US2139504A - Thermal responsive control mechanism - Google Patents

Description

2 Sheets-Sheet l WLM T/welnlopi/e Invent@- I Willian J. l/(ihg` bg WW1/7 5 E His Attol-neg.

Dec. 6, 1938. w. J. KING THERMAL RESPONSIVE CONTROL MECHANISM Filed Oct. 25, 1954 Irl@ l.

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W. J. KING THERMAL RESPONSIVE CONTROL MECHANISM Dec. 6, 19.38.

Filed Oct. 23, 1934 2 Sheets-Sheet 2 mLwi @L @Emi n .ma F

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Inventor.' Willianw J. King,

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by 'Mm/L1' 90ML@ His Attorljelg.

Patented Dec. 6, 1938 UNITED STATES f THERMAL nEsroNsIvr-z ooN'rnoL MECHANISM william J. King, Schenectady, N. Y., mince tc General Electric Company, a corporation of New York Application october-,23, 1934, sci-n1 No. 149,566

16 Claims.

This invention provides an improved form of thermal-responsive control mechanism particularly adapted for use in controlling an oil burner in response to the presence or absence of name 5 although not necessarily limited thereto.

One of the principal objects is to obtain a maximum speed of response upon a rapid variation of temperature. v.

Another object is to enable the maximum speed of response to be obtained upon a rapid increase of temperature between relatively low values and also upon a rapid decrease of temperature between relatively high values.

A further object is to obtain automatically-a follow up" action whereby the response of the mechanism is obtained during the initial part of a rapid temperature increase or decrease.

Another one of the principal objects is to provide an improved electrical type ofrthermal-responsive mechanism which is responsive to rate of change of temperature rather than to a mere change of temperature.

In the installation of heating equipments such as oil burners and the like it is common practice 25 to providecontrol apparatus for starting'and stopping the supplyof fuel and air to the burner and for energizing the fuel ignitionmeans, all in proper sequence and automatically in response to operation of a room thermostatic regulator. It is usual to include in such control apparatus a thermal-responsive mechanism for insuring that no appreciable quantity o f fuel is fed into the furnace when the flame is not burning and also for providing protection against ignition failure during the starting cycle and against flame failure at any time during the operation of the burner.

A specific object of this invention is the provision in an oil burner-'control of an improved ticularly characterized oy its speed of response lto ame failure, either initial or occurring subsequent to ignition, and which will operate in a simple and reliable, manner to prevent the supply to thel furnace combustion chamber of unbumed oil in an amount which would give rise to dangerous or undesirable conditions.

The desired rapidity of response and operation both upon ame ignition and ame failure is primarily secured in the mechanism of the present invention by the use of an electrically energized heat sensitive device the operation of which is not hampered by the inertiaof moving parts and v'which at the same time is so constructed thermal-responsive 'mechanism which isv par-- that it provides the desired follow up action necessary for a prompt response immediately subsequent to the initiation of either an increase or a decrease in temperature.

In its preferred embodiment the thermal-responsive mechanism of the present invention is adapted to eilect operation of a two-position switch, which may be either a start-run vor a start-stop" switch or the like, incorporated inY the electrical control apparatus of an oil burner furnace. In general the mechanism comprises an electrically energized heat sensitive device subjected to the heat of combustion and consisting of at least two electrical current controlling elements having thermal electric. characteristics which change at relatively different rates in response to change in a variable temperature condition to which the device is subjected, and apparatus electrically connected to said device for eecting predetermined operation of the above referred to two-position switch Vin response to predetermined differentials between'the values of current owlng through the respective elements of the heat sensitive device.

A -clear understanding of the details of the invention may be secured from thefollowing description taken in conjunction with the accompanying. drawings in winch are illustrated various aspects thereof.

- In the drawings, Fig. 1 diagrammatically illustrates an oil burner boiler furnace with electrical control incorporating one embodiment of the invention employing a thermopile as the heat sensitive device, Figs. 2 and 3 are respectively a perspective and a cross section showing the details of construction of the thermopile, and Figs. 4 and 5 are curves diagrammatically representing the characteristics of operation of the thermo;- pile and associated mechanism. Fig. 6 is a schematic showing of the electrical connections of a second embodiment of the invention employing a pair of temperature variable resistors as the heat sensitive device, Fig. '7 diagrammatically illustrates the manner of mounting vthe heat sensitive device in an oil burner furnace in electrical connection with the control apparatus of the furnace, Fig. 8 shows in detail the construction of the heat sensitive device employing the aforesaid variable resistors, and Figs. 9 and 10 are curves representing the characteristics of the operation of the Vthermal-responsive mechanism illustrated in Figs. 6 to 8 inclusive. Fig. 11 shows schematically the electrical connections of a modified form of the invention inwhich the variable resistor type of heat sensitive device is employed and Fig. 12 is a curve representing the characteristics of operation of this modification. Like parts are designated by the same numerals in all the figures.

Referring Ito Fig. 1, 20 represents a vertical type boiler furnace having a combustion chamber 2| in the top of which is an opening for the seating ofa burner head unit 22 provided with an oil and air nozzle 23 and ignition electrodes 24. Oil, atomizing air and primary combustion air are supplied under proper pressures to the nozzle 23 by thepcombined blower-compressor unit 25 which is energized from the electrical supply line 26 through the electrical automatic control apparatus represented at 21. 28 designates a room thermostat for initiating operation of the control apparatus and 30 designates a two-position switch having ixed start and run contacts, represented as S and R respectively, and a movable contact 3| incorporated in the control apparatus 21 for modification of the operation thereof in a manner to be described hereinafter. For purposes of illustration and description, the boiler furnace 20, burner head 22, blower-compressor 25, control apparatus 21, and room thermostat 28 are represented diagrammatically as beingof the forms disclosed respectively in the copending applications of H. S. Woodruff, S. N. 676,651, led June 20, 1933; W. O. Lum and J. Eaton, S. N. 691,320, iiled Sept. 28, 1933; W. O. Lum, S. N. 553,119, led July 25, 1931, and S. N. 737,063, filed July 26, 1934; -John Eaton, S. N. 735,103, filed July 14, 1934; and W. N. Mischler, S. N. 715,631, filed March 15, 1934, all of which applications are assigned to the assignee of the present invention. It is to beunderstood, however, that it is not intended that the field of application of the present invention be limited to association with these specific forms of apparatus. On the contrary, it is contemplated that the invention may be incorporated with equal advantages in other similar forms of control apparatus in conjunction with various forms of fluid fuel burner equipments. For a detailed description of the operation of the various elements of the burner and control apparatus referred to, reference should be had to the above identified applications, but sulcient description will be given hereinafter for a proper understanding of the present invention.

Operatively associated with the switch 30 of the control apparatus 21 is a thermal-responsive mechanism constructed in accordance with the present invention and comprising a heat sensitive device 32 located in the combustion chamber 2| in position to be subjected directly to the heat of combustion of the fuel and air discharged from the nozzle 23, and electrically connected by means of leads 33 and 34 to the operating coil of the polarized relay 35 which is arranged to operate the movable contact 3| between the fixed contacts S and R. Contact 3| is normally held by spring 36 in the start position engaging contact S.

The embodiment of the heat sensitive device 32 which is employed in the thermal-responsive mechanism illustrated in Fig. 1 and the construction of which is shown in detail in Figs. 2 and 3, comprises a number of thermocouples 40 connected in series to form a thermopile. In accordance with well known principles the couples generate a thermal electromotive force (thermal E. M. F.) which is nearly `proportional to the difference in temperatures of the hot andcold junctions and the E. M. F. generated by the thermopile as a whole is directly proportional to the number of such couples connected in series.

Since theE. M. F. per couple is small, suilicient -nisms employing thermocouples as the heat sensitive means, resides in the fact that both sets of the thermojunctions of the thermopile are mounted to be directly subjected to the heat of combustion of the fuel. However, for the sake of simplicity the oppositely disposed sets of thermojunctions of the thermopile 32 will be referred to respectively as the cold junctions, designated as A, and the` hot junctions, designated as B. In accordance with the invention, the thermopile is so constructed that the hot junctions heat up and cool oi rapidly so that their temperature follows closely a change in temperature of the surrounding conditions, while the cold junctions heat up and cool off at a relatively slower rate so that their temperature lags behind that of the hot junctions upon a given change in temperature of the surrounding conditions. This may be accomplished in a number of different Ways but in the embodiment illustrated the portions of the metal strips which comprise the hot junctions are made smaller than the portions which comprise the cold junctions and also are bifurcated as shown in Fig. 2 so that they have a relatively lsmall thermal capacity and a large ratio of heat transfer area to mass. 'I'he "cold junctions not only comprise the portions of the metal strips having the larger mass, but are mounted on the pins 4| which pass therethrough and are separated therefrom by electrical insulating tube 42, and also have interposed between them the metal washers 43 and insulating Washers 44. Hence the mass of the cold junctions is appreciably larger than that of the hot junctions and the ratio of heat transfer area to mass is appreciably smaller which will result in a decided lag of temperature of the cold junctions behind that of the hot junctions. When the thermopile 32 is subjected to a varying temperature condition, the temperatures of the respective sets A and B of the thermojunctions and hence the thermal effects produced therein, change at relatively different rates, resulting in the production of a variable differential E. M. F. in the thermopile. This differential E. M. F. is in one direction upon a temperature increase and in the opposite direction upon a temperature decrease, and produces electrical currents in corresponding directions in the circuit comprising leads 33 and 34 and the operating coil of polarized relay 35. The relay is set to operate in each direction upon a predetermined value of current flow of the respectively proper directional sense. It will be evident from the preceding description that it is possible for any one skilled in the art, keeping in mind the principles of the present invention, to readily vary the construction of the heat sensitive device-or thermopile 32 and the adjustment of the relay 35 to secure any desired range of operation of the thermal-responsive mechanism.

The operation of the mechanism hereinbefore described is as follows: Assuming that the contact 3| of the switch 30 is in its normal position engaging the start contact S, a call for heat by the thermostat 28 initiates operation of the control device 21 which in turn starts the blower-compressor 25 and energizes the ignition electrodes 24. Oil is supplied to the nozzle 23 from the blower-compressor and is atomized there by means of high pressure air likewise sup'- plied from the blower-compressor. ized oil is discharged from the nozzle into the combustion chamber 2| along with primary combustion air supplied by the blower-compressor and, providing the burner is properly operating, is ignited by means of the electrodes.

Fig. 4 illustrates the characteristics of operation of the thermal-responsive mechanism when the thermopile 32 is so constructed that the temperatures of the hot and cold junctions will eventually be the same although the cold junctions A lag behind the hot .junctions B in attaining this value of temperature, and the polarized relay 35 is so adjusted that the magnetic bias imposed upon the movable contact 3| is just sumcient to hold this contact in engagement with the contact R against the pull of the spring 36 but is not suihcient to move the contact from S to R. Initiation of iiame in the combustion chamber causes an immediate rapid rise of the temperature of the hot junctions B and, as illustrated in Fig. 4, a relatively slower rise of the temperature of the cold junctions A.. Within a few seconds, the differential E. M. F. generated in the thermopile becomes large enough to produce a current ow in the operating coil of the relay 35 sufilcient to move the contact 3| from engagement with the start" contact S into engagement with the run contact R. This point is designated on the curves by the words relay operates. The temperatures of the two sets of junctions of the thermopile continue to rise until that of the hot junctions eventually reaches a steady state value and that of the cold junctions also reaches the same steady state value whichiis approximately the temperature of the burning fuel. It will be evident that at this point there is no current flowing through the coil of the relay 35 but the contact 3| will continue to be held in the run position by means of the magnetic bias imposed thereon by the relay.

If, upon initiation of the operation of the oil burner mechanism, combustion is not established, no change takes place in the characteristics of the heat sensitive device 32 and the contact 3| remains in engagement with contact S whereupon the automatic control apparatus 21 goes through a predetermined number of recycling operations, as described in detail in the previously referred to application of John Eaton, S. N. '735,103 until combustion is established or completely shuts down the oil burner mechanism. If, however, combustion is established and the oil burner mechanism continues to operate in proper manner, the contact 3| is thrown into engagement with contact R by the thermalresponsive mechanism and is held there as previously described until the room thermostat 28 is satised or until for some reason there is a name failure within the combustion chamber. In case of subsequent iiame failure the hot junctions B immediately start to cool down at a in Fig. 4, which differential E. M. F. produces a current in the operating coil of the relay 35 in the proper direction and, after a few seconds, of suiilcient magnitude to overcome the magnetic bias imposed upon the contact 3| to hold it in engagement with contact R. thus permitting the spring 36 to Vpull the contact 3| back into engagement with contact S and allowing the control apparatus 2l to go through the recycling operations or to completely shut down the oil burner mechanism.

Fig. represents the characteristics of operation of the thermal-responsive mechanism when the relay 35 is so adjusted that the magnetic bias imposed upon the contact 3| is insuilicient v to hold the contact in engagement with the run contact R, and the heat sensitive device 32 is so constructed and mounted that the cold iunctions A will not catch up in temperature at any time with the hot junctions B upon a rise in temperature of the conditions to which the device is subjected. In this case the thermopile 32 is so mounted that the cold junctions A are in intimate heat conductive relation with the waterbacked surface of the combustion chamber through the mounting pins 4| as shown at 45 and therefore the heat will be radiated from the cold junctions at such a rate that they will be unable to catch up in temperature with the hot junctions. Upon initiation of combustion the temperatures of the hot and cold junctions diverge as shown in Fig. 5 until sutilcient differential E. M. F. Ais produced not only to operate the contact 3| from S to R but also to hold the contact in engagement with the contact R. The temperatures of the two sets of junctions A and B continue to risey until a steady state condition is reached whereat a suflicient differential E. M. F. is produced to hold in the relay or, in other words, to maintain the contact 3| in engagement with the contact R. 'I'his condition continues during proper operation of the burner and until flame failure at which time the temperatures of the hot and cold junctions begin to converge and a differential E. M. F. shortly is reached which is insuiilcient to hold the contact 3| in engagement with the contact R whereupon it is drawn by means of spring 36 into engagement with contact S. In all other respects the operation of the mechanism the characteristics of which are illustrated in Fig. 5, is the same as that of the mechanism the characteristics of which are illustrated in Fig. 4.

It is well known in the operation of oil burners that fluctuations may occur in the temperature in the combustion chamber due to irregularities in' the operation of the fuel and air delivery nozzle. Some degree of such fluctuation is to-be expected and is not harmful to the efficient operation of the furnace but if the fluctuation occurs to too great an extent it indicates that the burner mechanism is operating improperly and should be shut down until adjustments or repairs are made. How- 4ever, it is preferred lthat the flame detecting mechanism should not operate as quickly in the case of a dropin temperature due to these undesirable fluctuations of the combustion temperature as it would in the case of a drop of temperature due to complete iiame failure. Such a contingency is taken care of by the present invention -in that the characteristics of the heat sensitive device 32 are such that the changes in temperature of the combustion chamber attendant upon harmless uctuations are too slow to produce any divergence or produce such slight divergence of the temperatures of thel hot and cold junctions that the resultant differential E. M. F. is insuillcient to operate the relay 35. On

the other hand, if the fiuctuations attain'undesirable proportions, the changes in temperature in the combustion chamber take place at a rate which, though slower than that in the case of a complete flame failure, is rapid enough to produce a sufcient differential E. M. F. between the hot" and cold junctions to operate the' relay. The period of time required for operation of the relay upon the occurrence of undesirable fluctuations varies in inverse proportion to the seriousness of the fluctuation since the greater the degree of fiuctuation the more rapid the change in temperature in the combustion chamber and hence the shorter is the time required for the production of the required differential E. M. F.

The operational characteristics of the thermalresponsive mechanism which provide a safeguard against operation of switch contact 3| from R to Sjupon flame fluctuations which are not of sufllcient degree to be harmful, also provide a safeguard against operation of the contacts from S to R due to changes in the ambient temperature when the burner is not in operation and the flameis not ignited. Such changes in ambient temperature, due to weather conditions or other causes, ordinarily occur at such slow rates that either no divergence or only slight divergence 1s produced between the temperatures of the respective sets of thermojunctions A and B. Hence, no current, or insufficient current, is caused to `flow in the operating coil of relay 35 and contact 3| remains in engagement with contact S until `operation of the burner is initiated and flame is produced.

Another important characteristic of the thermal-responsive mechanism hereinbefore described is the follow up action which, after operation of the mechanism upon ame ignition, then places the mechanism in condition to operate quickly in a reverse direction upon flame failure. This has heretofore been accomplished with mechanical flame detectors by means of complicated clutch devices. As illustrated in Figs. 4 and 5, the required differential in E. M. F. between the hot and cold junctions is produced in a short period of time after flame failure, which period of time is appreciably less than that required for the heat sensitive device to cool down to the temperature at which it originally operated the relay 35 upon flame ignition.

In Figs. 6 to 8 is illustrated a second embodiment of the' invention, the principles underlying which are substantially the same as those upon which the first embodiment is based. The heat sensitive device of this second embodiment is represented at 46 as comprising a pair of temperature sensitive variable resistors R1 and Rz which have positive temperature coeflicients of resistance and which are connected respectively by means of leads 41 and 48 in series with the opposing coils C1 and C; of a differential relay 50 which is arranged to operate the movable contact 3| of the switch 30 electrically connected in circuit with the oil burner control apparatus 21. Electrical energy is supplied to the coils vof the relay through resistors R1 and R2 by means of lead 5| which, along with return lead 52, is connected tobeenergized from any suitable electrical source eithe directly or through the control apparatus 2 l The important characteristics of the thermalresponsive mechanism of this second embodiment are that the resistors R1 and R2 are so constructed that their electrical current controlling characteristics vary at relatively different rates in response to a given temperature change of the surrounding conditions, and that the switch contact 3|'is adjusted to operate in response to a certain predetermined value of differential fiux produced by the opposing coils C1 and Cz of the relay 50, which differential flux is the result of the difference in current ow through the unequally variable resistors. 'I'he construction and arrangement of the elements of the thermal-responsive mechanism to attain these characteristics are capable of wide variations, and for purposes of illustration several modifications are described and illustrated hereinafter. However, it is to be understood that these modifications are not intended as limiting the scope of the present invention but are set forth only for the purpose of clarifying the principles thereof and to make it possible for one skilled in the art to practice the invention which is capable of numerous modifications within the range of mechanical skill.

One practical form of construction for the heat sensitive device 46 is shown in detail in Fig. 8 from which it will be seen that the resistors Ri and R2, in the form of wires, are wound in alterhating turns about the supporting spools 53, which may be of soapstone or any other suitable electrical insulating and heat resisting material and which are held in place by means of metal spacing bars 54 and 55. 'I'he diameter of the wire forming the resistor R1 is greater than that of the wire forming the resistor Rz so that the former has a greater thermal capacity and a smaller ratio of heat transfer surface to mass than the latter. Furthermore, R2 is made of a very fine wire so that it heats up and cools off rapidly. Hence, the temperature, and therefore the resistance, of R1 lags appreciably behind that of R2 with a given 'change in temperature of the conditions to which the resistors are subjected. The relative diameters of the two Wires may be so selected as to give the desired degree of lag for any particular application. 'Ihe same results may be secured by making the wires of equal diameters and coating the surface of the wire R1 with heat insulating material so that it absorbs and gives off heat at a slower rate than the wire R2.

The Wires are brought together at one end and secured to the connecting stud 56 to which is secured also the electrical supply lead 5|. The other ends of the resistors R1 and lR2 are secured respectively to connection studs 51 and 58, to which also are secured respectively the relay leads 41 and 48. A mounting arm 60 is attached to the upper spacing bar 55 as shown-to provide means for mounting the heat sensitive device 46 beneath the shell of the combustion chamber 2| of the furnace and in proximity to the discharge end of the nozzle 23.

In the construction of the mechanism the characteristics of operation of which are illustrated in Fig. 9, the resistors R1 and Rz are so selected that they have the same value of resistance at the low `temperaturewhich prevails in the combustion chamber of the furnace when the pressor 25 and energize the ignition electrodes 24.

If the flame is properly ignited, the temperature and the resistance of R2 increase more rapidly than that of R1 so that more current is allowed to flow through the coil C1 of the relay 60 than through the coil Cz. Thus a differential ux is produced in the relay which in a few seconds is sufllcient to cause the contact 3| of the switch 30 to moveffrom engagement with the contact S into engagement with the contactR in opposition to the pull of the biasing spring 36. R1, which has a lower temperature coemcient of resistance than R2, has a lower value of resistance than Ra when the steady state temperature lcondition is nally attained in the combustion chamber and the two resistors attain the same temperature. Hence there is produced at all times during proper operation of the burner a suicient differential flux to hold the contact 3l of the relay in engagement with the run contact R as indicated by the curves of Fig. 9. Upon failure of the flame due either to improper operation of the f burner or to satisfaction of the room thermostat..

the resistors immediately start to cool down, the

temperature of R2 dropping at a faster ratethan that of R1 so that their values of resistance tend to converge and the differential flux produced in the 4relay 56 becomes so reduced that the spring 36 is able to return the contact 3| to engagement with the contact S.

In the modification of the ame detecting mechanism, the characteristics of operation of which are illustrated in Fig. 10, the resistors R1 and Ra are so selected that they have different positive temperature coeflicients of resistance and the resistance of R1 is greater than that of Rz at the low temperature normally prevalent in the combustion chamber of the furnace when the burner is not in operation. Furthermore, the directions orf pull of the relay 50 and of the spring 36 are reversed with relation to the contact 3| Aso that the contact is held normally in engagement with the contact S by the dierential flux produced in the relay as a result of the difference in values of resistance of the resistors R1 and Rz when cold. In this case, ignition of the ame causes the resistance of Rz to increase more rapicl` ly than that of R1 so that within aY few seconds the differential flux produced in the relay is reduced to such extent that the contact 3| is moved from engagement with contact S into engagement with contact R by means of the spring 36 thus breaking the start circuit and making the run circuit. The temperatures of R1 and Rz continue to rise' and their resistance values become equal at the operating temperature prevaient in the combustion chamber. Contact 3| then remains in the run position during continuation of proper combustion.v Upon failure of the flame, the resistance of R2 decreases more rapidly than that of R1 so that different values of current are again produced in the coils of the relay and the resultant differential flux moves the contact 3| back into engagement with the contact S.

Fig. 11 diagrammatically illustrates a modified form of electrical circuit to be used in the thermal-responsive mechanism of the present invention in conjunction with a heat sensitive device such as that illustrated in Fig. 8. The operational characteristics of this mechanism are'represented by the curves of Fig. 12. In this modification the resistors R1 and R2 are so selected that they have the same positive temperature coecient of resistance and equal values of resistance at both the lower and upper temperatures prevalent in the combustion chamber of the furnace during non-operation and operation respectively of the burner but, as previously described in conne'cuon with-ene other modifications of themvention, the resistance of `R1 lags behind thatof R2 in changing from the lower value to the lupper value in response to a change in the temperature of the conditions to which the heat responsive device is subjected. In the 'presently described modification the heat sensitive device 46 is mounted within the combustion chamber 2| of the furnace 20 in the same manner as illustrated in Fig. 7 and described hereinbefore. Referring particularly to Fig. 11, it will be seen that the resistors R1 and R2 are. connected by means of the leads 41 and 43 respectively so that they form two legs of a. direct current bridgel, the other two legs vof which are formed by the resistors Ra and R4 placed externally of the, fur-` The various resistors making up the bridge lines which may be connected to any suitable source of alternating current either directly or through the control apparatus 21 of the oil burner furnace. Across the supply lines 62 is connected the primary of transformer 63, one side of thel secondary of which is connected through lead 64 and rectifier 65 to the end of the bridge 6 I, which is common .to the resistors Ra and R4, and the other side of which secondary is connected throughleads 66 and 5| to\ the end of the bridge which isI common to the res tors R1 and R2, thus forming the input circuit or supplying direct current energization to the bridge. 10 represents a saturable core reactor the primary of which comprises two coils 1I and 12 connected in series with the operating coil of the relay 13 acrossY the altenating current supply lines 62. The `second-- ary of the reactor also comprises two separate coils one of which, the operating coil 14, is connected in the output circuit of the bridge 6|, and the other of which, the holding coil 15, is connected by means of leads 64 and 66 across the secondary of the transformer 63. Primary coils 1| and 12 of the reactor are placed in opposition to each other so that no'voltage will be induced in the secondary coils. The coil of relay 13 is connected in series with coils 1| and 12 and is so arranged that a predetermined magnitude of flux produced therein by the current flowing through the primary coils 1| and 12 moves the contact 3| of the switch 30 from engagement with the contact S into engagement with the contact R in opposition tothe pull of the spring 36. y

In the operation of the mechanism just described the saturating current normally flowing through the holding coil 15 lowers the impedance of the reactor 10 to such an extent that the alternating current flowing through the primary coils 1| and 12 is just sufficient to hold the contact 3| in engagement with the contact R after it has once been moved to this position, but is insumcient to move the contact from S to R. Upon ignition of the fuel discharged from the burner nozzle 23 the resulting flame heats up the resistos R1 and R2 so that their temperatures and their resistances increase and diverge as indicated in the-curves of Fig. 12. This divergence of the values of the resistances produces an unbalance of the bridge which sets up a current in the output circuit thereof and through the operating coil M in the same direction as the current already owing in the holding coil 15. Hence the saturating effect of the two currents is additive and the impedance of theA reactor is lowered sumciently to permit a current flow inthe coils 1| and 12 and the relay 13, which' current after a few seconds, is sumcient to move the contact 3| from engagement with the contact S into engagement with the contact R. kWhen the maximum temperature is reached in the combustion chamber the resistance of Ri is equal again to that of Rz and no current flows through the operatingcoil il since the bridge again becomes balanced. In this state the saturating effect of the holding coil 'i5 is suflicient to cause the relay 13 to hold the contact 3| in engagement with the contact R. When the flame fails the temperature and resistance of R2 fall more quickly than that of R1 with the result that a current is produced in the output circuit of the bridge which is opposite in direction to that produced upon a rise in temperature. 'I'he saturating eiects of the currents produced in the holding coil 'l5 and'operating coil 14 are now subtractive. Hence the impedance of the circuit of the relay 13 is increased to such an extent that the current flowing therein produces insufficient flux to hold the contact 3| in engagement with the contact R and the contact 3| is moved back into engagement with the contact S by means of the spring 36.

The time required for the operation of the 4 thermal-responsive mechanism employing the dissimilar resistors is inversely proportional to the rate of change of the temperature conditions to which the heat sensitive device is subjected which was also the case with the mechanism employing the thermopile heat sensitive device. In other words, the slower the rate of change of the temperature conditions the longer it takes for the resistances of the resistors R1. and Rz to diverge enough to produce a current flow in the relay sufiicient to operate the contact 3|. Furthermore, the required degree of divergence does not occur at all if the change in the temperature of the surrounding conditions is below a certain rate. The advantages of this characteristic were fully pointed out in connection with the mechanism employing the thermopile heat sensitive device. l

Likewise the heat sensitive device employing the dissimilar resistors has follow up characteristics similar to those of the heat sensitive device employing the thermopile as previously described. After the flame is ignited and sufficient time elapses for the required divergence of the resistances to cause operation of the relay, the temperatures of the two resistors continue to risei` and maintain with respect to each other the proper relationship so that upon flame failure they can produce operation of the relay in the reverse direction in a short period of time which is comparable to that required for operation of the relay upon ame ignition. Hence it is not necessary for the resistors to cool down to the temperatures at which they were when the relay operated upon flame ignition.

To one skilled in the art it will be evident from the description given hereinbefore that the thermal-responsive mechanism constructed in accordance with the present invention may be adjusted so that it will operate with great rapidity both upon flame ignition and flame failure. With a certain heat sensitive device, either of the thermopiie or variable resistor type, the

rat of ch ge of temperature, and therefore of electrical esponse, in the respective current controlling, e ments, i. e., the thermojunctions in the one c e and the variable resistors inthe other c produced by a given change in the temporre of the condition to which the device is subjected, will be known since in practice tlie elements of the device will be selected to have certain predetermined characteristics. Hence it will be possible to determine readily the different degrees of divergence between the temperatures and the electrical response of the respective current controlling elements after different periods of time have elapsed subsequent to flame ignition and flame failure. Curves similar to those diagrammatically illustrated in Figs. 4, 5, 9, 10 and 12 may be plotted showing the operational characteristics of the particular heat sensitive device which it is desired to use under a particular set of conditions. The relay for operation of the "start-run" switch, or the startstopf switch as the case. may be, of the burner control apparatus may then be adjusted to operate in response to the particular differential which occurs in the predetermined short period of time. In practice it has been found that the thermal-responsive mechanism of the present invention may be made to operate in a period of time which is in the order of two or three seconds subsequent to either flame ignition or flame failure. This is appreciably faster than the speed with which the flame detecting mechanisms heretofore commonly known in the art Will operate.

An additional advantage provided by the present invention is the securing of an eilcient follow up action by simple and reliable electrical means as contrasted to the common forms of mechanical follow up devices, such as complicated slipping clutches and the like, the operation of which may be rendered unreliable by heat warping, wear and improper adjustment.

A further advantage of the thermal-responsive mechanism constructed in accordance with the present invention arises from the fact previously pointed out that the mechanism operates in 'response to rate of change of the surrounding conditions. 'I'hat is, temperature changes at certain slow rates will not cause operation of the mechanism, but undesirably large fluctuations of combustion resulting in more or less rapid changes in temperature will cause the mechanism to operate at vrates which are inversely proportional to the rate of change of the temperature conditions, and an extremely rapid change in temperature such as produced by flame ignition and by complete fiame failure will produce practically instantaneous operation of the thermal-responsive mechanism.

It will be obvious that the advantages of the present invention may be secured by various modifications of the apparatus herein illustrated and described, and it is intended that such modifications as do not depart from the true spirit vand scope of the invention will be covered by the appended claims in which are set forth those features which are believed to be new and novel. What I claim as new and desire to secure by Letters Patent of the United States, is:

1. The combination with an electrically operated fuel burning apparatus and electro-responsive switch control mechanism including an operating winding therefor, of a thermocouple having both the thermojunctions thereof arranged to be heated simultaneously by combustion of the fuel. the thermocouple being electrically conamato-i gize the operating'winding of said switch mechanism, said device comprising a thermopile dis- -posed with both the hot and the cold thermojunctions thereof in position to be subjected to the heat of combustion of the fuel, and means for' effecting in the cold thermojunctions a v slower rate of change in the thermo-electric effect produced therein than in the hot thermojunctions upon initiation and termination of fuel burning. n

3. In combination with a fuel burning mechanism, an electric energizing circuit therefor, a thermopile having both the hot and cold thermojunctions thereof positioned to be exposed tothe heat of combustion of the fuel, said cold thermojunctions having a slower rate of thermo-electric response than said hot ther- 'mojunctions to a given change in the temperature to which the thermopile is subjected in order that thermo-currents of opposite directional sense will be generated in the thermopile in response to initiation and failure respectively of combustion, and electro-responsive switch mechanism, including a switch and an operating winding, for controlling said circuit, said operating winding being electrically connected to said thermopile to operate the switch in opposite directions in accordance with the directional sense of the thermo-currents generated in the thermopile.

4. In combination with a fuel burning apparatus, an electric energizing circuit therefor, a relay for controlling the circuit, said relay having biasing means tending to produce operation thereof in one direction and an operating winding, a thermopile positioned to be subjected to the heat of combustion of the fuel, certain of the thermojunctions of the thermopile having a slower rate of thermo-electric response to temperature` change than others of the thermojunctions whereby thermo-electric currents of opposite directional sense are produced in the thermopile in response to initiation and failure respectively of combustion, said thermopile being electricallyconnected to cause energization of the operating winding of the relay respectively in aid to andi in opposition to the biasing means in response to the thermo-electric currents of opposite directional sense. f

5. In combination with an electrically operated fuelV burning apparatus including means for producing a flame, means for controlling the operation of the apparatus in response to initiation and failure of the flame including a relay electrically connected to control the energlzation of the apparatus and having differentially connected windings, and a heat sensitive device for controlling the operation of the relay, said device including a pair of temperature-variable resistance resistors positioned to be commonly subjected to the temperature changes, resulting from flame initiation and flame failure, said resistors being electrically connected in parallel with each other and each in series with one of the windings of the relay, one of the resistors having a slower rate of temperature variation than the other in response to the temperature changes to which the' resistors are commonly subjected whereby current differentials are produced in the windings of the relay to operate the same upon ame initiation and flame failure.

6. In combination with a fuel burning apparatus and electro-responsive switch mechanism for controlling the operation thereof, a Wheatstone bridge and an electricA supply circuit therefor connected to energize the switch mechanism in response to `a predetermied current ow, produced in the output circuit of the bridge by electrical unbalance thereof, a pair of temperaturevariable-resistance resistors connected each in a separate leg of said bridge and both positioned to be commonly affected by changes in temperature resulting from changes in the burning of the fuel, said two resistors having different rates of temperature variation in response to said changes in temperature.

'1. In combination with an electrically operated fuel burning apparatus including means'for producing flame, a Wheatstone bridge and an electric supply circuit therefor, an electro-responsive switch mechanism for controlling the operation of said apparatus and electrically connected to be operable in opposite directions between two circuit controlling positions in response to predetermined current flow in the output circuit of the bridge, two of the legs of the bridge including temperature-variable resistance resistors positioned to be commonly subjected to the heat of the llame with the qvresistor of one of said two legs having a' larger thermal capacity and a smaller ratio of heat transfer surface to mass than the resistor of the other of. said two legs. v

8. The combination with `a furnace and combustion means for effecting temperature changes therein, of a device operable into a plurality of different positions for controlling said rst means, and means responsive to rate of change of temperature of combustion at any point within the range of said changes for effecting operation of said controlling device into' one of said positions during the initial part of a temperature increase within the furnace at a predetermined rate and for effecting operation of said controlling device into a second one of said positions during the initial part of a temperature decrease within the furnace at a predetermined rate.

9. The combination with a furnace having combustion means for producing temperature variation therein, of electro-responsive means for controlling said rst means, and means responsive to the rate of variation of the ltemperature of combustion within the furnace for effecting operation of said electro-responsive means including a plurality of temperature variable electric current controlling elements commonly subjected to the temperature Within the furnace, certain of said elements having lower rates of temperature change than other of said elements in response to variation in the temperature to which the elements are commonly subjected.

l0. The combination with a furnace having means operable for producing combustion therein, of means for controlling the operation of said `first means including at least two thermally responsive elements commonly subjected to the temperature within Ythe furnace and having temperature variable electric current controlling characteristics, said two elements'being variable in temperature at diiferent rates in response to variation in the temperature to which they are commonly subjected, an electroresponsive control device operable alternately into each of two positions for effecting predetermined control of said combustion producing means. and electrical circuits operatively, interconnecting said electroresponsive control device and said current controlling elements for eecting operation oi!j said control device into one of said positions in response to the occurrence of a predetermined differential between the current controlling characteristics of said elements resulting from a temperature rise due to combustion initiation and for effecting operation of said control device into the other of said positions in response to the occur-y rence of a second predetermined diilerential between the current controlling characteristics of said elements resulting from a subsequent temperature drop due to combustion failure.

11. The combination with a furnace having means operable for producing combustion therein, of means for controlling the operation of said first means including at least two thermally responsive elements commonly subjected to the temperature of combustion within the furnace, one of said elements having smaller thermal capacity and a largerratio of heat transfer surface to mass than the other of said elements whereby differentials between the current controlling characteristics of said two elements are set up in response to variations in the temperature to which the elements are commonly subjected, and electroresponsive means operatively connected in circuit with said elements for effecting predetermined control of said combustion producing means upon the occurrence of predetermined differentials between the current controlling characteristics of said elements.

12. The combination with a furnace having means for producing temperature variations therein, of electroresponsive means for controlling said first means and means responsive to a predetermined rate of variation of the temperature within the furnace for eecting operation of said electroresponsive means including a plurality of electric circuits connected for operatively energizing said -electroresponsive means and a pair of resistors commonly subjected to the temperature within said furnace and connected each in a separate one of said circuits, said resistors having different rates of temperature variation in response to variations in the temperature to which they are commonly subjected and the resistances of said resistors being variable with the temperature of the resistors.

13. The combination with a furnace having electrically operated fuel burner apparatus for producing temperature variations therein, of electroresponsive means for controlling the operation of said apparatus, a plurality of electric circuits connected for effecting actuation of said electroresponslve means, and a pair of temperature-variable-resistance resistors connected each in a separate one of said circuits for controlling the current flow therein and positioned to be subjected to the heat of combustion of the fuel, one of said resistors having a small thermal capacity and a large ratio of heat transfer surface to mass and the other of said resistors having a relatively larger thermal capacity and a relatively smaller ratio of heat transfer surface to area.

14. In combination, a combustion chamber. a pair of temperature responsive elements therein each responsive at a different rate to variations in the temperature within said chamber, heat generating means operable for varying the temperature within said chamber at rates above a predetermined value, and means responsive to a predetermined y differential in the response of said elements for controlling said heat generating means only when the rate of change of the temperature within said chamber exceeds said predetermined value.

15. In combination, a combustion chamber, a temperature responsive control device having a pair of cooperating control elements in said chamber each responsive at a diierent rate to variations in the temperature within said chamber and cooperating to effect response of said device only to rates of temperature change within said chamber greater than a predetermined maximum rate, heat generating means operable for varying the temperature within said chamber at rates above said predetermined maximum value, and means under the control of said temperature responsive control device for controlling operation of said heat generating means.

16. In combinatioma combustion chamber, a pair of temperature responsive elements Within said chamber, one of said elements being responsive` at a slower rate to temperature changes within said chamber than the other, electrical means under the control of said elements and operable only when a predetermined temperature differential exists between said elements corresponding to rates of temperature change within said chamber greater than a predetermined maximum rate, heat generating means operable for varying the temperature within said chamber at rates above said predetermined maximum value and means operable under the control of said electrical means for controlling operation of said heat generating means..

WILLIAM J. IUNG.

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