Paper making and fiber liberation – Processes and products – Running or indefinite length work forming and/or treating...
Reexamination Certificate
2001-10-22
2004-03-02
Fortuna, José A. (Department: 1731)
Paper making and fiber liberation
Processes and products
Running or indefinite length work forming and/or treating...
C162S198000, C034S444000, C034S445000, C034S465000, C034S459000, C239S008000, C239S011000
Reexamination Certificate
active
06699365
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an air atomizing nozzle intended for use with a rewet shower for the paper making industry.
DESCRIPTION OF THE PRIOR ART
A modern paper machine produces paper from a mixture of water and fiber through consecutive processes. Three machine sections named forming, pressing and drying play the most important roles in the making of paper. Pulp at the headbox of the paper machine normally consists of about 1% fiber and 99% water.
The forming section of the paper machine removes water from the pulp by gravity and vacuum suction to form a sheet. In the pressing section, the sheet is conveyed through a series of pressing nips where additional water is removed and the fiber web is consolidated. The water concentration is reduced to about 40% after pressing. The remaining water is further evaporated and fiber bonding develops as the paper contacts a series of steam-heated cylinders in the drying section. The moisture level drops down to about 5 to 10% after the drying section.
One of the important properties of a paper product is the moisture level. However the uniformity of moisture in the paper product in both the machine direction and the cross machine direction is even more important than the absolute moisture level. There are numerous influences on the paper machine that can cause variation of the moisture content, particularly in the cross machine direction. Wet edges and characteristic moisture profiles are common occurrences on paper sheets produced by a paper machine. Therefore a number of actuator systems have been developed to offer control of the moisture profile during paper production.
One such actuator system is a water rewet shower that selectively adds small water droplets onto the paper surface. The rewet showers, which are commercially available, employ actuator nozzle units that are mounted in sequential segments (or zones) across the paper machine direction. Water flow rate is controlled independently through each actuator nozzle unit. Hence the moisture profile on the paper sheet can be adjusted by the rewet system. Spray nozzles are normally used in those rewet showers to generate droplets small enough to produce effective rewetting.
One significant component in a rewet shower is the nozzle. Droplet sizes and water mass profiles across the nozzle jets are the most important parameters to evaluate the feasibility of a particular nozzle for a rewet shower. Water particles too small tend to evaporate before they can reach the paper sheet. Droplets too big can hardly produce uniformity on the paper sheet and in extreme cases they may cause problems such as strips on the web. The ideal mass profile for the paper rewet shower generated from a single nozzle is a square shape.
The width of the square determines the zone size of the rewet shower. The height of the square represents the moisture added through this single nozzle. The coupling effects between adjacent nozzle jets are minimal in this ideal case.
Two kinds of nozzles, hydraulic and air atomizing, are widely used for water sprays. A hydraulic nozzle uses energy from a highly pressurized source to break water into droplets at the nozzle. The flow rate passing through a hydraulic nozzle is a function of the source pressure. The spraying pattern, such as spraying angle and velocity profile, is affected by the pressure as well. The fact that the droplet size is related to the flow rate makes the hydraulic nozzle ideal for operation at a fixed design point.
An air-atomizing nozzle uses energy from pressurized air to break water into small droplets. Two types of atomizing nozzle are in wide use. The internal-mixing-type nozzles mix atomizing air with water within a mixing chamber before emitting the droplet. The dependence of water flow rate on the pressure of atomizing air makes this type of nozzle unsuitable for rewet showers. The external-mixing-type nozzles mix the water with the atomizing air in an opening area outside the nozzle. The water flow rate of external-mixing-type nozzles is independent of the atomizing air pressure. The spray patterns of the external-mixing-type nozzle are affected mostly by air pressure. The droplet size from an external-mixing-type nozzle depends more on the air pressure than the water flow rate. Separating droplet size and profile controls from water flow rate control substantially simplifies the controlling strategy of a spraying system. The characteristics of the external-mixing-type nozzle make this kind of nozzle most suitable for paper rewet applications.
A simple example of an externally mixing nozzle consists of a tube surrounded by an annulus as is described by M. Zaller and M. D. Klem in “Coaxial Injector Spray Characterization Using Water/Air as Simulants”, 28
th
JANNAF Combustion Subcommittee Meeting, CPIA Publication 573, vol. 2, pp151-160 (“Zaller et al.”). The water flows within the tube, and the atomizing air flows in the annulus surrounding the tube in the direction parallel to the water stream. As is described in Zaller et al. this nozzle configuration can produce water droplets less than 50 microns. However the drawback of this simple nozzle is the mass profile which takes a relatively sharp peak at the center of the nozzle jet as shown in
FIG. 1
by the profile labeled “Single Stream.” The pulse-shaped single stream profile limits the zone size of the rewet shower.
With the same nozzle geometry as described in Zaller et al., one can introduce swirling flow in the annulus surrounding the water tube. The atomizing air moves in a direction substantially perpendicular to the water stream. German Patent No. 952,765 describes one of the “single stream” nozzles that uses a swirl to break the water into droplets. The swirl generates relatively larger particles compared to the straight flow assuming that the same air pressure is employed. The drawback of the “single swirl” nozzle of German Patent No. 952,765 is that the mass profile has a recess in the center aligned with the nozzle and two peaks on both sides of the recess as is shown in
FIG. 1
by the profile labeled “Single Swirl.”
U.S. Pat. No. 4,946,101 which is owned by the owner of German Patent No. 952,765 discloses an apparatus combining a straight stream and a swirl in the annulus surrounding the water tube. A swirling member with square threads is used to produce the required swirling flow. The combined straight and swirling flows break the water into small droplets. Centrifugal force generated from the swirl acts on water droplets and pushes them away from the center of the jet. The peak from the straight stream compensates the recess created from the swirling flow. The resulting mass profile has a relatively flat portion in the center of the jet and two relatively steep slopes on both edges as shown in
FIG. 1
by the profile labeled “Stream-Swirl Combination.”
The present invention adds to the combined straight and swirling stream another straight stream outside of and surrounding the swirling stream. One of the purposes of adding another straight stream is to add axial momentum to the particles at the outer region of the swirl which makes the slopes on the edges steeper. The resulting water profile (shown in
FIG. 1
by the profile labeled “Stream-Swirl-Stream Combination”) created by the combination of the three atomizing air streams is closer to a square in shape than that generated from the combination of a straight stream and a swirl.
In the atomizing nozzle of the present invention a combination of three air streams is used to break the water into small droplets. A water stream with relatively low velocity is located in the center of the nozzle jet. A main air stream moving straight in the same direction as the water stream is located around the water stream. This main air stream moves much faster than the water flow inside the water stream. The shearing force generated by the large velocity gradient at the boundary of the two steams is the major force to break the water into small particles. As is described in Zaller et al. this major air stream deliv
ABB Inc.
Fortuna Jos'e A.
Rickin Michael M.
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