) is separated from the mixing chamber (42) by a second panel (44).

2. A mixing device according to clause 1 , in which the second panel (44) comprises a plurality of feed openings (46), in which the tube walls (37) comprise a first group which extend to the feed openings (46) of the second panel (44), and in which the discharge openings (66) of the elongate first channels (38) of the first group of tube walls (37) are connected to the feed openings (46) of the second panel (44), preferably in a substantially sealing or fluid tight manner, and in which the tube walls (37) comprise a second group, and in which the discharge openings (66) of the elongate first channels (38) of the second group of tube walls (37) are in fluid communication with or debouch into the mixing chamber (42). 3. A mixing device according to clause 1 or 2, in which each elongate first channel (38) comprises at least two discharge openings (66,81), and in which a first group of the discharge openings (66) of the elongate first channels (38) are connected to the feed openings (46) of the second panel (44), preferably in a substantially sealing or fluid tight manner so as to debouch into the fourth chamber (40), and in which a second group of the discharge openings (66) of the elongate first channels (38) debouch into the mixing chamber (42).

4. A mixing device according to one of the preceding clauses, in which the second panel (44) is provided with passage openings (86) which provide fluid communication between the fourth chamber (40) and the mixing chamber (42).

5. A mixing device according one of the preceding clauses, in which one of the openings (75) of each elongate second channel (58) is in fluid communication with the port (27) for supplying the second fluid, and in which the other openings (76) of the elongate second channels (58) are in fluid communication with the mixing chamber (42).

6. A mixing device according to one of the preceding clauses, in which the mixing chamber (42) is provided with the port (27) for supplying the second fluid, in which one of the openings (75) of each elongate second channel (58) is in fluid communication with the mixing chamber (42), and in which the other openings (76) of the elongate second channels (58) are in fluid communication with the port (26) for discharging the mixture of the first fluid and the second fluid. 7. A mixing device according to one of the preceding clauses, in which the tube walls

(37) of the heat exchanger tubes (36) are configured to separate the elongate first channels

(38) on the interior of the tube walls (37) from the elongate second channels (58) on the exterior of the tube walls (37). 8. A mixing device according to one of the preceding clauses, in which each tube wall (37) comprises a main portion (68) which is provided with a plurality of connecting members (50) and a first end portion (67) which extends from the main portion (68), in which the tube walls (37) of adjacent heat exchanger tubes (36) are connected to each other by means of the connecting members (50), and in which the elongated second channels (58) on the exterior of the tube walls (37) are separated from each other by means of the connecting members (50). 9. A mixing device according to one of the preceding clauses, in which the tube walls

(37) are made from plastic material.

10. A mixing device according to one of the preceding clauses, in which the connecting members (50) are made from plastic material.

1 1. A mixing device according to one of the preceding clauses, in which the connecting members (50) between the tube walls (37) of adjacent heat exchanger tubes (36) are configured to be connected to each other by means of a snap-fit arrangement.

12. An exhaust gas recirculation (EGR) system, comprising:

- a combustion chamber having a fuel inlet (3), a combustion air inlet (4), and a combustion gas outlet (6),

- a mixing device (10) according to one of the preceding clauses, in which the inlet (20) for supplying the first fluid is connected to the combustion gas outlet (6) of the combustion chamber, and the port (27) for supplying the second fluid is connected to a source of ambient air, and the port (26) for discharging the mixture of the first fluid and the second fluid is connected to the combustion air inlet (4) of the combustion chamber. 13. An exhaust gas recirculation (EGR) system according to clause 1 1 , in which the system comprises a steam boiler (2) having a number of fire tubes, a firebox, a water feed opening (8) and a steam discharge opening (9), and in which the firebox is provided with the combustion chamber. 14. A method for mixing a first fluid and a second fluid in a mixing device (10), in which the mixing device (10) comprising:

a housing (12) which is provided with:

- an inlet (20) for supplying the first fluid,

- a port (27) for supplying the second fluid,

- a port (26) for discharging the mixture of the first fluid and the second fluid,

- a plurality of heat exchanger tubes (36) having tube walls (37) which are situated next to each other and extend substantially parallel to each other, in which the interiors of the tube walls (37) define elongate first channels (38), in which each elongate first channel

(38) comprises a supply opening (65) and a discharge opening (66,81), in which preferably the supply opening (65) is situated at a first end of said elongate first channel (38), and preferably the discharge opening (66, 81) is situated at a second end of said elongate first channel (38) remote from said first end, in which the supply openings (65) of the elongate first channels (38) are in fluid communication with the inlet (20) for supplying the first fluid, and in which at least a number of the discharge openings (66,81) of the elongate first channels (38) are in fluid communication with the mixing chamber (42), and in which the tube walls (37) of adjacent heat exchanger tubes (36) are connected to each other, preferably by connecting members (50), so as to define elongate second channels (58) on the exterior of the tube walls (37), in which each elongate second channel (58) comprises a first opening (75) and a second opening (76), in which preferably the first opening (75) is situated at a first end of said elongate second channel (58), and preferably the second opening (76) is situated at a second end of said elongate second channel (58) remote from said first end, in which the elongate second channels (58) are in fluid communication with the port (27) for supplying the second fluid and with the mixing chamber (42) by means of said first and second openings (75,76) of the elongate second channels (58), and in which the method comprises:

- supplying the first fluid to the inlet (20) of the housing (12), the first fluid having a first temperature,

- supplying the second fluid to the port (27) for supplying the second fluid of the housing (12), the second fluid having a second temperature which is lower than the first temperature,

- lowering the temperature of the first fluid by means of heat transfer through the tube walls (37) from the first fluid flowing through the elongate first channels (38) to the fluid flowing through the elongate second channels (58),

- mixing the first fluid and the second fluid in the mixing chamber (42) after the temperature of the first fluid has been lowered by means of the heat transfer.

15. A method according to clause 14, in which the second fluid flows through the elongate second channels (58) in co-current with the first fluid which flows through the elongate first channels (38).

16. A method according to clause 14, in which the mixture of the first fluid and the second fluid flows through the elongate second channels (58) in counter-current with the first fluid which flows through the elongate first channels (38).

Claims

- a first inlet (20) for the first fluid,

- a second inlet (27) for the second fluid,

- a mixing chamber (42) for mixing the first fluid and the second fluid into a mixture of the first fluid and the second fluid,

- an outlet (26) for the mixture of the first fluid and the second fluid,

characterised in that the housing (12) is provided with a plurality of heat exchanger tubes

(36) having tube walls (37) which are situated next to each other and extend substantially parallel to each other, in which the interiors of the tube walls (37) define elongate first channels (38), in which each elongate first channel (38) comprises a first supply opening (65) and a first discharge opening (66,81), in which the first supply openings (65) are in fluid communication with the first inlet (20) for the first fluid, and in which at least a number of the first discharge openings (66,81) are in fluid communication with the mixing chamber (42), and in which the tube walls (37) of adjacent heat exchanger tubes (36) are connected to each other so as to define elongate second channels (58) on the exterior of the tube walls

(37) , in which each elongate second channel (58) comprises a second supply opening (75) and a second discharge opening (76), in which the elongate second channels (58) are in fluid communication with the second inlet (27) for the second fluid and with the mixing chamber (42) by means of the second openings (75,76).

2. A mixing device as claimed in claim 1 , in which the second supply openings (75) of the elongate second channels (58) are in fluid communication with the second inlet (27) for the second fluid, and in which the second discharge openings (76) of the elongate second channels (58) are in fluid communication with the mixing chamber (42).

3. A mixing device as claimed in claim 1 or 2, in which the mixing chamber (42) is provided with the second inlet (27) for the second fluid, in which the second supply openings (75) of the elongate second channels (58) are in fluid communication with the mixing chamber (42), and in which the second discharge openings (76) of the elongate second channels (58) are in fluid communication with the outlet (26) for the mixture of the first fluid and the second fluid.

4. A mixing device as claimed in one of the precedings claims, in which the tube walls (37) of the heat exchanger tubes (36) are configured to separate the elongate first channels (38) on the interior of the tube walls (37) from the elongate second channels (58) on the exterior of the tube walls (37).

5. A mixing device as claimed in one of the preceding claims, in which each tube wall (37) comprises a main portion (68) which is provided with a plurality of connecting members (50) and a first end portion (67) which extends from the main portion (68), in which the tube walls (37) of adjacent heat exchanger tubes (36) are connected to each other by means of the connecting members (50), and in which the elongated second channels (58) on the exterior of the tube walls (37) are separated from each other by means of the connecting members (50).

6. A mixing device as claimed in claim 5, in which the connecting members (50) are made from plastic material.

7. A mixing device as claimed in claim 5 or 6, in which the connecting members (50) between the tube walls (37) of adjacent heat exchanger tubes (36) are configured to be connected to each other by means of a snap-fit arrangement.

8. A mixing device as claimed in one of the preceding claims, in which the housing (12) comprises a first distributing chamber (32) which is provided with the first inlet (20) for the first fluid, in which the first distributing chamber (32) is delimited by a first panel (34) having a plurality of feed openings (46) which are connected to the first supply openings (65) of the elongate first channels (38).

9. A mixing device as claimed in claim 8, in which the housing (12) comprises a second distributing chamber (52) which is separated from the first distributing chamber (32) by the first panel (34), and in which the second distributing chamber (52) is situated adjacent to the first distributing chamber (32), in which the second distributing chamber (52) is provided with the second inlet (27) for the second fluid, and in which the second supply openings (75) of the elongate second channels (58) are in fluid communication with the second distributing chamber (52), and in which at least a number of the first discharge openings (66,81) of the elongate first channels (38) and the second discharge openings (76) of the elongate second channels (58) open into the mixing chamber (42), and in which the mixing chamber (42) is provided with the outlet (26) for the mixture of the first fluid and the second fluid.

10. A mixing device as claimed in claim 8, in which the mixing chamber (42) is provided with the second inlet (27) for the second fluid, and in which the housing (12) comprises a first collecting chamber which is separated from the first distributing chamber (32) by the first panel (34), and in which the first collecting chamber is situated adjacent to the first distributing chamber (32), in which the first collecting chamber is provided with the outlet (26) for the mixture of the first fluid and the second fluid, and in which the second supply

5 openings (75) of the elongate second channels (58) extend from the mixing chamber (42), and in which the second discharge openings (76) of the elongate second channels (58) open into the first collecting chamber, and in which at least a number of the first discharge openings (66,81) of the elongate first channels (38) open into the mixing chamber (42).

10 1 1. A mixing device as claimed in claim 9 or 10, in which the housing (12) comprises a second collecting chamber (40) which is provided with a discharge (28) for the first fluid, in which the second collecting chamber (40) is separated from the mixing chamber (42) by a second panel (44).

15 12. A mixing device as claimed in claim 1 1 , in which the second panel (44) comprises a plurality of feed openings (46), in which the tube walls (37) comprise a first group which extend to the feed openings (46) of the second panel (44), and in which the first discharge openings (66) of the elongate first channels (38) of the first group of tube walls (37) are connected to the feed openings (46) of the second panel (44), and in which the tube walls

20 (37) comprise a second group, and in which the first discharge openings (66) of the elongate first channels (38) of the second group of tube walls (37) are in fluid communication with the mixing chamber (42).

13. A mixing device as claimed in claim 1 1 or 12, in which each elongate first channel 25 (38) comprises at least two discharge openings (66,81), and in which a first group of the discharge openings (66) of the elongate first channels (38) are connected to the feed openings (46) of the second panel (44) so as to open into the second collecting chamber (40), and in which a second group of the discharge openings (66) of the elongate first channels (38) open into the mixing chamber (42).

30

14. A mixing device as claimed in one of claims 1 1-13, in which the second panel (44) is provided with passage openings (86) which provide fluid communication between the fourth chamber (40) and the mixing chamber (42).

35 15. A mixing device as claimed in one of the preceding claims, in which the tube walls (37) are made from plastic material.

16. An exhaust gas recirculation (EGR) system, comprising:

- a mixing device (10) as claimed in one of the preceding claims, in which the first 5 inlet (20) for the first fluid is connected to the combustion gas outlet (6) of the combustion chamber, and the second inlet (27) for the second fluid is connected to a source of ambient air, and the outlet (26) for the mixture of the first fluid and the second fluid is connected to the combustion air inlet (4) of the combustion chamber.

10 17. An exhaust gas recirculation (EGR) system as claimed in claim 14, in which the

system comprises a steam boiler (2) having a number of fire tubes, a firebox, a water feed opening (8) and a steam discharge opening (9), and in which the firebox is provided with the combustion chamber.

15 18. A method for mixing a first fluid and a second fluid in a mixing device (10) as claimed in one of the claims 1-13, in which the method comprises:

- supplying the first fluid to the first inlet (20) of the housing (12), the first fluid having a first temperature,

- supplying the second fluid to the second inlet (27) of the housing (12), the second 20 fluid having a second temperature which is lower than the first temperature,

- mixing the first fluid and the second fluid in the mixing chamber (42) after the 25 temperature of the first fluid has been lowered by means of the heat transfer.

19. A method as claimed in claim 18, in which the second fluid flows through the elongate second channels (58) in co-current with the first fluid which flows through the elongate first channels (38).

30

20. A method as claimed in claim 18, in which the mixture of the first fluid and the second fluid flows through the elongate second channels (58) in counter-current with the first fluid which flows through the elongate first channels (38).

35