EP2757193A1

EP2757193A1 - Spray nozzle and a method for spraying fluids in droplets for processing fibrous webs

Info

Publication number: EP2757193A1
Application number: EP13152188.2A
Authority: EP
Inventors: Topi Tynkkynen; Tapio Pitkäniemi
Original assignee: Valmet Technologies Oy
Current assignee: Valmet Technologies Oy
Priority date: 2013-01-22
Filing date: 2013-01-22
Publication date: 2014-07-23

Abstract

The invention is related to a spray nozzle (20, 30) for spraying fluids in droplets for processing fibrous webs. The nozzle comprises a housing having a first longitudinal central channel (24, 34) for the fluid to be sprayed and a second coaxially extending outer channel (21, 31) for a dispersing fluid. The velocity vector of the fluid to be sprayed exiting from the opening of the first channel has mainly a linear component, that is the fluid exiting from the nozzle has essentially rectilinear motion, whereas the velocity vector (V1, V2) of the dispersing fluid exiting from the opening of the second channel has both rotational and linear components, that is the dispersing fluid exiting from the nozzle has spiral motion. The nozzle (20, 30 is provided with means (25, 35) for feeding the dispersing fluid tangentially at the infeed end of the second channel (21, 31), the second channel having inner (22, 32) and outer walls (23, 33) defining an annular space therebetween. The space is dimensioned (α, β, D1, D2, H1, H2) such that the rate between the rotational and linear components of the velocity (V1, V2) of the dispersing fluid increases before it leaves the channel (21, 31) and contacts the fluid to be sprayed. The invention is related also to a method for spraying fluids in droplets for processing fibrous webs.

Description

FIELD OF THE INVENTION

The present invention relates to a nozzle for spraying fluids in droplets for processing fibrous webs, wherein the nozzle comprises a housing having a first longitudinal central channel for the fluid to be sprayed and a second coaxially extending outer channel for a dispersing fluid, wherein the velocity vector of the fluid to be sprayed exiting from the opening of the first channel has mainly a linear component, that is the fluid exiting from the nozzle has essentially rectilinear motion, whereas the velocity vector of the dispersing fluid exiting from the opening of the second channel has both rotational and linear components, that is the dispersing fluid exiting from the nozzle has spiral motion.
In the context of this application, the expression "processing fibrous webs" covers coating or sizing papers and paper boards with coating or sizing agents, respectively, adding fiber suspensions, e.g. nanocellulose, to the paper or paper board webs, as well as other spraying processes for making paper or paper boards, e.g. for moistening webs with water or water including PVOH, starch or similar.

BACKGROUND OF THE INVENTION

There are many different types of spraying nozzles for spraying different fluids in paper or paper board making processes. US 7820011 discloses a moistening nozzle for spraying water as mist to moisten a paper web. The moistening nozzle comprises a frame to which air and water are fed, a water nozzle which is arranged inside the frame and wherewith water is conducted to an outlet of the moistening nozzle and an air nozzle wherewith air is conducted to an outlet of the moistening nozzle. The air nozzle has an internal thread that brings the air into swirling motion, the internal threads being formed on the inner surface of the air nozzle. The air nozzle and the water nozzle are arranged one within the other to allow the air and the water to produce water mist that is sprayed out from the moistening nozzle. US 6969012 relates to an atomizer comprising a housing having three inlets, three channels each including a nozzle in communication respectively with said inlets. Said three inlets comprise a fluid-receiving first inlet, a fluid-receiving second inlet, a liquid-receiving third inlet, one of said channels being an inner channel. Said inner channel is associated with said third inlet and is uniform in diameter. The one of said nozzles associated with said inner channel extends outwardly of said housing beyond the other two of said nozzles. An angular swirling member is coaxially disposed in said housing with respect to said second nozzle. A problem with these prior art nozzles is that the swirling motion is achieved by using internal threads in a chamber in which the dispersing fluid is fed axially. These solutions have been useful as such, but there is still a need for more efficient dispersing of the fluid to be sprayed. It could be achieved by increasing the velocity of the dispersing fluid. With these prior art devices, this would require a higher fluid feed pressure with effective - and costly - compressors, and the higher velocity would mean higher linear and rotational components of the velocity vector of the dispersing fluid. With a higher linear velocity component the spray cone would be narrower and this is not desirable.

BRIEF SUMMARY OF THE INVENTION

An aim of the present invention is to provide an improved spray nozzle which produces an improved rotational to linear component rate of the dispersing fluid by using low pressure blowers. Another aim is to provide an improved method for spraying fluids in droplets for processing fibrous webs. These aims are achieved by means of a spray nozzle for spraying fluids in droplets for processing fibrous webs, wherein the nozzle comprises a housing having a first longitudinal central channel for the fluid to be sprayed and a second coaxially extending outer channel for a dispersing fluid, wherein the velocity vector of the fluid to be sprayed exiting from the opening of the first channel has mainly a linear component, that is the fluid exiting from the nozzle has essentially rectilinear motion, whereas the velocity vector of the dispersing fluid exiting from the opening of the second channel has both rotational and linear components, that is the dispersing fluid exiting from the nozzle has spiral motion, characterized in that the nozzle is provided with means for feeding the dispersing fluid tangentially at the infeed end of the second channel, the second channel having inner and outer walls defining an annular space therebetween, the space being dimensioned such that the rate between the rotational and linear components of the velocity of the dispersing fluid increases before it leaves the channel and contacts the fluid to be sprayed. The method of the present invention is characterized by what is defined in independent claim 11.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: is a cross-sectional view of a prior art moistening nozzle;
FIG. 2: is a schematic cross-sectional view of a nozzle of the invention;
FIG. 3: is a variation of the embodiment shown in FIG. 2, and
FIG. 4: shows dimensioning of the second channel.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a prior art moistening nozzle as disclosed in US 7820011 . The nozzle has a frame 1 and a water nozzle 2 connected to the inside of the frame 1 with a threaded joint 17. Inside the frame 1 is also arranged an air nozzle 3 such that the water nozzle 2 and the air nozzle 3 are concentric. The air nozzle 3 is secured to the frame 1 with a securing nut 4 that is connected to the exterior of the frame 1 with a threaded joint 18. The water nozzle 2 includes a water connector 5 to which water is fed through a pipe or a hose or the like. From the water connector 5, the water flows out of the water nozzle through a water duct 6. In the frame 1 is also arranged an air connector 7 to which a pipe or a hose or the like is connected for feeding air to the moistening nozzle. Water is fed into the moistening nozzle, i.e., in the middle of the moistening nozzle, from the rear part thereof, and air is fed into the moistening nozzle from the side of the moistening nozzle. Air is conducted from the air connector 7 to an air chamber 8 arranged around a shaft 14 of the water nozzle. From the air chamber 8, the air flows axially towards the air nozzle 3 through apertures 9 in the frame 1. The air nozzle 3 comprises an internal thread 10 which is provided on the inner surface of the air nozzle 3 and by means of which the air is brought into swirling motion. The air in swirling motion thus flows through an air gap 19 in the moistening nozzle to surround the water supplied from the water nozzle 2, whereby the mixture of water and air forms water mist. Due to the swirling motion, the water mist forms an even cone-shaped spray. The air nozzle 3 is arranged inside the frame 1 in such a way that the air nozzle is positioned in place against a control surface 11 of the air nozzle inside the frame 1. The control surface 11 of the air nozzle is located around the central axis in the circumferential direction. The water nozzle 2, in turn, is positioned in place against a first control surface 12 of the water nozzle provided on the inner surface of the frame 1. On the shaft 14 of the water nozzle 2 may be provided a collar 13 by which the water nozzle 2 is supported against the first control surface 12 of the water nozzle. The first control surface 12 of the water nozzle is also parallel to the circumference around the axis of the moistening nozzle. Due to this structure, the control surface 11 of the air nozzle and the first control surface 12 of the water nozzle can be provided with one attachment of a machining piece on the frame 1, whereby they can be made concentric with close tolerance and the air nozzle 3 and the water nozzle 2 can be mutually centered with very good accuracy. In the frame 1 of the moistening nozzle are provided second and third control surfaces 15, 16 of the water nozzle to fit the water nozzle accurately into place.
This solution has proven to be effective in certain applications, but if higher velocities are needed, this solution requires costly compressors to provide higher pressures for higher velocities. This would lead to a higher axial velocity which would result in a narrower cone shape of the water mist. Higher velocities are needed, e.g. when the speed of the web to be processed by a sprayed fluid is increased. This is due to the boundary air layer of the web also travelling at a higher speed and, thus, the fluid to be sprayed on the web should have a higher speed than the boundary layer.
Fig. 2 shows schematically a cross-sectional view of an embodiment of the nozzle of the invention. The nozzle 20 comprises a housing having a first longitudinal central channel 24 for the fluid to be sprayed and a second coaxially extending outer channel 21 for a dispersing fluid. The velocity vector of the fluid to be sprayed exiting from the opening of the first channel mainly has an axial component, whereas the velocity vector of the dispersing fluid exiting from the opening of the second channel has both rotational and axial components. The nozzle 20 is provided with means 25 for feeding the dispersing fluid tangentially at the infeed end of the second channel 21. The infeed end may have a feed chamber 26 into which the dispersing fluid is first fed tangentially to cause the dispersing fluid rotate therein and to enter therefrom to the second channel 21 in rotational motion. It is also possible to arrange the tangential feed of the dispersing fluid directly to the second channel at its infeed end without the separate feed chamber. The second channel has inner 22 and outer 23 walls defining an annular space therebetween, the space tapering towards the exit of the channel such that the rate between the rotational and axial components of the velocity of the dispersing fluid increases before it leaves the channel and contacts the fluid to be sprayed. In the embodiment of fig. 2, the inner and outer walls of the second channel taper towards the exit of the second channel at the same rate such that the distance W between the walls 22, 23, normally to the surfaces thereof, remains constant. R1 and R2 show the radius of the second channel at different locations showing that the radius decreases towards the exit end of the channel.
Fig. 3 shows a variation of the embodiment of fig. 2. The nozzle 30 differs from the nozzle 20 of fig. 2 in that the second channel 31 has a constant cross-sectional area. This is achieved by having the inner surface 32 taper more steeply towards the exit of the second channel than the outer surface 33. In fig. 3, D refers to the diameter of the inner surface 32 and D' to the diameter of the outer surface on a plane normal to the longitudinal central axis of the nozzle 30. The area of the channel 31 is thus Π(D²- D'²)/4. The nozzle 30 is provided with means 35 for feeding the dispersing fluid tangentially at the infeed end of the second channel 31. The infeed end may have a feed chamber 36 into which the dispersing fluid is first fed tangentially to cause the dispersing fluid rotate therein and to enter therefrom to the second channel 31 in rotational motion. It is also possible to arrange the tangential feed of the dispersing fluid directly to the second channel at its infeed end without the separate feed chamber.
The cross-sectional area may also be something between the embodiments of Figs. 2 and 3, or even increasing, depending on the desired manner of selecting the said ratio of rotational velocity and axial velocity.
Fig. 4 shows exemplary dimensioning of the second channel in the nozzle of the present invention.
In Fig. 4:

D2 is the diameter of the exit opening of the annular second channel at the central line of the opening (H2)
D1 is the corresponding diameter at the lower end of the second channel at the central line of the opening (H1)
H2 is the width of the annular exit opening of the second channel
H1 is the corresponding width at the lower end of the second channel
α is the conicity of the second channel
β is the opening angle of the cross section of the second channel
L is the length of the nozzle part containing the second channel

D2 is defined by the dimensions of the first channel for the fluid to be sprayed. In nozzles for spraying water the diameter of the inner channel is usually in the order of about 4 mm. In the nozzles of the present invention the diameter of the inner channel is preferably in the range of 2 to 8 mm and the diameter D2 of the second channel in the range of 4 to 12 mm, preferably in the range of 6 to 9 mm.
In order to have the dispersing fluid, e.g. air, speeded up as desired D1 is preferably about (1,3 - 4)*D2. More preferably D1 is about (1,5 - 2,5)*D2 and H1 is preferably 0,5 - 3 mm (0,8 - 1,5 mm).
H2 is preferably about (1 - 4)*H1, more preferably about (1,5 - 2,5)*H1). L is preferably 5 - 50 mm (more preferably 10 - 30 mm). The conicity α is preferably in the range of 30° - 85°, more preferably in the range of 45° - 75°, still more preferably in the range of 55° - 65°. The opening angle β is preferably in the range of 0° - 15° (more preferably in the range of 1°-5°).

In the calculations carried out it was discovered that the nozzles of the invention, as shown in figures 2 and 3, had substantially improved efficiency compared to the prior art nozzle of US7820011 . Table 1 shows the results of the calculations.

Table 1

	Prior art	Nozzle according to the invention		Nozzle according to the invention		Nozzle according to the invention
	Prior art	Tapering channel	Constant cross-sectional area	Tapering channel	Constant cross-sectional area	Constant cross-sectional area
Air feed pressure	0.300 bar	0.300 bar	0.300 bar	0.015 bar	0.015 bar	0.075 bar
Amount of air from nozzle	8 kg/h	11 kg/h	16 kg/h	8 kg/h	11 kg/h	8 kg/h
Rotational velocity of air	50 m/s	100 m/s	165 m/s	70 m/s	115 m/s	80 m/s
Axial velocity of air	155 m/s	145 m/s	100 m/s	100 m/s	70 m/s	50 m/s
Rotational velocity/axial velocity	0.3	0.7	1.6	0.7	1.6	1.6

In the table, the columns headed "Tapering channel" refer to the tapering of the cross-sectional area of the tapered (and annular) air channel. In the columns headed "Constant cross-sectional area", the cross-sectional area of the channel in question remains constant.
The axial velocity of air means the velocity of air in the direction of the liquid jet. All the velocities in the table have been measured at the point where the air exits from the nozzle, that is, immediately before the air and the liquid come into contact.
As shown in table 1, the rotational velocity of the air at the same feed pressure is about twice or three times higher with the nozzles of the invention than with the prior art nozzle. The last line in the table 1 shows the ratio of rotational velocity and axial velocity which is clearly increased from that of the prior art, that is, with the nozzle according to the invention, the velocity of air is "transferred" from axial velocity to rotational velocity compared with the prior art. This ratio can be adjusted/selected as desired through the tapering/opening of the cross-sectional area of the tapered channel. In Figs. 1-3 the velocity vectors of air are shown schematically by the arrows V, V1 and V2, the ratio of rotational velocity and axial velocity being 0.3, 0.7 and 1.6, respectively. These vectors depict the angle of ascent of the air which is moving spirally.

Claims

Spray nozzle (20, 30) for spraying fluids in droplets for processing fibrous webs, wherein the nozzle comprises a housing having a first longitudinal central channel (24, 34) for the fluid to be sprayed and a second coaxially extending outer channel (21, 31) for a dispersing fluid, wherein the velocity vector of the fluid to be sprayed exiting from the opening of the first channel has mainly a linear component, that is the fluid exiting from the nozzle has essentially rectilinear motion, whereas the velocity vector (V1, V2) of the dispersing fluid exiting from the opening of the second channel has both rotational and linear components, that is the dispersing fluid exiting from the nozzle has spiral motion, characterized in that the nozzle (20, 30 is provided with means (25, 35) for feeding the dispersing fluid tangentially at the infeed end of the second channel (21, 31), the second channel having inner (22, 32) and outer walls (23, 33) defining an annular space therebetween, the space being dimensioned (α, β, D1, D2, H1, H2) such that the rate between the rotational and linear components of the velocity (V1, V2) of the dispersing fluid increases before it leaves the channel (21, 31) and contacts the fluid to be sprayed.
The spray nozzle of claim 1, characterized in that the inner (22) and outer walls (23) of the second channel (21) taper towards the exit of the second channel at the same rate such that the distance (W) between the walls, normally to the surfaces thereof, remains constant.
The spray nozzle of claim 1, characterized in that the inner wall (32) of the second channel (31) tapers towards the exit of the second channel more steeply than the outer wall (33) such that the cross sectional area of the second channel (31) remains constant.
The spray nozzle of any of claims 1 to 3, characterized in that the second channel (21, 31) has an annular infeed opening having first width H1 and an annular exit opening having second width H2, wherein H1 is in the range of 0,5 to 3 mm and H2 is (1 - 4)*H1.
The spray nozzle of claim 4, characterized in that H1 is in the range of 0,8 - 1,5 mm.
The spray nozzle of claim 4 or 5, characterized in that H2 is (1,5 - 2,5)*H1.
The spray nozzle of any of claims 1 to 6, characterized in that the second channel (21, 31) has conicity a (Fig. 4) in the range of 30° - 85°.
The spray nozzle of claim 7, characterized in that the conicity a is in the range 45° - 75°.
The spray nozzle of claim 7, characterized in that the conicity a is in the range 55° - 65°.
The spray nozzle of any of claims 1 to 9, characterized in that the diameter (D2) of the exit opening of the annular second channel is smaller than the corresponding diameter (D1) at the infeed end of the second channel.
Method for spraying fluids in droplets for processing fibrous webs, characterized in that in the method is used the spray nozzle (20, 30) of any of the claims 1 to 10, and that the rotational velocity of the dispersing fluid at the exit of the second channel is within the range from 50 to 180 m/s.
The method of claim 11, characterized in that the rotational velocity is within the range from 80 to 165 m/s.
The method of claim 11, characterized in that the rotational velocity is within the range from 100 to 120 m/s.
The method of any of the claims 11 to 13, characterized in that the ratio of rotational velocity to axial velocity is in the range from 0,5 to 2.
The method of claim 14, characterized in that the ratio of rotational velocity to axial velocity is in the range from 0,7 to 1,6.
The method of any of the claims 11 to 15, characterized in that the feed pressure of the dispersing fluid in the second channel is within the range from 0.050 to 0.300 bar.