Patent Application
20230128530
This disclosure relates to systems for detecting the presence of a burner flame using flame rectification, and more specifically, improved methods and systems using flame rods and hot surface igniters forming part of the rectified current pathway when a flame is present.
When operating combustion burners, it is desirable to provide some means of determining whether a flame is present to ensure that uncombusted combustion gas is not supplied to the burner and its surroundings and does not create an explosion hazard. One known device for detecting the presence of a flame is a “flame rod” or “flame rectification rod.”
A flame rod is a conductive rod with a ceramic insulator which serves as a first electrode and is positioned to contact the flame when the burner is ignited. The burner housing serves a second electrode. When it is present, and an excitation voltage is supplied to the flame rod, the flame provides a conductive pathway that allows current to flow from the flame rod to the burner housing. Conversely, when an excitation voltage is supplied and no flame is present, no current flows from the flame rod to the burner housing. A sensing circuit is typically connected to the flame rod to detect the presence of current from the flame rod to the burner housing so that an indication that a flame is or is not present may be provided. The combustion process typically produces soot or other deposits that foul the flame rod. The deposits act as an insulator, increasing the impedance of the flame rod and reducing the current to the burner at a given voltage. As a result, flame rods must be replaced or serviced at some frequency as their impedances reach too high a level. Thus, a need has arisen for an improved means of using flame rectification to detect the presence of a burner flame.
In accordance with a first aspect of the present disclosure, a burner flame detection system is provided which comprises a conductive flame sensor comprising a conductive terminal and a flame sensing circuit comprising a flame detection signal output node. The conductive flame sensor conductive terminal is positioned proximal to a burner having a conductive body. The burner has an ignited state and an unignited state such that when the burner is in the ignited state, the burner and the conductive flame sensor are in electrical communication with one another. The flame sensing circuit is configured to supply an alternating current having a frequency of from about 24 kHz to about 300 KHz to the conductive flame sensor conductive terminal, and when the burner is in an ignited state and the alternating current is supplied to the conductive terminal, the flame sensing circuit generates a rectified current from the conductive flame sensor conductive terminal to the burner.
In accordance with a second aspect of the present disclosure, a method of determining if a burner is ignited is provided. The method uses a conductive flame sensor comprising a conductive terminal and positioned proximate a burner. The method comprises providing a flame sensing alternate current source operatively connected to the conductive terminal, the alternating current having a frequency of from about 24 kHz to about 300 kHz; and generating a rectified current from the conductive flame sensor to the burner when a source of the alternating current supplies the alternating current to the flame sensor conductive terminal, and the burner is in an ignited state
In accordance with a third aspect of the present disclosure, a burner flame detection system is provided which comprises a hot surface igniter comprising a conductive pattern connected to a conductive terminal and positioned proximal to a burner having a conductive body. The burner has an ignited state and an unignited state, such that when the burner is in the ignited state, the conductive terminal and the burner conductive body are in electrical communication with one another. In a preferred example, the burner flame detection system includes a flame sensing circuit configured to supply a flame sensing alternating current to the hot surface igniter conductive terminal, wherein when the burner is in the ignited state and the flame sensing alternating current is supplied to the hot surface igniter conductive terminal, the flame sensing circuit generates a rectified current from the hot surface igniter conductive terminal to the burner.
In accordance with a fourth aspect of the present disclosure, A method of determining if a burner having a conductive body is ignited is provided. The method comprises providing a conductive flame sensor having a conductive terminal and positioned proximate the burner and providing a flame sensing alternate current source operatively connected to the conductive terminal and having an alternating current with a frequency of from about 24 kHZ to about 300 kHz and generating a rectified current from the conductive flame sensor to the burner when the flame sensing alternating current source supplies flame sensing alternate current to the conductive flame sensor's conductive terminal, and the burner is in an ignited state.
Like reference numerals refer to like parts in the figures.
The systems and methods herein use the property of “flame rectification” to determine whether a burner flame is present. In flame rectification, an active flame defines an electrical path from a flame sensor to a burner body. As is known to skilled artisans, based on Mollberg's flame model, a conductive path through a flame may be modeled as a high resistance (megaohms) resistor in series with a diode. Thus, when subjected to an alternating current, the flame conducts electricity when the AC signal is positive and acts as an open circuit when the AC signal is negative.
Flame rods are conductive rods that are typically used as flame rectification sensors. The rod is typically positioned inside the flame when the burner is lit. Over time, soot from the combustion process and other particulate matter accumulate on the flame rod and “foul it” causing it to diminish in its sensitivity. It has been found that a flame rod may be modeled as a capacitor. The accumulated deposits may be modeled as an insulator of varying thickness. As is known to those skilled in the art, the complex impedance of an RC circuit is a vector sum of a resistance and a “reactive capacitance”:
where Z=complex (vector) impedance (ohms)
R=resistance (ohms)
f=frequency (sec−1)
C=capacitance (farads)
Xc=capacitive reactance
As equations (1) an (2) suggest, as deposits accumulate on a flame rod, its capacitance decreases, which increases the contribution of the reactive capacitance Xc to the impedance Z. At a standard (US) AC frequency of 60 Hz, the contamination that develops on flame rods produces a significant impedance. As equations (1) and (2) also suggest, as the frequency f of an applied AC signal increases, the capacitive reactance Xc decreases, and the complex impedance approaches the resistance. It has been discovered that by sufficiently increasing the frequency of an AC excitation signal supplied to a flame rod, the sensitivity of the flame rod's impedance to the accumulation of soot or other deposits can be significantly diminished.
Hot surface igniters are a well-known means of igniting combustion gas. Silicon nitride hot surface igniters typically comprise two insulating tiles with a printed conductive, heat generating pattern printed on one of the inside faces of the two insulating tiles. When connected to a voltage source, the conductive, heat generating pattern generates heat. It has also been discovered that a hot surface igniter of this construction can also function as a flame rectifying sensor and be modeled as a capacitor. Relative to flame rods, hot surface igniters have the added advantage of generating combustion temperatures which allows them to burn off accumulated deposits and avoid the replacement cycles that are necessary for flame rods. Disclosed herein are circuits intended to generate a binary (ON/OFF) signal of a voltage range suitable for a commercial microcontroller based on the presence of a flame rectified signal generated when a flame rectification sensor is exposed to a flame.
Referring to
Flame sensing circuit 24 is designed to provide a signal at flame sensing circuit 24 flame detection signal output node 36 that is suitable for input to a flame presence indicator. The indicator is preferably visual and/or audible. The flame presence indicator may be a stand-alone indicator or may be integrated with a controller, such as a commercially available microcontroller. In preferred examples the controller is operatively connected to a gas valve that is operative to selectively supply combustion gas to burner 21. In certain preferred examples, flame sensing circuit 24 is designed to provide a binary signal (ON/OFF) at flame sensing circuit output node 36 even though the signal generated by flame sensor 22 is not a binary signal. In the example of
Flame sensor 22 is preferably a flame rod or a hot surface igniter. In certain examples in which flame sensor 22 is a hot surface igniter, the heating conductive pattern generates heat during an ignition operation and detects the presence of a flame during a flame detection operation. In other examples, the hot surface igniter includes a flame sensing conductive pattern separate from the heating conductive pattern so that heating and flame sensing can occur simultaneously.
The flame sensing circuit 24 receives an AC voltage signal at input node 38, which is an output from AC generating circuit 31. The AC signal generated by the AC generating circuit may be a sine wave but is preferably a square wave.
In preferred examples, and as shown in
In the example of
When a DC source 33 supplies a DC voltage to resistors 35a-35d, capacitors 39a and 39b will charge and discharge. As capacitors 39a and 39b charge and discharge, BJTs 37a and 37b will alternate being ON and OFF, causing the BJT 37a and 37b collector voltages to rise and fall, thereby recharging capacitors 39a and 39b. Capacitor 39c and resistor 35e serve as a high-pass filter that removes the DC output that would otherwise be observed at AC generating circuit 31 output node 34. DC source 33 provides a supply voltage of from about 10V to about 48V, preferably from about 12V to about 36 V, and more preferably about 24V. DC source 33 is shown in greater detail in
Capacitors 39a and 39b—along with resistors 35b and 35c—are selected to achieve a desired frequency of the AC signal at output node 34 by adjusting the current flow across capacitors 39a and 39b. Resistors 35a and 35d are selected to achieve a desired rising edge time of the AC signal at output node 34 by adjusting the current through BJTs 37a and 37b. In preferred examples, AC generating circuit 31 is a balanced multivibrator with the resistances of resistors 35b and 35c being equal, the resistances of resistors 35a and 35d being equal, and the capacitances of capacitors 39a and 39b being equal.
In preferred examples, the component values of AC generating circuit 31 are selected to produce a square wave AC voltage signal at output node 34 having a frequency range of from 24 kHZ to 300 kHZ, more preferably 40 kHz to 200 kHz, and still more preferably from 70 kHZ to 100 kHZ, and more preferably from 80 kHz to 90 kHZ. In preferred examples, these frequencies yield a stable signal (logical high or low) at flame sensing circuit output node 36 when burner 21 is lit. It has been found that when flame sensor 22 is a flame rod and the AC signal at AC generating circuit output node 34 has a frequency in these ranges, the signal at flame sensing circuit output node 36 is stable when burner 21 is lit even when significant deposits have accumulated on the flame rod. Exemplary component values for AC generating circuit 31 for achieving he preferred frequencies referenced above are as follows:
Flame sensing circuit 24 comprises a flame sensor driver circuit 26, a signal conditioning circuit 28, and a load circuit 30. The flame sensor 22 includes a conductive terminal 32 that receives an output signal from an output node (not separately shown) of the flame sensor driver circuit 26. In the case of a flame rod, conductive terminal 32 is electrically connected to the flame rod body. In the case of a hot surface igniter, conductive terminal 32 is electrically connected to a heating conductive pattern or a flame sensing conductive pattern in the hot surface igniter.
Flame sensor driver circuit 26 provides a means of amplifying a DC offset introduced when the flame sensor 22 is subjected to an alternating current while a flame is present. The flame sensor driver circuit 26 comprises resistor 42 which is in series with flame sensor 22 and which has an input node 45 connected to capacitor 40 and resistor 44. Capacitor 40 has an input connected to input node 38. Input node 38 is connected to BJT 46 emitter 48 and AC generating circuit output node 34. BJT 46 emitter 48 is also the output node of the flame sensor driver detection circuit 24. Resistor 44 is connected to BJT base 60 and resistor 42 input node 45. BJT 46 collector 50 defines a flame sensor driver circuit flame detection output node that is connected to signal conditioning circuit 28. BJT 46 acts as a switch that supplies current from collector 50 to signal conditioning circuit 28 when a flame is present.
BJT 46 is a PNP BJT in which there is a positive offset from the emitter 48 to base 60 having a fixed voltage when there is a path for current flow through the base 60. When no flame is present, there is effectively an open circuit from flame sensor 22 to burner 21, and there is no path for current flow from emitter 48 to base 60. However, when a flame is present, and there is a path for current flow through the base to resistor 42, BJT 46 is turned ON, which allows current to flow from emitter 48 to collector 50. When there is no flame, collector 50 floats on the emitter 48 voltage during the positive AC cycle and is connected to ground during the negative AC cycle. Current flow through resistor 42 and flame sensor 22 produces a DC offset voltage at node 45 which causes the BJT 46 offset between emitter 48 and base 60 to exceed the threshold required to turn BJT 46 ON. Preferred examples of component values for flame sensor driver circuit 26 are as follows:
When a flame is present, with BJT 46 ON current flows from the AC generating circuit output node 34 to diode 52, which is part of signal conditioning circuit 28. Signal conditioning circuit 28 includes an RC low pass filter and converts the input signal at diode 52 to a positive and less variable signal at signal conditioning circuit output node 66 when a flame is present. The reduced variability ensures that the load circuit 30 can more reliably supply a DC voltage having discrete binary values at load circuit output node 36, which is also the flame sensing circuit 24 output node.
Signal conditioning circuit 28 comprises diode 52 which his connected to a parallel combination of resistor 58 and capacitor 54. Resistor 58 and capacitor 54 define an RC low pass filter. When the AC signal at BJT 46 collector 50 is positive, diode 52 is forward-biased and allows current to pass. When the AC voltage at BJT 46 collector 50 is negative, diode 52 is reverse-biased and does not allow current to pass. Thus, resistor 58 and capacitor 54 only see positive voltages. When the AC voltage at BJT 46 collector 50 is more than the voltage of capacitor 54, capacitor 54 charges until reaching its peak voltage. When the AC voltage at BJT 46 collector 50 is below the capacitor voltage, capacitor 54 discharges. Thus, the capacitor 54 smooths the ripple created by the half-wave rectification provided by diode 52. Resistor 58 provides a path to ground to remove excess charge across capacitor 54. Resistor 58 input node 56 is connected to current limiting resistor 62.
Signal conditioning circuit 28 also comprises a current limiting resistor 62 which, along with Zener diode 64, is connected to signal conditioning circuit output node 66. Zener diode 64 is reverse biased and protects load circuit 30 against power surges because once it reaches its breakdown voltage, Zener diode 64 allows current to flow to ground, thus capping the signal conditioning circuit output node 66 at the breakdown voltage. It is generally desirable to maximize the voltage at the signal conditioning circuit output node 66. Thus, diode 52 is preferably selected to have a small voltage drop. It is also desirable to filter out the AC noise and have a faster response at signal conditioning circuit output node 66, therefore, capacitor 54 is selected to have a low capacitance. Resistor 62 is preferably selected to have a resistance that will protect against voltage surges at signal conditioning circuit output node 66, and Zener diode 64 is selected to have a breakdown voltage at the maximum desired voltage at signal conditioning circuit output node 66.
Preferred examples of component values for signal conditioning circuit 28 are shown in Table 3:
Signal conditioning circuit 28 is connected to load circuit 30. In the example of
DC source 74 supplies direct current to load circuit 30. Load circuit 30 adjusts the voltage from the signal conditioning circuit 28 to match the input requirements of a controller connected to flame sensing circuit output node 36. In an alternate implementation, DC source 33 supplies both the AC generating circuit 31 and the load circuit 30. In certain examples, the voltage at signal conditioning circuit output node 66 ranges from 0-25V, preferably from 0-12V, and more preferably from 0-9V, depending on the frequency of the voltage signal at AC generating circuit output node 34.
The output from node 66 of the signal conditioning circuit 28 is not a current that flows into the load circuit 30, but rather a voltage that turns the MOSFET 68 ON and OFF. When MOSFET 68 is ON it acts like a low resistance resistor, providing a low impedance path to ground. As a result, without Zener diode 78 and resistor 80, the input voltage to a controller (i.e., the voltage at flame sensing circuit output node 36) would be approximately zero when a flame is present, and approximately the voltage of DC supply 74 (e.g., 24V) when no flame is present, which is too high for many commercially available microcontrollers. Zener diode 78 and resistor 80 form a parallel combination and are selected to match the microcontroller (not shown) input requirements. Zener diode 78 also protects the controller against excessive voltages by effectively capping the voltage at flame sensing circuit output node 36 at the breakdown voltage of Zener diode 78.
Using the following relationship, the resistance of resistor 80 can be determined based on a desired maximum input voltage to the controller:
In an example where R76 is 47 kΩ, and the maximum controller input voltage is 3.3V, equation (3) yields a resistance of 7.5 kΩ for R80. Zener diode 78 may also be selected to have a breakdown voltage equal to the maximum input voltage of the microcontroller to protect it from surges. Preferred exemplary component values for the load circuit 30 are provided in Table 4:
As mentioned previously, in certain examples, a hot surface igniter may be used in place of a flame rod for flame sensor 22. The hot surface igniter comprises at least two ceramic insulating tiles having a heating conductive pattern disposed between them, such as by printing the pattern on one of the inner faces of one of the insulating tiles. In preferred examples, the ceramic tiles comprise silicon nitride. Examples of such silicon nitride igniters are shown in U.S. patent application Ser. No. 16/366,479, the entirety of which is hereby incorporated by reference.
In certain examples, silicon nitride igniters comprising only a heating conductive pattern are used as flame sensor 22, while in other examples hot surface igniters are provided which include both a heating conductive pattern and a separate flame sensing conductive pattern that is used as flame sensor 22. In the former case, the igniter operates in both a heating mode and a flame sensing mode. In the heating mode, the heating conductive pattern generates heat when a voltage is applied across its conductive terminals. In the flame sensing mode, a voltage is not applied across the terminals of the igniter. Instead, one of the terminals is connected to resistor 42 of flame sensor driver circuit 26 and the other is disconnected from ground so that the flame sensing conductive circuit acts as an electrode that conducts electricity to the burner flame when the burner 21 is lit. A schematic illustrating an implementation of this type of igniter is shown in
In
In a heating mode, switches 86 and 88 are in a closed position and make electrical contact with positive igniter terminal 113a and ground igniter terminal 113b, respectively. In the heating mode, current flows from AC mains supply 82 positive terminal 83 through igniter 84 and to node 90 which connects igniter ground terminal 113b to AC mains ground terminal 85. In a flame sensing mode, switches 86 and 88 are in an open position so that the igniter 84 is disconnected from both AC mains supply 82 and ground. It has been discovered that disconnecting ground terminal 113b from ground is important when igniter 84 is in a flame sensing mode because otherwise current from the flame sensor driver circuit 26 may short to ground instead of flowing through the burner flame and to the body of burner 21.
Referring to
Referring to
As discussed in the '479 App., ceramic hot surface igniters used in the gas burner systems described herein are prepared by sintering ceramic compositions. In certain examples, post-sintering, the ceramic insulating tiles used to form the igniter (not including conductive ink circuit) have a room temperature resistivity that is no less than 1012 Ω-cm, preferably no less than 1013 Ω-cm, and more preferably, no less than 1014 Ω-cm. In the same or other examples, the tiles have a thermal shock value in accordance with ASTM C-1525 of no less than 900° F., preferably no less than 950° F., and more preferably, no less than 1000° F.
In other examples, the conductive ink comprising the heating conductive pattern has a (post-sintering) room temperature resistivity of from about 1.4×10−4 Ω·cm to about 4.5×10−4 Ω·cm, preferably from about 1.8×10−4 Ω·cm to about 4.1×10−4 Ω·cm, and more preferably from about 2.2×10−4 Ω·cm to about 3.7×10−4 Ω·cm. In the case of a material with a constant cross-sectional area along its length, resistivity ρ at a given temperature T is related to resistance R at the same temperature T in accordance with the well-known formula:
R(T)=ρ(T)(l/A), where (4)
In the case of a cross-sectional area that varies along the length of the conductive circuit, the resistance may be represented as:
where, L=total length of circuit along direction of current flow (cm), and the remaining variables are as defined for equation (4).
In certain examples, the ceramic bodies comprising the ceramic hot surface igniters described herein preferably comprise silicon nitride and a rare earth oxide sintering aid, wherein the rare earth element is one or more of ytterbium, yttrium, scandium, and lanthanum. The sintering aids may be provided as co-dopants selected from the foregoing rare earth oxides and one or more of silica, alumina, and magnesia. A sintering aid protective agent is also preferably included which also enhances densification. A preferred sintering aid protective agent is molybdenum disilicide. The rare earth oxide sintering aid (with or without the co-dopant) is preferably present in an amount ranging from about 2 to about 15 percent by weight, more preferably from about 8 to about 14 percent by weight, and still more preferably from about 12 to about 14 percent by weight of the ceramic body. Molybdenum disilicide is preferably present in an amount ranging from about 3 to about 7 percent, more preferably from about 4 to about 7 percent, and still more preferably from about 5.5 to about 6.5 percent by weight of the ceramic body. The balance is silicon nitride.
The conductive ink circuit is preferably printed onto the face of one of the ceramic tiles to yield a ceramic hot surface igniter (post-sintering) with heating properties that are tailored to the specific application for which the igniter is intended as well as to the voltage at which the igniter will operate. Listed below in Table 5 are some exemplary room temperature resistance (RTR) values for various applications.
The conductive ink used for the heating conductive circuit may comprise tungsten carbide in an amount ranging from about 20 to about 80 percent, preferably from about 30 percent to about 80 percent, and more preferably from about 70 to about 75 percent by weight of the ink. Silicon nitride is preferably provided in an amount ranging from about 15 to about 40 percent, preferably from about 15 to about 30 percent, and more preferably from about 18 to about 25 percent by weight of the ink. The same sintering aids or co-dopants described for the ceramic body are also preferably included in an amount ranging from about 0.02 to about 6 percent, preferably from about 1 to about 5 percent, and more preferably from about 2 to about 4 percent by weight of the ink. In certain examples, the flame sensing conductive pattern comprises is formed from an ink of the same composition as the heating conductive pattern.
Referring to
Referring to
Second insulating ceramic tile 124 includes a first face 130a that faces ceramic insulating tile 122, and a second face 130b that faces third insulating ceramic tile 126. Heating conductive pattern 136 is printed on the second face 130b of the second ceramic insulating tile 126 and faces the inner face 132a of third ceramic insulating tile 132a. The heating conductive pattern is connected to two terminals 134a and 134b for connection to a power source and ground, respectively. The three ceramic insulating tiles 122, 124, and 126 are laminated together using the techniques described in the '479 App. to create a unitary hot surface igniter structure.
In the example of
In the example of
In this example, the operation of the flame sensor driver circuit 26 of
A 24 kHZ voltage signal is supplied to the emitter 48 of BJT 46, and voltages at node 45, BJT collector 50 and BJT base 60 are determined via simulation.
When burner 21 is not lit, there is an open circuit between flame sensor 22 and burner 21. Because there is no current path through resistor 42, there is no current path available from BJT emitter 48 to BJT base 60. Thus, there is no emitter-base offset in BJT 46, and BJT remains OFF. As a result, the node 45 voltage floats on the emitter 48 voltage, and signals 180 and 182 are the same. During positive AC cycles at emitter 48, collector 50 is remains 0V and is essentially grounded. During negative AC cycles, collector 50 sees the voltage of emitter 48. Thus, as shown in
When burner 21 is lit, there is an active current path from flame sensor 22 to burner 21 which, in accordance with Mollberg's flame model, can be modeled as a relatively lower resistance resistor in series with a diode, the series combination of which is in parallel with a relative higher resistance resistor. As a result, positive current passes from the flame sensor 22 to the burner 21, but only a small negative leakage current passes from the burner 21 to the flame sensor 22.
When the positive current flows from the flame sensor 22 to the burner 21, the rectification effect of the flame causes a negative (time-varying) DC offset in the voltage at node 45 relative to node 38 because capacitor 40 (like capacitors in general) cannot pass DC.
As illustrated by
As discussed previously, higher frequency AC voltages supplied top flame rod can reduce the impact of accumulated deposits on the impedance of the flame rod. They can also reduce the impact of the ceramic insulating tiles of a hot surface igniter on the igniter's impedance. In this example, the circuit of
A “Monster Mash” is a formula used to simulate extreme soiling conditions by mixing a wide variety of diverse food ingredients typically used in—and creating soils in—household ovens. In this example, the Monster Mash comprises cherry pie filling, tomato puree, egg yolks, whole milk mozzarella cheese, pasteurized cheese spread, lard, and tapioca. The Monster Mash is applied along 2.249 inches (57.13 mm) of a flame rod having a length of 3.249 inches (82.53 mm) and a diameter of 0.114 inches (2.89 mm). It is applied by running the flame rod through a volume of the Monster Mash applied to a flat surface to obtain a thin layer that is slightly translucent and smoothed around the flame rod until even. The flame rod is placed in a pan but supported above the surface of the pan to prevent burning and placed in an oven preheated to 375° F. for 7-8 minutes, until the Monster Mash is golden brown. The baked layer diameter of the flame rod is 0.127 inches (3.23 mm). Burner 21 is lit, the AWG frequency is varied from 10 Hz to 3 MHz, and the “on-rate” is measured. The peak to peak voltage at flame sensor driver circuit input node 38 is measured as is the mean voltage. The peak to peak voltage at signal conditioning circuit output node 66 is also measured as is the mean voltage.
The LED is ON 99 percent of the time once the frequency reaches 20 kHZ but is not consistently lit 100 percent of the time except when the frequency is in the range of 40 kHz-2 MHz. Because it is not rectified, the voltage at node 38 remains at zero or very close thereto during the entirety of the runs (
The differences in the runs of
This application claims the benefit of U.S. Provisional Application No. 63/252,663, filed on Oct. 6, 2021, the entirety of which is hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 63252663 | Oct 2021 | US |
