Abstract:
An atomizing nozzle for two substances, which is used for spraying a liquid with the aid of a compressed gas, is provided. The atomizing nozzle includes a mixing chamber, a liquid inlet that extends into the mixing chamber, a compressed gas inlet which extends into the mixing chamber, and an outlet located downstream from the mixing chamber. An annular gap is provided which surrounds the outlet and discharges compressed gas at a high speed. The atomizing nozzle is used for purifying flue gas.

Description:
FIELD OF THE INVENTION 
     The invention relates to a two-substance atomizing nozzle for spraying a liquid with the aid of a compressed gas, comprising a mixing chamber, a liquid inlet opening out into the mixing chamber, a compressed gas inlet opening out into the mixing chamber and an outlet opening downstream of the mixing chamber. 
     BACKGROUND OF THE INVENTION 
     In many process engineering installations, liquids are distributed in a gas. In such cases, it is often of decisive importance that the liquid is sprayed in drops that are as fine as possible. The finer the drops, the greater the specific surface area of the drops. This can give rise to considerable process engineering advantages. For example, the size of a reaction vessel and its production costs depend considerably on the average drop size. However, it is often by no means adequate for the average drop size to be below a certain limit value. Even a few significantly larger drops can lead to considerable operational malfunctions. This is the case in particular whenever the drops do not evaporate quickly enough on account of their size, so that drops or even pasty particles are deposited in downstream components, for example on filter fabrichoses or fan blades, and lead to operational malfunctions due to encrustations or corrosion. 
     In order to spray liquids finely, either high-pressure one-substance nozzles or medium-pressure two-substance nozzles are used. An advantage of two-substance nozzles is that they have relatively large flow cross sections, so that even liquids containing coarse particles can be sprayed. 
     The representation of  FIG. 1  shows a two-substance nozzle with internal mixing according to the prior art. A basic problem with such nozzles results from the fact that the walls of the mixing chamber  7  are wetted with liquid. The liquid which wets the wall in the mixing chamber  7  is driven to the nozzle mouth as a liquid film  20  by the shearing stress and compressive forces. It is tempting to assume that the walls toward the nozzle mouth are blasted dry because of the high flow velocity of the gas phase, and that only very fine drops are thereby formed from the liquid film. However, theoretical and experimental work by one of the inventors, have shown that liquid films on walls may still exist as stable films without drop formation even when the gas flow that drives the liquid films to the nozzle mouth reaches supersonic speed. And this is indeed also the reason why it is possible to use liquid film cooling in rocket thrust nozzles. 
     The liquid films  20  that are driven by the gas flow to the nozzle mouth  8  may even migrate around a sharp edge at the nozzle mouth on account of the adhesive forces. They form a bead of water  12  on the outside of the nozzle mouth  8 . Outer drops  13 , the diameter of which is many times the average diameter of the drops in the jet core or the core jet  21 , break away from this bead of water  12 . And although these large outer drops only contribute to a small proportion of the mass, they are ultimately determinative for the dimensions of a vessel in which, for example, the temperature of a gas is to be lowered by evaporative cooling from 350° C. to 120° C. without drops entering a downstream fan or downstream fabric filter. 
     A liquid is introduced into the prior-art nozzle represented in  FIG. 1  in the direction of the arrow  1 , parallel to a center longitudinal axis  24 . The liquid is passed through a lance tube  2 , extending concentrically with respect to the center longitudinal axis  24 , and enters a mixing chamber  7  at a liquid inlet  10 . The lance tube  2  and the mixing chamber  7  are concentrically surrounded by an annular chamber  6 , which is formed by means of a further lance tube  4  for the feeding of the compressed gas to the two-substance nozzle. Compressed gas is introduced into this annular chamber  6  according to the arrow  15 . A circumferential wall of the mixing chamber  7  that is radial with respect to the center longitudinal axis  24  has a number of compressed gas inlets  5 , which are arranged radially with respect to the center longitudinal axis  24 . Through these compressed gas inlets  5 , compressed gas can enter the mixing chamber  7  at right angles to the liquid jet entering through the liquid inlet  10 , so that a liquid/air mixture is formed in the mixing chamber  7 . The mixing chamber  7  is adjoined by a frustoconical constriction  3 , which forms a convergent outlet portion, which is followed after an extremely narrow cross section  14  in turn by a frustoconical widening  9 , which forms a devergent outlet portion. The frustoconical widening  9  ends at the outlet opening or the nozzle mouth  8 . 
     SUMMARY OF THE INVENTION 
     The invention is intended to provide a two-substance atomizing nozzle with which a uniformly fine drop spectrum can be achieved both in the outer region and in the jet core. 
     Provided for this purpose according to the invention is a two-substance atomizing nozzle for spraying a liquid with the aid of a compressed gas, comprising a mixing chamber, a liquid inlet opening out into the mixing chamber, a compressed gas inlet opening out into the mixing chamber and an outlet opening downstream of the mixing chamber, in which nozzle an annular gap surrounding the outlet opening is provided for compressed gas to be discharged at high speed. 
     By providing the annular gap that surrounds the outlet opening and is subjected to atomizing gas, for example air or water vapor, a liquid film on the wall of the nozzle mouth, in particular the divergent outlet portion, is drawn out into a very thin liquid lamella, which breaks down into small drops. In this way, the formation of large drops from liquid films on the wall in the nozzle outlet region can be prevented or reduced to an acceptable degree, and at the same time the fine drop spectrum in the jet core can be maintained, without the compressed gas consumption of the two-substance nozzle or the associated self-energy requirement having to be increased for this. Experimental studies conducted by the inventors have shown that provision of an annular gap allows the maximum drop size to be reduced to about a third for the same expenditure of energy. This may be considered to be a minor effect. However, it must be borne in mind that the volume of a drop of a diameter reduced by a factor of 3 is only one twenty seventh of that of the large drop. Without going here into the interrelated aspects that are known to all, it should be clear to a person skilled in the art that this gives rise to considerable advantages with respect to the required overall volume of evaporative coolers or sorption systems, for example for flue-gas purification. With the additional annular-gap atomization, a much finer drop spectrum can therefore be produced with the same expenditure of energy. The amount of air passed through the annular gap is advantageously 10% to 40% of the total amount of air that is atomized. In process engineering installations in which atomized substances are introduced into vessels or channels that are at approximately the same pressure as the surroundings (1 bar), the total pressure of the air in the annular gap is advantageously 1.5 bar to 2.5 bar absolute. The total pressure of the air in the annular gap should advantageously be at such a level that, when expansion takes place to the pressure level in the vessel, approximately the speed of sound is reached. 
     In a development of the invention, the outlet opening is formed by means of a peripheral wall, the outermost end of which forms an outlet edge and the annular gap is arranged in the region of the outlet edge. 
     In this way, the compressed gas discharged from the annular gap at high speed can leave directly in the region of the outlet edge and, as a result, reliably ensure that a liquid film at the nozzle mouth is drawn out into a very thin liquid lamella, which is then divided up into fine drops. 
     In a development of the invention, the annular gap is formed between the outlet edge and an outer annular gap wall. 
     In this way, the outlet edge itself can be used for forming the annular gap. This simplifies the structure of the two-substance atomizing nozzle according to the invention. 
     In a development of the invention, an outer end of the annular gap wall is formed by an annular gap wall edge and the annular gap wall edge is arranged after the outlet edge, as seen in the outflow direction. The annular gap wall edge is advantageously arranged after the outlet edge by between 5% and 20% of the diameter of the outlet opening. 
     In this way, the creation of coarse liquid drops at the rim of the outlet opening can be prevented particularly reliably. 
     In a development of the invention, control means and/or at least two compressed gas sources are provided, so that a pressure of the compressed gas supplied to the annular gap and a pressure of the compressed gas entering the mixing chamber through the compressed gas inlet can be set independently of each other. 
     Separate pipelines for admitting compressed gas to the mixing chamber and for subjecting the annular gap to compressed gas offer advantages to the extent that the pressure in a gap air chamber arranged upstream of the annular gap can then be prescribed independently of the pressure of the atomizing gas that is fed to the mixing chamber. This is of significance with regard to the self-energy requirement if compressors with different back pressures or steam networks with matching different pressures are available in an installation. However, generally only one compressed gas network with a single pressure is available. In this case, pressure reducers may be used for example. When the annular gap is supplied with compressed gas by means of a separate line, the amount of air passed through the annular gap is set by means of separate valves, independently of the amount of air in the core jet that is introduced into the mixing chamber. 
     In a development of the invention, the mixing chamber is surrounded at least in certain portions by an annular chamber for supplying the compressed gas and a gap air chamber arranged upstream of the annular gap is connected in terms of flow to the annular chamber. 
     If only one gas network with a single pressure is available, it is necessary to take atomizing gas that is supplied to the annular gap from the same network. The configuration of the two-substance atomizing nozzle can be simplified by taking the atomizing gas that is supplied to the annular gap from the annular space from which the mixing chamber is fed with atomizing gas. Suitable dimensioning of the flow connection between the annular chamber and the gap air chamber allows the energy requirement of the nozzle according to the invention to be minimized. The flow connection is formed, for example, by means of bores in a dividing wall between the annular chamber and the gap air chamber that are to be suitably dimensioned in cross section, including in relation to the bores forming a compressed gas inlet into the mixing chamber. 
     In a development of the invention, a veil-of-air nozzle which surrounds the outlet opening and the annular gap at least in certain portions is provided. 
     The provision of a veil-of-air nozzle leads to a further improvement in the spray pattern of the two-substance atomizing nozzle according to the invention; in particular, it is possible to avoid backflow vortices, by which drops and dust-containing gas are mixed together and lead to troublesome deposits at the nozzle mouth. 
     In a development of the invention, the veil-of-air nozzle has a veil-of-air annular gap which surrounds the outlet opening and the annular gap and the outlet area of which is very much larger than an outlet area of the annular gap. The veil-of-air nozzle is advantageously fed with compressed gas of a pressure that is much lower than a pressure of the compressed gas supplied to the annular gap. 
     In this way, the veil-of-air nozzle, which encloses the nozzle mouth in an annular form, can be subjected to air at low pressure in an energy-saving manner. This is very important because the veil-of-air annular gap of the veil-of-air nozzle is to be made very much larger than the annular gap for the liquid film atomization to avoid a backflow vortex. 
     In a development of the invention, means are provided to impart a swirl about a center longitudinal axis of the nozzle to a mixture of compressed gas and liquid in the mixing chamber. 
     The fact that it is possible with the two-substance atomizing nozzle according to the invention to spray the liquid film that exists on the inner wall in the nozzle outlet part into small drops at the nozzle mouth as a result of the additional annular gap atomization offers further interesting starting points for nozzle design. In particular, it is hereby admissible to impart a swirl to the two-phase flow in the mixing chamber, and consequently also in the outlet part of the nozzle. This does admittedly have the effect that rather more drops are flung onto the inner wall of the outlet part. However, this is not detrimental because of the very efficient annular gap atomization. One advantage of the swirling is that a swirled flow in the mixing chamber and in the outlet part tends to be centrally symmetrical. This can scarcely be achieved with conventional two-substance nozzles with internal mixing and has previously led to the formation of a particularly high number of large drops in certain regions at the nozzle mouth. As a result, the average drop size can be reduced considerably by swirling the core jet. 
     In a development of the invention, the compressed gas inlet has at least a first inlet bore, which opens into the mixing chamber and is aligned tangentially in relation to a circle around a center longitudinal axis of the nozzle, to produce a swirl in a first direction. 
     The provision of tangential inlet bores allows a swirl to be produced in the mixing chamber in a way that is simple and scarcely liable to blockage. 
     In a development of the invention, a number of first inlet bores, in particular four, are provided in a first plane perpendicularly in relation to the center longitudinal axis and spaced apart in the circumferential direction. 
     An evenly spaced-apart arrangement of such tangential inlet bores allows a clear swirl to be achieved in the mixing chamber. 
     In a development of the invention, at least a second inlet bore, which is aligned tangentially in relation to a circle around the center longitudinal axis of the nozzle, is provided parallel to the center longitudinal axis and at a distance from the first inlet bore, to produce a swirl in a second direction. 
     In this way, opposing swirling directions can be imparted to the flow in the mixing chamber in the different planes of the inlet bore or air supply bore. Opposing swirling directions have the effect of producing very pronounced shearing layers in the mixing chamber, contributing to the formation of particularly fine drops. 
     In a development of the invention, a number of second inlet bores, in particular four, are provided in a second plane perpendicularly in relation to the center longitudinal axis and spaced apart in the circumferential direction. 
     In a development of the invention, at least three planes with inlet bores are provided, spaced apart parallel to the center longitudinal axis, the inlet bores of successive planes producing an oppositely directed swirl. 
     For example, a first plane, counting from the liquid inlet, may have left-turning inlet bores, the second plane right-turning inlet bores and the third plane again left-turning inlet bores. The opposing swirling directions have the effect of producing very pronounced shearing layers in the mixing chamber, contributing to the formation of particularly fine drops. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages of the invention emerge from the claims and the following description of preferred embodiments in conjunction with the drawings. Individual features of the individually represented embodiments can be combined with one another in any way desired without going beyond the scope of the invention. In the drawings: 
         FIG. 1  shows a two-substance atomizing nozzle according to the prior art, 
         FIG. 2  shows a two-substance atomizing nozzle according to a first embodiment of the invention, 
         FIG. 2   a  shows an enlarged detail of  FIG. 2 , 
         FIG. 2   b  shows an enlarged detail of an alternative embodiment, 
         FIG. 3  shows a sectional view of a two-substance atomizing nozzle according to a second preferred embodiment of the invention, 
         FIG. 4  shows a portion of a sectional view of the nozzle of  FIG. 2  in which different sectional planes are marked, 
         FIG. 5  shows a sectional view of the plane I of  FIG. 4 , 
         FIG. 6  shows a sectional view of the plane II of  FIG. 4  and 
         FIG. 7  shows a sectional view of the plane III of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     The sectional view of  FIG. 2  shows a two-substance atomizing nozzle  30  according to the invention, according to a first preferred embodiment. The two-substance atomizing nozzle  30  according to the invention is constructed in a way similar to the known nozzle according to  FIG. 1 , at least as far as the introduction of the liquid and the compressed gas into the mixing chamber and the shaping of the nozzle adjoining the mixing chamber are concerned. A liquid to be atomized is supplied in the direction of an arrow  32  from a liquid source  32   a  by way of an inner lance tube  34 , which extends parallel to a center longitudinal axis  36  of the nozzle  30 , and passes to a liquid inlet  38 , which has a reduced cross section in comparison with the tube  34 . After passing the liquid inlet  38 , the liquid then passes in the form of a liquid jet extending concentrically with respect to the center longitudinal axis  36  into the cylindrical mixing chamber  40  arranged concentrically with respect to the center longitudinal axis  36 . The tube  34  and the mixing chamber  40  are surrounded by an annular chamber  42 , which is formed by the intermediate space between an outer lance tube  43  and the inner lance tube  34  and into which compressed gas, for example compressed air, is introduced in the direction of an arrow  44  from a source of compressed gas  44   a . A circumferential wall of the mixing chamber  40  that extends concentrically with respect to the center longitudinal axis  36  has a number of inlet openings  46   a ,  46   b ,  46   c , all of which together form a compressed gas inlet into the mixing chamber  40 , that is to say for supplying what is known as the core air. The compressed gas inlet openings  46  are arranged offset in relation to one another in the direction of the center longitudinal axis  36  and also in the circumferential direction. As a result, compressed gas is introduced into the mixing chamber  40  in different layers. The precise arrangement of the compressed gas inlet openings  46  is further explained below on the basis of  FIGS. 4 to 7 . 
     Provided so as to adjoin the mixing chamber  40  is a frustoconical constriction  48 , which forms a convergent outlet part and, after passing an extremely narrow cross section, goes over again into a frustoconical widening  50  of a smaller aperture angle, which forms a divergent outlet part. The divergent outlet part ends at an outlet opening  52  or a nozzle mouth. The outlet opening  52  is formed by a peripheral outlet edge  54 , which forms the end of the outlet part situated downstream in the direction of flow. 
     The frustoconical constriction  48  and the frustoconical widening  50  are surrounded by a funnel-like component  56 , so that an annular gap air chamber  58  is formed between the funnel-like component  56  and an outer wall of the outlet part. This annular gap air chamber  58  is supplied with compressed gas from the annular chamber  42  by means of a number of inlet bores  60 . A lower end of the funnel-shaped component  56  in the representation of  FIG. 2  is formed by an annular gap wall edge  62 , which runs around the outlet opening  52 . Formed between the annular gap wall edge  62  and the outlet edge  54  is an annular gap  64  surrounding the outlet opening  52 , which consequently surrounds the outlet opening  52  in an annular form. 
     Through this annular gap  64 , which is represented once again in an enlarged manner in the representation of  FIG. 2   a , compressed gas is discharged at high speed. In this way, a liquid film  66 , which forms on an inner wall of the conical widening  50 , is drawn out at the outlet opening  52  of this divergent nozzle outlet part into a very thin liquid lamella  68 , which breaks down into small drops. Experimental studies conducted by the inventors have shown that in this way the maximum drop size of the two-substance atomizing nozzle  30  can be reduced to about a third for the same expenditure of energy as compared to the case of the prior-art nozzle according to  FIG. 1 . The amount of air passed through the annular gap is between 10% and 40% of the total amount of air that is atomized. 
     As can be seen from the representations of  FIGS. 2 and 2   a , the annular gap outlet edge  62  protrudes somewhat from the outlet edge  54  in the direction of flow. Therefore, a further improvement in the atomization and a guard for the sharp outlet edge  54  are achieved by making the outer annular gap nozzle protrude somewhat beyond the nozzle mouth of the central nozzle. The annular gap outlet edge  62  advantageously protrudes beyond the outlet edge  54  by 5% to 20% of the diameter of the outlet opening 
     As a departure from the embodiment of the atomizing nozzle  30 , the annular gap air chamber  58  may be supplied with compressed gas from a separate line. For this purpose, for example, the bores  60  are closed and compressed gas from source  44   a  is introduced directly into the annular gap air chamber  58 ′ from a separate line as shown in  FIG. 2   b . Alternatively, a separate compressed gas source  44   b  may be utilized in addition to source  44   a , which source  44   b  is connected via a line to chamber  58 ′ as shown in  FIG. 2   b  in dotted lines. 
     The sectional view of  FIG. 3  shows a further two-substance atomizing nozzle  70  according to a second preferred embodiment of the invention. With the exception of an additional veil-of-air nozzle  72 , the two-substance atomizing nozzle  70  is constructed in the same way as the two-substance atomizing nozzle  30  of  FIG. 2 , so that there is no need for a detailed explanation of the basic functional principle and the same components are provided with the same reference numerals. 
     In the case of the two-substance atomizing nozzle  70 , the funnel-shaped component  56  is surrounded by a further component  74 , which in principle is constructed in a tubular form, forms a further lance tube and narrows in the manner of a funnel in the direction of the outlet opening  52 . In this way, a veil-of-air annular gap  76  is formed between the component  74  and the component  56 . The veil-of-air gap  76  ends approximately level with the outlet opening  52  and a lower, peripheral edge of the component  74  is arranged level with the annular gap wall edge  62 . However, a cross-sectional area of the veil-of-air gap formed as a result is much larger than the annular gap  64 , in order that backflow vortices can be avoided when the veil of air is introduced. The veil-of-air nozzle  72  enclosing the nozzle mouth or the outlet opening  52  in an annular form can be subjected to air at low pressure, which is supplied according to an arrow  78 , in an energy-saving manner. 
     The two-substance atomizing nozzle  30  and the two-substance atomizing nozzle  70  of  FIGS. 2 and 3 , respectively, may be arranged at the lower end of what is known as an atomizing lance, which protrudes into the process space. 
     The representation of  FIG. 4  shows a portion of a sectional view of the two-substance atomizing nozzle  30  of  FIG. 2 . Sectional planes that are respectively denoted by I, II and III are taken through the various planes with compressed gas inlet openings  46   a ,  46   b ,  46   c.    
     The fact that it is possible with the two-substance atomizing nozzle  30 ,  70  according to the invention with additional annular gap atomization to spray the liquid film  66  that exists on the inner wall in the divergent nozzle outlet part  50  into small drops at the nozzle mouth offers further interesting starting points for nozzle design. In particular, it is admissible to impart a swirl to the two-phase flow in the mixing chamber  40 , and consequently also in the outlet part  48 ,  50  of the nozzle  30 ,  70 . This does admittedly have the effect that rather more drops are flung onto the inner wall of the outlet part. However, this is not detrimental because of the very efficient additional annular gap atomization. One advantage of the swirling is that a swirled flow in the mixing chamber  40  and in the outlet part  48 ,  50  tends to be centrally symmetrical. This can scarcely be achieved with conventional two-substance nozzles and has previously led to such nozzles having a tendency to “spit”, in that a particularly high number of large drops were formed in certain regions at the nozzle mouth. Previously, the center lines of the air supply bores  5  of the conventional nozzle according to  FIG. 1  were aligned with the center longitudinal axis  24  of the two-substance nozzle. It is tempting to assume that a centrally symmetrical flow configuration must result from this. This is not the case, however; rather, even very small disturbances in the supply of liquid or air to the mixing chamber are sufficient to make the jet deviate to the side. 
     According to the invention, on the other hand, it is envisaged to align the bores for forming the compressed gas inlet openings  46   a ,  46   b ,  46   c  in each case tangentially in relation to a circle around the center longitudinal axis  36  of the nozzle. As a result, the jet that is swirled in this way centers itself of its own accord in the mixing chamber  40  as well as in the convergent outlet part and in the divergent outlet part of the nozzle  30 ,  70 . 
     The tangential alignment of the compressed gas inlet openings  46   a  can be seen more precisely from the sectional view of  FIG. 5 . Altogether, four bores evenly spaced apart from one another in the circumferential direction, which form a flow connection from the annular chamber  42  into the mixing chamber  40 , are arranged in the plane I. All these bores are arranged tangentially in relation to an imaginary circle  80  around the center longitudinal axis  36  of the nozzle. A swirl, which in the representation of  FIG. 5  is indicated by means of a circular arrow in the counterclockwise direction, forms as a result in the plane I. 
     The representation of  FIG. 6  shows the arrangement of four bores for the formation of the compressed gas inlet openings  46   b  in the plane II. The compressed gas inlet openings  46   b  are likewise arranged tangentially in relation to a circle around the center longitudinal axis  36  of the nozzle, but in such a way that a flow around the center longitudinal axis  36  in the clockwise direction is obtained in the plane II. 
     As can be seen from  FIG. 7 , the compressed gas inlet openings  46   c  in the plane III are again arranged in the same way as the compressed gas inlet openings  46   a  in the plane I, so that a flow around the center longitudinal axis  36  in the counterclockwise direction is again obtained in the plane III. 
     According to the invention, it is therefore envisaged to impart opposite directions of swirl to the air supply bores in the different planes I, II, III. So, the first air supply bore plane I, counting from the liquid inlet, is arranged so as to be left-turning, the second bore plane II right-turning and the third bore plane again left-turning. The opposing swirling directions in the different planes I, II, III have the effect of producing very pronounced shearing layers in the mixing chamber  40 , contributing to the formation of particularly fine drops. 
     Furthermore, the two-substance atomizing nozzles  30 ,  70  may be optimized by the solid liquid jet that enters the mixing chamber being divided up even before it interacts with the atomizing air. This can take place in various ways that are in themselves conventional, for example by providing baffle plates, swirl inserts and the like.