Patent Application
20080124667
The present invention relates generally to the field of gas-fired appliances. More specifically, the present invention pertains to systems, methods, and controllers for regulating gas pressure to gas-fired appliances such as warm air furnaces.
Warm air furnaces are frequently used in homes and office buildings to heat intake air received through return ducts and distribute heated air through warm air supply ducts. Such furnaces typically include a circulation fan or blower that directs cold air from the return ducts across a heat exchanger having metal surfaces that act to heat the air to an elevated temperature. An ignition element such as an AC hot surface ignition (HSI) element or direct spark igniter may be provided as part of a gas burner unit for heating the metal surfaces of the heat exchanger. The air heated by the heat exchanger can be discharged into the warm air ducts via the circulation fan or blower, which produces a positive airflow within the ducts. In some designs, a separate inducer fan or blower can be used to remove exhaust gasses resulting from the combustion process through an exhaust vent.
In a conventional warm air furnace system, gas valves are typically used to regulate gas pressure supplied to the burner unit at specific limits established by the manufacturer and/or by industry standard. Such gas valves can be used, for example, to establish an upper gas flow limit to prevent over-combustion or fuel-rich combustion within the appliance, or to establish a lower limit to prevent combustion when the supply of gas is insufficient to permit proper operation of the appliance. In some cases, the gas valve regulates gas pressure independent of the inducer fan. This may permit the inducer fan to be overdriven to overcome a blocked vent or to compensate for pressure drops due to long vent lengths without exceeding the maximum firing rate of the appliance.
In some designs, the gas valve may be used to modulate the gas firing rate within a particular range in order to vary the amount of heating provided by the appliance. Modulation of the gas firing rate may be accomplished, for example, via pneumatic signals received from the inducer fan, or via electrical signals from a controller tasked to control the gas valve. While such techniques are generally capable of modulating the gas firing rate, such modulation is usually accomplished via control signals that are independent from the control of the combustion air flow produced by the inducer fan. In some two-stage furnaces, for example, the gas valve may output gas pressure at two different firing rates based on control signals that are independent of the actual combustion air flow produced by the inducer fan. Since the gas control is usually separate from the combustion air control, the delivery of a constant gas/air mixture to the burner unit may be difficult or infeasible over the entire range of firing rate.
In some systems, supply air temperature and pressure sensors are employed to sense the combustion air flow produced by the inducer fan. Typically, the temperature and pressure sensors will sense the supply air fed to the burner box, which can then be used by the controller to compute mass flow through the combustion side of the furnace. In some designs, a mass flow sensor may also be used in lieu of, the temperature and pressure sensors to compute mass flow.
The addition of these sensors require additional power to operate the furnace, decreasing overall power efficiency. In some cases, the performance of these sensors can degrade over time, causing the furnace to operate at a lower efficiency or to shut-down due to a system fault. The complexity associated with installing these sensors can also increase the level of skill and time required to install and service the furnace system.
The present invention pertains to systems, methods, and controllers for controlling gas-fired appliances such as warm air furnaces. A furnace system in accordance with an illustrative embodiment can include a burner unit in communication with a combustion air flow conduit and heat exchanger, a variable speed inducer fan or blower adapted to provide combustion air flow to the burner unit, a furnace controller and motor speed control unit adapted to regulate the speed of the fan or blower, and a pneumatically modulated gas valve adapted to variably output gas pressure to the burner unit based at least in part on the combustion air flow.
The furnace controller can include a processor adapted to compute the combustion mass air flow at the burner unit, and a motor speed control unit adapted to regulate the speed of the fan or blower based at least in part on the computed air mass flow. In some embodiments, the motor speed control unit can comprise a separate unit from the furnace controller. In other embodiments, the motor speed control unit can be a part of the furnace controller. During operation, the furnace controller can be configured to receive heat demand signals from one or more thermostats that can be utilized by the motor speed control unit to either increase or decrease the combustion air flow in order to modulate the gas valve.
An illustrative method of controlling the gas-fired appliance can include the steps of receiving a heat request signal and activating the inducer fan or blower to produce a combustion air flow at the burner unit. Once the combustion air flow is initiated, the gas valve can be activated to provide fuel to the burner unit, which can then be ignited via an ignition element. To modulate the gas pressure fed to the burner unit, the speed of the inducer fan or blower can be adjusted based on the heat request signals. During operation, the rotational speed of the inducer fan or blower can be sensed via a sensor or switch, or alternatively the voltage or current to the inducer fan or blower motor can be measured in order to determine the supply air mass flow. Using the computed supply air mass flow, the speed of the inducer fan or blower can then be adjusted upwardly or downwardly in order to modulate the gas pressure outputted by the gas valve.
The following description should be read with reference to the drawings, in which like elements in different drawings are numbered in like fashion. The drawings, which are not necessarily to scale, depict selected embodiments and are not intended to limit the scope of the invention. Although examples of furnace systems methods, and controllers are illustrated in the various views, those skilled in the art will recognize that many of the examples provided have suitable alternatives that can be utilized. While the furnace systems and methods are described with respect to warm air furnaces, it should be understood that the systems and methods described herein could be applied to the control of other gas-fired appliances, if desired. Examples of other gas-fired appliances that can be controlled can include, but are not limited to, water heaters, fireplace inserts, gas stoves, gas clothes dryers, gas grills, or any other such device where gas control is desired. Typically, such appliances utilize fuels such as natural gas or liquid propane gas as the primary fuel source, although other liquid and/or gas fuel sources may be provided depending on the type of appliance to be controlled.
Referring now to
The inducer fan 28 can be configured to produce a positive airflow through the heat exchanger 20 forcing the combusted air within the burner box 16 to be discharged through the exhaust vent 28. A pressure switch 34 can be attached to the combustion side of the heat exchanger 20 at the input of the inducer fan 28 to sense the pressure of combustion air flow present on the combustion side of the furnace. The pressure signals from the pressure switch 34 can be fed to a controller 40 that can be used to enable the gas valve 12 and initiate ignition.
On the non-combustion side 22 of the heat exchanger 20, a heated air blower or fan 36 blows heated air through a separate path in the heat exchanger 20 into the warm air ducts 24, the heated air space 26, and back through cold air return ducts 38. One or more thermostats 42 located in the heated air space 26 may provide input back to the controller 40. The feedback from the thermostats 42 may be in the form of temperature set-points inputted by an occupant of the space 26.
During operation, a supply of gas can be fed to the gas valve 12, which, in turn, outputs a metered gas pressure to the gas manifold 14 for combustion in the burner box 16. The fuel fed to the burner box 16 can then be ignited via an AC hot surface ignition element, direct spark igniter, or other suitable ignition element 44. A flame sensor 48 can be employed to provide an indication when a flame is present. The flame sensor 48 signals and signals from a flame rollout switch 46 can be inputted to the controller 40, which can be configured to shut down the gas valve 12 upon the occurrence of a fault condition. A thermal limit sensor 50 can be used to sense the temperature within the heat exchanger 20, which can be used by the controller 40 to shut down or limit the gas supplied to the burner box 16 via the gas valve 12 or to change the speed of the inducer fan 28 or heated air blower 36 in order to reduce the heat exchanger temperature.
On the non-combustion side 64 of the heat exchanger 62, a heated air fan or blower 76 can be configured to blow heated air through a separate path in the heat exchanger 62, similar to that described above with respect to furnace system 10. In the illustrative embodiment of
The furnace controller 80 can include a motor speed control unit 82 capable of varying the speed of the inducer fan 70. The inducer fan 70 can comprise a multi-speed or variable speed fan or blower capable of adjusting the combustion air flow between either a number of discrete airflow positions or variably within a range of airflow positions. In certain embodiments, for example, the inducer fan 70 can vary the combustion air flow 74 through the combustion side 60 of the furnace between an infinite number of positions within the speed range of the fan 70, allowing the furnace to draw in supply air into the burner box 58 and heat exchanger 62 at a variable rate. In some embodiments, the motor speed controller unit 82 can also vary the rate at which the heated air fan or blower 76 discharges heated air into the warm air ducts 66.
Although the furnace controller 80 depicted in
In the illustrative embodiment of
In some embodiments, and as shown in
In use, gas supplied to the gas manifold 56 and burner box 58 is automatically modulated based on the pressure differential of the combustion air across the heat exchanger 62. If, for example, the combustion air flow through the heat exchanger 62 is increased, the corresponding increase in pressure differential between the pneumatic conduits 88,90 causes the gas valve 54 to increase the firing rate in order to maintain a particular air/fuel ratio at the burner box 58. If, conversely, the combustion air flow through the heat exchanger 62 is decreased, the corresponding decrease in pressure differential between the pneumatic conduits 88,90 causes the gas valve 54 to decrease the firing rate. Typically, the gas firing rate outputted by the gas valve 54 will be linear with respect to the combustion air flow produced by operation of the inducer fan 70, although other non-linear configurations are possible.
The pressure metered fuel outputted from the gas valve 54 can be fed to the gas manifold 56, which injects the fuel into the burner box 58 for combustion. An ignition element 96 such as an AC hot surface ignition element, direct spark igniter, or other suitable igniter can then activated via the controller 80 to ignite the air/fuel mixture within the burner box 58. If desired, a flame rollout switch 98 and flame sensor 100 can be used by the controller 80 to monitor the presence of a flame within the burner box 58.
The motor speed control unit 82 can be configured to control the firing rate of the gas valve 54 at a desired value or within a range of values by adjusting the rotational speed of the inducer fan 70. The motor speed control unit 82 can include a microprocessor that calculates the air flow (CFM) based at least in part by sensing the fan speed and/or by measuring the motor voltage and/or current within the inducer fan 70. For example, in some embodiments the voltage and/or current used to operate the inducer fan motor can be measured and then correlated with a conversion factor or map stored within the motor speed control unit 82 in order to compute the combustion air flow produced by the inducer fan 70. From this calculation, the heat input to the heat exchanger 62 can then be determined, and based on the heat transfer properties of the system, can be used to determine the supply air temperature.
By sensing and computing the supply air temperature via feedback signals received from the inducer fan 70 and/or the heated air blower 76, the furnace system 52 obviates the need for additional sensors such as thermal sensors, mass flow sensors, and/or pressure sensors in the combustion air flow or non-combustion air flow path. With respect to the furnace system 10 described above with respect to
The fan speed signal 108 can be utilized by the on-board motor speed control unit 82 compute the temperature of the supply air fed to the burner box based on the combustion air flow, as discussed above. The fan speed signal 108 can be sensed, for example, via a sensor (e.g. a Hall effect sensor, reed switch, magnetic sensor, optical sensor, etc.) in order to compute the combustion air flow produced by the inducer fan or blower wheel. In some embodiments, for example, rotational speed of the inducer fan can be determined via a sensor or switch located adjacent the blower wheel used in some fan or blower configurations. The manner in which the speed signal 108 is obtained will differ, however, depending on the type of fan configuration employed. From the fan speed signal 108, the controller 80 can be configured to compute the supply air temperature from the heat transfer properties of the heat exchanger.
A fan voltage/current signal 110 can also be received in addition to, or in lieu of, the fan speed signal 108 for computing the combustion air flow through the combustion side of the furnace system. In some embodiments, for example, the fan voltage/current signal 110 can be determined by directly measuring the power drop across a resistive element (e.g. a high-precision resistor) coupled to the fan motor or by other methods such as via a resistive bridge circuit. As with the fan speed signal 108, the fan voltage/current signal 110 can be used to compute the heat provided to the heat exchanger, which, in turn, can be used to compute the supply air temperature.
As indicated generally by reference number 112, the furnace controller 80 can be configured to receive one or more other signals for controlling other aspects of the furnace system. Examples of other types of signals 112 can include actuator signals from other furnace components such as any dampers or shut-off valves as well as power signals from the other furnace components. It should be understood that the types of signals fed to the controller 80 will typically depend on the type of gas-power appliance being controlled.
The outputs 114 of the controller 80 can include a thermostat signal 116 for communicating with each thermostat, a gas-shut-off signal 118 for controlling the supply of gas to the gas valve, and an igniter signal 120 for ignition of fuel within the burner box. An inducer fan speed signal 122 outputted to the inducer fan can be provided to control the speed of the fan to either increase or decrease the combustion air flow. A heated air blower speed signal 124, in turn, can be outputted to the heated air fan or blower to control the operational times and/or speed of the heated air discharged into the warm air ducts. As indicated generally by reference number 126, the controller 80 can also be configured to output one or more other signals, if desired.
As illustrated diagrammatically in
Once the inducer fan 70 is at its proper ignition speed and the ignition element 96 is at the proper ignition temperature, the controller 80 may then power the gas valve 54, as indicated generally by block 142, forcing metered fuel into the burner box 58 for combustion. Upon activation, the ignition element 96 may ignite the fuel causing a flame to develop, which can then be sensed via the flame sensor 100, as indicated generally by block 144. After the heat exchanger 62 warms for a predetermined period of time (e.g. 15 to 30 seconds), the heated air fan or blower 76 can then be activated to direct cold air across the heat exchanger 62 and into the warm air ducts 66, as indicated generally by block 146.
Once ignition is proven, the ignition element 96 can then be deactivated and the controller 80 tasked to adjust the speed of the inducer fan 70 to meet the heat demand set-points received by the thermostats 78, as indicated generally by block 148. The furnace controller 80 can be configured to sense and/or measure the speed of the inducer fan 70, as indicated generally by block 150. Sensing of the inducer fan speed can be accomplished, for example, with a sensor, switch, or other suitable means for sensing rotation of the blower wheel or other component of the inducer fan 70.
In an alternative method 158 depicted in
From the sensed speed at block 150 in
Once the supply air temperature has been computed at block 152, the furnace controller 80 may next adjust the speed of the inducer fan 70 in order to achieve the temperature set-point received by the thermostats 78, as indicated generally by block 154. If, for example, the controller 80 determines that an increase in air flow is necessary based on the calculated temperature of the supply air fed to the heat exchanger 62, the controller 80 can increase the rotational speed of the inducer fan 70. Conversely, if the controller 80 determines that a decrease in air flow is necessary based on the calculated supply air temperature, the controller 80 can decrease the rotational speed of the inducer fan 70.
As the controller 80 adjusts the speed of the inducer fan 70 either upwardly or downwardly depending on the heating demand, the combustion air flow will likewise fluctuate causing a change in air pressure across the heat exchanger 62. This change in pressure can then be sensed by the gas valve 54 via the pneumatic conduits 88,90. As indicated generally by block 156, the gas valve 54 can then modulate the fuel fed to the burner box 58 based on these pressure signals. The process of sensing and/or measuring the speed of the inducer fan 70 or the voltage/current of the inducer fan motor, computing the supply air temperature, and then adjusting the speed of the inducer fan 70 based on the calculated supply air temperature in order to modulate the gas valve can then be repeated, as necessary, to achieve or maintain the desired temperature set-point.
Once the gas valve 54 is initially opened at point 164, the gas pressure Pg outputted by the gas valve 54 increases in proportion to the pressure change ΔPair produced by the pressure signals received from the pneumatic conduits 88,90, as illustrated generally by ramp 168. In those embodiments employing an amplifier 92, the slope of the ramp 168 will typically be greater due to the amplification of the pressure differential ΔPair fed to the gas valve 54.
In some embodiments, the gas valve 54 can be equipped with a high-fire pressure regulator in order to limit the gas pressure outputted from the gas valve 54 once it reaches a particular point 170 along the ramp 124. When a high-fire pressure regulator is employed, and as illustrated generally by line 172, the gas pressure Pg outputted by the gas valve 54 will not exceed a maximum gas pressure Pg(max), thus preventing over-combustion at the burner box 58.
Having thus described the several embodiments of the present invention, those of skill in the art will readily appreciate that other embodiments may be made and used which fall within the scope of the claims attached hereto. Numerous advantages of the invention covered by this document have been set forth in the foregoing description. It will be understood that this disclosure is, in many respects, only illustrative. Changes can be made with respect to various elements described herein without exceeding the scope of the invention.
