Patent Application
20060275720
The present invention generally relates to fuel-fired heating apparatus and, in a representatively illustrated embodiment thereof, more particularly provides a fuel-fired, natural draft water heater having a specially designed, low power control system which includes a flammable vapor sensor and is operatively connected to a pilot valve millivolt circuit portion of the water heater.
Fuel-fired, natural draft water heaters typically maintain a standing pilot flame which is used to ignite a main burner flame when the control system of the water heater calls for heat to be added to its tank-stored water from the main burner. The pilot burner is supplied with fuel gas through a normally closed valve having an electrically operated solenoid portion used to keep the valve open during the presence of the pilot flame.
The valve solenoid portion is part of what is customarily referred to as a “millivolt” circuit and is wired in series with a thermoelectric device, such as a thermocouple or a multi-thermocouple thermopile structure, which is impinged upon by the pilot flame. Such thermoelectric device operates to convert pilot flame heat to a relatively small amount of electrical current that flows through the millivolt circuit and, via the solenoid, holds the normally closed pilot valve in an open position to maintain the standing pilot flame. Conventionally, the pilot valve is linked to the main burner valve in a manner such that if the pilot valve shuts off the main burner valve automatically does so as well to thereby shut off the water heater combustion process.
In recent years considerable design effort has been expended to provide fuel-fired water heaters with flammable vapor ignition resistance (FVIR) of various sorts in an attempt to prevent the ignition by the water heater of extraneous flammable vapors that may be present (from, for example, spilled gasoline on the floor) adjacent the water heater. One suggested technique to achieve this protective result is to use a flammable vapor sensor which detects flammable vapors adjacent the water heater and terminates or precludes combustion initiation in the water heater combustion chamber. A common type of flammable vapor sensor used in this application is of a chemiresistor type in which the electrical resistance of the sensor increases as a function of the concentration of flammable vapors to which the sensor is exposed. It is this flammable vapor-created sensor resistance increase which is utilized to prevent ignition of such flammable vapors.
One previously proposed technique for using a variable resistance flammable vapor sensor in this application is illustrated and described in published U.S. Patent application 2001/0042564 to Abraham et al, which is hereby incorporated by reference herein, in which a variable resistance flammable sensor 22 is placed in the water heater millivolt circuit in series with the valve solenoid 28 and a thermocouple 36 or thermopile that is position so as to be heated by a standing pilot flame 34. Electrical energization of the solenoid coil 28 with sufficient voltage holds the fuel valve 24 open against the force of a spring 26 urging the valve to its normally closed position.
A disadvantage of this approach of interposing a variable resistance flammable vapor in a fuel-fired water heater millivolt circuit is noted in U.S. published Patent Application 2001/0042564 as being “ . . . it is necessary to increase the internal resistance of the solenoid coil. This can be accomplished by for example the use of higher gauge wire (smaller diameter) and an increase in number of coils in the electromagnet”.
Thus, if it desired to add the flammable vapor ignition protection of a variable resistance type flammable vapor sensor by interposing the sensor in the millivolt circuit in a retrofit application it is necessary to modify or replace the valve solenoid to accommodate the additional electrical resistance (and the corresponding decrease in voltage available to operate the valve solenoid) created by the sensor. This, of course, undesirably adds to the overall installation and materials cost of this retrofit effort. And, of course, a modified solenoid structure would also have to be provided if the flammable vapor sensor was incorporated in the millivolt circuit in the original manufacture of the water heater.
From the foregoing it can readily be seen from the foregoing that it would be desirable to provide a technique for associating a variable resistance flammable vapor sensor with the millivolt circuit of a fuel-fired water heater without the previous necessity of modifying the valve solenoid portion of the millivolt circuit. It is to this goal that the present invention is primarily directed.
In carrying out principles of the present invention, in accordance with a representative embodiment thereof, a fuel-fired heating apparatus is provided which is illustratively a water heater but could alternatively be another type of fuel-fired heating apparatus such as, for example, a boiler or air heating furnace.
The water heater has a burner, representatively a pilot burner, adapted to receive fuel and combustion air and create a flame therefrom, and a normally closed fuel valve coupled to the burner for supplying fuel thereto when opened. A thermoelectric circuit portion of the water heater has connected in series therein (1) a thermoelectric device positioned to receive heat from the burner flame and responsively generate an electrical voltage, (2) a solenoid structure operative to receive electrical power from the thermoelectric device and responsively hold the normally closed fuel valve in an open position, and (3) a normally open switch device operative to receive an electrical signal and responsively close to thereby permit thermoelectric current flow through the solenoid structure for the duration of said electrical signal.
A low power electrical control circuit, operable by an electrical power source, preferably a long-life DC battery, is coupled to the thermoelectric circuit only via the normally open switch device and is operable to protect the heating apparatus against an undesirable operating condition. The low power electrical control circuit has connected therein a variable resistance sensor operative to detect the predetermined undesirable operating condition which is representatively the presence of a predetermined concentration of extraneous flammable vapors adjacent the heating apparatus. The control circuit, in the absence of detection by the sensor of the undesirable operating condition, outputs an electrical signal to the switch device to maintain it in a closed state, and terminates the electrical signal upon detection by the sensor of the undesirable operating condition to return the switch device to its normally open state which automatically returns the fuel valve to its normally closed position or precludes it from being opened.
By associating the control system and switch with the thermoelectric circuit in this unique manner, which forms a method of the present invention that may be carried out in the initial fabrication of the heating apparatus or later as a retrofit method, the resistance of the sensor, illustratively a flammable vapor sensor, is not incorporated into the thermoelectric circuit. Accordingly, it is not necessary to modify the fuel valve solenoid coil in any manner.
According to various other aspects of the present invention, the normally open switch device is a solid state switch device, preferably a field effect transistor, and the thermoelectric device is preferably a thermopile structure. The low power electrical control circuit preferably includes a multi-resistor bridge section, with the variable resistance sensor forming a resistive portion of the bridge section. Preferably, the control circuit further includes an operational amplifier of the open collector output type and has an input portion operatively connected across the circuit bridge section, and an output portion coupled to the normally open switch device. The control circuit also preferably includes a first electrical lead interconnecting the output portion of the operational amplifier to the normally open switch device, a second electrical lead interconnected between the first electrical lead and the circuit bridge section, and a pull-up resistor connected in the second electrical lead.
As schematically illustrated in
Beneath the bottom end of the tank 14 is a combustion chamber 22 in which a main fuel burner 24 and an associated pilot burner 26 are operatively disposed. In a conventional manner, each burner is operative to receive fuel and combustion air which it combusts to create a burner flame. A main fuel supply line 28 is connected to the main burner 24, and a pilot fuel supply line 30 is connected to the pilot burner 26. Normally closed main and pilot fuel supply valves 32,34 are respectively installed in the fuel supply lines 28,30. The pilot valve 34 is linked to the main valve 32 in a conventional manner such that when the pilot valve 34 closes the main valve 32 also closes.
The pilot valve 34 has an electric solenoid coil 36 (see
With reference now to
Turning now to
It is important to note at this point that the flammable vapor sensor 46 is not directly connected in the millivolt circuit 50. Thus, the valve solenoid coil 36 does not have to be modified in any manner to accommodate the extra electrical resistance of the flammable vapor sensor 46. Instead of being connected in the millivolt circuit 50, the variable resistance flammable vapor sensor 46 is connected in the separate control circuit 52 as will now be described.
Control circuit 52 includes a pair of electrical leads 62,64 between which, from left to right in
An operational amplifier 73, representatively of the open collector output type, is interposed in lead 69 and has an input lead 74 interconnected between its positive input terminal and the lead 70 between the resistors R3 and R4, an input lead 76 interconnected between its negative input terminal and the lead 68 between resistor R1 and the flammable vapor sensor 46, and an output lead 78 connected to gate G of the transistor 61. Thus, the input side of the operational amplifier 73 is connected across the bridge section of the control circuit 52. Additionally, an electrical lead 80 having a 100 kΩ pull-up resistor R5 therein is interconnected between leads 62 and 78 as shown. Pull-up resistor R5 functions to substantially reduce the electrical current outflow from the control circuit 52.
When the flammable vapor sensor 46 is not exposed to flammable vapor 48, its resistance is substantially less than the 49.9 kΩ resistance of resistor R4 so that the total resistance of the R1,R2 circuit leg is similarly substantially less than the total resistance of the R3,R4 circuit leg. Accordingly, under this condition the operational amplifier 73 outputs an electrical signal via lead 78 to the gate “G” of the transistor 61, thereby maintaining the transistor 61 in its closed state and permitting current flow through the transistor 61 to hold the pilot valve 34 open. However, if the flammable vapor sensor 46 is exposed to flammable vapor 48 and its electrical resistance rises to above 49.9 kΩ, the operational amplifier output signal in lead 78 terminates. This causes the transistor 61 to return to its normally open state in which it blocks electrical current flow therethrough, thereby causing the pilot valve 34, and thus the main valve 32, to close.
It should be noted that the control circuit 52 is electrically coupled to the millivolt circuit 50 only by the single electrical lead 78 extending between the operational amplifier 73 and the field effect transistor 61. The electrical power source for the control circuit 52 (the battery 72) is separate from and not coupled to the electrical power source (the thermopile 58). Thus, to retrofit a fuel-fired water heater with the control circuit/transistor structure of the present invention all that is necessary is to connect the transistor 61 in the water heater's millivolt circuit, and then provide the single lead interconnection between the control circuit 52 and the installed transistor. In accordance with principles of the present invention, the control circuit 52, by itself or with the associated transistor 61, may conveniently be provided in module form for original or retrofit installation on an associated water heater.
Instead of the illustrated long-life battery 72 used to power the control circuit 52, other suitable electrical power sources, such as a thermoelectric source or an AC power source converted to DC power, could be alternatively utilized if desired without departing from principles of the present invention. Moreover, in the representatively illustrated control circuit 52, resistors of other suitable values could be utilized, and other circuit components and arrangements could be utilized without departing from principles of the present invention. Similarly, electrical switch structures other than the illustrated field effect transformer 61 could be utilized without departing from principles of the present invention.
Although the present invention has been representatively illustrated as being incorporated in a fuel-fired, natural draft water heater, it will be readily appreciated by those of skill in this particular art that principles of the present invention could alternatively be utilized to advantage in other types of fuel-fired heating apparatus such as, by way of example and not of limitation, fuel-fired boilers and heating furnaces. Additionally, various types of variable resistance type sensors other than a flammable vapor sensor could be incorporated in the control circuit 52, to detect an undesirable operating condition of the associated fuel-fired heating apparatus, could be utilized without departing from principles of the present invention. For example, but not by way of limitation, various other types of gas sensors, or a temperature sensor, could be used.
The foregoing detailed description is to be clearly understood as being given by way of illustration and example only, the spirit and scope of the present invention being limited solely by the appended claims.
