Patent Application
20210054999
This disclosure relates to gas control valves that are thermally-actuated by a ceramic heater and to gas heating systems comprising such control valves with ceramic igniters.
Many oven cavities in the United States and abroad are heated by gas using a ceramic igniter, such as a silicon carbide hot surface igniter. Silicon carbide ceramic igniters include a semi-conductive ceramic body with terminal ends across which a potential difference is applied. Current flowing through the ceramic body causes the body to heat up and increase in temperature, providing a source of ignition for the combustion gases. In such oven heating systems, it is standard to include a thermally-actuated gas control valve assembly, sometimes referred to as a bimetal gas valve assembly, to ensure that combustible gas is only supplied to the silicon carbide igniter once it has reached a surface temperature at which a combustible mixture of the combustion gas and air will ignite.
The gas valve assembly and the silicon carbide igniter are connected in series to the AC (nominal 120 VAC) mains through a switch or relay that controls the flow of electricity to the circuit. When the oven calls for heat the switch is closed and electricity flows to the silicon carbide igniter first and then the bimetal valve assembly. The igniter has a negative temperature coefficient of resistance and a high resistance at room temperature which limits the voltage and current to the bi-metal valve assembly. The high initial resistance prevents the valve from opening before the hot surface igniter has reached the ignition temperature of the combustion gas. As the hot surface igniter begins to heat up its resistance begins to drop (due to the negative coefficient of resistance) and eventually stabilizes at approximately 35 ohms and 2700° F. (maximum temperature) at 116 VAC.
As the resistance of the igniter drops the current begins to flow to the bimetal valve assembly. Inside the assembly are a wire resistance element and a thermally deflectable bimetal strip. The wire resistance element is wrapped around a portion of the bimetal strip and as current begins to flow, the resistance element begins to heat up. As the resistance element heats up, the bimetal strip reaches a deflection temperature at which it deflects to unseat a valve plug from the gas valve assembly's gas outlet, thereby placing the interior of the assembly and its gas inlet in fluid communication with the gas outlet and allowing gas to flow. The required voltage of 3.03-3.30 VAC and 3.2-3.6 Amps is not provided by the circuit until the silicon carbide igniter is at the desired operating temperature. The bimetal strip comprises two metals having different coefficients of expansion. The differing coefficients of expansion cause the strip to bend so the valve plug end of the bimetal strip deflects away and is unseated from the gas outlet.
The advantage of this design is that the current required to open the bimetal gas valve will not be present until the hot surface igniter is at its operating temperature. This ensures that no flow of a combustible gas is allowed until the hot surface igniter is at a temperature that will insure ignition of the gas.
Unfortunately, there are disadvantages to known thermally-actuated gas control valve assemblies and gas heating systems that utilize them in combination with silicon carbide igniters. First, silicon carbide igniters, in particular the M-circuit design, are very fragile and easily broken during installation at the factory, shipping and installation of the oven in the end users' home. In addition, silicon carbide igniters are slow to heat up and, in most cases, take 10-20 seconds to get to their desired operating temperature.
Moreover, the bimetal valve is also slow, requiring an additional 20-40 seconds to open after the silicon carbide igniter reaches its desired operating temperature. As a result, the overall time to ignition is somewhere between 30-60 seconds. Further, the silicon carbide hot surface igniter will form a silicon dioxide insulating layer on the surface of silicon carbide grains and leads, producing an increase in the room temperature resistance of the igniter over time. This increase in resistance further increases the overall time to temperature, degrading the overall performance of the system. Also, silicon carbide is a semiconductor, and the entire igniter is conductive. This requires that the operator of the oven must be protected from inadvertent contact to prevent burns or electrical shorting.
Silicon nitride igniters have long been used in water heater and furnace applications and have several advantages over silicon carbide igniters. First, silicon nitride igniters have superior strength and fracture toughness, making them very durable in their various applications. Moreover, the surface of the silicon nitride igniter is insulating so the risk of electrical shorting is eliminated. In addition, the time to temperature is 50-75% faster compared to silicon carbide, and the power draw is 80% less than silicon carbide. It is also worth noting that silicon nitride igniters have a positive temperature coefficient of resistance, like most materials.
The obstacle to the adoption of silicon nitride igniters in an oven cavity has been the expense of the control system required to turn the igniter on and off. The current drawn by the typical silicon nitride igniter is not sufficient to cause the gas valve assembly to open with typical wire resistance elements. Therefore, the bimetal valve would need to be replaced with a valve like a solenoid, and a control board would need to be added to turn the igniter on, sense when it has reached temperature and send a signal to the solenoid to open. The combination of these features makes adoption cost prohibitive. In contrast, the silicon carbide igniter in combination with the bimetal gas valve is very cost competitive. Thus, a need has arising for a gas valve assembly that addresses the foregoing.
In accordance with a first aspect of the present disclosure, a thermally-actuatable gas valve assembly is provided which comprises a housing, a thermal actuator, a valve plug, and a ceramic heater. The housing has a gas inlet, a gas outlet, and an interior volume that is in selective fluid communication with the gas outlet. A thermal actuator is disposed in the interior volume, and the valve plug is operatively connected to the thermal actuator. The valve plug is positioned to selectively seal the gas outlet from the interior volume, and the ceramic heater is in thermal communication with the thermal actuator. In certain examples, the thermal actuator comprises a bimetal member or bimetal member assembly that deflects when heated to a deflection temperature. In the same or other examples, the ceramic heater comprises a ceramic body and a conductive ink pattern disposed in the ceramic body. In the same or other examples, the ceramic body comprises silicon nitride. At the same time or in other examples, the ceramic heater's conductive ink pattern has a room temperature resistivity of from about 6.5×10−5 Ω·cm to about 2×10−4 Ω·cm. At the same time or in other examples, the conductive ink pattern has a room temperature resistance of from about 5Ω, to about 15Ω.
In accordance with a second aspect of the present disclosure, a gas heating system is provided which comprises a ceramic igniter and a thermally-actuated gas valve assembly comprising a housing, a thermal actuator, a valve plug, and a ceramic heater. The housing has a gas inlet, a gas outlet, and an interior volume that is in selective fluid communication with the gas outlet. A thermal actuator is disposed in the interior volume, and the valve plug is operatively connected to the thermal actuator. The valve plug is positioned to selectively seal the gas outlet from the interior volume of the gas valve assembly, and the ceramic heater is in thermal communication with the thermal actuator. In certain examples, the thermal actuator comprises a bimetal member or bimetal member assembly that deflects when heated to a deflection temperature. In certain examples, the ratio of the ceramic igniter's room temperature resistance to the ceramic heater's room temperature resistance is from about 1.9 to about 4.0. At the same time, the sum of the ceramic igniter's room temperature resistance and the ceramic is from about 25Ω to about 65Ω.
In accordance with a third aspect of the present disclosure, a gas heating system is provided which comprises a ceramic igniter comprising a conductive ink pattern having a positive temperature coefficient of resistivity and a thermally-actuated gas valve assembly. The thermally-actuated gas valve assembly comprises: i) a housing having a gas inlet, a gas outlet, and an interior volume that is in selective fluid communication with the gas outlet; (ii) a thermal actuator disposed in the interior volume; (iii) a valve plug operatively connected to the thermal actuator and positioned to selectively seal the gas outlet from the interior volume; and (iv) a heater in thermal communication with the thermal actuator. In certain examples, the heater is a ceramic heater comprising a conductive ink pattern. At the same time or in other examples, the ceramic heater has a positive temperature coefficient of resistivity.
In accordance with a fourth aspect of the present disclosure, a method of igniting gas is provided. The method comprises providing a source of combustion gas in selective fluid communication with a ceramic igniter, providing a gas valve assembly operable to selectively place the source of combustion gas in fluid communication with the ceramic igniter, energizing the ceramic igniter such that it reaches a surface temperature of no less than an ignition temperature of the combustion gas, and energizing the ceramic heater to place the source of combustion gas in fluid communication with the ceramic igniter. The gas valve assembly comprises a thermal actuator and a ceramic heater in thermal communication with the thermal actuator. In certain examples, the thermal actuator is a deflectable member, and the step of energizing the ceramic heater to place the source of combustion gas in fluid communication with the ceramic igniter comprises heating the thermal actuator such it deflects. At the same time or in other examples, the ratio of the ceramic igniter room temperature resistance to the ceramic heater room temperature resistance is from about 1.9 to about 4.0. At the same time or in other examples, the sum of the room temperature resistance of the ceramic igniter and the room temperature resistance of the ceramic heater is from about 25Ω to about 65Ω. At the same time or in other examples, the step of energizing the ceramic heater comprises applying a potential difference of at least about 15V AC rms across the ceramic heater. At the same time or in other examples, the step of energizing the ceramic igniter comprises applying a potential difference of at least about 75V AC rms across it. At the same time or in other examples, the gas valve assembly comprises a gas inlet and a gas outlet, the thermal actuator is fixed at one end relative to a ceramic insulator in the gas valve assembly and has a free end connected to a valve plug, and the valve plug is removably seated in the gas outlet, such that when the thermal actuator deflects, the valve plug becomes unseated from the gas outlet to place the gas inlet in fluid communication with the gas outlet. At the same time or in other examples, the gas inlet is placed in fluid communication with the gas outlet no sooner than when the ceramic igniter reaches an autoignition temperature of the combustion gas.
Like reference numerals refer to like parts in the figures.
Described below are examples of thermally-actuated gas valve assemblies comprising a ceramic heater and gas heating systems comprising such gas valve assemblies and ceramic igniters. The gas valve assemblies include a thermal actuator that deflects when subjected to a deflection temperature, thereby unseating a valve plug from an outlet port of the gas valve assembly and placing the outlet port in fluid communication with the inlet port. In certain examples, the ceramic igniter is a silicon nitride igniter. In the same or other examples, the ceramic heater is a silicon nitride heater. As compared to known gas valve assemblies, those described herein are more resistant to fracture and ignite combustion gas more quickly.
Referring to
A thermal actuator 26 is attached to a valve plug 42 that selectively and sealingly engages the inlet 43 of gas outlet port 40. When valve plug 42 sealingly engages inlet 43, the gas outlet port 40 is not in fluid communication with the gas inlet port 38 of the gas valve assembly 20 or the interior 24 of the housing 22 of the gas valve assembly 20, in which case combustible gas will not flow from gas valve assembly 20 to the burner to which the gas outlet port 40 is connected.
The thermal actuator 26 preferably deflects in response to heat to move the valve plug 42 in and out of engagement with the inlet 43 of gas outlet port 40. In certain preferred examples, thermal actuator 26 comprises a bimetal member assembly 23 formed from two metals having different coefficients of thermal expansion. In the example of
The bimetal member assembly 23 has a first end 34 and a second end 37 spaced apart from one another along the x-axis. The bimetal member assembly 23 is cantilevered. First end 34 is fixedly attached to insulator block 36 via rivet 35. Insulator block 36 is fixedly attached to the interior of housing 22. Second end 37 is attached to valve plug 42 which is not fixedly attached to the housing, either directly or indirectly. When the bimetal member assembly 23 is heated to a deflection temperature, the second end 37 moves away from inlet 43 of gas outlet port 40 in a direction along the z-axis to unseat valve plug 42 from the gas inlet 43 of gas outlet port 40 and provide combustible gas to the burner(s) to which gas outlet port 40 is connected.
First bimetal member 28 is attached to insulator block 36 at a first end 34 and to a second bimetal member 30 at a second end 39. The first bimetal member 28 is attached to and overlaps second bimetal member 30 in a direction along the x-axis. A wire resistance heater 44 is provided along and wraps around first bimetal member 28 and is selectively energizable to heat the bimetal member assembly 23 to a temperature above the deflection temperature (the temperature at which the bimetal member assembly 23 deflects sufficiently to unseat valve plug 42 from gas inlet 43 of gas outlet port 40). In one known example, the wire resistance heater 44 comprises a nickel-chromium alloy coil wrapped around at least a portion of the first bimetal member 28 and extending along at least a portion of the member's 28 length along the x-axis.
Insulator block 36 preferably comprises a ceramic material and includes two rivets (not separately identified) having upper rivet heads 45a and 45b used to place the wire resistance heater 44 in an electric circuit and in electrical communication with a source of alternating current. Each of the two rivets extends through the insulator block 36 along the z-axis direction. The upper rivet heads 45a and 45b are disposed on the ends of respective rivets which extend through insulator block 36 and through respective silicone o-ring seals 51a and 51b. Underneath the insulator block (
Referring again to
Referring to
In the second operative configuration (
The prior art gas valve assembly 20 of
In accordance with the present disclosure, a thermally-actuatable gas valve assembly is used with a silicon nitride igniter. Unlike a silicon carbide igniter, silicon nitride igniters have a positive temperature coefficient of resistance. If placed in series with a wire resistance heater of the type found in currently available thermally-actuated gas control valves and a source of 120 VAC (rms) current, the valve plug 42 will never unseat from the gas inlet 43 of the gas outlet port 40 because the current draw will be too low. Known wire resistance heaters would have to be extended in length significantly and impractically to provide sufficient heat to deflect the bimetal member assembly 23.
It has been discovered that a ceramic heater may be used in place of a wire resistance heater to generate sufficient heat to unseat the valve plug 42 from the gas outlet port 40 and supply combustible gas from the valve assembly to a fluidly coupled burner. In accordance with an embodiment, a silicon nitride igniter 52 is provided and placed in series with a source of alternating current 50 and the ceramic heater 54 as shown in
Ceramic igniters useful in connection with the gas valve assemblies described herein include those described in U.S. patent application Ser. No. 16/366,479, the entirety of which is hereby incorporated by reference.
Although represented as a resistor 52 in
The ceramic igniters described herein are generally in the shape of a rectangular cube and include two major facets, two minor facets, a top and a bottom. The major facets are defined by the first (length) and second (width) longest dimensions of the ceramic igniter body. The minor facets are defined by the first (length) and third (thickness) longest dimensions of the igniter body. The igniter bodies also include a top surface and a bottom surface which are defined by the second (width) and third (thickness) longest dimensions of the igniter body.
The igniter tiles are ceramic and preferably comprise silicon nitride. The conductive ink circuit is disposed between the tiles and generates heat when energized. The ceramic tiles are electrically insulating but sufficiently thermally conductive to reach the outer surface temperature necessary to ignite combustible gases such as natural gas, propane, butane, and butane 1400 (a butane and air mixture with a heating value of 1400 Btu/ft3) within the desired period of time.
As described in greater detail below, in certain examples, the ceramic tiles comprise silicon nitride, ytterbium oxide, and molybdenum disilicide. In the same or other examples, the conductive ink circuit comprises tungsten carbide, and in certain specific implementations, the conductive ink additionally comprises ytterbium oxide, silicon nitride, and silicon carbide.
In certain examples, when subjected to a potential difference of 120V AC, the ceramic igniters described herein reach a surface temperature of at least 1400° F., preferably no less than 1800° F., more preferably no less than 2100° F., and even more preferably no less than 2130° F. These temperatures are preferably reached in no more than eight seconds, more preferably reached in no more than six seconds, and still more preferably, reached in no more than four seconds after the potential difference is applied.
In the same or additional examples, the surface temperature of the ceramic igniters herein does not exceed 2600° F., preferably does not exceed 2550° F., more preferably does not exceed 2500° F., and still more preferably 2450° F. at any time after a full wave 132V AC potential difference is applied, including after a steady-state temperature is reached.
In the same or other examples of ceramic igniters in accordance with the present disclosure, when subjected to a potential difference of 102V AC, the ceramic igniters described herein reach a surface temperature of at least 1400° F., preferably at least 1800° F., and still more preferably at least 2100° F. in no more than seventeen, preferably more than ten, and more preferably no more than about seven seconds after the 102V AC potential difference is first applied. These temperatures are preferably reached in no more than four seconds and are more preferably reached in no more than three seconds.
In the same or additional examples, the thickness of the conductive ink circuit of the hot surface igniter (taken along the thickness axis) is not more than about 0.002 inches, preferably not more than about 0.0015 inches, and more preferably, not more than about 0.0009 inches. In the same or additional examples, the thickness of the conductive ink circuit (taken along the thickness axis) is not less than about 0.00035 inches, preferably not less than about 0.0003 inches, and more preferably, no less than about 0.0004 inches.
The hot surface igniters of the present disclosure also preferably have a green body density of at least 50 percent of theoretical density, more preferably, at least 55 percent, and still more preferably at least 60 percent of theoretical density.
As discussed in U.S. patent application Ser. No. 16/366,479, ceramic igniters used in the gas heating systems described herein are prepared by sintering ceramic compositions. Post-sintering, the tiles used to form the igniter 52 (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 and 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.
The conductive ink comprising the conductive ink circuit 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 p at a given temperature T is related to resistance R at the same temperature Tin accordance with the well-known formula:
R(T)=ρ(T)(l/A), where (1)
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, and the remaining variables are as defined for equation (1).
The ceramic bodies comprising the ceramic igniters 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 of the ceramic body, more preferably from about 8 to about 14 percent by weight, and still more preferably from about 12 to about 14 percent by weight. 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 ceramic tiles to yield a ceramic igniter (post-sintering) room temperature resistance (RTR) of from about 20Ω to about 60Ω, preferably from about 25Ω to about 55Ω, and more preferably from about 30Ω to about 50Ω. At the same time, the ceramic igniter high temperature resistance (HTR) over the temperature range 2138° F. to 2700° F. is preferably from about 115Ω to about 280Ω, preferably from about 120Ω to about 270Ω, and more preferably from about 128Ω to about 260Ω.
The conductive ink in the igniter should 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.
The ceramic heaters 54 (
Examples of thermally-actuatable gas valve assemblies in accordance with the present disclosure will now be described with reference to
Valve plug 72 and connector 67 define an integrally formed elastomeric structure that is resistant to high temperatures. Valve plug 72 is connected to bimetal member 66 at bimetal member second end 77, with connector 67 being inserted through a hole (not shown) at second end 77 of bimetal member 66 to connect second end 77 to valve plug 72. Rivet 69 connects first end 74 of the thermal actuator 66 to insulator block 76. Rivet head 68 (
Ceramic heater 64 is preferably similar to the silicon nitride hot surface igniters described in U.S. patent application Ser. No. 16/366,479 and made in accordance with the methods and techniques described therein. Ceramic heater 64 is provided proximate first end 74 of the thermal actuator 66 and is spaced apart from the thermal actuator 66 along the z-axis such that thermal actuator 66 is between the top 25 (
Bimetal member 66 is preferably located inboard of the y-axis ends of ceramic heater 64 along the y-axis by a margin sufficient to ensure that connectors 84a and 84b do not short circuit with bimetal member 66. Ceramic heater 64 has a length along the y-axis of from about 0.4 inch to about 1.0 inch, preferably from about 0.5 inch to about 0.8 inch, and more preferably from about 0.55 inch to about 0.75 inch. Ceramic heater 64 has a width along the x-axis of from about 0.15 inch to about 0.35 inch, preferably from about 0.18 inch to about 0.30 inch, and more preferably from about 0.24 inch to about 0.26 inch and a thickness along the z-axis of from about 0.030 inch to about 0.08 inch, preferably from about 0.040 inch to about 0.070 inch, and more preferably from about 0.05 inch to about 0.06 inch.
Ceramic heater 64 preferably comprises ceramic tiles that define a ceramic body with a conductive ink embedded therein. The ceramic body comprises at least one selected from a nitride ceramic, a carbide ceramic, and an oxide ceramic. Preferred carbide ceramics include silicon carbide, titanium carbide, and tantalum carbide. Preferred oxides are selected from the group consisting of alumina and cordierite. Preferred nitrides include silicon nitride and aluminum nitride. Ceramic heater 64 preferably has a positive temperature coefficient of resistance and a positive temperature coefficient of resistivity.
The conductive ink pattern between the ceramic tiles comprising the ceramic heater 64 preferably has a pre-firing thickness of from about 0.0002 inch to about 0.003 inch, more preferably from about 0.0003 inch to about 0.0025 inch, and still more preferably from about 0.0004 inch to about 0.002 inch before sintering. The ink comprising the conductive ink pattern comprises silicon nitride in an amount no greater than about 30 percent by weight of the conductive ink, and at least one conductive component in an amount no less than about 70 percent by weight of the conductive ink, wherein the conductive component is selected from the group consisting of tungsten, tungsten carbide, molybdenum, molybdenum disilicide, and titanium nitride.
Sintering aids may also be used in an amount that is no greater than about 8 percent by weight of the conductive ink, preferably no greater than about 7 percent by weight of the conductive ink, and still more preferably no greater than about 6 percent by weight of the conductive ink. In the same or other examples, sintering aids may be present in an amount of at least about 0.01 percent by weight of the conductive ink. Suitable sintering aids are selected from the group consisting of oxides, metals, and rare earth oxides. Suitable oxides include Y2O3, MgO, Al2O3, and SiO2). Suitable metals include Ni, Co, Cu, Pd, Ru, and Rh. Suitable rare earth oxides include Yb, Sc, La, and Hf. The conductive ink of the ceramic heater 54 has a post-sintering room temperature resistivity of from about 6.5×10−5 Ω·cm to about 2×10−4 Ω·cm, preferably 8.0×10−5 Ω·cm to about 1.8×10−4 Ω·cm, and more preferably from about 1.0×10−4 Ω·cm to about 1.2×10−4 Ω·cm. The conductive ink has a room temperature resistance of from about 5Ω to about 15Ω, preferably from about 6Ω to about 11Ω, and more preferably from about 8Ω to about 10Ω. The high temperature resistance at steady state (i.e., at a temperature of 2138° F. to about 2700° F.) is from about 17Ω to about 28Ω, preferably from about 19Ω to about 26Ω, and more preferably from about 23Ω to about 25Ω. The conductive ink pattern is selected to achieve the desired resistance in light of the resistivity of the ink.
In the example of
The bimetal member preferably has a deflection temperature of from about 150° F. to about 1000° F., preferably from about 200° F. to about 800° F., and more preferably from about 250° F. to about 750° F. When implemented in the circuit of
The bimetal material is preferably selected based on the dimensions of the gas valve apparatus and the desired deflection temperature and properties. In one example, ASTM Type™ 4 bimetal may be used (ASTM D388-06). TM4 is supplied as Truflex™ E4 by Engineered Materials Solutions of Attleboro, Mass.
In one example, bimetal member 66 comprises a first metal that comprises nickel, chromium, and iron, preferably consists essentially of nickel, chromium, and iron, and more preferably consists of nickel, chromium, and iron. At the same time, bimetal member 66 comprises a second metal that comprises nickel and iron, preferably consists essentially of nickel and iron, and more preferably consists of nickel and iron. The first metal is present in an amount ranging from about 40 percent to about 60 percent by weight of the bimetallic member, preferably from about 45 percent to about 55 percent by weight of the bimetallic member, and more preferably from about 48 percent to about 52 percent by weight of the bimetallic member. In preferred examples, the first metal has a coefficient of thermal expansion greater than the second one.
In the same or other examples, bimetallic member 66 has a density of from about 0.25 lb/in3 to about 0.35 lb/in3, preferably from about 0.27 lb/in3 to about 0.33 lb/in3, and more preferably from about 0.28 lb/in3 to about 0.32 lb/in3. In the same or other examples, bimetallic member 66 has a modulus of elasticity of from about 23×10−6 psi to about 27×10−6 psi, preferably from about 24×10−6 psi to about 26.5×10−6 psi, and more preferably from about 25×10−6 psi to about 26×10−6 psi.
In the same or other examples, bimetallic member 66 has a flexivity at 100° F. to 300° F. (measured in accordance with ASTM D388-06) of from about 7.0×10−6° F.−1 to about 11.0×10−6° F.−1, preferably from about 7.5×10−6° F.−1 to about 10.5×10−6° F.−1, and more preferably from about 8.5×10−6° F.−1 to about 9×10−6° F.−1.
In accordance with such examples, the bimetal member 66 has a length (along the x-axis) of from about 1.0 in. to about 3.0 in., preferably from about 1.25 in. to about 2.75 in., and more preferably from about 1.5 in. to about 2.375 in. At the same time, the bimetal member 66 has a width of from about 0.200 in. to about 0.625 in. and a thickness (along the z-axis) of from about 0.012 in. to about 0.022 in., preferably from about 0.014 in. to 0.020 in., and more preferably from about 0.016 in. to about 0.018 in.
A gas heating system may be provided by placing gas outlet port 40 of gas valve assembly 60 in fluid communication with a burner and by placing a ceramic igniter 52 (
A second example of a gas valve assembly 70 (with housing 22 and cover 25 removed) is shown in
In the case of
A simple cantilever element (such as bimetal member 66 of
B=(0.53F(T2−T1)L2)/t (3)
wherein,
Referring to
B=0.53F(T2−T1)[(Fbb2)/tb−(Fa(a2+2ab))/ta] (4)
wherein,
By varying the length and thickness of each bimetal member 98 and 100, the z-axis deflection (B) can be optimized by utilizing the above equation. The z-axis deflection in the designs of
Referring to
The second bimetal member 108 is unitary save for a distal end opening through which rivet 67 is disposed to secure the distal end 77 of the bimetal member assembly 104 to the valve plug 72. Each bimetal member 105 and 108 is preferably constructed of the same bimetal material as bimetal member 66 (
In the exemplary gas valve assemblies 60, 70, and 75, the ceramic heater 64 is oriented with its length (longest dimension) orthogonal to the length (longest dimension) of bimetal member 66 or bimetal member assemblies 96, 104 such that the longest dimension of the ceramic heater 64 extends along the y-axis while the longest dimension of the bimetal member 66 or the bimetal member assemblies 96, 104 extends along the x-axis. One benefit of this arrangement is that the heater terminals 88a and 88b do not have to be bent to contact the rivets 90a and 90b. As shown in each of
Referring to
A method of igniting gas will now be described with referenced to
The ceramic igniter is energized such that it reaches a surface temperature of no less than an ignition temperature of the combustion gas, and the ceramic heater is energized, causing the bimetal member 66 to deflect and pull the valve plug 72 out of sealing engagement with the inlet 43 of the gas outlet port 40, at which point the gas outlet port 40 is in fluid communication with the interior 24 of the gas valve assembly (
The ceramic igniter and ceramic heater are energized by placing them in series with a source of alternating current having an rms voltage of from 102V to 132V as shown in
As reflected in
In the examples that follow, a ceramic heater is placed in series with a ceramic igniter and a source of alternating current as shown in
According to the Ohm's law, the rms input voltage to the ceramic heater can be related to the rms current to each of the ceramic heater and ceramic igniter as follows:
V
in
=I(R1+R2) (5)
where,
The input voltage to the ceramic heater and the output voltage from the ceramic heater (which is the input voltage to the ceramic igniter) may be related using the voltage divider equation, as follows:
V
out
=V
in[R2/(R1+R2)] (6)
Referring to
In the following examples, the ceramic igniter is used in an oven cavity and has a desired steady state temperature of from 2138° F. to 2700° F. at input rms voltages ranging from 102V rms AC to 130V rms AC. At the same time, the ceramic heater (R1 in
A ceramic heater (
A ceramic igniter (
In this example and those that follow, the ceramic bodies for both the igniter and heater consist of 82 weight percent silicon nitride, 13 weight percent ytterbium oxide, and 5 weight percent molybdenum disilicide. Referring to
In this case, the igniter power draw P2 at 100V rms is only about 38 W, less than what is needed to reach the desired steady state temperature. In addition, at 130V the heater power draw P2 is 21 W, which corresponds to an excessive bimetal member temperature of about 997° F. Thus, the combination of resistances R1 and R2 does not meet the requirements for the ceramic igniter and the thermally-actuated gas valve assembly.
A ceramic heater with a room temperature resistivity of 1.1×10−4 Ω·cm is provided. Its conductive ink circuit comprises 100 weight percent tungsten and has a thickness of 17 microns. The room temperature resistance R1 is 9Ω.
A ceramic igniter with a room temperature resistivity of 3.5×10−4 Ω·cm is provided. Its conductive ink circuit comprises 75 weight percent tungsten carbide, 20 weight percent silicon nitride, 3 weight percent ytterbium oxide, and 2 weight percent silicon carbide. The circuit is about 25 microns thick. The room temperature resistance R2 is 32Ω. The ceramic heater and ceramic igniter are placed in series with one another and with a source of alternating current as shown in
From 100V to 130V AC rms the igniter power draw P2 exceeds 45 W, and the heater power draw P1 exceeds 8 W to allow gas to flow to the burner yet remains below 20 W to prevent the thermal actuator from overheating. Thus, the igniter has sufficient power to reach its desired ignition temperature while the heater does not exceed the maximum bimetal member deflection temperature. Thus, this combination of R1 and R2 in which the ratio of the room temperature resistances R2/R1 is 3.6 and the sum of the room temperature resistances R1+R2 is 41Ω achieves the desired igniter and thermally-actuated gas valve requirements.
A ceramic heater with a room temperature resistivity of 2.9×10−4 Ω·cm is provided. Its conductive ink circuit comprises 100 percent tungsten carbide and is about 17 microns thick. The room temperature resistance R1 is 25Ω.
A ceramic igniter is provided with a room temperature resistivity of 3.5×10−4 Ω·cm. Its conductive ink circuit comprises 75 weight percent tungsten carbide, 20 weight percent silicon nitride, 3 weight percent ytterbium oxide, and 2 weight percent silicon carbide. The circuit is about 20 microns thick. The room temperature resistance R2 is 42Ω. The ratio of room temperature resistances R2/R1 is 1.7, and the sum of the room temperature resistances R1+R2 is 67Ω. The input voltage Vin is varied from 0 to 130V AC rms, and the power draws, current, voltages, and resistances are determined as in Example 2. The results are provided in Table 3 and in
While the heater does not exceed its maximum desired power draw of 20 W, even at a source voltage Vin of 130V, the igniter fails to reach its minimum desired power draw of 45 W to reach its desired ignition temperature. This is due primarily to the high total resistance R1+R2 in the circuit. Thus, this combination of R1 and R2 does not meet the desired igniter and thermally-actuated gas valve criteria.
A ceramic heater with a room temperature resistivity of 2.8×10−4 Ω·cm is provided. Its conductive ink circuit comprises 84 weight percent tungsten carbide, 12 weight percent silicon nitride, 3 weight percent ytterbium oxide, and 2 weight percent silicon carbide. The circuit is about 17 microns thick. The room temperature resistance R1 is 37Ω.
A ceramic igniter is provided with a room temperature resistivity of 3.5×10−4 Ω·cm. The conductive ink circuit comprises 75 weight percent tungsten carbide, 20 weight percent silicon nitride, 3 weight percent ytterbium oxide, and 2 weight percent silicon carbide. The circuit is about 20 microns thick. The room temperature resistance R2 is 42Ω. The ratio of room temperature resistances R2/R1 is 1.1, and the sum of the room temperature resistances R1+R2 is 79Ω.
The input voltage Vin is varied from 0 to 130V AC rms, and the power draws, current, voltages, and resistances are determined as in Example 2. The results are provided in Table 4 and in
The igniter power draw P2 is too low to meet the igniter's ignition requirements across the range of input voltages (Vin) from 102V to 130V AC rms. In addition, the heater power draw is too high at and above 120V AC rms and would result in a temperature exceeding the bimetal member's maximum deflection temperature. Thus, this combination of resistances R1 and R2 does not meet the requirements of the ceramic igniter or the thermally-actuated gas valve.
The foregoing examples show that a room temperature resistance ratio R2/R1 of 3.6 and room temperature total resistance R1+R2 of 41Ω achieves the desired igniter and thermally-actuated gas valve performance. A ratio R2/R1 of 2.2 at a total resistance of 45Ω was not sufficient. However, if the thermal actuator of Example 2 were made from a bimetal member with a maximum deflection temperature of more than 1000° F., the ratio of 2.2 at a total resistance of 45Ω may be satisfactory.
This application claims the benefit of U.S. Provisional Application No. 62/888,872, filed on Aug. 19, 2019, the entirety of which is hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 62888872 | Aug 2019 | US |
