US2277621A - Control system for forced draft stokers - Google Patents

Description

March 24, 1942. w. J. WEBER CONTROL SYSTEM FOR FORCED DRAFT STOKE-RS Filed April 15, 1959 ,fimnntor Wilbur J- Weber Patented Mar. 24,1942

2,271, 21 CONTROL SYSTEM FOR FORCED pm Wilbur J. Weber,

OKERS Philadelphia, Pa... assignor to Minneapolis-Honeywell Regulator Company,

Minneapolis, Minn.,

Application April 13,

Claims.

My invention relates to control of combustion systems and particularly those adapted to burn solid fuel.

The object of my invention is to prevent the accumulation of a large amount of fuel in the combustion chamber which would remain there to burn after the demands for combustion have ceased, that is, when a controlling thermostat for example, becomes satisfied.

When solid fuel is fed continuously during a demand for combustion it obviously is not all burned as fast as it is supplied because usually the fire must be built up from a small one to a relatively large one during the demand for combustion. Hence when the demand for combustion ceases a large amount of burn and give off unneeded and wasteful heat.

My invention comprehends terminating the fuel supply before the demand for combustion is satisfied and allowing a forced draft device to burn the fuel already in the combustion chamber and thereby maintain the required rate of heating without supplying additional fuel. If the accumulated fuel and forced draft will not maintain the required amount of heating the fuel supply is again initiated. With this control arrangement the amount of fuel remaining in the combustion chamber when the demand for combustion is satisfied is relatively small and material economies are thereby effected.

My invention is particularly advantageously applicable to those types of fuel feeding apparatus which take the form of a stoker of the overfire feed type and which supply fuel intermittently in charges, the charges of fuel being deposited upon the fuel bed. This form of fuel feed mechanism incorporates fuel carrying means which are moved into and out of the combustion chamber. To prevent the fuel carrying means from stopping in a position over the fire bed an automatic switch operated by the fuel feed mechanism is provided whereby the mechanism after having once been started will go through a full cycle and can only stop in the fully retracted" position. With my improved control arrangement, the stoking mechanism will usually be antomatically stopped in response to a condition indicative of the amount of heat in the combustion chamber before the demand for combustion is satisfied. Thus the demand for combustion will not become satisfied at a time when there.

is practically a fresh charge of fuel in the combustion chamber or shortly before a fresh charge is added. The forced draft will cause the accumulated fuel to burn up and satisfy the ourfuel remains to a corporation of Delaware 1939, Serial No. 267,625

(01. zsa-s) rent demand for combustion without piling up unnecessary fuel which would remain to burn during periods when combustion is not necessary.

With this type of stoking apparatus there is a period of time which elapses after there is a demand for combustion before fuel is delivered into the combustion chamber. This delay period enables the forced draft to suitably agitate the 10 fire and condition the fire bed for efficiently igniting the fuel charge when it is deposited thereon.

While my invention is particularly adapted to use with the type of fuel feeding mechanism referred to above, it is not limited to such use.

The salient objects of my invention are apparent from the foregoing general explanation. Stating the principal object of my invention in another way, it is to burn more of the fuel which is supplied to a combustion chamber duringa demand for combustion before the demand becomes satisfied, that is, to burn more of the supplied fuel while heat is needed rather than after heat is not needed.

Another object is to proportion the total heat of the combustion to the thermostats requirements without lag in delivery or overshooting.

Another object is to provide a control arrangement for solid fuel burning apparatus wherein the draft and fuel supply are automatically controlled and the fuel supply is stopped before the draft is cut off while a demand for combustion is in progress.

Another object is the provision of a control 85 system for automatic stokers of the type which always go through a complete fuel delivery cycle after having been started wherein the fuel delivery means may be automatically stopped during a demand for combustion and combustion may be continued under forced draft to satisfy the demand.

- Another object is the provision of an automatic stoker control system having fuel and draft supply devices and means operable to stop the fuel feed and draft in response to different values of a condition or different conditions.

The drawing represents diagrammatically an overfire feed stoker having the control system of my invention applied thereto.

Referring to thedrawing, at I0 I have shown a steam boiler in a stoker fired system, the fuel being fed to the boiler by means of a motor driven feeding mechanism designated generally at H.

The fuel feeding mechanism is supported by a frame structure l2 which may be made up of angle irons or the like. The framework or supporting structure I2 supports a coal hopper I3 and beneath the hopper is a reciprocable grate or scoop designated at I4. The lower part of the hopper extends into the scoop I4. The scoop I4 is arranged so that it can be moved to a position extending through an opening I5 in the boiler and into the combustion chamber. The scoop is supported on a rail or guideway I6 and travels back and forth on the guideway and into and out of the combustion chamber. The scoop I4 has a downwardly extending member I'| having a slot I8 therein, and engaged in the slot is a pin I9 on the end of a rocker arm 20 which is pivoted at 2| to an integral portion 22 of the framework l2. The rocker arm has a longitudinal slot 23 and engaged in this slot is a crank pin 24 on a cam wheel 25. The cam wheel 25 has a cam surface 26 and another cam surface 21 which is of slightly smaller radius than the cam surface 26. The cam wheel 25 is driven by an electric motor 28, the motor 28having a pulley 29 and cam wheel 25 having a pulley 3|) with a belt 3| running over the two pulleys. The cam wheel 25 is driven in a counter-clockwise direction by the electric motor and as it turns the crank pin 24 moves in the slot 23 causing the rocker arm 20 to be oscillated back and forth with the pin I9 moving in the slot I8 and thereby reciprocating the scoop I4 on the guideway I6.

While the cam wheel 25 is oscillating the rocker arm 20 to the right the scoop I4 is moving into the combustion chamber and continues to move in that direction until the cam wheel 25 begins to oscillate the rocker arm 28 to the left. At the lower part of the hopper I3 there is an opening to permit coal to pass into the scoop I4 and which may be covered by means of a sliding door 35. The door is carried on a vertical shaft 36 which extends through suitable guide bearings 31 and 38 forming part of the framework |2. Numeral 39 designates a collar on the shaft or rod 36 and interposed between this collar and the guide bearing 37 is a coil spring 40 which biases the rod 36 downwardly so as to tend to move the door 35 downwardly to cover the opening in the lower part of the hopper I3. The lower end of the rod 36 is attached to one arm of a bell crank lever 4| by means of a link 42. The bell crank lever is pivoted at 43 and has a shorter arm 44 carrying a roller cam follower 45 which is arranged to follow out the contour of the cam wheel 25. When the scoop I4 is moving inwardly the cam follower 45 follows out the surface of the cam wheel 25 of greater radius and the bell crank lever 4| is held in a counterclockwise rotated position with the coil spring 40 compressed and the door 35 moved upwardly so as to permit coal to pass from the hopper into the scoop I4. When the scoop I4 has been moved to its extreme right position or shortly before this the cam follower 45 moves to the surface of cam wheel 25 of smaller radius and the bell crank lever 4| is caused to move in a clockwise direction due to the coil spring 48 urging rod 36 downwardly. This action causes the door 35 to move downwardly and cover the opening in the lower part of the hopper I3. With the parts in the position shown in the drawing, the cam wheel is just about to start retracting or moving the scoop outwardly from the combustion chamber. While the scoop is moving inwardly it fills up with fuel passing out of the hopper I3. When the scoop is all the Way in, the door 35 is in closed position as stated and as the scoop is retracted or moved outwardly the fuel in the scoop is prevented from being carried back out again because of the door 35 being in its downward position. As the scoop is retracted the fuel is forced oil the scoop and is deposited on the fuel bed in the combustion chamber.

As it is not desired to stop the scoop I4 in a position over the fire bed exposed to the heat therefrom an arrangement is provided whereby the motor 28 driving the stoking mechanism is connected directly across the line-forming the source of power whenever the scoop I4 is in any position except its fully retracted position. Numeral 46 designates a switch having a movable switch blade 41 carrying a contact" which is engageable with a fixed contact 43. The switch 46 is normally closed and as will presently be pointed out, completes a circuit connecting the motor 28 directly to the line whenever the scoop I4 is not in a fully retracted position wherein a finger attached to the scoop may engage with the blade 41 and disengage the contacts 48 and 49.

Forced draft for the boiler is supplied by a forced draft fan 6| driven by an electric motor designated at 52.

The system is arranged to be controlled primarily from a room or space thermostat indicated at and this thermostat may be of a conventional type comprising a bimetal element 56 arranged to actuate a movable switch blade 51 carrying a contact 58 cooperable with a fixed electrical contact 59. Further control is provided by a pair of switches which may take the same form as pressure switches known to the art and which are conventionally used as high limit switches. These switches in the present instance are designated at 60 and 6| and comprise mercury switches 62 and 63 which are operable between open and closed positions by means of elements responsive to thepressure within the boiler. These switches open at predetermined relatively high pressures. For purposes of maintaining a minimum fire in the boiler or for maintaining a minimum boiler water temperature for domestic hot water purposes, a temperature responsive switch 64 is provided which comprises a mercury switch 65 actuable by a thermostatic element responsive to the temperature of the water within the boiler. This switch closes at a predetermined relatively low temperature. The operation of the various switches will be explained in the description of operation following.

Whenever there is a demand for heating in the room or space in which the thermostat 55 is located the blade 51 causes the contacts 58 and 59 to engage, which completes a circuit forthe electric motor 28 as follows from wire 18, to wire 69, to the thermostat 55 through the element 56, blade 51, contacts 58 and 59, wire II, wire I2, switch 62, wire 13, wire 14, motor 28', wire I5, and wire 16 back to wire I1. Wires I0 and 11 may be connected to a suitable source of external power, not shown. It is assumed that the boiler pressure is low enough at this time so that it is below th settings of the switches 60 and 6| so that these switches are both closed. The switch 6| is set at a higher setting than the switch 60 for a reason which will presently be pointed out. At the same time as the above described circuit is completed, an additional circuit will be completed for energizing the motor 52 and starting the draft fan 5! in operation. This circuit is as follows: from wire 10, wire 69 through element 56 of thermostat 55, through blade 51, contacts -45 is opened. The stoking mechanism will After the switch 52 opened, continued opera* 1 58 and 58, wire 'Il. wire I8, wire I8; switch 58. wire 88, motor 52, wire wire 11. As soon as the above circuits are completed the stoking mechanism begins to operate, that is, to start to move the scoop l4 inwardly while draft from the draft fan agitates the fire during the time the scoop is moving inwardly,

As soon as there has been a small amount of movement of the scoop l4 the finger 58 moves away from the switch blade 41 and the switch 48 becomes closed, (the scoop-having been in full retracted position upon starting). When the switch 48 closes, motor 28 is connected directly to the source of power and its operation is then independent of the thermostat and it will COR- tinue operating at least until the stoking mechanism has gone through-one complete cycle and the scoop 14 has been moved to its fully retracted position. The circuit through switch 45 is as follows: from wire ID to wire 59 to wire 82, wire 88, switch 45, wire 84, wire 14, motor 28', wire 15 and wire 15 back to wire 11.

While the stoking mechanism and draft fan continue in operation fuel will be intermittently deposited in the combustion chamber in charges as the scoop I4 is moved in and out. Thus the fire will be gradually built up as long as the thermostat 55 is calling for heat and at a predetermined boiler pressure the switch 52 will open and the original circuit for motor 28 will be interrupted. The pressure at which switch 52 is opened may be adjusted to a suitable value depending upon the characteristics of the particular system. This switch is set to open, however. at a lower pressure than the pressure at which switch 53 is opened. When the switch 52 is opened the stoking mechanism will be at some point in its cycle and it will continue in operation until that cycle is completed and the scoop I4 is in fully retracted position wherein the switch then there will ordinarily be a considerable amount of fuel in the combustion chamber of th boiler which has accumulated during the time that the pressure has been building up to the setting of the switch52. Inasmuch as the switch 53 is still closed the draft fan will continue in operation and will continue burning up the accumulated fuel in the combustion chamber. This accumulated fuel may be suificient so that with the forced draft, heating will continue to increase and the boiler pressure will continue to rise while the accumulated fuel is buming. Normally the thermostat 55 should now bestop. At this time come satisfied and stop the draft fan. Due to the fact that the fuel supply has been cut oil! before the draft there will be a considerably smaller amount of accumulated fuel in the combustion chamber after the thermostat is satisfied than there would be if the stoking mechanism were kept in operation as long as the thermostat werecalling for heat. The result is that there is a smaller amount of accumulated fuel to be burned during times when the thermostat is not calling for heat. If instead of becoming satisfied thermostat 55 continues to call for heat, operation of the draft fan might cause the fire to increase sufflciently so that the boiler pressure would rise high enough to open the switch 53. The switch 53 thus acting as a high limit would shut off the draft fan so that now both the fan and the stoking mechanism would be inoperative.

Upon the boiler pressure again dropping to the setting of the switch 53 the draft fan would again be started.

8|, and wire I8 back to tion of the draft fan might not cause the boiler pressure to rise high enough to open the switch 53 before thermostat 55 became satisfied. While the draft fan is operating the boiler pressure might again drop to the setting of the switch 52 which would again start the stoking mechanism in operation and it would of course remain in operation for at least one complete cycle. In the manner described, the switch 52 might cycle on and off one or more times before the thermostat 55 finally became satisfied with the draft fan continuing to operate all the time.

When there are relatively long intervals during which there are no thermostatic demands for combustion switch 54 will intermittently operate the stoking mechanism and draft fan to maintain the fir or to maintain a minimum boiler water temperature as desired. At a predetermined relatively low temperature mercury switch 55 will be closed, energizing motor 28 through the following circuit: from wire 10. to

wire 59, wire 82; wire 85, switch 55, wire 81, wire 18, wire 12, switch 52, wire 13, wire 14, motor 28, wire 15, wire 15, back to wire 11. The stoking mechanism will now go through at least one complete cycle and possibly more if switch 55 remains closed long enough. When motor 28 is energized through the circuit just described the fan motor 52 is also energized through the following circuit: from wire 10, to wire 59, wire 82, wire 85, switch 55, wire 81, wire 19, switch 58, wire 88, motor 52. wire 8|, wire I5, back to wire 'I'I. Switch 54 will thus act as a low limit or fire maintaining controller.

From the foregoing operation it is readily seen that with my improved control arrangement the pressure switches can be so set that normally the stoking mechanism will be stopped before the space thermostat becomes satisfied. Thus normally the fuel feeding mechanism will not be depositing a charge of fuel in the combustion chamber shortly before or after the thermostat becomes satisfied. Furthermore, that fuel that has been fed into the combustion chamber by the stoking mechanism during the time that the thermostat has been calling for heat will to a great extent be burned up by reason of the operation of the draft fan after the stoking mechanism has been stopped.

The stoking mechanism which I have disclosed is representative of other similar forms of stoking apparatus wherein the fuel is fed into the combustion chamber in periodic charges or intermittently rather than continuously. While my invention is particularly adaptable and advantageous when applied to this particular type of stoking mechanism, its application is not limited thereto inasmuch as it is apparent that my control arrangement possesses the advantage that more of the fuel that is supplied while the thermostat is calling for heat is burned during that time than would ordinarily be burned with conventional control arrangements now in use. a

In accordance with the statutesmy disclosure is of a preferred form which the invention may take and is exemplary of other forms which it may have and of other applications which may be made of it. The invention is therefore to be limited only in accordance with the appended claims.

I claim as my invention:

1. In a control system, in combination, means forming a combustion chamber, means comprising an automatic stoker for feeding fuel into said combustion chamber, said stoker means being of a type arranged to repeatedly go through an operating cycle and having means whereby it can stop itself at a point in its cycle, means comprising a device for supplying forced draft to said combustion chamber, meanscomprising a thermostat for starting said stoker means and draft supplying means into operation, and means responsive to a condition indicative of the amount of combustion in said combustion chamber operable to bring about termination of operation of said stoker means, said condition responsive means normally acting before said thermostat becomes satisfied, said forced draft means continuing in operation after termination of operation of said stoker means and stopping upon said thermostat becoming satisfied.

2. ma control system, in combination, means forming a combustion chamber, means comprising an automatic stoker for. feeding fuel into said combustion chamber, said stoker means being of a type arranged to repeatedly go through an operating cycle and having means whereby it can stop itself at a point in its cycle, means comprising a device for supplying forced draft to said combustion chamber, means comprising a thermostat for starting said stoker means and draft supplying means into operation, means responsive to a condition indicative of the amount of combustion in said combustion chamber operable to bring about termination of operation of said stoker means, said stoker means normally stopping before said thermostat becomes satisfied, and means responsive to a condition indicative of increased combustion in said combustion chamber for stopping the supply of draft by said forced draft means.

3. In a control system, in combination, means forming a combustion chamber, means comprising an automatic stoker for feeding fuel into the combustion chamber, said stoker means being of a type arranged to repeatedly go through operating cycles so as to periodically deposit charges of fuel in the combustion chamber and after having been started always going through at least one cycle, means for supplying draft to said combustion chamber, temperature responsive means controlling said stoker means in accordance with demands for heat, control means for preventing said stoker means from depositing a charge of fuel in the combustion chamber after said temperature responsive means is satisfied, said control means comprising a device responsive to a condition indicative of the amount of combustion in the combustion chamber, said device being operable to stop said stoker means and being normally-set to operate at a value of said condition such that the stoker means is shut down before the temperature responsive means becomes satisfied, and means operative at a different value of said condition for discontinuing operation of said draft supply means, said last mentioned value of the condition being such that normally the draft supply means remain in operation until the temperature responsive means is satisfied.

4. In a control system, in combination, means forming a combustion chamber, means comprising an automatic stoker for feeding fuel into the combustion chamber, said stoker means being of a type arranged to repeatedly go through operating cycles so as to periodically deposit charges of fuel in the combustion chamber and after having been started always going through at least one cycle, means for supplying draft to said combustion chamber, temperature, responsive means controlling said stoker means in accordance with demands for heat, control means for preventing said stoker means from depositing a charge of fuel in the combustion chamber after said temperature responsive means is satisfied, said control means comprising a device responsive to steam pressure generated by heat from said combustion chamber, said device being operable to stop said stoker means and being normally set to operate at such a pressure that the stoker means is shut down before the temperature responsive means becomes satisfied, and

means operative at a different value of steam pressure for discontinuing operation of said draft supply means, said last value of steam pressure being such that normally the draft supply means remain in operation until the temperature means is satisfied.

5. In apparatus of the character described, in combination, means forming a combustion chamber, means comprising an automatic stoker for feeding fuel into the combustion chamber, means comprising a fan for delivering air to said chamber, temperature responsive means for starting and stopping said stoker in accordance wth demands for heat, means responsive to a temperature indicative of the amount of heat in said combustion chamber for stopping said stoker, said last means being set to normally stop the stoker before said first temperature responsive means becomes satisfied to prevent the piling up of excessive fuel in the combustion chamber, and another means operable to stop said fan in response to temperature indicative of the amount of heat in the combustion chamber, said other means being set so that the fan continues to operate after the stoker is stopped when the said first thermostatic means are yet unsatisfied but so that the fan also may be stopped before the first thermostatic means are satisfied depending on the amount of heat in the combustion chamber.

WILBUR J. WEBER.

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