Note: Descriptions are shown in the official language in which they were submitted.
CA 02547102 2006-05-16
Docket No.: RHWH-0139
LOW POWER CONTROL SYSTEM AND ASSOCIATED METHODS
FOR A WATER HEATER WITH FLAMMABLE VAPOR SENSOR
BACKGROUND OF THE INVENTION
The present invention generally relates to fuel-fired heating
1o 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
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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
lo 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 previousiy 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,
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.
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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
1o 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.
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SUMMARY OF THE INVENTION
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
1o 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,
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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
lo 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.
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BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic cross-sectional view through a representative
fuel-fired natural draft water heater embodying principles of the present
invention; and
FIG. 2 is a schematic diagram of a millivolt circuit portion of the
water heater to which a specially designed low power control system,
incorporating a variable resistance flammable vapor sensor, is operatively
coupled.
DETAILED DESCRIPTION
As schematically illustrated in FIG. 1, this invention provides a fuel-
fired, natural draft water heater 10 which representatively rests on a floor
12 and includes the usual insulated tank 14 that holds a quantity of
pressurized water 16 for on-demand delivery to various plumbing fixtures
through a tank-mounted outlet fitting 18. Pressurized hot water 16
delivered to such plumbing fixtures is automatically replaced by
pressurized cold water delivered to the tank 14 via a tank inlet fitting 20.
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.
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The pilot valve 34 has an electric solenoid coil 36 (see FIG. 2) which
circumscribes a movable metal core portion 38 of the pilot valve 26 and,
when electrically energized as later described herein, moves the core 38 in
a first direction (representatively upwardly as indicated by the arrow 40 in
FIG. 2) to open the normally closed pilot valve 34. When coil 36 is de-
energized, the solenoid core 38 is spring-driven in the opposite direction
42 to thereby close the pilot fuel supply valve 34, thus also closing the
main fuel supply valve 32.
With reference now to FIGS. 1 and 2, according to key aspect of this
1o invention, the water heater 10 is provided with a specially designed low
power fuel valve control system 44 which includes a flammable vapor
sensor 46. The vapor sensor 46 is positioned near the floor 12 and is
operative to sense flammable vapor 48 (such as from spilled gasoline) near
the floor 12 adjacent the bottom end of the water heater 10 and
responsively cause the pilot valve 34, and thus the main valve 32, to close.
The flammable vapor sensor 46 is of the chemiresistor type and has an
electrical resistance which increases with increases in the concentration of
the flammable vapor 48 to which the sensor 46 is exposed.
Turning now to FIG. 2, the specially designed low power fuel valve
control system 44 of the present invention comprises electrical circuitry
that includes a millivolt circuit portion 50 and a control circuit portion 52.
The millivolt circuit 50 includes a pair of electrical leads 54,46 which are
coupled as shown to the pilot valve solenoid coil 36, and to a thermopile
device 58 in series with the coil 36. Thermopile device 58, which may
alternatively be a thermoelectric device of another suitable type such as a
thermocouple, is positioned to be impinged upon by the standing flame
60 of the pilot burner 26 to thereby thermoelectrically generate a small
electrical current in the millivolt circuit 50. A conventional ECO
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(emergency cutoff device) is connected in lead 54 between the coil 36 and
the thermopile 58, and a field effect transistor 61 is connected in the lead
54 between the ECO and the coil 36. As will be seen, transistor 61
functions as a normally open solid state electrical switch structure in the
millivolt circuit 50. When it is in its normally open mode, the transistor 61
blocks electrical current flow through the solenoid coil 36. When it is in its
closed mode the transistor 61 permits electrical current flow through the
solenoid coil, and does not add any appreciable additional resistance to
the millivolt circuit 50.
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 FIG. 2, electrical leads 66,68,69,70 are
connected. A 3.6V long life lithium DC storage battery 72 is connected in
lead 66. A 150k= resistor R1 and the flammable vapor sensor 46 (forming a
variable resistor R2) are connected in series in the lead 68, and a 150k=
resistor R3 and a 49.9k= resistor R4 are connected in series in lead 70. As
can be seen, the resistors R1-R4 form a bridge section of the control
circuit 52, with the flammable vapor sensor forming a resistive element in
one of the legs of such bridge circuit.
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
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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 100k= 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 49.9k= 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 49.9k=, 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
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thermopile 58). Thus, to retrofit a fuel-fired water heater with the control
circuitltransistor 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 convenientiy 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
1o control circuit 52, other suitable electrical power sources, such as a
thermoelectric source or an AC power source converted to DC power,
could be alternativeiy 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 transistor 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
readlly 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
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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.
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