Abstract:
An exhaust duct for cooling an exhaust flue. One or more modular coolant coils are disposed about the outer circumference of or within the flue interior cavity for thermal communication with exhaust gas. Each coil has a helical spiral profile extending along the flue axial dimension, an interior lumen there through for passage of coolant provided by a cooling system, and a respective inlet and outlet for respective intake and discharge of coolant. Optionally at least one remotely adjustable valve is coupled to the coolant coil and an industrial automation controller, for regulation of coolant flow rate within the coil. The coolant coils may incorporate one or more coolant temperature sensors in communication with the controller. A plurality of exhaust duct cooling coils may be under common control of the controller, for allocation of coolant among the coils and other portions of the cooling system.

Description:
BACKGROUND OF THE DISCLOSURE 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to fluid-cooled exhaust flues or ducts for transferring heat from an exhaust gas to a second cooling fluid flowing in cooling conduits that are in thermal communication with the duct. The present invention is suited for application to combustion or process exhaust flues, such as are found in electric arc steel mill furnaces, petroleum or other chemical refining plants or electric power generation plants. 
         [0003]    2. Description of the Prior Art 
         [0004]    Exhaust flues, such as found in electric arc furnaces in steel mills, require reduction in exhaust gas temperature before the exhaust is released to the atmosphere, for conformity with environmental regulations or to reduce damage to exhaust flues that may result from prolonged exposure to high temperatures. In some applications heat extracted from the exhaust gas is used for cogenerative power generation or other thermal energy needs, including building or factory process heating. 
         [0005]    Traditionally, exhaust flues or ducts have been cooled with fluids, such as treated water, flowing through cooling conduits in thermal communication with the flue. Water passing through the cooling conduit absorbs heat from the flue exhaust, is re-cooled to a lower temperature by an air cooling tower or other heat exchanger, and recycled through a continuous loop to the exhaust flue. 
         [0006]    United States Patent Application Publication No. US 2006/0291523 shows arrays of axially-oriented cooling tubes or channels about the periphery of an exhaust duct, wherein the cooling fluid is pumped parallel to the exhaust gas. Proximally adjoining cooling tubes are welded together along their axial lengths to form a unitary circumference of the cooling duct flue. Adjoining tube fluid carrying interiors are interconnected by elbow bends at each end of the exhaust flue, forming a serpentine, undulating cooling fluid flow path. In other embodiments the cooling fluid flows in U- or C-shaped channels formed about the outer periphery of the duct. As one skilled in the art can appreciate, such tight elbow bends between proximal adjoined cooling tubes creates relatively higher cooling fluid flow resistance than an equivalent length of straight tube. The higher fluid flow resistance must be overcome by use of higher power consuming cooling flow pumps. 
         [0007]    The axially oriented cooling tubes of the US 2006/0291523 publication will also require relatively high cooling water flow rates in order to avoid overheating cooling water proximal the exhaust flue inlet region. More heat must be transferred out of the exhaust flue near its inlet than near its outlet, because the exhaust gas cools as it flows through the flue. If an operator wishes to follow a common cooling practice to maintain the cooling water below its boiling point one must maintain a relatively high flow rate through the axially oriented tubes so that the cooling water does not overheat proximal the exhaust flue inlet. Given the tube orientation, cooling water heated proximal the flue intake in a cooling tube flowing toward the exhaust must travel a circuitous path along the entire duct length and back before it is exhausted to an outlet manifold. During such a circuitous path the heated fluid has limited remaining capacity to absorb heat from flue at the downstream end. The long, circuitous flow path in turn increases cooling water pumping power requirements, in addition to the higher pumping requirements attributed to higher flow rate and need to overcome pumping resistance in tight elbow bends. 
         [0008]    U.S. Pat. No. 4,556,104 references a heat exchanger for heating especially an organic liquid transfer fluid by way of combustion gasses from a burner of fossil fuel. It states that a flue or cooling conduits proximal the inlet hot combustion gasses can be shielded with a refractory material or by spirally winding a single continuous loop of cooling coil about the flue interior at varying winding pitch rates, with closer winding near the flue intake and wider winding proximal the flue exhaust. While in theory tighter coil winding proximal the flue intake would enable a greater rate of heat transfer, the disclosure appears to be in the context of intentionally heating the fluid in the cooling coil. Logically if fluid in the cooling deviates from a desired temperature all one would do would be to adjust the heater exhaust temperature up or down to achieve the desired fluid temperature. This is not possible in the context of a steel mill, power generation plant or other industrial process application, where the exhaust temperature of the flue gas cannot be adjusted without compromising process efficiency or quality. 
         [0009]    It is also noted that the continuous cooling coil shown in the U.S. Pat. No. 4,556,104 must be replaced in toto, or a section of which must be replaced in situ in case of cooling coil leak or other failure. 
         [0010]    Thus, a need exists in the art for a an exhaust flue gas cooling duct that selectively varies cooling fluid circulation rate in different zones of the duct, so that for example, more heat can be transferred away from the duct proximal the relatively hotter duct intake region and a lower circulation rate can be utilized in the relatively cooler duct exhaust region, thereby conserving fluid flow capacity and cooling pumping power requirements. 
         [0011]    Another need exists for an exhaust cooling duct cooling coil geometry that reduces fluid pumping resistance than required for previously known axially oriented cooling coils with relatively tight elbow radius between adjoining axial coil sections. 
         [0012]    Yet another need exists for an exhaust cooling duct having modular cooling coils that can be field installed and repaired with relatively lower effort than known integrated, single coil cooling systems, preferably without disrupting adjoining associated cooling system structure, manifolds and valving. 
       SUMMARY OF THE INVENTION 
       [0013]    Accordingly, an object of the invention is to create a duct cooling system that enables selective variation of duct cooling parameters in separate cooling zones, so that cooling fluid water usage can be optimized and cooling water pumping power can be reduced. 
         [0014]    Another object of the present invention is to create a duct cooling system that reduces cooling coil pumping resistance. 
         [0015]    Yet another object of the present invention is to create a duct cooling system employing modular multi-zone cooling subsystems that can be selectively repaired or replaced without the need to disrupt other unaffected duct cooling subsystems and related components. 
         [0016]    These and other objects are achieved in accordance with the present invention by the duct cooling system of the present invention which employs modular, multi-zone spirally oriented cooling conduits. 
         [0017]    One aspect of the present invention is an exhaust duct cooling system having an exhaust flue defining a interior cavity for passage of exhaust gas there through along an axial dimension thereof. A coolant coil is disposed about the flue external circumference or interior cavity for thermal communication with exhaust gas. The coolant coil has a helical profile extending along the flue axial dimension, an interior lumen there through for passage of coolant, and a respective inlet and outlet for respective intake and discharge of coolant. Optionally, at least one flow regulator, which is preferably but is not required to be an adjustable valve, is coupled to the coolant coil, for regulation of coolant flow rate within the coil. Optionally the adjustable valve may be remotely controlled, such as by a controller of an industrial automation system. 
         [0018]    Another aspect of the present invention is directed to an exhaust duct cooling system, having an exhaust flue defining an interior cavity for passage of exhaust gas there through along an axial dimension thereof. A plurality of coolant coils are disposed serially about the flue external circumference or within the flue interior cavity for thermal communication with exhaust gas. Each respective coil has a helical profile extending along the flue axial dimension, an interior lumen there through for passage of coolant, and a respective inlet and outlet for respective intake and discharge of coolant. Optionally, at least one adjustable valve is coupled to each respective coolant coil, for regulation of coolant flow rate within the coil. This aspect of the invention optionally may also feature an intake manifold in common parallel fluid communication with the inlets and an exhaust manifold in common parallel fluid communication with the outlets. 
         [0019]    Yet another aspect of the present invention is directed to a method for cooling an exhaust duct cooling system having an exhaust flue that defines an interior cavity for passage of exhaust gas there through along an axial dimension thereof. The method includes orienting at least one coolant coil about the flue external circumference or within the flue interior cavity for thermal communication with exhaust gas. The coil has a helical profile extending along the flue axial dimension, an interior lumen there through for passage of coolant, a respective inlet and outlet for respective intake and discharge of coolant, and a flow regulator that is optionally at least one adjustable valve coupled to the coolant coil. As an additional option the adjustable valve may be remotely controlled by a controller of an industrial automation system coupled thereto, for regulation of coolant flow rate within the coil. The method includes feeding coolant through the intake and discharging the cooling through the outlet at a flow rate; measuring coolant temperature at least the outlet with a temperature sensor and regulating coolant flow rate with the flow regulator. Optionally the temperature sensor is remotely coupled to the controller; the controller regulating coolant flow rate with the adjustable valve in order to achieve a desired outlet coolant temperature. Optionally a plurality of coolant coils, associated valves and temperature sensors may be in communication with the industrial automation controller, so that the controller can optimize coolant utilization within the aggregate cooling system. 
         [0020]    One or more of the objects, aspects and features of the present invention may be selectively employed jointly in combinations or severally by one skilled in the art when practicing the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
           [0022]      FIG. 1  is a schematic view showing application of the present invention in an exemplary steel mill; 
           [0023]      FIG. 2  is a plan view of an embodiment of an exhaust duct of the present invention; 
           [0024]      FIG. 3  is an elevational sectional view of the exhaust duct of the present invention taken along 3-3 of  FIG. 2 ; 
           [0025]      FIG. 4  is a detailed elevational sectional, view of the exhaust duct of the present invention taken along 4-4 of  FIG. 2 ; 
           [0026]      FIG. 5  is a schematic view of a plurality of exhaust ducts of the present invention with optional remote control valves thereof operated by a controller of the present invention within an industrial automation communication and control system; 
           [0027]      FIG. 6  is a detailed elevational sectional view similar to that of  FIG. 4 , of another embodiment of the exhaust duct of the present invention; 
           [0028]      FIG. 7  is a schematic view similar to that of  FIG. 5 , of the embodiment of  FIG. 6 ; and 
           [0029]      FIG. 8  is a schematic view similar to that of  FIG. 1 , showing application of an alternative embodiment of the present invention in an exemplary steel mill. 
       
    
    
       [0030]    To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
       DETAILED DESCRIPTION 
       [0031]    After considering the following description, those skilled in the art will clearly realize that the teachings of the present invention can be readily utilized in exhaust cooling ducts for different industrial applications. 
         [0032]    General Cooling System Overview 
         [0033]      FIG. 1  shows schematically a steel mill  10  including an electric arc furnace  15  with an exhaust flue  20 . The exhaust, arrow F, is routed to a particulate drop out box  25 , and thereafter to a forced draft cooler  30  where finer particulates are extracted from the exhaust. The plant  10  has a main cooling supply header  40  and a main return header  45  that is responsible for routing coolant in a defined portion, of the plant  10  that may include the entire plant. Coolant is recycled from the return header  45  to a coolant recycling heat exchanger  50  (here shown as an exemplary air cooling tower) that lowers the return coolant temperature so that it may be returned to the plant via the supply header  40 . The exhaust flue  20  includes a plurality of exhaust ducts  100  of the present invention. Here in the exemplary embodiment of  FIG. 1  there are shown five such separate exhaust duct  100  sections, though it should be understood by those skilled in the art that the number and sizing of such ducts will be specific application dependent. 
         [0034]    The exemplary embodiment of the present invention is shown herein in a steel mill application. However, as previously stated, it should be understood by those skilled in the art that the present invention may be applied to other exhaust flue environments, such as by way of nonlimiting example in power plants or chemical processing plants. 
         [0035]    As a practical matter any industrial plant utilizing cooling water or other cooling fluid does not have an infinite supply of coolant, thereby necessitating recycling of coolant after subsequent cooling. Therefore coolant must be monitored and allocated within the plant often in accordance with dynamically changing cooling needs. Ideally coolant should not be wasted, overheated to a mixed fluid-gas thermodynamic phase, cooling dwell time necessary to reduce coolant temperature for recycling back to the plant should be minimized and coolant pumping should be minimized in order to reduce plant operating costs. 
         [0036]    Exhaust Duct and Cooling System Structure 
         [0037]      FIG. 2  shows a cooling duct assembly  100  of the present invention, including flue  110 . The flue  110  is of known construction, having a cylindrical or other desired cross-sectional profile, with intake flue flange  112  and exhaust flue flange  114 . Exhaust flows through the flue  110  in the direction of the double arrow F. The flue may be constructed of any known material for the application, including exemplary rolled sheet steel, and may include an insulative lining of refractory material or ceramic in any portion thereof. 
         [0038]    The exhaust duct  100  includes at least one and preferably a plurality of spiral wrapped coolant coils  120  about the flue  110 . In a preferred, but not required embodiment, the individual serial coils form separate circuit zones (C 1 , C 2 , C 3  . . . C(N−1), CN), and may have a varying number of winding turns and winding pitch as selected by the designer. As one skilled in the art can appreciate, the heat absorption capacity (and conversely flue cooling capacity) of any individual cooling coil  120  is a function of the number of windings, their pitch, coil tubing material, tubing diameter, thermal capacitance properties of the coolant and coolant flow rate, among others. The coils  120  may be wrapped about the exterior circumference of the flue  110  and in other applications about the interior of the flue. 
         [0039]    Each respective coil  120  may have any desired cross-section and be constructed of any known material suitable for exhaust flue cooling applications. An exemplary cross-section and material for coolant coils shown in the figures herein may be round steel tubing that can be readily formed into a helical spiral shape. The relatively gentle spiral bends of larger winding diameter have lower fluid flow resistance than the relatively tighter radius 180 degree sharp elbow bends and long tube runs required at the ends of previously known axially oriented cooling tube constructions, thereby reducing pumping power needed to pump coolant through the cooling tube coils  120 . 
         [0040]    Exemplary dimensions for coolant coils of the present invention as applied in steel mill exhaust flues are: 
         [0041]    helical winding profile inside diameter of 63-87 inches (1.6-2.2 meters), preferably constructed of 2 inch (50 mm) or 2.5 inch diameter (64 mm) schedule  80  or schedule  160  pipe; or 3 inch (76 mm) schedule  40  or schedule  30  pipe; 
         [0042]    any desired helical profile axial length, but often 17-20 feet (5.2-6.2 meters); 
         [0043]    2-N (often 5-9) zone coil circuits within the helical profile; and 
         [0044]    each coil circuit having an internal surface area of 43-76 square feet (3.9-6.9 square meters). 
         [0045]    The coils  120  are of modular construction and individually replaceable or serviced in the field after exhaust duct construction without interaction or disabling of other neighboring coils. Referring generally to  FIGS. 2-4 , each modular coil  120  has a cooling tube inlet  122  and a cooling tube outlet  124 . Cooling tube caps  126  seal the respective ends of each coil  120 , for maintaining coolant retention integrity without leaks. For additional exhaust duct cooling system modularity, in each cooling zone, C 1 -CN, the cooling coil tubes preferably are coupled in parallel to a common coolant supply header  130  that has a coolant supply inlet  132  connected to the cooling system coolant supply and a coolant supply manifold  134 . Each cooling zone preferably includes a manual shut off valve  136  having an inlet coupled to the coolant, supply manifold  134  and having an outlet coupled to the cooling tube inlet  122  by way of a reinforced flexible hose  138 , shown schematically in the figures. Completing the cooling flow circuit in each cooling tube  122 , the cooling tube outlet  124  is coupled in parallel to cooling water exhaust manifold  140 , having a cooling water exhaust outlet  142  that is connected to the cooling system return loop to the cooling tower, for cooling of the coolant and eventually recycling to the cooling system supply. 
         [0046]    In the preferred embodiment shown the exhaust manifold  140  is coupled in parallel to all of the cooling tube outlets  124  in an associated set of zones by each respective reinforced flexible hose  148 , shown schematically in the figures and in turn to cooling water exhaust manual shut-off valve  146 . 
         [0047]    Any cooling tube  122  can be included or isolated from the cooling system by actuation of the respective intake and exhaust manual shut-off valves  136 ,  146 , for removal and replacement or servicing, without impacting other zones or the respective supply or exhaust manifold structures  134 ,  144 . The manifolds  134 ,  144  are coupled to the exhaust duct  100  by header supports  150 . 
         [0048]    Coolant Flow Control 
         [0049]    Referring to  FIG. 5 , optional coolant flow regulation and/or calibration in each zone C 1 -CN is preferably accomplished by actuation of flow control valve  160 . In alternative configurations of the present invention coolant flow regulation can be accomplished by other flow regulator structure, including initial selection of cooling tube  122  diameter, or by adjustable regulator structure, including by way of non-limiting example a flow restrictor plate, venturi or orifice in series with the cooling tube or the manual shut-off valves  136  or  146 . In order to achieve the benefits of optional automated plant process control afforded by modern industrial control systems each of the flow control valves  160  is preferably remotely actuated by an industrial automation controller  180  via a communications pathway, depicted schematically as  182 . A controller may be implemented by way of example through a programmable logic controller (PLC) or a general purpose digital computer emulating a PLC, also known as a “soft PLC” having a processor that executes stored process control software commands via a software operating system. The controller  180  and valve  160  communication pathway may be implemented by any way known in the industrial automation and control field, including by way of example hard wired twisted cable pair, shielded coaxial cable, computer communications bus, Internet, Intranet, or wireless remote communication. 
         [0050]    Advantageously the controller  180  also monitors temperature in each coolant coil  120  by way of a temperature sensor, such as outlet temperature sensor  170  via communications pathway  182 . An inlet temperature sensor  172  may also be employed. The controller  180  preferably adjusts coolant flow rate via valve  160  in each coolant coil based in at least part by the temperature measurements obtained from temperature sensors  170 ,  172  or a combination of measurements from both, such as via a known temperature control feedback loop. The controller may also utilize other plant operational information in regulating coolant flow rates in each cooling coil  120 . For example, as shown in  FIG. 5 , if another part of the plant requires higher priority allocation of coolant, permissible maximum temperatures in one or more of the regulated cooling zones C 1 -CN may be raised to free up coolant capacity in higher priority zones. 
         [0051]    Preferred multiple zone C 1 -CN construction of the duct system of the present invention enables more precise heat transfer with overall lower coolant pumping effort than known designs that incorporate axially oriented parallel tube cooling. For example, known axial oriented cooling tube constructions require long pumping pathways through a serpentine tube layout, thereby generating more coolant pumping resistance than the relatively shorter, large diameter helical windings of the individual zone cooling tubes  120  of the present invention. 
         [0052]    In practice of the present invention, an exhaust duct  100  may have a single cooling zone C that is coupled to a common plant cooling system with other exhausts ducts individually having one or more separate serial coil  120  cooling zones CN. Each “zone”, whether jointly or severally part of a single exhaust duct  100  assembly or a consolidation of zones in multiple exhaust duct assemblies, may be controlled separately or as part of an aggregate combination or sub combination by an industrial plant coolant control system. 
         [0053]    The present invention cooling system enables precise fine tuning of flow rates in each zone C 1 -CN. For example in  FIG. 2 , zone C 1  is closest to the exhaust intake of the duct section  100 . The flow rate through the zone C 1  tube  120  may be established by its associated flow control valve  160  (or if the flow control valve  160  is not used, by adjustment of the manual valves  136  or  146 , or by adjustment or replacement of other flow regulation structure, including remote actuated valves actuated by solenoids), so that cooling water measured by outlet temperature sensor  170  does not exceed a desired maximum temperature: for example 140 degrees Fahrenheit (60 degrees Celsius). Exhaust gas cools as it traverses flue  110  from zone C 1  to zone C 2 . Assuming the coolant coil for zone C 1  is constructed the same as that of zone C 1 , less coolant flow should be required to remain within the maximum temperature setpoint and so on as the exhaust traverses the flue. By maximizing flow rate efficiency for each zone below the maximum temperature, aggregate coolant pumping for all zones is reduced, freeing up coolant for other applications within the plant as well as reducing pumping costs. 
         [0054]    As an additional option to conserve coolant and provide additional coolant allocation flexibility within an industrial plant, the alternative embodiment of the present invention shown in  FIGS. 6 and 7  provide for coolant recirculation within any one or more of the cooling zones within a single exhaust duct assembly or in a plurality of exhaust duct assemblies within a plant. Coolant bypass  190  includes a remote actuation valve  192  of known design, actuated by a bypass valve solenoid control  194  in communication with PLC  180 . In this exemplary embodiment the manual shut-off valves  136 ,  146  for supply and exhaust shown in the embodiments of the prior figures are also replaced by respective remote actuated supply valve  196 , actuated by supply valve solenoid  198 , and exhaust valve  200 , actuated by exhaust valve solenoid  202 . As one skilled in the art can appreciate, any of the embodiments of the present invention can substitute different flow restrictors than those shown in the figures. For example a solenoid controlled remote actuated valve may be substituted for the flow control valve  160  in the embodiment of  FIG. 2  and conversely, flow control valves can be substituted for one or more of the solenoid controlled remote actuated valves  192 ,  196 , and  200  in the embodiment of  FIG. 6 . 
         [0055]    in normal operation of the embodiment of  FIGS. 6 and 7 , bypass valve  192  is closed, and coolant circulation is from the supply manifold  130  with return to the exhaust manifold  140 . When coolant recirculation is desired, for example when fresh coolant must be allocated to another portion of the plant cooling system, supply valve  196  may be closed in part or totally to reduce fresh coolant supply from the supply manifold  130  to the cooling tube  120 . The bypass valve  192  is opened in part or totally, in order to cause coolant recirculation within cooling tube  120 . Exhaust valve  200  also is closed in part or totally to reduce flow of recirculated coolant to the exhaust manifold  140 . Coolant recirculation may be modified based on coolant temperature feedback from the temperature sensor  170  or in response to remote commands from the PLC. For example, if a measured temperature by temperature sensor  170  exceeds a maximum threshold the percentage of recirculated coolant may be reduced by opening further the coolant supply and exhaust valves  196 ,  200  and/or closing further the bypass valve  192 , or any combination of the above. 
         [0056]    An optional advantage of the present invention is that the concept of a coolant bypass  190  with bypass valve  192  may be incorporated into an exhaust duct assembly  100  having a single cooling loop  120 , so that a user may install a reduced cost cooling duct with a single zone. As shown in  FIG. 8 , this single zone per duct inventive coolant bypass concept may be incorporated into multiple cooling duct assemblies  100 , in which case each separate duct assembly  100  constitutes a “zone” for coolant control purposes, and may be commonly controlled in any combination by the plant coolant flow regulation system. 
         [0057]    Although various embodiments which incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.