Atomizers and nozzle inserts therefor

Abstract: An atomizer (10) for discharging a jet of a mixture of one fluid and another (compressible) fluid comprises a nozzle head (12) having at least one nozzle hole (20) therein for discharging the jet. The nozzle hole (20) has an inwardly tapering inlet end and an outwardly tapering outlet end for reducing recirculation and wetting at the exit of the hole (20). The outlet end tapers at a taper angle (ϑ) no greater than 7° and the inlet end is advantageously curved at a radius (R) which is from 2 to 10 times the minimum diameter (Q₁) of the hole (20). The length (L₂) of the hole (20) is also preferably from 1 to 5 times the minimum diameter (Q₁). The nozzle hole (20) may conveniently be provided in a nozzle insert (30).

Description: This invention relates to dual fluid atomizers and to nozzle inserts for such atomizers. Dual fluid atomization is a technique which uses the momentum supplied by a compressible fluid (usually air or steam) to break a liquid up into very fine droplets. For the case described here, this is done by internally mixing the liquid and compressible fluid and spraying the mixture into the surrounding gas through small orifices. These orifices are typically sharp edged at both their inlets and their outlets. Often, processes which utilize dual fluid atomizers involve spraying into a dusty environment, and problems may then arise with dust deposition around the outlets of the atomizer orifices. Current methods of dealing with these problems include physically cleaning the atomizers in situ, shutting down the process to physically clean the atomizers, or using vent air (namely a clean air flow around the immediate vicinity of the atomizers) to reduce the deposition of dust on the atomizers. Nozzle designs are known which incorporate a single discharge hole with a conical outlet. (See, for example, US Patent US-A-4 625 916 and J M Beer & N H Chigier, Combustion Aerodynamics , Robert E Krieger Publishing Company, Malabar, Florida, 1983 (pp. 124-127 and p. 187). There is, however, no suggestion in these references to shape the nozzle to maintain spray quality (i.e. drop sizes) at constant consumption and pressure of a compressible fluid or for the purpose of controlling atomizer deposition. US Patent US-A-3 419 220 depicts a tapered nozzle on the entrance side to make the nozzle more wear-resistant. US Patent US-A-4 625 916 provides a nozzle having a bore which diverges on the exit side. There is nothing in either of these two references which suggests a combination of these features or suggests a resulting reduction in deposition or a decrease in irrecoverable pressure losses. Other references of interest are G M Blythe, et al, Evaluation of a 2.5-MW Spray Dryer/Fabric Filter SO 2 Removal System , EPRI Report #CS-3953, May, 1985 (pp. 9-10); and M Babu, et al, Duct Injection Technologies for SO 2 Control , First Combined FGD and Dry SO 2 Control Symposium, Paper No 10-2, October, 1988 (p. 73). US Patent US-A-2 175 160 discloses a nozzle for oxygen cutting blowpipes in which an inlet portion of the nozzle passage is curved, terminating in a straight conical profile portion. It is specified that the curved portion should not in general be shorter than about one third of the total nozzle length. The wall between the throat and discharge mouth of the passage is initially tapered at 14° or more. US Patent US-A-4 160 526 describes a liquid fuel atomizer nozzle with an internal mixing chamber in which the incoming liquid fuel to be atomized is subjected to high velocity air streams both within the liquid's central core and about its periphery to provide effective atomization. According to the invention there is provided a nozzle insert for a dual fluid atomizer, the nozzle insert comprising an insert member having a hole therethrough with an inlet end for receiving a mixture of first and second fluids, an outlet end for discharging a jet of the mixture, and a minimum diameter therebetween, the inlet end being tapered inwardly towards the outlet end and the outlet end being tapered inwardly towards the inlet end; characterised in that the outlet end tapers at an angle no greater than 7° to a central axis of the hole, the inlet end tapers at a curved radius, the ratio between the minimum diameter and the curved radius being between 2 and 10. The invention also provides an atomizer for discharging a jet of a first fluid and a compressible second fluid, the atomizer comprising a nozzle insert as set out above, and: a nozzle head defining a space for receiving a mixture of the first and second fluids; an aperture in said nozzle head for receiving the insert member; first fluid supply means connected to the nozzle head for supplying the first fluid to the nozzle head; and second fluid supply means connected to the nozzle head for supplying the second fluid to the nozzle head. Embodiments of the invention involve altering the design of existing dual fluid atomizers to reduce deposition on the atomizer and to reduce irrecoverable pressure losses while maintaining spray quality. The exit holes are made using tapered expansions on the outlet end rather than the sharp edged exits that are generally used. Bell mouths can be used on the inlets to these holes further to reduce irrecoverable pressure losses. The outlet taper on the exit hole is designed to reduce wetting of the atomizer tip and thereby minimise atomizer deposition. In addition, this taper reduces irrecoverable pressure losses associated with straight drilled holes. The cone angle of the tapered discharge holes is no greater than 14°, since flow through larger angle expansions can cause recirculation in the hole and reduce the desired benefit. Similarly to that set out above, a common problem arises when a liquid or slurry is sprayed into dust laden flue gas. In these applications, it is not uncommon to find large deposits on the atomizers which have to be removed. As deposits are formed, the atomizer performance suffers. Larger droplets are made and the rate of atomizer wetting increases because of the disturbance to the system caused by these deposits. Therefore, a reduction in atomizer deposition can be expected to allow the process to run more reliably as well as at lower operating costs. The feasibility of embodiments of the present invention for reducing atomizer deposition has been established by actual test results using a water and air mixture. The nozzles were shaped to keep the flow of fluid more streamlined throughout and reduce turbulence of the jets at the nozzle exits. The existence of this turbulence causes wetting of the atomizer which promotes the growth of deposits. Reduced wetting of the atomizer tip was seen with the shaped holes which should mean a reduction in deposit formation. Any reduction in deposition should lower vent air requirements and/or atomizer cleaning requirements. When dual fluid atomizers are operated to obtain small droplet sizes, the limiting factors are typically air pressure and air consumption. These factors are limited both in terms of availability and the expense associated with them. One objective is to maintain the spray quality for a given atomizer at given flowrates with a reduction in air pressure/flow requirements. Energy savings are realised because there is less irrecoverable pressure loss with embodiments of the invention than with straight hole nozzles. Although the embodiments will be described in the context of a single dual fluid atomizer design, the same technique can be used in atomizers with multiple nozzles and in any other dual fluid design. There is also the potential for a reduction in atomizer wear with the shaped holes of the embodiments of the invention. This is based on the idea that the flows should be more streamlined and less frictional forces would exist in the atomizer internals. In a preferred embodiment of the invention, the air holes are also shaped. The inlets to the air passages are tapered or rounded inwardly in a direction towards the outlet end, and the outlets are tapered inwardly in the direction of the inlet. The object of the shaped air holes is further to reduce irrecoverable pressure losses. The shaped air holes do not contribute to the reduction in atomizer deposition or wear. Accordingly, embodiments of the invention provide an atomizer which is simple in design while avoiding turbulence which leads to wetting, which has been recognised by the present inventors as a source for the build-up of undesirable deposits, and as an area of irrecoverable pressure loss. There is also theoretical evidence that the shaped holes could allow outlet velocities above sonic. Therefore, it may be possible to reduce droplet sizes still further by increasing the jet velocity without increasing flow and pressure requirements. The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which; Figure 1 is a sectional view, partially in elevation, of an atomizer in accordance with an embodiment of the invention; and Figure 2 is a sectional view of a nozzle insert, on an enlarged scale, which can be used in the atomizer of Figure 1. Referring to Figure 1 of the drawings, an atomizer 10 includes a hollow nozzle head 12 which defines a vestibule 26 for receiving a mixture of a first fluid (such as water or other liquid or slurry) and a second fluid (such as air or other gas). The second fluid is generally compressible and expandable to help disperse the first fluid and help discharge a jet of finely atomized fluid through a nozzle hole 20 extending through the nozzle head 12. The first fluid, such as water, is supplied through a first fluid supply means in the form of a liquid passage 16 to a mixing chamber 24 which is followed by the vestibule 26. The expandable second fluid, in this case air, is supplied through a conical or rounded inlet 17 of an air passage 18. Supply lines (not shown) for the water and air are connected to supply conduits 14 which also mechanically support the nozzle head 12. The shaped holes are provided by using shaped inserts. A nozzle insert 30 according to the invention is best shown in Figure 2. The nozzle insert 30, which can be fixed to the nozzle head, defines the nozzle hole 20. The nozzle hole 20 has an inlet end and an outlet end, the latter having an internal diameter Q 2 . The inlet end tapers, by means of curved surface, inwardly towards the outlet end. The outlet end tapers inwardly, by means of a conical surface, towards the inlet end. Thus the nozzle hole 20 has a minimum diameter Q 1 where the two tapered surfaces meet. According to the invention, the tapered extent of the outlet end is at a cone angle of no more than about 14°. The inlet end tapers in a curve having a radius R. In order to provide a sufficiently smooth and streamlined entry condition for the jet mixture, the ratio between the inlet end diameter Q 1 and the radius R is from 2 to 10. So as to allow flow within the nozzle hole 20 enough time to become streamlined, the nozzle insert 30 should also be sufficiently long. It is advantageous for the ratio between the total nozzle length L 2 and the nozzle inlet diameter Q 1 to be from 1 to 5. The taper angle , which is one-half of the cone angle, is advantageously between 1.5° and 7°. For installation, the nozzle insert 30 has a smaller diameter inlet end portion 32 having an outer diameter D 1 , a step 36 near the middle of the nozzle at a distance L 1 from the inlet end, and a larger diameter outlet end portion 34 having an outer diameter D 2 . This reduces the quantity of material required for constructing the insert which is often of a hardened material that is more expensive than the material of the nozzle head and other portions of the atomizer. Actual experiments for verifying the feasibility of the present arrangement were conducted with an insert having the following specific dimensions: L 1 = 3.175mm (0.125") L 2 = 6.350mm (0.250") D 1 = 3.912mm (0.1540") D 2 = 4.750mm (0.1870") Q 1 = 2.705mm (0.1065") Q 2 = 3.914mm (0.1541") R = 0.597mm (0.0235")  = 6°.

Claims: A nozzle insert for a dual fluid atomizer (10), the nozzle insert comprising an insert member (30) having a hole (20) therethrough with an inlet end for receiving a mixture of first and second fluids, an outlet end for discharging a jet of the mixture, and a minimum diameter (Q 1 ) therebetween, the inlet end being tapered inwardly towards the outlet end and the outlet end being tapered inwardly towards the inlet end; characterised in that the outlet end tapers at an angle () no greater than 7° to a central axis of the hole (20), the inlet end tapers at a curved radius (R), the ratio between the minimum diameter (Q 1 ) and the curved radius (R) being between 2 and 10. A nozzle insert according to claim 1, wherein the taper angle () of the outlet end is between 1.5° and 7°. A nozzle insert according to claim 1 or claim 2, wherein the ratio between the length (L 2 ) of the nozzle hole (20) and the minimum diameter (Q 1 ) of the nozzle hole (20) is between about 1 and 5. A nozzle insert according to claim 1, claim 2 or claim 3, wherein the insert member (30) has a smaller diameter inlet portion (32) defining the inlet end and a larger diameter outlet portion (34) defining the outlet end. An atomizer for discharging a jet of a first fluid and a compressible second fluid, the atomizer (10) comprising a nozzle insert according to any one of the preceding claims, and: a nozzle head (12) defining a space (26) for receiving a mixture of the first and second fluids; an aperture in said nozzle head (12) for receiving the insert member (30); first fluid supply means (16) connected to the nozzle head (12) for supplying the first fluid to the nozzle head (12); and second fluid supply means (17, 18) connected to the nozzle head (12) for supplying the second fluid to the nozzle head (12). An atomizer according to claim 5, wherein the first fluid supply means includes a flow passage (18) having an inlet and an outlet, and wherein the inlet of the flow passage (18) tapers inwardly towards the outlet and the outlet tapers inwardly towards the inlet.

Inventors: BAILEY RALPH T, HOLMES MICHAEL J, BAILEY, RALPH T., HOLMES, MICHAEL J.
Applicants: BABCOCK & WILCOX CO, THE BABCOCK & WILCOX COMPANY
Application year: 1992
Main classification: B05B1
IPC codes: B05B7, B05B1