Patent application title: Boiler system stabilizing damper and flue control method
John Robert Weimer (Stacy, MN, US)
IPC8 Class: AF23L1300FI
Class name: Stoves and furnaces dampers electric
Publication date: 2011-08-25
Patent application number: 20110203569
One embodiment of a boiler system stabilizing damper for boiler flue
systems comprising a cylindrical main housing (418), a pressure sensing
means composed of sensor cap (414) and sensor hole (416), a pressure loss
generating means composed of a single blade damper (410), a shaft (406)
and motor (404), and a controller (402) comprising a pressure transducer
and electronic control. Other embodiments are described.
1. A stabilizing damper for flue control and boiler system stability,
comprising: a.) a fluid conduit enabling the transmission of a gaseous
fluid, b.) a means near the inlet of said fluid conduit for sensing the
static pressure of said gaseous fluid, c.) a means near the outlet of
said fluid conduit for generating a variable pressure loss in said
gaseous fluid, and d.) a controller in communication with said static
pressure sensing means, and in communication with said means for
generating a variable pressure loss, the controller comprising electronic
circuitry capable of controlling said variable pressure loss.
2. The stabilizing damper of claim 1 wherein said conduit is of cylindrical geometry.
3. The stabilizing damper of claim 1 wherein said conduit is of oval geometry.
4. The stabilizing damper of claim 1 wherein said conduit is of rectangular geometry.
5. The stabilizing damper of claim 1 wherein said sensing means is a hole through said conduit and covered by a rectangular is of cylindrical geometry.
6. The stabilizing damper of claim 1 wherein said pressure loss means is a single blade damper.
7. The stabilizing damper of claim 1 wherein said pressure loss means is a butterfly damper.
8. The stabilizing damper of claim 1 wherein said pressure loss means is an iris damper.
9. The stabilizing damper of claim 1 wherein said controlling means comprises a stepper motor, a pressure transducer and an electronic control.
10. The stabilizing damper of claim 1 wherein said controlling means comprises a brushless DC motor, a pressure transducer and an electronic control.
CROSS-REFERENCE TO RELATED APPLICATIONS
 This application claims the benefit of provisional patent application Ser. No. 61/338,727, filed 2010 Feb. 23 by the present inventor.
FEDERALLY SPONSORED RESEARCH
 Not Applicable
SEQUENCE LISTING OR PROGRAM
 Not Applicable
 1. Field of Invention
 This invention, the stabilizing damper and boiler flue control method, applies to flue gas venting systems, and is used to control and stabilize both the flue venting system and the boiler/heater operations.
 2. Prior Art
 For purposes of discussion in the remainder of this document, unless otherwise stated, we will refer to all combustion heating equipment that incorporates a flue system, as a boiler. The particular type of heating equipment does not affect the operation of this invention.
 This section explores boiler flue venting systems that incorporate one or more boilers. Current control functions for flue venting systems, and methods for stabilizing boiler operations via a venting system will be examined. To illustrate the prior art and its limitations, we begin with a discussion of the overall structure of a boiler system as illustrated in FIG. 1, and the functions of a boiler system as depicted in the schematic representation in FIG. 7. FIG. 1 illustrates a typical example of a boiler system comprising three boilers 102 connected in a common flue architecture that includes a common breach section 108. This example includes barometric dampers 104 on each boiler which is one method used with current boiler designs for stabilizing boiler and flue operations. These barometric dampers are connected between the flue outlet of the boilers 103 and the riser inlets 105 to the breach section. The breach section acts as a manifold connecting the multiple boilers to the common chimney 110. This example shows a mechanical venting system which includes a flue exhaust fan 112, and a breach pressure sensor 106. The part of this entire boiler system originating from the flue outlet of the boilers and ending at either the chimney outlet, or the exhaust fan in a mechanical venting system, is the boiler flue system.
 The ultimate purpose of a boiler system is to provide building heat, domestic hot water (DHW), or process heat. Except for certain minor cases, there is always a variable demand rather than a constant demand for the quantity of building heat, DHW heat, and process heat. As example, the heat demand in a building will vary depending on the outdoor temperature. An increase or decrease in the outdoor temperature will require a corresponding change in the building heat demand and the heat output of the boiler system. A typical boiler provides heat at a constant output rate. Because of this, a boiler cannot generate heat that continuously matches the process or building heat demand. A constant output will result in either too little or too much heat. In order to match the heat being generated by the boiler system to the required heat demand of the building or process, a boiler is operated cyclically as an ON/OFF device over a certain period of time. One exception to ON/OFF cycling is the use of a modulating output boiler. The average heat generated over the full cyclic operating period when the boiler is ON will equal the total required heat demand over that period. As example, this is a common method employed in a home using a thermostatically controlled forced air furnace. When designing a boiler system, the smallest size boiler is selected that will meet the minimum heat demand without running the boiler in a damaging short cycle. Short cycling is the excessive switching of a boiler between the ON and OFF state over relatively short periods of time in an attempt to equal heat demand. In order to meet the maximum heat demand for the building or process, multiple boilers are added until enough heat generating capacity is reached to meet that maximum demand. This required continuous boiler cycling in a multi-boiler system with a common flue is the root cause of boiler stability problems. Although modulating boilers circumvent the ON/OFF cycling problem, they still have this stability problem associated with flue venting. It is this stability problem that needs to be solved.
 There are only two possible ways to connect boilers to a boiler flue system. One way is for a single boiler to be connected into its own individual boiler flue system. The other way is for multiple boilers to be connected into a single boiler flue system, referred to as a common flue configuration. This common flue configuration in combination with boiler cycling is the source of boiler stability problems arising from multi-boiler systems. Because this is the only way to connect multiple boilers into a single boiler flue system, the stability issue is always inherent to the design. This creates a problem difficult to correct using current methods. In addition to the stability problem, this configuration tends to reduce boiler efficiency. Although less complicated, stability issues also exist with single boilers incorporating a single flue. In this case, the efficiency problem is the overriding, one.
 The stability and efficiency issues can be understood from the key functional aspects of how a boiler system operates which are illustrated in FIG. 7. FIG. 7 is a schematic representation of the processes inherent to a complete boiler system. These boiler system processes can be divided into three sub-processes. They are the combustion sub-process CC, the heat transfer sub-process HE, and the draft sub-process FM. Everything begins with the combustion of the fuel/air mixture, which produces the hot flue gases that are the ultimate source of heat for the boiler system. This is the first sub-process CC. The hot flue gases then move into the heat exchanger, which transfers the heat from the flue gases to a heat transfer medium such as water. A heating system that uses water as a heat transfer medium is referred to as an hydronic heating system. This heat transfer process constitutes the second sub-process HE. The extracted heat has many uses such as heating a building or providing heat to a manufacturing process. Finally, the flue gases are removed from the boiler(s) and moved into the flue piping, which can include a chimney and/or exhauster fan(s). We call this third sub-process the flue mover FM. The flue mover is commonly referred to in this industry as a flue gas venting system, but for our purposes "flue mover" is more descriptive of its actual function. The flue gases are transported through the flue mover and are subsequently disposed. This is usually into the outside air or atmosphere, but could also be into some post processing application such as a gas scrubber to remove contaminants or CO2. The key to making this entire boiler system process operate correctly and efficiently is the proper control of the volumetric flow rate of the fuel/air mixture (combustion gases) and flue gases through the boiler itself. The volumetric flow rate of the combustion and flue gases is ultimately controlled by the flue mover sub-process FM. This is why we prefer calling the flue gas venting system a "flue mover", because that is its exact function. This flue mover is the key to the proper, efficient operation of the entire boiler system. This flow rate establishes the residence time and pressures of the combustion gases in the combustion chamber and the flue gases in the heat exchanger. The residence time and the pressures in the combustion chamber determine the reaction mechanisms and efficiency of the combustion process. The residence time in the heat exchanger determines the efficiency of the heat transfer process from the combustion (flue) gases to the heat transfer medium.
 The discharge static pressure at the flue outlet of the boiler is a measure of the flow rate of the fuel/air mixture and flue gases through the boiler equipment (the combustion chamber, and heat exchanger). Measuring this pressure with a magnehelic gage is one technique used to set up a boiler during boiler commissioning, or to diagnose problems during operation. In order to move the flue gases from the discharge point of the boiler through the flue section (flue mover) and maintain the correct discharge static pressure, the static pressure throughout the flue section must be continuously more negative in the direction of flow from the discharge of the boiler to the discharge of the flue section (the chimney outlet). The problem with the current design of flue sections which causes the instability problems lies with the inability to maintain the correct static pressures at all times in every location throughout the flue system. This occurs as a result of boiler cycling.
 As previously mentioned, two basic types of flue systems currently exist: a single boiler incorporating a single flue, and multiple boilers incorporating a single flue. The flue system may or may not include a chimney. There are four possible ways that flue gases can be forced through the flue system. The first is with a chimney and is called a natural draft system. In this case, the required negative pressures relative to the discharge point of the boiler are generated by the stack effect in the flue system. This creates the necessary forces within the flue mover to move the flue gasses and the combustion gases. The second and third methods involve two types of mechanical draft or mechanical venting systems. One type of mechanical venting system uses an exhaust fan(s) alone. In this case, the negative flue pressures with respect to the boiler flue outlet are created by the venting or exhaust fan(s). The third type uses an exhaust fan(s) in combination with a chimney. This approach uses a combination of both a fan and the stack effect to create the negative flue pressure in relation to the boiler discharge point. The fourth method involves the use of a fan assist within the boiler itself to force the combustion and flue gases through the boiler. This method is commonly used with category III and IV (positive pressure) flue movers. This fourth method creates a new problem in multi-boiler systems in maintaining the correct, required, negative pressure gradient within the flue mover. All flue movers must use a negative relative pressure gradient through the flue system to provide the correct driving force for moving the flue gases.
 There are several methods currently used to control flue gas flow rates in the flue mover as operating conditions change within the system. For natural draft systems this is normally done with a barometric damper. This works by pulling in cooler room air and mixing it with the hot flue gases, thus cooling those flue gases and increasing their density, which in turn reduces the stack effect. The result is a reduction in the flue gas flow rate. The assumption for this method to work is that an excess stack effect always exists within the flue system. The barometric damper works in a Category I, or non-condensing negative pressure, flue system only. This technique will not work with high efficiency boilers and low NOX positive pressure boilers. The excess heat in the flue gases, plus the use of indoor air to cool the flue gases is a source of inefficiency for this type of system. Natural draft systems can also incorporate a draft hood to initiate the flow of flue gases when the boiler first turns on. If a chimney does not contain hot, low density, flue gases, there will be no stack effect to create flue gas movement. A draft hood works by starting the process of filling the chimney with hot flue gases thus creating the stack effect. As with the barometric damper, this creates boiler inefficiency. Another method for initiating the stack effect is to use a two stage or multistage boiler firing system. The first stage, or low fire stage, provides a means for slowly filling a chimney with hot flue gases in order to start the natural draft process. Another method currently used to control boiler flue gas flow rates, which applies to mechanical draft systems, uses a variable speed fan to move the flue gases. A single pressure sensor in the main breach is used for controlling fan speed in an attempt to regulate breach pressures. This works by varying the fan speed in order to hold at least one section of the main breach at a pressure set point. The problem with this approach is that it does not control the required pressure gradient throughout the flue system. This in turn fails to control the all important pressures in the boiler branch sections of the flue system during boiler cycling. The net effect is boiler system instability. Another method used in an attempt to control boiler instability is to increase the breach diameter. This tends to reduce the effects of boiler instability to a tolerable level, but does not eliminate it. In addition to failing to eliminate instability, there are restrictions imposed on boiler layouts using this method. This limits boiler system design options and results in increased building costs.
 An improper flow rate, or a flow rate of flue gases that is not within specifications, is referred to as an uncontrolled draft. An uncontrolled draft results in flue gas flow rates being higher or lower than specifications. An uncontrolled draft can be in either a fixed or constantly fluctuating state between too high and too low. An uncontrolled draft that is too high is an overdraft case. An uncontrolled draft that is too low is an under draft case. A controlled draft is often referred to as a balanced draft, and this is the flue state that is desired for the proper operation of the boilers.
 The following are some of the consequences of uncontrolled draft:  1. Poor combustion efficiency.  2. Unstable pilot.  3. Pilot and main flame ignition problems.  4. Flame retention and flame failure problems.  5. Unstable flame pulsations.  6. Incorrect fuel/air ratios that result in problems such as sooting, CO and NOx.  7. Heat transfer problems within the heat exchanger.  8. Incomplete combustion carried into the breach piping.  9. Damage to the boiler/heating unit, and/or the flue piping.
 An actual case history illustrates the substantial problems that exist using the current flue control methods. FIG. 2 shows a boiler layout employed in a boiler heating system. It consists of four boilers in a common flue architecture with a chimney and employing mechanical draft exhausters. By definition, a boiler system firing state is one of the possible combinations of ON OFF conditions for each and every boiler making up a boiler system at some point in time. For example, considering the boiler system in FIG. 2, one firing state would be boiler #1 ON and all others OFF at a point in time. Another state would be Boiler #2 ON and all others OFF at a point in time. The full gamut of firing states would be all possible combinations of ON OFF states for the boilers in the system.
 FIG. 3 is a table showing operating data for some of the boiler operating states for the example boiler system illustrated in FIG. 2. Each column in this table gives the operating data for one of the four boilers, labeled 1 through 4 in a particular boiler system firing state. The table shows four columns, one for each boiler. Each row, labeled 1 through 7, is the full operating data for the boilers during a firing state at a point in time. This data consists of the ON/OFF state of the boiler with the boiler discharge pressure below measured in inches of water column (InWC). The required boiler discharge pressure, as established by the manufacturer of the boilers in this system, was in the range from -0.05 InWC to 0.0 InWC.
 For a stable system, the boiler discharge pressure must be within the recommended operating range as established by the boiler manufacturer for all possible boiler firing states of the boiler system. As shown by the data in this table, the magnitudes of the boiler discharge pressures in this system range widely from 0.0 InWC to -0.6290 InWC depending on the firing state of the boilers. The extremely high negative pressures are well out of the acceptable operating limits, in some cases by over a factor of ten. Furthermore, as the boiler system changes from one boiler firing state to another, the boiler discharge pressures fluctuate wildly outside the recommended operating pressures as established by the manufacturer. This is a classic example of a seriously unstable boiler system, and is not uncommon in the industry. The remedies for such a problem are typically very expensive. This expense is compounded by the fact that these types of instability problems do not normally show up until the system has been fully installed and is in the process of being commissioned. Often, the end result is to completely replace the system using a trial and error approach to find some workable solution. Many times, if the system instability does not create a situation that is too far out of tolerance, the system is left as is. This often results in premature failure of equipment, and operating inefficiency.
 Current attempts to solve the balance and instability problems consist of using fixed position dampers, barometric dampers, redesigning the boiler system and building to employ multiple single boiler/single flue systems, and increasing the diameter of the breach piping. The best one can do with these techniques is minimize the problem, but at additional costs. Fixed position dampers usually don't work. A barometric doesn't work with high efficiency heating units and decreases the efficiency of less efficient units. Increasing the breach pipe diameter adds costs and has size limitations without actually eliminating the stability problem. This is the most common approach. An increase in the breach diameter necessitates an increase in the chimney diameter. A common method used today for a mechanical drafting system employs a pressure sensor to control draft. Some boilers incorporate a non-modulating damper that is used to retain the residual heat of the boiler in its OFF state. When the boiler is ON, the damper is fully open. When the boiler is OFF, it is closed to prevent heat from being naturally drafted out the chimney and thus wasted to the outside. This does nothing to eliminate the stability problem. In a nutshell, the way this is currently handled is to simply find an acceptable way to live with the problem.
 In accordance with one embodiment a stabilizing damper comprises a main tube for housing the unit and providing a means for conducting flue gases through the damper, and incorporating a means located on the damper inlet end for connecting to the boiler flue output point, and another means located on the damper outlet end for connecting to the riser inlet to the breach section of the flue system, also incorporating a pressure sensing cap with a sensor hole in the main tube on the stabilizing damper inlet side for determining the boiler discharge pressure, and incorporating on the outlet side a damper mechanism capable of varying a damper blade angle resulting in a continuous change in static pressure loss in the flue gas flow of the main tube, a motor for varying the blade angle or position, and a main controller for interfacing to the boilers, and controlling the operation of the stabilizing damper.
 FIG. 1 shows a standard layout for a three boiler flue system with barometric dampers.
 FIG. 2 shows a layout for the example boiler heating system including the boilers, the breach and a partial chimney section.
 FIG. 3 shows boiler operating state data for the boiler system illustrated in FIG. 2.
 FIG. 4A shows a side view of the stabilizing damper.
 FIG. 4B shows an inlet view of the stabilizing damper.
 FIG. 4C shows an outlet view of the stabilizing damper.
 FIG. 5 shows a perspective 3D view of the stabilizing damper.
 FIG. 6 shows a flow chart for the control operation of the stabilizing damper.
 FIG. 7 shows a functional diagram for a boiler flue system.
 FIG. 8 shows a layout for a three boiler flue system with the stabilizing dampers.
 102 Boiler  103 Boiler flue outlet or discharge point  104 Barometric damper  105 Riser inlet to breach section  106 Breach pressure sensor  108 Breach section of boiler flue system  110 Chimney section of boiler flue system  112 Flue exhauster fan  402 Controller for the stabilizing damper  404 Damper blade motor for stabilizing damper  406 Blade shaft  408 Standoff support  410 Damper blade  412 Pressure sensor tubing  414 Pressure sensor cap on main housing  416 Pressure sensor hole in main housing  418 Main housing  420 Stabilizing damper inlet  422 Stabilizing damper outlet  424 Damper shaft seal bushing  426 Sensor cap tube fitting  428 Mounting plate  CC Combustion sub-process of boiler system  HE Heat transfer sub-process of boiler system  FM Draft sub-process of boiler system (flue mover)  802 Boiler  803 Boiler flue outlet or discharge point  804 Stabilizing damper  806 Breach pressure sensor  808 Breach section  810 Chimney section  812 Flue exhauster fan
FIGS. 4A, 4B, 4C, and 5--Preferred Embodiment
 One embodiment of the stabilizing damper is illustrated in FIG. 4A (side view), FIG. 4B (bottom view), FIG. 4C (top view), and the isometric view in FIG. 5. FIG. 5 illustrates the mechanical mechanism of this embodiment, while FIG. 4A and FIG. 4C also include the control components. The stabilizing damper has a tubular main housing 418 made from uniform sheet material. In this embodiment the main housing is a rolled cylinder of constant diameter along the main length of the cylinder. This sheet material can be a metal such as galvanized steel, stainless steel, etc., the choice of which depends on the environment in which the stabilizing damper will be employed. A stainless steel such as AL29-4c is preferable in hot, acidic environments to avoid corrosion problems. Other types of stainless steel or galvanized steel can be used in more tolerable environments. In a cool, condensing, acidic environment a plastic material could be employed provided it was resistant to attack by the acidic environment, and had sufficient structural strength for the application. It is preferred that the stabilizing damper inlet 420 be shaped as a standard female opening to facilitate connection to the boiler discharge piping. It is preferred that the stabilizing damper outlet 422 be shaped as a standard male opening to facilitate connection to the riser inlet of the flue piping.
 One embodiment of the damper mechanism is a damper blade 410, as a flat circular plate, attached to a round shaft 406. Other damper mechanisms could be a multi-blade butterfly or an iris. The material choices for construction of the damper blade and the shaft would follow the same reasoning as that applied to materials for the main housing. Damper shaft bushings 424 on each end of the damper blade are used for providing a seal between the shaft and the housing, and to provide a smooth rotation of the shaft and blade. The shaft is connected to a motor 404 which acts as a means for rotating the damper blade. One embodiment uses a stepper motor to provide an accurate positioning control of the damper blade. A brushless DC motor is an example of another type of motor that can provide positioning control. A mounting plate 428 provides a means of support for the motor mechanism and the controller 402, and provides shielding from the potentially hot surface of the main tubular section. Standoff supports 408 are attached to the main housing and the mounting plate 428, and provides a means of support for the mounting plate. In this embodiment the standoff supports were constructed from as thin a piece of sheet metal as mechanically and structurally possible. Since the outlet flue gases can reach high temperatures, sometimes on the order of 650 degrees Fahrenheit, a means is required to protect the electronic controls and motor from excessive heating. The thin material and cutaway sections for the standoff supports eliminate overheating from heat transfer by minimizing the cross sectional area needed for significant heat transfer to take place. The small cross sectional area of the standoff support limits heat transfer to the mounting plate, and a large surface area provides a dissipative heat transfer path to the surroundings rather than transmission to the mounting plate. In this embodiment all of the longitudinal edges were folded to increase the structural strength of the standoff support while minimizing the thickness of the construction material.
 A pressure sensing cap 414 is located at the inlet side of the stabilizing damper. For this embodiment, the pressure sensing cap is placed above the damper inlet approximately a length equal to one radius of the main housing diameter. The cap and its position provide a stable static pressure reading from which the damper operates. In order to keep a stable flue gas flow field, and thus a stable static pressure reading, the pressure sensing cap is kept a length of at least 2 main tube diameters away from the fully open inside edge position of the damper blade. The sensing cap for this embodiment is approximately 2 inches by 2 inches in the base dimensions, and approximately 1.5 inches in height. A pressure sensor hole 416 of approximately 3/4 inch in diameter for this embodiment is centered under the pressure sensing cap, and into and through the main housing. A sensor cap tube fitting 426 is placed in the side of the pressure sensing cap centered at approximately 1/2 inch from the top of the cap. A pressure sensor tubing 412 is attached to the sensor cap tube fitting and runs to a pressure sensing means which is part of the controller 402 for the stabilizing damper. This pressure sensor tubing can be made of a flexible material such as rubber tubing, or a rigid material such as stainless steel metal tubing. The appropriate sensor cap tube fitting is used depending on the type of pressure sensor tubing used. A controller for the stabilizing damper would include a means for sensing the pressure at the pressure sensing cap and then activating the motor to move the damper mechanism.
Operation--FIGS. 4A, 4B, 4C, 6, and 8
 FIG. 8 shows the system in FIG. 1 with the barometric dampers 104 replaced by the stabilizing dampers 804 of this invention. Each of the stabilizing dampers controls the discharge pressure at each of the boiler discharge points 803 in order to hold these pressures at their required operating points.
 To stabilize the boiler system and provide flue control, the volumetric flow rate of the flue gases from the boiler needs to be controlled within specifications established by the boiler manufacturer. This can be accomplished by adjusting or controlling three variables at the boiler discharge point: the flue gas velocity pressure, the static pressure, and the flue gas density.
 The main housing acts as a pipe for the transport of the flue gases, as well as the support for components of the stabilizing damper. The diameter of the flue outlet of the boiler at the boiler discharge point is set by the boiler manufacturer from the boiler design specifications. These specifications would include the flue gas density and volumetric flow rate. The volumetric flow rate at any point is determined by the flue gas density, the flue gas velocity pressure and the static pressure at that discharge point. The flue gas density within the stabilizing damper is the same as that set by boiler specifications. If the diameter of the stabilizing damper is the same as the diameter of the boiler discharge, the velocity pressure at the pressure sensor hole 416 will be the same as that set by the boiler specifications. In this case, the static pressure measured at the pressure sensor hole is the only remaining variable needed to control the volumetric flow rate of the flue gases. The set point static pressure is used for controlling the volumetric flow rate and is measured at the pressure sensor hole. It is also the same static pressure as that at the boiler discharge point and set by the boiler specifications. The stabilizing damper controls this static pressure by varying the position of the damper mechanism as the flow conditions vary in the flue system. This in turn controls the flue and stabilizes the boiler system irrespective of the fluctuations in flow properties within the flue system itself.
 If the diameter of the balancing damper is different from that of the boiler discharge point, it will be necessary to calculate or measure a new static pressure set point and velocity pressure in order to maintain the correct volumetric flow rate for the flue gases. Calculating the new static pressure and velocity pressure is within the ability of persons skilled in the discipline of fluid mechanics. Instead of calculating the pressure set point, a method for measuring this pressure, after the damper system has been installed, is presented here. When a stabilizing damper with a diameter different from the diameter at the boiler discharge point is installed, it will be necessary to attach a short section of straight pipe, typically one pipe diameter in length, at the discharge point of the boiler followed by either a pipe reducer or diffuser finally followed by the stabilizing damper. A small hole is placed into the short section of pipe at the discharge of the boiler, and a magnehilic is used to measure the static pressure at this point. This is a standard technique currently practiced by boiler installers. A second magnehelic would be attached to the pressure sensing tube from the pressure sensing cap of the stabilizing damper. With the stabilizing damper maintained open, in the full unrestricted flow position, the boilers are fired and adjusted to the correct operating point using the magnehelic pressure at the discharge of the boiler as a reference. The magnehelic pressure measured at the stabilizing damper would then give the correct set point operating pressure for the stabilizing damper.
 The damper mechanism creates a varying resistance, and thus a varying pressure loss in the flow of flue gases, as the damper mechanism changes position. In this embodiment the damper mechanism is a simple single blade damper, and a change in the blade angle would constitute a change in position of the damper mechanism. The pressure loss created by the damper mechanism is equal to the static pressure at the outlet of the stabilizing damper less the static pressure at the pressure sensor hole. Thus, the static pressure at the pressure sensor hole can be held constant by a simple adjustment of the damper mechanism as the static pressure at the outlet of the stabilizing damper fluctuates. These fluctuations are a result of variations in the operating conditions in the flue system, which manifest as instability.
 Any flue system will require a means for moving the flue gases though the system. This means has been previously described as the stack effect and/or mechanical venting. All flue systems, whether with the stabilizing damper or not, require a sufficiently more negative static pressure through the flue system, which is provided by the flue moving means. This normally required flue moving means also provides the more negative pressure at the outlet of the stabilizing damper that enables the damper to work correctly.
 The controller requires a control signal used to adjust the damper mechanism that controls the discharge pressure of the boiler. The control signal is supplied through a pressure sensing port made up of the sensor hole in the main housing plus the sensor cap, which is attached to a pressure transducer that provides the pressure signal used by a controller to adjust the damper mechanism. The pressure signal is constantly monitored by the controller. If the pressure is too low or too high, the damper mechanism is adjusted to a more closed or more open position, creating more or less static pressure losses from the damper mechanism until the boiler discharge pressure is within the proper operating range for the boiler. Electronic control means and controllers are readily available for this control purpose. A wireless controller is ideal for this purpose. An example of a control strategy for this invention that can be incorporated within an electronic controller is shown in the flow chart in FIG. 6.
 As the pressures within the flue section fluctuate from variations in boiler firing cycles and atmospheric changes, creating conditions for instability and inefficiency in the boiler system, the stabilizing damper holds the boiler discharge pressure within its proper, optimal operating range.
 From the description above, a number of advantages of some embodiments of the stabilizing damper become evident:  (a) First and foremost, this damper provides a means to eliminate the system instability problems currently associated with boiler room flue system.  (b) The stabilizing damper affords a significant reduction in design, material and installation costs for boiler systems.  (c) The stabilizing damper provides a means for improving the efficiency of boiler operations.  (d) The stabilizing damper simplifies boiler system design methods and is more forgiving of boiler system design errors.  (e) The stabilizing damper will permit Category I II III and IV boilers to be installed in a Category I breach flue system. This is impossible by current methods.  (f) The stabilizing damper will permit the mixing of Category I II III and IV boilers in a common flue system. This is impossible by current methods.
Conclusion, Ramifications, and Scope
 Accordingly, the reader will see that the stabilizing damper of the various embodiments solves the current boiler instability problems that plague this industry. Furthermore, along with the stabilization of boiler operations comes an improvement in the efficiency of boiler operations. Another unexpected result of the stabilizing damper is the potential for reduced design and installation costs for the boiler system, and also potential reduced costs for the building itself. The use of the stabilizing damper provides for a boiler design that is far more forgiving of design errors.
 Although the description above contains many specificities, these should not be construed as limiting the scope of the embodiment but as merely providing illustrations of some of the presently preferred embodiments. For example, the main housing although presented as a cylindrical device, could be of another shape such as a square, oval, etc.; the controller can be a wireless electronic device rather than the usual wired controller.
 Thus, the scope of the embodiment should be determined by the appended claims and their legal equivalents, rather than by the examples given.