US2704571A - Safety control circuit for forced draft - Google Patents

Description

March 22, 1955 C. A. REICHELDERFER SAFETY CONTROL CIRCUIT FOR FORCED DRAFT HEATING SYSTEMS Filed Oct. 20. 195

United States Patent SAFETY CONTROL CIRCUIT FOR FORCED DRAFT HEATING SYSTEMS Charles A. Reichelderfer, Columbus, Ohio, assignor to Norman Products Company, Columbus, Ohio, a corporation of Ohio Application October 20, 1950, Serial No. 191,293

2 Claims. (Cl. 158-1) The present invention relates to heating apparatus, and more specifically to an improved safety control circuit for forced draft heating systems of the type embodying a fluid fuel burner, electrically-actuated valve means for controlling the flow of fuel to the burner, and an electric motor driven blower or fan arranged to forcibly circulate air across the burner to effect an elficient heat transfer within the heating system.

In the past, numerous types of forced draft heating systems embodying a fuel burner and a motor driven fan or blower operable to pass air across the burner have been proposed for use. One particular type of heating system of this character comprises the so-called unit heater of the type normally employed in heating relatively large areas such as factories, auditoriums, or other large rooms which are usually difiicult to heat efiiciently through a central heating plant. Generally, in previously known forced draft heating systems, the fuel control valve for the main burner of the system is arranged to be opened concurrently or simultaneously with the energization of an electric motor employed in driving a forced air fan or blower unit arranged to circulate air past the burner. Various control circuits have been proposed to effect the simultaneous opening of the fuel valve and the energization of the blower motor. In such previously known circuits, should the fan motor fail to operate by reason of various mechanical dlfilCllltlBS, the associated fuel valve of the heating system would nonetheless be opened to supply fuel to the burner, with the result that localized overheating of the system 'would be encountered, which in many instances proved dangerous and resulted in the destruction of the heating system.

It follows, therefore, that the primary object of the present invention is to provide a control circuit for forced draft heating systems which functions to positively prevent opening of the associated fuel supply valve of the burner element in the absence of a positive circulation of air across the burner by the associated blower or fan of the system, thereby preventing destructive and dangerous overheating of the system in the event of failure of the motor driven blower.

It is another object of this invention to provide a control system for forced draft heating systems which functions automatically in response to temperature conditions within the zone to be heated by the system to establish parallel operating circuits for an electrically-actuated fuel supply valve and the motor of a blower or fan; which makes provision for energization of the blower prior to the opening of the fuel supply valve to assure a positive flow of air across the burner prior to combustion of fuel within the system; and which further provides for continued operation of the fan or blower following the extinguishment of fuel at the burner, in order that the system may be sufliciently cooled after operation, and that useful heat developed within the system may be discharged to the zone being heated by the system, rather than being retained in the heating apparatus.

For a further and more detailed understanding of the present invention and the various additional objects and advantages realized therefrom, reference is made to the following description and the appended drawing, wherein the single figure illustrates in diagrammatic form the present forced draft heating system and control circuit therefor.

Referring now to the drawing, the numeral 5 designates generally a gaseous fuel burner which is positioned within an internal combustion chamber 6 defined by a plurality Patented Mar. 22, 1955 of walls 7 disposed within an open ended casing 8. The walls 7 defining the combustion chamber 6 are preferably positioned within the casing 8 directly ahead of a rotary forced air fan or blower 9 which is driven upon energization of an electric motor 10. Preferably, the walls 7 defining the combustion chamber are spaced relative to one another to provide a cellular construction within the casing 8 over and through which a portion of the air circulated by the fan 9 may pass to be heated and thereafter discharged through the grilled discharge opening 11 of the casing 8. The combustion chamber 6 is open, as at 12, adjacent the fan 9, and terminates at its opposite end in a flue gas outlet 13 from which gaseous products of combustion emanating from the burner 5 may be suitably piped to the outside atmosphere. Connected with the burner 5 is a fuel supply conduit 14 in which is interposed a control valve 15 of the type which is opened in response to the energization of a solenoid winding 16, and which is normally closed when the winding 16 remains deenergized. It should here be understood that the preceding apparatus and combination of parts are common to many types of forced draft heating systems and in and of themselves constitute no part of the present invention.

In accordance with this invention, a safety control circuit is provided which prevents the opening of the fuel valve 15 in the absence of a positive circulation of air by the fan 9, and as will hereinafter be more fully described, provision is made for the continued operation of the fan 9 following closure of the fuel valve, in order that heat built up within the system may be efficiently utilized and distributed to the zone being heated, rather than being retained within the heating apparatus and permitted to dissipate therein.

The reference characters L1 and L2 designate the main power supply lines for the present control circuit. Connected with the power supply lines L1 and L2, as by the leads 17, is the primary winding of a transformer T. Connected with one side of the secondary winding of the transformer T, as by the lead 18, is the thermally-responsive bimetallic spiral 19 of a room thermostatic switch 20. The thermostatic switch 20 is provided with a stationary contact 21 arranged to be engaged by the spiral 19 as the temperature of the zone to be heated by the system falls below a predetermined temperature. The stationary contact 21 is electrically connected by a conductor 22 with one side of a relay switch winding 23, while the opposite side of the winding 23 is connected by a lead 24 with the secondary of the transformer T to complete an operating circuit for the relay winding. The winding 23 of the relay is provided with a movable armature 25 which is connected to actuate a double-pole, single-throw switch 26. Normally, the switch 26 occupies an open position so long as the winding 23 is deenergized and is provided with the switch blades 27 and 28 which are electrically connected with the separate current supply lines L1 and L2. The switch 26 is provided with stationary contacts 29 and 30 which, upon energization of the winding 23, are engaged by the blades 27 and 28.

Electrically connected with, and leading from, the stationary terminal 29 of the switch 26 is a lead 31 which is connected to one end of the winding 16 of the solenoidactuated fuel valve 15. The opposite end of the winding 16 is electrically connected with a lead 32 extending to the stationary contact terminal 33 of a normally open time delay switch 34. The contact 33 is arranged in the path of a movable thermally-responsive bimetal arm 35 which is arranged in close thermal proximity to a resistance-type heating element 36 electrically connected respectively with the lead 31, as at 37, and a second lead 38. The opposite end of the bimetal arm 35 is electrically connected through a conductor 39 with a stationary contact terminal 40 of a normally closed limit control switch 40a arranged within, or in close thermal proximity to, the combustion chamber 6 of the heater casing. The limit control switch embodies a movable bimetal arm 41 whose outer end is movable to engage the contact 40. The opposite or stationary end of the bimetallic arm 41 is electrically connected with the lead 38, and in operation, the arm 41 is arranged to flex outwardly out of engagement with the stationary terminal 40 of the switch in response to a predetermined high temperature within the combustion chamber 6. As such, the limit control switch functions to break the operating circuit for the fuel supply valve 15 in the event of overheating within the combustion chamber. The opposite end of the lead 38 is connected with a stationary contact terminal 42 of a normally open draft-responsive switch 42a which also comprises a bridge 43 carried on a movable draft-responsive arm 44 which extends within the flue outlet 13 in the path of combustion products being circulated by the fan 9, and which is mounted for pivotal movement to an open position in the absence of a flow of air upwardly through the flue 13, and which is movable to a circuit closure position, as shown in full lines in the drawing, in response to the circulation of air by the fan 9 upwardly through the fiue 13. Located adjacent to, but spaced from, the stationary contact 42 of the draft switch is a second stationary contact 45 which is also arranged to be electrically engaged by the bridge 43 of the draft-responsive switch arm 44. The second terminal 45 of the draft switch is electrically connected by a lead 46 with the stationary terminal 30 of the double-pole, single-throw switch 26. Thus, it will be seen that the solenoid winding 16 for the fuel supply valve 15 is disposed in series circuit with the draft switch 42a, the limit control swtich 41, and the time delay switch 34, and the opening of any one of these three switches serves to break the operating circuit for the winding 16 and thereby prevent the opening of the fuel valve 15 and the flow of fuel to the burner 5.

One of the terminals of the fan motor 10 is connected with the current supply line L2 by the conductor 47, while the other terminal is connected with a second lead 48 which extends to the stationary contact terminal 49 of a normally open thermally-responsive switch 50 which is disposed within the casing 8 of the heating apparatus and which is responsive to the temperature within the heating system to control the operation of the fan motor 10. The switch 50 is provided with a spiral bimetallic element 51 which is electrically connected by way of a lead 52 with the opposite current supply line L1. Normally, the thermally-responsive switch 50 is arranged to remain open so long as the temperature within the heater casing 8 remains below a predetermined relatively low temperature, and is closed when the atmosphere within the casing 8 reaches a predetermined high temperature slightly below the normal operating temperatures within the casing 8 during burning of the fuel at the burner 5. Connected in bypassing relation to the switch 50 is a conductor 53 which extends between the lead 31 and the second lead 48 of the motor 10. Thus, an operating circuit for the motor 10 may be established from the current supply lines L1 and L2 either by way of the conductor 53 upon closure of the switch 26, or by way of the lead 52 and switch 50 upon closure of the latter.

In operation, assuming the system to be cool, an operating cycle is initiated upon the attainance of a predetermined low temperature within the zone to be heated by the system. The room temperature-responsive thermostat 20 closes to establish an operating circuit for the winding 23 which, upon energization, closes the blades 27 and 28 to establish parallel operating circuits for the fan motor 10 and the solenoid winding 16 of the fuel valve. Upon closure of the double-pole, single-throw switch 26, an operating circuit is established for the motor 10 by way of the leads 53, 48, and 47. Following energization of the motor 10, the fan 9 is driven to forcibly circulate air across the burner and in and around the walls 7 defining the combustion chamber. As air passes from the fan 9, the same swings the movable arm 44 of the draft switch 42a from its dotted line position as shown in the drawing to its full line position of circuit closure with respect to the contact terminals 42 and 45. Thus, current flows through the lead 46, the terminals 42 and 45, the lead 38, and through the normally closed limit control switch 41. Upon closure of the draft switch, the resistance heating element 36 of the time delay switch 34 is energized to heat the bimetal arm 35 which, with a predetermined time lag. engages the stationary contact 33 completing an energizing circuit to the solenoid winding 16 of the fuel valve 15. Upon energization of the winding 16, the valve is opened to permit fuel to flow to the burner 5 and to be there ignited by a suitable pilot flame, not shown. Fuel is continuously supplied to the burner 5 until the atmosphere of the room or zone to be heated by the system reaches a desired and predetermined high temperature. At this time, the room temperature-responsive thermostat 20 opens, thus deenergizing the relay winding 23 and opening the double-pole, single-throw switch 26. Upon opening of the switch 26, the winding 16 is deenergized and the fuel valve 15 closes, thus shutting off the fuel supply to the burner 5. However, the fan motor 10 remains energized due to the closure of the thermally-responsive switch 50 so long as the temperature within the casing 8 remains above a predetermined de sired level, thus permitting the fan 9 to continuously operate after extinguishment of the flame at the burner 5, in order that the system may be cooled by circulation of air therethrough, and in order that more efficient use may be made of the heat within the unit by utilizing the heat built up within the system during operation of the burner to increase the temperature of the zone to be heated. After a predetermined time of operation of the fan 9 following extinguishment of the burner 5, the thermally-responsive element 51 of the switch 50 is cooled sufiiciently to open the operating circuit for the motor 10, thus stopping the operation of the fan 9.

The purpose of the time delay switch 34 is to provide a predetermined and relatively short delay in the energization of the winding 16 of the solenoid valve 15 following closure of the draft-responsive switch 42a. The provision of this time delay switch is deemed advantageous to prevent energization of the solenoid valve 15 in the event of intermittent fluttering or oscillation of the movable arm 44 caused by up drafts through the flue outlet of the system. It will be understood that certain adverse weather conditions, or improper installations might result in periodic fluttering of the member 44 in the absence of a positive flow of air through the system established by the fan 9, and in such event, the time delay provided by the switch 34 prevents accidental energization of the valve 15.

In view of the foregoing, it will be seen that the present invention provides an improved and mechanically efficient safety control circuit for forced draft heating systems, wherein a motor driven fan or blower is utilized to forcibly circulate air to the burner unit of the system. Control circuits formed in accordance with this invention are characterized by their ability to positively prevent the burning of fuel within the system in the absence of the forced circulation of air therethrough, and in the event of back pressures or downdrafts or restrictions within the flame outlet of the heating system, thus preventing dangerous and destructive overheating within the unit in the event of mechanical failure of the motor, or its operating circuit, employed to drive the fan or blower unit, and the possibility of combustion products being forced outwardly to the zone being heated. Further, the present control system provides for a predetermined time delay in the opening of the fuel valve of the system following the initial energization of the blower unit, thus assuring a ready supply of forced air across the burner prior to the ignition of fuel within the system. A further advantage of the present control system resides in the provision of a thermally-responsive switch in series circuit with the blower unit by which the blower is maintained in an energized condition for a predetermined time following extinguishment of the burner to enable the system to be cooled and to drive off excess heat within the system into the atmosphere to be heated and thereby to make efficient use of all fuel burned in the system.

It should be borne in mind that the individual switch elements, as shown and described in the appended drawing and the foregoing description, are illustrative of one form or type of switch suitable to accomplish the ends of this invention, and that other suitable types of switch instrumentalities may be employed in substitution for those specifically described without departing from the spirit of the invention or the scope of the following claims.

I claim:

1. A forced draft heating apparatus embodying a fuel burner; electro-responsive valve means for controlling the passage of fuel to the burner; a motor-driven blower arranged to circulate air to said burner; and a control system therefor comprising, in combination: a first switch means responsive to the temperature of a zone to be heated by said apparatus for establishing parallel operating circuits for said electro-responsive valve means and for the motor of said blower; second switch means connected in series with said 'electro-responsive valve means and arranged to normally maintain the operating circuit for said valve means open in the absence of circulation of air by said blower; a time delay device including a switch connected in series with said valve means and electrically connected with said second switch means and including an electro-responsive actuator connected to be energized through said second switch means for closing the operating circuit for said valve means a predetermined time following closure of said second switch means, whereby to delay energization of said valve means upon closure of said first switch means; and a thermally-responsive switch connected in series with the motor of said blower and in bypassing relation to said first switch means and responsive to temperature conditions within said apparatus for closing an operating circuit for the motor of said blower independently of said first switch means, said thermally-responsive switch being effective to maintain an operating circuit for the motor of said blower so long as temperatures within said apparatus remain above a predetermined level.

2. In a forced draft heating system; a casing defining a combustion chamber having inlet and outlet ends; a fluid fuel burner positioned in said chamber between the inlet and outlet ends thereof; electro-responsive valve means for controlling the flow of fuel to said burner, said valve means being energizable to permit free flow of fuel to said burner; a blower arranged to positively circulate combustion air through said chamber from the inlet end toward the outlet end thereof; an electric motor for driving said blower; a first thermally-responsive switch means positioned in a zone to be heated by said systemand having electrical connections with said electro-responsive valve means and said motor and operable to open or close parallel operating circuits for said valve means and said motor; a draft-responsive switch including an actuating element positioned in said chamber and elec trically connected in series with said valve means, said draftresponsive switch being operable in response to a positive flow of air through said chamber to establish an operating circuit for said electro-responsive valve means; a time delay device including a switch connected in series between said valve means and said draft-responsive switch and including an electro-responsive actuator connected to be energized through said draft-responsive switch for closing the operating circuit for said valve means a predetermined time following closure of said draft-responsive switch, whereby to delay energization of said valve means upon closure of said first thermallyresponsive switch means; and a second thermally-responsive switch means positioned in thermal proximity to said combustion chamber and electrically connected with said motor in by-passing relation to said first thermally-responsive switch means and operable in response to a predetermined temperature within said casing to maintain an operating circuit for said motor independently of said first thermally-responsive switch means.

References Cited in the file of this patent UNITED STATES PATENTS 1,732,174 Sweatt Oct. 15, 1929 2,137,581 Myler Nov. 22, 1938 2,168,859 Bergey Aug. 8, 1939 2,188,565 Bower Jan. 30, 1940 2,251,055 Howard et a1. July 29, 1941 2,259,299 Dewey Oct. 14, 1941 2,412,990 Kruse Dec. 24, 1946 2,432,314 Holthouse Dec. 9, 1947

Images