Patent Application
20070042884
This invention relates to heated rolls used in web processing operations such as: calendering; drying; laminating; embossing; pre-heating; corrugating; curing; heat-setting, shrinking; bonding; etc. It has particular application to roll surface materials responsive to high temperature induction heating, where energy losses associated with conventional, indirect heating systems can be significantly reduced.
The surface layer of most heated rolls is typically thick-walled and made of a ferrous alloy, while other more specialized, usually unheated rolls use other metals (e.g. aluminum) as well as non-metal materials such as granite. Heated rolls are typically heated internally, using hot water, hot oil, or steam, and may also be heated externally using steam jets, gas flames, hot air impingement, infra-red radiation, or magnetic induction. The depth of the surface layer of conventional heated rolls is typically greater than is necessary for the application at hand. While this may be due to strength considerations, it is often due to a lack of appreciation of the how this depth affects the process response time, energy consumption, and required heating system size. The applied heat may also be free to migrate in the cross-direction through the roll's thick outer wall at a rate higher than is optimal for the specific application. Or, conversely, the relatively low thermal conductivity of ferrous metals may limit the lateral heat conduction to a rate lower than is optimal for a specific application. The following are examples of these various situations;
Web Calendering Applications
In some continuous sheet producing industries (such as papermaking and metal sheet or foil manufacturing), in one of the final steps, commonly referred to as calendering, the web is passed between heavy, stiff rolls that are loaded against one another to compress the web and make it more dimensionally uniform. Calendering rolls are typically thick-walled and made of a ferrous alloy and are most commonly heated by a hot internal fluid flow, using hot water, hot oil or steam. The historical use of internal heating fluids, and the need for high roll stiffness during calendering has resulted in conventional heated calendering rolls being relatively thick-walled (typically with wall thickness of more than one inch, or solid throughout), with elaborate and expensive cross-bored fluid channels and rotating seals. Furthermore, conventional heated calendering rolls are made of an essentially homogeneous, moderately thermally-conductive material, such as steel, throughout their full wall thickness, to permit unimpeded heat conduction from the inside to the outside. The external fluid heating systems that accompany conventional heated calendering rolls are also relatively inefficient due to piping circuit heat losses and energy conversions losses (their original source of energy is often natural gas or heating oil, requiring energy conversion with attendant combustion and heat exchanger inefficiencies). Requisite external fluid temperature control systems must also be able to cool the fluid (to enable precise roll temperature control, and to remove the heat quickly during stoppages), which adds to the overall complexity and cost.
A primary limitation of conventional heated calendering rolls is that they are relatively thermally-conductive throughout their substantial depth, requiring that their whole mass be heated up to attain a desired outer surface temperature. Furthermore, the large mass of internal fluid used by conventional heated calendering rolls also has to be heated before it can heat the roll, or cooled before it can cool the roll. The large roll and fluid mass, and the long radial conductive path throughout the depth of the roll, thus produces a large thermal inertia that reduces the response time of roll temperature controls to process disturbances on continuous operations, and which furthermore thus imposes a severe setup and change-over delay on discontinuous, batch-type calendering processes. A further consequence of this large thermal inertia is that external fluid cooling and heating systems must be significantly over-sized to cool and heat the rolls fast enough before and after stoppages. Over-sizing these fluid-cooling and heating systems beyond what steady-state conditions require adds significantly to their size, complexity and initial cost, and further aggravates their inherent energy inefficiencies. A further limitation of conventional heated calendering rolls is that they are homogenous across their width, and the internal fluid paths are continuous across their entire width, making it impossible to locally heat just one cross-direction region of the roll. Even if a localized external heating means, such as magnetic induction or forced-convection heating using hot air impingement, is used to heat just one cross-direction region of the roll, the applied heat will conduct laterally through the roll, diminishing the localized effect.
Web Drying Applications
While there are many types of webs that are dried using heated rolls or heated metal cylinders, a typical one is a wet paper web, where in the case of paper manufacturing, it is upstream of the calendering process described above, and the web is typically first dried by passing it around steam-heated metal cylinders that are typically referred to as dryer cans. To permit heat conduction from the inside to the outside, dryer cans are usually made of a ferrous material such as cast iron, and are usually relatively thick-walled to support their own weight and that of the internal condensate, and to withstand the internal steam pressure.
The response time and cross-direction heat migration limitations mentioned above for web calendering applications apply equally to web drying applications.
Web Converting Applications
In certain web converting processes, such as web embossing, web laminating, or paper corrugating, a common manufacturing step is to pass individual web layers around one or more heated, ferrous, relatively thick-walled preheating cans or rolls to dry and pre-heat the layers before they are embossed or laminated together. The two or more rolls that form the embossing or laminating nips through which the web passes are also typically heated.
In embossing applications the purpose may be to thermally soften the web to make it more malleable. On laminating applications the purpose may be to preheat and dry the individual layers after rewetting or coating to reduce or prevent curl of the final laminated product, or to simply add heat to the process to facilitate intra-layer bonding during laminating. On all laminating and embossing applications it is generally desirable to ensure a very uniform (i.e. +/−1% of process) roll surface temperature profile across the width of the process. If the roll temperature is not sufficiently uniform it may impart a non-uniform temperature profile to the web(s) before or during laminating or embossing. This in turn may affect the web's localized compressibility, malleability, and dimensional stability, leading to variable finished product quality. While on high throughput web manufacturing operations such as papermaking it is cost effective to measure and control web properties in narrow zones across the width, such investments are usually not viable on converting applications that are typically much narrower and slower. Consequently, narrow zone control of effective roll heating means such as external magnetic induction, is often not commercially viable on converting applications. The common solution is then to use enhanced internal heat conduction means, such as helical fluid channels or phase change heat pipes, to promote lateral heat conduction and minimize lateral temperature variations. Unfortunately, such rolls are relatively expensive to build, they often require fluid connections, and they typically require a significant internal structural mass, which along with the internal fluid itself, adds to their thermal inertia to lengthen their heat-up response.
In paper corrugating applications the purpose is typically to heat and dry the individual paper layers in a controlled manner before gluing them together. Controlled drying of the outer paper layers during corrugating prevents or reduces warp of the combined final product, while controlled heating of both the inner and outer paper layers supplies the precise amount of process heat to gel, but not prematurely crystallize, the starch-based adhesive.
The response time and cross-direction heat migration limitations mentioned above for web calendering applications apply equally to web converting applications, especially in that preheating cans are homogenous across their width, with a single internal steam chamber, making it difficult to locally heat just one lateral region of the preheater can to localize the drying or heating effect in a given lateral region of the web.
Web Curing and Heat-Setting Applications
In certain other web producing and handling industries, many of which are discontinuous, batch-type processes (such as for bonded nylon fabric production for mesh or textile manufacturing), a common manufacturing step is to pass the web around one or more internally-heated metal rolls to dry and cure the material, to achieve a final target moisture, set and cure adhesive bonds, and shrink the web to its final, stable dimensions.
As with the previously described heated calendering applications, the historical use of internal fluids has resulted in conventional curing/heat-setting rolls being relatively heavy-walled with elaborate and costly internal fluid channels and rotating seals. These curing/heat-setting rolls are also made of a homogeneous, thermally-conductive material, such as steel, throughout their depth, and are also typically accompanied by an expensive and over-sized external fluid cooling and heating system to quickly remove and add heat before and after stoppages. Curing/heat-setting rolls are also homogenous across their width, and the internal fluid paths are continuous across their entire width, making it impossible to locally heat just one cross-direction region of the roll. This has a unique, negative implication on curing/heat-setting applications. Web shrinkage is typically a somewhat non-uniform phenomenon, occurring more freely and uniformly at the edges, and less easily and uniformly near the center of the web due to friction between it and contacting machine elements, such as rolls. This cross-direction non-uniformity often produces wrinkles in the web as it shrinks, which in turn may have a deleterious effect on the final quality of the web. If the curing/heat-setting roll could be heated in the center first, and then the heating application gradually broadened out toward the edges at an optimum, controlled rate, web wrinkling problems could perhaps be reduced or eliminated.
The above examples clearly illustrate that on many web manufacturing and converting applications the design of conventional heated rolls, and their method of heating, imposes significant limitations, and that these limitations will apply equally or in part to other web applications involving heated rolls.
Induction Heating
As mentioned above, and as disclosed in U.S. Pat. No. 4,384,514, ferrous metal rolls used on web manufacturing and converting applications can be externally heated by magnetic induction, whether or not the roll is also simultaneously heated by an internal heated fluid flow. Recent advancements in induction heating technology permit very reliable operation at high power densities (>50 kW power transmission/meter of roll width), to enable very reliable and efficient, rapid external heating of the surfaces of ferrous metal rolls to much higher temperatures than was previously attainable. Unfortunately the above-described conventional rolls cannot fully exploit the benefits of this new induction heating technology.
The heavy mass of conventional steel rolls or cans, the heat capacity of their thick steel walls, and the volume and heat capacity of internal cooling and heating fluid, all add to the substantial thermal inertia of these systems. Even though state-of-the-art induction technology heats just the surface region of a steel roll, where the thermal energy is actually needed by the process, that heat must unfortunately migrate into the roll and internal fluid, and heat up the entire combined mass before the surface temperature can be stabilized at a target value.
It is therefore an object of the present invention to provide a new apparatus using any one of a family of rolls, all consisting of a relatively thin outer layer that can be rapidly heated by any of the various external heating modes mentioned above, and a supportive core structure that will not easily absorb or conduct heat, that is fabricated of lighter-weight material with a much lower specific heat and thermal conductivity.
Another object of the present invention is to provide heated rolls in which the supportive core structure is fabricated of a material with a relatively high thermal conductivity, such as aluminum.
Still another object of the present invention is to provide heated rolls fabricated of a single contiguous material that is both responsive to eternal magnetic heating and also relatively lightweight.
A still further object of the present invention is to provide heated rolls that are of minimal diameter so that its surface and end wall heat losses are minimized, thereby allowing it to be heated to a higher temperature.
Another object of the present invention is to provide heated rolls that have a relatively lower thermal inertia than conventional rolls presently used for the same purposes, so that substantially less power will be needed to heat their surfaces to a given temperature in a given time period, and then maintain it there.
Yet another object of the present invention is to provide rolls with minimal thermal mass to facilitate more rapid cooling (which will be particularly advantageous on discontinuous, batch-type processes) by simple means, such as an external, inexpensive, forced-air convection cooling plenum, thereby eliminating the need for complex and expensive internal fluid cooling systems.
A further object of the present invention is to provide composite rolls that can be segmented in the cross-direction dimension to allow adjacent external heating elements to selectively heat given regions of the roll.
A web processing apparatus of the present invention includes (A) a roll consisting of (i) a composite annular structure, having a thin outer shell or sleeve, typically less than a ¼ inch thick, made of a first material capable of being heated to the desired temperature by a known external heating method, and an inner, typically thicker sleeve or core, made of a second material that is highly non-conductive in both electrical and thermal respects and able to withstand said desired temperature; said composite annular structure being suitably constructed and reinforced so that the exterior surface of the outer sleeve is exposed, and the outer sleeve and inner sleeve or core are adequately anchored to one another, enabling the composite annular structure to perform the desired function effectively and reliably; and (ii) a final suitable structure supporting said composite annular structure to permit mounting and rotation of the roll; (B) a device for externally heating the outer sleeve of the composite roll.
In another embodiment of the present invention the inner sleeve or core of the web processing apparatus is made of a second material that is highly thermally conductive and able to withstand said desired temperature. The web processing apparatus could also include (A) an optimized roll manufactured from a contiguous material such as carbon-fiber composite, which thereby has a significantly lower thermal mass, higher strength-to-weight ratio, and higher surface emissivity than conventional rolls manufactured from ferrous alloys, and (B) a device for externally or internally heating such optimized roll.
In another embodiment of the present invention, the web processing apparatus includes (A) an optimized roll manufactured from a material responsive to heating by external magnetic induction (such as steel or carbon-fiber composite), that has a minimal outside diameter (typically <12″ diameter) in order to minimize its thermal mass, so that it can be heated by a given induction-generated heat input (typically >20 kW/meter roll width) to a higher temperature (typically >>150° C.) than would be possible with a larger, conventional heated roll, and (B) an external magnetic induction heating device with a power output (typically >20 kW/meter roll width) that is sufficient to heat the roll of the present invention to a relatively high temperature (typically >>150° C.).
The optimized heated roll apparatus of the present invention may be beneficially applied and heated by any external (i.e. steam jets, gas flames, hot air impingement, and infra-red radiation) or internal method (i.e. internal magnetic induction, or hot fluids such as hot water, hot oil or steam). However, the various embodiments described herein of the present invention are particularly suited to heating by external magnetic induction, which is typically simpler to apply and/or more energy efficient than other potential heating methods.
The optimized heated roll apparatus of the present invention is generally designed to enable faster, more controllable heating, to a higher temperature, with lower energy expenditure, and cooled more quickly with a simpler cooling system, than conventional rolls that are predominantly manufactured from ferrous alloys. The preferred embodiments of the present invention therefore each provide some combination of the following core advantages;
Specific applications where the various embodiments of the present invention would apply include numerous continuous and batch-type web manufacturing processes, such as:
While the embodiments of the present invention apply particularly well to the applications listed above, they will apply equally or in part to other web manufacturing applications involving heated rolls.
Having thus generally described the nature of the invention, reference will now be made to the accompanying drawings that schematically illustrate embodiments thereof.
a is a sectional-view of a length-wise portion of the embodiment of
b is an elevational view of the embodiment shown in
a is a cross-sectional view of a preferred arrangement of an alternate embodiment of the present invention, showing an optimized roll and an adjacent magnetic induction heating device, wherein the roll is manufactured from a relatively thin outer layer consisting of a material that is particularly responsive to external induction, such as steel or carbon-fiber composite, and a relatively thick inner layer of a material that is less dense and that has a relatively high thermal conductivity.
b is a sectional plan-view of the embodiment shown in
a is a graph illustrating the steady-state surface temperature produced by a variable heat input profile, derived using a finite element analysis of a composite roll consisting of a thin outer ferrous layer and a thicker inner aluminum layer, of the type shown in
b is a graph illustrating the steady-state surface temperature produced by the same variable heat input profile, derived using the same finite element analysis, of a steel roll with the same outside and inside diameter as the composite roll analyzed in
c is a graph illustrating the steady-state surface temperature produced by the same variable heat input profile, derived using the same finite element analysis, of a steel roll with the same outside diameter and weight as the composite roll analyzed in
a is a cross-sectional view of a another embodiment of the present invention, showing an optimized roll manufactured of a suitable contiguous material such as carbon-fiber composite, a web in contact with said roll, and a suitable, adjacent external heating device such as a sectionalized magnetic induction actuator.
b is a plan-view of the arrangement shown in
a is a cross-sectional view of embodiment of the present invention, showing an optimized, small diameter roll manufactured from a suitable material such as steel or carbon-fiber composite, a web in contact with said roll, and an adjacent magnetic induction heating device.
b is a plan-view of the embodiment shown in
Referring to
Referring again to
As mentioned above, those skilled in the art of manufacturing rolls could provide other ways to achieve the desired, annular, composite roll structure, and to design and manufacture an additional supportive structure to permit mounting and rotation of the roll.
Referring still to
Referring now to
As in the case of the previous arrangement shown in
The ability to localize the external heating effect, whether the heat is generated by induction or other means, could be quite beneficial on paper drying applications, to facilitate cross-directional profiling of the paper's moisture content. This ability could also be quite beneficial on laminating and corrugating applications, to facilitate cross-directional profiling of the moisture and temperature of the incoming paper layers, so as to maximize the flatness, bond strength and dimensional stability of the final combined laminate or board. Furthermore, this ability could be extremely beneficial on some web curing/heat-setting applications where it may be preferable to start heating in the center of the machine and progress outwards at a controlled rate, to shrink the web symmetrically, and from the center outward, to minimize or eliminate the formation of wrinkles.
Referring now to
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Referring now to
As described above with respect to other arrangements, the individual disks 17 could also be lined up with segmented, external heating means, such as individual inductors, to facilitate zonal temperature control, and/or inclined to prevent exposure of the web to continuous “cold” seams between adjacent disks 17.
One could also modify the foregoing arrangements to make effective and responsive use of internal heating, by locating fluid-carrying chambers or channels between a first, relatively thin, thermally-conductive outer sleeve, and a thicker, thermally-insulating layer below it. To ensure heat transfers easily, and essentially solely, to the outer sleeve, the fluid-carrying chambers or channels could be in direct thermal contact with the underside of the outer sleeve, while being otherwise surrounded and embedded within the inner, thermally-insulating layer below, prior to connecting at the end of the roll to an external source of steam, hot water, or hot oil.
Referring now to
A finite difference model that assumes the conventional roll dimensions noted above, and accounts for induction heat input, convection and radiation losses to ambient from the surface of the roll 100, and contact conduction losses to the fabric being cured/heat-set, and which also assumes the fluid channels are empty and filled with air, produces the results plotted in
The same finite difference model, when applied to either of the arrangements shown in
Referring to
The outer, typically ferrous layer 43 can be formed by suitable means such as either spray or plasma coating of steel 43 onto the underlying, typically aluminum substrate 44, or by shrink-fitting a thin, typically steel tube 43 around a heavier-walled, typically aluminum core 44.
The roll's outer ferrous layer is typically heated by an external magnetic induction heating device 45. Said magnetic induction heating device 45 may be sectionalized, as shown in
Referring now to
Arrangement “X”: An 8 inch outside diameter static roll, with surface emissivity 0.3, in a surrounding 70° F. environment, comprising a 0.020 inch thick steel outer surface layer and a 0.313 inch thick inner aluminum layer, and weighing 35 lbs/meter, heated to 300° F. by an average heat input rate of 3.6 kW/meter, that varies across the width by +/−5%.
Arrangement “Y”: The scenario of Arrangement “X”, but where the roll comprises a 0.020 inch thick steel outer surface layer and a 0.313 inch thick inner steel layer, and weighs 90 lbs/meter.
Arrangement “Z”: The scenario of Arrangement “X”, but where the roll comprises a 0.020 inch thick steel outer surface layer and a 0.106 inch thick inner steel layer, and weighs 35 lbs/meter.
Referring again to
Referring to
Referring now to
Referring still to
Referring now to both
The ability to localize the external heating effect, whether the heat is generated by induction or other means, could be quite beneficial on paper drying applications, to facilitate cross-directional profiling of the paper's moisture content. This ability could also be quite beneficial on laminating and corrugating applications, to facilitate cross-directional profiling of the moisture and temperature of the incoming paper layers, so as to maximize the flatness, bond strength and dimensional stability of the final combined laminate or board. Furthermore, this ability could be extremely beneficial on some web curing/heat-setting applications where it may be preferable to start heating in the center of the machine and progress outwards at a controlled rate, to shrink the web symmetrically, and from the center outward, to minimize or eliminate the formation of wrinkles.
Referring now to
Referring now to
Referring now to both
Although suitable materials from which to manufacture the various layers of the embodiments of the present invention would be steel, ceramic, carbon-fiber composite, and aluminum, those skilled in the arts of advanced material science and/or roll manufacturing may identify other suitable materials that will satisfy the objectives of the present invention, and which would fall within the scope of the present invention.
While the foregoing invention has been described with respect to its preferred embodiments, various alterations and modifications are likely to occur to those skilled in the art. All such alterations and modifications are intended to fall within the scope of the appended clause.
| Number | Date | Country | |
|---|---|---|---|
| 60818739 | Jul 2006 | US | |
| 60710069 | Aug 2005 | US | |
| 60710110 | Aug 2005 | US |
