WO2008106977A1

WO2008106977A1 - A system and method for dosing fluid

Info

Publication number: WO2008106977A1
Application number: PCT/DK2008/050045
Authority: WO
Inventors: Niels Torp Madsen; Peter Rosenbeck Mortensen
Original assignee: Grundfos Nonox AS
Current assignee: Emitec Denmark AS
Priority date: 2007-03-02
Filing date: 2008-02-26
Publication date: 2008-09-12
Anticipated expiration: 2009-09-02

Abstract

The present invention relates to a dosing system for dosing a first fluid into a stream of a second fluid, the system comprising a fluid guide and a fluid feeding device (7). The fluid guide comprises a channel part (1) having an inlet arranged to receive the first fluid from the fluid feeding device (7) and an outlet arranged to deliver the first fluid to the second fluid. Furthermore, the channel part (1) and the outlet are shaped and arranged so that heat conducted from the second fluid may evaporate first fluid present in the channel part (1) and generate an interface between a gas phase located downstream of a liquid phase, said interface being sufficiently stable to keep the generated gas phase distinct from a liquid phase of the first fluid when no first fluid is fed to the inlet. The invention further relates to an exhaust system comprising such a dosing system and to a method utilising such a dosing system or such an exhaust system.

Description

A SYSTEM AND METHOD FOR DOSING FLUID

The present invention relates to a system and method for dosing fluid. The system comprises a fluid guide for guiding fluid from a feeding device, typically being a pump, and to a delivery position typically being an exhaust system of a combustion engine.

BACKGROUND OF THE INVENTION

Attempts have been made in order to avoid formation of deposits such as urea derivatives in piping and nozzle systems used for atomizing and introducing the so atomised urea into the exhaust system of a combustion engine. Typically, the urea is dissolved in a fluid (water) and deposits are typically formed based on urea and/or urea derivatives. The deposits are often formed on surfaces of the piping and/or nozzle, and as these deposits growth in size they start to block fluid passages resulting in a still poorer atomization and control of the delivered amount. Eventually the passages are totally blocked and the passages must be cleaned in some way which typically requires dismantling and manually removal of the deposits.

In connection with the present invention, it has been found that some or all of the following conditions should be met for creating deposits:

dry urea must be present (the water has to evaporate) • dry such as dewatered urea has to reach temperatures above 175°C without water present the flow passages of the piping and/or nozzle must be non-smooth in order to make an attachment point for the deposits presence of cavities, dead ends or re-circulating flow regions; e.g. regions where in-flowing or over-flowing fluid does not come directly into contact with the surface of the regions, such as flow over a rectangular cavity.

The fluid is preferably a urea in water solution (liquefied urea), typical 32,5% urea in demineralised water, Adblue (DIN 70070 / AUS32) or denoxium. Thus, by feeding liquefied urea (urea dissolved in water) or denoxium to a nozzle for atomization may often result in formation of deposits. Such deposits are typically, but not limited to, in the form of crystals or amorphous structures and occur if the piping and nozzle is not designed to take into account such formation. It is therefore an object of the present invention to provide a dosing system for dosing of fluid, and in particular urea, into a stream of hot fluid, typically being an exhaust system which dosing system seeks to at least mitigate problems pertaining to formation of deposits, typically, but not limited to, based urea or urea derivatives.

DESCRIPTION OF INVENTION

In accordance with the object of the present invention, the invention relates in a first aspect to a dosing system for dosing a first fluid into a stream of a second fluid, the system preferably comprising, a fluid guide and a fluid feeding device, wherein the fluid guide comprises a channel part having an inlet arranged to receive the first fluid from the fluid feeding device and an outlet arranged to deliver the first fluid to the second fluid, and wherein - the channel part and the outlet are shaped and arranged so that heat conducted from the second fluid may evaporate first fluid present in the channel part and generate an interface between a gas phase located downstream of a liquid phase, said interface being sufficiently stable to keep the generated gas phase distinct from a liquid phase of the first fluid when no first fluid is fed to the inlet.

In systems according to the present invention deposits may in some situations be formed in the gas containing region. However, such deposits are removed automatically e.g. by being dissolved or eroded by liquid flowing into the gas containing region when liquid is fed into the channel part. Thus, while many of the known systems seek to avoid formation of deposits, the present invention may be viewed as accepting such formations and flushing them away.

Although the invention is disclosed in connection with feeding liquefied urea to an exhaust system of a combustion engine, it is envisaged that the invention is applicable in a broader sense and with other fluids. The invention may also be used for feeding other reducing agents to an exhaust system. Such fluids may react with the exhaust gasses in the same way or differently from reactions with liquefied urea.

While deposits have shown to be a challenge in connection with the present invention, gas inclusions, such as small air bubbles, in the channel part has also shown to be an issue to consider. Such gas inclusions in the piping may result from liquid evaporating locally or be introduced when liquid is fed to the channel part. These gas inclusions act as damping means in the channel part resulting in a slow response in the outlet of the piping to e.g. a rapid increase in pressure at the inlet of the piping. The present invention has also shown to have the potential to overcome problems related to such gas inclusions.

As the channel part and the outlet are shaped in accordance with the present invention, gas inclusions will have a tendency to fill up an entire cross section of e.g. the channel part (and thus not only at one side of the channel part) whereby the flow through the channel part will have a tendency to push such gas inclusions out of the channel part and the outlet. Furthermore, as it is aimed at avoiding or minimising cavities, dead end and re-circulating flow regions, gas inclusions in such regions will be minimised accordingly.

The channel part of a dosing system according to the present invention may comprise only one outlet, the only one outlet being the one arranged to deliver the first fluid to the second fluid, so that fluid flowing into the channel part may only leave the channel part through its outlet.

The channel part may comprise only one inlet, the only one inlet being the one arranged to receive the fluid from the feeding device.

In preferred embodiments of the invention, the channel part may be shaped so that no cavities, dead ends or re-circulating flow regions are present. Hereby the risk of deposits staying within the channel part can be minimised. A dosing system may further comprise a strainer arranged upstream of the outlet or at the inlet of the channel part. Such a strainer can be used to prevent impurities, such as particles, present in the feeding device from entering the nozzle.

In preferred embodiments of the invention, the outlet further comprises an atomization device. The atomization device may comprise at least two converging nozzle channels. The at least two converging channels may be formed in a circular disc attached to the downstream end of the channel part. Such a device may be used to atomize the liquefied urea by letting fluid jets from the converging channels impinge each other.

The circular or oval disc may be an end of a tubular member, the tubular member being adapted to receive a downstream end region of the channel part. The parts may be held together e.g. by welding, press-fitting, gluing, or mechanical fastening means. Other appropriate ways of assembling the channel part and the atomization device will be well-known to a person skilled in the art.

The channel part may preferably have an internal circular cross section with an internal diameter being smaller than or equal to 4.0 mm, such as smaller than or equal to 2.8 mm, preferably smaller than or equal to 2.3 mm, such as smaller than or equal to 1.3 mm.

The channel part may have an external circular cross section with an external diameter being larger than or equal to 2 mm.

The longitudinal extension of the channel part may be shorter than 10 m, such as shorter than 5 m, preferably shorter than 2 m, or even shorter than 0.2 m.

In some embodiments of the invention, the surface roughness on the surface of the channel part (1) measured by Rz may be smaller than,

( jU(G)0,008 - 2,5 / Rz5(l 6%)25 ) such as smaller than

/RzI 6 ( ^U(G)0,008 - 2,5/Rz5(16%)16 ), preferably smaller than

The surface roughness should preferably be so small that deposits are not easily formed on the surfaces.

In some embodiments of the invention, the first liquid is liquefied urea, such as urea dissolved in water, or denoxium. However, other types of fluids, such as other reducing agents, are also possible within the scope of the invention.

The channel part may be made from stainless steel, aluminium or plastic. For some applications, such as for specific types of reducing agents, other types of materials may be more appropriate, e.g. to prevent disadvantageous chemical reactions with the material.

In embodiments of the invention comprising an atomization device, the atomization device may be made from stainless steel or ceramics.

Preferred embodiments of the invention further comprise a reservoir for storing the first fluid, the reservoir being in fluid communication with the fluid feeding device, preferably by a tube made from stainless steel, aluminium, or plastic.

In a second aspect the present invention relates to an exhaust system comprising a dosing system according to the first aspect and an exhaust pipe through which the second fluid flows, the outlet of the fluid guide being arranged so as to feed the first fluid into the second fluid.

In such an exhaust system according to the present invention, the outlet may be arranged flush with or protruding the interior surface of the exhaust pipe. The outlet may protrude the interior wall to an extent being smaller than half the diameter of the exhaust pipe through which the second fluid flows, and preferably smaller than 1.5 times the external diameter of the channel part. Hereby it may be ensured that no deposit of first fluid deviates (urea deviates) is created on or around the outlet of the first fluid. In preferred embodiments of an exhaust system, a downstream region of the channel part may be arranged in the wall of the exhaust pipe in such a manner that thermal contact between said region of the channel part and the wall of the exhaust pipe surrounding the channel part is established. In alternative embodiments, a downstream region of the channel part may be arranged in the wall of the exhaust pipe in such a manner that the said region is thermally isolated from the wall of the exhaust pipe. Hereby heating of the nozzle is limited and the thermal decomposition of the first fluid inside the nozzle is reduced.

In a third aspect the present invention relates to a method utilising a dosing system according to the first aspect of the present invention or an exhaust system according to second aspect of the present invention. The method preferably comprises feeding first liquid into the channel part through its inlet during a first period at a first flow rate, and feeding first liquid into the channel part through its inlet during a succeeding second period at a second flow rate or not feeding first liquid into the channel part through its inlet during a succeeding second period, wherein - the duration of the second period is not sufficiently short or the second flow rate, if different from zero, is not sufficiently high to prevent evaporation of first fluid out of the outlet during the second period, said evaporation taking place in regions of the channel part (1) located in or in the vicinity of the second fluid.

In some embodiments of the invention, the duration of a first period, succeeding a second period, may be sufficiently long or a first flow rate, succeeding a second flow rate, may be sufficiently high to ensure that the whole channel part is filled with liquefied urea. Hereby it is secured that atomisation pressure in the first fluid is achieved and small droplets are formed, securing a fast evaporation and homogenous distribution of first fluid in the second fluid.

The first flow rate may be greater than the second flow rate, and the second flow rate may be zero. The first and/or the second flow rate may be constant during the first and/or second period respectively. Alternatively it may vary during a first and/or a second period.

In some embodiments of the invention, the duration of a first period may be between 0.1 and 1 second, and the duration of a second period may be between 0.1 and 1 second.

It is envisaged that many of the advantages obtained by the dosing system according to first aspect of the present invention are also obtainable by the second and third aspect of the present invention.

The present invention, and in particular preferred embodiments thereof, will now be disclosed in further details in connection with the accompanying drawings in which:

Fig. 1 shows a cross sectional view of a fluid guide integrated in an exhaust pipe according to the present invention,

Fig. 2 shows the temperature conditions in the channel part,

Fig. 3 shows a dosing system according to the present invention,

Fig. 4 shows a cross sectional view of an atomization device attached to a channel part according to the present invention,

and

Fig. 5 shows seven embodiments of arrangements according to the present invention of the fluid guide in an exhaust pipe.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

With reference to fig. 1, a fluid guide according to a first preferred embodiment will now be described in greater details. The fluid guide which guides fluid from a feeding device to an exhaust gas comprises a channel part 1 having at its inlet a strainer 2 and at its outlet an atomization device 3. The outlet of the fluid guide and the atomization device 3 are arranged in a wall of pipe 5 leading exhaust gasses from a combustion engine to a catalyst. The atomization device 3 is arranged so that it is flush with or protrudes slightly the interior surface of the pipe 5. It should be noted that fig. 1 is not executed to scale; the longitudinal extension of the channel part 1 is typically many times larger than the diameter of the channel part 1.

The downstream region of the channel part 1 is, as indicated in fig. 1, arranged in the wall 5 of the exhaust pipe in such a manner that thermal contact between said region of the channel part 1 and the wall 5 of the exhaust pipe surrounding the channel part 1 is established. This may be provided in a number of ways, for instance by shaping the channel part 1 and the hole in the exhaust pipe so that the channel part 1 is press-fitted into the wall of the exhaust pipe, the channel part is welded, soldered, glued, or the like to the wall. However, the arrangement of the channel part 1 in the wall 5 of the exhaust system is preferably so that if the respective surfaces do not abut each other, the thermal conductivity between the channel part 1 and the wall 5 of the exhaust pipe is similar to, such as equal to, or higher than the thermal conductive of the wall 5 or the channel part 1.

In a further embodiment, a flange adapted to receive the channel part 1 for attachment thereof to the exhaust pipe is applied. In such embodiments, the flange is made of a material having a thermal conductivity similar to, such as equal to, or higher than the conductivity of the channel part 1 or wall 5 the exhaust pipe.

In still a further embodiment, the channel part 1 is thermally isolated from the wall 5 of the exhaust pipe. Such isolation is typically established by manufacturing the flange for receiving the channel part in a material having a low conductivity, such as plastic.

As indicated above, deposits occurs e.g. on surfaces that are non-smooth and that do not contain cavities, dead ends or re-circulating flow regions. In accordance with this, the channel part 1 is shaped with a constant circular cross section so that no cavities are present in the channel part 1. Furthermore, the interior surface of the channels part has a roughness of smaller than Λ}RZ25

(jU(G)0,008 - 2,5/Rz5(16%)25 ), such as smaller than

(jU(G)0,008 - 2,5/Rz5(16%)16 ), preferably smaller than 4RZ4 ( jU(G)0,0025 - 0,8/Rz5(16%)4 ), where the roughness is determined according to ISO 4288.

Although the channel part 1 in the embodiment of fig. 1 is disclosed as having a constant cross sectional area throughout its longitudinal extension, the cross sectional area may decrease or increase in the direction towards the atomization device 3. Furthermore, the cross section may deviate from circular and may be e.g. elliptic. However, a cross section with corners, such as a square-shaped is typically less attractive as it may produce re-circulating flow or areas with no flow in or at the vicinity of such corners.

When the internal diameter of the channel part 1 is not constant, at least a region of the channel part 1 in the vicinity of the outlet should be sufficiently small to assure that the stable interface is generated. Typically, in the 10 cm of the channel part 1 (measured from the outlet) located at the outlet, the internal diameter of the channel part 1 is smaller than 4.0 mm, such as smaller than 2.8 mm, preferably smaller than 2.2 mm, such as smaller than 1.3mm.

Furthermore, the atomization device 3 is also shaped so that substantially no cavities, dead ends or re-circulating flow regions are present. In accordance with the embodiment shown in fig. 1 this is accomplished by forming the atomization device 3 with two converging nozzle channels 4a, 4b each extending from the interior surface Ia of the channel part 1 and to the outer surface 3a of the atomization device 3. However, the region between the inlets of the two nozzle channels 4a, 4b facing upstream the channel part 1 may be exposed to deposits. If deposits occur in this region, they will not influence the flow as the region is small compared to the diameter of the channel part. Furthermore, the fluid flowing past such deposits will remove the deposits by dissolving or eroding them thereby generating a flow path to the inlets of the nozzle channels 4a, 4b. Atomization is provided when fluid flowing out of the nozzle channels 4a, 4b impinges which results in formation of droplets.

The fluid guide may in many situations work with no atomization device 3 provided, and the outlet of the fluid guide is thereby provided by the end of the channel part 1. A typical scenario where no atomization device is provided is where the internal diameter of the channel part 1 is so small and the flow into which the urea (or in general the liquid) is introduced has sufficient momentum to disperse the jet emerged from the end of the channel part 1.

It is contemplated that the fluid guide may be equipped with other types of atomization devices than the one shown in fig. 1. For instance, a venturi-shaped outlet may be applied to the end of the channel part 1 to provide atomization, or the outlet may be shaped as a hollow cone, full cone, flat fan, solid stream atomizer, or the like.

As indicated, the fluid guide comprises a strainer 2 at its inlet. This strainer 2 filters particulates from the fluid flowing into the channel part 1, which particulates have a size which could block the nozzle channels 4a, 4b if not filtered off.

The strainer may be considered as a cavity as the cross section of it may be larger than the internal diameter of the channel part 1 in order to allow a demanded flow through it without giving rise to a large pressure drop. The temperature of the strainer should be kept below 175°C to avoid creation of deposits, such as urea derivatives. Accordingly, the strainer is arranged sufficiently distant from the exhaust system and other heat source in order to avoid heating of the strainer.

During use of the fluid guide, the demands for urea will typically vary to a large extent. In order to handle very broad ranges of atomization, feeding of urea through the fluid guide is typically done with pulse width modulation typically so that the flow through the fluid guide stops completely at some instants in time. During the periods where no fluid is flowing through the fluid guide, heat from the hot exhaust gasses will start to evaporate fluid present in the vicinity of the atomization device 3. Thereby a liquid-gas interface 6 will be created above which the fluid is a liquid and below which the fluid is a gas.

The size of the cross section of the channel part 1 is selected so that the surface 5 tension of the fluid-gas interface will be sufficient to keep the liquid-gas interface 6 stable at least when the interface is orientated facing in the direction of the gravity. In other embodiments, the cross section of the fluid guide is selected so that the fluid-gas interface 6 is stable irrespectively of its orientation relative to the gravity. Thus, the internal diameter of the tube used for the channel part is 10 typically below 4.0 mm, such as smaller than 2.8 mm, preferably smaller than 2.2 mm, such as smaller than 1.3 mm. Such diameters make it possible for the surface tension of liquid, e.g. liquefied urea, to "block" the tube, and thereby make a sharp distinction between dry and wet side in the tube.

15 The presence of a gas comprising urea may have a tendency to create deposits in the form of urea crystals or urea derivatives on interior surfaces of the downstream end of the channel part 1 and on the surface of the atomization device 3 facing upstream. However, such deposits are removed either as the temperature in the regions of the deposits reaches the decomposition temperature

20 of the deposits, as the deposits are decomposed or eroded when liquid flows past these regions during a pulse, by purging of the evaporated liquid or a combination thereof.

As shown in figure 2, the sharp distinction between liquid and gas by the liquid- 25 gas interface 6 represents a sharp change in temperature. The temperature in the liquid above the interface 6 will be below or at the boiling temperature of the liquid, which means that for urea the temperature of the liquid will not exceed the boiling point of 103⁰C. As formations of the urea and/or urea derivative, such as cyanuric acid, ammeline and ammelide, as deposits requires a temperature of 30 175°C or above, formation of such deposits will be unlikely in the liquid.

Boiling of liquefied urea (a urea and water solution) will create large volumes of gas (the expansion factor is greater than 1000), and this large volume of gas will escape through the nozzle channels 4a and 4b and purge the gas filled part of the 35 fluid guide. The liquefied urea in the boiling zone will boil away in one shot resulting in a high velocity of the gas escaping through the nozzle. This purging reduces the amount of material, e.g. urea, in the gas filled part of the fluid guide that may create deposits to an amount that this is too small to make disturbing deposits. The gas primarily consists of ammonia, water (vapour) and carbon dioxide. Other gases such as biuret and iso-cyanuric acid can be present.

In fig. 3, the fluid guide is shown in connection with a urea feeding device 7. This urea feeding device 7 is adapted to feed urea into the fluid guide at a controllable rate typically in a pulse width modulated manner and may be constituted by a metering pump (not shown) or a similar pumping device. The urea feeding device 7 delivers the fluid to the channel part 1 through the strainer 2 arranged at the inlet of the channel part 1. The strainer may be arranged in another location e.g. before the pump as long as it is ensured that impurities are filtered off.

The channel part 1 with the feeding device 7 connected to its inlet has typically no fluid connection to the ambience upstream of liquid-gas interface 6 whereby this fluid tight system will further stabilise the liquid-gas interface 6. The fluid tightness is preferably provided by a valve (not shown) located upstream of the channel part 1, and preferably in the feeding device 7 where the fluid tightness is provided by e.g. a pump (not shown) of the feeding device 7.

The feeding device 7 is typically connected to a reservoir (not shown) storing the water solution of urea so that the feeding device 7 receives urea from this reservoir. The connection between the reservoir and the feeding device 7 is typically also designed so as to avoid deposits in a manner similar to the fluid guide. The connection is typically a tube made of plastic, stainless steel or aluminium.

The tube used for the channel part is made of material of sufficient strength and has a wall thickness of sufficient size to avoid expansion in radial direction of the channel part 1. Such dimensioning of the channel part 1 is made in order to assure that a pressure increase of the fluid present in the tube, e.g. when a fluid pulse is feed into the tube, will not result in a substantial buffering of fluid in the fluid guide. An acceptable buffer size is related to the time for creating sufficient atomization pressure. The time is preferably less than 25 ms. Typical dimensions of the channel part are an internal diameter of below 4.0 mm, such as smaller than 2.8 mm, preferably smaller than 2.2 mm, such as smaller than 1.3 mm and a wall thickness of 1 mm. The material of the channel pipe is preferably stainless steel, but may be other materials such of aluminium or plastic.

The strainer 2 and channel part 1 are connected to the urea feeding device 7 by a flange 8 as indicated in the close-up of fig 3. The connection by the flange 8 may be detachable so that the strainer may be removed for cleaning or replacement if being clogged. However, it is preferred to include the strainer 2, the channel part 1 and the atomization device 3 in a single unit which unit is not separable.

The fluid feeding device 7 is located at a distance from the point where the atomised urea is to be delivered (the location of the atomisation device 3). In order to avoid formation of cavities or re-circulating flow regions (where deposits may occur and no flushing with fresh fluid takes place), the channel part 1 is guided uninterrupted, typically as a single piece, from the strainer 2 to the atomisation device 3. A typical length of the channel part 1 is between 0.04 m and 10 m. It should be mentioned that although it is preferred to manufacture the channel part 1 as a one piece element, the channel part 1 may be assembled from various pieces. However, in such embodiments the flow passage between such various parts should be made sufficiently smooth and should not contain cavities, dead ends or re-circulating flow regions which could result in formation of deposits.

In the embodiment shown in fig.s 1 and 2, the atomization device 3 is typical made from a circular disc, round or oval, with nozzle channels 4a, 4b which are attached to the end of the channel 1 by soldering, welding, gluing, or press- fitting. However, the atomization device may alternatively be made as shown in fig. 4. In this embodiment, the atomization device 3 is made as a tubular part which closely fits the exterior of the channel part 1. In such embodiments, the atomization device 3 and/or channel part 1 may be provided as a spare part whereby a damaged atomization device 3 may easily be replaced. The atomization device 3 is typically made of stainless steel, but may be made from other materials, such as ceramics. In a further embodiment, the end of the channel is chamfered and the disc with nozzle channels 4a 4b is consequently oval (it is assumed that the channel part has a circular cross section which need not to be the case).

As mentioned above, feeding of liquefied urea into the exhaust system is typically done in a pulse width modulated manner. Typically, the pulse width modulation is divided into periods where no fluid is fed to the channel part 1 and periods wherein fluid is fed into the channel part 1. During a first period, fluid is flowing through the outlet resulting in a cooling of the channel part 1 in the vicinity of the exhaust pipe 5. When the fluid flow is stopped in a succeeding second period, heat from the exhaust gasses and the exhaust pipe will start to heat up fluid present in the channel part 1. If the duration of the second period is sufficiently long, the fluid being present in the channel part 1 in the vicinity of the exhaust pipe 5 will boil and evaporate out of the nozzle channels 4a and 4b, and the interface 6 will be created and travel upstream in the channel part 1 as shown in fig. 2.

When fluid is fed to the channel part during a succeeding pulse, the liquid will move the interface 6 downstream in the channel part 1 and down to the nozzle channels 4a, 4b where after liquid will flow through the nozzle channels 4a, 4b. Thus, the duration of the pulse where liquid is fed to the channels part 1 is preferably selected so that the volume of the channel part 1 filled with gas is filled with liquid during a pulse. However, at very low demands for liquid, the pulse width and flow rate may not be sufficient to fill the volume filled with gas. However, this is not considered as being a problem as the heat will evaporate liquefied urea in the channel part 1 resulting in a flow of urea out of the nozzle channels 4a and 4b.

Amplitude modulation may provide the same or similar effects as the flow rate through the channel part 1 may be insufficient to avoid evaporation of liquid and generation of the interface 6 during some periods. Later, however, the flow rate will be sufficient to fill the channel part 1.

Fig. 5 shows seven embodiments according to the present invention pertaining to arranging a fluid guide in an exhaust system. The figure discloses that the channel part 1 with atomization device 3 may be arranged in a flange 9 used for attaching the channel part 1 with atomization device 3 to the wall of the exhaust pipe 5. Furthermore, the figures show that the orientation of the spray in the exhaust pipe 5 is controllable by the orientation of the atomization device relative to the wall of the pipe 5 as shown in the examples of fig. 5. Furthermore, the fluid guide may be arranged in a bend of the pipe 5. It should be noted that although fig. 5 shows different embodiments, two or more of these embodiments may be used together.

Claims

1. A dosing system for dosing a first fluid into a stream of a second fluid, the system comprising a fluid guide and a fluid feeding device (7), wherein - the fluid guide comprises a channel part (1) having an inlet arranged to receive the first fluid from the fluid feeding device (7) and an outlet arranged to deliver the first fluid to the second fluid, and wherein the channel part (1) and the outlet are shaped and arranged so that heat conducted from the second fluid may evaporate first fluid present in the channel part (1) and generate an interface between a gas phase located downstream of a liquid phase, said interface being sufficiently stable to keep the generated gas phase distinct from a liquid phase of the first fluid when no first fluid is fed to the inlet.

2. A dosing system according to claim 1, wherein channel part comprises only one outlet, the only one outlet being the one arranged to deliver the first fluid to the second fluid, so that fluid flowing into the channel part (1) may only leave the channel part (1) through its outlet.

3. A dosing system according to claim 1 or 2, wherein the channel part comprises only one inlet, the only one inlet being the one arranged to receive the fluid from the feeding device (7).

4. A dosing system according to any of the preceding claims, wherein the channel part is shaped so that no cavities, dead ends or re-circulating flow regions are present.

5. A dosing system according to any of the preceding claims, further comprising a strainer (2) arranged upstream of the outlet or at the inlet of the channel part (1).

6. A dosing system according to any of the preceding claims, wherein the outlet further comprises an atomization device (3).

7. A dosing system according to claim 6, wherein the atomization device (3) comprises at least two converging nozzle channels (4a, 4b).

8. A dosing system according to claim 7, wherein the at least two converging channels are formed in a circular or oval disc attached to the downstream end of the channel part (1).

9. A dosing system according to claim 8, wherein the circular or oval disc is an end of a tubular member, the tubular member being adapted to receive a downstream end region of the channel part (1).

10. A dosing system according to any of the preceding claims, wherein the channel part (1) has an internal circular cross section with an internal diameter being smaller than or equal to 4.0 mm, such as smaller than or equal to 2.8 mm, preferably smaller than or equal to 2.3 mm, such as smaller than or equal to 1.3 mm.

11. A dosing system according to any of the preceding claims, wherein the channel part (1) has an external circular cross section with an external diameter being larger than or equal to 2 mm.

12. A dosing system according to any of the preceding claims, wherein the longitudinal extension of the channel part (1) is shorter than 10 m, such as shorter than 5 m, preferably shorter than 2 m, or even shorter than 0.2 m.

13. A dosing system according to any of the preceding claims, wherein the surface roughness on the surface of the channel part (1) measured by Rz is smaller than,

( jU(G)0,008 - 2,5 / Rz5{\ 6%)25 ) such as smaller than yJRzlβ ( ^U(G)0,008 - 2,5/Rz5(16%)16 ), preferably smaller than

[RZA ( jU(G)0,0025 - 0,81 Rz5(\ 6%)4 ) .

14. A dosing system according to any of the preceding claims, wherein the first liquid is liquefied urea, such as urea dissolved in water, or denoxium.

16. A dosing system according to any of the preceding claims, wherein the 5 channel part (1) is made from stainless steel.

17. A dosing system according to any of the claims 1-14, wherein the channel part (1) is made from aluminium.

10 18. A dosing system according to any of the claims 1-14, wherein the channel part (1) is made from plastic.

19. A dosing system according to any of the claims 6-9 or any of the claims 10-18 when dependent on claim 6, wherein the atomization device (3) is made from stainless steel.

15

20. A dosing system according to any of the claims 6-9 or any of the claims 10-18 when dependent on claim 6, wherein the atomization device (3) is made from ceramics.

20 21. A dosing system according to any of the preceding clams, further comprising a reservoir for storing the first fluid, the reservoir being in fluid communication with the fluid feeding device, preferably by a tube made from stainless steel, aluminium, or plastic.

25 22. An exhaust system comprising a dosing system according to any of the preceding claims and an exhaust pipe through which the second fluid flows, the outlet of the fluid guide being arranged so as to feed the first fluid into the second fluid.

30 23. An exhaust system according to claim 22, wherein outlet is arranged flush with or protruding the interior surface of the exhaust pipe.

24. An exhaust system according to claim 23, wherein the outlet protrudes the interior wall to an extent being smaller than half the diameter of the exhaust pipe through which the second fluid flows, and preferably smaller than 1.5 times the external diameter of the channel part (1).

25. An exhaust system according to any of the preceding claims 22-24, wherein a 5 downstream region of the channel part (1) is arranged in the wall of the exhaust pipe in such a manner that thermal contact between said region of the channel part and the wall (5) of the exhaust pipe surrounding the channel part (1) is established.

10 26. An exhaust system according to any of the preceding claims 22-24, wherein a downstream region of the channel part (1) is arranged in the wall (5) of the exhaust pipe in such a manner that the said region is thermally isolated from the wall of the exhaust pipe.

15 27. A method utilising a dosing system according to any of the preceding claims 1-21 or an exhaust system according to any of the preceding claims 22-26, the method comprising, feeding first liquid into the channel part (1) through its inlet during a first period at a first flow rate, and feeding first liquid into the channel part (1) 20 through its inlet during a succeeding second period at a second flow rate or not feeding first liquid into the channel part (1) through its inlet during a succeeding second period, wherein the duration of the second period is not sufficiently short or the second 25 flow rate, if different from zero, is not sufficiently high to prevent evaporation of first fluid out of the outlet during the second period, said evaporation taking place in regions of the channel part (1) located in or in the vicinity of the second fluid.

30 28. A method according to claim 27, wherein the duration of a first period, succeeding a second period, is sufficiently long or a first flow rate, succeeding a second flow rate, is sufficiently high to ensure that the whole channel part is filled with liquefied urea.

29. A method according to any of the claim 27 or 28, wherein the first flow rate is greater than the second flow rate.

30. A method according to claim 29 wherein the second flow rate is zero.

31. A method according to claim 27-30, wherein the first and/or the second flow rate is constant during the first and/or second period respectively.

32. A method according to any of the preceding claims 27-31, wherein the duration of a first period is between 0.1 and 1 second, and the duration of a second period is between 0.1 and 1 second.