US2390902A - Control system - Google Patents

Description

CONTROL SYSTEM Filed Aug. 28, 1942 2 Sheets-Sheet 1 INVENTOR.

l 35 JOSEPH PVOLLRATH ATTORN Y.

Dec. 11, 1945. w J. P. VOLLRATH CONTROL SYSTEM Filed Aug 28, 1942 2 Sheets-Sheet 2 STEAM H Y M M TR NL L V0 T NV T I A H m L w n E until the waste shpplied to maximum. This Patented Dec. 11, 1945 UNITED STATES PATENT OFFICE CONTROL SYSTEM Joseph P. Vollrath, Glenside, Pa., assignor to The Brown Instrument Company, Philadelphia, Pa., 7 a corporation of Pennsylvania 9 Claims.

The present invention relates to the control of regenerative furnaces, and more particularly to a control system for controlling the supply of fuel to a regenerative furnace of the open hearth type.

It is an object of the present invention-to maintain the desired fuel flow at a constant, equal rate to both ends of the furnace irrespective of variations in fuel pressure, the injection steam pressure or temperature changes in the burner piping.

It is an object of the invention to provide a control system which will maintain a heat balance in the checker system of the furnace when the latter is reversed at periodic intervals. It is a further object of the invention to prevent entry of fuel into the furnace after reversal takes place gas dampers are completely resuflicient combustion airhas. been the burner ports. It is a further ob- Ject of the invention ,to eliminate flooding of the furnace with unburnt fuel immediately following a reversal operation 'by automatically opening the versed and fuel valve slowly at a predetermined but adiustwable rate fromthe timeair begins to flowto the burner ports.

It is a. further object of vide a controlsystem that the invention to prois instantly transferable from automatic control to manual control' by. means of the operation of suitable selector valves.

perature in the furnace reaches a predetermined prevents damage to the furnace roof, saves fuel and shortens .the heating time for the charge.

In the control system of this invention, when it is time to reverse the operation of the furnace, is suitable switch is thrown to initiate the reversring the combustion air dampers are reversed and upon completion of this operation, the injection steam to the opposite side of the furnace is .quickly eration of the control valves will in no wise effect the speed with which the fuel is cut off and turned on. This insures that the method of starting the fuel supply will effectively prevent harmful and wasteful flooding of the furnace with unburned fuel and will admit fuel as rapidly as it can be burned. Also, the rate of fuel admission is correct for each furnace and is independent of variations caused by different operators.

The various features of'novelty which characterize my invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, however, its advan- 15 tages and specific objects obtained with its use,

reference should be had to the accompanying drawings and descriptive matter in which I have illustrated and described a preferred embodiment of the invention. i

In the drawings:

Figure 1 is a schematic showing, partly in section, of an open hearth reversal control system,

Figure 2 isa view showing more in detail the manner in which the control instruments operate to control the supply of fuel and the sequence of the reversing operations, and 1 Figure 3 are curves showing the rateof steam and fuel supply and cut-oil.

Referring to Figure 1, there is shown at I an openhearth furnace having checkers 2 which can t be connected with a combustion airsupply 3, or a stack 4 by means of a suitable damper 5. This damper is moved from the position showntoa position in which it will connect the left checker with the air supply and the right checker with the stack by means of a reversible electric motor 6 that serves to drive the damper through a trans mission mechanism 1, shown herein as being a belt. As the motor 6 operates, an arm 8 attached to the shaft thereof can be used to operate switch unit 9 or switch unit ill at the limits of operation of themotor when the damper has been properly positioned.

The fuel forthe pe ll past a valve I2 erated reversing valve l3 to either pipe H or pipe lli. depending upon whether the right or left end of the furnace is being fired. The injection steam for the fuel is supplied through a pipe 18 having in it a pneumatically controlled valve l7 and a manually or automatically operated reversing valve l8, to pipes I 9 or 20 depending upon whether the right or the left end of the furnace-is being fi ed. The control valves l2 and 11' are of the 45 time is supplied through the operation of the furnace and energize the motor 9 to reverse the damper 9.

Located in the fuel supply pipe H is an orifice 2|, the pressure across which is measured by a manometer or other flow measuring device 22. This manometer-forms part of a flow controlling instrument 23 to which it is connected by a cable 24, the controller acting to adjust the valve l2 in a manner to be described in response to variations in the fuel to keep the latter constant. The

temperature of the furnace is measured by means of a radiation pyrometer 29 which is directed through an opening 29 in the back wall of the furnace toward the roof thereof. Thispyrometer is connected to a potentiometer type recording control instrument 21 by means of a cable 28.

The furnace pressure is regulated by means of a furnace pressure controller 29 that is connected with the interior of the furnace bymeans of a pressure line 39. This controller is also provided'with a compensating line 3| which runs parallel to the line 39 and terminates adjacent the opening in the furnace through which the line 39 extends. The furnace pressure controller serves to operate a damper 32 in the stack 4 to control the pressure in the furnace. This damp- 49 and an e'qualiiing connection 50, in which the valve 39 is located, to the pipe 42.

Air is supplied to the steam valve H by means of a continuation of the pipe 49. This pipe has in it a normally open solenoid valve 5! and a normally closed solenoid valve 52 which upon occasion can be closed and opened respectively to control the operation of the injection steam supply. Beyond valve 52 is a restriction 53 that is adjustable to vary the speed with which air can be exhausted from the line 49 when the valve 92 has been opened. There is also provided in the line 49 ahead of the valve ii an adjustable restriction 54 that is used as a constant bleed in the air line when the mechanism is on manual control.

The instrument 2! may be a potentiometric instrument of the type shown and described in Krogh Patent, No; 2,159,971, granted May 30, 1939, and includes a motor actuated relay mechanism through which the deflection of the pointer 59 of a galvanometer connected to and responsive to the voltage of the pyrometer 25, adjusts a potentiometer recorder carriage 59 to different positions, corresponding to the different values of said voltage. The carriage 59 cooperates with 'a normally stationary control table 51, which may be adjustable along the path of movement ofthe carriage 59, and the position of which, along said path, is a measure of the normal value of the voltage to which the galvanometer of the instru ment responds.

er is normally biased in a closing direction bya weight 33 and may be opened by a pneumatic piston 34 which is controlled by the instrument 28. A hand operated winch 35 is also used to adjust the damper 32. 1

Each of the instruments 23, 21, and 29 is of the air operated type, and they are supplied with air from a line 39 through a filter 31 and pressure "regulator 38. The system is so set up that the fuel valve may be operated by either the fuel controller and the temperature controller o the fuel controller alone. These operations may be obtained by means of proper manipulation of a bypass valve 39 and a manual-automatic valve 49 in a manner to be set forth in detail below.

Air is supplied from the supply 39 through the filter 31 and pressure regulator 38 to a pipe 4'! which leads to the instrument 21. From this in 1 strument, air passes through pipe 42 to the valve" 49 and through pipe 43 to the instrument 23.

a pipe 44 to the valve 49 and a pipe to the fuel The adjustment of the carriage 99 relative/t0 the stationary control table 91, gives upand down movements to a link 98 and thereby oscillates a rocking element 59. The latter is connected by a link 99 to an actuating leve 9| by which the flapper valve 92 of the instrument 2! is adjusted.

40 The instrument 2'! as shown, also comprises a mechanism 93 through which an initial change in the control pressure due to the adjustment of the flapper 92, produced by a change in the po-' sition of the carriage 59, results in an immediate follow-up adjustment of the control pressure, partially neutralizing the eflect of the'initial change therein, and in a delayed compensating adjustment'neutralizing in whole or in part, the

effect on the control pressure of the follow-up adjustment. 9

As shown'in Fig. 2, the air under pressure is supplied by the pipe 4| to the inlet side of a restricted orifice 94 to actuate the mechanism 93.

control valve l2. The modulated air is also supplied through a pipe 49 to the follow-up provisions of the control instrument that is located in casing 23. Located in the pipe 45 between the valve 49 and the valve I2 is a restriction '41 that v is adjustable to vary the speed with which changes in pressure set up by the instrument 23' can be applied to the valve l2. Also in this pipe is a normally closed solenoid valve 48 that can be opened upon occasion to exhaust the air line and permit the valve 12 to close quickly. The instrument 2'! can be by-passed to cut out its effect on the control of the fuel by means of air' which is supplied directly from the source through pipe and to actuate .thecontrol valve i2.

The air is passed to the valve and to. the mechanism 92 through a pilot valve chamber 95 which receives air from the pipe 41, and bleeds air through a vent 99, as required to maintain a pressure in the chamber 95 proportional at all times to the pressure in'the nozzle 92.

For the purpose of the present invention, the

- only important characteristic of the instrument 21, is that it varies the pressure in the pilot valve chamber 95, in predetermined accordance with variations in the roof temperature of the furnace v as measured by pyrometer 29. Further reference to the details of construction and operationpatent, is thus unnecessary.

The instrument 23, which, as previously stated, regulates .the air pressure suppliedto th pipe 49, is responsive to th rate of fuel flow through the main II to the pipes l4 and IS. The instrument 23 may be of any suitable type and is made responsive to the rate of fuel flow, by means comprising the measuring orifice 2| formed in the pip I I, and the U tube manometer'n. The latter has one leg connected by a pressure transmitting pipe to the pipe I I at the inlet side of the orifice 2|, and has its other leg cc nnectedby a pressure transmitting pipe 1| to the pipe II at the outlet side of the orifice 2|. A float 12 resting on the manometer sealing liquid 13,,which may be mercury, is given rising and falling movements as a result of the sealing liquid displacement produced 10 transmitter element comprises two end to end vertically disposed coil; and 16 surroundings.

manometer leg portion in which the armature 14 is axially movable. The instrument 23 includes the receiver element of the inductance bridge, which comprises vertically disposed end to end coils 11 and 1 8, in which the core 19 is axially movable. The remote ends of the coils 11 and 18 are connected by conductors in cable 24, to the remote ends of the coils 15 and, 16., The adjacent ends of the coils 11 and 18are connected by other conductors also in cable 24 t0 the adjacent ends of the coils 15 and 16. The bridge is energized by an alternating current supply.

With the inductance bridge arranged and energized as described, an upward movement, for example or the transmitter armature 14 increases the impedance of the coil 16 and decreases the dependent upon the pressure in the nozzle 82, and hence upon'the rate at which fuel passes through impedance of the coil 15. Those impedance changes result in a change inthe relative current flows through the transmitter and receiver coils and thereby cause an increase of current flow through the coil 11 relative to the current flow through the coil 18, and causes the armature 19 to move upward until the resultant'increase in the impedance of the coil 11 relative to theri r'npedance of the coil 18, makes the impedance'ratio of coils 11 and 18 equal 'to the impedanceratio of. the coils 15 and 18. This reestablishesithe balancedcondition of the bridge. The movementgiven to the armature 14, and the position got; thereceiver armature 19 is thus dependent upon the position of the armature 14, for all values'oithe diiferentiai of the pressures at the oppositesides of the orifioe2I. I v a The instrument 23 includes a; flapper valve 8| and a nozzle 82, the latter being connected to the air supply pip 43 through a restrictedorifice 83. The relative adjustment of the flapper 8| and nozzle 82 is dependent upon the position of the nges in position of the armature 19, effect the adjustment oi the flapper valve armature 19. C

8| through a, lever 84 pivoted at 85, and from one end of which the armature 19 is suspended. The.

opposite end of the lever 84 is connected by a link 86 to an oscillating element which may carry an exhibiting arm 81 for indicating or recording the fuel fiow. The oscillatory movements of the lever 84 effected by movements of the armature 19, give movements to the flapper 8| relative to the nozzle 82. In the commercial form of the instrument 23, shown in outline in Fig. 1, the flapper. valve 8|, is thus actuated through the link 84 and means through which the up and down movements of that link give up and down movements to the upper end of link 88. The lower end of the link 88 is pivotally connected to a flapper actuating lever 89, which is pivoted at 90 and provided with a pin or shoulder 9| which engages the flapper 8 I.

50 double throw switch trol the operation orthe damper 5 by the motor .9.cts as a limit switch for the motor 6. box 9 is biased flapper is moved away from the nozzle 82 by a clockwise adjustment of the lever 89, and is spring biased for movement toward the nozzle when the v duced the initial control pressure change. Fur-, l5 ther reference'to the follow-up adjustment mechanism is unnecessary herein, because that mech anism is now known, is in commercial use, and is fully disclosed in the above mentioned Moore patent, and also because of the fact that for the general purposes of the present invention, the,pivot 90 may be stationary, and the lever 89 may b operated through a link 88 directly connecting the lever 89 to the lever 84.

The instrument 23, as shown in Fig. 2, includes a pilot valve mechanism comprising a chamber The pilot valve mechanism receives air from the pipe 43 and discharges air 1 through the vent 93 as required to maintain an the pipe I l to the furnace. I The remaining structure which is necessary in order to produce proper operation of the steam and fuel valves upon the reversal of theiurnace' consists of switches which are used to controlthe various solenoid valves that have beenmen- 'tioned. To this end, the valve I3 that is used a to reverse the supply of fuel from one end of the 5 furnace to the other has formed on .it cam I00 that serves to position a. cam follower IuI which is pivoted at, I02 andnormally biased into engagement with the cam by means of a spring. This follower has mounted on it a single pole,

I04 which is used to con- 8, and, indirectly, the operation of the solenoid valves.

As this motor moves to' the limitsoi' its travel after. operating the dampenswitch boxes a and I0; are operated by the rod 8-that is carried by, the motor, as has previously been described. Each of these switches boxes has in it two single pole, single throw switches. .The box 9 has in it a switch I05 thatis used to'control the opera tion of thesolenoid valves and has in it a, switch 106 that serves as a limit switch for the motor 8.

The box I0 has init a switch I01 that serves to I control the solenoid valves and a switch I08 that The in o, clockwise direction by a spring I09 and the box I0 is biased in a, counterclockwise direction by a spring Il0.

In the normal operation of the instrument the air supply comes through pipes 4| and 42 to the instrument 23, where it is modulated in accordance with the flow of the fuel to adjust the opensupply-constant. iii the roof temperature exthat upon reversal,

. be opened as shown by curve from the zero point to point a. The fuel "if Qvaive will be opened slowly as shown by the slow I of the solid curve to point b. This means that the injection steam is turned on it -,-drop in the curve at .fuel will be turned off and be used to completely prior to Place.-

. and steam valves I:

known manner to riage 56 will move up-scale to a point where it will engage the control carriage 51 and act through the mechanism 58 to operate the controller 63. This serves to reduce the pressure supplied through pipe 42 to the instrument 23 and therefore, to partially close the valve I2 and reduce the supply of fuel, normally the fuel is controlled entirely by the controller controller 21 accordingly acts as a limit controller to reduce the supply of fuel when the furnace roof gets too away with the be moved from the position shown in Figures 1 and 2 counter-clockwise to a; position in which the equalizer connection 50 is open. Air will then pass through pipes 49 and 50 and valve 38 directly to the pipe 42;

If it is desired to manually control the supply of fuel to the furnace, the valve 39 will be turned to a position in which the equalizer connection 50 isopen and the valve 40 will be rotated to connect pipes 42 and 45. Thereafter, the pressure applied to the fuel valve I2 can be regulated by means of the pressure regulator 38 to adjust the opening of the valve I2. During this time, the bleed restriction 54 will permit a reduction in pressure in order to permit closing of the valve. Without this restriction, pressure could only be built up in the system.

Upon reversal of the furnace, it is desired to quickly cut off the fuel supply from, for example,

the right end of the furnace, reverse the damper 5, and slowly turn on the fuel on the left end. After the fuel has been turned oil, from the right end, that burner must be purged in order to prevent a flare-back and there must be sufficient combustion is turned on at the left end. This operation is best shown by the curves in Figure 3 in which the solid line curve is that of the opening of the 'fuel valve and the dotted line curve is that of steam valve. It will be seen the steam valve will quickly the abrupt raise of the the opening of the the fuel is turned on so that as the fuel will immediately begin to burn, and the furnace will not be flooded. when reverssl of the furnace takes place, the fuel valve will be turned off quickly, as shown by the abrupt drop in the curve at ewhile the steam valve will be shut on more slowly as shown by the gradual d. This means that the the remaining steam before time that reversal actually takes wneagtne-rurnace is to be reversed, the fuel and II are'simultaneously rotated, either manually or automatically in any pipes I4 and I5, respectively to pipes-"Fahd 20. During this operation the low. portion of cam IIIII on the'handle of valve I3 will come under the follower IIII to permit. the follower to move in aclockwise direction. The switch I04 is therefore moved so that the contacts in its left end will be opened and those 23 and the hot. If it is desired to do' limit control, the valve 50 can air in the furnace before fuel I will be turned on full purge the bumersafter the motor 6 begins to rotate energize each of the solenoid end of the furnace and to connect the right end of the furnace with the stack. Immediately the arm 8 will move to the right enough to permit switches I" and I08 to close,

When the switch I01 closes, a. circuit will be completed through that switch and switch IIII t valves 48, I and When the solenoid valve 48 is energized, this normally closed valve is opened and air is exhausted from the pipe beyond the restriction 41 so that valve I2 can close, thus quickly shutting off the supply of fuel to the furnace. When solenoid valve 5| is energized, this normally opened valve will be closed to cut off the supply of air through pipe 48 to the valve I'I. At the same time, normally closed valve 52 will be opened so that air can bleed from the line 45 and valve I'I through the restriction 53 to the atmosphere. Thus the valve I! will be closed at a rate dependent upon the speed with which air can bleed through restriction 53. Thus the fuel valve is quickly closed, and the steam valve is closed at a slower rate permitting the burner to be purged.

The valves I2 and I1 remain closed until such time as arm 8 on the motor 8 moves to a position in which switch box 9 is moved in a counterclockwise direction. This opens limit switch III to stop the motor 6, and at the same time opens switch I05 to deenergize the three solenoid valves. When valves 5| and 52 are deenergized, air can immediately come through line 49 to valve H to quickly open this valve. Upon the closure of valve 48, air can be applied to the diaphragm of valve I2 at a speed depending upon the opening of restriction 41 thereby slowly opening this valve. At this time, the-furnace is firing from the opposite end, or the left end in Figure 1, at a rate depending upon the fuel controller 25. The steam has been'turned on immediately and the fuel oil has been turned on slowly so that the furnace will not be flooded before combustion of the fuel starts,

During the time that the furnace is operating the furnace pressure controller 29 operates to branch of the pipe noid valve 1 I2 which will permit this pipe to bleed to the atmosphere through an adjustable restriction II5.

Depending upon the fuel that is being used and the manner in which the furnace I is being operated. it may be desirable to lock the damper l! in place while the furnace is being reversed or it may be desirable to completely shut oi! the stack 4 during reversal since the reversing operation may cause a major disturbance in the furdirect the fuel and steamfrom a close at the same time in the right end will i be closed. This closes a circuit through those contacts and switch I06 to energize motor 5 for rotation in a direction to reverse the valve 5 to nace pressure. If it is desired to lock the damper 52 in place a switch III will be closed so that the normally open valve III will be energized to the pressure in. the lower part 34 is maintained durhowever, it is In this fashion, of pipe IIS and in cylinder ing the reversing operation. If, desired to completel close the damper the reversing operation, the switch II4 will also be closed so that when valve -I II is closed the normally closed valve III will be open to permit air to escape from the cylinder and to permit the reversal takes place.

durinl the damper 32 to be closed by operation of the weight 33. The speed with which the damper closes will depend upon the adjustment of restriction I I5.

From the above detailed description of the operation of my control system, it will be seen that I have provided a system which is adapted to control the nace at all times during the operation of this furnace, and to reduce the supply of fuel when the temperature of the roof reaches some dangerous point. I have also provided means to quickly turn off the fuel at one side of the furnace and to slowly turn it on at the other side when a reversal operation takes place. I have also provided means to control the furnace pressure during operation of the furnace and to interlock this control with the reversing mechanism to either lock the damper in place or completely close the damper during the reversal of the furnace.

While in accordance with theprovisions of the statutes, I have illustrated and described the Best form iof my invention now known to me, it will be apparent to those skilled in the art that changes may be made in the form of the apparatus disclosed without departing from the spirit of my invention as set forth in the appended claims, and that in some cases certain features of my invention may sometimes be used to advantage without a corresponding use of other features.

Having now described this invention, what I claim as new and desire to secure by Letters Patentis:

1. In a regenerative furnace controlsystem in which fuel and an injection fluid are supplied alternately to each end of the furnace, a pipe line to direct fuel to the furnace ends, a pipe line to direct injection fluid to the furnace ends, a pneumatic control valve in each of said pipe lines, a valve in each of said pipe lines to direct the material therein to either end of the furnace,

said latter valves being located between said pneumatic valves and the furnace whereby a single pneumatic valve in each of said fuel and injection fluid pipe lines may control the flow of material to both ends of the furnace, a supply of air to each of said pneumatic valves, electrically actuated means separately operative to control the supply of air to said pneumatic valves and means to simultaneously energize said electrically actuated means.

2. In a regenerative furnace-control system in which fuel and a fuel injection medium are supplied alternately to each end of the furnace,'a pipe line to supply fuel to both ends of the furnace, a reversing valve to direct fuel through said pipe line to one end or the other of the fur.. nace, a pneumatic control valve located in said pipe line on the supply side of said reversing valve, a supply of air under pressure to operate said pneumatic valve, means to restrict the flow of air to said pneumatic valve whereb said pneumatic valve may not be opened faster than some predetermined rate, and means operative to quickly exhaust the air supplied to said pneumatic valve whereby said pneumatic valve may close quickly, the arrangement being such that the fuel on either end of the furnace may be turned on slowly and turned oil. quickly by oper ation of the same pneumatic valve, depending upon the position of said reversing valve.

3.-In a regenerative furnace reversing system the combination of fuel supply lines to the opfuel supplied to a regenerative fur:

s end of the connect said means and stack to opposite ends of posite ends of the furnace, a pneumatic valve to control the supply of fuel, a supply of air for said valve, electrically operated means to control the supply of air to said valve, a pipe line to sup-- ply an injection medium to ends of the furnace,

a second pneumatic valve in said pipe line to control the supply of injection medium, other electrically operated means to control the supply of air to said second pneumatic valve, an air sup, ply duct for the ends of said furnace, a damper in said supply duct to direct combustion airto one end or the other of said furnace, a motor to, operate said damper, means to operate said motor and means operated by said motor to actuate said electrically operated means to close both of said pneumatic valves while said motor is running, s

4. In a regenerative furnace reversing system, the combination of fuel and injection medium supply lines, a pneumatic valve in each of said supply lines, a supply of air for each valve operative to open the same when applied to the valve, electrical means to control the application of air to each of said valves, an air supply duct for said I motor is running whereby said valves will close when said damper means is beingoperated.

5. In a regenerative furnace control system, the combination of means to supply air to either end of the furnace, a stack to be connected toelther furnace, damper means operative to the furnace, means to measure the pressure in the furnace, a second damper in said stack .to regulate the pressure of the furnace, means operated by said furnace pressure measuring means to adjust said second damper, means to reverse the furnace andmechanism operated by said last means to close said second damper while the furnace is being reversed.

6. In a regenerative furnace control system, means to supply combustion air to either end of the furnace, a stack adapted to be connected to either end of the furnace, damper means movable from a first position in which the combustion air is directed to one end of the furnace and the stack is connected to the other end to a second position in which the combustion air is directed to the second end and the stack is connected to the first end, means to measure the furnace pressure,

means to regulate the furnace pressure operated by said measuring means, a motor to drive said damper means from its first position to its second position and mechanism operated by said motor to lock said regulating means in place while said motor is operating. I

7. In a regenerative furnace reversing system, the combination of fuel and injection medium supply lines, a pneumatic valve in each of said supply lines, a supply of air for each valve and adapted to open the same when applied to each valve, electrical means to control the application of air to each of said valves, a damper to control the pressure in the furnace, a supply of air to con trol the operation of said damper, additional electrical means to control the application of air to said damper, reversing means to reverse the supoperate said reversing means, and means operated by said motor to actuate each of said electrical means while said motor is running.

8. In a regenerative furnace control system, the combination of means to supply fuel, an injection medium and air to each end of the furnace, a stack to be connected to either end of the furnace, air operated valve means to control individually the supply of fuel and injection medium to the furnace, reversing means operative to connect the supply of air to either end of the furnace, an air operated damper to regulate the furnace pressure, motor means to operate said reversing means, and

mechanism controlled by said motor to control' the supply of air to said valves and damper during the time that said motor is running to reverse the furnace.

9. line. regenerative furnace reversing system L having pipes to supply fuel to each end of the furnace, pipes to supply an injection medium'to each 1 end of the furnace, and havin an air duct to supply combustion air to each end of the furnace, a

valve in each of the pipes to supply fuel and injection medium to the furnace ends to direct the fuel and injection medium to one or the other of the furnace ends, a pneumatic valve in each pipe upstream from said previously mentioned valves, said pneumatic valves being opened by the application of air thereto, electrical means to control the application of air to said pneumatic valves, a damper in the air duct to direct the combustion air to one end or the other of the furnace, motor means to operate said damper, means operated upon operation of said first mentioned valves to reverse the fuel and injection medium to the furnace to start said motor means operating to operate said damper, and means operated by said motor means to enemize said electrical means during the time said motor means is operating to relieve the air pressure in said pneumatic valves.

JOSEPH P. VOLLRA'I'H.

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