WO2000050815A1

WO2000050815A1 - Oxygen depletion sensor

Info

Publication number: WO2000050815A1
Application number: PCT/US2000/004851
Authority: WO
Inventors: Bill Kwok; Peter Montana, Jr.
Original assignee: Robertshaw Controls Company
Priority date: 1999-02-26
Filing date: 2000-02-24
Publication date: 2000-08-31
Also published as: JP2002538402A; CA2362615A1; AU3007500A

Abstract

An oxygen depletion sensor relies on the shift of a pilot flame away from a nozzle as the oxygen content of the combustion air decreases. A thermocouple is positioned so as to be in the flame during combustion at acceptable oxygen levels and in the unburned zone when the oxygen content is below an acceptable level. The large voltage difference between conditions of high and low oxygen content provide for a more reliable sensor.

Description

OXYGEN DEPLETION SENSOR

BACKGROUND OF THE INVENTION

The present invention generally relates to controlling the flow of gas to a gas-fired

appliance and more particularly pertains to an oxygen depletion sensor that enables a gas

controller to shut off the supply of gas to the appliance when the oxygen content of the

combustion air falls below a preselected value.

Obviating the need to vent a gas burning appliance provides a number of advantages

for various applications. First and foremost, it obviates the need to route combustion gases

to the outside of a building or room thereby greatly simplifying the installation of certain

appliances. Additionally, elimination of a vent may enhance the efficiency of certain gas-

fired appliances as it precludes the escape of heat through such vent.

Combustion of certain gases such as natural gas (methane) or liquified petroleum gas

can yield very clean, substantially toxin-free exhaust gases, essentially limited to C02 and

H20, if the supplied combustion air has an adequate oxygen content. However, as the

oxygen content drops below a critical level, an increasing amount of undesirable pollutants

are created during the combustion of the fuel. Additionally, as the oxygen is depleted in an enclosed space, less and less will be available for the occupants thereof to breathe as well.

It is therefore most desirable for combustion in a non-vented gas-fired appliance to

immediately and automatically be discontinued should the oxygen content of the combustion

air fall below a critical level.

A number of different approaches have been devised in an effort to address this

problem. In its most fundamental form, a standard pilot light and thermocouple combination

may serve as a crude oxygen depletion sensor in view of the fact that a substantial drop in

the oxygen level will eventually cause a pilot flame to extinguish. This in turn will cause the

voltage generated by the thermocouple to drop below the threshold level required for

maintaining the associated gas valve in its open position. However, substantially more

sensitivity is desirable and indeed is required by currently applicable industry standards.

One approach that has been employed in an effort to enhance the sensitivity of the

pilot light/thermocouple combination to low oxygen levels has exploited the fact that the

pilot flame temperature decreases as the oxygen level drops off. By enhancing the sensitivity

of the gas valve to respond to the slightly reduced voltage generated by a thermocouple being

subjected to a slightly cooler flame, the gas supply can be shut off long before a drop in oxygen level would actually cause the pilot flame to become extinguished. However, such

devices nonetheless suffer from certain disadvantages. As an example, the heightened

sensitivity of the valve to slightly reduced thermocouple voltages may give rise to

unnecessary shut downs when a slight reduction in voltage is caused by something other than

an unacceptably low oxygen level. A shifting of the thermocouple relative to the pilot flame

could cause the thermocouple to be subjected to a cooler region of the flame. Conversely,

a shifting of the flame relative to the thermocouple, such as when it is distorted by a draft,

could similarly cause the thermocouple to be subjected to a cooler region of the flame. In

either event, a slightly reduced voltage is the result that may not be distinguishable from the

reduced voltage that results from an overall cooling of the flame caused by a decrease in the

oxygen level. Additionally, such systems are vulnerable to the radiated heat generated by

the main burner of a gas-fired device as the temperature actually sensed by the thermocouple

may therefore be higher than that actually generated by the pilot flame and as a result, a

cooling of the pilot flame caused by a drop in the oxygen level could go undetected.

The device described in USPN 5,674,065 to Grando et al. appears to be a

representative example of heretofore known thermocouple and oxygen depletion sensor

configurations and as such would suffer from the disadvantages described above. The thermocouple is described as being conventionally installed as part of an oxygen depletion

safety device wherein the thermocouple is positioned so as to be heated by the gas flame of

the pilot. When the flame becomes unstable or diminished in size due to a reduction in the

oxygen level, its temperature drops off causing a commensurate reduction in the voltage

generated by the thermocouple. Any voltage reduction below the 14-22 open circuit millivolt

range will cause the gas valve to close. As a consequence, a reduction of the output voltage

to slightly below the nominal range for any reason would cause the shut down of the gas

supply even if the oxygen level were not significantly depleted. Conversely, the sensitivity

of such system could conceivably allow the radiant heat of the burner to maintain the output

voltage within the nominal range even if the oxygen level were depleted to an unacceptable

level. Either situation is undesirable.

Other disadvantages inherent in prior art devices are that they generally require a

substantial reconfiguration in order to accommodate the use of different gaseous fuels.

Different nozzles and different relative positions of the nozzle and thermocouple require a

multiplication of the number of parts that must be manufactured and on hand for the various

applications. As an example, in the apparatus described in Grando et al. the thermocouple must be repositioned from out in front of the pilot to an attachment point directly on the side

of the pilot in order to accommodate a change from natural gas to liquified petroleum gas.

A simple, reliable oxygen depletion sensor is needed that overcomes the shortcomings

of the heretofore known sensors. Such sensor should be easy to manufacture and should be

readily installable into existing appliance configurations.

SUMMARY OF THE INVENTION

The present invention provides an oxygen depletion sensor that offers substantial

advantages over previously known devices. The sensor is easily manufactured, very reliably

responds to a drop in the oxygen content to below a preselected level, is substantially

unresponsive to slight shifts in pilot flame or thermocouple positions and additionally

provides a visual indication of the oxygen level. In general terms, the present invention takes

advantage of the effect that the oxygen content of combustion air has on flame velocity and

causes the difference between the voltage generated by thermocouple during high and during

low oxygen level conditions to be greatly exaggerated. In contrast to the previously known

devices wherein the thermocouple output gradually drops off as the oxygen level is depleted and the difference between the voltage that is generated during combustion at acceptable

oxygen levels and at unacceptable oxygen level conditions is very slight, the device of the

present invention needs only to recognize a substantial drop in the generated voltage.

The flame velocity of a particular gaseous fuel is indicative of how fast such gas burns

and depends to a large extent on the amount of oxygen that is available for combustion.

Flame velocity is therefore a direct function of the oxygen/fuel ratio in a mixture subject to

combustion. By holding the air/fuel ratio constant, the flame velocity becomes a direct

function of the oxygen content in the combustion air.

During combustion, the position of the flame, or more accurately, the periphery of the

base of such flame will stabilize where the flow velocity of the combustible mixture

substantially equals the flame velocity. As the oxygen level decreases, the flame velocity

drops off causing the base of the flame to move away from the nozzle emitting the

combustible mixture. The unburned zone will tend to increase as the oxygen level continues

to decrease until eventually, the flame is extinguished. The velocity of a gas flowing from a nozzle is substantially a function of the volume

of the gas flow and of the cross-sectional area of the nozzle through which such gas flow

issues. The amount of combustion air that is drawn into a flow of gaseous fuel prior to its

emission from a nozzle is substantially dependent upon the pressure drop generated by the

flow as it passes through a mixing chamber and upon the size of the primary air intake

opening through which air is drawn into such mixing chamber in response to the pressure

drop. The volume and velocity of the gaseous fuel flowing into and through such mixing

chamber will determine the pressure drop relative to ambient conditions. The pressure at

which the gaseous fuel is supplied and the size of the inlet orifice through which the fuel

flows into a mixing chamber will in turn determine the volume and velocity of the gaseous

fuel as it passes through the mixing chamber. The combination of all these parameters will

determine where the flame resulting from the combustion of the air/fuel mixture issuing

from the nozzle will be positioned relative to such nozzle. As the oxygen content of the

combustion air is decreased, the flame will move away from the nozzle.

The oxygen depletion sensor of the present invention exploits these fluid dynamic

principles of gas flow and flame velocity. By advantageously positioning a thermocouple

adjacent to an appropriately configured nozzle emitting an air/fuel mixture, such that the thermocouple is enveloped by the flame during combustion of such mixture at oxygen levels

above a preselected value and is wholly outside the flame at oxygen levels below such

preselected level, a very substantial change in output voltage is thereby realized as a function

of only a relatively minor change in the combustion air's oxygen content. An associated

valve that controls the flow of gas to the appliance is configured such that only a voltage in

the range generated by the thermocouple while enveloped by the flame is sufficient to hold

the valve in its open position. The substantially lower voltage generated by the

thermocouple when located in the unburned zone is substantially below the threshold voltage

required for maintaining the valve in its open position. Such system is substantially

impervious to slight shifts in the relative positions of the thermocouple and pilot flame as the

voltage generated in even the coolest region of the flame is still well above the threshold

temperature at which the valve shuts off. Conversely, the heat radiated by the main burner

is incapable of raising the voltage generated by the thermocouple while positioned outside

the flame to a level sufficient to maintain the valve in its open position. Only the substantial

drop in output voltage commensurate with the temperature differential between a location

inside and a location outside the flame is recognized as being indicative of an unacceptable

drop in oxygen content. The present invention provides an additional benefit in that the position of the flame

relative to the thermocouple gives a visual indication of the oxygen content of the

combustion air. The oxygen content will be at or above an acceptable level when the

thermocouple is observed as being fully enveloped by the flame. Any shift of the periphery

of the flame toward the thermocouple that is apparent or any obvious instability or lifting of

the flame is an indication of a depleted oxygen level. A gap between the flame and the

thermocouple during for example manual override of an automatic shutoff valve is an

indication of an unacceptably low oxygen level.

Finally, the present invention provides the additional advantage of being readily

adaptable for use with different gaseous fuels. Rather than requiring the use of a different

nozzle configuration and/or the repositioning of the thermocouple relative to the nozzle in

order to accommodate the use of a different gas, the same nozzle and the same relative

placement of the nozzle and thermocouple may be employed by simply resizing the inlet

orifice and air hole sizes.

These and other features and advantages of the present invention will become

apparent from the following detailed description of a preferred embodiment which, taken in conjunction with the accompanying drawings, illustrates by way of example the principles

of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is semi-schematic view of an oxygen depletion sensor of the present invention

as used in a gas-fired appliance;

Fig. 2 is an enlarged cross-sectional view of the flame forming component of the

sensor shown in Fig. 1 ;

Fig. 3 is side view of the oxygen depletion sensor of the present invention with a pilot

flame burning in combustion air having a high level of oxygen content;

Fig. 4 is a side view of the oxygen depletion sensor of the present invention with a

pilot flame burning in combustion air having a low level of oxygen content; and Fig. 5 is a graphic plot of the voltage versus oxygen level for a device constructed in

accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The Figures generally illustrate the present invention and more particularly, are

directed to a preferred embodiment thereof. The oxygen depletion sensor of the present

invention simultaneously serves as a pilot light for a gas-fired appliance and functions to shut

off the flow of gas to such appliance should the oxygen content of the combustion air fall

below a preselected level. The device is readily adaptable for use with different gaseous

fuels. Additionally, the device provides a visual indication of the oxygen content that is

present in the combustion air.

Fig. 1 is a semi-schematic representation of a preferred embodiment of the oxygen

sensor device 12 of the present invention as used in a gas-fired appliance. A bracket 14

maintains a thermocouple 16 in position adjacent the mouth of nozzle 18 of a flame forming

component 20. A gas valve 22 controls the flow of gas from a supply line 21 to both the flame forming component 20 via line 24 and to the main burner 26 of the appliance via line

28. The valve is configured such that the range of voltages generated by the thermocouple

while subject to temperatures encountered within a pilot flame is sufficient to hold the valve

in its open position while voltages generated by the thermocouple at temperatures below

those encountered within a pilot flame are insufficient to hold the valve open and therefore

allow the valve to close and shut off the flow of gas to the appliance. The general

configuration of such a valve may conform to that which is well known in the art wherein

a spring loaded valve element is held in its open position by an electromagnet that is

energized by the thermocouple. The particular performance parameters of such valve

configuration are selected such that the threshold voltage for holding the valve open is below

the voltage generated by the thermocouple while positioned in a pilot flame and above the

voltage generated by such thermocouple while positioned in the unburned zone adjacent a

pilot flame.

The bracket may additionally hold an ignitor 30 in position so as to enable the gaseous

mixture issuing from the nozzle to be ignited to establish a pilot flame. Additionally, the

bracket may hold a thermopile 32 in position above the flame. The voltage generated by the

thermopile may be used by the valve 22 to regulate the flow of gas to the main burner 26. Fig. 2 is a cross sectional view of the flame controlling component 20 of the oxygen

depletion sensor and illustrates its essential features. Gas line 24 supplies the component

with gaseous fuel which enters a mixing chamber 34 through inlet orifice 36. An air hole 38

is formed in the wall of the mixing chamber and the gaseous fuel flowing from inlet orifice

as well as the air drawn in through the air hole exits through nozzle 18. The line pressure

of the gas and the cross-sectional area of inlet orifice 36 dictate the velocity with which the

gas emanates from the orifice. The velocity of the gas flow and size of the cross-sectional

area of the air hole in turn determines the rate at which combustion air is drawn into the

mixing chamber. The respective flow rates of the gaseous fuel and the combustion air in turn

determine the air/fuel ratio of the resultant mixture. Finally, the diameter of the nozzle

determines the velocity with which the air/fuel mixture issues from the nozzle.

By properly selecting the various parameters, i.e. type of gaseous fuel, line pressure,

inlet orifice size, air hole size and nozzle size, a stable flame forms adjacent the mouth of the

nozzle upon combustion of the mixture issuing from the nozzle. As the oxygen content of

the combustion air that is drawn in through the air hole decreases, the position of the flame

shifts away from the end of the nozzle. Thermocouple 16 is positioned so as to be located

within the flame in the position the flame assumes when the combustion air contains what is considered to be an adequate oxygen content and outside of the periphery of the flame, i.e

in the unburned zone, when the flame is in the position it assumes when the combustion air

contains what is considered to be an inadequate oxygen content.

The following dimensions and relationships are representative of a configuration that

functions in accordance with the present invention:

gas type: methane

inlet pressure: 3.5 to 10.5" water column

inlet orifice: 0.013" diameter

air hole: 0.166" diameter

nozzle: 0.125" diameter

TC from nozzle: 0.35"

TC output at 600-700 F: 13mN - 16mN

min. to hold valve open: 2.5mN

An oxygen depletion sensor constructed in accordance with the above specifications

has been found to automatically shut off the flow of gas when the oxygen content in the combustion air drops from the normal 20.8% to between 18.2 - 19.4%. Further depletion

of the oxygen content is thereby curtailed to prevent the formation of noxious products of

combustion and to leave an adequate oxygen content for breathing.

Fig.3 illustrates the oxygen depletion sensor 12 in operation when the combustion air

contains an adequate content of oxygen. The pilot flame 40 forms and stabilizes a short

distance adjacent to the mouth of nozzle 18. The base of the flame, or more accurately, the

proximal end of the periphery of the flame approximately conforms to the loci of where the

flame velocity is substantially equal to the flow velocity of the combustible mixture of

gaseous fuel and combustion air issuing from the nozzle. The thermocouple 16 is positioned

within the flame and generates a voltage commensurate with the 600-700 F temperature it

is thereby subjected to. Such voltage is well above the threshold voltage selected to hold the

valve 22 in its open position thus allowing it to supply gas to the flame forming component

20 as well as to the main burner 26 as needed.

The configuration of the device has the added benefit of maintaining a fairly high flow

velocity of the combustible mixture which allows the main burner 26 to be distanced further

away from the nozzle 18 and thermocouple 16 than is possible when using a low velocity configuration. This serves to reduce the amount of radiated heat sensed by the thermocouple.

Additionally, the higher velocity stabilizes the flame which is thereby less susceptible to

distortion by drafts and less susceptible to buoyancy effects making it feasible to mount the

device in inclined and even vertical orientations as well as the illustrated horizontal

orientation.

Fig. 4 illustrates the oxygen depletion sensor 12 in operation in an environment

wherein the combustion air has a depleted oxygen content. The flame 40a forms at a position

further removed from the mouth of the nozzle 18 which causes the thermocouple 16 to be

positioned outside the flame in the unburned zone. The temperature outside the periphery

of the flame is substantially less than inside the flame, on the order of several hundred

degrees Fahrenheit, resulting in a commensurately reduced voltage output. The lower

voltage output is well below the threshold voltage required to maintain the gas valve 22 in

its open position which causes it to shut off all gas flow. Additionally, a visual inspection

of the pilot flame offers a clear indication of the oxygen content by virtue of the position of

the flame relative to the thermocouple. Any gap between the visible periphery of the pilot

flame and the thermocouple or any instability or lifting of the pilot flame is an indication of

an unacceptably low oxygen content and that shut off is imminent. Fig. 5 is a representative plot of thermocouple voltage versus oxygen content of the

combustion air for a oxygen sensor device of the present invention. As is apparent, the

voltage generated by the thermocouple gradually decreases as the oxygen content of the

combustion air becomes depleted. In the zone identified by reference numeral 50, the

thermocouple is enveloped by the flame and therefore subject to very high temperatures, as

is evidenced by the relatively high voltage output. As the oxygen content is depleted, the

flame velocity of the mixture decreases which causes the flame temperature to drop off

slightly as is evidenced by the slight slope in the curve. The depletion of the oxygen content

also causes the flame to gradually move away from the nozzle and thermocouple until its

periphery is substantially aligned with the thermocouple. At such point, the temperature

sensed by the thermocouple is subject to a precipitous decline as is evidenced by the abrupt

drop in the voltage in zone 52. Further depletion of the oxygen content would cause the

voltage to continue to decrease (zone 54) as the flame moves further away from the

thermocouple and its temperature continues to decrease. The flame forming component of

the present invention is configured such that the abrupt transition zone 52 substantially

includes the preselected limit 56 below which the oxygen content is considered unacceptable.

Additionally, the gas valve is configured such that the threshold voltage necessary to

maintain the valve in its open position is selected to correspond to a voltage 58 encountered in the transition zone 52. As a consequence, maximum sensitivity and repeatability is

simultaneously achieved.

The oxygen depletion of the sensor of the present invention can be readily modified

to accommodate the use other gaseous fuels by either altering the inlet orifice diameter, the

air hole diameter, or both. The same thermocouple may be used, the same spacing between

the thermocouple and the mouth of the nozzle may be maintained and the same overall flame

forming component may be employed.

While a particular form of the invention has been illustrated and described, it will also

be apparent to those skilled in the art that various modifications can be made without

departing from the spirit and scope of the invention. Accordingly, it is not intended that the

invention be limited except by the appended claims.

Claims

WHAT IS CLAIMED IS:

1. An oxygen depletion sensor device for discontinuing the flow of a gaseous fuel

to a fuel-burning appliance when the oxygen content of combustion air available for the

combustion of said gaseous fuel falls below a preselected limit, comprising:

a flame forming component for providing a flame of combusting gas adjacent

thereto, said flame being subject to a change in position as a function of the oxygen content

of combustion air available for combustion of said gaseous fuel, wherein said flame shifts

away from said flame forming component as said oxygen content decreases;

a thermocouple positioned so as to be located within said flame when said

oxygen content is above said preselected limit and adjacent such flame when said oxygen

content is below said preselected limit, wherein voltage generated by said thermocouple

while said thermocouple is within said flame is greater than a threshold level and voltage

generated by said thermocouple while said thermocouple is adjacent said flame is below said

threshold level; and

a valve for controlling the flow of gaseous fuel to said appliance, wherein said

valve is responsive to voltage generated by said thermocouple such that said flow is

discontinued when the voltage generated by said thermocouple is below said threshold level.

2. The oxygen depletion sensor of claim 1 , wherein said flame forming component

comprises:

a mixing chamber;

an inlet orifice formed in said mixing chamber for allowing said gaseous fuel

to enter;

an air hole formed in said mixing chamber for allowing combustion air to be

drawn into said mixing chamber; and

an exit nozzle through which said gaseous fuel and said combustion air exit said

mixing chamber.

3. The oxygen depletion sensor of claim 1, wherein said flame serves as a pilot

flame for igniting a main burner of said appliance.

4. The oxygen depletion sensor of claim 3, further comprising:

a thermopile operative to generate a voltage when subjected to said flame

sufficient to allow said valve to regulate the flow of gaseous fuel to said main burner.

5. The oxygen depletion sensor of claim 4, wherein said thermopile is positioned

directly above said flame.

6. An oxygen depletion sensor, comprising:

a flame forming component for causing combustion of a mixture of a gaseous

fuel and combustion air to form a flame which shifts in position as a function of the oxygen

content of said combustion air; and

a thermocouple operative to generate a voltage as a function of temperature,

positioned so as to be located within said flame when said combustion air has an acceptable

oxygen content and outside said flame when said combustion air has an unacceptable oxygen

content.

7. The oxygen depletion sensor of claim 6, further comprising:

a valve for controlling the flow of said gaseous fuel therethrough, responsive

to the voltage generated by said thermocouple and configured to shut off said flow of gaseous

fuel at voltages below those generated when said thermocouple is located within said flame.

8. The oxygen depletion sensor of claim 7, wherein said valve controls the flow

of said gaseous fuel to said flame forming component and to a main burner of a fuel-burning

appliance and wherein said flame serves as a pilot flame for igniting said main burner.

9. An oxygen depletion sensor, comprising:

a thermocouple operative to generate a voltage as a function of temperature; and

a flame forming component for causing combustion of a mixture of a gaseous

fuel and combustion air to form a flame, wherein such flame envelops said thermocouple only

when said oxygen content of said combustion air is above a preselected level while said flame

is positioned adjacent said thermocouple when said oxygen level of said combustion air is

below a preselected level.

10. The oxygen depletion sensor of claim 9, further comprising a valve for

controlling a flow of said gaseous fuel therethrough, responsive to the voltage generated by

said thermocouple and configured to shut off said flow of gaseous fuel at voltages below those

generated when said thermocouple is enveloped by said flame.

11. The oxygen depletion sensor of claim 10, wherein said valve controls the flow