EP0665937A1

EP0665937A1 - Rotational atomizer and air supply system for fuel burners using the rotational atomizer

Info

Publication number: EP0665937A1
Application number: EP93924783A
Authority: EP
Inventors: Péter KOLTA; Mihaly Lantos
Original assignee: Individual
Current assignee: Individual
Priority date: 1992-10-29
Filing date: 1993-10-29
Publication date: 1995-08-09
Also published as: AU5432494A; HUT68763A; WO1994010502A1; HU211806B; HU9203395D0

Abstract

Rotational atomizer preferably for fuel burners that uses two stage atomization, in which a first atomizer (13) sprays out atomized liquid along paths closing acute angles with a central axis, the second stage comprises a bladed wheel (10) with slots (15) arranged around the first atomizer (13) so that the slots (15) intersect said paths, and a motor (4) coupled to and rotating the bladed wheel (10) with high speed so that the particles created in the first stage enter the slots (15), impact on the slot walls and form thin films flowing in radially outward direction and flying out in finely atomized state past reaching outer edges of the slots (15). An air supply system is also provided for fuel burners using a rotational atomizer. The system comprises a ring-shaped channel (7) for primary air with axial airflow to a circular zone, in which fuel particles leave the atomizer (10, 13) along tangential paths, and a secondary airflow channel (24) around the primary airflow channel (7), wherein a ring-shaped space (25) is formed between mouth openings of the primary and secondary airflow channels (7, 24) closed by a screen (26), and predetermined amount of air is fed in the space (25) behind the screen (26) to counterbalance any vacuum that would otherwise build up in front of the screen (26).

Description

^*•*

ROTATIONAL ATOMIZER AND AIR SUPPLY SYSTEM FOR FUEL BURNERS USING THE ROTATIONAL ATOMIZER

The invention relates to a rotational atomizer consisting of two stages, in which the first stage is a first atomizer that has a central axis and sprays out atomized liquid along paths closing acute angles with the axis. The rotational ato- mizer can be used primarily for fuel burners.

The invention also relates to an air supply system for fuel burners using a rotational atomizer, that comprises a ring-shaped channel for providing axial airflow of primary air to a circular zone in which fuel particles leave the ato- mizer along substantially tangential paths, and a secondary airflow channel that surrounds the primary airflow channel and is spaced therefrom in radial direction.

In rotary fuel burners a bell-like hollow body is rotated and the supplied oil flows along the rotating inner surface while being accelerated thereby. At the mouth opening oil flies out in tangential direction in the form of small drop¬ lets. Publications DE 3.047582 C2 and DE 2.706520 Al il¬ lustrate a few number of embodiments of such atomizers.

In such designs the average size of the droplets cannot be decreased below a threshold value, since there is a maxi¬ mum speed of rotation whereafter the oil film cannot be acce¬ lerated any more, since the film will slip relative to the surface. The droplets have a wide size distribution. This might be one of the grounds that limit decreasing the amount of unwanted components in the exhaust gases.

In addition to the characteristics of the atomization process the operation of rotational oil burners largely depends on the way how air supply is provided. The fine drop¬ lets which fly away from the rotating bell in tangential di- rection should be diverted in forward direction towards the burning zone, and air should be admixed in proper quantity to the atomized fuel. In case of variable fuel supply rate e.g. up to 1:5 or 1:10, then the conditions regarding air supply must be provided within the whole operational range.

In the burner described in the above referred publication DE 2.706.520 Al the nozzle supplying primary air is provided with a narrow ring-shaped opening at the departure zone of the droplets, and the supplied primary air, which has been forced previously to rotate around its axis of flow,thrusts the flying fuel particles in forward direction. The secondary air is supplied from a mouth opening that closely encircles the nozzle of primary air, therefore due to the differing speeds of the two airflows a whirling takes place at the con¬ tact zone of the two flows.

The supply of primary and secondary air through adjacent concentric ring channels is described in the book of Endre KISS: "Olajtuzeles" (Mviszaki Kδnyvkiadό, Budapest, 1970) in figure 99 and figures 178-180 and in the associated descrip¬ tions. In such air supply problems arise from the fact that the amount of the air should be defined by the rate of fuel supply and the velocity component of the airflow should be determined by the size and speed vector of the flying par- tides. With any predetermined cross-section of the air chan¬ nels the optimum conditions cannot be maintained within the whole adjustment range of the fuel supply. This can be the ground why oil burners using rotational atomization have limited adjustment range of fuel supply or several burners with differing fuel supply rate are used in a system.

The primary object of the invention is to provide a rota¬ tional atomizer, which has an improved quality of atomiza¬ tion, i.e. wherein the size of particles is smaller and more uniform than in conventional atomizers. A further object lies in decreasing the problems connect¬ ed with the supply of primary and secondary air, whereby fa¬ vorable exhaust gas parameters and wide adjustment range can be reached.

According to a first aspect of the invention it has been discovered that a two-stage atomization should be provided, in which the path of the flying droplets of a first atomizer is crossed by a bladed wheel rotating with high speed around the central axis of the first atomizer so that the particles impact walls of radial slots in the wheel and form thin films at the wall that stream away from the axis, and after reach¬ ing the outer edge of the wall particles of uniform size will be formed that fly out in substantially tangential direction. In a preferable embodiment the first atomizer is of a ro¬ tational type with an atomizing element that rotates in op¬ posite direction than the bladed wheel. The opposite rotation increases the speed of the impact and ensures that particles reach the slots in optimum direction. By using a rotational type atomizer at the first stage a wider adjustment range will be obtained for the flow rate of the liquid than in case of using a pressurized nozzle type atomizer, since the opera¬ tion of this latter type depends on the liquid pressure, and optimum conditions are provided only in the close vicinity of a predetermined flow rate.

It is preferable if the atomizer element of the first atomizer is formed by a pair of axially spaced discs coupled to and supported by a feeding tube so that the interior of the tube communicates with the space between the discs, and the feeding tube is coupled to a motor running with a lower speed than the motor which drives the bladed wheel. The low speed motor runs typically with a speed of 3.000-6.000 r.p. . while the speed of the other motor depends on the flow rate and the minimum value thereof at low liquid rates is between about 15.000 - 30.000 r.p.m. which can go up to 80.000 r.p.m. or still higher speeds at the upper end of the adjustment range.

In a preferable embodiment the high speed motor has a hollow shaft coupled at one end to the bladed wheel, and the low speed motor is arranged axially behind the high speed motor, the feeding tube extends through and is spaced from the hollow interior of the shaft of the high speed motor and it is connected to a liquid supply. in oil burner applications it is preferable if a tube is arranged around the high speed motor with a spacing there¬ between to form a channel for the introduction of primary air, this tube has a conically tapering frontal member that extends up to the zone of the slots in the bladed wheel, and a second tube is arranged around the first tube that defines with the first tube a channel for introducing secondary air. ₅ Here it is preferable if the secondary air channel has a conically tapering end portion with a ring-like mouth opening spaced in radial direction from the mouth opening of the channel for the primary air, and a screen is arranged at the zone between the mouth openings of the primary and secondary ₁₀ air channels, furthermore air inlet means with predetermined cross-section are arranged behind the screen to balance any vacuum that would otherwise build up in front of the screen.

In a preferable embodiment an air directing sleeve is attached to the frontal portion of the inner tube surrounding _ιg said conical member, the screen is fixed between frontal portions of the sleeve and of the conical member, and the air inlet means are openings at the rear part of the sleeve com¬ municating with the secondary air channel.

According to a further aspect of the invention an air ₂₀ supply system has been provided for fuel burners using a ro¬ tational type atomizer. This system comprises a ring-shaped channel for primary air providing axial airflow at a circular zone, in which fuel particles leave the atomizer along sub¬ stantially tangential paths, and a secondary airflow channel 25 around the primary airflow channel, wherein a ring-shaped space is arranged between mouth openings of the primary and secondary airflow channels closed by a screen, and predeter¬ mined amount of air is fed in the space behind the screen to counterbalance any vacuum that would otherwise build up in 2_Q front of the screen.

In a preferable embodiment of the air supply system the secondary channel has a tapering end section and the direc¬ tion of flow of the secondary air follows the form of this section, while the direction of the primary airflow is sub- _₅ stantially axial.

In an embodiment a chamber is formed behind the screen communicating through a plurality of openings with the secon- dary airflow channel.

When this air supply system is combined with the two stage rotational atomizer, an oil burner is formed which can be adjusted within a wide range of oil supply and has favor¬ able exhaust gas parameters reducing thereby unwanted en¬ vironmental load.

The invention will now be described in connection with preferable embodiments thereof, wherein reference will be made to the accompanying drawings. In the drawing:

FIG. 1 is the sectional elevation view of the rotational atomizer; FIG. 2 is the front view of the rotational atomizer; and FIG. 3 shows the frontal portion of a preferred embodi- ment in half sectional view.

The rotational atomizer shown in FIG. 1 is designed to atomize liquid fuels supplied through a supply line 1. The end portion of the supply line 1 is connected through a ro- tating connection to feeding tube 2. The feeding tube 2 ex¬ tends through and attached to an axial bore made in the shaft of a motor 3 preferably of the contactless type. The opera¬ tional speed of the motor 3 is about 5000 r.p.m. and the feeding tube 2 rotates also with this speed. A three-phase motor 4 is arranged in front of the first motor 3 which rotates with a high speed (about 40.000 r.p.m.) in the opposite direction. The shaft of the second motor 4 is also provided with an axial bore and a section of the feeding tube 2 which extends out from the first motor 3 is lead through and spaced from this second bore. In the embodiment shown the feeding tube 2 is not beared to the bore of the shaft of the high speed motor 4 but it is protected against vibration by means of teflon spacer rings 5. In axial direc¬ tion the spacer rings 5 are spaced with non-uniform distances from each other and from bearing 12 at the outer end of the shaft, and the spacing corresponds to roots of a Bessel func¬ tion. This specific spacing minimizes the hazard of periodic vibrations.

A sleeve 6 is arranged around the three-phase motor 4 which is kept by means of radial support ribs 7 in the inte¬ rior of a first axial tube 8. In this way a ventillation channel is formed between the tube 8 and the sleeve 6 which has a radial size that corresponds to the height of the sup¬ port ribs 7. This channel is used for guiding the flow of primary air. Behind the high speed motor 4 the first motor 3 is also attached to the tube 8 by means of radial ribs. An outer tube 9 is arranged coaxially around the tube 8, and a secondary air ventillation channel is formed between the tubes 8 and 9 attached to each other by discrete radial ribs (not shown in the drawing) so that the assembly consti¬ tutes a single constructional unit. A cylindrical support member 11 is attached to the free outer end of the shaft of the three-phase motor 4, and a larger number (e.g. 48...52) of radial slots are provided in the wall of the support member 11. In this way the cylindric¬ al portion of the support member 11 looks like a bladed wheel 1° which has several atomizer plates formed by the walls bet¬ ween neighboring slots.

The end of the feeding tube 2 for the fuel is supported by means of a high speed bearing 12 attached to the support member 11. A pair of spaced pre-atomizer discs 13 are attach- ed to the free end of the feeding tube 2 defining a thin, ring-like channel with a width of about .3-.5 mm and communi¬ cating through a mouth opening (not shown) with the interior of the feeding tube 2. The blades of the bladed wheel 10 are arranged uniformly and concentrically around and being spaced from the outer edge of this channel. A conically tapering end portion of the inner tube 8 diverts primary air to the bladed wheel 10.

FIG. 3 shows the frontal portion of a further embodiment of the rotational atomizer according to the invention. A cone-frustum shaped support member 11 is attached to the end of shaft 14 of the motor 4, and the wider frontal end of the cone is attached to a cylindrical extension member. A plurality of slots 15 are made through the wall of the cy¬ linder by spark machining technique to form a circular row, and the walls between the slots constitute the bladed wheel

10. A conical closure member 16 is attached to the frontal end of the support member 11 to close the interior of the bladed wheel 10 and to protect the bearing 12 and the disc from the backwardly radiating heat of the burning space.

The feeding tube 2 extends in the hollow space of the shaft 14, and a hub portion of the inner disc 13a is tightly fitted to the end of feeding tube 2.The bearing 12 is arranged between the outer surface of this hub portion and a cylindrical nest made in the interior of the support member

11. The spaced outer disc 13b is attached by means of bolts 17 to the inner disc 13a. The fist axial tube 8 is extended by a conically tapering member 18 which has a short cylindrical end portion 19. The frontal end of the cylindrical portion 19 extends till the beginning of the row of slots 15. A sleeve 20 supported by the motor 4 is arranged coaxially within the conical member 18 which extends till the end of the cylindrical portion 19. The sleeve 20 is spaced both from the cylindrical portion 19 and from the support member 11 by respective ring channels. A plurality of openings 21 are defined in the rear wall of the sleeve 20 to divert a fraction of primary air flowing in the tube 8 in forward direction along the mantle surface of the end of the shaft 14 and of the support member 11 to cool these elements and the bearing 12 through the support member 11. The dominant portion of the primary air flows along the ring channel defined between the cylindrical portion 19 and the sleeve 20, and this flow intersects the path of atomized fuel droplets flying tangentially out through the slots 15. The flow of primary air is illustrated in FIG. 3 by dash-dot arrows.

The secondary air flows also in frontal direction between the interior of the outer tube 9 and the exterior of the inner tube 8. The frontal outer end of the outer tube 9 is arranged in the exemplary embodiment as a flame directing tube 9a. In the embodiment shown in FIG. 3 a connecting mem¬ ber 22 is attached to the tube 9, and the flame directing tube 9a is fitted to the end of this member 22. This member 22 has a hollow interior consisting of a conically flaring section 22a and a tapering section 22b. An air-diverting sleeve 23 is attached to the inner tube 8 at the zone of the inner end of the conical member 18 which has an outer profile that extends in parallel to the profile of the sections 22a and 22b. The sleeve 23 defines together with the members 22a, 22b a conically tapering secondary air channel 24 that has a uniform width. A chamber 25 is defined between the interior of the sleeve 23 and the exterior of the conical member 18, and the outer end of the chamber 25 is closed by a screen 26 which extends normal to the central axis immediately behind the beginning of the slots 15. The rear part of the chamber communicates through openings 27 with the secondary air chan¬ nel 24. The screen 26 is made preferably as a metal grid but it can also be a perforated metal plate or a heat-resistant member provided with appropriate openings. A flange 28 is provided on the outer tube 9 to enable attachment of the assembly to an oven.

The operation of the .rotational atomizer according to the invention is as follows:

The liquid fuel flows along the interior of the feeding tube 2 which rotates together with the first motor 3 and reaches the ring-shaped channel between the discs 13a and 13b. Owing to the rotation of the discs 13a and 13b the li¬ quid gets accelerated by the disc-walls and will flow in a spiral path. After reaching the outer edge of the discs the liquid flies out in small droplets. The angle of the path depends on the speed of rotation and from the viscosity of the liquid. In normal operation this path is closer to tan¬ gential than to radial direction.

The support member 11 is rotated by the motor 4 with high speed in the opposite direction and the air flowing outwardly through the slots exerts a sucking effect on the flying fuel droplets which increases the speed of flow. The spacing between the walls and the radial height of the slots have been chosen so that each drop should impact a wall. This will be a non-elastic impact and will occur by a speed of about at least 78-80 m/s. This speed will be the result of the speed of the flying droplets (being between about 3-5 m/s) and of the wall moving in opposite direction (being about 75-77 m/s) . As a result a very thin liquid film will be formed on the wall (with a thickness of a few microns) which flows outwardly with a high speed. The final atomization takes place when the thin liquid film reaches the edge of the wall and gets released. Owing to the uniform thickness of the film and to the high speed, the size-distribution of the particles will be more uniform than in case of conventional rotary atomizer systems. The fuel particles fly out in tangential direction from the slots. In an exemplary embodiment with a liquid flow of 2 1/h the fly¬ ing angle was 85°.

In the embodiment shown in FIG. 3 the primary air flows in axial direction when it intersects the tangentially flying liquid particles so that the flow directions close substan¬ tially a right angle. The supply of the primary air occurs in two ways. The first is the channel between the inner wall of the cylindrical portion 19 and the outer wall of the sleeve 20, this supplies about 60-70% of the primary air. The other flow arrives through the interior of the sleeve 20 to the slots and this flow provides a cooling for the element 11 and the bearing 12. The constructional design determines the ratio of these two flows.

In an alternative embodiment, in which the feeding tube 12 is sufficiently rigid so that there is no need for the bearing 12 i.e. the tube 2 rotates freely in the interior of the shaft 14, there will be no need for the sleeve 20 and the primary air can arrive through a single channel.

The fuel particles will be mixed with the primary air and will take a helical path flowing mainly in forward direction. The smaller particles will have higher axial speed component, while the larger ones get to higher radial distances from the central axis. In this way the atomized particles cannot re- combine, since they flow along differing, non intersecting paths.

The secondary air flows along the extension of the chan- nel 24. The presence of the radial spacing between the prima¬ ry and secondary airflows, which is higher at the beginning and decreases in axial direction, is preferable, since the two flows with differing speed and direction do not interfere in the most critical zone where the particles fly out of the slots. The presence of the air diverting sleeve 23 and the additional air supply through the screen 26 has a high signi¬ ficance just in this critical zone. If a wall were made in¬ stead of the screen 26, then a sudden drop of pressure would take place in the space between the flows of the primary and secondary air resulting in a sucking effect in axially re¬ verse direction, which would result in the formation of fuel drops on the wall and recombination of atomized particles. To eliminate this phenomenon, such an amount of air is lead through the screen 26 that blocks the formation of any suck- ing effect in the critical zone. This inflowing additional air improves the mixing of the air with fuel particles. With such a design of the air supply and in case of a unity rate of the primary airflow i.e. v=l, the rate of the air flowing out through the screen 26 is about v=.5-.06, while the flow rate in the secondary air channel 24 lies between v=l-1.5. The conical design of the primary and secondary air channels with differing angles affects the direction of the velocity vector of air molecules so that their paths cross each other. These two factors result in a whirling effect. Since the burning process starts already in the proximity of the sur¬ face of the screen 26, this whirling ensures not only a good fuel-air mixing effect but also a certain extent of exhaust gas recirculation.

The air supply shown in FIG. 3, i.e. the use of the air directing sleeve 23 and the screen 26 in the radial zone bet¬ ween the primary and secondary air supply channels, is not restricted to the counter-rotating atomizing system accord- ing to the invention but it can be used in conjunction with any similar burning system.

The rotational atomizer according to the invention en¬ ables the continuous adjustment of the rate of the fuel supp- ly within a wide range, and each selected supply rate can be associated with an optimum primary and secondary air-supply rate and with an optimum motor speed.

Experimental measurements were carried out with the rota¬ tional atomizer shown in FIG. 3. The system was capable of burning fuel within a range of 1-10 kg/h. The test results with 1.8 kg/h fuel rate and using industrial burning oil were as follows:

Temperature of the exhaust gases (°C) 220

C0₂ (Vol%) 15.1 0₂ (Vol%) .6

CO (ppm) 3.5

N0_X (ppm) 13.0

These test data demonstrate that both the amount of CO and NO_x in the exhaust gas are surprisingly low, while the values of C0 and oxygen are very close to the stoichiometric equilibrium associated with ideal burning. The oil burned as if it were a gas, and the flame was favorably short.

Claims

Clai s :

1. Rotational atomizer, comprising a first atomizer de¬ fining a central axis and capable of spraying out atomized liquid along paths around said axis, said paths close acute angles with said axis, characterized by comprising a bladed wheel (10) with slots (15) arranged around said first atomiz¬ er so that the slots (15) intersect said paths, and a motor (4) coupled to and rotating the bladed wheel (10) with high speed so that said particles enter said slots (15) , impact on the slot walls and form thin films flowing in radially out- ward direction and flying out in finely atomized state past reaching outer edges of the slots (15) .

2. The rotational atomizer as claimed in claim 1, charac¬ terized in that said first atomizer is of a rotational type with an atomizing element rotating in opposite direction than said bladed wheel (10) .

3. The rotational atomizer as claimed in claim 2, charac¬ terized in that the atomizer element of the first atomizer is formed by a pair of axially spaced discs (13a, 13b) coupled to and supported by a feeding tube (2) so that the interior of the tube (2) communicates with the space between the discs (13a, 13b) , said feeding tube is coupled to a motor (3) running with a lower speed and in opposite direction than said motor (4) that drives the bladed wheel (10) .

4. The rotational atomizer as claimed in claim 3, charac¬ terized in that the high speed motor (4) has a hollow shaft (14) coupled at one end to the bladed wheel (10) , said lov; speed motor (3) is arranged axially behind the high speed mo¬ tor (4), said feeding tube (2) extends through and being spaced from the hollow interior of the shaft (14) of the high speed motor (4) and being connected to a liquid supply.

5. The rotational atomizer as claimed in any of claims 2 to 4, characterized in that the relative speed of the two ro¬ tating system depends on the rate of liquid supply and falls in the range of about 30.000 to 80.000 r.p.m.

6. The rotational atomizer as claimed in any of claims 2 to 5, characterized in that a tube (8) is arranged around the high speed motor (4) with a spacing therebetween to form a channel for the introduction of primary air, said tube (8) has a conically tapering frontal member (18) extending up to the zone of said slots (15) , a second tube (9) is arranged around the first tube (8) to define with the first tube (8) a channel (24) for introducing secondary air.

7. The rotational atomizer as claimed in claim 6, charac¬ terized in that said secondary air channel (24) has a coni- cally tapering end portion with a ring-like mouth opening spaced in radial direction from the mouth opening of the channel for the primary air, and a screen (26) is arranged at the zone between the mouth openings of the primary and secon¬ dary air channels, air inlet means with predetermined cross- section are arranged behind the screen (26) to counterbalance any vacuum that would otherwise build up in front of the screen (26) .

8. The rotational atomizer as claimed in claim 7, charac- terized in that an air directing sleeve (23) is attached to the frontal portion of the inner tube (8) surrounding said conical member (18) , said screen (26) is fixed between front¬ al portions of the sleeve (23) and of the conical member (18) , said air inlet means are openings at the rear part of the sleeve (23) communicating with the secondary air channel (24).

9. The rotational atomizer as claimed in any ofclaims 1 to 8, characterized by being used as a burner and said liquid is fuel.

10. Air supply system for fuel burners using a rotational atomizer, comprising a ring-shaped channel for primary air providing axial airflow at a circular zone, in which fuel particles leave said atomizer along substantially tangential paths, and a secondary airflow channel around said primary airflow channel, characterized in that a ring-shaped space is arranged between mouth openings of said primary and secondary airflow channels closed by a screen (26) , and predetermined amount of air is fed in the space behind the screen (26) to counterbalance any vacuum that would otherwise build up in front of the screen (26) .

11. The air supply system as claimed in claim 10, charac¬ terized in that said secondary channel (24) has a tapering end section and the direction of flow of the secondary air follows the form of this section, while the direction of the primary airflow is substantially axial.

12. The air supply system as claimed in claim 11, charac¬ terized in that a chamber (25) is formed behind the screen (26) communicating through a plurality of openings (27) with the secondary airflow channel (24) .