COMBUSTION CONTROL SYSTEM

Abstract: A method and apparatus for controlling combustion in a process heater (10) in which the flow of fuel (20) and oxidant are controlled pneumatically includes adjustable valve (82, 84) means to modify a signal to either an air flow control damper (50) or a fuel control valve (58) to maintain maximum thermal efficiency. Typically, a fuel control valve (58) and an air flow control valve (50) are pre-set, and then, in response to a signal according to the demands of the system, move in parallel. The ratio controller (82, 84) of this invention operates to allow an easy and efficient means of varying the ratio of fuel (20) to oxidant.

Description: Description COMBUSTION CONTROL SYSTEM Technical Field This invention relates broadly to furnace, boilers, incinerators, and like equipment wherein fuel is burned. More particularly, this invention relates to combustion equipment which regulates the flow of combustion air and fuel in response to a signal generated from a condition of a process. Still more particularly, this invention relates to combustion equipment which will provide a fuel- lean mixture of combustion components as process conditions modify the demand for process heating. The efficient control of combustion relates to the efficient control of the fuel and combustion oxidant, usually air. Efficient control of combus¬ tion is essential in the power industry, chemical industry, and various other process industries. The amount of heat furnished by the combustion process depends upon the amount of heat needed by the operation requiring the heat, whether for the generation of power, or for the chemical process, or for an oil refinery process. In addition, the amount of heat needed will continually vary, thus requiring continual control of the combustion. In some industries, particularly the power industry and the chemical industry, the relative efficiency of the combustion process may be accom- plished by the use of elaborate control systems, such as computers, various kinds of automatic valves, and electronic controls. As a result, numerous control systems have been designed to control the combustion processes. And, with time, these systems tend to become more elab¬ orate. Then, as a system becomes more elaborate, the number of components, such as dials, gauges, recorders, computers, and the like, requiring the attention of the operator, increases. With increased complexity, the probability of someone making a mis¬ take increases greatly. The operator has an increased number of recording devices to observe and mtaintain, dials to continually observe, and switches to be ready to operate, for example. Disclosure of Invention Therefore, the primary object of this invention is to provide a method and apparatus for controlling combustion in a process which is efficient, easy to operate, and economical. Another object of this invention is to provide a method and apparatus for controlling combustion in a process which will require only a minimum number of control components in order to regulate the combus- tion effectively. Still another object of this invention is to provide a method and apparatus for controlling com¬ bustion in a process in which fuel and combustion air are easily regulated for more efficient energy con- sumption. Still another object of this invention is to provide a method and apparatus for maintaining close control of the combustion in a process. Another object of this invention is to provide a method and apparatus which can be easily adjusted to regulate a combustion temperature. Another object of this invention is to provide a method and apparatus for controlling combustion in a process heater in which a combustion temperature may be easily maintained. Still another object of this invention is to provide a method and apparatus for maintaining close control of the energy consumption in a pro¬ cess heater. Still another object of this invention is to provide a method and apparatus to modify quickly and efficiently the ratio of air to fuel of a com¬ bustion system. These and other objects of the invention will become apparent from the accompanying description and drawings and attached claims. This invention describes a greatly simplified and efficient method and apparatus for combustion control by eliminating many sophisticated and highly technical controlling devices commonly shown in other combustion control systems. By simplifying the combustion control system, this invention reduces the number of control devices which the operator must understand and closely ob- serve. Consequently, the risk of accident caused by inattention of the operator is greatly reduced. Further, this invention provides for a more efficient combustion control system by assuring a fuel-lean combustion during a change in combustion requirements. All of these advantages are obtained by the application of very few, and simple, low-cost com¬ ponents. The method and apparatus of this invention include generating a signal from a condition of a process stream and adapting that signal to actuate various fuel and combustion oxidant control com¬ ponents. The type of signal generated includes a fluid pressure signal, and is usually a pneumatic signal. The condition of the process stream from which the signal is generated is preferably a temp- erature of the process, or the pressure of a boiler. In one typical application, a temperature-re¬ sponsive device measures a temperature of the pro¬ cess and emits a fluid-pressure signal which may be either an increasing pressure signal or a de¬ creasing pressure signal. The signal is communicated in parallel to a signal-restrictive member, such as a needle valve; to a first signal-responsive device, such as a first fluid-responsive relay; and to a second sig¬ nal-responsive device, such as a second fluid-re¬ sponsive relay. The first fluid-responsive device is adapted to respond to a signal of a first char¬ acterization, and the second fluid-responsive de- vice is adapted to respond to a signal of a second characterization. The signal-restrictive member is further adaptable to communicate a signal to a sig¬ nal to a signal-collecting device, such as a volume tank, and the signal-collecting device is adaptable to provide a signal of a first characterization and a signal of a second characterization. The signal- collecting device is adaptable to communicate in parallel with the first signal-responsive device and with the second signal-responsive device. The first signal-responsive device is further adaptable to communicate with an air control device which is adaptable to control a flow of combustion air in response to a signal from the first signal-respon¬ sive device. The second signal-responsive device is further adaptable to communicate with a fuel control device which is adaptable to control a flow of fuel in response to a signal from the second signal-responsive device. The inventio teaches a method and apparatus for controlling combustion in which fuel-lean com¬ bustion is provided during an interval in which the firing rate is changed. For example, when the process sytem demands more heat, the flow of com¬ bustion air is increased prior to an increase in the flow of fuel, thereby maintaining a fuel-lean mixture during the interval of change. Or, when the process system requires the heat to be reduced, the flow of fuel is decreased prior to a decrease in the flow of combustion air, thereby maintaining a fuel-lean mixture during this interval of change. In a typical combustion operation, a fuel flow control valve and an air flow control valve, or damper, react in parallel in proportion to the signal generated by the temperature controller. Thus, after being initially set, at chosen points, a fuel control valve and an air flow con¬ trol valve maintain a given ratio of fuel to oxi¬ dant as they move in response to the signal gen¬ erated by the temperature controller. I had become aware of the manner in which fuel and oxidant valves react proportionately in a typical combustion system, and described in my U. S. Patent Number 4,261,508, issued April 14, 1981, and my PCT Application PCT/US 81/00367, pub¬ lished September 30, 1982, a method and apparatus which provide for a fuel-lean mixture during an interval of change in the heat demand of a process system. Now, in this application, I am describing a method and apparatus in which an adjustable valve is in the system in order to modify, or bias, the signal to either the fuel flow control valve or the air flow control valve. An adjustable valve, or relay, which I have found useful for this pur¬ pose is one known as Fairchild Model 21212 Adjust- able Ratio Relay. The adjustable flow control valve, which I also refer to as a ratio controller, can serve JTTREAIT O PI /fa ~ 1PO ^ ?NATlO^ the following purposes: To compensate for changes in the heating value of the fuel being utilized; To modify the fuel/air ratio at various furnace load conditions to maintain maximum thermal efficiency; To offset an undesirable fuel/air ratio that might exist at a particular time as a result of the relative fixed flow character- istics of the flow control devices.. The ability to bias one of the output sig¬ nals from the combustion system of this invention is desirable in that it provides, within the lim¬ its of the combustion system, the apparatus to easily and quickly change the ratio of air to fuel. For example, by raising or lowering the air output signal relative to the fuel output signal, or vice versa, more or less combustion air is introduced to the burners per unit of fuel. Brief Description of Drawings In the accompanying drawings: FIGURE 1 is a schematic drawing of a typical process heating system. FIGURE 2 is a diagrammatic drawing of a com- bustion control system according to this invention. FIGURE 3 is a schematic drawing of a combus¬ tion control system according to this invention. FIGURE 4 is a schematic drawing of a combus¬ tion control system according to this invention in one particular mode of operating condition. FIGURE 5 is a schematic drawing of a combustion control system according to this invention in a second particular mode of operating condition. y^ Q HEA FIGURE 6 is a schematic drawing of a combus¬ tion control system according to this invention in¬ cluding a ratio control device. Best Mode for Carrying Out the Invention FIGURE 1 describes a typical combustion and heating system for a conventional process industry, which, in its basic arrangement, comprises a fur¬ nace or heater 10, generally, having a process in¬ let stream 12 entering therein, a typical heating coil 14, a process outlet stream 16, and a burner 18 fired by a fuel line 20 and air supplied as through opening 22. FIGURE 2 describes, diagrammatically, for added simplicity, one manner in which my invention may be utilized in any type of equipment involving combustion of a fuel and an oxidant. In one embodiment of my invention as described in FIGURE 3, line 24 provides communication between process outlet stream 16 and a temperature control- ler 26. Temperature controller 26 may be one of any conventional devices for measuring temperature and providing a signal to other equipment in re¬ sponse to that temperature. Thus, temperature controller 26 measures a temperature of the process stream and provides a fluid-pressure signal through lines 28, 30, 32, and 34, in parallel to a first fluid-responsive relay 36, a needle valve 38, and a second fluid-responsive relay 40. First fluid-responsive relay 36 responds to an increasing pressure signal in lines 28 and 30 and transmits a signal through line 42 to an air flow control device 44, which, in this embodiment of my invention, further communicates a signal, through line 45, to a conventional pneumatic de- vice 46, generally, which operates, through con- * !ΪEXt* OMPI R TA_ * ° nection 48, louvers 50, positioned in air duct 52, to regulate the amount of combustion air. Second fluid-responsive relay 40 responds to a decreasing pressure signal in lines 28 and 34 and transmits a signal through line 54 to a fuel control device 56, generally, which further transmits a signal through line 57 to fuel control valve 58, generally, which operates to control the amount of fuel transmitted to burner 18. Needle valve 38 is preferably an adjustable valve, and is subject to the pressure condition, or change in pressure, in lines 28 and 32. Then, needle valve 38 may be adjusted to transmit, at a reduced rate, the pressure condition in line 32 to volume tank 60 through line 62. Thus, an in¬ creasing pressure level in volume tank 60 is de¬ layed for a period of time dependent upon the de¬ gree of restriction provided by the adjustment of needle valve 38. Also, when there is a decreasing pressure signal in line 28, a decrease in the pres¬ sure level in volume tank 60 is delayed for a per¬ iod of time dependent upon the degree of restric¬ tion in needle valve 38. Volume tank 60 communicates in parallel with first fluid-responsive relay 36 and second fluid- responsive relay 40 through lines 62, 64, and 66. Essentially, air flow control device 44 and fuel control device 56 are automatic/manual control devices which may be set at a particular point to respond to particular signals presented to them through lines 42 or 57 in order to control the flow of air and fuel. One example of such a control would be Foxboro Model 135S. An example of a first fluid-responsive relay 36 os Foxboro Model B0114YL pneumatic relay. An example of a second fluid-responsive relay 40 is Foxboro Model B0114BZ pneumatic relay. FIGURE 4 describes a pach of increasing pres¬ sure transmission 68 which originates from the sig¬ nal generated by temperature controller 26. This path is to show the cooperation of the various con¬ trols and does not suggest that the type of signal and strength of signal are the same at all points along that path. FIGURE 5 describes a path of decreasing pressure transmission 70 which also originates from a sig¬ nal generated by temperature controller 26. In a typical operation, temperature controller 26 is set at a chosen temperature so that it may emit a signal of increasing pressure when the tem- perature measured is below the set point, and so that it may emit a signal of decreasing pressure when the temperature measured is above the set point. Thus, when the temperature is at the chosen, or set, point, the pressure magnitude remains con¬ stant. When the temperature of the process stream 16 falls below the set point of temperature controller 26, temperature controller 26 emits a signal of increasing pressure which follows the path 68 shown in FIGURE 4. This signal of increasing pressure is immediately transmitted through first fluid- responsive relay 36 to operate pneumatic device 46 to cause the flow of combustion air to increase promptly, before the flow of fuel is increased. This signal of increasing fluid pressure is simultaneously transmitted through needle valve 38 and into and through volume tank 60, where, be¬ cause of the restriction caused by needle valve 38, the pressure builds up gradually, and finally is relayed to the high pressure side of second O PI _. fluid-responsive relay 40 to actuate relay 40 and be transmitted through relay 40 to actuate fuel control device 56. The signal of increasing pres¬ sure actuates fuel control device 56 to open or further open, fuel control valve 58 and increase the flow of fuel to burner 18. If the temperature indicated by temperature controller 26 is above its set point, then temper¬ ature controller 26 emits a signal of decreasing pressure which follows the path 70 shown in FIGURE 5. This signal of decreasing fluid pressure is transmitted simultaneously through the low pres¬ sure side of fluid-responsive relay 40 and fuel con¬ trol device 56 as well as through needle valve 38, through volume tank 60, and the low pressure side of first fluid-responsive relay 36. In this manner, the signal of decreasing pressure actuates fuel control device 56 immediately which acts- to close fuel control valve 58 to some degree to reduce the flow of fuel to burner 18 before the flow of air to burner 18 is decreased. The signal of decreas¬ ing pressure is transmitted through needle valve 38 to volume tank 60, but only gradually reduces the fluid pressue in volume tank 60 so that the signal of decreasing pressure presented to first fluid-responsive relay 36 lags behind the time of the full signal presented to second fluid-respon¬ sive relay 40. In either situation, as outlined above, an increasing pressure signal or a decreasing pres¬ sure signal, the arrangement of the needle valve and volume tank causes a delay in the response of either the first or second fluid-responsive relay so that a fuel-lean mixture is maintained during any change in the operating conditions of the pro¬ cess. It is readily seen, however, that opposite responsiveness of the temperature control and fuel and air control devices could be utilized and the same effectiveness of a fuel-lean mixture still maintained, the important responsiveness being a delay in the transmitting of the fuel control sig¬ nals It is also clear that following any interval in which the firing rate is changed, the pressure level in the volume tank 60 reaches equilibrium and becomes constant with the fluid pressure sig¬ nal and no change is induced in the flow of either the combustion air or the fuel. In the combustion control system described in FIGURE 6, I show an adjustable valve, or ratio control relay 82, in line 42, as it would be suit¬ ably placed for modifying the signal to the air flow control device 44. As an alternative, I also show in FIGURE 6, a ratio controller 84 in line 54, as it would be suitably placed for modifying the signal to fuel control device 56. I have provided for the choice of a ratio controller in either air flow control line or fuel control line. It should be understood that I do not require the ratio controllers 82 and 84 to be present in the system at the same time. Since many different embodiments of this in¬ vention may be made without departing from the spirit and scope thereof, it is to be understood that the specific embodiments described in detail herein are not to be taken in a limiting sense, since the scope of the invention is best defined by the appended claims.

Claims: Clai s 1. In a combustion control system having means to control a flow of fuel and means to control a flow of oxidant in response to a signal gen- erated from a condition of a process, the im¬ provement in combination therewith comprising: means for controlling the ratio of fuel to oxidant. 2. A combustion control system as described in Claim 1, wherein: the means for controlling the ratio of fuel to oxidant includes a ratio controller. 3. A combustion control system as described in Claim 2, wherein: the ratio controller is adjustable. 4. A combustion control system as described in Claim 3, wherein: the ratio controller is manually adjust¬ able. 5. A combustion control system as described in Claim 1, wherein: the means for controlling the ratio of fuel to oxidant is a control of the flow of oxidant. 6. A combustion control system as described in Claim 5, wherein: the control of the flow of oxidant is adjustable. 7. A combustion control system as described in Claim 6, wherein: the control of the flow of oxidant is manually adjustable. JTJREA O PI 8. A combustion control system as described in Claim 1, wherein: the means for controlling the ratio of fuel to oxidant is a control of the flow of fuel. 9. A combustion control system as described in Claim 8, wherein: the control of the flow of fuel is adjustable. 10. A combustion control system as described in Claim 9, wherein: the control of the flow of fuel is manually adjustable.

Inventors: BROACH GEORGE C, BROACH, GEORGE, C.
Applicants: BROACH CO G C, THE G. C. BROACH COMPANY
Application year: 1982
Main classification: G05D23
IPC codes: F23N1