Solid State Safety Control For Fuel Burning Apparatus

Abstract

A solid state flame sensing control circuit for a fuel burning device incorporates a thermal sensitive circuit breaker, a heating coil to energize the circuit breaker, a variable resistance device arranged to monitor and respond to a given physical condition or absence of the flame of the fuel burning device and a thyrister type unit which when energized permits sufficient current to pass through the heating coil to trip the circuit breaker. The elements are so connected that as long as the variable resistance device senses a proper flame condition the thyrister will be non-conductive; the thyrister becoming conductive when the variable resistance device, dependent on its nature, senses an absence of flame or a flame shift, or a temperature condition of such flame within the combustion chamber of the fuel burning device indicative of an absence of or improper burning. In preferred embodiment means are also provided to render the circuit insensitive to normal flame flicker.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a safety control for a fuel burning device, and more particularly to a solid state flame sensing control circuit capable of constantly monitoring the operation of the fuel burning device and of shutting down the fuel burning device upon the occurrence of faulty ignition, flame loss or improper burning, irrespective of the cause.

2. Description of the Prior Art

While the control circuit of the present invention is generally applicable to fuel burning systems, for purposes of an exemplary illustration it will be described in connection with portable space heaters of conventional and well known types frequently used, for example, by contractors for heating and drying purposes.

The nature of the fuel burning portable space heater does not constitute a limitation on the present invention. In general, such a space heater typically comprises an outer housing surrounding a combustion chamber. Means are provided to introduce air into the combustion chamber. A burner is located at one end of the combustion chamber. The burner normally has a fuel nozzle, frequently incorporating eductor means providing jets of air to draw, mix and atomize the fuel delivered by the nozzle. The nozzle, together with the eductors, discharge a combustible fuel-air mixture into the combustion chamber. Means are provided to ignite the mixture, and after initial ignition continuous burning occurs. Typically, during the continuous combustion, convection heat currents issue from the end of the heater opposite the burner and additional heat radiates from the surface of the heater housing.

Portable space heaters of the general type described are frequently provided with an ignition transformer and a motor. The motor normally runs a fan supplying air to the combustion chamber and the eductors and operates a fuel pump.

When the portable space heater is functioning properly, fuel burning will occur near the end of the combustion chamber at which the burner is located. In the event of improper air flow, however, the flame will move toward the opposite end of the combustion chamber, the combustion becoming unsteady and inadequate for proper heating. Under such a circumstance, it is desirable to shut down the heater. Inadequate air may result from a malfunction of the fan or a blocking of the passages for the air into the combustion chamber.

Inadequate and possibly dangerous conditions may also be indicated by a lower than normal temperature of the burner flame, representing improper combustion conditions.

It is also desirable to shut down the portable space heater when there is a flame failure. This can occur by virtue of faulty ignition, a blockage of the fuel nozzle or exhaustion of the fuel supply.

In any case the malfunction can cause insufficient or incomplete burning or a failure to burn issuing fuel, producing a dangerous existence of highly flammable liquid or noxious fumes. Prior art workers have devised a number of safety control circuits for fuel burning devices proposed to avoid the many and often disastrous results of improper burning or failure of flame in apparatus such as portable space heaters. For example, circuits have been proposed incorporating a thermal sensitive circuit breaker, a heating coil to energize the circuit breaker and a cadmium sulfide cell to monitor the flames of the fuel burning device. The heating coil and the cell were connected in parallel, the cell being shunted across the heating coil. The theory was that so long as the cadmium sulfide cell sensed a proper flame, insufficient current would pass through the heating coil to trip the thermal sensitive circuit breaker. In such a circuit, however, the cadmium sulfide cell would sometimes react to flame flicker causing the tripping of the circuit breaker when shut down of the fuel burning device was not actually required.

In another prior art embodiment, a thermal sensitive circuit breaker and heating coil were again used, together with a cadmium sulfide cell. In this embodiment a relay was provided having normally closed contact points in series with the heating coil. When the cadmium sulfide cell sensed a proper flame, the armature of the relay would react so the normally closed contact points would open removing sufficient power from the heating coil to prevent tripping of the circuit breaker. Such a circuit, however, has proven expensive to manufacture, requiring a relay (a relatively large component) and was subject to mechanical failure of the relay.

The safety control circuit of the present invention is a solid state circuit which is very small in size as compared to those of the prior art making it easy to apply in the most convenient manner and is very inexpensive to manufacture. The circuit is reactive, and more effectively in any case, both to improper burning and loss of flame, irrespective of the cause. Nevertheless, the circuit is so designed that in preferred embodiment it is not reactive to ordinary flame flicker so that unnecsary and sometimes highly damaging shut down of the fuel burning device will not occur. The circuit contains no relays or similar devices of prior art apparatus which have been often subject to mechanical failure. Finally, the circuit is extremely simple, containing a small number of elements and is characterized by quick response in the event of improper burning or flame loss lending it a high safety quotient. Its use will obviate the high incidence of problems such as noted above to occur in prior art apparatus.

SUMMARY OF THE INVENTION

The device providing the solid state flame sensing control circuit of the present invention comprises first and second leads capable of providing its connection to a power source. The electrical power means of the fuel burning device is connected across the first and second leads in parallel. Such electrical power means may comprise, for example, a motor and an ignition transformer.

In a preferred embodiment of the invention here illustrated a thermal sensitive circuit breaker has the contacts thereof connected in the first lead and across the first and second leads is connected an additional lead incorporating, in series, a heating coil for actuating the circuit breaker, a first resistor and a silicon controlled rectifier. Yet another lead is provided incorporating a second resistor and a cadmium sulfide cell.

This last mentioned lead is connected at one end to that lead containing the heating coil, first resistor and silicon controlled rectifier at a position between the heating coil and the first resistor. The other end of the lead containing the second resistor and the cadmium sulfide cell is connected to the second lead. Finally, the gate of the silicon controlled rectifier is connected by a lead to that lead containing the second resistor and the cadmium sulfide cell, at a position between the last two mentioned elements. The silicon controlled rectifier gate lead contains a break-over device.

The circuit is such that when the cadmium sulfide cell senses a proper flame, sufficient voltage for firing the break-over device does not exist and the gate of the silicon controlled rectifier is not energized. Therefore, the silicon controlled rectifier will, under such conditions, be rendered non-conductive and the heating coil will then carry an insufficient amount of current to trip the thermal sensitive circuit breaker. Upon the occurrence of a flame failure or improper burning, the resistance of the cadmium sulfide cell will responsively increase to produce the firing voltage of the break-over device. Once this occurs, the gate of the silicion controlled rectifier will be energized and the silicon controlled rectifier will be rendered conductive, thereby permitting a flow of current through the heating coil sufficient to cause it to trip the thermal sensitive circuit breaker. As provided, the break-over device insures that the circuit will not react to normal flame flicker.

BRIEF DESCRIPTION OF THE DRAWING

The drawing is a diagram illustrating the circuit of the present invention in the aforementioned preferred embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the Figure leads 1 and 2 are connectable to a suitable source of power (not shown). This connection may be made through the use of a simple plug. In addition, one of leads 1 and 2 may be provided with a conventional start switch.

In the Figure, the fuel burning apparatus is not shown, but exemplary electrical power means for the fuel burning apparatus are represented by the motor 3 and the ignition transformer 4. The motor 3 and transformer 4 are connected in parallel across the leads 1 and 2 by leads 5 and 6, respectively. As mentioned above, the transformer 4 will serve in the ignition of the air-fuel mixture. The motor 3 will operate a fan providing air for the combustion chamber and the eductors. The motor may also drive a fuel pump, as the fuel burning device is provided with one.

A thermal sensitive circuit breaker and heating coil unit are generally indicated at 7. The thermal sensitive circuit breaker 8 thereof is located in lead 1 while the heating coil 9 is located in a lead 10 connected across leads 1 and 2. Lead 10 also incorporates, in series following relation, a first resistor 11 and a silicon controlled rectifier 12.

An additional lead is indicated at 13, connected at one end to the lead 10 between the heating coil 9 and the first resistor 11. The other end of lead 13 is connected to lead 2. The lead 13 incorporates, in series relation, a second resistor 14 and a cadmium sulfide photo conductive cell 15.

Finally, the gate of the silicon controlled rectifier 12 is connected by a lead 16 to lead 13 at a point 17 between the resistor 14 and the cell 15. The lead 16 incorporates a break-over device such as a neon tube 18.

The operation of the circuit of the present invention may be understood from the following. Leads 1 and 2 are connected to a source of electrical current. This, in turn, will normally cause energization of the motor 3 and the ignition transformer 4. Since at the very outset there is no flame for the cadmium sulfide cell 15 to sense, the cell will have a very high resistance. Therefore, a voltage will appear at point 17 sufficient to fire the neon tube 18. This, in turn, will energize the gate of the silicon controlled rectifier 12, rendering the silicon controlled rectifier conductive. Since the silicon controlled rectifier 12 is in a conductive state, current sufficient in a very short period to heat the coil 9 to the point of tripping the circuit breaker 8 will begin to flow. However, if ignition takes place properly, the cadmium sulfide cell will be energized by the light of the flame and its resistance will drop substantially. Consequently, the voltage at 17 will drop below the break-over voltage of the neon tube 18 and the gate of the silicon controlled rectifier will be deenergized. Therefore, if proper ignition occurs quickly, the silicon controlled rectifier 12 will be rendered non-conductive before the heating coil 9 will have attained a temperature sufficient to trip the circuit breaker.

As long as proper burning continues, the cadmium sulfide cell 15 will remain energized by exposure to the light of the flames, the silicion controlled rectifier 12 will remain in a non-conductive state and the circuit breaker 8 will remain closed. However, should the flames shift due to improper burning or extinguish for any reason, the cadmium sulfide cell will no longer be properly energized by exposure to the light of the flames, its normally high resistance will be reinstated causing the break over voltage of the neon tube 18 to be reached. In this manner the silicon controlled rectifier is rendered conductive once more. If the apparent malfunction does not immediately self correct, this, in turn, will cause the coil 9 to almost immediately heat to a sufficiently high temperature level to trip the thermal sensitive circuit breaker 8, and the motor 3 and ignition transformer 4 will be deenergized.

The neon tube 18 adds a safety element of important significance. Since it has a definite firing voltage, the variation in resistance of the cadmium sulfide cell 15 due to a normal type of flame flicker will have no effect on the gate of the silicon controlled rectifier 12. This is true for two reasons, the inherent slight delay in the response of the cadmium sulfide cell and the fact that the firing voltage of the neon tube will not be reached at point 17 when the position of a displaced flame is quickly reestablished. It will be understood by those skilled in the art that the neon tube 18 could be any appropriate break-over device such as a diac or the like.

The selection of components of the proper characteristics and ratings for the circuit of the present invention will depend upon the application of the circuit and, having the benefit of the present disclosure, the same can be made by the worker skilled in the art. The cadmium sulfide cell 15 must be located in a position to properly sense the flames so as to receive the proper amount of illumination from them. For example, the cell 15 may be mounted on the casing of the portable space heater adjacent a port in the combustion chamber. The port in the combustion chamber should be located at a position adjacent the position the flames would normally assume in the combustion chamber when proper combustion is taking place. The resistor 14 should be chosen in conjunction with the cadmium cell 15 and the break-over device 18, based on their respective characteristics. Note in respect to the resistance involved, if any perceptible current should flow through the heating coil 9 during normal operation of the space heater, it will be in an extremely small amount. The rectifier as above described and here employed in conjunction with the heater coil 9 and resistor 11 is an element of such a nature that it will bring a cold coil 9 to a tripping condition in about 15 seconds. The resistor 11 should have a rating such as to assure the presence of a gate voltage when the silicon controlled rectifier 12 is in the conductive state. The silicon controlled rectifier 12 must be capable of carrying sufficient current to enable the proper heating of coil 9. It should also be of such sensitivity as to properly respond when combined with the other components of the circuit.

In an exemplary form of the circuit of the present invention, not intended to constitute a limitation on the present invention, excellent performance was achieved where the resistor 11 was chosen to have a rating of 400 ohms, the resistor 14 was chosen with a rating of 100,000 ohms and the breakover device 18 was a readily available neon tube of the designation NE2A.

Modifications may be made in the invention without departing from the spirit of it. For example, the circuit of the Figure may incorporate a thermal sensitive switch which will open the circuit should too high a temperature be achieved in the combustion chamber.

In some instances, while not preferred, where the problem of momentary flame flicker or shift is not of serious concern, the neon tube 18 may be eliminated from the circuit which will then function in an obvious manner with highly desirable results which have the aforementioned advantages.

The illustrated use and application of a variable resistance device and the thyrister in the form of the rectifier described in any case contributes a significant advance to the art of safety controls of the type described.

The foregoing description of a preferred embodiment and application of the invention concept highlights its advantages and distinct features of novelty. However, as indicated previously, it is not intended that the invention embodiments be thereby limited to the elements as comprised in the described circuit nor to a control circuit wherein only a photo-sensitive control cell is employed as a sensing element. For example, while in the preferred embodiment here illustrated the sensing device exposed to the heater flame has been described as a cadmium sulfide cell, it may be an equivalent device having the criteria to meet the requirements of a variable resistance of the nature characteristic of the cadmium sulfide cell. Thus, the flame sensing device may be another type of light sensitive unit. Provided it has the variable resistance characteristic, it even may be a unit which is temperature sensitive, reacting to change its resistance when the temperature sensed indicates a dangerous condition. Therefore, the invention contemplates a flame sensing unit which on the one hand may respond to light or absence of light or on the other hand to a given temperature level or its absence to vary its resistance in the circuit provided and thereby react similarly to the cadmium sulfide cell above described. In either event, the presence or absence of flame light or the inadequacy of or the non-existence of a predetermined flame temperature level will be a criteria indicative of faulty heater operation and possible dangerous conditions.

To carry the concept a bit further, it is noted that the silicon controlled rectifier is a unit in the thyrister category. On this basis, one can substitute a Triac which will function similarly.

The potential scope of the invention and the variables in applying the same should now be self-evident. Of course, the embodiment illustrated and described has been found to have most significant and effective benefit in use and lends particular advantage in its novel application to the art described.

In any case, the physical embodiment of the invention will be highly economical and the resulting package so small in size as to facilitate its application. As a matter of fact the cost of the invention embodiment is in many cases as little as about one third that of prior art packages of similar usage.

Moreover, the response of the unit is such as to make it substantially fail safe and to ignore false signals. It obviates the chance of disaster due to incomplete fuel burning or flame failure.

From the above description it will be apparent that there is thus provided a device of the character described possessing the particular features of advantage before enumerated as desirable, but which obviously is susceptible of modification in its form, proportions, detail construction and arrangement of parts without departing from the principle involved or sacrificing any of its advantages.

While in order to comply with the statute the invention has been described in language more or less specific as to structural features, it is to be understood that the invention is not limited to the specific features shown, but that the means and construction herein disclosed comprise but one of several modes of putting the invention into effect and the invention is therefore claimed in any of its forms or modifications within the legitimate and valid scope of the appended claims.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A safety control circuit for a fuel burning device of the type producing a flame in normal operation and having electrical power means, said circuit comprising first and second leads connected to said electrical power means and connectible to a power source for energizing said electrical power means, a thermal sensitive circuit breaker connected to one said lead having in connection therewith means for tripping breaker contacts included therein, a device of the thyrister category connected to said tripping means, a variable resistance device operatively related to said thyrister device and arranged to monitor and sense a flame condition in the fuel burning device to which it is applied, means including a device having a defined firing voltage included to render said thyrister device non-responsive to said variable resistance device when the latter is energized by a predetermined flame condition during proper combustion conditions in the related fuel burning device and responsive to a deenergizing of said variable resistance device in instances of improper flame, flame absence or failure, faulty ignition or improper combustion in said fuel burning device to trip said tripping means and open said breaker contacts.

2. The structure as set forth in claim 1 characterized by said device having a defined firing voltage being arranged to render said circuit insensitive to ordinary momentary changes in flame condition in the related fuel burning device.

3. The structure as set forth in claim 1 characterized by said device having a defined firing voltage being a neon tube or other device having equivalent function.

4. Apparatus as set forth in claim 1 characterized by said tripping means including a heater type coil in series with said thyrister and said variable resistance device being connected across said thyrister.

5. Apparatus as set forth in claim 4 characterized by said thyrister being a silicon controlled rectifier and said variable resistance device being a photo-sensitive device.

6. Apparatus as set forth in claim 1 characterized by said device having a defined firing voltage including means rendering the circuit insensitive to momentary change in flame condition connected between a gate of said thyrister device and said variable resistance device.

7. Apparatus as set forth in claim 6 including complementary resistor means in said circuit, connected between said tripping means and said variable resistance device and said tripping means and said thyrister device.

8. Apparatus as set forth in claim 4 characterized by said thyrister device bieng inserted in a third lead across said first and second leads in series with said tripping means and said variable resistance device being connected between said third lead and the other of said first and second leads.

9. Apparatus as set forth in claim 8 wherein said thyrister device is a silicon controlled rectifier and said variable resistance device is a photo-sensitive device sensing flame condition.

10. A structure as set forth in claim 9 including a resistor interposed between and in series with said heating coil and said rectifier, a fourth lead containing a second resistor and said photo-sensitive device in series, one end of said fourth lead being connected to said third lead between said heating coil and the said first resistor, the other end of the said fourth lead being connected to said second lead and the gate of said rectifier being connected to said fourth lead between said second resistor and said photosensitive device.

11. A structure as set forth in claim 10 characterized by said device having a defined firing voltage including a break-over device in a fith lead connecting between said gate and said fourth lead and providing means operable to render said circuit insensitive to ordinary flame flicker or momentary flame displacement.

12. A structure as set forth in claim 11 wherein said break-over device comprises a neon tube or the like having a defined firing voltage.