US3364972A - Combustion control system - Google Patents

Description

1968 J. A. DEUBEL ETAL 3,364,972

COMBUSTION CONTROL SYSTEM Filed Jan. 17, 1966 MOTOR .r q 9? 4, 49 F I NV EN T0125 MA 1/444 H (Jo/v55 cfusmv 4. DEUBEL fem-"5r E. 045W United States Patent 3,364,972 COMBUSTION CONTROL SYSTEM Justin A. Deubel, Franklin, Wis., and William H. Jones, Villa Park, and Ernest E. Olson, Mount Prospect, 11]., assignors to International Telephone and Telegraph Corporation, New York, N.Y., a corporation of Maryland Filed Jan. 17, 1966, Ser. No. 520,969 1 Claim. (Cl. 158-28) ABSTRACT OF THE DISCLOSURE A silicon controlled rectifier is serially arranged between a power source and a relay, the contacts of which control the operation of a burner fuel supply and igniter means. A gate signal for the rectifier is provided via a gas tube diode that is shunted by a circuit including a photoconductive cell exposed solely to burner radiation. With the cell dark (burner out), the gas tube diode conducts gating the rectifier to pick the relay and ignite the burner. With the cell illuminated (burner on), the gas tube diode is placed in the non-conductive state, and the rectifier as well. A hold circuit maintains the relay for supplying fuel to burner.

This invention relates to combustion control systems for oil burners and the like, and more particularly to an improved safety system which eliminates the use of flame detectors having moving parts.

In oil burners for residential and commercial heating equipment, demand for heat is determined by the operation of a room thermostat or temperature controller. Upon closure of the contacts of the thermostat or temperature controller, it is the function of a control system in which such contacts are located to establish, and to effect combustion of, an oil and air mixture. In such systems as heretofore known, typically there is a combustion thermostat in the form of a bimetal element positioned in the vicinity of the flame or the flue gas passages. The combustion thermostat is a flame detector, and initially works in conjunction with the thermostat to permit the establishment and ignition of the oil and air mixture. After ignition the thermostat contacts open in response to the flame. If ignition does not take place a safety device interrupts the flow of fuel and air.

The use of such a detector is subject to a number of disadvantages. It is inherently a slow acting device, requiring several seconds before its movable parts respond to a flame. Furthermore, because of repeated stressing of such mechanical parts, and the changes in physical and electrical characteristics of such electrical contactswhich are brought about by their numerous make-andbreak operationsthe response time for such a detector upon exposure to a flame is not a constant, but is always changing. As will be appreciated, such variations in characteristics are objectionable for a control circuit in which events are to be made to occur in a precise sequence, at specified intervals.

Another disadvantage of such detectors is that they fail with age, and ultimately are unable to respond to heat. When this point is reached, it may be that the detector is in one or the other of an open or closed circuit condition. In one condition, it would prevent the establishment and ignition of the desired oil-air mixture. In the other, it would permit combustion to continue only until operation of a cut-out safety switch shuts off the burner and prevents the room thermostat from controlling the burner. The safety switch normally must be manually reset.

Attempts have heretofore been made to replace the bimetal type combustion thermostat with a photocell.

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However, because of the low current carrying capacity of a photocell, it was found necessary to employ in conjunction therewith-for use in establishing and effecting ignition of the oil-air mixturea relay of sufiicient sensitivity to be actuated by the small current to which circuits in which photocells are located must be limited. Relays of such extreme sensitivity are difiicult to manufacture and to keep in satisfactory working order. The costs of manufacture, service and maintenance of such-devices is prohibitive.

It is an object of this invention to provide an improved safety control system for oil burners and the like which overcomes the above and other disadvantages of the prior art.

It is another object of this invention to provide a fail safe control system for oil burners which eliminates the use of bimetal combustion thermostats, and which avoids the use of low current carrying devices in the relay control circuit, thereby eliminating the use and cost of unusually sensitive relay devices.

A further object of this invention is to provide an improved combustion control system for oil burners and the like, comprising a minimum number of component parts of simple design and rugged construction, capable of reliable operation over a long operating life.

Another object of this invention is to provide a solid state safety circuit in which the failure of any component will not result in unsafe operation of the burner.

The above and other objects and advantages of this invention will become apparent from the following de scription taken in conjunction with the accompanying drawing of an illustrative embodiment thereof, in which:

FIG. 1 is a fragmentary schematic view of portions of an oil burner combustion system in which this invention is utilized;

FIG. 2 is a fragmentary view of the blast tube of FIG. 1, partly broken away to show the arrangement of the parts therein; and

FIG, 3 is a schematic diagram of the combustion control system of this invention.

Referring to FIG. 1, there is shown a blast tube 10 which extends at one end into the combustion chamber 11 of heating equipment 12. At its outer end, the blast tube 10 is connected to the outlet of a blower housing 13, in which the blower shaft 14 is adapted for operation by a motor 15. As is apparent, operation of the motor 15 causes air to be forced from the blower housing 13 through the blast tube 10.

In addition to the foregoing, the blast tube 10 is adapted to carry an oil line 16, through which oil is supplied under pressure. The oil-air mixture is adapted to be ignited, as by ignition current indicated at the arrow 17. Also, supported within the blast tube 10, at a distance from the combustion chamber, is a detector element 18,

from which leads 19 extend to the control circuit of this invention.

As will be seen in FIGURE 2, the oil line 16 extends through the center of a multi-blade airditfuser element 20, the blades of which engage the inner wall of the blast tube 10. The airditfuser 20 also supports spark control means, shown as a pair of rods 21, 22 which have their ends in near touching engagement adjacent the nozzle 23 of the oil line 16. The rods 21, 22 extend along through tubular support members 24, 25 through which electrical connections (not shown) are made to the rods from the source of ignition current.

The airdiffuser 20 has its blades shaped to impart turbulence to the air forced through the blast tube 10. Thus, air on reaching the vicinity of the nozzle 23 of the oil line 16 is markedly turbulent. Accordingly, the pressurized oil is ejected from the nozzle 23 in a fine mist which immediately, by virtue of the turbulence of the air,

is dispersed in a manner such that, upon establishing a spark between the adjacent ends of the rods 21, 22, the

air-oil mixture is ignited substantially instantaneously.

' Referring to FIGURE 3 along with FIGURES 1 and 2, there is shown a transformer 30 having its primary winding 31 connected to a power source, which may be an ordinary GO-cycle, 115 volt house supply. Both the motor 15 and ignition control network 32 are adapted for simultaneous connection across the power lines through a normally open relay switch R Closure of the relay switch R establishes operation of the motor 15 to force air through the blast tube operation of the ignition control network 32 to supply current through the rods 21, 22; and operation of means for causing oil to pass through the high pressure line 16. For example, a fuel oil pump driven by the blower motor may be used to force oil through the line.

The transformer 30 has a secondary winding 33, to one end of which is connected a heater element 34, and to the other of which the control coil of a relay R is connected through a normally closed bimetallic switch 35 which is controlled by heat from the heater 34. The secondary 33 is tapped, as at 36, and a normally open relay switch R is connected to the tap connection 36. Both the normally open switches R and R are respective sets of contacts of the relay R, and are adapted to be closed in sequence when the relay R is energized with R closing first so that the relay holding circuit is closed before the load is introduced.

A room thermostat or temperature controller 37 has one of its contacts connected to the control coil of the relay R, and its other contact connected to the normally open relay switch R In accordance with the invention, a semiconductive device, shown as a silicon controlled rectifier 38, is provided with its main current path connected between the heater 34 and the junction 39 of the relay switch R and the last-mentioned contact of the room thermostat 37.

The silicon controlled rectifier 38 has its control electrode or gate 40 coupled through a gas tube 41 to the sliding contact 43 of a voltage divider 44. Although other types of gas tubes may be found to function satisfactorily in this capacity, best results to date have been obtained with so-called neon tubes in view of the compatibility of their operating parameters with those of the silicon controlled rectifier 38. As shown, the detector 18 has one of its leads 19 connected to one end of the voltage divider 44, and its other lead 19 connected to the junction 45 of the heater 34 and the cathode of the silicon controlled rectifier 38. Also, a biasing resistor 42 between the gate 40 and the cathode of the rectifier 38 is included to improve reliability of operation. The other end of the voltage divider 44 is connected to an additional secondary winding 46 of the transformer 30.

The conditions of the switches R R and S shown in FIG. 3 are those which exist prior to establishing a flame in the combustion chamber. The detector 18, which may be a cadmium sulphide cell, exhibits a high resistance when in the non-illuminated state, being many times the resistance of the voltage divider 44, the total resistance of which may be of the order of 25,000 ohms. Under such circumstances, the voltage at the sliding contact 43 of the voltage divider with respect to point 40 is sufficiently high to effect firing of the gas tube 41, thereby to establish a voltage at the control electrode 40 with respect to point 45 to etfect conduction of the silicon controlled rectifier 38. For example, the secondary windings 33 and 46 may be such as to cause a voltage of 90 volts to appear between the sliding contact 43 and point 40. Using a suitable gas tube, such voltage is suflicient to fire the gas tube to eflfect conduction of the rectifier 38.

Although the aforementioned operation is sufficient to place the silicon controlled rectifier in its conductive state, no current will flow until the contacts of the room thermostat or controller 37 are closed, to signal a dethat there is immediately established a completed circuit through the secondary winding 33, the normally closed heater switch 35, the relay R, the room thermostat 37, the rectifier 38, and the heater 34. This is the pull-inf circuit for the relay, the current through it being sufi'icient to energize the relay R, and cause its two switches R R to close.

Immediately upon the relay R being energized, and the switches R R closing, the motor 15 is set into operation to force air through the blast tube 10 and to pump fuel oil through nozzle 23, and the ignition control network 32 is rendered operable to supply current to the rods 21, 22 for establishing the spark necessary to effect combustion of the air-oil mixture. The blades of the airdiffuser 20 are shaped to insure that light from the flame passes rearwardly through the blast tube 10 and.

can be readily detected by the detector cell 18.

Within a short time after establishing the flame, the detector cell 18 is lowered in resistance to a very low resistance value, and becomes essentially a conductor. This event results in the voltage at the sliding contact 43 being lowered below that which is needed to maintain conduction of the gas tube 41, whereupon the gas tube is extinguished. This removes the signal from the gate of the SCR, and when the AC. voltage applied across the anode and cathode of the diode next passes through zero, the diode is returned to the non-conduct ing state. However, since the relay switch R is closed, there is established a holding circuit for the relay R, through the upper part of the secondary 33 the normally closed switch 35, the relay R and the room thermostat or temperature controller 37.

It should be noted that during normal operation, the

rectifier 38 does not conduct long enough to cause heater 34 to generate significant heat with the result that switch 35 remains closed. Also, even though the resistance of detector 18 is reduced when a flame exists and there is still a complete circuit through heater 34, the current flow is very limited in view of the resistance of voltage divider 44 so that insufiicient heat is generated by heater 34 to open switch 35.

The relay R is kept energized in this manner until the contacts of the room thermostat or temperature controller 37 open. Immediately upon opening of these contacts, the holding circuit for the relay is broken, Whereupon the relay is de-energized, and its switches R R open. This immediately stops the motor 15, and shuts off power to the ignition control network 32, thereby extinguishing the flame. The control system then returns to the condition previously described prior to initiation of the flame.

It should be apparent that the basic purposes and advantages of the control circuitry of the invention are its reliable fail safe features. Even though the various components of the circuitry are manufactured and assembled with great care, it is always possible that various failures may take place; and in that event, the circuitry of the invention will prevent an unsafe condition from occurring.

First of all, since a certain threshold amount of voltage is needed across the gas tube 41 before it will conduct, it acts very much like a switch. That is, the gate for the diode is not biased to initiate conduction until the gas tube ionizing voltage has been reached and the tube is conducting. The. importance of controlling the diode generated by temperature effect on the detector, for example) except those sufficient to fire the tube 41.

Furthermore, if proper ignition does not take place or if the flame should be extinguished due to the lack of fuel or combustion air, the detector element 18 will immediately sense the absence of light causing its resistance to greatly increase. The resulting voltage increase on the gas tube triggering device will cause the tube to conduct and render the rectifier 38 conductive so that heater 34 is energized. Continued current flow for a short time through heater 34 will generate suflicient heat to cause bimetallic switch 35 to open. The opening of switch 35 de-energizes the relay and causes the opening of switches R R thereby stopping the motor and the ignition control network 32.

The detector 18 utilized in the invention is not susceptible to failure in the same manner as bimetal elements heretofore used. However, should a failure occur wherein the resistance of detector 18 does not decrease in the presence of a fiame, the resulting continued high voltage on the trigger device will cause a similar reaction to that described immediately above so that an unsafe condition will not result.

Upon further analysis, it will be seen that if various other broken connections or short circuits should occur, the circuit will function in each instance to prevent a dangerous situation. As another example, if an open circuit condition exists adjacent heater 34 and the room thermostat 37 calls for more heat, the relay pull in circuit would not be completed and hence ignition would not take place.

In the event rectifier 38 were bypassed by a short circuit, the closing of the room thermostat contacts would cause heater 34 to be energized, thereby opening bimetallic switch 35, and the relay would once more drop out. If gas tube 41 were shorted, the controlled rectifier turn on current would be established, the basic pull in circuit would be completed, and the control would lockout on safety; i.e. heater 34 would open switch as the SCR would continue to conduct. Should the gas tube or the rectifier fail causing an open circuit during heating, the next succeeding closure of the room thermostat contacts results in the pull in circuit being an open circuit, and combustion cannot take place. Similarly, a break in the circuit adjacent resistor 44 or a short around resistor 44 would either prevent the establishment of conduction through the rectifier and thus prevent pull in of the power relay or would cause continuous energization of heater 34 until safety lockout occurred.

From the foregoing, it will be apparent that various modifications can be made in the structure illustrated and described herein without departing from the spirit and scope of our invention. For example, trigger devices other than the gas tube disclosed may be employed, such as unijunction transistors and triggering diodes. Accordingly, it is intended that such modifications be included in the appended claim.

We claim:

1. In a heating system, the combination comprising: a burner; electrically actuable means for supplying fuel to the burner; a thermostat responsive to the heat output of the burner; burner igniting means fixed adjacent said burner; a source of A.C. power; a relay including a coil and at least one set of normally open contacts, said coil having one end related to a first side of said power source, said set of contacts interconnecting said power source to said fuel supplying means and to said burner igniting means; a controllable rectifier having an anode, cathode and control gate, the anode and cathode of which serially interconnect the other end of said relay coil with the second side of said power source; a series circuit including a slidewire potentiometer and a photoconductive cell, said potentiometer having a resistor interconnecting the first side of said power source and one terminal of said photoconductive cell, said cell being disposed to receive radiation from the lighted burner; a gas tube diode interrelating the gate of said rectifier and the first side of said power source, said gas tube diode activating said gate to energize said relay and cause said burner to be supplied with fuel and be ignited when said cell is in a darkened condition, said gas tube diode and rectifier being placed in a non-conductive state when said cell receives radiation from the lighter burner, said potentiometer slidewire being connected to said gas tube diode and serving to relate the same to the first side of said power source; a thermally operated switch including a heater resistor in series with said rectifier and the second side of said power source, said switch having normally closed contacts in series connection with said relay coil and the first side of said power source, said thermostat having a pair of contacts serially arranged between the other end of said relay coil and said rectifier; and a further set of normally open relay contacts interrelating the common point of said rectifier and thermostat to the second side of said power source to provide a hold circuit for said relay coil.

References Cited UNITED STATES PATENTS 2,265,904 12/1941 Herr 13828 2,85 2,702 9/ 1958 Pinckaers. 3,109,910 11/1963 Foglernan. 3,161,759 12/1964 Ganibill et a1. 3,174,528 3/1965 Staring 158-128 X 3,276,507 10/1966 Eldridge et al 158-28 JAMES W. WESTHAVER, Primary Examiner

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