US2567702A - Automatic burner control - Google Patents

Description

Sept. 11, 1951 P. G. FRERER AUTOMATIC BURNER CONTROL v Filed April 16, 1948 I I I l l I I I l I l I l l I 3mm PAUL G. FRERER Patented Sept. 11, 1951 AUTOMATIC BURNER CONTROL Paul G. Frerer, Baltimore, Md., assignor to Bendix Aviation Corporation, Baltimore, Md., a corporation of Delaware Application April 16, 1948, Serial N0. 21,402

4 Claims. 1

This invention relates to the operation of heating installations and, more particularly, to an improved control governing the starting and operating conditions, adapted to discontinue operation of the system in the presence of abnormal conditions.

Acceptable control units for automatically fired heating installations must react reliably in a number of situations, some of which are potentially dangerous if not handled properly. When the space temperature responsive controller calls for heat, the burner must be quickly and safely started, and thereafter its operation must be continuously monitored to detect the presence of any abnormal condition. If there is any defect in the burner starting or operation, the system should be removed from service until the defect has been located and remedied by the operator. In addition, manipulation or vibration of the thermostat must not fill the combustion chamber and ilue passages with explosive vapors which may give rise to a disastrous pull if ignited. This may occur if the householder, deciding it is warm enough, turns back the thermostat thereby shutting oil' the heating system but shortly thereafter changes his mind and resets the thermostat to a higher value. At this time the combustion chamber may still be hot and, if a fuel-air spray is delivered into it at this time, the hot walls vaporize the liquid fuel to generate a highly explosive gaseous mixture. If an ignition impulse is nowapplied with the combustion chamber and associated passages in this condition, a puil' may result severely damaging or destroying the furnace and associated smoke pas sages.

Accordingly, it is an object of the invention to provide a new and novel control unit charac terized by a minimum purge or scavenging period after an interruption in the temperature control circuit.

Another object of the invention is to provide a new and novel control unit for heating installations which will automatically start burner operation and lock the burner out of service if combustion does not occur within a selected period.

Other objects and advantages of the invention will in part be obvious and in part described when the following specification'is read in conjunction with:

The single figure drawing schematically illustrating a heating system embodying the principles of the invention.

The basic heating unit includes a combustion chamber l0 provided with a stack or flue l2 through which the combustion products pass. It is to be understood that a suitable medium, such as air or water, is situated in heat exchanging relationship with the combustion chamber ill, the details of the heat exchanger being omitted from the drawing. The heating unit is fired by a burner l4 which may be of the pressure atomizing type illustrated, with the inner end of its blast tube projecting within the combustion chamber l0 adjacent a pair of spaced igniting electrodes l6. When the system is oil-fired, a suitable storage tank and connecting line are associated with the burner [4. In the interest of simplicity, however, these details have been omitted from the drawing as they constitute no part of the invention. The ignition electrodes it are connected with the high voltage secondary of the ignition transformer it and, when this transformer is energized, a high voltage discharge passes between the electrodes 8 to ignite the fuel-air mixture delivered from the burner blast tube.

' The operation of the heating system is monitored by the control unit 26, which may be manufactured and distributed as an independent unit designed for shipment separate from and convenient connection to the burner installation. The system is powered by the connection of the leads 22 and 24 to an electric power main of suitable voltage. The power lead 24 is connected by lead 24 with the line or mains terminal 28 of the control unit 26, and intermediate branch connections 30 and 3| connect the lead 24 respectively with one terminal of the burner motor l4 and one terminal of the ignition transformer IS. The power lead 22 is connected with the line terminal 33 of the control unit 26 through the normally closed limit switch 20 and the line 32. The limit switch 20 is of the type adapted to respond to excessive temperatures, excessive pressures, or any other selected index of abnormal system operation. The control unit 26 is provided with a main relay 34, a flame relay 35 and an auxiliary relay 36. Two sets of normally disengaged contacts, 31 and 38, are operatively associated with the main relay 34, and adapted to close in response to energization thereof. One of the contacts 31 is connected with the line terminal 33 by the power bus 39, while the other contact 31 is connected with the burner terminal 4| by the burner bus 40. The burner terminal 4| is in turn connected with the second lead from the burner l4 by the line 42. The contacts 38 participate in a burner running circuit to be later described. The control element of the main relay 34 is its operating winding which is connected at one end with the active bimetal 43 of the normally closed safety switch 44. The safety switch 44 is of a well-known type comprisng a pair of spaced parallel bimetais 43, 45 with a. latch bar 46 pivoted to the compensating bimetal 45 at 41 and biased for clockwise rotation about this point. The latch bar 46 is normally restrained from such motion, however, by the engagement of its end with the latch tip 48 aflixed to the active bimetal 43, which has an electrical heater resistor 49 thermally associated therewith. The bimetals 43,45 are so arranged that their deflections in the presence of temperature variations have the same sense whereby the disstance between their free ends remains the same in the presence of ambient temperature changes despite the flexing of the respective bimetals. If an electric current be passed through the heater resistor 49, however, the active bimetal 43 is heated independently of the compensating bimetal 45, whereupon the ends of bimetals 43 and 45 assume a diverging position to disengage the latch bar 46 from the latch tip 48, permitting it to move quickly to the dashed-line position where it remains until reset to open the circuit through the safety switch 44. The end of the operating winding of main relay'34 remote from the safety switch 44 is connected with the thermostat terminal 56 by the lead 5|.

The remaining thermostat terminal 52 is connected with the line terminal 28 through the power bus 53.

The operating winding of the flame relay 35 has one lead connected with the power bus 53 and its other lead connected with the sensing impedance terminal 56, which is in turn connected with the thermally responsive resistor 54 situated in the stack l2. The other terminal of this resistor is directly connected with the power lead 22. For proper operation in the system shown, the resistor 54 has a high negative temperature coeflicient and presents substantially an open circuit at the temperature prevailing in the absence of burner operation. It may be composed of a mixture of 80% ferric oxide with the remainder bentonite, ball clay and titanium dioxide or zirconium oxide, an example of an equivalent composition being that composition described in U. S. Patent No. 2,407,750. When current flows through the operating winding of the flame relay 35, it attracts the flexible member 51 provided at its free end with a contact 58 and also bearing the laterally displaced contact 59 on the extension 68. The contact 58 cooperates with a stationary contact 6|, while the contact 59 cooperates with the stationary contact 62. The degree of movement of the flexible member 51 toward the left is governed by the magnitude of current in the operating winding of the flame relay 35, and the spacing of the contacts is so adjusted that, with increasing current, contact 59 first engages contact 62, after which contact 58 engages contact 6|.

A number of contacts are associated with the auxiliary relay 36. These include the movable contact 1| normally in engagement with back contact 12 and also engageable in an overlapping manner with front contact 13. In addition, there is movable contact 14 normally engaged with back contact 15 and overlappingly engageable with front contact 15. The term, overlappingly, in connection with the engagement of these contacts, indicates that the front contact is engaged before the corresponding back contact is disengaged. The auxiliary relay 38 is also provided with a movable contact 11 normally in engagement with a back contact 16.

The front contact 13 is connected with the contact 62 of the combustion responsive switch whose flexible contact carrying member 51 is linked with the power bus 39. In addition the combustion switch contact 8| is connected through the lead 19 with one of the contacts 38 on the main relay 34, as well as the back contact 12 on the auxiliary relay 38. The other of the contacts 38 on the main relay 34 is linked with the front contact 16 on the auxiliary relay 36 by the lead 80. The associated back contact 15 connects to one end of the safety switch heater 48 via the lead 8|, the other terminal of the safety switch heater 49 being linked with the power bus 38.

The movable contact 11 on the auxiliary relay 36 is connected with the burner bus 48 while its cooperating back contact 18 is linked with the ignition terminal 83 through the ignition bus 84. The ignition terminal 83 is connected externally of the control unit 26 through the lead 85 with the second terminal of the primary winding of the ignition transformer i8.

The operating winding of the auxiliary relay 38 is connected between the power bus 53 and the movable contact 1| controlled by said relay. The similarly controlled movable contact 14 is connected with the compensating bimetal 45 of the safety switch 44 through the lead 88.

A thermostat 81 of the type presenting a closed circuit in the presence of temperatures lower than the value it is set to maintain is located in the space to be monitored and connected through appropriate leads with the thermostat terminals 50, 52.

With the foregoing elements and interconnections thereof in mind, the operation of the unit under a variety of conditions may readily be followed. In the schematic illustration, all units are shown in their standby position, which is to say the thermostat 81 presents an open circuit as does also the temperature sensitive stack element 54. The temperature in the controlled space now gradually decreases causing the thermostat contacts to move toward each other until an electric circuit is completed therethrough, at which time the following energizing circuit is completed for the main relay 34: line terminal 33, power bus 39, safety switch heater 49, lead 8|, back contact 15, movable contact 14, lead 86, compensating bimetal 45, latch bar 48, active bimetal 43, the operating winding on main relay 34, lead 5|, thermostat terminal 58, thermostat 81, thermostat terminal 52, power bus 53 and line terminal 28. The current flowing through this circuit energizes the main relay 34, closing contact sets 31 and 38. The closing of contacts 31 completes the following energizing circuit for the burner l4: power lead 22, limit switch 20, lead 32, line terminal 33, power bus 39, contacts 31, burner bus 40, burner terminal 4|, lead 42, burner l4, branch line 38 and power lead 24. At the same time, due to the energization of the burner bus 40 following the closure of contact 31. exciting current is supplied to the primary of the ignition transformer l8 though the following circuit: power lead 22, limit switch 20, lead 32, line terminal 33, power bus 39, contacts 31, burner bus 40, contact 11, back contact 18, ignition bus 84, ignition terminal 83, lead 85, primary of transformer I8 and branch line 3| to the power lead 24. The joint energization of the burner and of the ignition system results in the delivery of a fuel-air spray from the end of the blast tube adjacent the spaced electrodes l9 between which there now exists an electric discharge. This discharge ignites the fuel mixture to produce a flame in the combustion chamber l9 accompanied by the evolution of combustion gases passing upwardly through the flue passage I2 past the thermally responsive resistor 54. This resistor presents a very high impedance at the normal ambient temperature, but with the rising temperature produced by the hot stack gases resulting from combustion its impedance steadily diminishes. It will be noted that the starting energizing circuit for the main relay 34 includes the safety switch heater 49 thermally associated with the active bimetal 43, and the continued passage of current through this circuit for a selected period, usually of the order of ninety seconds, results in a warping movement of the active bimetal 43 away from the compensating bimetal 45 to disengage the latch bar 46 and thereby open the circuit through the safety switch, de-energizing the main relay 34.-

As will be later seen, however, suitable combustion control circuits are incorporated in the apparatus to remove the safety switch heater 49 from the operating circuit of main relay 34 in response to the establishment of combustion, thereby preventing this lock-out reaction.

The operating circuit for the flame relay 35 extends from the power lead 22 through the thermally responsive resistor 54 to the sensing impedance terminal 59, operating winding of flame relay 35, power bus 53 and line terminal 29 to power lead 24. As the temperature of thermally responsive resistor 54 gradually rises, the current through this circuit increases slowly, gradually drawing the flexible contact carrying member 51 toward the operating winding of the flame relay 35. The first effect of this movement is to bring contacts 59 and 62 into engagement, but this is without effect, as it will be seen that contact 62 of the flame relay 35 is connected with the front contact I3 of the auxiliary relay 36, which is disengaged at this time due to the nonenergization of auxiliary relay 36. The continuing movement of the flexible member 51 ultimately brings the contact 59 into engagement with the contact 6| at a higher temperature level than that at which contacts 59 and 62 meet. The

auxiliary relay 36 is then energized through the following circuit: line terminal 29, power bus 53. operating winding of auxiliary relay 36, movable contact 1|, back contact 12, lead 19, contact 6|. contact 58, flexible member 51 and power bus 39 to th line term nal 33. The resulting operation of the auxiliary relay 36 brings the movable contact H into engagement with its front contact 13, the contact 13 being engaged prior to the disengagement of back contact 12. The energizing circuit for the auxiliary relay 36 is thereby altered to the extent that it is complet d through at this time. A new operating circuit for the main relay 34, independent of the safety switch heater 49, is thereby set up through the following connections: line terminal 33, power bus 39, flexible member 91, contact 59, contact 6|, lead 19, contacts 39, lead 99, front contact 16, movable contact I4, lead 99, compensating bimetal 45; latch bar 46, active bimetal 43, operating winding of main relay 34, thermostat terminal 59, thermostat 91, thermostat terminal 52 and power bus 53 to line terminal 29. The connections just described are those corresponding to the normal running condition of the apparatus, and continue so long as the burner functions normally and the thermostat 91 continues to call for heat. The temperature at the thermostat 91 now rises gradually in response to the heat developed by the operation of the burner until the thermostat contacts separate to open the circuit between thermostat terminals 59 and 52 which interrupts the operating circuit for the main relay 34 at this point. a

After such a shutdown, in response to an opening of the thermostat circuit, it is desirable to prevent the resumption of burner operation until the combustion chamber has cooled sufficiently to avoid interference with the normal conditions of operation. It is to be noted that, although in the foregoing description the thermostat 81 has opened to de-energize the main relay 34, the flame relay 35 and auxiliary relay 36 are still energized because of the time required for the cooling of the thermally responsive resistor 54. Therefore, if the thermostat 91 be immediately reset to a higher temperature by the user of the equipment, there will be no immediate energization of the burner I4, since the starting circuit is interrupted by the separation of movable contact 14 from the back contact I5 arising out of the continued energization of the auxiliary relay 36, and the running circuit, extending from the front contact 16 through the lead 99 and contacts 39, is interrupted by the separation of the contacts 39 attendant upon the de-energization of the main relay 34. As the thermally responsive resistor 54 cools, the flexible member 51 moves toward the right, flrst permitting the contacts 58 and 6| to disengage. This is without effect on the energizatlon of the auxiliary relay 36, since its operation completed a second operating circuit through the contacts 59 and 62. Not until contacts 59 and 62 disengage, as a result of a further decrease in the temperature of the thermally responsive resistor 54, is the auxiliary relay 36 de-energized to permit the movable contact I4 to reengage the back contact 15 completing the starting energizing circuit for the main relay 34. Thus, it is impossible for the operator of the equipment to bring about a starting of the burner with a hot combustion chamber I9, by the manipulation of the room thermostat 91.

Let it be again assumed that the system is in its normal running condition with all relays fully energized. If air or water now appears in the fuel line, the flame in the combustion chamber |9 will cease, again permitting the temperature of the thermally responsive resistor 54 to drop slowly. Under these conditions, however, the main relay 34 remains in operative position causing continued operation of the burner l4. As the stack temperature drops, the current through the flame relay 35 is gradually diminished permitting a correspondingly gradual movement of the flex-- ible member 51 toward the right until contacts 59 and 6| disengage. It will be recollected that the running circuit energizing the main relay 34 the movable contact 14 out of engagement with the back contact 15, holding the starting circuit open at this point. The stack temperature now continues to drop permitting a further movement of the flexible member 51 toward the right until contacts 59 and 62 disengage at a relatively lower temperature, de-energizing the auxiliary relay 36 at this time, which re-engages movable contact 14 with back contact 15 to again complete the starting circuit extending through the safety switch heater 49 at this point. If the combustion failure arose from a cause temporary in nature, the fuel-air mixture now delivered by the burner M is ignited to establish a flame in the combustion chamber followed by a normal starting sequence of operation within the control unit 26. -On the other hand, if flame is not established, lock-out will occur within ninety seconds.

The description of the invention in the foregoing environment has made its principles clear, and there will be apparent to those skilled in the art many minor deviations and modifications thereof not departing essentially from its essence. The system described provides for a trial restart in the event of a combustion failure during the normal portion of the running cycle before the heating system is permanently removed from service, and also prevents starting of the burner with a hot combustion chamber in the event of manipulation of the thermostat by the user of the heating system. In addition, if the heating system should fail to start properly at the beginning of a heating cycle it is at once removed from service.

What is claimed and desired to be secured by United States Letters Patent is:

1. In burner control apparatus, a combustion sensing element in the form of a temperature responsive resistor, a combustion relay, means including the winding of said combustion relay for connecting said resistor to a source of electrical energy, said resistor serving to control the current in the winding of said combustion relay, a first normally open switch controlled by said combustion relay and closing at a first combustion temperature, a second normally open switch controlled by said combustion relay and closing at a second combustion temperature higher than said first combustion temperature; a normally closed time delay switch; an electrically responsive actuator for opening said time delay switch at a predetermined time interval after initiation of current fiow through said actuator; an electric control relay having its winding connected at one end with a source of electric energy; a first movable contact controlled by said relay and normally engaged with a second contact and engageable with a third contact in response to said first switch with said third contact, means connecting the other terminal of said second switch with said second and sixth contacts, means connecting said fifth contact with said source of electric energy through said actuator, and means connecting the other terminal of said time delay switch with said fourth contact.

2. In burner control apparatus, a combustion sensing element in the form of a temperature responsive resistor, a combustion relay, means including the winding of said combustion relay for connecting said resistor to a source of electrienergization of said control relay; a fourth movend of said control relay winding with said first contact. means connecting the other terminal of cal energy. said resistor serving to control the current in the winding of said combustion relay, a first normally open switch controlled by .said combustion relay and closing at a first combustion temperature, a second normally open switch controlled by said combustion relay and closing at a second combustion temperature higher than said first combustion temperature; a first electric control relay; a third normally open switch adapted to close in response to energization of said first electric control relay; a normally closed time delay switch connected at one end with a source of electric.energy through the winding of said first control relay; an electrically responsive actuator for opening said time delay switch at a predetermined time interval after initiation of current flow through said actuator; a second electric control relay having its winding connected at one end with said source of electric energy; a first movable contact controlled by said second relay and normally engaged with a second contact and engageable with a third contact in response to energization of said second control relay; a fourth movable contact controlled by said second relay and normally engaged with a fifth contact and engageable with a sixth contact in response to energization of said second control relay; means connecting one terminal of each of said first and second switches with said source of electrical energy, means connecting the other end of said second control relay winding with said first contact, means connecting the other terminal of said first switch with said third contact, means connecting the other terminal of said second switch directly with said second contact and through said third switch with said sixth contact, means connecting said fifth contact with said source of electric energy through said actuator, and means connecting the other end of said time delay switch with said fourth contact.

3. The combination defined by claim 2, wherein said normally closed time delay switch is connected at one end with a source of electrical energy through the winding of said first control relay and through a fourth normally open temperature responsive switch.

4. In burner control apparatus, a combustion sensing element in the form of a temperature responsive resistor, a combustion relay, means including the winding of said combustion relay for connecting said resistor to a source of electrical energy, said resistor serving to control the current in the winding of said combustion relay, a first normally open switch controlled by said combustion relay and closing at a first combustion temperature, a second normally open switch controlled by said combustion relay and closing at a second combustion temperature higher than said first combustion temperature; a first electric control relay, a third normally open switch adapted to close in response to energization of said first electric control relay; a fourth normally open switch adapted to close in response to energization of said first relay; a normally closed time delay switch connected at one end with a source of electric energy through the winding of said first control relay; an electrically responsive actuator for opening said time delay switch at a predetermined time interval after initiation of current flow through said actuator; a second electric control relay having its winding connected at one end with said source of electric energy; a first movable contact controlled by said second relay and normally engaged with a second contact and engageable with a third contact in response to energization of said second control relay; a fourth movable contact controlled by said second relay and normally engaged with a fifth contact and engageable with a sixth contact in response to energization of said second control relay; a seventh movable contact controlled by said second relay normally engaged with an eighth contact and adapted to open in response 10 connecting the other terminal of said second switch directly with said second contact and through said third switch with said sixth contact, means connecting said fifth contact with said source of electric energy through said actuator, means connecting the other end of said time delay switch with said fourth contact, means connecting one terminal of said fourth switch with said source of electric energy, means connecting the other terminal of said fourth switch to said seventh contact, and means connecting said eighth contact with said source of electric energy.

PAUL G. FRERER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,800,412 Shivers Apr. 14, 1931 1,876,281 Eiseman Sept. 6, 1932 1,910,721 Taylor May 23, 1933 2,066,413 Miller Jan. 5, 1937 2,154,041 Gille Apr. 11, 1939 2,217,886 Baak Oct. 15, 1940

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