Patent Application
20120125595
1. Field of the Invention
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.
2. Description of the Prior Art
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Another object of the present invention is to create a duct cooling system that reduces cooling coil pumping resistance.
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.
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.
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.
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.
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.
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.
The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which:
To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures.
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.
General Cooling System Overview
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.
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.
Exhaust Duct and Cooling System Structure
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 (C1, C2, C3 . . . 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.
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.
Exemplary dimensions for coolant coils of the present invention as applied in steel mill exhaust flues are:
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;
any desired helical profile axial length, but often 17-20 feet (5.2-6.2 meters);
2-N (often 5-9) zone coil circuits within the helical profile; and
each coil circuit having an internal surface area of 43-76 square feet (3.9-6.9 square meters).
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
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.
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.
Coolant Flow Control
Referring to
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
Preferred multiple zone C1-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.
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.
The present invention cooling system enables precise fine tuning of flow rates in each zone C1-CN. For example in
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
in normal operation of the embodiment of
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
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.
