Abstract:
The present invention relates to novel dryer systems that incorporate two-stage processes for heating air for drying a traveling web. The present invention is operable within a drying system having a drying hood containing a dryer. The drying hood receives heated air through an intake and expels system air through an exhaust. The portion of system air that is maintained in the system is divided into two portions and directed into separate parallel conduits for two-stage heating that results in greater temperature uniformity and efficiency within the system. One loop includes a mixing chamber for the initial mixing of system air with the combustion products of a burner. A second loop includes an injection chamber that receives the initially mixed air and injects it into the other portion of the system air, resulting in greater temperature uniformity within the drying hood and increased operating efficiency for the entire system.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the field of fluid dynamics and heat transfer, and more specifically to a system and method for mixing fluid streams within an industrial drying machine. 
     2. Description of the Prior Art 
     Industrial machines, such as those common in the textile, nonwovens and paper manufacturing industries, commonly utilize heated air to dry a newly formed product, as well for thermal bonding, curing and other processes that require an air stream with a uniform temperature profile. Typically, air is heated through conventional combustion means and then directed in various fashions towards the web of wet material. The heated air passes through or impinges the web, losing some of its heat in the drying process. The cooled air, referred to as system air, is then divided into portions that are re-circulated through the drying machine and portions that are exhausted into the atmosphere. 
     Drying machines in the aforementioned industries are generally of three types: through-air-dryers (TAD), impingement dryers, or floatation dryers. Each of these types of dryers is typically contained within a drying hood, which supplies and directs heated air to the surface of the web. A vacuum or pressure differential pulls the heated air through or onto the surface of the web and exhausts the cooled air into the system at large, at which point a portion of the cooled air will be exhausted into the atmosphere while the remainder is reused for drying applications. The direction of travel of the web is referred to as the machine direction, and the direction perpendicular to the machine direction and coplanar with the web is referred to as the cross-machine direction. 
     A typical dryer system  100  is shown in  FIG. 1 . As noted, the system  100  includes a dryer  110  that is partially surrounded by a dryer hood  112 , through which air is drawn from the surrounding structures. A web of goods enters the hood  110  on the wet end  114  and proceeds through the dryer  110 , where heated air is drawn through it, to the dry end  116 . The heated air is pushed in through an intake  118  and is drawn out of an exhaust  120  by a main fan  122  which drives partially closed circuit as shown in  FIG. 1 . A portion of the system air is exhausted into the atmosphere through duct  124 . 
     The remaining system air is directed to an air heater  126  that combines the system air with combustion products from a burner  128 . The burner  128  is driven by a combustion air source  130 , such as a fan, and fuel  132 . The mixed air  134  is a combination of combustion products and system air that will be used to dry the web passing through the dryer hood  112 . Those skilled in the art will recognize that the combination of the system air and the combustion products will not necessarily produce a uniformly profiled stream of heated air. On the contrary, the introduction of a secondary stream of combustion products into the system air may produce non-homogenous profile for the mixed air  134 . As a result, a typical dryer system  100  generally incorporates a static mixer  136  for inducing turbulence and mixing into the mixed air  134  stream so as to maximize thermal uniformity prior to entering the drying hood. 
     The foregoing example demonstrates both the strengths and weaknesses of the state of the art heating systems. While the current art is able to make remarkable use of system air through the re-circulation mechanisms, the necessary mixing of that air with combustion products is potentially hazardous to the end product. An essential aspect of textile and paper manufacturing is that the air that is drawn through or impinged upon the product must have a substantially uniform temperature profile along the cross-machine direction. Particularly for the manufacture of lightweight materials, such as tissue paper, any deviation in the temperature profile can irreversibly damage the finished product. The economic effects of non-uniform heating are multiple, including the energy required to replace the lost product, the costs of replacing the wasted raw materials, and the labor necessary to fix, maintain, manage and operate the dryer through a new production cycle. As such, one of the paramount concerns in the paper industry is designing a dryer that reliably maintains a uniform temperature profile in the cross-machine direction. 
     As noted above, it is common practice to re-circulate spent system air and reuse it in the drying cycle. Typically, the system air is combined with newly heated air and then the air is mixed as it passes through the machine ductwork towards the web of goods. Although the industry has made several attempts at efficiently re-circulating the air exhausted through the roll, the current state of the art requires a significant distance between the mixing point and the web in order to ensure that the temperature profile of the mixed stream is sufficiently homogenous. 
     For example, attempts have been made to introduce a heated fluid stream into a cooler fluid stream by using a baffling structure. Such a mechanism was contemplated in the invention described in international publication WO/0012202 published on Mar. 9, 2000. Although that invention describes a mechanical means for inducing turbulence, and hence mixing, in the combination of two fluid streams, it still does not do so with optimal efficiency of space and energy. In particular, the baffle design does create a large eddy that induces mixing of the fluid streams, but it does not do so in a symmetrical or uniform manner. Thus, the designers must either remix the turbulent air with a second device such as a static mixer; or alternatively, they must maximize the distance between the baffle location and the intake into the drying hood. Each of these two solutions involves non-trivial modifications to the drying systems described above, and both solutions would cost the producer in terms of energy efficiency and space utilization. 
     Given the foregoing, it is readily apparent to those skilled in the art that there is a need for a system and method for mixing fluid streams that is compact, energy efficient and produces a reliably uniform temperature profile across the web. Moreover, there is a need in the art for solutions that can be easily integrated into current drying system design without greatly expanding the hardware and space necessary to manufacture textiles. Lastly, there is a need in the art for a drying system that will minimize energy expenditures while deriving the greatest benefits from the raw materials processed therein. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention relates to a novel drying system that incorporates two-stage processes for heating air for drying a traveling web. In its various embodiments, the present invention operates within a system having a drying hood containing a dryer. The drying hood receives heated air through an intake and expels system air through an exhaust, a portion of which is directed into the atmosphere. In one embodiment, the portion of system air that is maintained in the system is divided into two portions and directed into separate parallel loops for two-stage heating that results in greater temperature uniformity and efficiency within the drying system. 
     The first portion of the system air is directed into a first conduit, and the second portion of the system air is directed into a second conduit. The first conduit includes an injection chamber that is disposed serially, or incorporated into, the drying hood intake. The second conduit includes a mixing chamber that is coupled to a burner for heating the air within the system. 
     The mixing chamber includes an arrangement of passages that effectively and efficiently mix the second portion of the system air with the combustion products from the burner. This mixed air stream is directed towards the injection chamber, where an injector or series of injectors induce further mixing by injecting the mixed air stream into the first portion of the system air. The injection chamber can also be integrated into the drying hood and controlled in such a manner so as to provide homogenous or non-homogenous air temperature across the running web, as determined by the user and the particular drying application. 
     By dividing the heating process into two stages, the present invention greatly increases the drying efficiency of a drying system. Notably, although one embodiment of the present invention utilizes a pair of distinct conduits for the heating process, the physical size of the drying system will not be affected. On the contrary, because of the increased mixing and heating efficiency of the present invention, it is possible to construct a drying system that is both smaller in size and more energy efficient that those presently used in the industry. Moreover, as described further below, the two-stage process of the present invention can also be utilized in a single conduit dryer configuration, in which the injection chamber is used for injecting an external source of heated air into the stream of mixed air from the mixing chamber. Numerous sources of external heated air, described below, can be utilized for improving the performance and efficiency of industrial dryers. 
     Further details and advantages of the present invention will become readily apparent from the detailed description of the preferred embodiments that refers specifically to the following drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a through-air-dryer system typical of the prior art. 
         FIG. 2A  is a schematic representation of a drying system in accordance with one embodiment of the present invention. 
         FIG. 2B  is a schematic representation of a drying system in accordance with another embodiment of the present invention. 
         FIG. 3  is a perspective view of a mixing chamber of the drying system of the present invention. 
         FIG. 4  is a cross-sectional view of the mixing chamber shown in  FIG. 3  along line  5 - 5 . 
         FIG. 5  is a cross-sectional view of the mixing chamber shown in  FIG. 3  along line  4 - 4 . 
         FIG. 6  is a perspective view of an injection chamber of the through-air-dryer system of the present invention. 
         FIG. 7  is a partial cut-away plan view of the injection chamber shown in  FIG. 6  in accordance with one embodiment of the present invention. 
         FIG. 8  is a partial cut-away side view of the injection chamber shown in  FIGS. 6 and 7  in accordance with one embodiment of the present invention. 
         FIG. 9  is a partial cut-away side view of the injection chamber shown in  FIG. 6  in accordance with another embodiment of the present invention. 
         FIG. 10  is a partial cut-away plan view of the injection chamber shown in  FIG. 9 . 
         FIG. 11  is a perspective view of a partial manifold of the injection chamber in accordance with the present invention 
         FIG. 12  is a cross-sectional view of the manifold of the injection chamber in accordance with the present invention. 
         FIG. 13  is a schematic diagram of a dryer system having an integrated injection chamber in accordance with one embodiment of the present invention. 
         FIG. 14  is a partial cut-away view of a dryer hood having an integrated injection chamber in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention includes both a system and method for mixing fluid streams, particularly those associated with contemporary drying systems. As described below, the present invention solves a number of problems noted in the textiles, paper and non-wovens industries. Most notably, the present invention includes a significant redesign of the drying system that efficiently utilizes system air and mixes it with combustion products in order to produce uniformly heated air for the web of goods. The mixing efficiencies of the present invention allow for a compact dryer design that is more economical in terms of raw materials, energy and space utilization. 
     Turning to  FIG. 2A , the system  10  for drying a textile web is shown. As shown, the system  10  is represented schematically, thus it should be understood that the novel features of the present invention are equally applicable to all types of industrial mixers, including at least TAD&#39;s, floatation dryers and Yankee impingement dryers, as well as any other dryer that uses heated air for drying goods. The system  10  includes a dryer  12  disposed within a drying hood  14 . The dryer  12  is typically one of the aforementioned dryers commonly used for drying goods, although it should be understood that the present invention is operable with any and all kinds of dryers that utilize heated air. A web enters the drying hood  14  at a wet end  16  and exits the drying hood  14  at a dry end  18 . As discussed in detail above, air drawn through an intake  48  passes through the dryer  12  and the drying hood  14  and is expelled through an exhaust  20 , which is in turn coupled to a pair of parallel conduits that embody the system  10  of the present invention. 
     The exhaust  20  is coupled to a first air conduit  22  in circuitous communication with the exhaust  20  and the intake  48  and a second air conduit  24  in communication with the first air conduit  22 . The air expelled through the exhaust  20  is referred to as system air, i.e. air that is not introduced from outside the system  10 . The system air (not shown) is divided into a first portion  32  and a second portion  34 , which are directed into the first conduit  22  and the second conduit  24 , respectively. 
     A first fan  26  is part of the first air conduit  22  for receiving the first portion  32  of the system air and directing it through an injection chamber  46 . A second fan  28  is part of the second air conduit  24  for receiving the second portion  34  of system air and directing it through to a mixing chamber  36 . An exhaust port  30  is preferably disposed in the second conduit  24  for optionally expelling some of the second portion  34  of the system air into the atmosphere. 
     The mixing chamber  36  is adapted for receiving the second portion  34  of the system air and mixing it into combustion products  40  emanating from a burner  38 , which is fed by a source of combustion air  41  and fuel  42 . The combustion products  40  are too hot for direct introduction into the system  10 . For example, the combustion products  40  may typically be between 1100 and 1550 degrees Celsius. Accordingly, the system  10  of the present invention introduces a two stage mixing process in order to efficiently temper the combustion products  40  into a readily usable stream of air heated to a range typically between 400 to 1500 degrees Celsius, i.e. a stream of mixed air  44 . 
     The resulting mixed air  44  is directed towards the injection chamber  46 , where it is injected back into the first portion  32  of the system air. After injection of the mixed air  44  into the first portion  32  of the system air, the intake  48  of the system  10  directs the uniformly profiled air into the dryer hood  14 . The specific means for mixing and means for injection are discussed in detail below. 
       FIG. 2B  is a schematic representation of another embodiment of the present invention, wherein identical reference numerals refer to similar elements as described with reference to  FIG. 2A . As in the previous embodiment, the system  10  includes a dryer  12  disposed within a drying hood  14 . The web enters the drying hood  14  at a wet end  16  and exits the drying hood  14  at a dry end  18 . Air drawn through an intake  48  passes through the dryer  12  and the drying hood  14 , from whence it is expelled through an exhaust  20 . Unlike the prior embodiment, however, that shown in  FIG. 2B  has a single conduit for recycling the system air. 
     The exhaust  20  is coupled to a conduit  24 ′, which is in circuitous communication with the exhaust  20  and the intake  48 . The air expelled through the exhaust  20  is still referred to as the system air. The system air (not shown) consists solely of a portion  34 ′, which is directed into the conduit  24 ′, as noted above. 
     A fan  26 ′ is part of the conduit  24 ′ for receiving the portion  34 ′ of system air and directing it through to a mixing chamber  36 . An exhaust port  30  is preferably disposed in the conduit  24 ′ for optionally expelling some of the portion  34 ′ of the system air into the atmosphere. 
     As in the prior embodiment, the mixing chamber  36  is adapted for receiving the portion  34 ′ of the system air and mixing it into combustion products  40  emanating from a burner  38 , which is fed by a source of combustion air  41  and fuel  42 . As previously noted, the combustion products  40  are too hot for direct introduction into the system  10 . Thus the system  10  of the present invention introduces another two stage mixing process in order to efficiently temper the combustion products  40  into a readily usable stream of air heated to a typical range of 150 to 600 degrees Celsius referred to as the stream of mixed air  44 . 
     The resulting mixed air  44  is directed towards the injection chamber  46 , where it receives an injection of heated air  45  from an external source  47 . For purposes of the present invention, the heated air  45  may include air that is heated by a turbine, a second burner, exhaust from the machinery of the system  10 , as well as certain types of naturally occurring volumes of air, such as those derived from geothermal processes. Thus as defined herein, the term external source  47  should be understood to refer to a source of heated air that is not derived from a burner located within the system  10 . For example, the external source  47  may be typified as waste heat from another process or heat from another, lower cost source. Accordingly, the burner  42  used in the present invention can be smaller and more fuel efficient, thereby reducing the overall space and energy consumption associated with heating the air. As in previous embodiments, after injection of the heated air  45  into the mixed air  44 , the intake  48  of the system  10  directs the uniformly profiled air into the dryer hood  14 . 
       FIG. 3  is a perspective view of the mixing chamber  36  of the system  10  of the present invention. The mixing chamber  36  includes a first passage  50  directing combustion product  40  from the burner  38 , a second passage  52  carrying the second portion  34  of the system air, and a third passage  54  directing the mixed air  44  to the injection chamber  46 . Preferably, the first passage  50  and second passage  52  are in fluid communication and oriented in an orthogonal manner, as shown in  FIG. 3 . 
       FIG. 4  is a cross-sectional view of the mixing chamber  36  shown in  FIG. 3  along line  4 - 4 . As shown, the mixing chamber  36  is preferably outfitted with a perforated sleeve  56  that selectively places air from the second portion  34  in fluid contact with the combustion product  40  that is traveling through the first passage  50 . In the cross-sectional view along line  5 - 5  shown in  FIG. 5 , the first passage  50  has a circular cross-section. The second passage  52  terminates near the intersection between it and the first passage  50 , and the perforated sleeve  56  is disposed between the respective passages. 
     A volume is defined between the perforated sleeve  56  and the interior surface of the second passage  52 , and the second portion  34  of the system air must of course occupy this volume as it passes through the perforated sleeve  56 . In a preferred embodiment, the volume so defined is variable about the perforated sleeve  56 , such that the pressure gradient along the surface of the perforated sleeve  56  will also be variable. For example, a volume along section  60  is greater than a volume along section  62 , which in turn is greater than a volume along section  64 . By varying the volume defining the intersection between the combustion product  40  and the second portion  34  of the system air, the designers can tailor the mixing rate of the two fluid streams as they form the mixed air  44 . 
       FIG. 6  is a perspective view of an injection chamber  46  of the drying system of the present invention. The injection chamber  46  includes a third passage  70  for directing the first portion  32  of the system air. The third passage  70  is intersected by at least one injector  72  that directs the mixed air  44  into the first portion  32  of the system air. The means for injection are described in full detail below in conjunction with alternative embodiments of the system  10 . 
       FIG. 7  is a partial cut-away plan view of the injection chamber  46  shown in  FIG. 6  in accordance with one embodiment of the present invention.  FIG. 8  is a partial cut-away side view of the injection chamber  46 . As shown in  FIGS. 7 and 8 , an arrow pointing leftwards represents the first portion  32  of system air. Each of the injectors  72  includes a projection  73 , which in the embodiment shown is defined by a first tubular portion  74  and a second tubular portion  75 . The injectors  72  are arranged orthogonal to the flow of the first portion  32  of system air, which is to say that they are also orthogonal to the third passage  70  described above. 
     The first tubular portion  74  and second tubular portion  75  cooperate to define an obtuse structure in the third passage  70  so as to create pockets of low pressure  77  in the flow of the first portion  32  of system air. The projections  73  defined by the first tubular portion  74  and second tubular portion  75  are purposefully obtuse in order to maximize the turbulence in the airflow and thereby induce mixing between the mixed air  44  and the first portion  32  of system air. A plurality of ports  78  (depicted as small arrows) are defined on the second tubular portion  75  for transmitting the mixed air  44  into the pockets of low pressure  77 . The flow of mixed air  44  into the third passage  70  is controlled by at least one throttle valve  76  disposed between each of the first tubular portions  74  and second tubular portions  75 . The throttle valves  76  are controllable by a system operator either mechanically or electronically, depending upon the configuration of the system  10 . 
       FIG. 9  is a partial cut-away side view of the injection chamber shown in  FIG. 6  in accordance with another embodiment of the present invention. As shown, the injector  80  includes a manifold  82  having a plurality of nozzles  84  disposed thereon.  FIG. 10  is a partial cut-away plan view of the injection chamber shown in  FIG. 9  better demonstrating the aerodynamic properties of the manifolds  82 , and  FIG. 11  is a perspective view of a partial manifold  82  of the injection chamber  46 . Each manifold  82  defines a leading edge  86 , a central portion  88  that includes the nozzles  84 , and a trailing edge  90 . As used herein, the terms leading and trailing refer to the standard orientation of an object in a fluid stream, i.e. the leading edge  86  is the first edge to contact the fluid, while the trailing edge  90  serves to smooth out any turbulence in the fluid. 
       FIG. 12  is a cross-sectional view of the manifold  82  of the injection chamber  46  in accordance with the present invention. As shown, the nozzles  84  are disposed on the surface of the central portion  88  for directing a fluid in a direction normal to the surface of the central portion  88 . In particular, the nozzles  84  are configured for injecting the mixed air  44  into the first portion  32  of the system air. The aerodynamic profile of the manifolds  82 , as detailed in  FIG. 12 , creates small-scale turbulence in the air stream, as opposed to the large pressure drop described above with respect to the obtuse projections  73 . In particular, for each manifold the surface of the leading edge  86  defines an angle θ relative to the central portion  88  and the trailing edge  90  defines an angle φ relative to the central portion  88 . In preferred embodiments, the angle θ is less than twenty degrees, and is most preferably less than fifteen degrees for optimum aerodynamics. The angle φ is preferably less than twelve degrees, and is most preferably less than eight degrees. 
     As the manifolds  82  described herein are specifically designed to reduce turbulence in the system  10 , the only turbulence created in a manifold-style injection chamber  46  is by the injection of the mixed air  44  into the first portion  32  of system air through the nozzles  84 . It follows that in order to maximize the mixing activity of the two streams, each manifold  82  must have a number of nozzles  84  disposed thereon, preferably arranged in multiple rows and on both surfaces of the central portion  88 . As the nozzle velocity of each nozzle  84  can be optimized for variable conditions, a system operator can fine-tune the mixing performance of the injection chamber  46  for particular needs. 
     One particular benefit of the manifold approach to fluid injection is that the temperature profile of the air entering the intake  48  can be readily controlled using a control loop for varying the injection rate of the manifolds  82 . This increased control over the air profile near to or within the drying hood  14  allows for customized and optimized temperature control, which in turn permits engineers and manufacturers to develop improved goods at lower costs. Control over the manifolds  82  is precise enough that it is possible to dispose the injection chamber  46  close to, or even integrated into, the intake  48  of the drying hood  14 . In particular, electronic control over the manifolds  82  permits a manufacturer to locate the injection chamber  46  at any point in the system  10  that is downstream from the mixing chamber  36 , including of course integrating the injection chamber  46  into the drying hood  14 . 
     By way of example,  FIG. 13  is a schematic diagram of a dryer system  10  having an integrated injection chamber  11  in accordance with one embodiment of the present invention. While similar reference numerals refer to similar elements, the system configuration shown in  FIG. 13  illustrates an injection chamber  46  integrated into the drying hood  14 . A controller  49  is coupled to the drying hood  14  and the injection chamber  46 , and is preferably configured to receive feedback signals from the drying hood  14  in order to monitor and adapt the nozzle velocity of the manifolds  82  of the injection chamber  46 . The manifolds  82  of the injection chamber  46  can be controlled to create particular temperature profiles in the drying hood  14  in both the machine and cross-machine directions. Moreover, the controller  49  can be adapted to provide instantaneous response from the feedback signals, thus providing an effective bias against unwanted variations in the temperature profile of the hood. 
       FIG. 14  is a partial cut-away view of a dryer hood  14  having an integrated injection chamber illustrating the precision and capabilities of the aspect of the invention described above. A web  19  of material is shown disposed within the hood  14 . The web  19  defines three zones of differing dryness, a first zone  190 , a second zone  192  and a third zone  194 . The injection chamber  46  and intake  48  are integrated into the drying hood  14  and disposed in close proximity to the web  19 . The controller  49  receives signals indicative of the dryness/temperature or alternative measurement of the web, and in response to those signals directs the manifolds  82  within the injection chamber  46  to respond in an appropriate fashion. 
     For example, the manifolds  82  within the injection chamber  46  can be controlled to produce three streams of differing temperature, a first stream  200 , a second stream  202  and a third stream  204 . The nature of the feedback through the controller  49  ensures that the first stream  200  corresponds to the first zone  190 , the second stream  202  to the second zone  192 , and the third stream  204  to the third zone  204 . Accordingly, the integration of the injection chamber  46  not only provides means for homogenizing the air temperature within the drying hood  14 , it also provides means for biasing the air temperature within the drying hood  14  in a manner that is readily controllable. That is, the injection chamber  46  can be biased to inject hot air into an area correlating with a wet portion of the web  19 , and conversely, the injection chamber  46  can be controlled to inject cooler air towards a dryer portion of the web  19 . In short, by integrating the injection chamber  46  into the drying hood  14 , the present invention enables users to optimize the drying of the web  19  in the most efficient manner. 
     The benefits of the present invention, in particular those achieved through the control over the manifolds  82  as well as the integration of the injection chamber  46  into the drying hood  14 , result from the two-stage mixing processes described in detail above, which in turn reduces the length of the conduits necessary to direct the first portion  32  of the system air. Moreover, the usage of an external source, such as heated air from an ancillary process or machine, further lessens the costs associated with heating a uniform stream of air. As illustrated above, the present invention will enable engineers and designers to manufacture industrial dryers that utilize this process, which in turn will increase the drying efficiency of any number of commercial operations. 
     While the present invention has been described in detail with respect to its preferred embodiments, these should be understood to be exemplary in nature and not limiting as to the scope of the present invention. It is certain that design modifications could be readily devised by those skilled in the art, and that any such modifications would fall within the scope of the present invention as defined herein by the following claims.