“Through air technology” is a term used to describe systems and methods enabling the flow of heated air through a nonwoven web for the purpose of drying or bonding fibers or filaments. Examples include the drying of nonwoven products (e.g., tea bags and specialty papers); drying and curing of fiberglass mat, filter paper, and resin-treated nonwovens; thermobonding and drying of spunbond nonwovens; drying hydroentangled webs; thermobonding geotextiles with or without bicomponent fibers; drying and curing interlining grades; and thermobonding absorbent cores with fusible binder fibers. The drying of tissue paper is also a particularly important application of through air technology. Systems and methods related to through air drying are commonly referred to through the use of the “TAD” acronym.
The present disclosure is applicable to the genus of through air technology systems (including dryers and bonders) and methods. As used herein, “TAD” may refer to “through air drying” or “through air dryer” depending on the context in which it is used. As used herein, “TAB” may refer to “through air bonding” or “through air bonder” depending on the context in which it is used.
A significant challenge relating to TAD/TAB systems is the introduction of large quantities of energy (e.g., 1 to 60 MW) into a TAD/TAB system without compromising performance, controllability, and reliability, enlargement of the TAD/TAB system, pressure drop, air mixing, turndown, and achieving target air temperature to a TAD/TAB from heat exchange devices.
For through air technology systems, the air temperature that passes through the material to be dried or bonded may need to be uniform, sometimes with less than about +/−1° C. variation. This level of temperature uniformity may be required to achieve uniform bonding or drying at the material's edges and across the full width of the material to meet process or product quality requirements.
The width of the material to be dried may be greater than 6 meters at times. Uniformity can be difficult to achieve over such a span. Even with uniform heating, good mixing, and insulated ducting, there is often cooler air in a boundary layer(s) of the air due to heat loss experienced while the air passes through ducting.
The present disclosure provides techniques for heating one or more boundary layers of air in ducting to keep the boundary layer(s) at or near a desired temperature for drying or bonding material. The present disclosure also provides techniques for heating some area of the boundary layer(s) more than others to overcome heat losses in the system.
Electrical heating tape(s) may be placed proximate to or coupled to an outside of a duct wall inside skin. The heating tape(s) may cover about 50% to about 100% of some of the duct walls depending on output (watts/in2) of one or more heating elements used by the system. The heating tape(s) may be implemented based on locations of anticipated heat losses and the system's duct arrangement.
An aspect of the present disclosure relates to a system including a fan, an air heater, a mixing element, ducting, and a hood. The air heater heats air received from the fan to produce first heated air. A mixing element operates on the first heated air to generate second heated air of a desired uniform temperature distribution. Ducting, coupled to the mixing element, includes at least one heating tape located proximate to two parallel walls of the ducting. The at least one heating tape is selectively operated to compensate for heat loss experienced by the second heated air while traveling through the ducting. A hood, including an air inlet, couples to the ducting at the air inlet. The hood surrounds a foraminous cylinder. The foraminous cylinder provides an air outlet that is in fluidic communication with the fan.
Another aspect of the present disclosure relates to a method including outputting unheated air from a fan. The unheated air is manipulated to produce heated air having a desired uniform temperature distribution. The heated air is sent, through ducting, to an air inlet of a hood. At least one heating tape, located proximate to two parallel walls of the ducting, is used to compensate for heat loss experienced by the heated air while traveling through the ducting. The heated air is communicated to a material on a foraminous cylinder in the hood. The heated air becomes unheated air as it dries or bonds the material. This unheated air is then circulated to the fan.
A further aspect of the present disclosure relates to a system including ducting, a hood, and a foraminous cylinder. The ducting receives heated air. The ducting has at least one heating tape located proximate to two opposing walls of the ducting. The at least one heating tape is selectively operated to compensate for heat loss experienced by the heated air while traveling through the ducting. The hood receives the heated air from the ducting. The hood at least partially surrounds the foraminous cylinder. The foraminous cylinder moves material through the hood, with the material being contacted by the heated air.
While the present disclosure is described with respect to through air systems including dryers and bonders, other systems may be used, such as Yankee air systems, flatbed dryers, floater dryers, and other dryers and ovens.
For a more complete understanding of the present disclosure, reference is now made to the following description taken in conjunction with the accompanying drawings.
Certain systems include a TAD/TAB having a hood and foraminous cylinder that are wider than a width of the material being dried or bonded. Such a configuration allows for cooler air, in the boundary layer(s) of heated air, to mostly bypass the edge of the material. This results in a substantial amount of the system's energy being wasted. Certain systems may also or alternatively include ducting specially designed to minimize a heat transfer path, which minimizes the decrease in temperature of the boundary layer.
Certain systems add bypass air into the airflow. Adding bypass air adds to the width of the TAD/TAB. Such may be unbeneficial because it requires increased space, capital cost, and energy consumption. Systems adding bypass air into the airflow have been used by Valmet, Inc. for more than two decades.
The present disclosure improves upon such systems by using heating tape(s) to ensure the boundary layer(s) of air input to a TAD/TAB is at a desired temperature for drying or bonding material. Heating tape(s) is placed at particular locations of ducting and selectively operated (e.g. activated) to maintain air traveling through the ducting at a desired uniform temperature distribution. Heat generated by the heating tape(s) may be used to counteract heat loss experienced by air traveling through the ducting.
The TAD/TAB system may include a TAD/TAB 100 including a foraminous (e.g., porous) cylinder 104 at least partially surrounded by a hood 106, a main fan(s) 108, an air heater(s) 110, and a mixer(s) 112. A width of the hood 106 may be commensurate with a width of material, to be dried or bonded, moved along the foraminous cylinder 104. While only one main fan 108, one air heater 110, and one mixer 112 are illustrated, one skilled in the art will appreciate that the TAD/TAB system may include more than one main fan 108, more than one air heater 110, and/or more than one mixer 112.
Material to be dried or bonded is carried along the foraminous cylinder 104 through the hood 106. Heated air of a desired uniform temperature distribution is input to the hood 106 and exposed to the material to be dried or bonded. Air is cooler after it travels through the material than it was when it first contacted the material. The cooled air travels through holes in the foraminous cylinder 104 and is output from the TAD/TAB 100 as cooled (or exhaust) air.
At least some of the cooled air output from the TAD/TAB 100 may be recirculated to the TAD/TAB 100. As illustrated, cooled air that is output from the TAD/TAB 100 may be passed through the main fan 108 to the air heater 110. The air heater 110 may heat the cooled air via combustion of fossil fuels. The air heater 110 heats the cooled air and outputs the heated air to the mixer 112. The air heater 110 may include various types of air heating elements known in the art and not yet created. For example, the air heater 110 may include one or more electric heaters, one or more steam coils, one or more glycol/air heat exchangers, and/or one or more combustion-based heating elements. The air heating element(s) implemented in the air heater 110 may depend on system configuration and a desired temperature of the air to be output by the air heater 110.
The mixer 112 receives heated air from the air heater 110 and outputs heated air of a desired uniform temperature distribution. The heated air of the desired uniform temperature distribution is input to the TAD/TAB 100 (and more particularly to the hood 106).
While not illustrated, one skilled in the art will appreciate that the system may include an exhaust whereby at least some air in the airflow is removed from the system. The exhaust may be located between the main fan 108 and the heater 110 in an example configuration.
While
The lines between components of the TAD/TAB system, illustrated in
Ducting may include walls (202/204). When air is output from the mixer 112, the air may exhibit a perfect (or nearly perfect) desired uniform temperature distribution (illustrated by the linear temperature profile 206). As the air travels through the ducting, the desired uniform temperature distribution deteriorates. That is, the boundary layer(s) decreases in temperature as the air travels through the ducting (i.e., more and more of the air located at or proximate the ducting walls 202/204 decreases in temperature as the air travels through the ducting). This is illustrated by a comparison of temperature profiles 206, 208, and 210. The boundary layer(s) may grow due to man doors, flanges, or other locations in the ducting walls (202/204) whereby heat loss is capable of occurring. As a result, the air goes from having a desired uniform temperature distribution (as illustrated by the linear temperature profile 206) to a distribution including a desired temperature for drying or bonding material at a location away from the ducting walls (202/204), and a cooled temperature at the boundary layer(s) (as illustrated by the arcuate temperature profiles 208 and 210 in
By using heating tape(s) on one or more walls of the ducting, the boundary layer(s) of the air may be maintained (or substantially maintained) at a desired temperature for drying or bonding material during the entirety of the air's travel through the ducting. This would result in the air maintaining the linear temperature profile 206 while traveling through the ducting.
Ducting includes an inside skin 302 and an outside skin 310. An insulation layer 308 may be located between the inside skin 302 and the outside skin 310. For example, the insulation layer 308 may be located between protrusions 304 extending from the surface of the inside skin 302.
Heating tape(s) 304 may be placed proximate to or coupled to at least one outer surface of the inside skin 302 of the ducting. In an example, the heating tape(s) 304 is located between an outer surface of the inside skin 302 and the insulation layer 308.
Multiple strips of heating tape 304 may be placed proximate to or coupled to the inside skin 302. The strips of heating tape 304 may be controlled as a single unit (e.g., may be selectively operated as a single unit) or a subset of the heating tape 304 may be selectively operated separate from other strips of the heating tape 304.
The heating tape(s) 304 may be placed proximate to or coupled to the inside skin 302 of ducting located between the mixer 112 and the TAD/TAB 100 (and more particularly an air inlet of the hood 106). If the system includes more than one mixer 112, the heating tape(s) 304 may be placed proximate to or coupled to the inside skin 302 of ducting located after the last mixer 112 with respect to airflow (e.g., placed proximate to or coupled to the inside skin 302 of ducting located between the last mixer 112 and the TAD/TAB 100). However, one skilled in the art will appreciate that the heating tape(s) 304 may be implemented with other ducting of the system.
The heating tape(s) 304 may be implemented along an entire distance (or implemented along a substantial distance) of the ducting between the mixer 112 and the TAD/TAB 100. Alternatively, the heating tape(s) 304 may only be placed proximate to or coupled to the ducting proximate to an air inlet of the hood 106.
The amount of heating tape(s) 304 placed proximate to or coupled to a particular section of ducting (e.g., implemented along a particular length of the ducting) may depend on energy cost and/or a strategy for controlling the temperature of the boundary layer(s). For example, heating tape(s) 304 may be uniformly implemented along all or nearly all of the length of the ducting from the mixer 112 to the TAD/TAB 100. In this implementation, the heating tape(s) 304 may maintain the boundary layer(s) at or substantially at a desired drying or bonding temperature over the duration of the ducting (e.g., the heating tape(s) 304 may be operated to maintain a desired uniform temperature distribution along the duration of the ducting). In another example, heating tape(s) 304 may only be placed proximate to or coupled to ducting proximate to the air inlet of the hood 106, or a thicker amount of heating tape(s) 304 may be placed proximate to or coupled to ducting proximate to the air inlet of the hood 106 than is implemented distant from the air inlet of the hood 106. In this implementation, the heating tape(s) 304 may gradually increase the temperature of the boundary layer(s) of the air as the air gets closer to the air inlet of the hood 106 such that the air experiences a desired uniform temperature distribution by the time the air reaches the air inlet of the hood 106. Better control of the boundary layer(s) may be experienced by implementing heating tape(s) 304 over the length of the ducting, as compared to simply implementing heating tape(s) 304 to ducting proximate to the air inlet of the hood 106. In some systems, implementing heating tape(s) 304 over the entire length of the ducting between the last mixer 112 and the air inlet of the hood 106 may be beneficial because heat loss may be relatively constant along the length of the ducting. The cost of operating the heating tape(s) 304 may be a minimal consideration because the difference between the temperature of the boundary layer(s) and the desired temperature for drying or bonding material may only be a few degrees (e.g., +/−2° C.).
As illustrated in
Heating tape(s) 306 may be placed proximate to or coupled to the outside surface of each of the four walls of the inside skin 302. Such implementation may result in the temperature profile of the air being completely planar (as illustrated by 206 in
In at least some systems, it may not be necessary to implement heating tape(s) 306 with respect to every wall of the inside skin 302. Two opposing (e.g., parallel) walls of the inside skin 302 may communicate with air that is ultimately exposed to edges of material to be dried or bonded on the foraminous cylinder 104 (as illustrated in
It may be beneficial to implement heating tape(s) 306 with respect to the walls of the inside skin 302 that communicate with the air that is exposed to the edges of the material to be dried or bonded, but may not be necessary to implement heating tape(s) 306 with respect to the walls of the inside skin 302 that communicate with air that is exposed to the non-edge portions of the material to be dried. By using heating tape(s) 306 to heat the air that communicates with the edges of the material to be dried or bonded, the temperature profile of the air in the ducting may be arcuate. The arcuate temperature profile of the air may include two boundary layers, of decreased temperature, that extend along the plane of the paper on which
Nonetheless, it may be beneficial to implement at least some heating tape(s) 306 with respect to the opposing walls of the inside skin 302 that do not communicate with air that contacts the edges of the material. Such heating tape(s) may be used to control the size of the decreased temperature boundary layers of the arcuate temperature profile and, as a result, change the amount of decreased temperature air and desired temperature air applied to the material to be dried or bonded.
Heating tape(s) 306, coupled to at least two walls of the ducting, is operated (508) to compensate for heat loss experienced by the heated air while traveling through the ducting. The heating tape(s) 306 may be operated using at least one temperature sensor and a control loop. The at least one temperature sensor may be used to monitor the temperature of the heated air at or proximate at least one wall of the ducting.
Heating tapes of different outputs may be used. As such, one skilled in the art will appreciate that the amount of heating tape(s) 306 used may depend on the output of the heating tape(s) 306, the amount of insulation in the ducting, material make-up of the ducting, etc. For example, heating tape(s) 306 capable of producing stronger outputs may only need to cover about 50% of an area of ducting wall between the mixer 112 and the TAD/TAB 100, whereas heating tape(s) 306 capable of producing lesser outputs may need to cover more than about 50% (e.g., up to about 100%) of the area of the ducting to affect the boundary layer of the air in the same manner. One skilled in the art will also appreciate that the amount of output of heating tape(s) 306 need to maintain the boundary layer(s) at the desired temperature for drying or bonding may depend on the temperature of the air output by the mixer 112, the amount of insulation in the ducting, the material make-up of the ducting, etc.
The heated air, after passing through the ducting, is communicated (510) to material on the foraminous cylinder 104 in the hood 106. The heated air becomes unheated air after it passes through the material. At least some of this unheated air is circulated (512) to the main fan 108. This results in an airflow loop as illustrated in
While the present disclosure has been particularly described in conjunction with specific embodiments, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. It is therefore contemplated that the appended claims will embrace any such alternatives, modifications, and variations as falling within the true spirit and scope of the present disclosure.
Number | Date | Country | |
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62678643 | May 2018 | US |