Patent Application
20110203497
Not Applicable
Not Applicable
1. Field of Invention
This invention, the boiler system mechanical combustion air unit, applies to combustion heating equipment, and is used to supply the correct amount of combustion air to combustion heating equipment. This invention is termed a molecular balance combustion air unit.
2. Prior Art
For purposes of discussion in the remainder of this document, unless otherwise stated, we will refer to all combustion heating equipment as a boiler. The particular type of combustion heating equipment does not affect the operation of this invention.
The combustion process for heating equipment requires both fuel and combustion air. The ultimate source of combustion air for a boiler, or a system of boilers, is the surrounding atmosphere. More directly, that ultimate source is the outside air. Boilers or boiler systems are typically placed within an enclosed space such as a boiler room, or heating plant, which is in turn located within a building. There are currently two basic methods for supplying combustion air, often referred to as makeup air, to the boilers located within an enclosed space. Firstly, there is open combustion wherein the heating unit draws the combustion air into the boiler room naturally from its outside surroundings. This requires the boiler room, and thus the building, to be open to the free flow of outside atmospheric air into the building. Louvered openings are commonly used for this purpose. This method for introducing combustion air is a source of inefficiency for combustion heating systems. It introduces excess air in addition to the required combustion air. For open combustion, there is not only a need to heat the cold outside air used for the combustion process, but also to heat the associated excess air brought into the building.
In an effort to increase energy efficiency, buildings are being more tightly sealed creating a resistance to the free flow of atmospheric air into the boiler room. This limits the usefulness of the open combustion method by restricting the natural supply of combustion air required for the heating process. Beyond tight sealing, the design of the building itself may not incorporate a means to provide for the free flow of outside air into the boiler room. A boiler plant located several floors below ground level in a central basement area of a building is an example of this situation. To compensate for this problem, a mechanical means for supplying combustion air is employed. This is the second approach to supplying combustion air to boilers. A fan is used to move combustion air from the outside atmosphere, usually through a duct system, directly into the boiler room. The ability to supply the correct amount of combustion air is the main problem with this mechanical combustion air method. The amount of combustion air required will depend on the heat demand to the boilers at any time.
The current technique for providing mechanical combustion air to a boiler or a boiler system is to use a fan to pressurize the boiler room with outside atmospheric air. As the boilers consume air from the boiler room to meet their combustion needs, the pressure in the room will tend to fall relative to the outside surroundings. For open combustion, this reduced pressure relative to the outside air is the driving force that draws the makeup air into the boiler room. The extent to which the pressure drops relative to the outside surroundings will depend on how well the boiler room is sealed, the relative temperatures of the outside and inside air (the relative air densities), and the outside atmospheric pressure. For mechanical combustion, the fan will pump makeup air into the boiler room attempting to compensate for the reduced room pressure. The problem with this approach is that pressurizing a boiler room does not directly translate to supplying the correct amount of combustion air to meet the needs of boilers or a boiler system. Again, the result is either an excess or shortage of combustion air to the boilers. This creates inefficiencies for building heating, and boiler operations. Currently, this is the only known method for providing mechanical combustion air to the boilers.
The overall problems with these methods are a combination of high installation cost, inefficiency and the overall complexity of running the system. For mechanical combustion air sourcing, these problems result from the control strategy used to implement this method. An efficient control strategy must be able to supply the correct amount of combustion air needed to meet solely the demand of the boilers, and that type of control function currently does not exist. The current method for controlling the pressurization of the boiler room requires the tight sealing of the boiler room. Tightly sealing a boiler room is expensive and difficult to achieve. Unless the boiler room is tightly sealed, two major problems result. First, excess cold air is pumped into the general building space while simultaneously supplying combustion air to the boilers. The excess cold air needs to be heated, increasing the heating costs, and thus the inefficiency inherent to the current method. The second problem comes from the technique for pressurizing the boiler room. This method requires a pressure transducer to reference some space outside the boiler room to create a differential pressure signal. Then using that differential pressure reading as the feedback signal to control the fan speed, outside air is pumped into the boiler room in an effort to control the room pressure. This approach is difficult to set up and to stabilize because of the very low differential pressures being measured, usually on the order of 0.001 Inches of Water. If the room is not tightly sealed it becomes difficult to control air flow and room pressurization, and creates an additional avenue for excess air to be brought into the building.
The control function used in current practice for pressurizing a boiler room requires a stable control signal generated from the pressure differential between the boiler room pressure and some external reference pressure. The external signal is usually the inside building pressure somewhere outside the boiler room, or the atmospheric air pressure outside the building. Directly referencing the outside air has always been a considerable problem, and is avoided if possible. Wind, natural air flows and barometric pressure variations create stability problems when referencing pressure to the outside of the building. Using the inside building space creates a more stable signal, but is still inefficient because of the excess cold air brought into the building. There is also the cost associated with the room pressurization method, which includes the cost of sealing the boiler room plus the total cost for installing and setting up the system as a whole.
This industry has an unmet need for another method for delivering combustion air that is more stable, efficient, and easier to employ and install; a method that will supply the correct amount of combustion air to meet the exact demand of the boilers.
In accordance with one embodiment, a molecular balance combustion air unit comprises a main tube for housing the unit and providing a means for conducting combustion air from the outside atmosphere to boilers or a boiler room and including a means for connecting to piping used for conducting air from the outside atmosphere, a pressure and temperature sensing means for use in determining the flow rate of combustion air on a volume and mass basis, a variable speed motor driven fan within the main housing to move the correct volume of combustion air required by the boilers on an O2 and air density basis, and a main controller for interfacing to the boilers and controlling the operation of the unit.
One embodiment of the combustion air unit is illustrated in
The combustion air unit comprises a tubular main housing 102 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 standard steel, galvanized steel, stainless steel, etc., the choice of which depends on the environment in which the combustion air unit will be employed. Standard steel would satisfy a typical installation. A plastic material could be employed provided it had sufficient structural strength for the application. The combustion air unit in this embodiment has a flanged inlet connection 122 for easy attachment to standard piping used to convey outside air to the combustion air unit, and a flanged outlet connection 126 which is the discharge point for the combustion air to the boiler room or boilers. The discharge end could include metal grating as a protective finger guard attached to the flange.
This embodiment of the combustion air unit includes a fan 104 attached to a variable speed fan motor 106. The fan motor is contained within the main housing in this embodiment, but could also be positioned outside the main housing. With the motor contained within the main housing, a direct drive shaft to the fan itself can be employed thus simplifying the design. The material choices for construction of the fan would follow the same reasoning as that applied to materials for the main housing. A variable speed motor drive device 108 powers the motor and receives a control signal from a main control device 116. One embodiment uses a three phase motor connected to a variable speed three phase motor drive to provide speed control of the fan. A brushless DC motor and appropriate controller is an example of another type of power means that can provide a variable speed drive to the fan. In this embodiment, the motor drive uses a standard 120 VAC power source allowing for the incorporation of a simple installation means. This embodiment incorporates a standard electrical plug allowing for a simple plug and run installation. It could also be hardwired to the electrical power source.
Pressure and temperature control signals are provided through a static pressure sensing cap 110, and a temperature sensor 114 respectively. For this embodiment, a pressure sensing port 124 consisting of an approximately ¾″ hole is placed in the main housing centered under the sensing cap. The static pressure sensing cap provides a pressure reading to a pressure transducer incorporated with the main control device via a pressure sensor tube 112 that is attached between the pressure transducer and the pressure sensing cap. The main controller receives a pressure signal from this pressure sensing means. Any available temperature sensing device can be used for the temperature sensor and is connected to the main control device sending a temperature signal to the controller. This temperature sensor measures the temperature of the air as it moves through the main housing. If a heat conductive material such as steel is used for the housing, the temperature sensor can be attached directly to the housing itself and will sense the temperature of the housing unit. This would give an equivalent temperature reading for the combustion air itself. This embodiment uses a Dallas Semiconductor DS600U+ integrated circuit chip on a circuit board as a temperature sensor. This circuit is mounted in direct contact with the metal housing. A motor mounting plate 118 provides support for the fan and motor assembly, and additionally acts as a flow, straightener. This flow straightening means is usually sufficient for most applications. Additional flow straighteners 120 can be added between the fan motor and the pressure sensing port as needed. Flow straighteners, if necessary, can help to provide a more stable static pressure reading at the pressure sensing port, and can also help to eliminate fan system effects.
The controller 116 in this embodiment is an electronic controller which could incorporate any of a number of current methods for executing a sensing and control action for the proper functioning of the combustion air unit. These methods could be with a microcontroller, a microprocessor, an FPGA, a CPLD or an ASIC, to name a few of the most common means. The controller for the combustion air unit also includes a means, usually a pressure transducer, for converting the pressure from the sensor tube into a signal useable by the controller, and a means for connecting to the temperature signal from the temperature sensor. Included in the controller is a means for connecting to each of the boilers which will allow for the identification of the boiler and its current operating state. The controller has a means for connecting to and transmitting a speed control signal to the motor speed controller. The abilities and techniques for designing such a controller are within the current state of the art. What is new is the control strategy behind the algorithm incorporated within the controller for executing the sensing and control functions.
The basis for the operation of this molecular balance combustion air unit is controlling the volumetric flow of combustion air as required by the boilers within the boiler system by adjusting the fan speed of the unit, and using the measured pressure at the pressure sensing port as a feedback control signal. Given a controller as mentioned above, it is easily within the art to readily develop an algorithm for the selected controller that executes the sensing and control functions, provided a workable control strategy is known. This section teaches that strategy.
A key point in devising the control strategy for the combustion air unit is the realization that the heat demand from a boiler system is constantly changing, resulting in a constantly changing firing state for the boilers and consequently, a constantly changing demand for combustion air. A unique aspect of the control function devised for this combustion air unit is that the control function set point is the combustion air demand, and is consequently constantly changing. This is a unique approach in that a typical control function has a fixed set point and a control strategy that adjusts the operating state of the system to match that set point. For the combustion air unit of this invention, the set point changes as the combustion air demand changes, and then the fan speed is controlled to deliver the correct amount of combustion air by using the inlet pressure and the related fan curves as the basis for measuring and controlling the combustion air flow rate.
For discussion, the control strategy behind the operation of the combustion air unit is described in three parts.
The first part is the means for determining the quantity of combustion air needed by each boiler for each of the boiler firing states. This data, entered into the controller during setup, is required in two forms: as a volumetric flow rate, and as a mass flow rate. One of the complex features inherent to the operation of this combustion air unit is that the boilers require combustion air to be delivered on a mass basis, but a fan can only control the delivery of combustion air on a volumetric basis. A unique feature of this invention is that a means has been created to implement this type of operation. Once the combustion air data, used to operate the combustion air unit, has been determined, it can be entered into the controller.
The second part is the means for establishing the control set point, which is the combustion air demand of the boilers in the boiler system, and is required in two forms as described above, a mass flow rate and a volumetric flow rate.
The third part to the strategy is the actual method used to control the delivery of combustion air based on the constantly changing set point.
For the first part, a description of a method for determining the correct amount of combustion air volume required by the boilers for each boiler firing state is provided. Then a means for entering the data into the controller is described. Finally, the means for informing the controller of the current firing state of each boiler is described. The volumetric data for this part can either come directly from the boiler manufacturer, if available, or it can be calculated. The following describes the basis of a method for calculating the combustion air requirements for a boiler from the standard boiler specifications. Standard boiler specifications are the fuel type, the boiler MBH (thousands of BTU's per hour) and the % CO2 in the flue gas, for each of the boiler firing states. In some instances the % excess air is provided instead of the % CO2. This data is standard information from boiler manuals, readily available from the manufacturer.
To determine the required mass and volumetric flow rate in lbm/min and CFM (ft3/min), respectively, for all the firing states of a boiler, begin with the fuel type, the heating capacity in BTU's for each boiler firing state, and the % excess air required for each boiler firing state. If % CO2 instead of % excess air is provided, the % excess air can be calculated from the % CO2. Using the standard density of air, the mass flow rate in lbs mass/min can be determined from the volumetric flow rate. This flow rate data would be entered into the controller during the installation and setup of the boiler system.
The basis for calculating the combustion air requirements comes from the chemistry of combustion. Take the case for the fuel type being standard natural gas with a composition of 80% methane plus 20% ethane. The following is the balanced chemical reaction for combustion of this fuel with k being the excess air fraction. For example, 30% excess air gives a k=0.3. The reaction basis is 1 cubic foot of natural gas at STP.
(1+k)9.2N2+(1+k)2.3O2+0.8CH4+0.2C2H6→(1+k)9.2N2+k2.3O2+1.2CO2+2.2H2O
Solving the right side of this equation provides the relation for % CO2 versus % excess air.
% CO2=120.0/(11.5k+12.6) or (e1)
k=(120.0/(11.5*%CO2))−(12.6/11.5). (e2)
Equation e1 provides the % CO2 given the excess air fraction. Equation e2 provides the excess air fraction k given the % CO2 in the flue gas. From the left side of the balanced chemical reaction, the amount of combustion air required for each cubic foot of natural gas, NG, burned is (1+k)9.2N2+(1+k)2.3O2=(1+k)*11.5 air. The required combustion air at STP per cubic foot of fuel is
1 ft3NG=(1+k)*11.5 ft3. (e3)
The combination of equation e3, the heat demand for the particular boiler firing state, and the standard heat content of this natural gas will give the total amount of combustion air required. The heat content for this standard natural gas at STP is 1142 BTU/ft3. The method is the same for all other fuel types. What changes are the hydrocarbon constituents and the coefficients in the equation and the heat content of the fuel, which normally leads to different results for the excess air equations, e1 and e2. The required data is readily available from standard engineering handbooks.
One more procedure will be required to complete this part of the strategy, and allow for the efficient operation of the combustion air unit. As shown by the chemical reaction above, the basis is the mass of constituents and not the volume. The fan within the unit, on the other hand, delivers on a volume basis. A means is needed to determine the mass flow rate through the combustion air unit, and convert this to the correct corresponding volumetric flow rate that the fan will be capable of delivering. This feature of this invention is a new method not currently employed at this time. The combustion air unit of this invention delivers the correct amount of combustion air on a molecular basis rather than simply pumping volumes of air into the boiler room, and is why it is called a molecular balance combustion air unit.
From the combustion air temperature, measured by the combustion air unit, the density of the combustion air can be determined. For this case, dry air can be assumed, and any error in calculating the density based on this assumption will be within acceptable limits. From the measured temperature T in ° F., the density can be determined from the following relation:
ρ=39.666804/(T+459.58). (e4)
The boiler specifications provide the required mass flow rate of air in lbm/minute to the boiler depending on the firing state of the boiler. The next step is to sum up all the required mass flow rates for all active boilers which we give as M. From these two pieces of data, the required volumetric flow rate Q in ft3/minute that must be supplied by the combustion air unit can be determined using the relation:
Q=M/ρ. (e5)
This Q is the set point for the combustion air unit control function from which the fan will operate. This number is derived from the mass flow rate of combustion air as required for correct boiler operation.
An example will clarify all the steps in this method.
This describes the means for determining the mass and volumetric flow rate of combustion air that must be delivered for each of the boiler firing states. The total amount of combustion air at any point in time would be the sum of the required amounts of combustion air for each individual boiler.
The second part needed to implement a control strategy is the method by which the controller senses the boiler firing states, and then establishes the set point for the control function. Individual boilers are activated to a particular firing state by the boiler call signals, which come from a sequencing controller. A sequencing controller is a standard component of a boiler control system to which all of the boilers are connected. The combustion air controller is similarly connected to these same signals. Methods for electronically interfacing these boiler call signals to the electronic control of the combustion air unit are well known in the industry. Wireless controls are ideal for this type of application. By this method, the combustion air unit always knows the firing state of the boilers and can determine the required amount of combustion air to deliver. This amount of combustion air is the set point used in the control strategy. During installation, the combustion air unit is set up through the controller 116 by entering the data for the required amount of combustion air for every firing state for every boiler. This is typically done through keypad entry. This constitutes part two, the means by which the controller senses the boiler firing states, and establishes the set point for the control strategy.
To recap the first two parts, an electronic controller is connected to the call for heat signals sent to the boiler units by the sequencing controller. A wireless controller is ideal for this type of application. Data for the correct amount of combustion air on a molecular basis needed by the boilers is calculated and entered into the electronic controls during installation by keypad entry to the main controller. As boilers are activated to a particular firing state by the boiler call signals from the sequencing controller, the combustion air controller adjusts the set point accordingly, using the actual measured temperature of the combustion air stream.
The third part is the method by which the combustion air unit delivers the correct amount of combustion air using the measured static pressure.
Combustion air flow control is carried out by controlling the pressure at the pressure sensing port 110 and 124 through the fan speed adjustments. The static pressure is a measure of the volumetric flow rate through the unit based on the standard fan curves. Via the fan curves, the measured static pressure is converted to the volumetric flow rate through the unit. The basic technique is as follows. If the volumetric flow rate, as measured through the static pressure and converted via the fan curves, is less than the set point flow rate, the fan speed is increased. If the measured flow rate is more than the set point flow rate, the fan speed is decreased. The trick is in the technique for determining the volumetric flow rate from the pressure measurement.
A standard fan curve typical of the fan system in this invention is shown in
The controller will need the fan curves in a mathematical form. A common method used to create a mathematical form is to build a piece wise linear curve fit of the fan data from the measured data points. This has the form P=kQ+n on data intervals such as Pa,Qa to Pb, Qb. The piece wise curve is determined from the following relations:
k=(Pa−Pb)/(Qa−Qb) (e6)
and
n=Pa−kQa. (e7)
An AMCA standard fan curve is derived at a fixed fan speed, and at standard air density, usually ρ=0.075 lbm/ft3. When satisfying the condition of dynamic similitude, the fan curve for all other speeds and densities can be determined from the following fan relations
Q=Q
b(N/Nb) (e8)
and
P=P
b(N/Nb)2(ρ/ρb). (e9)
In these relations, Q is the volumetric flow rate, N is the fan speed, P is the fan static pressure, and ρ is the air density. The subscript b indicates the data from the measured base fan curves. The new P versus Q curve is created from the base measured P versus Q data. In
In
From the description above, a number of advantages of some embodiments of the molecular balance combustion air unit become evident:
Accordingly, the reader will see that the combustion air unit of the various embodiments solves current combustion air sourcing problems that plague this industry. Improved sourcing of combustion air improves the efficiency of boiler operations. Another result of the molecular balance combustion air unit is the potential for reduced design and installation costs.
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.
Thus, the scope of the embodiment should be determined by the appended claims and their legal equivalents, rather than by the examples given.
This application claims the benefit of provisional patent application Ser. No. 61/338,712, filed 2010 Feb. 23 by the present inventor.
| Number | Date | Country | |
|---|---|---|---|
| 61338712 | Feb 2010 | US |
