US2002693A - Draft control - Google Patents

Description

E. T. DAHL DRAFT CONTROL May 28, 1935.

Filed Oct. 7. 19:52

4 Sheets-Sheet l May 28, 1935. i DAHL 2,002,693

DRAFT CONTROL Filed Oct. 7, 1932 4 Sheets-Sheet 2 lave/2601'."

May 28, 1935. E. T. DAHL 2,002,593

DRAFT CONTROL Filed Oct. 7, 1952 4 Sheets-Sheet 5 .Trzbezzior:

May 28, 1935. E. T. DAHL 2,002,693

DRAFT CONTROL Filed Oct. 7, 1952 4 Sheets-Sheet 4 In yen for!" By MQLZWL W WQA liar/a e ya.

Patented May 28, 1935 UNITED STATES PATENT OFFICE mesne assignments, to

wood, Mass.

Frank A. Morrison,

Eldon Macleod, West- Newton,

Mass.; Cameron Macleod, Berwyn, Pa.; Leslie Soule; Dedham, Mass.;

trustees, doing business as Needham, Mass.,

Sullivan A. Sargent, Jr.,

Mason-Neilan Regulator Company, Boston,

Mass.

Application October 7, 1932, Serial No. 636,709.

22 Claims.

This invention relates to control mechanism and more particularly to mechanism for controlling the draft in a furnace in accordance with variations in external draft conditions and in changes in the amount of fuel supplied to the furnace.

This invention is a modification and adaptation of the basic principle of control disclosed in my application, Serial No. 635,356 filed September 29, 1932, for Dual control mechanism, to which reference is hereby made. The invention herein may be classified as rate of flow control of furnace draft and is adapted nace and in stationary furnace, burning both solid and fluid fuels.

The object of the invention is to provide the proper air supply for complete combustion at all times. Another'object is to automatically main.- tain any desired draft differential between the stack and the air intake duct from the blowers or differential across the boiler tubes themselves necessary to' insure a high CO2 and a low CO content in the stack gases, and to control this draft differential in accordance with variations in the supply of fuel to the burner. Thus as the amount of fuel is increased and more fuel is burned, the draft control mechanism automatically increases the amount of air in the proper ratio to the fuel being burned to insure complete combustion. Likewise when less fuel is consumed, the mechanism automatically reduces the air supplied to again provide complete combustion without excess air. The amount of air supplied is also varied in accordance with external draft conditions.

As is well known, the natural stack draft varies considerably in accordance with external draft conditions such as the velocity and direction of i the wind and the atmospheric pressure. If no draft control mechanism is used, the blowers running at a constant speed will create an excessive draft differential whenever the stack draft increases, as when strong head winds are blowing. With my draft control mechanism, when external conditions increase the stack draft, the blowers may be run at slower speeds with resulting blower steam savings. My draft control mechanism is sensitive to the slighest variation in stack draft and automatically controls the supply of steam to the blowers so as to maintain the desired draft differential at all times. 7

An important advantage of my draft control mechanism is that it definitely eliminates the smoke nuisance. As all engineers know, the

presence of smoke belching from a ships stacks for use in marine furindicates incomplete combustion. Combustibles are passing through furnaces and out through the stacks without giving up their energy, and much fuel is thereby wasted. Furthermore, in many harbors, fines and other penalties are imposed upon ships for excessive smoking and these penalties may be avoided by the use of my draft control mechanism. Hitherto it has been common practice to eliminate the smoke nuisance by excessive air, and this practice is difficult to detect until the excess air becomes sufficient to create the tell-tale white smoke. But eliminating the smoke nuisance by use of excessive air is tremendously costly in its heat loss. It has been ascertained that an oil fired furnace temperature with a C02 of 13.7% in the fuel gases (15% indicating perfect combustion) is 3743 F., while with a C02 content of 8%, for example, the furnace has a temperature of 2302 F. Thus while the use of excessive air may avoid smoke, it conceals a heat loss of the furnace of 1441 F., or almost one third. By means of my draft control mechanism I am able to maintain the correct draft and air supply in accordance with variations in external draft conditions and in fuel supply at 25 all times and eliminate the smoke nuisance without heat loss.

It is impossible to maintain this correct draft differential manually except with extreme vigilance on the part of ciency of trial runs the fireman. The high cmof steamships, where expert control is provided, is seldom equalled in practice. Accurate indicating instruments must first inform the fireman what the boiler draft differential is before he can make the necessary blower speed changes. The fireman must then be constantly alert to note the stack draft, the blower speeds, the amount of oil, coal, or other fuel passing through the burner, and keeping all these factors in mind, set the blowers in accordance therewith. Except in trial runs, manually controlled blowers are not changed in accordance with every blower condition as is necessary for maximum economy and efficiency. My mechanism greatly simplifies boiler operation by automatically providing the correct draft differential in accordance with varying conditions .which is the most important and most diificultpart'of boiler operation.

Briefly, in the fluid fuel burning installationa furnace. The proper, preferably provided by I my mechanism includes draft for the furnace is a steam-driven blqwer mounted on the steam and a control valve is line to the blower. instrument responsive to rate of flow, such as a. 55

pair of 'sylphon bellows, is placed in the draft passage line of the furnace, one connection being in the air intake duct and the other at the stack, so that the instrument moves in accordance with the draft differential between the blower and the stack. A second instrument, sensitive to pressure, is placed in the fuel supply line to the burner. The rate of flow and pressure .responsive instruments then operate on the control valve through an independent pressure system, and through their resultant action to control the supply of steam operating the blower.

In the coal burning installation, the mechanism is the same, except that coal is supplied preferably by a steam-operated stoker, and the second instrument, sensitive to pressure, is connected to the steam pressure generated by the boiler, rather than to the fluid fuel supply line.

Before explaining in detail the present invention it is to be understood that the invention is not limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawings, since the invention is capable of other embodiments and of being practiced or carried out in various ways. Also it is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation, and it is not intended to limit the invention claimed herein beyond the requirements of the prior art.

In the drawings:

Fig. 1 is a diagrammatic view of my dual control mechanism as adapted for-rate of flow control of furnace draft in a stationary installation burning fluid fuel.

Fig. 2 is a diagrammatic view of my dual control mechanism as adapted for rateof flow control of furnace draft in a stationary iaistallation burning fluid fuel. 1

Fig. 3 's' a modification of the exhaust valve mechani in, utilizing, a diaphragm chamber.

Fig. 4 is adiagrarfmatic view ofmy' dual .control mechanism as'adapted'for rate of. flow con-. trol of furnace draft in amarine or stationary installation burning solid fuel.

- In Fig. 1 is shown my dual control mechanism adapted to a marine installation. I0 represents the mechanism for maintaining adequate and constant pressure of air for operating the independent pressure system which in turn operates the control valve on the steam line to the blower.

. the rate words, by the differential in pressure in the draft The air is supplied through pipe II and passes into the reservoir -I3 through reducing valve I2, which is preferably set for 15 pounds pressure. The air from reservoir I3 passes down the pipe I6 leading to a diaphragm mechanis '20 of the type described in Annin application erial No. 463,034 and a flexible branch pipe I'I having a fixed orifice I8 bypasses thestream of air. The mechanism III in Fig. 1 differs slightly from the mechanism I0 in Fig. 2, but in operation the two are the same. In Fig. 2 the mechanism is exactly the sameas that disclosed and described in my application, Serial No. 635,356, previously referred to. In mechanism I0 in Fig. 1, IIwrepresents a horsehair or felt strainer through which the operating air is passed and which removes all dirt and moisture from the air. The independent pressure system ordinarily exhausts to the atmosphere through the exhaust or orifice I9, but is controlled by the flapper 56 actuated by of flow of furnace draft or, in other between the stack and air intake duct from the blower. The independent pressure is diverted in 25 from increasing. Air

I8 which leads into a central diaphragm chamber 22. A stem 24 abutting the top side of the diaphragm chamber 22 forms a valve between pipe I6 and chamber 25. A similar stem 25 abutting the bottom side of the diaphragm chamber 22 forms an exhaust valve between the chamber 21 and the atmosphere. Both chambers are connected on the outside by pipe 28 which leads through pipe 29 to the control valve 30. A gauge 3| is connected to pipe 29 and records the pressure in said pipe. Control. valve 30, which is preferably a diaphragm motor valve, controls'the admission of steam or other heating medium to the heating tank 36.

'On the closing of exhaust orifice I9, pressure in diaphragm chamber 22 builds up, forcing valve 26 closed and valve 24 open, allowing pressure from pipe IE to enter directly into chamber 25, flow through connecting pipe 28 into chamber 21 and equalize the pressures therein. When pressure in chamber 25 balances the force in central chamber 22, valve 24 closes to prevent pressure then flows through pipes 28 and 29 to control valve 30, the pressure of which on the diaphragm 30a. opens the valve 30 and permits the steam to flow to the pump I02. Valve 30 as here shown opens on movement down.

Liquid fuel is supplied to the burner I40 through the line I30. This fluid fuel may be oil, gas or oil combined with pulverized coal or any other suitable fluid fuel, but for simplicity the fuel will be spoken of herein as oil. A pressure responsive instrument (which may be merely a connection which transmits the original pressure of the oil) is placed in the fuel supply line I30 before the fuel passes to the burner I40, and connects with the helical coil 39, which may be in the form of a Bourdon tube or other pressure responsive device. Coil 39 is held rigidly fixed at one end, and tothe other end of the coil which is free to move is rigidly attached an arm 40 with a sliding wedge 4| thereon placed in abutting relation to one end of lever 42 which is fulcrumed about point 43. (Fulcrum the exhaust valve comprising orifice I 9 and flapper 56, as distinguished from the disclosure in my application, Serial No. 635,356, wherein fulcrum 43 is between the exhaust valve I9 and 56 and the sliding wedge 4|.) secured to lever 42. The position of wedge 4| on arms 40 is adjustable to provide various ranges of control. An instrument I60 responsive to rate of flow is connected in the draft passage through the boiler. The instrument I60 may be any suitable mechanism, sensitive to rate of flow or a difference in pressure, such as a mercury manometer, diaphragm or a pair of sylphon bellows.

Herein the instrument I50 comprises a pair of' and is pivoted to one end of arm 53 at the point I68 and on the other end of arm 53 is placed the sliding wedge 54. Arm I69 rests on the top of bellows I6I and is also pivotally connected to arm 53 at the point I10 between sliding wedge 54 and the point I68.

43 this time is outside a Exhaust orifice I9 is rigidly The device operates in the same manner as that disclosed in my application, Serial No.

635,356, previously referred to, except that the action is transmitted through instruments responsive to different characteristics. As pressure in fuel line I30 tends to rise, helical coil 39 unwinds, and acting through arm 40 and lever 42 causes orifice I9 to open partially, permitting control valve 30 to partially open, thereby increasing the amount of steam supplied to blower I50, and consequently the amount of air to take care of the increased amount of fuel being supplied to the burner I40. On a decrease in pressure on the fuel supply line I30, the action is opposite.

If the draft in the air intake duct -I66 from the blower I50 tends to increase, this effect is transmitted to the bellows I6I, raising arm I69. This elevates-sliding wedge 54 rotating the arm 53 about the point I68 as a fulcrum so as to cause flapper 56 to partially close nozzle I9. The independent air pressure is then diverted, causing control valve 30 to partially close. This reduces the speed of the blower I50 sufficiently to maintain a constant draft condition in air intake duct 30 and the proper resultant or differential in pressures between air intake duct I66 and the stack I64. If, on the other hand, external wind conditions cause the stack draft to decrease (the pressure at the stack normally being negative), this will cause bellows I62 to contract so as to allow arm I61 to move downwardly, and thereby elevating the wedge 54, with the point I10 now acting as the fulcrum. Flapper 56 is again caused to partially close orifice I3, again diverting the independent pressure to control valve 30, partially closing the same and again reducing the speed of the blower I50 sufficiently to maintain the constant desired draft differential between the stack and the air intake -duct I66.

As can be readily seen, this balanced draft system controls the speed of the blower I50 so as to provide the required draft differential in accordance with changes in the fuel oil pressure, and with changes in the stack draft and the air intake duct pressure. The system is entirely self-compensating, and the variable factors caused to-counterbalance one another so as to prevent over-controlling, which would be noticeable in fluctuations of the draft differential and consequent CO2 content of the fiue gases. Fluctuations in the pressure and consequent rate of flow of the fuel supply are compensated for by changing the speed of the blower I50, through the action of the independent pressure system operating on the control valve 30-on the steam supply line 35. The rate of speed of the blower I50 is likewise changed from the same independent pressure system acting on the'control valve 30 in accordance with the changing differential in pressure between the stack draft and the air intake duct, as caused by external wind conditions. The speed of the blower is controlled therefore to provide the proper air supply for complete combustion at all times through the joint and resultant action of the two instruments, one being responsive to changes in pressure of the fuel supply, and the other responsive to changes in the draft through the furnace. The ratio between the changes in fuel oil pressure and the corresponding increments of changein the blower speeds is readily adjusted. Similarly, the sliding wedge 54 may be moved along so as to obtain any desired change of blower speed in relation to an increment of draft variation. A In' Fig. 2,'I have shown the same rate of flow control of furnace draft adapted to a stationary installation. The construction and operation of this installation is exactly the same except that the forced draft supplied by the steam-operated blower is eliminated, and the control valve 30 instead of operating the supplyof steam 'to the blower operates a simple damper mechanism I80, thereby controlling the draft through the furnace. Also the supply of fuel oil to the burner is iere regulated by the fuel regulator I85, of any suitable type. The regulator shown herein is actuated by the boiler drum steam pressure, and it automatically supplies additional fuel oil with a decrease in boiler steam pressure, and vice versa. Therefore, the desired CO2 content of the flue gas-es is automatically maintained. The mechanism I0 for maintaining a constant and adequate pressure of air for operating the independent pressure system is shown exactly as that disclosed in my application, Serial No. 635,356, and differs slightly from the same mechanism shown in Fig. 1 herein. However, the functions of the two mechanisms are the same and it is understood that they are interchangeable. The damper mechanism I80 may be placed in the stack if desired, and it should be understood that the forced draft can be dispensed with even in a marine installation.

Again in the stationary installation, the draft control mechanism automatically maintains the desired draft differential between the air intake duct and the boiler stack by automatically resetting the damper I80. ilt the same time it is so inter-connected that afvariation in pressure or rate of flow of the fuel oil to the burners also automatically resets the damper so as to provide the correct supply of air for the fuel consumption as varied. Again my draft control mecha nism correlates the movement of the damper so as to provide the required draft in accordance with the boiler steam pressure, the fuel oil supply pressure, the stack draft and the air intake duct draft. It is entirely self-compensating, as one condition tends to balance the other, and this prevents the over-controlling common in other installations.

In Fig. 3, I have shown a modification of the exhaust valve mechanism of the independent pressure system, which utilizes a diaphragm chamber instead of a pair of sylphon bellows. The operation of the diaphragm chamber, however, is exactly the same in effect. Diaphragm chamber 200 has a centrally flexible membrane 20I hinged at point 20Ia and dividing the chamber into two parts, upper chamber 202 and lower chamber 203. Suitable springs or plates should be connected with membrane 20I to make it incline normally downwardly from the hinge 20Ia, as is well known in'mechanism of this type. The pipe I63 connects upper chamber 202 to the stack I64,

and lower chamber 203 connects through pipe I65 to the air intake duct I66 from the blower. A stem 204 extends downwardly through the top of upper chamber 202 and rests on the membrane 20I. At its upper end stem 204 is pivotally attached to lever 42, the fulcrum on 43 (Fig. 1) being omitted. Lever 42, bearing flapper 56, is fulcrumed therefore atthe point 205 at the top of stem 204 and at the contact point with sliding wedge 4I, nozzle I9 and the cooperating flapper 56 being intermediate the two fulcrum points. Therefore, as lever 42 is tilted by either arm 40 the steam header of the boiler,

or stem 204, nozzle I9 is partially opened or closed in accordance with the resultant or diiferential between the movements of arm and stem 204.

Diaphragm chamber 200 operates exactly as the pair of sylphon bellows IGI and I62 of Fig. 1, the upper chamber 202 corresponding to bellows I62, and lower chamber 203 corresponding to bellows l6l. The proper resultant or diflerential in pressure between air intake duct l 66 and stack I64 is maintained as before, but by using the diaphragm chamber 200 in place of the sylphon bellows.

In Fig. 4, I have shown the same rate of flow control of furnace draft adapted to a solid fuel burning installation. The construction and operationof this installation is similar to that shown in Fig. l, in that a forced draft is supplied by the steam operated blower I50 but the pressure coil 39 instead of being operated by the pressure of fuel oil is operated by the steam pressure from A steam operated stoker 220 is also provided for supplying coal or other solid fuel, as needed. The exhaust valve mechanism of the independent pressure system is shown operated by a diaphragm chamber 200 as in Fig. 3, but it is understood that a pair of sylphon bellows as in Figs. 1 and 2 may be substituted.

The furnace 2l0, of any suitable construction, has steam tubes 2| I therein, baflies 2I2 to provide a circuitous passage for the burning gases around the steam tubes 2| I, and a stack 2 l3 with adamper 2M therein. A steam header 2 l5 0on nects with the pressure coil 39 to operate the same. The air line 29 from the independent pressure system now operates three motor valves 30, preferably of the diaphragm type, one valve 30 operating the damper 21 4 in the stack, another controlling the supply of steam for operating the blower I50, and the third controlling the supply of steam operating the stoker 220. The blower I50 and stoker 220 may be operated from the same engine if desired. Again the diaphragm chamber 200 is connected to two different points in the furnace to register the rate of flow between the two points. The pipe I63, for instance, connects the upper chamber 202 to the point 202:; in the furnace and pipe I65 connects the lower chamber 203 to the point 2030. in the furnace.

The operation of the coal-burning installation is similar to the others. The draftcontrol mechanism automatically maintains the desired draft differential between the two points 202a and 203a. in the furnace 2l0 by automatically resetting the damper 2 l4 and changing the speed of the blower and the stoker. At the same time, it is so interconnected that a variation in the steam pressure generated by the furnace or boiler also automatically resets the damper, blower and stoker so as to increase or decrease the heat units supplied to the furnace together-with the corresponding proper supply of air for combustion, as required to maintain a given pressure of steam in the header 2l5. Again my draft control mechanism correlates the movement of the damper, blower and stoker so as to provide the required draft in accordance with the boiler steam pressure and changes in external draft conditions.

I claim:

1. In a draft control system for a fluid fuel operated furnace, a steam driven blower for supplying air to the furnace, a'source of steam supply for operating the blower, a control valve on the steam supply line controlling the supply of steam to said blower, a pilot mechanism utilizing an independent source of pressure for operating said control valve and ordinarily exhausting to the atmosphere, an exhaust valve governing the exhaust from said pilot mechanism comprising a movable orifice and flapper, levers for moving said orifice and flapper toward and away from each other and so operating the exhaust valve, an instrument responsive to the pressure of the fuel supply, an instrument responsive to the rate of flow of air through said furnace, a

coil actuated by the pressure responsive instrument for moving the orifice lever, an arm actuated by the rate of flow responsive instrument I 2. In a draft control system for a fluid fuel operated furnace, a steam driven blower for supplying air to the furnace, a, source of steam supply for operating the blower, a control valve on the steam supply line controlling the supply of steam to said blower, a pilot mechanism utilizing an independent source of pressure for operating said control valve and ordinarily exhausting to the atmosphere, an exhaust valve governing the exhaust from said pilot mechanism comprising a movable orifice and flapper, levers for moving said orifice and flapper toward and away from each other and so operating the exhaust valve, an instrument responsive to the pressure of the fuel supply, a pair of sylphon bellows responsive to the rate of flow of air through said furnace, a helical coil actuated by the pressure responsive instrument for moving the orifice lever, an arm actuated by the'difference in pressures between said bellows for moving the flapper lever and so actuating the exhaust valve of the independent pressure system through the resultant action of said pressure responsive instrument and pair of sylphon bellows.

3. In a draft control system for a fluid fuel operated furnace, a damper controlling the supply of air to the furnace, a control valve regulating the position of said damper, a source of fuel supply for the furnace, a pilotmechanism for utilizing an'independent source of pressure for operating said control valve and ordinarily exhausting to the atmosphere, an exhaust valve governing the exhaust from said pilot mechanism comprising a movable orifice and flapper levers for moving said orifice and flapper toward and away from each other and so operating said exhaust valve, an instrument responsive to pressure of the fuel supply, an instrument responsive tothe rate of flow of air through said furnace, a coil actuated by the pressure responsive instrument for moving the orifice lever, an arm actuated by the rate of 'flow responsive instrument for moving the flapper lever and so actuating the exhaust valve of the independent pressure system through the resultant action of said governing the exhaust from said pilot mechanism comprising a movable orifice and flapper on'levers for moving said orifice and flapper toward and away from each other and so operating said exhaust valve, an instrument responsive to pressure of the fuel supply, a pair of sylphon bellows responsive to the rate of flow of air through said furnace, a helical coil actuated by the pressure responsive instrument for moving the orifice lever, an arm actuated by the difference in pressures between said bellows for moving the flapper lever and so actuating the exhaust valve of the independent pressure system through the resultant action of said pressure responsive instrument and pair of sylphon bellows.

5. In a draft control system for a fluid fuel operated furnace, a damper controlling the supply of air to the furnace, a control valve regulating the position of said .damper, a source of fuel supply for the furnace, automatic means for governing the fuel supplyto the furnace in accordance with boiler steam pressure, a pilot mechanism for utilizing an independent source of pressure for operating said control valve and ordinarily exhausting to the atmosphere, an exhaust valve governing the exhaust from said pilot mechanism, and means responsive to the resultant effect of pressure of the fuel supply and difference in pressure in said furnace at the air intake duct and at the stack for actuating said exhaust valve.

6. In a draft control system for a furnace, means for supplying air to the furnace, a control valve governing said air supply means, a pilot mechanism utilizing an independent source of pressure for operating said control valve and ordinarily exhausting to the atmosphere, an exhaust.

valve governing the exhaust of said pressure to the atmosphere, a supply and waste valve in said pilot mechanism for supplying and wasting pressure to and from said control valve, diaphragm means in said pilot meclfanism acted on by said pressure for actuating the supply and waste valve, and joint means responsive to the resultant effect of the pressure of the fuel and the difference in pressure in said furnace at the air intake duct and at the stack for actuating said exhaust valve.

7. In a draft control system for a furnace, means for supplying air to the furnace, a control valve governing said air supply means, a pilot mechanism utilizing an independent source ofpressure for operating said control valve and ordinarily exhausting to the atmosphere, an exhaust valve governing the exhaust of said pressure to the atmosphere, a supply and waste valve in said pilot mechanism for supplying and wasting pressure to and from said control valve, diaphragm means in'said pilot mechanism acted on by said pressure for actuating the supply and waste valve, an instrument responsive to the pressure of the fuel supply and another instrument responsive to the difference in pressure in said furnace at the air intake duct and at the stack, and joint means operated by the resultant action of said instruments for actuating the exhaust valve of the pilot mechanism.

8. In a draft control system for a fluid fuel operated furnace, a steam driven blower for supplying air to the furnace, a source of steam supply for operating the blower, a control valve on the steam supply line controlling the flow of steam to said blower, a pilot mechanism utilizing an independent source of pressure for operating said control valve governing the exhaust of said pressure to the atmosphere includingan orifice and flapper, a supply and waste valve in said pilot mechanism for supplying and wasting pressure to and from said control valve, a diaphragm chamber in said pilot mechanism acted on by said pressure for actuating said supply and waste valve, levers for moving said orifice and flapper toward and away from each other and so operating the exhaust valve, an instrument responsive to the pressure of the fuel supply, an instrument responsive to the rate of flow of air through said furnace, a coil actuated by the pressure responsive instrument for moving the orifice lever, an arm actuated by the rate of flow responsive instrument for moving the flapper lever and so actuating the exhaust valve of the independent pressure system by the resultant action of said instruments.

9. In a draft control system for a fluid fuel operatedfurnace, a damper controlling the supply of air to the furnace, a control valve regulating the position of said damper, a source of fuel supply. for the furnace, a pilot mechanism utilizing an independent source of pressure for operating said control valve governing the exhaust of said pressure to the atmosphere including an orifice and flapper, a supply and waste valve in said pilot mechanism for supplying and wasting pressure to and from said control valve, a diaphragm chamber in said pilot mechanism acted on by said pressure for actuating said supply and waste valve, levers for moving said orifice and flapper toward and away from each other and so operating'the exhaust valve, an instrument responsive to the pressure of the fuel supply, an instrument responsive to the rate of flow of air throughsaid furnace, a coil actuated by the pressure responsive instrument for moving the orifice lever, an arm actuated by the rate of flow responsive instrument for moving the flapper lever and so actuating the exhaust valve of the independent pressure system by the resultant action of said instruments. 7

10. In a draft control system for a furnace, means for supplying air to the furnace, a control valve governing said air supplying means, an independent pressure system for operating said valve, means for supplying fuel to the furnace, an instrument responsive to the rate of fuel supply, an instrument responsive to the difference in pressure in said furnace at the air intake duct and in the stack, and means operated by the resultant action of said instruments controlling the indeplying means. 5

11. In a draft control system for a furnace, means for supplying air to the furnace, a control valve governing said air supplying means, a pilot mechanism utilizing an independent source of pressure for operating said control valve and ordinarily exhausting to the atmosphere, an exhaust valve governing the exhaust from said pilot mechanismmeans for supplying fuel to the furnace and means responsive to the resultant effect of the rate of fuel supply and of the difference in pressure in said furnace at the air intake duct and in the stack for actuating said exhaust valve.

12. In a draft control system for a furnace, means for supplying air to the furnace, a control valve governing said air supplying means, a pilot mechanism utilizing an independent source of pressure for operating said control valve and by the resultant action of said instruments for actuating the exhaust valve of the independent pressure system.

13. In a draft control system for a furnace, means for supplying air to the furnace, a control valve governing said air supplying means, a pilot mechanism utilizing an independent source of pressure for operating said control valve and ordinarily exhausting to the atmosphere, an exhaust valve governing the exhaust from said pilot mechanism, means for diverting said pressure for operating said control valve on the closing of said exhaust valve means for supplying fuel to the furnace, an instrument responsive to the rate of fuel supply, an instrument responsive to the difference in pressure ,in' said furnace at the air intake duct and in the stack, and joint means operated by the resultant action of said instruments for actuating the exhaust valve of the independent pressure system.

14. In a draft control system for a furnace, means for supplying air to the furnace, a control valve governing said air supplying means, a pilot mechanism utilizing an independent source of pressure for operating said control valve and ordinarily exhausting to the atmosphere, an exhaust valve governing the exhaust from said pilot mechanism, a supply and waste valve in said pilot mechanism, diaphragm mechanism acted on by said pressure for actuating said valve to divert said pressure to the control valve to operate the same on the closing of the exhaust valve means for supplying fuel to the furnace, an instrument responsive to the rate of fuel supply, an instrument responsive to the diiference in pressure in said furnace at the air intake duct and in the stack, and joint means operated by the resultant action of said instruments for actuating the exhaust valve of the independent pressure system.

15. In a draft control system for a furnace,

, means for supplying air to the furnace, a control valve governing said air supplying means, a pilot mechanism utilizing an independent'source of pressure for operating said control valve and ordinarily exhausting to the atmosphere, an exhaust valve governing the exhaust from said pilot mechanism comprising a relatively movable orifice and flapper, levers for moving said orifice and flapper toward and away from each other and so operating the exhaust valve means for supplying fluid fuel to the furnace, an instrument responsive to the pressure of the fuel supply, an instrument responsive to the difierence in pressure in said furnace at the air intake duct and in the stack, a coil actuated by the pressure responsive instrument for 'moving the orifice lever, an arm actuated by the instrument responsive to the difference in pressure in the furnace for moving the flapper lever and so actuating the exhaust valve and the independent pressure system by the resultant action of said instruments.

16. In a draft control system for a furnace, means for supplying air to the furnace, a control valve governing said air supplying means, a pilot mechanism utilizing an independent source of pressure for operating said control valve and ordinarily exhausting to the atmosphere, an exhaust valve governing the exhaust from said pilot mechanism, said pilot mechanism including a casing, a pair of diaphragms dividing said casing into two outer chambers and an inner chamber, a supply valve and a waste, valve in said outer chambers governing the supply waste of said pressure in said system, said pressure expanding said inner chamber and actuating said supply and waste valves to divert said pressure to the control valve for operating the same on the closing of said exhaust valve means for supplying fuel to the furnace, and joint means responsive to the rate of fuel supply and to the difference in pressure in said furnace at the air intake duct and at the stack for actuating said exhaust valve.

17. In a draft control system for a furnace, means for supplying air to the furnace, a control valve governing said air supplying means, a pilot mechanism utilizing an independent source of pressure for operating said control valve and ordinarily exhausting to the atmosphere, an exhaust valve governing the exhaust from said pilot mechanism means for supplying fuel to the furnace, an instrument responsive to the rate of fuel supply, aninstrument responsive to the difference in pressure in said furnace at the air intake duct and in the stack, said pilot mechanism including a casing, a pair of diaphragms dividing said casing into two outer chambers and an inner chamber, a supply valve and a waste valve in said outer chambers governing the supply and waste of said pressure in said system, said pressure expanding said inner chamber and actuating said supply and waste valves to divert said pressure to the control valve for operating the same on the closing of said exhaust valve, and joint means operated by the resultant action of said instruments for actuating the exhaust valve of the independent pressure system.

18. In a draft control system for a solid fuel burning furnace, a steam driven blower and stoker supplying air and fuel to the furnace, a damper in the stack of said furnace a source of steam supply for operating said blower, stoker and damper, control valves on the steam supply line controlling the supply of steam to said blower, stoker and damper, a pilot mechanism utilizing an independent source of pressure for operating said control valve and ordinarily exhausting to the atmosphere, an exhaust valve' governing the exhaust from said pilot mechanism, means for diverting said pressure for operating said control valve on the closing of said exhaust valve, an instrument responsive to pressure of the steam generated in the furnace, an instrument responsive to the difference in pressure in said furnace at the air intake duct and at the stack, joint meansoperating upon the resultant action of said instruments for actuating the exhaust valve of the independent pressure system.

19. In a draft control system for a solid fuel burning furnace, a steam driven blower and stoker supplying air and fuel to the furnace, a damper in the stack of said furnace, a source of steam supply for operating said blower, stoker and damper, control valves on the steam supply line controlling the supply of steam to said blower, stoker and damper, a pilot mechanism utilizing an independent source of pressure for operating said control valve and ordinarily exhausting to the atmosphere, an exhaust valve governing the exhaust from said pilot mechanism utilizing an independent source of pressure for operating said control valve and ordinarily exhausting to the atmosphere, a supply and waste valve in said pilot mechanism for supplying and wasting pressure to and from said control valve, diaphragm means in said pilot mechanism acted on by said pressure for actuating the supply and waste valve, an instrument responsive to the pressure of steam generated in the furnace, an instrument responsive to the difference in pressure in said furnace between the air intake duct and the stack and joint means operated by the resultant action of said instruments for actuating the exhaust valve of the pilot mechanism,

20. In a draft control system for a solid fuel burning furnace, a steam driven blower and stoker supplying air and fuel to the furnace, a damper in the stack of said furnace, a source of steam supply for operating said blower, Stoker and damper, control valves on the steam supply line controlling the supply of steam to said blower, stoker and damper, a pilot mechanism utilizing an independent source of pressure for operating said control valve and ordinarily exhausting to the atmosphere, an exhaust valve governing the exhaust from said pilot mechanism utilizing an independent source of pressure for operating said control valve and ordinarily exhausting to the atmosphere, a supply and waste valve in said pilot mechanism for supplying and wasting pressure to and from said control valve, levers for moving said orifice and flapper toward and away from each other and so operating the exhaust valve, an instrument responsive to the pressure of steam generated, an instrument responsive to the difference in pressure in said furnace at the air intake duct and at the stack, a coil actuated by the pressure responsive instrument for moving the orifice lever, an arm actuated by the differential pressure instrument for moving the flapper lever and so actuating the exhaust valve of the independent pressure system by the resultant action of said instruments.

21. In a draft control system for a solid fuel burning furnace, a steam driven blower and stoker supplying air and fuel to the furnace, a damper in the stack of said furnace, a source of steam supply for operating said blower, stoker and damper, control valves on the steam supply line controlling the supply of steam to said blower, stoker and damper, a pilot mechanism utilizing an independent source of pressure for operating said control valve and ordinarily exhausting to the atmosphere, an exhaust valve governing the exhaust from said pilot mechanism utilizing an independent source of pressure for operating said control valve and ordinarily exhausting to the atmosphere, said pilot mechanism including a casing, a pair of diaphragms dividing said casing into two outer chambers and an inner chamber, a supply valve and a waste valve in said outer chambers governing the supply and waste of pressure in said system, said pressure expanding said inner chamber and actuating said supply and waste valves to divert said pressure to the control valve for operating the same on the closing of said exhaust valve and means responsive to the resultant effect of the pressure of steam generated in said furnace and to the difference in pressure in said furnace at the air intake duct and the stack for actuating said exhaust valve.

22. In a draft control system for a solid fuel burning furnace, a steam driven blower and stoker supplying air and fuel to the furnace, a

damper in the stack of said furnace, a source of steam supply for operating said blower, stoker and damper, control valves on the steam supply line controlling the supply of steam to said blower, stoker and damper, a pilot mechanism utilizing an independent source of pressure for operating said control valve and ordinarily exhausting to the atmosphere, an exhaust valve governing the exhaust from said pilot mechanism utilizing an independent source of pressure for operating said control valve and ordinarily exhausting to the atmosphere, an instrument responsive to the pressure of steam generated in the furnace, an instrument responsive to the difference in pressure in said furnace at the air intake duct and at the stack, said pilot mechanism including a casing, a pair of diaphragms dividing said easing into two outer chambers and an inner chamber, a supply valve and a waste valve in said outer chambers governing the supply and waste of pressure in said system, said pressure expanding said inner chamber and actuating said supply and waste valves to divert said pressure to the controlvalve for operating the same on the closing of said exhaust valve, and joint means operated by the resultant action of said instrument for actuating the exhaust valve of the independent pressure system.

. EDWARD T. DAHL.

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