Patent Application
20120145347
The present invention relates to a system for transporting a fibre web through a dry end of a fibre web processing machine from a drying section to a reeling section of the machine. In particular, the invention relates to such a system in a tissue paper machine.
The invention also relates to an air supply module and an air exhaust module for use in such a system.
The invention also relates to method of preventing dust laden air from escaping a localized zone in a dry end of a fibre web processing machine.
Regarding the dry end of a tissue machine, the conventional methods of controlling the web and containing dust are disappointing. The conventional methods are several layers of solutions. Each new successive solution (layer) has been devised without fully re-evaluating what has been previously done.
The present invention relates to a detailed outline of a dry end design that efficiently integrates the key processes and objectives of the dry end of a tissue or towel grade machine while using new and existing technology.
There are many aspects of the invention included herein that can be used as stand-alone devices or processes. However, they are more valuable when used in unison.
The objectives of sheet handling include sheet stability, preservation of the crepe in the sheet, and effective containment of dust. The present invention satisfies these needs.
According to the invention, careful attention is given to balancing of the airflows. Air is not only exhausted at critical locations, it is also supplied at strategic locations.
Experience has shown that systems that require intensive or complicated maintenance will eventually fail due to lack of maintenance. This new system is designed to have minimal maintenance requirements. Automated cleaning devices are incorporated where needed.
The majority of known dust control strategies are based on the premise that you cannot extract air from too close to the web. The majority of sheet handling solutions are based on “stabilize the sheet and don't worry about the dust”.
The conventional strategies employed thus far are largely guess work based on misperceptions.
Scanners, glue turn-up at the reel drum, tail threading, sheet knockdown showers, calendering, etc. are poorly integrated with the key components of sheet handling and dust control.
Consequently, prior art techniques have the following drawbacks:
A common problem associated with conventional systems is the sheet instability which occurs along the edges of the sheet run. This is a result of the incoming air flows.
The incoming air currents are intentional and are used for the purpose of containing the dust in the machine. Unfortunately, they result in edge instability. In areas where the web is unsupported, this inwards velocity normally cannot exceed a magnitude of 100 ft/min (0.5 m/s).
The incoming air currents will exist in certain areas without a dust collector present. When the web exits the creping doctor or exits a calender, air is pumped with the web's boundary layer and air must enter the sides of the machine to compensate. In these cases, exhausting from the zone only worsens the web stability problem.
In areas such as the entrance to a calender or the lead-in to a reel drum, air is pushed out of the sides of the machine. Simple containment velocities are not sufficient.
Another common issue is that conventional pulper exhaust systems are greatly oversized. Containment is attempted with high exhaust flowrates. In many cases, mist and vapour still escapes as variations in conditions occur. The conventional systems waste energy.
Another important issue is cleaning of the web. There are several variations of web cleaners available for tissue grades. They are all meant to be retrofit into a machine and there is no holistic approach in their concept. All known existing types may have one or more problems as specified in the following list:
In many cases, they function for a short period before they become plugged and the paper machine operator eventually removes them. Regardless if they work, there is little effort in preventing dust and debris from entering the nip at the reel. This renders useless any effort at web cleaning.
The objective of the present invention is to alleviate these problems.
The system according to the invention is characterized in that it comprises one or a plurality of groups of air modules, wherein each group of air modules is arranged for supplying and evacuating air from a predetermined, localized zone along the web run of the dry end, and wherein each group of air modules comprises:
The method according to the invention is characterized by the steps of:
Generally, the problems associated with web transportation in a dry end of a conventional tissue machine relate to the following technical areas: the handling of the boundary layer on a moving surface, the handling of flooded nip, the handling of linear free jets, the active airfoils, the passive air foils, sheet tensioning considerations, the spreading of the tissue web, the containment and removal of dust, wet scrubbing and the issue of containment at the pulper.
In the following, some of these areas will be discussed in more detail.
Boundary Layer on a Moving Surface
A moving surface carries air with it. There are theoretical correlations available for laminar flows and empirical correlations available for turbulent flows. The correlations used originate from those derived for applications involving a stationary plate and a moving airstream and are adopted for use in a given configuration. In a typical application, the vast majority of the boundary layer air is very close to the moving surface.
Flooded Nip
When two rotating rolls are in contact, a region of high pressure exists at the closing nip between them. Boundary layer air is carried into the nip area from the surfaces of both rolls. A larger roll will naturally have a stronger boundary layer flow. A jet of air exits the nip. The average angle of this jet is influenced by the relative magnitude of the two boundary layer flows.
It is possible to influence the magnitude of either boundary layer flow by using linear supply-air nozzles to augment their flows. In this way, it is possible to change the angle of the jet of air that exits the nip. This is useful when one roll carries a boundary layer that is clean while the other has a boundary layer that contains dust or debris. By augmenting the flow of the clean boundary layer flow, it is possible to prevent dust or debris from entering the nip.
Linear Free Jet
A linear free jet is a jet of air emerging perpendicularly from a slot in a surface.
Many of the important variables associated with linear free jets, e.g. the expansion angle, the core length zone, the transition zone length and the profile similarity zone length, can be determined using published correlations by scientists such as Baturin, Nielson and Rajaratnam.
Linear free jets can be used for air curtains. Some critical issues to consider include:
The effectiveness is roughly proportional to the nozzle velocity squared.
Active and Passive Foils
A foil comprising a nozzle releasing air along the foil surface in the direction of the travelling web in order to stabilize the web is called an active airfoil. A foil lacking such a nozzle is called a passive foil. Passive foils are the most common in the industry. They are low cost and adjustable.
Flat plates rely upon a slight wrap angle at it's trailing and leading edges to make sure that the web sticks to it.
Dust accumulation on the leading and trailing edges tends to fall off in large pieces. This usually results in web breaks. The dust accumulation on the leading edge is due to the impaction of dust particles that originate from the dust laden boundary layer that is above the web and upstream of the foil. The dust particles on the trailing edge originate from the dust-laden air that is drawn into the zone at the trailing edge where the web's boundary layer causes air to be drawn in. The dust particles that accumulate on the leading edge will tend to be smaller as there are greater accelerations involved in the airstream and thus increased separation due to impaction.
The top surface of the web is exposed to allow for pressure relief upstream and downstream of the foil. This results in dust release above the web. In conventional configurations, dust is released between the foils.
The length of the foil surface in the direction of web travel is limited because air accumulates between the web and foil. The web is held to the underside of the foil due to the low pressure that is generated on the foil surface due to the movement of the web. It is also held to the foil due to the sheet tension coupled with the wrap on the leading and trailing edges of the foil. Web permeability contributes to the accumulation of air between the web and the foil.
The allowable foil length (in the machine direction) increases with:
Because of friction, a passive foil will induce a drag force on the web which results in increased sheet tension requirements.
Increased sheet tension results in increased drag on a passive foil.
Sheet Tension Considerations
The sheet tension can be predicted through-out the sheet run. A sheet handling system must be designed such that the sheet tension varies within an acceptable range through-out the sheet-run.
The sheet tension varies depending on the location in the dry end.
If the tension is too high at any particular location, the crepe will be pulled out or, worse yet, the web will be torn. If the tension is too low, the web will locally bag or drop off of the foils.
The following factors apply loads to the web along it's path and affect the sheet tension:
Consider the following example. The start of the sheet run is at the creping doctor. The web is pulled at the end of the sheet run by the reel. Passive foils drag on the web while active foils push on the web. Consequently, there is a need for strategically locating passive and active foils in the sheet run.
The consequence of grouping too many passive foils at the start of the sheet run is that the sheet tension will become too high downstream of the passive foils.
The consequence of grouping too many active foils at the start of the sheet run is that the sheet tension will become too low downstream of the active foils. This configuration may also lead to sheet tensions that are too high at the start of the sheet run.
The frictional load on the passive foils can be predicted using experimentally determined friction factors. The ‘push’ provided by active foils can be predicted using experimentally determined correction factors and a simple energy balance.
Careful attention must be given to selecting the appropriate locations for the passive and active foils in order to achieve favourable tensions at certain locations while maintaining tensions within the acceptable range. Preferably, active and passive foils are alternately positioned along the sheet run.
Assuming that the web properties are uniform in the cross-machine direction and neglecting adverse air currents, cross-machine variations in sheet stability can be attributed to cross-machine variations in sheet tension. When sheet tension is not high enough in the center of the machine, the web sags in the center of the machine. This is referred to as ‘bagging’. When the web tension is too loose on the edges, it is referred to as ‘edge flutter’.
Edge flutter is also caused by adverse air currents or cross-machine variations in the web's properties.
The sheet tension curve is affected by the crepe ratio. Variations in reel speed (crepe ratio) will shift the tension curve vertically.
The friction load on passive foils is partly a function of sheet tension. Increased sheet tension due to reduced crepe ratio will result in higher friction loads on the foils closest to the reel. The web massflow is of course constant as a function of location. The web acts as a linear tension spring. The velocity of the web varies as a function of sheet tension or location.
Sheet tension affects product quality and the runnability at the dry end. Consideration must be given to sheet tension in the design of the foils.
The Containment and Removal of Dust
Air conveys dust. This is an easy concept to accept when you consider transporting dust within ductwork.
Consider a short piece of duct with an inlet and an outlet. A certain volumetric flow of air is used to carry a certain massflow of dust with a minimum carrying velocity. Dust and air enter one end of the duct. The same dust and air exit the other end of the duct.
Now consider a point source of dust in a tissue machine near a moving tissue web. There are key considerations for effectively conveying the dust away:
Historically, the problem is that little consideration has been given to where the air comes from. If you simply evacuate the required amount of air from near the web, you will draw in the web. If you reduce the airflow to ensure that the web is stable, you can't capture all of the dust. The web is permeable, but it is not permeable enough.
A common approach is to install an exhaust hood over top of the sheet run and provide adequate containment velocity around the sheet run. However, sheet edge stability is always an issue. There is a limit to the inwards velocity at the edge of a moving web. This low containment velocity is not adequate to combat normally occurring air currents in the dry end. It also results in high exhaust flow requirements which is a waste of energy.
The answer to the problem is to strategically introduce the air that will be evacuated and account for airflows that exist due to moving surfaces. We need to supply some air to the source of dust and evacuate the same air along with the dust. Air is simply a tool for conveying the dust. There must be balance.
This invention takes web handling and dust control to a new level. In addition to controlling the evacuation of air, the supply of air is controlled and the natural airflows due to moving surfaces are favourably manipulated or allowed for.
Containment at the Pulper
The purpose of exhausting air from the pulper is to prevent a large bubble of hot humid air from escaping from the pulper. The required pulper exhaust flowrate is not only related to the volumetric flowrate of vapour that is being released by the hot agitated stock.
The air flow patterns near the pulper floor openings are difficult to predict. Conventional pulper exhaust systems achieve modest downwards air velocities at the floor openings. It is difficult to overcome other significant influences such as:
The aforementioned influences vary with time. The conventional answer has been to oversize the pulper exhaust system. This results in wasteful air scrubbing requirements and needless machine room make-up air requirements.
The buoyancy forces acting on the humid pulper air must be counteracted by the downwards velocity of air at the floor openings.
Neglecting localized high air velocities resulting from various forced air currents, buoyancy forces are the dominant factor that must be controlled. Theoretically, if there is no air forced into the pulper and it is completely sealed, no exhaust is required as the air will reach saturation and no more water vapour will be liberated from the stock or showers.
The required Pulper Exhaust varies depending on the operating condition. The following two steady state conditions must be considered.
The pulper exhaust flowrate must satisfy both conditions. In terms of Pulper Exhaust Flowrate requirement, the worst case condition occurs when the web is directed into the pulper. It is essentially the same as discharging air into the pulper and relying on the Pulper Exhaust to accommodate it. When the web is directed into the pulper from the Yankee, it carries boundary layer air with it on both of it's sides. This flow of additional air may be sufficient to overcome the predicted containment flowrate and spillage from the reel pulper opening would occur.
The following expression summarizes the various contributions to the required Pulper Exhaust Flowrate for a conventional system.
Q
pulper
=Q
containment
+Q
vapour
+Q
trim
+Q
boundary layer air
Qcontainment The flowrate of air required to achieve containment velocities at the floor opening(s) into the pulper to offset buoyancy effects.
Qvapour The flowrate of water vapour generated from the pulper liquid surface
Qtrim The flowrate of air into the pulper from other sources such as a trim conveying system
Qboundary layer air The flowrate of air that is carried with the web into the pulper when the web is diverted to the pulper from the Yankee at machine speed.
Water Vapour Generation
The flowrate of water vapour in the pulper due to the evaporation of water can be approximated using many methods. It is quite low in magnitude in comparison to the other contributing factors in the sizing of the required pulper exhaust flowrate.
In the following, the present invention will be disclosed with reference to the accompanying drawings, wherein:
In the web transport system according to the present invention, air is used for conveying dust. The web transport system introduces supply air and evacuates dust-laden air at strategic locations. The sheet run is enclosed and the air flows are balanced such that flows into and out of the control volume are accounted for. If deemed beneficial, some of the exhaust air is re-used as supply air; It is re-circulated. This reduces the amount of exhaust air out of the machine room thereby decreasing machine room make-up air requirements.
This embodiment of the web transport system comprises a plurality of foils or plates 7, 68 being arranged to support the web during the travel from the
Yankee cylinder 1 to the reeling station 2. Also, the web transport system comprises air supply modules 9, air exhaust modules 10, a dry end enclosure 61 (see
The following sub-sections will describe each of the main components of the web transport system according to the invention in more detail.
The web transport system is comprised of:
The flexibility in the sizing and configuration of the various in-machine components allow for the use of this system on almost any tissue machine dry end regardless of it's size, speed or configuration.
The supporting air systems are customized to suit the application.
The sizing and selection of the components is well defined. It is repeatable. Regardless of who applies the system, for a given application, the same solution is devised.
Air supply modules in the form of nip flooding air jets allow for the control of air flow patterns at a closing nip, whereby the entrance of unwanted materials into a closing nip such as that of a calender or reel can be hindered.
The top of the web does not need to release dust as it does in a conventional tissue machine dry end. Exposure of the top of the web needlessly results is dust escaping from above the web and forces the building of a big box around the dry end. All conventional efforts at containing the dust above the web are merely inefficient solutions that are costly to implement, operate and maintain while failing to get to the root of the problem.
The web transport system according to the invention comprises a multitude of sub-systems that can work in unison to efficiently enclose the sheet run from the creping doctor to the reel. Using the web transport system according to the invention, the following objectives are met:
The following items have been the same for many years and essentially remain the same in this new concept:
These items stay the same because when the web transport system is adopted, it is desirable to have the ability to retrofit it to existing machines.
Airfoil nozzles 14 are optional in the web transport system according to the invention. They are used when the maximum allowable sheet tension is low.
In some cases, active foils are not required. This reduces the complexity of cost of a system. Active type foils usually can be omitted in applications where:
There are two main purposes for the airfoil nozzle.
Air supply modules or nozzles 32 having a self cleaning mechanism according to one aspect of the invention will now be described with reference to
For applications that require uninterrupted air flow from an air supply nozzle 32, a cleaning plate or blade 33 is oscillated longitudinally in the slot, opening or outlet 34 of the nozzle 32 (see
In this case the nozzle 39 comprises a cleaning plate or blade 40 having a linear edge 41. Also, the nozzle displays a V-shaped guide slot 42 in which a stationary pin 43 is arranged. An actuator 44 is arranged to bring the cleaning blade 40 to travel back and forth such that the cleaning blade 40 travels a V-shaped path and such that the edge 41 moves in and out of the air supply slot, opening or outlet 45 of the nozzle 39 when the blade 40 is oscillated longitudinally in the air supply slot 45. When the blade 40 is in its outermost position, i.e. the position disclosed in
In the web transport system according to the invention, dust is contained by exhausting and supplying air between adjacent foils 7a and 7b, as is shown in
Air is supplied to the zone between the foils 7a and 7b via an air supply module 9 having a downstream facing cross-machine air supply nozzle 46 ejecting fresh air onto the trailing edge of the first, upstream foil 7a. This airflow is a uniform laminar flow that has a velocity field with a maximum magnitude that is much lower than the velocity of the web 47. The air supply nozzle 46 is adjustable such that the velocity and the volumetric flow of the ejected air can be changed. Also, on the leading edge of the second, downstream foil 7b there is an air exhaust module 10 being arranged to evacuate dust-laden air from the zone between the foils 7a and 7b. The air exhaust module 10 comprises an upstream facing cross-machine air exhaust slot 49.
The air supply module 9 and the air exhaust module 10 are connected to an air duct system (not shown) being arranged for leading fresh air to the air supply module 9 and for leading dust-laden air away from the air exhaust module 10.
The exhaust flow is slightly greater than the supply flow. This creates a very low under-pressure in the zone between the foils 7a and 7b. The air from the supply nozzle 46 combines with bleed-air relieved from the end of the first foil 7a and the induced machine room air from above the foils, as is shown in
Preferably, the gap g (see
Preferably, the supply air is primarily re-circulated exhaust air that has been mixed with some fresh air. The required flowrate of the supply air is approximately 75% of the exhaust flowrate. This equates to approximately 13 cfm/inch (14.5 (m3/min)/m).
As with conventional foil applications, there is preferably a wrap on the trailing edge of the upstream foil 7a and on the leading edge of the downstream foil 7b.
This configuration allows for great flexibility during design, construction and start-up as it allows for adjustments to the foil geometry and airflows.
The distance between the foils 7a and 7b can vary depending on the application. A distance between the foils 7a and 7b of approximately 3 to 4 inches (76 to 102 mm) has proven to be suitable.
There are no hidden passages or inaccessible locations in the air supply and air exhaust modules 9, 10. The tending side and drive side ends of the air exhaust module 10 have a round inspection port with a quick release latch. This allows for easy inspection of the interior of the air exhaust module 10.
The air exhaust module 10 can have a constant profile regardless of the width of the machine. There are semi-tangential outlets 49 located along the width of the module 10 approximately every 2500 mm. Also, the air supply module 9 can have a constant profile regardless of the width of the machine. There are rectangular inlet connections 50 located along the width of the module 9 approximately every 2500 mm.
The air supply and exhaust modules 9, 10 can be retrofitted to existing foils as required in a rebuild situation.
Preferably at least one, more preferably a majority and most preferably all of the air exhaust modules 10 of the web transfer system comprises a self-cleaning air exhaust slot 48.
A preferred embodiment of an air exhaust module having such a self-cleaning air exhaust slot 48 will be described in the following with reference to
An air exhaust module comprising this type of self-cleaning exhaust slot is suitable for evacuating dust-laden air having a relatively low moisture content.
The air exhaust module 10 comprises a rotatable section 51 being formed by a cylindrically shaped body 52 and a tangentially extending lip 53, which is fixed to the envelope surface of the body 52. The body 52 may be comprised of pipe sections. The section 51 is rotatably mounted in a support structure 54 such that when the section 51 is rotated, the lip 53 traverses the exhaust slot 48 of the module 10 and clears the exhaust slot 48. A scraper blade 55 removes any material that may be attached to the body 52. When clearing the exhaust slot 48, the lip 53 pushes any material blocking the slot 48 into the module 10 where the loosened material is carried away by the flow of the exhaust air.
The rotatable section 51 can be rotated more than 180 degrees between a first, idle position and a second position, which is illustrated using broken lines in
The support structure 54 comprises equally spaced support struts 56 being arranged along the cross-direction of the module 10. The lip 53 and the scraper blade 55 extends continuously along the cross-direction of the module 10 while the body 52 preferably is comprised of several pipe sections, each terminating at a support bearing carried by a support strut 56. The pipe sections are linked with a short piece of round bar that passes through each bearing and is fastened to the ends of the pipe sections with pins.
When traversing the exhaust slot 48, the lip 53 temporarily halts the flow of air and consequently the self-cleaning function cannot be used in applications where the sheet stability would be upset during it's use. In applications where the self-cleaning function would affect the sheet stability, it's use could be restricted to sheet-off conditions such as during crepe blade changes.
Alternatively, the rotatable section 51 can be divided into two part-sections that are actuated independently. This would allow for half of the slot 48 to be functioning while the other half is being cleaned. However, this would require the use of two actuator assemblies instead of one, in which case there would preferably be one actuator on the tending side and one actuator on the drive side of the module 10.
The design of the air exhaust module 10 disclosed in
Another type of known air exhaust modules are long semi-cylindrical devices that have a longitudinal exhaust slot where dust-laden air is drawn into the module. Often, the exhaust slot of these modules becomes blocked by relatively large pieces of broken web material. In many cases, the exhaust slot is not easily accessible and a simple means of maintaining cleanliness of the slot is required.
The normal exhaust slot of such a dust collector is a long, narrow and semi-rectangular opening running in the cross-machine direction of the module. The opening may be slightly tapered in the direction of the flow of the exhaust air. The cross-machine length of the exhaust slot is usually within the range of 10 to 30 feet (3.0 to 9.1 m) and the normal slot gap range is approximately ½″ to 2″ (1.3 to 5.1 cm). If a wad of broken web material is lodged in the exhaust slot, it is sometime difficult for it to break free. In this type of dust collector applications, water may ‘undermine’ the back of the wad to help it become dislodged, but this is not always successful. With time, the performance of the collector is compromised as the effective exhaust slot area is reduced due to the blockage or blockages.
Up until now, only the following methods have been used for removing blockages from dust collector intake slots:
Manual cleaning is not always practical as the collectors are often located in areas that are difficult to access. The compressed air showers are also not effective since there is a high vacuum level at the intake slot which causes difficulties for the wads to become dislodged. If the air showers do dislodge any wads, the wads become airborne in the dry end of the machine and this is not desirable. Large wads that are allowed to fall outside of the collector may cause sheet breaks. Consequently, there is a need for a wet dust collector that allows for the wads to be drawn into the collector so that disturbances outside of the collector are avoided.
Another embodiment of a dust collector having a self-cleaning air exhaust slot will be described in the following with reference to
The lower slot plate 59 is adjustably fixed in the collector 57 as in a conventional dust collector. The upper slot plate 58, however, is movably arranged in the collector 57 such that it can move back and forth in the longitudinal direction of the slot 60 with a length of stroke equal to or larger than P.
Consider a wad of broken web material at an arbitrary location along the slot 60. The effective slot gap varies as the upper slot plate 58 moves back and forth. The wad is exposed to this variation in the slot gap. As the slot gap increases at this location, the wad gets drawn into the slot 60 by the air sucked into the dust collector 57. As the effective slot gap decreases, the wad becomes compressed or sheared. With repeated cycles, the wad is completely drawn into the slot 60.
The open area of the intake slot 60 remains constant and the airflow remains approximately constant as the upper slot plate 58 moves back and forth. Thus, the airflow balance of the entire dust control system remains unchanged.
The oscillation of the upper slot plate 58 can be constant or cycled on and off depending on the rate at which material accumulates on the slot 60. The rest position of the upper slot plate 58 is preferably at the mid-point of it's travel, as is shown in
A dry end enclosure according to one aspect of the invention will now be described with reference to
The dry end enclosure of the web transport system is an assembly of physical barriers, air curtains and controlled air flow patterns that effectively envelope the dry end of the machine to prevent the escape of dust.
The boundaries of this dry end enclosure are shown in
With several features as described in this document, the sheet run is enclosed. The dry end enclosure 61 is a control volume in which massflows and energy must be balanced.
Air curtains according to one aspect of the invention will now be described with reference to
Part of the dry end enclosure may be accomplished by of air curtains, as is shown in
In conventional dust control systems, air moves inwards to the machine near the edge of the web in an attempt to contain the dust. In the web transfer system according to the invention, the air moves outwards from the machine near the edge of the web.
An air curtain is a non-intrusive dynamic barrier between adjoining zones. A physical barrier such as a wall or strip curtain is perhaps lower cost but they inhibit the physical and visual access of the operators.
Air is introduced along the sides of the web or sheet run to stabilize the edges of the web or sheet 47 and contain the dust laden air to the dry end enclosure. Favourable air flow patterns are generated.
With the application of vertically downward linear air curtains that are oriented in the machine direction along the edges of the sheet run, two important results are achieved:
At first glance, one would think that the web 47 would get pushed away from the foil 7. This is not the case. The airflow patterns induced by the nozzles 62 in the area of the web 47 are as shown in
In areas that air curtains are not practical, enclosure panels are preferably used. Where air curtains and panels are both impractical, the system shall rely upon containment velocities of 100 to 150 fpm (0.5 to 0.75 m/s) to contain the dust laden air.
A creper lead-out foil 13 according to one aspect of the invention will now be described with reference to
The creper lead-out foil 13 is used to transfer the web from the creping doctor.
A cross-machine seal nozzle 64 in the up-stream end of the lead-out nozzle 13 is used to blow air onto the surface of the Yankee cylinder 1 such that the boundary layer air is removed from the Yankee cylinder 1 before the web 47 reaches the creping blade 65 of the creping doctor 21. This seal nozzle 64 is preferably of the self cleaning type described above in relation to
In the majority of cases, the dry end toe of the Yankee cylinder 1 will leak air.
This leaked air is hot, humid and contaminated with dust and products of combustion from the direct fired heating system of the Yankee Hood. It is important that this air be contained and removed. In addition to the normal boundary layer air that exits due to the moving surface, the hood may be pressurized. Every machine has different leakage rates as it depends on the design and balance of the Yankee Hood. This moisture laden air is normally carried with the top surface of the web. It usually results in condensation on the downwards-facing surfaces of the foils. This leads to accumulations of sheet additives or ‘stickies’ on the foil surfaces.
A common problem encountered in this area is edge instability. This is mostly caused by an inflow of air on the sides of the machine as the web 47 pumps air towards the dry end. This symptom is usually worsened by a pulper exhaust system. The lead-out foil 13 is preferably equipped with side air-curtain nozzles of the type described above in relation to
An exhaust module 10 acting as a dust collector having a cross-machine intake slot is located on the upstream end of the foil 68 that follows the creper lead-out foil 13 (see
Air is drawn in to the area 66 below the web 47 at the creping doctor 21. This is a result of the movement of the web 47 as it ‘pumps out’ air with it's boundary layer. In high speed machines, it may be an advantage to introduce low-velocity air below the web 47 at the creping doctor 21.
This first foil 13 should not be an active foil because the sheet tension must be relatively low at the creping doctor 21.
The lead-out foil 13 is equipped with a knock-down air shower 67. If the web 47 needs to be directed to a pulper 125 (see
The cross-machine seal nozzle 64 stops the boundary layer air that is carried with the Yankee cylinder 1. This allows the required wrap angle on this foil 13 to be less than that used on conventional creper lead-out foils. This results in decreased sheet tension downstream of the foil 13.
Pressurized air is used to induce an airflow from the edges of the Yankee cylinder and direct it into the pulper opening. The misting showers are used to ensure that the trim or Yankee fuzz is knocked down into the pulper.
As with conventional designs, a web or sheet support is required directly after the creping doctor lead-out foil. In the following, this foil will be referred to as the ‘second foil’ of the web transport system. A second foil 68 according to one aspect of the invention will now be described with reference to
This foil 68 is an important variable to be used in the design of a dry end. The geometry of this foil 68 is adjusted to suit the specific requirements of the machine. The leading edge of this foil is equipped with an exhaust module 10 that has a self-cleaning exhaust slot similar to the type described above in relation to
When there is a cut-off doctor 15, the second foil assembly must be retractable, as is shown in
A cross-machine knockdown air shower pipe 69 is located on the strong-back of the cut-off doctor 15. Favourable air currents are generated to draw the web 47 downwards when needed.
Fixed curtain panels 70 hang from the dry end toe of the Yankee Hood are close to the top end of the second foil 68 when the foil is retracted. This seals the area 70 next to the Yankee cylinder 1 to help contain dust while the web 47 is directed down to the pulper 125 (see
A calender lead-in according to one aspect of the invention will now be described with reference to
The majority of machines do not use a calender. For machines that do use a calender, special transition zones are incorporated in the foils to allow for transfer of the web into and out of the calender rolls while enclosing the web from above.
A calender lead-in foil 71 leads the web 47 into the gap formed between the counter-rotating calender rolls 72, 76 of the calender 3. At the entrance to the calender, there is a wrap on the lower calender roll 72. An air supply module in the form of an air curtain nozzle 12 producing a nip-flooding air jet is used to accomplish two main tasks.
An optional sheet spreading airfoil on the calender lead-in foil 71 may be used.
A dust collector 10 with a self-cleaning intake slot is strategically located at the downstream end of the web supporting foil 71 and removes dust laden air from the trailing edge of the foil 71.
A calender roll dust collector 16 and a calender roll air deflector 17 according to one aspect of the invention will now be described with reference to
The deflector 17 and the dust collector 16 are used on the wet end side of the lower calender roll 72. This is similar in design to the unit used at the reel drum (to be described below).
Boundary layer dust-laden air is normally pumped into the zone 73 between the moving calender roll 72 and the underside of the web 47.
The minimum required flowrate of air from this collector 16 for relief of the over-pressure in this zone 73, can be predicted with the following equation.
Q1=B1+B2
where:
B1=the volumetric flowrate rate of the boundary layer on the underside of the web 47.
B2=the volumetric flowrate rate of the boundary layer on the rotating lower calender roll 72.
The boundary layer airflows can be approximated using CFD analysis tools.
A scanner passage according to one aspect of the invention will now be described with reference to
The dust laden air is effectively contained above the web while it passes through the scanner 4.
The concept employed here is similar to the foil to foil transfer as described above in relation to
The web 47 is supported by a scanner lead-in foil 77 at the entrance of the scanner 4. The trailing edge of this foil 77 is close to a scanner head 80 as shown in
A scanner lead-out foil 78 is positioned downstream of the scanner 4. The foil 78 is sloped such that approximately 2 degrees of wrap is achieved to ensure good transfer. The leading edge of this foil 78 has a smooth leading edge. The exhaust slot 48 is located directly above this edge. The exhaust slot 48 is self-cleaning. The volumetric flow rate of the exhaust air is slightly higher than the volumetric flow rate of the air supply module 9. The web is not drawn in as the zone is balanced.
An enclosure panel 79 is oriented cross-machine and extends from the top of the dust collector 10 to the upper dry end face of the scanner frame. Adequate clearance to the moving scanner head is provided.
As the scanner head passes in the cross-machine direction, a portion of the supply nozzle 46 and a corresponding portion of the exhaust slot 48 are covered simultaneously. The exhaust slot self-cleaning mechanism is prevented from operating (with controls logic) while the scanner head is in the sheet run.
A reeling station 2 comprising a reel drum 87 according to one aspect of the invention will now be described with reference to
The area on the wet end side of the reel drum is a zone that is naturally pressurized due to the incoming boundary layer air that moves with the underside of the web 47 and the outer surface of the reel drum 87. The boundary layer air on the underside of the web 47 has a relatively high concentration of dust.
A deflector 19 is oriented cross-machine and is located on the wet end side of the reel drum 87. It is equipped with a wiper blade 88. The wiper blade 88 cuts off the boundary layer air that moves with the reel drum 87 and helps prevent the web 47 from ‘ballooning’ above the reel drum. The air is deflected downwards with the curved cross-machine deflector 19.
A reel drum air exhaust module 18 in the form of a dust collector is used to evacuate air from the area. If this air is not evacuated directly from this zone, the zone would be pressurized and it would squeeze the dust laden air out the sides of the machine. It would also cause the web 47 to ‘balloon’ as it is transferred to the reel drum 87.
The dust collector 18 is sized to suit the flowrate of boundary layer air that comes with the web 47 and the rotation of the reel drum 87.
The intake of the collector 18 may be the self-cleaning type described above in relation to
The minimum required flowrate of air from this collector 18 for relief of the over-pressure in this zone, can be predicted with the following equation.
Q1=B1+B2
where:
B1=the volumetric flowrate rate of the boundary layer on the underside of the web 47.
B2=the volumetric flowrate rate of the boundary layer on the rotating reel drum 87.
The boundary layer airflows can be approximated using CFD analysis tools.
Web or sheet pressurization at the reel drum according to one aspect of the invention will now be described with reference to
A sheet spreader 6 is located directly before the reel drum area. Air is introduced above the web 47 directly after the sheet spreader 6 and before a reel shield 101. This zone is covered by an enclosure 102 and is pressurized to help keep the web 47 tight to the surface of the reel drum 87.
To transfer the web from the sheet spreader 6 to the reel drum 87 while preventing ‘ballooning’ of the web, air is released at the trailing edge of the spreader 6. The geometry of the enclosure 102 helps create a pressurized zone or volume 103 above the web 47. Initially, the air has a low velocity and a positive static pressure. The air travels with the web 47 and passes through the restriction between the shield 101 and the reel drum 87. The air is released to the area above the reel drum 87. The static gauge pressure is zero where it is released and the dynamic pressure is positive.
Creating an over-pressure on top of the web 47 is the same as creating an under-pressure below the web 47. Importantly, there is a pressure differential that pushes the web 47 downwards. The pressure differential is easily adjustable by changing the air flowrate into the zone 103 or adjusting the gap between the shield 101 and the reel drum 87. The air is introduced at low velocity thus avoiding web stability issues.
Bernoulli's theorem (the conservation of energy) shows that the static pressure in the low velocity zone is converted to velocity pressure at the flow restriction where the air ultimately exits the zone. The static pressure is essentially zero at this location. ‘Static pressure’ is a term that actually means static gauge pressure. Ambient air has a ‘static pressure’ of zero because this is the datum or reference pressure.
A reel shield with a dust collector and a nip flooding air curtain according to one aspect of the invention will now be described with reference to
Favourable airflow patterns are generated in the reel drum area with the use of air curtains 104, a dust collector 105 and enclosure panels 105. The special combination will achieve a stable web that is wrapped cleanly while dust is contained. The equipment is not intrusive on the normal mechanical motions that occur at the reel.
The reel shield 101 extends across the machine and is oriented vertically above the reel drum 87. An air exhaust module in the form of an area dust collector 105 is attached to the top of the reel shield 101. Preferably, the dust collector 105 comprises a self-cleaning slot as described in connection with
The nip flooding air jet concept is employed in a similar fashion as used with the entrance to the calender. An air supply module in the form of a nip flooding or air curtain nozzle 12 is located above the reel drum 87. This linear free jet is oriented cross-machine and is directed at the roll 107.
The nip flooding air nozzle 12 is preferably fitted with a self cleaning mechanism as described above in relation to
The air curtain nozzle 12 air floods the nip 110 across the full width of the web. The majority of the air that enters the nip originates from the nozzle 12 and the boundary layer air of the jumbo roll 107. When this air enters the converging nip 110, it is rejected opposite the direction of the web as it passes over the reel drum 87. Particulate and small pieces of broke that may be carried with the top of the web over the reel drum 87 are rejected by the strong air currents that exit the closing nip 110.
An intense rotational flow field is generated in the zone bounded by the air curtain 104, the top of the reel drum 87, and the reel shield 101.
The air flows in this zone are balanced to achieve containment of dust and debris.
Q1=Minimum required exhaust flow rate from the reel shield collector 105
S1=Supply flow from reel above-sheet pressurization
S2=Supply flow from the nip flooding air nozzle 12
B1=Boundary layer flow rate on full jumbo roll 107
C1=Air flow required at tending side and drive side for containment=Area×Containment Velocity
Q1=S1+S2+B1+C1
The boundary layer airflows can be approximated using CFD analysis tools.
The reel collector 105 has a relatively large exhaust volumetric flowrate and it has a large intake slot gap of 3 inches (76 mm). It is capable of drawing in large amounts of broke. A broke trap is used downstream of this collector to separate the large pieces of broke and deliver them to the pulper.
A lower reel drum air curtain according to one aspect of the invention will now be described with reference to
A lower reel drum air curtain nozzle 117 is used to contain dust that would otherwise get carried out with the boundary layer air of the roll 107. This nozzle 117 can also clean the web immediately after it has been rolled up. It blows air against the roll 107 immediately after it touches the reel drum 87. The nozzle 117 is a fixed cross-machine nozzle and preferably comprises a self cleaning mechanism as described above in relation to
A soft curtain 118 extends down from the nozzle plenum to the floor. This curtain is made of former fabric or similar material. It allows for the passage of broke which is normally pushed into the pulper opening. If there is no pulper opening below the reel, then rigid panels are used for the curtain below the nozzle 117.
As an option, this nozzle 117 can be actuated if deemed necessary. It could be actuated such that it follows the varying jumbo roll 107 diameter as it is wound. With the nozzle 117 close to the surface of the roll 107 at all times, the web is cleaned. This is the best time to clean the sheet.
A floor flush system according to one aspect of the invention will now be described with reference to
After knocking the sheet into the pulper 125 at the cut-off doctor 15 or creping doctor 21, and prior to re-establishing the sheet to the reel, a clean-up is required.
The web transport system according to the invention is designed such that there are very few locations where broke can hang-up or accumulate below the sheet run.
In the event of a sheet break, the majority of the broke falls on the floor below the web.
Compressed-air blow-pipes (not shown) can be installed to help ensure that all broke goes to the floor.
The floor 119 is sloped towards the pulper opening(s) 120. The floor area is curbed along the sides of the machine near the sole plates.
A cross-machine spray header pipe 121 is located on the floor 119 and oriented cross-machine. Water is released from this pipe 121 with a low velocity and hits a curved plate 122. The water ‘fans-out’ and produces a uniform flow of water on the floor 119 in the machine direction. This shower can wash across the entire machine width of the floor. The water flowrate is sufficient to wash the broke down into the pulper 125 (see
The entire process of broke disposal and dry end clean-up may be automated.
The floor 119 can be sloped to one or more other broke chute openings 120.
It is a function of existing machine geometries.
A pulper enclosure 127 according to one aspect of the invention will now be described with reference to
The escape of mist and hot humid air from the pulper 125 into the enclosed zone below the web, i.e. into the dry end enclosure 61 (see
This aspect of the innovation allows for the containment of the ‘bubble’ of hot humid air and mist in the pulper 125. The exhaust, to be directed outside of the building, is much less than that of a conventional system.
Normally, the pulper 125 has up to three broke chute openings 120. One is located below the creping doctor 21 and optional openings 120 are under the calender 3 and under the reeling station 2. Sometimes, an additional opening (not shown) is included beside the machine for manual disposal of broke.
A pulper module 20 is positioned at the floor opening below the creping doctor 21.
Preferably, pulper doors 124 are arranged on all broke chute openings 120 with the exception of the opening below the Yankee cylinder 1, where an air-curtain 128 (see
This system allows for complete control of the pulper's mist and vapour.
A pulper module 20 according to another aspect of the invention will now be described with reference to
The pulper module 20 seals the pulper 125 at the operating floor below the creping doctor 21 while allowing for the occasional passage of broke.
A cross-machine air curtain 128 is used at the pulper floor opening. The air curtain nozzle 129 is directed slightly downward from horizontal. During production, the air curtain nozzle 129 is in operation. It accomplishes two tasks:
A cross-machine opening 130 on the dry end side of the pulper module 20 allows for the passage of broke from the floor flush system as described above in relation to
When the web is directed into the pulper at full machine speed, it carries a boundary layer flow of air with it. In this situation, the supply air to the nozzle 129 would be temporarily halted using a divert damper (not shown).
The air curtain nozzle 129 could have water mist introduced internally to ensure that it remains clean in situations where the supply air is not very clean. A mechanical self-cleaning nozzle design, e.g. of the type shown above in relation to
An air exhaust module in the form of a cross-machine pulper exhaust header or collector 131 is located above the operating floor. An array of vertically oriented intake or pick-up tubes 132 extend from the header 131 and into the top of the pulper 125 adjacent the floor opening 130.
Each of the intake tubes 132 have a concentric tapered section at it's intake 134 located at the bottom end of the tube 132 (see
During normal operating conditions, water is prohibited from spilling from the intake 134 (and into the pulper) due to the high magnitude of the vertically upwards velocity of the air at the intake 134.
In the event of the occurrence of a blockage at the intake 134 due to the attempted entrance of broke or other materials, the water is no longer prevented from spilling into the pulper 125. The water accumulates above the blockage until there is sufficient weight to dislodge the materials and cause them to drop into the pulper 125. The blockage either gets drawn into the collector 131 or it must drop into the pulper.
If fibre accumulates on the rim (edge) of the intakes 134, water will accumulate along the edge and wet the materials causing them to drop.
Inspection ports (not shown) are included in the main cross-machine header 131. They are located on top of the header concentrically above each tube 132.
A pulper door according to one aspect of the invention will now be described with reference to
In applications where there is more than one pulper opening, and where these openings are used primarily for manual broke disposal-and clean-ups, a gravity operated pulper door 135 is preferably used.
The hinged door 135 is made of light-weight stainless steel sheet metal. It is held closed by gravity but it easily opens to allow for the passage of wetted broke 136. It functions like a check-valve. A flushing shower 137 preferably having a manual valve is needed to wet the broke and ensure that it moves through the door. A manual button and a controls system timer can be used to give a few minutes of flushing with a control valve. Alternatively, water from a floor flush system would suffice.
In large cross-machine openings such as those below the reel or below the calender, the axis of rotation for the door is preferably oriented cross-machine.
A sloped panel 138 may be added as shown in
Various types of actuated doors have been used in the past, however gravity has proven to be more reliable than actuators.
An air exhaust module in the form of a roll dust collector according to one aspect of the invention will now be described with reference to
A moving web carries air with it. When a web is wrapped around a roll (such as a turning roll or a spreading roll), some of the boundary layer air gets trapped between the web and the roll. This phenomena reduces traction.
The roll dust collector according to the invention has been devised to capture dust at it's source.
The roll dust collector improves traction while removing the dust laden boundary layer air.
The roll dust collector 199 (see
The intake slot 200 has an out-going nip. Broke which may land on the intake slot, is dislodged due to the movement of the roll 196. If the exhaust flow is temporarily halted with a divert damper, the air will flow out of the slot due to the pumping action of the rotating roll.
The roll dust collector housing 201 is sealed to the roll with a sealing device in the form of a doctor blade 202. The doctor blade is a light-weight cleaning doctor comprised of plastic or composite material. It is lightly loaded against the roll. As an alternative, the sealing device can be made with an airknife.
Inside the roll dust collector 199, an internal vortex airflow pattern is generated because of the internal moving surface of the roll 196. As with conventional dry type dust collectors, the internal vortex serves to help maintain internal cleanliness.
Bridging of the intake slot 200 by accumulated dust particulate is impossible due to the movement of one side of the intake slot, i.e. the roll surface. Thus, small intake gaps are feasible.
It is known by experience that in tissue/towel grade dust control applications, slot intake gaps smaller than ½″ (12.6 mm) are normally very susceptible to pluggage and bridging due to particulate accumulations. It is also known that a minimum required intake velocity of 4,000 ft/min (20.3 m/s) is necessary to minimize the likelihood of plugging. In many cases, exhaust flowrates are dictated by the minimum required intake velocity and the minimum required slot gap. In these cases, the capacity required is in excess of actual capacity required to control dust. With the roll dust collector 199, slot gaps as small as ¼″ (6.3 mm) can be achieved. Thus the exhaust flowrate capacity can be as much as 50% lower than that of a conventional dust collector.
The volumetric flowrate is determined by predicting the volumetric flowrate of air carried with the boundary layer of the moving web plus any supplemental airflows introduced to the zone upstream of the roll.
The roll dust collector according to the invention can be applied to almost any in-going nip where a web wraps a roll. It improves traction and contains dust.
As an example of a roll dust collector application,
Upstream of the spreader 203 and roll dust collector 199, an air supply module 9 of the type previously described is used to introduce supplemental air to the zone.
Dust from the saw blade slitters is contained and the web traction is improved thus improving the spreading capacity of the roll.
In some tissue machines, foil plates are not used and the web is almost fully supported by turning rolls and spreading rolls in the dry end of the machine.
The dry end comprises four rolls 206, each being equipped with a roll dust collector 199. Airflows to the roll dust collector intakes can be primarily boundary layer air or it can be supplemented with low velocity air supply modules that serve as containment curtains. External blowing nozzles can be added to maintain cleanliness of the roll dust collector doctor blades.
There are many turning rolls and spreading rolls used in unwinders/re-winders and also in grade re-winders. It is understood that the roll dust collector according to the invention can be applied to many of the rolls in such apparatus.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP09/67794 | 12/22/2009 | WO | 00 | 2/22/2010 |
