US2308318A - Stoker control - Google Patents

Description

Jan. 12, 1943.

H; T. SPARROW STOKER CONTROL Filed Oct. 16, 1940 Ill v v 'Hulocrk T. Sparrow lNVENTOR 3 ATTORNEY lines l3.

Patented Jan. 12 1943 STOKER CONTROL Hubert T. Sparrow, Minneapolis, Minn., assignor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn.,a corporation of Delaware I Application October 16, 1940, Serial No. 361,399 14 Claims. '(01. 236-'91) My invention relates to automatic controls for fuel burning apparatus and is particularly concemed with controls for fuel feeding. devices, principally stokers,vwherein a control device is used in the form of a variable electrical resistance element exposed directly to combustion temperature. My invention comprehends particular improved arrangements using such an element, it being an improvement 'over the invention of Bertram P. Pond disclosed in application Serial No. 361,427, filed October 16, 1940.

The principal object of my invention is to utilize an element as described above in a bal- My invention is primarily concerned with control mechanism for' the stoker and thecontrols include a primary relay I comprising a pull-in winding l6 and a bucking or drop-out winding- There is an armature associated with the relay l5 which is-connected to switch blades [8, l9, and 20, and which causes the said switch blades to move to the left into engagement with anced relay circuit wherein the balancing and unbalancing of the relay depends on the temperature of the element.

A further object is to provide an arrangement as set forth in the preceding object wherein a space thermostat may shunt one of the relay coils for causing the stoker to be started.

Another object is to provide means in the arrangement set forth in the preceding objects for adjusting the relative energization of the relay coils to control the temperature at which the stoker is started by the temperature responsive element.

Another object is to provide a heat anticipating thermostat in the combination set forth and meansfor adiusting either or both the-temperatures to be maintained by the said element and the amount of heat anticipation in accordance with outdoor temperature. Y

Another object is to provide means for stopping the stoker in response to the resistance element under high fire conditions by unbalancing the relay, with particular means for maintaining the high fire'setting constant even though the temperature to be maintained in the combustion chamber is adjusted.

Another object is to provide particular means and arrangements preferably including another balanced relay for preventing operation of the stoker when the fire is out.

Many more detailed objects and advantages of my invention willbecome apparent from-the following detailed description and annexeddraw- .ing wherein the single figure is a diagrammatic illustration of a system embodying a preferred form of my invention.

Referring to the drawing, numeral lo, desigwindings.

fixed electrical contacts 2|, 22, and 23, respectively, when the coil I6 is energized. The magnetomotive forces produced by the coils I6 and I1 oppose each other and whenever both of the coils are energized their .magnetomotive forces neutralize each other to cause deenergization of the relay, that is allowing the armature to per- 'mit its associated switch blades to drop away from their contacts. v

The controls'also comprise a balanced relay 25a and a balanced relay 25b which is the same 'as relay 250 except for the character of the The relay 25a includes an armature 2611 which is generally U-shaped having a leg-21a and a leg 28a. The armature 26a is'pivoted on a pivot 29a and carries a switch blade 30a mo vable between fixed electrical contacts 3: and 32a. Associated with the leg 21a is a winding 33a and associated with the leg 28a is a winding 34a. The position of the armature 26a: on its pivot and consequently the position of switch arm 30w with respect to its associated electrical contacts is determined by the relative energization of the windings 33a and 34a. Also asso-' ciated with the leg 38a is a booster winding a which, as will presently be described, becomes energized when the relayis moved to its in position, the relay being in its in" position when itswindings are unbalanced to such an extent as to cause switch blade 30a, to engage contact The relay 25b, as pointed out above, is like relay 2511 except for the character of its windings, its elements are distinguished by the identifying characteristic b. Further description of this relay is therefore not necessary.

If desired, booster windings may be associated with legs 21a and 21b of relays 25a and 25b.

nates a stoker of conventional type havingfuel conveying means H which conveys fuel to a retort [2 located within a combustion space indicated by the enclosure formed by the broken Such windings would be connected in series'with contacts 3m and 3lb, respectively. v v The control apparatus includes a'spa'ce temperatureresponsive thermostat 40 comprising a "coiled bimetal element 4| to which are attached movable switch blades 42 and 43 cooperating with fixed electrical contacts 44- and 45, respectively. The thermostat 40 acts to bring blade 43 into engagement with contact 45 at a fixed temperature and to bring blade 42 into engagement with contact 44 at a fixed lower temperature which may be one or two degrees lower, for example, the blade 43 being flexible. The blades disengage from their associated contacts successively in reverse order as the temperature rises. Associated with the bimetal element 4| i an electrical heating resistance 46 which acts to produce what is known as heat anticipation as will be described hereinafter.

The control apparatus also includes an outdoor temperature responsive controller 58 including an expansible and collapsible bellows 5| connected to a bulb 52 by a capillary tube 53, the bulb 52 and bellows 5| and capillary tube 53 being filled with a volatile liquid so that the bellows 5| expands and contracts in accordance with the temperature afiecting bulb 52. The bulb 52 is positioned so as to be responsive to outdoor temperature. The movable end' of the bellows 5| carries and the energization of winding 33b depends upon the resistance of element 68 which is in series with it and which of course depends upon the temperature within the combustion spac l3, the element 68 having a positive coefficient of resistance such that its resistance increases as the temperature in the combustion space I3 increases.

The winding 33a of relay 25a is normally energized through the following circuit: from the secondary 66 through wire 13, wire 88, fixed resistance 8|, wire 82, resistance 58, slider 51, wire 83, winding 33a, wire 84, wire 85, wire 86, ' wires 81, 18, 18, and 1| back to secondary 66. It will a stem acting on a pivoted arm 54 which has an integral portion 55 forming a slider adapted to slide over an electrical resistance 56. The arm 54 also has a right angular portion 51 forming a slider adapted to slide over an electrical resisance 58. The arm 54 is normally biased in a clockwise direction by a coil spring 59. the temperature aifecting bulb 52 decreases the bellows 5| contracts causing slider 51 to move to the right along resistance 58 and causing'slider 55 to move upwardly along resistance 56. Reverse operation results from a rise in temperature.

Within the combustion space is positioned a temperature responsive element 68 in th form of an element having an electrical resistance which varies in accordance with the temperature to which it is exposed. The element 68' in preferred forms of the invention takes the form of a length of wire which may be any of various known kinds whose resistance varies in accordance with temperature. The combustion space is indicated generally by the character l3, and

it is to be understood that the element 68 may be positioned at any suitablelocation within the combustion space, or at any place where the hot combustion gases will impinge it.

Power for the system is supplied by line conductors 6| and 62 and by step-down transformers 63 and 64. The transformer 63 comprises a primary winding 65 which is connected to a suitable source of power and a secondary winding 66 having fewer number of turns than the primary winding. The transformer 64 comprises a primary winding 61 connected to a source of power and a secondary 68 having'fewer number of turns than the primary winding.

With the parts in the position shown, the winding 34b of relay b is connected across the secondary 66 of transformer 63 through a resistance 49, the circuit being as follows: from secondary 66, through wire 69, resistance 49, wire 18, winding 34b and wires 12 and 1| back to secondary 66. Winding 34b is at all times connected across the secondary 66. windings 64a and 33b are connected in series with each other and with the element 68 across the secondary 66 by the following circuit: from secondary 66 through wire 13, wire 14, winding 33b, wire 15, winding 34a, wire 16, element 68, wire 11, wires 18, 18 and 1| back to secondary 66. It will be seen that both windings 33b and 34b of relay 25b are energized, the position of this relay de- Y pending upon their relative energization. The energization of winding 34b remains constant When be seen therefore that both the windings 33a and 34a of relay 25a are also energized, the energization of winding 34a depending upon the resistance of element 68 and the energization of winding 33a depending upon the position of slider 51 with respect to resistance 58. In other words the energization of winding 33a depends on controller 58.

.With the parts in the position shown on the drawing the thermostat 48 is satisfied so that none of its contacts are made. The outdoor I temperature is at an'intermediate value which might be 55 or 60 for example, at which heating is required in the spaces. cumstances the arm 51 of controller 58 is substantially at the mid point of resistance 58. The stoker is not in operation and the temperature within combustion space I3 is at an intermediate value and is decreasing inasmuch as the stoker is not in operation. The relay 25a is in a balanced position in which neither of its pairs of contacts is made and the relay 25b is in its normal position wherein it is unbalanced to the right having blade 38b in engagement with con-.- tact 32b. This is the normal position of relay 2511, this relay acting to shut down the system under circumstances when the fire has gone out entirely as will presently be described. The relative resistance values of windings 34b and resistance 49 are such that whenever the temperature within the combustion space I3 is not atan abnormally low value relay 25b is unbalanced to the right as shown so as to maintain blade 38b in engagement with contact 32b.

The operation of the system is as follows: as the temperature within the combustion space l3 falls due to the stoker not operating, when it reaches a predetermined value which is dependent upon the position of controller 58 inasmuch as winding 33a is in series with resistance 58, the resistance of the element 68 will have decreased to such a'value that by reason of the resultingincreased current flow through the winding 34a the magnetic pull on leg 28a of relay 25a will unbalance this relay .to the right, that is, so that blade 38a will be brought into engagement with contact 32a. The change in the resistance of element 68 which produces this last result is not suflicient however to ailect the position of relay 25b which remains in the position shown. When blade 38a of relay 25a engages contact 3211a circuit is completed for the pull in winding of relay I 6 as follows: from secondary winding 68 of transformer 64 through wire 88, wire 89, winding l6, wire- 98, wire 8|, blade 38a, :ontact 32a, wire 92, booster winding 35a, wire 93, blade 38!: of relay 25b, contact 32b, wire 94, booster winding 35b, wire 95, wire 16, wire 81, wire 96, and wire 81 back to secondary 68. Completion of the latter circuit which energizes the booster windings 36a and 65b causes Under these cirblades 30a and 30b to be held more tlrmly in engagement with their respective contacts 320,- and 32b to prevent chattering of the relays. From the foregoing it will be seen that energization of relay I5 depends upon relay 25 being unbalanced and in the position shown.

As soon as winding I6 is energized switch blades I8, I9, and 20 move to the left into en-' gagement with their associated contacts. Engagement of switch blade I8 with its associated contact does not have any effect as long as ther-' mostat 40 is satisfied. Engagement of blade I9 with contact 22 completes a maintaining circuit for winding I5 as follows: from secondary 68 through wire 88, wire 89, winding I6, wire 90, wire 98, contact 22, blade I9 and wire 91 back to secondary 68. It will be seen that this maintaining circuit shunts the booster windings 35a and 35b. Engagement of blade .20 with its associated contact 23 completes a circuit for the stoker as follows: from line conductors 6| to blade 20 through contact 23, wire 99, stoker I to line conductor 02.

stantially enough heat to maintain the space temperature at the desired value. With the controller 50 in the position shown when the outdoor temperature is at the above mentioned value of 55 F. or 60 F. for example, the temperature in the combustion space will be maintained at such a v value that heat will be supplied to the spaces at the proper rate to maintain or nearly maintain the space temperature at a uniform value of 70 F. for example. '11 now the outdoor temperature ,should fall the bellows will contract moving slider 51 to the right along resistance 58, increasing the resistance in the above described circuit of winding 33a. This will decrease the energization of winding 33a tending to unbalance the relay 25a towards a position wherein blade 30a engages contact 32a. Therefore the temperature within the combustion space will not have .to fall to as low a value to cause the ener- The stoker will now operate to supply addi-' Y tional fuel to the combustion space and the additional fuel supplied normally will cause the temperature in the combustion space I3 to rise,

and as the temperature rises the resistance of energized by reason of its maintaining circuit.

When the combustion space temperature has further risen so as to reach a higher predeters mined value the energization of winding 34a will gization of winding 34a to be increased sufliciently to unbalance relay 25 to its position wherein blade a engages contact 32a. In other words, as the outdoor temperature falls the controlling temperature values of resistance element. 60 will be raised. On the other. hand when the outdoor tempertaure rises slider 51 will be moved to the left along resistance 58 thereby increasing the energization of winding 33a necessitating that the combustion spa'ce temperature fall to lower values before the stoker willbe started. From' the foregoing it will be understood that as the,

outdoor temperature falls-a progressively larger minimum fire 'will be maintained in the combustion. chamben As pointed out above the conbe decreased to such an extent that relay 25awill be unbalanced to the left, the winding 33a now exerting a preponderating magnetic pull on leg 21a so as to cause blade 30a to engage contact 3Ia. When this occurs a circuit will be completed for the bucking winding l1 of relay I5 so as to deenergize the relay l5, this circuit being as follows: from secondary 68 through wire 88, wire I00, winding I1, wire I0l, wire I02, contact 3|a, blade 30a; wire 9 I, wire 98, contact 22, blade I9, and wire 91 back to secondary 68. Completion of this latter circuit is only momentary but it is sumcient to neutralize the magnetomotive force of winding I6 so as to cause relay I5 to assume its out position. as a relay I5 has been deenergized the maintaining circuit for winding I0 is interrupted and the circuit-for winding I1 is also interrupted.

From the foregoing it will be. seen that the stoker is started and stopped at predetermined values of temperaturein the combustion space, these temperatures at which the stoker is operated being dependent upon outdoor temperature which governs the position of controller 50.

It is desirable that the fire in the combustion chamber be maintained at such a size that substantially enough heat will be supplied to the spaces so, that the space temperature will be maintained at a uniform value. The controller is therefore so adjusted with respect to the relay 25a and resistafnce element 80 that as the outdoor temperature varies the combustion space temperature and correspondingly the minimum size of the fire will be so varied as to supply sub- .55 As'soon troller -50 is so adjusted with respect to relay 25a and resistance element that as the outdoor temperature varies a fire will be maintained of such a size as to maintain or nearly maintain the required space temperature. The outdoor controller will maintain a minimum fire calcu-.

lated to substantially meet the heating 'requirements of the space but if the space temperature falls below the required value the stoker will be started and operated by the thermostat 4830 that the flre is increased in size. 'I'hus whenever the temperature in the space has fallen to 'a relatively low value at which bothblades of thermostat 40 are in engagement with their associated contacts a circuit for shunting the winding 33a is completed as follows: from winding 330 through wire 83, slider 55, resistance 56, wire I08, resistance 40,

element4 I thermostatic blade 42, contact 44, wire I I01, and wire 84 back to winding 33a. 'When the shunt circuit is completed it carries a certain amount of current and therefore substantially reduces the current flow through the winding 33a. Thus the energization of winding 33a'is appreciably. reduced and the relay 25a immediately moves to its position wherein blade 30a engages contact 32a so as to start the stoker in-the manner described above.

As soon as the relay I5 is energizedin response to the space thermostatin the manner just described an additional shunt circuit across coil 33a is completed which is independent .of the fixed contact 44 and it is as followsz from winding,33a through wire 83, slider 55, resistance 50, wire I06, resistance 46, element 4|, blade 43, contact 45, wire I08, blade I8, contact 21, wire H0 and wires and 84 back to winding 33a.

From the foregoing it is obvious that normallywhen the relay 25a has been unbalanced so as to start the stoker in response to the thermostat 40 it will remain in that position keeping the stoker energized until the shunt circuit across have to be decreased relatively more.

the terminals of winding 33a has been interrupted, that is, when the temperature in the space has risen to a value so as to cause both blades 42 and 43 to disengage from their associated contacts.'

If during the time that the stoker is energized when it has been energized in response to the space thermostat 40 the temperature in the combustion space should rise to an unduly high value the stoker may be deenergized in response to the element 60. Thus when relay 25a is in its in position as a result of a call for heat from the space thermostat, it is relatively heavily unbalanced towards its in position by reason of the shunt circuit across winding 33a. If however the temperature in the combustion space now rises to a relatively high value tending to increase the resistance of element 60 a relatively great.

amount, the energization of winding 34a will be so reduced as to rebalance relay 25a and move it to its out position causing the stoker to shut down by reason of the completion of circuits already described. Shut down from high fire conditions in the manner just described normally will occur at a predetermined'relatively high temperature in the combustion space. If the stoker has been shut down in this manner, it may be restarted upon a fall in combustion temperature providing the space thermostat remains in a position calling for heat to the extent that contact 44 remains-in contact with blade 42, that is, if the combustion chamber temperature falls sufliciently, the energization of winding 34a may again be increased to such a value as to move relay 25a back to its in position.

It is desirable that the temperature at which the system is shut down under high fire conditions should remain nearly constant regardless of the hold fire temperature: being controlled by the position of slider 51 on resistance 58 and I have provided means to accomplish this result.

From the foregoing description it will be understood that as explained, as long as the outdoor teniperature remains at a given value the apparatus will maintain a given temperature in the combustion space and when there is a call for heat from the space thermostat the apparatus will be shut down in response to a predetermined high temperature in the combustion space. Under these circumstances the energization of winding 340. will be decreased to a givenamount depending upon the energization the resistance in circuit with winding 33a, slider 55 moves upwardly along resistance 56 so as to increase the resistance in the shunt circuits. Thus after the outdoor. temperature has fallen, the increased resistance in the shunt circuit will tend to reduce the relative amount of current the shunt circuit carries and to tend to increase the relative amount of current that flows through the winding 33a. when the shunt circuits are completed. The relationship of resistances 58 and 56 is such as to afiect the energization of winding 330. so that at any outdoor temperature the system will be shut down at nearly the same relatively high temperature in the combustion space under high fire conditions, that is, the energization of winding 33a will always be substantially the same when it is shunted.

The resistance 45 as described above is a heating resistance which is adjacent the temperature time that 'heat reaches the thermostat it will have already opened its contacts. In other words, the period of time that the stoker remains on when it is being controlled by thermostat 40 is of winding 33a to produce a shutdown in response to high fire conditions. Now if the outdoor temperature should fall so as to increase the temperature which the apparatus will maintain in the combustion space, the amount of resistance in circuit with winding 33a will be increased over what it previously was. Thus its energizationwill be relatively less. Now if under these conditions there should be a call for heat from the space thermostat so as to shunt winding 33a, the energization of winding 33a would normally be relatively less than it was when there was a call for heat from the space thermostat before the outdoor ,temperature fell. Thus the combustion space temperature would ordinarily now have to rise to a relatively higher value to produce a shutdown from high fire conditions because the energization of winding 340. would To compensate for the change in temperature to be maintained in the combustionsp'ace, I have provided the resistance ,56 and'slidejr 55. Thus when the outdoor temperature falls'so as to increase determined partly by the amount of heat given out by the heating resistance 46. It will be noted that the outdoor temperature responsive controller 50, by means of the slider 55 and resistance 56 controls the amount of resistance in circuit with heating resistance 46. Thus as the outdoor temperature falls, slider 55 moves upwardly along resistance 56, increasing the amount of resistance in circuit with heating resistance 46 so that there will be less heat given off by the latter resistance. It follows therefore that as the outdoor temperature falls the thermostat 40 will not open its contacts as soon as it otherwise would, that is, the operating periods will tend to be longer which is of course desirable inasmuch as during colder weather it is desirable that the tendency be for the operating periods to be lengthened.

From the foregoing it can be seen that the outdoor controller 50 functions not only to adjust the temperature which the apparatus will maintain in the combustion space, but also to maintain the temperature at which it will be shut down under high fire conditions at a constant value as well as to adjust the operation of the thermostat 40 so as to shorten or lengthen the operating periods, depending upon the outdoor temperature.

If the fire in the combustion space should go out entirely so that after the relay 25a is unbalanced so as to assume its position wherein blade 30a engages contact32a, the temperature in the combustion space continues to fall, the resistance of element 60 will continue to decrease and the energization of coils 34a and 33b will increase. When the temperature in the combustion space has fallen to a predetermined relatively low value indicating that the fire is out the energization of winding 33b will have increased to such an extent as to cause relay 25b to move to a position wherein blade 36b engages contact 3"), that is, when the combustion space temperature reaches a predetermined. low value, the energization of winding 33b will exert a magnetic pull on leg 21!) of relay 25b preponderating over the magnetic pull on leg 28b and armature 260 will be rotated in a clockwise direction brin ing blade 30b-into engagement with contact 3lb. At this time blade 30a of relay 2511. will be in engagement with contact 32a and as soon as blade 30b engagescontact 3") a circuit will be completed for energizing winding ll so as to deenergize relay l5, this circuit being as fol lows: from. secondary 68 through wire 88, wire I00, winding I'I, wire llll, wire I05, contact-3lb, blade 30b, wire 93, booster winding 35a, wire 92, contact 320, blade 30a, wire 9|, wire 98, contact 22, blade l9, and wire 91 back to secondary 68. When this last circuit is completed relay IE will be deenergized as described above. From the foregoing it will be seen that when relay 25b moves out of its position wherein blade 30'!) engages contact 32b it is impossible to complete the energizing circuit for relay l inasmuch as this circuit extends through blade 30b and contact 32b of relay 25b. Moreover whenever relay 25b assumes an unbalanced position wherein blade 30b engages contact 3lb, relay I5 is immediately deenergized and it cannot be again reenergized to start the stoker until the fire has been reestablished so as to cause relay 25b to againmove to its position wherein blade 30b engages its contact 3%.

' means being responsive to outdoor temperature,

- ticipating heater controlling said relayand means The element 60 in combination'with the relay 25b therefore constitutes mechanism for shutting down and preventing restarting of the stoker under conditions indicating that the fire is out and does so regardless of the position of the thermostat 40 and outdoor controller 50.

It is believed that the numerous constructional and operating advantages of my improved arrangement will be readily appreciated by those skilled in the art. By my particular use and arrangement of balanced relays the system is relatively sensitive and will produce responses whenever there are changes of temperature afiecting the resistance element 60. The system has a variety of desirable functional characteristics and .does not make use of parts which are untrustworthy in operation or which are subject to rapid deterioration in use. The single embodiment of my invention which I have disclosed is representative of. its preferred form. It is understood that there are various modifications and changes which may be made by those skilled in the art and the invention is therefore not to be limited by my disclosure which is exemplary but only in accordance with the appended claims. a

I claim as myinvention:

1. In a system of the class described, in combination, an automatic stoker, a combustion chamber containing i'uel fed by the stoker, control apparatus for the stoker comprising an element responsive to heat within the combusion chamber, said element carrying an electric current and being of sucha nature that its'resistance to current having another winding,'means' for adjusting the relative energization of said windings to adjust the temperature at which said relay is affected by said element to start the stoker, said adjusting a space thermostat of the type having an anwhereby said adjusting means controls the heating produced by said heater.

2. In a system of the class described, in combination, an automatic stoker, acombustion chamber containing fuel fed by the stoke /control-apparatus for the stoker comprising an element responsive to heat within the combustion chamber, said element carrying an electric current and being'of such a nature that its resistance to current flow varies with temperature, means associated with said element responsive to variation in current flow therethrough controlling the stoker, said last means comprising a balanced varying the current flow through one winding, a variable resistance for controlling the current carried by the shunt circuit, said element being operable to afiect the relay in response to a relatively high temperature in the combustion space when the shunt circuit is completed to stop the stoker, mechanical means-for simultaneously adjusting said variable resistances, and said resistances hearing such a relationship to each other thatlwhen the shunt circuit is completed the element causes stopping at the stoker at substantially the same relatively high temperature irrespective of the temperature setting at which the element aifects the relay to start the stoker;

3. In a system of the class described, in coinbination, an automatic stoker, a comblustion chamber containing fuel fed by the stoker, control apparatus for the stoker comprising an element responsive to heat within the combustion chamber, said element carrying an electric current and being of such a nature that itsresistance to current flow varies with temperature, means associated with said element responsive to Variation in current flow'therethrough controlling the stoker, said last means comprising a balanced relay having a winding controlled by said ele-, ment, said relay having another winding and temperature resopnsive means controlling the relative energiaztion of said windings, said temperature responsive means being arranged to shunt one of said windings so as to unbalance the relay for starting the stoker, means for adjusting the relative energization of said windings to adjust the temperature at which said relay is affected by said element to start thestoker, said last means comprising a variable resistance for varying the currentflow through one winding, 9.

variable resistance for controlling the current carried by the shunt circuit, said element being operable to affect the relay in response toa relatively high temperature in the combustion space when the shunt circuit is complete to stop the stoker, mechanicalmeans for simultaneously adjusting said variable resistances, and said resistances hearing such arelationshipto each other that whenthe shunt circuit is completed the element causes stopping of the stoker at substantiallythe same relatively high temperature irrespective oi the temperature setting at which the element affects the relay to start the stoker, said temperature responsive means being of the type having an anticipating heater, said heater being in said shunt circuit, outdoor temperature responsive means for operating said mechanical means, and said variable resistances also bein so correlated that as the outdoor temperature falls, the element controls the relay to maintain a higher temperature in the combustion chamber and the heat produced by said heater is reduced so as to tend to lengthen the operating periods .of'

to variation in current flow therethrough controlling the stoker, said last means comprising a balanced relay controlling the stoker and having a winding controlled by said element, said control apparatus including means responsive to the current flow through said element for stopping the stoker upon occurrence of a relatively low temperature in the combustion space indicative of the absence of fire, said last means comprising a second relay means controlled by .said element, said second relay having a winding in circuit with said element.

5. In a system of the class described, in combination, an automatic stoker, a combustion chamber containing fuel fed by the stoker, control apparatus for the stoker comprisingan element responsive to heat within the combustion chamber, said element carrying an electric current and being of such a naturethat its resistance to current flow varies with temperature.

and means associated with said element responsive to variation in current fiow therethrough controlling the stoker, said last means comprising abalanced relay for starting the stoker and having a winding controlled by said element, said control apparatus including means responsive to the current flow through said element for stop-- ping the stoker upon occurrence of a relatively low temperature in the combustion space indicative of the absence of fire, saidlast means comprising a second relay means controlled by said element, said second relay having a winding in circuit with said element, said last winding being in series with said first mentioned winding.

6. A system of the class described, comprising, in combination, a stoker, a room thermostat for operating the same upon fall in room temperature, electric heating means associated with the room thermostat and controlled thereby for locally heating the same while the stoker is being operated by said room thermostat, means responsive to the temperature of combustion for operating the stoker when the temperature of combustion falls to a predetermined minimum, and means responsive to outdoor temperature for simultaneously reducing the heating effect of said electric heating means and for raising the value of the temperature of combustion at which said combustion temperat vre responsive means operates said stoker.

'7. In a system of the class'described, in combination, means forming a combustion space, means for supplying fuel to said space, control apparatus for the fued supplying means comprising an element responsive to heat of combustion in said space, said element carrying an electric current and being ofsuch a nature that its resistance to current flow varies with temperature, means associated with said element responsive to variation in current flow therethrough con-'- trolling the fuel supplying means, means responsive to outdoor temperature associated with said third mentioned means to vary the efl'ect thereon of changes in resistance of said element to vary the amount of fuel supplied,,an indoor thermostat controlling said third means, heating means associated with the thermostat for heating it when it is demanding heat and means whereby said heating means is controlled by said outdoor temperature responsive means.

8. In a system of the class described, in combination, means forming a combustion space, means for supplying fuel to said-space, control apparatus for the fuel supplying means comprising an element responsive to heat of combustion in said space, said element carrying an electric current and being of such a nature thatits resistance to current flow varies with temperature, means associated with said element responsive to variation in current flow therethrough controlling the fuel supplying means, variable resistance means responsive to outdoor temperature so arranged as to vary the responsiveness of said third mentioned means to changes in resistance of said element, an indoor thermostat controlling said third means, said thermostat having a heater for heating it when it is demanding heat and means whereby said variable resistance means responsive to outdoor temperature controls said heater to vary the length of heat demand periods of,said thermostat in accordance with outdoor temperature.

9. In a system of the class described, in combination, an automatic stoker, a combustion chamber adapted to have fuel red thereto by the stoker, control apparatus for the stoker. comprising an element responsive to heat within the'combustion chamber, said element carrying an electric current and being of such a nature that its ,resistance to current flow varies with temperature, means associated with said element responsive to variation in current flow therethrough controlling the stoker, said last means comprising balanced control means having a current flow responsive device controlled by said element, said balanced control means having another current flow responsive device, means including a device responsive to outdoor temperature for controlling said other device for adjusting the relative energization of said devices to adjust in accordance with outdoor temperature the temperature at which said balanced control means is effected by said element to start the stoker, and means comprising a space thermostat controlling said balanced control means.

10. In a system of the class described, in combination, an automatic stoker, a combustion chamber adapted to have fuel fed theretoby the stoker, control apparatus for the stoker comprising. an element responsive to heat within the combustion chamber, said element carrying an electric current and being of such a nature that its resistance to current flow varies with'tempera- ,ture, means associated with said element responsive to variation in current flow therethrough controlling the stoker, said last means compris ing balanced control means having a current flow responsive device controlled by said'element, said I heater.

balanced control means having another current flow responsive device, means includinga device responsive to outdoor temperature for controllin said other device for adjusting the relative energization of said devices to adjust in accordance with outdoor temperature the temperature at which said balanced control means is eifected by said element to start the stoker, means comprising a space thermostat of the type having an anticipating heater controlling said balanced control means, and means whereby said adjusting means controls the heating produced .by said 11. In a system of the class described, in combination, means forming a combustion space, means for supplying fuelto said space, control means for the fuel supplying means comprising an element responsive to the heat of combustion in said space, said element carrying an electric current and being of such a nature that its re-' sistance to current flow varies with temperature,

first and second relays each having a winding bustion space temperature risingabove-a desired operating temperature and the other relay opens the one of said pair of contacts associated with it only upon the combustion space temperature dropping to a value indicating the cessation of combustion. r

12. In a system of the class binaizlon, means forming a combustion space, means for supplying fuel to said space, control means for the fuel supplying means comprising described, in coman element responsive to the heat of. combustion in said space, said element carrying an electric current and being of such a nature that its resistance to current flow varies with temperature, first and second relays each having a winding associated with said'element and having its energization controlled by said element. each relay having a pair of contacts adapted to be closed.

means dependent upon said pairs of contacts of both rel ys being closed to cause operation of said fuel supplying means, and ,means associated with said relays and controlling their operation so that a first of said relays opens the one of said pair offcontacts associated with it upon the com-' bustion space temperature rising above-a desired operating temperature and the second of said relays opens the one of said pair of-contacts associated with it only upon the combustion space temperature dropping to a value indicating the sponsive to space temperature for alsocontrolling said first relay. 13. In a fuel burner control system, fuel feeding .means, a combustion chamber fuel fed by said fuel feeding means, relay means for controlling the operation of said fuel feeding means, means responsive to the temperature within said combustion chamber to vary the current flow in said relay means to control the operation of said fuel feeding means to maintain a predetermined temperature within said combustion chamber, a space condition responsive switch associated with said relay means and operative to cause operation of said fuel feeding means upon the space condition assuming an undesirable value even though said combustion chamber temperature may, be

above the desired range but not above a high limit value, said combustion chamber temperature responsive means being operative if the combustion chamber temperature assumes said high limit value to operate as a high limit controller to cause said relay means to interrupt operation of said fuel feeding means in spite of said space condition responsive switch, and means responsive to outdoor temperature to adjust the combustion chamber temperature maintained by said combustion chamber temperature responsive means while maintaining constant the operation of said combustion chamber temperature responsive means as a high limit controllerwhen said space condition responsive switch is calling for operation of said fuel feeding means.

14. In a fuel burner control, system, fuel feeding 4 means, a combustion chamber containing fuel fed by said fuel feeding means, relay means for con-- trolling the operation of said fuel feeding means,

temperature sensitive resistance means responsive to the temperature within said combustion chamber and connected to a winding of said relay means to' vary the current flow in said relay means to control the operation of said fuel feeding means to maintain a predetermined temperature within said combustion chamber, a space condition responsive switch associated with said relay means and operative to cause operation of said fuel feding means upon the space condition assuming an undesirable value even though said combustion chamber temperature may be above the desired range but not above a high limit value, said combustion chamber temperature responsive resistance being operative if the combustion chamber temperature assumes said high limit value to operate as a high limit controller to cause said relay'r'neans to interrupt operation of said fuel feeding meansin spite of said space cessation of combustion, and a thermostat m.

condition responsive switch, an adjustable resistance responsive to outdoor temperature to ad.-

Just the combustion chamber temperature maintained by said combustion chamber temperature responsive resistance, and a compensating resistance adjusted simultaneously with said outdoor temperature responsive resistance .to prevent said outdoor temperature responsive resistance from,

affecting the high limit operation of said combustion chamber temperature responsive resistance when said space condition responsive switchis calling for operation of said fuel feeding means nunm'r r. srarmow.

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