US2328498A - Combustion control system - Google Patents

Description

Aug. 31, 1943.

G. w. SAATHOFF 2,328,498

COMBUSTION CONTROL SYSTEM Filed April 3; 1941 3 Sheets-Sheet l Aug. 31, 1943. w. SAATHQFF 2,328,498

COMBUSTION CONTROL SYSTEM Filed April 3, 1941 5 Sheets-Sheet 3 4 r 148/ =li 9. A

. Iv m 54 P/ l F fi 712 Patented Aug. 31, 1943 UNITED STATES PATENT OFFICE COMBUSTION CONTROL SYSTEM George W. Saathofl, South Orange, N. J.

Application April 3, 1941, Serial No. 386,663

7 Claims.

Thi invention relates to control systems and more particularly to control systems adapted to automatically control the rates of delivery of fuel and air to the furnace of a steam boiler or generator in accordance with the demand for steam and which will include means modifying the combustion rate in accordance with the rate of supply of feed water and variations in heat value in the fuel as reflected by steam pressure in the boiler drum and steam header.

Eificient operation of modern boilers requires that the heat input to a boiler shall follow as closely as possible the heat output of a boiler in the form of steam with the maintenance of substantially constant steam pressure at the point where steam is consumed, which in the case of a turbine would be at or near the inlet to the control valve of the turbine. To accomplish this nicety of balance between heat input and heat output as steam, the combustion rate and the rate of delivery of water to the boiler must be accurately coordinated from the standpoint of control. The combustion rate, of course, is determined by the rate of delivery of fuel and air for combustion of the fuel, but this rate is subject to change brought about by variations in the heat value of the fuel, and consequently provision should be made to correct for such variations, as well as other variations in rate of B. t. u. delivery to the furnace.

When the rating on a boiler is increasing, that is when the demand for steam is increasing, the rate of delivery of feed water must be increased also; but to take care of the increase in demand for steam and at the same time heat the feed water to the temperature of the water in the boiler, the combustion rate must be so stepped up or increased that enough heat is developed to generate the amount of steam required with the least possible drop in steam pressure in the steam header and also to raise as quickly as possible the temperature of the incoming feed water to the temperature of the water in the boiler at the pressure existing therein. Therefore, when the boiler load is increasing the control of the combustion rate, i. e. the rates of delivery of fuel and air, must be increased not only from steam pressure conditions which reflect the demand for steam but also partly from the rate of delivery of feed water to the boiler. The control system must also be so designed that changes in the air and fuel delivery rates may be modified to compensate for variations in the B. t. u. value of the fuel. Where solid fuels are employed it is impractical to continuously determine B. t. u. value,

but since changes in B. t. u. value are reflected in steam pressure the control system may be so designed that the fuel feed rate is modified until the combustion rate is restored to the value required to maintain the rate of steam generation and the pressure of the steam at the required values. Therefore a feature of the control system should be that it is capable of correcting the B. t. u. delivery rate until it is in balance with the air delivery rate required by the boiler load without disturbing either the rate of delivery of feed water or air for combustion.

When the load on a boiler is decreasing, the control system must respond quickly to so de crease the firing rate that th balance between heat input and heat output is not materially disturbed, other than temporarily and then for as short a time as possible, in orderthat the steam pressure at the point of use by turbines or other equipment will not depart too much from the value desired, and so that the deviation from the desired value will not be of long duration.

It is quite common practice to operate a number of small boilers in parallel or two or more large boilers in parallel. In either case, and particularly With the larger modern boilers, high over-all efiicienc requires that each boiler deliver its proportionate share of the total steam load. The features of the control system above outlined which have been stated in terms of function and performance must therefore be designed to control one or more boilers delivering steam to a common distribution header in such fashion that each boiler will pick up or drop load in the proportion that its capacity bears to the total load demand for steam.

It is, therefore, an object of this invention to provide a control system that shall be capable of controlling the combustion rate of a boiler furnaceby controlling the constituents which are supplied to support and maintain combustion and to so correct or modify the combustion rate to compensate for heat requirements of feed water delivered to the boiler as the load increases to the end that a predetermined balance may be maintained between the demand for steam and the heat input to the boiler or boilers.

Another object of the invention is to incorporate in the aforesaid control system, means for compensating for variations in the B. t. u. delivered and to supplement the control of the firing or combustion rate so that when the boiler load is increasing and calling for an increase in the rate of delivery of water to the boiler or boilers the firing rate will be increasedover and above that required to generate the steam demanded to oifset, thereby the tendency to lower the temperature of the water which is in the boiler and which has been heated to a temperature corresponding to the steam pressure. This supplemental control shall be so designed and connected up with the control system that its control function will diminish as the demand for steam levels off to a steady value and when this steady value is reached will no longer partially control the combustion rate. This supplemental control shall also be operative when the steam demand is decreasing so as to promptly decrease the combustion rate and prevent the steam pressure in the distribution header from rising above a certain predetermined value.

A still further object of the invention is to provide a control system that will primarily control the draft in accordance with steam pressure in the distribution header, which control is supplemented from the rate of delivery of feed water to the boiler, and which will control the rate of delivery of fuel to the furnace in accordance with the value of the furnace draft and to supplement the control of the fuel delivery rate in accordance with the steam pressure in the boiler drum and the rate of air flow through the boiler.

Other objects of the invention will in part be obvious and will in part be apparent from the following description taken in conjunction with the accompanying drawings in which:

Figure l is a more or less diagrammatic view of a steam boiler provided with a control system also more Or less diagrammatically illustrated, embodying what now appears to be a preferred form of the invention;

Fig. 2 is a diagrammatic view, partly in section, of a steam pressure-responsive master sender embodied in the control system;

Fig, 3 is a view in section of a sending head associated with the feed water control valve shown in Fig. 1;

Fig. 4 is a view'partly in section of the uptake draft damper regulator shown in Fig. i;

Fi 5 is a partially detailed view showing the operating mechanism for the pilot valve of the regulator in Fig. 4;

Fig. 6 is a partially detailed view of the operating mechanism for the pilot valve of the primary air damper regulator, the latter being as shown in Fig. 4;

F g. '7 is a view partly in section of the forced draft damper regulator shown in the system of Fig. 1;

Fig, 8 is a view in section showing the construction of the air flow and furnace combustion chamber pressure regulators shown in the system of Fig. l; and

Fig. 9 is a view in section of a ratio relay embodied in the system of Fig. l.

Throughout the specification and drawings like reference characters indicate like parts.

In Fig. 1 of the drawings a boiler furnace is illustrated to which a control system embodying the invention may be applied.

The steam generated in the boiler is collected in a steam drum 2 from which it flows through a superheater 3 to the steam-distribution header *1. Where two or more boilers are operating in parallel the outlets of the superheaters of these boilers are connected to header l. In order that steam-consuming equipment connected to the distribution header may operate more eiiiciently and the operation'thereof more accurately controlled the steam pressure in the distr bution header should be maintained substantially constant under all conditions or rates of demand for steam.

Since the flow of steam through the superheaters results in a drop in pressure it is obvious that unless the boiler is so operated that the pressure in the steam drum of the individual boilers is caused to increase with the flow of steam to the distribution header, the pressure in the distribution header will fall.

The control system which will be described hereinafter, is designed to so control the combustion rate of each furnace that the pressure in the steam drum of each boiler will increase as the rate of steam delivered to the distribution header increases so that the steam drum pressures will exceed the steam header pressure by an amount equal to the pressure drop through the superheaters of the boilers. If it is assumed that the load on the boiler or boilers is zero or at stand-by rating then there will be substantially no flow of steam from the steam drum of each boiler through its superheater to the distribution header. Under such circumstances, the pressure drop through the superheater of each boiler will be substantially zero and the pressure in the sl-istribution header and in the steam drum of each individual boiler will be substantially equal. However, the pressure drop througheach individual superheater is of the order of pounds per square inch or higher when each boiler is delivering its maximum of steam, then it will be apparent that as the load on a boiler increases the pressure in the steam drum of each boiler must be increased by an vmnount equal to the pressure drop through the superheater.

The arrangement of the control system and its novel features which will accomplish the objecti stated above will be apparent as the desc'lipt. n of the invention'progresses.

Air for supporting combustion is delivered a forced draft fan 5 into a duct which leads to a preheater I wherein the air is heated by gases of combustion discharging into the uptake 8 of the furnace. The heated air then discharges into a header 9 from which it flows into a duct H] to a wind box H serving the burners i the furnace. Aportion of this air goes directly into the furnace and this portion is referred to by boiler furnace operators as secondary air. The secondary air is that which is supplied for maintaining combustion at the "desired standard of efficiency. A portion of the air supplied by forced draft fan 5 is delivered into a pipe 134170 which the intake of a primary fan '54 is connected and which delivers this air to a pulverizer mill l5 and serves to carry the pulverized fuel into a pipe i6 to the burners of the furnace. The :primary air may therefore be called the carrier :air and it represents ordinarily about :15 per cent of the total amount of air required for combustion. The air delivered tothe primary air fan is controlled by .a damper ll operated by a primary air damperregulator H3.

The rate of new of air through the furnace is regulated by dampers i9 loc'ated'in uptake ductffi which leads to the intake of .aninduced draft fan 25. The induced :draft fan discharges the products of combustion to a stack not shown.

The dampers lie in the uptake duct-are positioned by a receiving regulat which is controlled from a control impulse transmitted by a master sender 22 that responds to the pressure of steam 'in steam header 4. For each value-of steam'pressure in thesteam-Theader there will :be

a different but definite value of control impulse so that the receiving regulator will adjust the position of draft dampers [9 to a definite position corresponding to the particular value of steam pressure in the steam header.

If it be assumed that the boiler is delivering steam at a rate corresponding to one-half capacity, then the pressure in the steam header should be at the particular value desired, in which case the draft dampers should be approximately in their mid-controlling position. With these assumed conditions it will be apparent that if the steam output of the boiler increases to maximum capacity, receiving regulator 2| will move draft dampers l9 to their Wide-open position or to the maximum position that they can be moved to and still have a control effect on the draft. Likewise, if the steam demand decreases below the assumed one-half capacity receiving regulator 21 will move draft dampers l9 progressively as required to their ultimate minimum open position. Thus, the change in steam pressure in the distribution header will decrease slightly from the value desired at one-half steam-generating capacity as the steam load increases and will increase only slightly above this value when the load on the boiler is reduced to zero or stand-by condition. Thus, if the maximum change in steam pressure in the steam header is 50 pounds per square inch then by setting the control system in so far as it affects draft damper regulator 2|, the steam pressure will decrease only say 25 pounds below the value desired at one-half steam-generating capacity of the boiler and will rise only 25 pounds per square inch above this value when the load on the boiler is reduced to zero or stand-by rating.

The amount of air delivered by forced draft fan 5 to the wind box of the furnace should be just enough to maintain the pressure in the combustion chamber of the furnace substantially constant, and this pressure is usually carried at about atmospheric. The rate of forced draft air is controlled by a damper 23.

The forced draft fan damper is positioned by means of a receiving regulator 24 which in turn is under the control of a furnace combustion chamber pressure regulator 25. Regulator responds to difference between the pressure in the combustion chamber of the furnace and atmospheric pressure and transmits a control force to the forced draft damper regulator 24, causing it to adjust the forced draft damper in such direction that the pressure in the combustion chamber of the furnace is maintained substantially constant.

Any change in position of the draft dampers in uptake duct 8 of the furnace will, of course, change the pressure in the combustion chamber of the furnace, and this change in pressure is responded to by the furnace draft regulator 25 which in turn sends a control impulse to the forced draft damper regulator 24, causing it to so position forced draft damper 23 that the pressure within the combustion chamber is restored to the value desired.

The primary air regulator i3 is controlled jointly in accordance with the rate of air flow through the furnace, which flow conveniently may be measured by an air flow regulator 26 that is connected to respond to the pressure drop across preheater i, and in accordance with variations in steam pressure in steam drum 2. A master sender 21 which responds to steam pressure in the steam drum of the boiler transmits control forces to the primary air control regulator. The operation of this master regulator is modified in accordance r drum 2.

3 with the air flow impulse set up by regulator 26 and this loading so modifies the combustion rate at all boiler ratings above stand-by that the pressure in the steam drum will be higher than the pressure in the steam header by an amount substantially equal to the pressure drop through the superheater.

By causing the primary air damper regulator l 8 to respond jointly to the pressure impulse sent out by steam drum master sender 21 and the air flow regulator 25, the primary air will be delivered to the furnace at a rate determined not only from the instantaneous value of air flow through the furnace but also from the actual pressure in steam From the above it will be seen that the control system functions to so perform the following operations that the rates of delivery of fuel and air to the furnace are maintained at values determined by the demand for steam at the point of use while holding the steam pressure at the points of use substantially constant. If it be considered that when the demand for steam is at the minimum value, the pressure in steam header 4 is at the value desired to be maintained at all times, then it follows that as soon as the load or the demand for steam increases there will be a tendency for the pressure in the steam header to decrease because of the drop through the superheater. This drop in pressure is responded to by steam header master sender 22 which in turn sends a control impulse to the receiving regulator 2| causing this regulator to open draft dampers 19 to a position corresponding to the value of the impulse sent out by the master sender. This results in an increase in rate of air flow through the boiler and such increase in air flow is reflected in a change in pressure drop across preheater 7. Air flow regulator 26 responds to this pressure drop and sends an impulse to the primary air damper control regulator 18, causing it to open up the primary air damper I 7 whereby the flow of primary air through the primary air fan is increased. Fan Id, therefore delivers more air through the pulverizer and this increase in air flow increases the flow of pulverized fuel to the burners of the furnace. The control impulse from air flow regu lator 26 also acts on a fuel-air ratio regulator 29 that sends out a loading pressure whose value or magnitude is in predetermined proportion to the air flow impulse. The effect of such increase in loading on master sender 2"! is as though the steam pressure in the steam drum had decreased. This causes master sender 27 to send out an impulse to the primary air damper control regulator l8 which tends to further increase the flow of primary air to the primary air fan 14, whereby still further increase in the flow of fuel to the burners of the furnace is accomplished and in advance of a drop in pressure in drum 2.

When the flow of steam from the boiler to the point of use increases the water level is responded to by a water level regulator W. L. R. which through its lever V. L. operates a feed water control valve 3! in a direction to produce an in-- crease in the flow of feed water to the boiler. A regulator P. D, V. R. of known type may be utilized to hold the pressure drop across valve 3| constant by actuating a valve P. D. V. in accordance with variations from a predetermined valuev of pressure drop across valve 3 l. Thus each position of the valve stem of valve ill will be a measure of the rate of flow of water into the boiler. A pressure-sending device 3| is operatively connected to stem 32 of the feed water valve so that as the feed water valve opens a pilot valve 33 on this sending head is positioned to send an impulse to receiving regulator 2| and cause it to move draft dampers l9 farther towards open position. The control force from device 3! tends to cause the receiving regulator 2| to open the dampers H! by an amount that will increase the air flow sufficiently to raise the combustion rate in the furnace by an amount required to raise the temperature of the incoming feed water to the temperature of the water in the boiler. Thus, the increased combustion rate takes care of not only the heat required to raise the temperature of the boiler water which is introduced as feed water to the temperature of the water in the boiler, but it also takes care of the heat required to maintain demand for heat output in the form of steam taken from the steam header. Thus the pressure in boiler drum 2 will not fall off but will tend to rise as the load increases, whereby the pressure in the steam header will be maintained substantially constant. In other words, the firing rate is increased at such a rate that the pressure in the steam drum will be increased by an amount sufficient to take care of the pre sure drop through the superheater.

In order that the loading on the steam drum master sender 21 shall not be increased too rapidly when a load increase takes place, means are provided whereby the pressure impulse sent out from the air flow regulator to. the fuelair ratio relay 2!? is delayed for a predetermined length of time. This is accomplished by placing a time-delay device such as a needle valve 342 and a check valve in parallel in a sending line 36 which transmits the control impulses from regulator 2,6 to. the fuel-air ratio relay 29. A volume chamber 3'! is placed between check valve 3.5 and needle valve 34 and fuel-air ratio relay 29. so that the fuel-air ratio relay will operate more accurately and responsively to changes in the air-flow impulses sent out from the air flow regulator 26.

The control impulses sent out by the flow regulator 2% (see Fig. 8) increase as the air flow through the furnace increases, and decrease as the air flow decreases. Thus, it will be observed, when the impulses from this regulator are increasing the check valve closes, causing the air impulse to pass through delays the building up of pressure in volume chamber Ill. As soon as the pressure increases in volume chamber 31 relay 29 is unbalanced,

causin it to send out an increased pressure to the loading element of the steam drum master it, This loading pressure operates to produce an effect on the master sender similar to that which would be produced if. the pressure in the steam drum had decreased. Thus the loading pressure acts as an anticipator for changes in pressure in steam drum 2, which stated another way, means that the system anticipates the de mand for steam and begins to increase the combustion rate before the increased output of steam has decreased the pressure in steam drum 2. A steam boiler has an accumulatoreffect so that increase in steam output does. not immediately reduce the steam drum pressure; therefore, theanticlpator feature of the control system has time to function in a direction to increase the steam pressure in drum. 2.

Control of the draft on the furnace, as stated previously, is effected by the uptake draft receiving regulator 25 which positions dampers l9; in the uptake ofthe furnace. in accordance-with needle valve 34 which r control impulses delivered thereto both by the steam header master sender 22 and the sending head 31 operated by the feed water valve 31. The uptake draft dampers can be moved in either direction in response to a change in either the pressure transmitted by master sender 22 or by sending head 3|.

Control of primary air, which in turn determines the rate of delivery of fuel to the furnace, may be accomplished through change in control impulses sent out by either master sender 21 and by air flow regulator 26, or both. Regulator 26 comes into play to change the control position of regulator l8 and damper H as soon as the air flow through the furnace has been changed by dampers l9, and master sender 21 modifies this control position, under normal conditions, as soon as the time-delay of the needle valve 34 has been wiped out. But if the pressure in steam drum 2 decreases from the value which should obtain for the rate of air flow called for the steam delivered by the boiler, master sender 2'! will operate to set regulator l8 and damper H in a position that will increase the rate of fuel delivery and thereby increase the firing rate to a value that will increase the pressure in drum 2 to the value required. The above-described modifying action of master 2? will compensate for change in B. t. u. value in the fuel, as where the B. t. u. value decreases, and it will also compensate for changes in steam drum pressure not attributable to decreasing B. t. u, value of the fuel.

The general description of the control ystemso far stated has been confined mostly to a single boiler; however, it will be obvious that a bank of boilers may be provided with a similar system and operated in parallel. \Vhere a bank of boilers is involved the. header master would control each uptake regulator 23 and this control would be supplemented by a sending head 3 l associated with the feed water control valve provided for each boiler, but in other respects each boiler would have a drum master 2?, air flow regulator 25, furnace pressure regulator a primary air regulator it, a ratio relay 29 with the valves 34 and 3.0 and volume tank 3".

Also it will be understood that each boiler furnace may have more than one pulverizing mill, which is indicated by the notations on pipes 53a and i317, and these additional mills would serve burners for the furnace.

Having thus described the general arrangement or" the control system and its mode of op-- eration, the regulators, master senders and ratio relay embodied therein Will now be described from which description the operation of each apparatus will be readily understood.

The steam header master sender and the steam drum master sender are both similar in construc-- tion and operate in substantially the same manner. The difference between them, however, is merely inthisthat the steam drum master sender is provided with a loading element which exerts a loading'iorce on the master sender in accordance with the value of the sending control force sent out by the fuel-air ratio relay. The con- Sl-lfllClliQllOfl'thES-G two master senders may, therefore, be illustrated in one view as in Fig. 2.

The uptake draft regulator 25 andv the primary air damper'regulator it are similar in construc-- tion and function in the same manner except that in the case of the primary damper controlregulator, the control impulse sensitive members are arranged. a little differently in their mounting. The construction of these two regulators is shown in Fig. 3 and in Figs. 4 and 5 the arrangement of the mechanisms that operate the pilot valves thereof.

The forced draft damper regulator 24 is similar in construction to regulators l8 and 2|, except that the mechanism for operating the pilot valve is different.

In order to simplify the specification and to explain the function of the control system, the master senders which are responsive to the steam header and steam drum pressures respectively, the uptake draft damper regulator, the primary air damper regulator, the fuel ratio relay, the furnace draft and other regulators will now be described.

Master senders 22 and 27 (see Figure 2) The master sender which responds to the steam header pressure comprises a beam 38 which is mounted on a knife edge 39 carried by push rod 40 which is secured to a steam-pressure responsive bellows 4| disposed within a steam pressuretight housing 42. When the steam pressure is zero or below the value at which it is desired to maintain the steam header pressure, the bellows tends to expand and moves the beam downwardly (clockwise) under the influence of a tension spring 43. This tension spring resists counterclockwise movement of the beam when steam pressures are increasing and tends to turn the beam clockwise when steam pressures are decreasing. At a point above the beam and a little to one side of the knife edge 39 a knife-edged fulcrum 44 is mounted which bears on the top side of the beam when the beam is being actuated counterclockwise by the steam pressure-responsive bellows 4|. Movement of the beam as above described is utilized to actuate a pilot valve 45 that controls the value of pressure control force transmitted to a sending line 46 leading to a pressure-sensitive element associated with a valve actuating mechanism for the uptake damper regulator 2|. The source of supply of pressure impulse controlled by valve 45 is received from a pressure supply pipe 48 in which a pressure medium such as compressed air is maintained at a substantially constant value, say a value of 60 pounds per square inch. The particular value of pressure in the supply pipe is one dictated by design of the apparatus to be actuated by these pressure impulses, and is usually such as will give the range of pressures required.

When the pressure acting on bellows 4| is a minimum, beam 38 is urged clockwise in which case pilot valve element 49 closes inlet port 5|] and opens an exhaust port 5|. Under these conditions the pressure medium in the sending pipe 46 exhausts to the atmosphere and will reach a minimum value equal to atmospheric pressure so long as the inlet of the pilot valve is closed.

As the steam pressure in the housing .2 rises, bellows 4| is compressed, turning beam 38 counterclockwise, whereby inlet port 50 is uncovered and exhaust port 5| is throttled, thereby allowing pressure medium to flow through the inlet port into the valve body and thence to sending line 46. A certain amount of the pressure medium escapes through the exhaust port so that the pressure established within the valve body in the space between the inlet and exhaust ports will be of a value determined by the relative JISSSUIB drops through the inlet port and the exhaust port. As the steam pressure rises or continues to rise and bellows 4| is further compressed, beam 38 is turned counterclockwise still farther until eventually pilot valve element 49 is in a position to completely close exhaust port 5|, in which case inlet port will be fully open and the pressure within the valve body and sending line 43 will have reached its maximum value. Therefore, when beam 38 is in a position intermediate the positions corresponding to minimum and maximum pressures in the sending line 46, the pressure in the sending line will be of a value intermediate these maximum and minimum values. Therefore, for each position of the steam pressure-responsive bellows 4| and beam 38, the pressure in the sending line will be correspondingly different but definite in value.

From the above it will be observed that when the steam pressure is at a maximum value the sending pressure in sending line 45 will be at maximum value and this value of sending pressure reduces the combustion rate by reducing the uptake draft to minimum value. Likewise when the steam pressure is such that pilot valve element 49 is positioned to completely close inlet port 53, the pressure in sending line 45 will be at its minimum value and the uptake draft dampers will be positioned to establish maximum draft for the boiler and maximum firing rate.

The steam drum master sender 21 is similar in construction to the master sender 22 just described except that it is provided with a loading element 53 which tends to oppose the force exerted by the steam pressure-responsive bellows on the beam. This leading element is preferably in the form of a pressure-flexible bellows 54 disposed Within a pressure-tight housing 55 to be actuated by impulse pressures delivered to a line 56 by the fuel-air ratio relay 29. As the pressure on this loading bellows increases, it exerts a force on a plunger 51 having a knife edge 58 bearing on beam 38 and tends to urge the beam in a clockwise direction, thereby moving the pilot valve in a direction to throttle inlet port 55 thereof and reduce the pressure transmitted through its sending line 46 to the primary air damper control regulator. Loading bellows 54 is loaded in accordance with the pressure impulse transmitted by air-flow" regulator 26. The impulses transmitted by the air-flow regulator are directly proportional to the pressure drop across air preheater l so that as the air flow through the boiler increases in response to opening-up of the uptake draft dampers l9 the load on loading bellows 54 will be increased.

The function of fuel-air flow ratio relay 29 is to provide a range of loading impulses for loading bellows 54 bearing a certain predetermined relationship to the range of values of im pulses transmitted by air-flow regulator 26 into line 35. In other words, the impulses transmitted to the loading bellows 54 may be made equal to the impulses transmitted by the air-flow regulator 25, or the ratio relay, by preadjustment may be caused to send out higher pressure impulses to loading bellows 54 which impulses in turn bear a definite relationship to the impulses sent out by the air-flowregulator, or by different preadjustment fuel-air ratio relay 29 may be caused to deliver pressure impulses to loading bellows 54 or master sender 21 that are less in magnitude than the impulses sent out by the air-flow regulator but still bearing a predetermined relationship to control impulses sent out by air-flow regulator 25.

If the steam demand on the boiler is increasing from a certain steady value the tendency is for the pressure in steam drum 2 to decrease and it would decrease if the primary air and fuel feed were not increased before this pressure drop actually occurs, and in some cases the pressure in the steam drum might drop too far and this in turn would cause a corresponding drop in the steam header. The control system herein disclosed is so arranged that as soon as the air flow through the boiler is increased by opening dampers I9 either in response to action by master sender 22, or feed-water-actuated sending head 3!, or both, master sender 21, being loaded in accordance with air flow by action of regulator 2S and relay 29, supplements the fuel delivery rate by causing damper H to be opened wider than it would have been opened in response to the air-flow impulse of regulator 26 alone. Thus, the steam pressure drop in steam drum 2 is anticipated and the firing rate is stepped up to a rate that will result in an increase in steam pressure in steam drum 2 as the steam output of the boiler increases.

Sending head 31 (see Figure 3) As shown in Fig. 3 sending head 3| connected operated as to increase the pressure impulses delivered to the pilot-valve-operating mechanism of uptake draft damper regulator 2!. When maximum pressure sent out by this sending head occurs, the feed water rate to the boiler is at a minimum and minimum pressure occurs when the feed water rate delivered to the boiler is at a maximum.

Sending head 3| comprises a pressure-tight housing 60 through the upper end of which a rotatable shaft Bl extends, a stufiing box 62 being provided to prevent control impulse medium from escaping past the shaft to the atmosphere. The inner end of this shaft is threaded and carries a traveling nut 53 to which a tension sprint,

64 is connected. This tension spring is connected 0 at its free end to a pressure-sensitive diaphragm 65 disposed to close the opposite end of the housing. The interior of housing 60 is connected to the sending impulse line through the tapped opening 68.

Pilot valve 33 is disposed for operation by a lever El connected to stem 32 of the feed Water valve and this lever is pivotally supported on a bracket 53 carried by the diaphragm. The other end of the lever 6'! is connected to stem 69 of pilot valve 33. Valve 33 has an inlet port 1i to which impulse medium such as a source of compressed air at constant pressure, is connected an exhaust port H and an outlet port 12 which is connected to the interior of the housing. When valve stem 69 is in neutral position, the inlet and the exhaust ports of this valve are closed. When the feed water valve is being moved towards closed position to reduce the quantity of water delivered to the boiler, lever 61 is turned counterclockwise, whereby valve stem 69 is moved upwardly, allowing air under pressure to flow from the supply source through inlet port 10 and outlet port 12 into the interior of the housing from which this pressure is communicated to sending line 59 and thence to the pilot-valveoperating mechanism of uptake draft damper regulator 2|. As the pressure in the housing builds up, diaphragm 65 is pushed downwardly against the tension of spring 64 until the pilot valve stem 59 is returned to neutral position. The pilot valve will remain in this position until the feed water valve has moved again in a closing direction. When this occurs the pilot valve stem 69 will again move upwardly, admitting more air pressure to the interior of housing 65, thereby increasing the pressure therein and in sending line 59 connected thereto. This pressure increase moves diaphragm 65 downwardly again until pilot valve stem 69 is returned to neutral. This operation will be repeated over and over until the feed water valve has reached its minimum closed position. When the feed water rate to the boiler is increasing, lever 6! is turned in a clockwise direction, in which case the air supply port 10 is closed and exhaust port ll is uncovered, thereby allowing pressure medium to escape from sending line 59 and the interior housing Bil to the atmosphere. When the pressure has decreased a certain amount, tension spring 84 will pull diaphragm 65 upwardly and shift pilot valve 69 to neutral position. The pilot valve will remain in this position until the feed water valve is opened wider. Thus, as the feed water valve continues to open wider and wider, pilot valve stem will be moved from neutral to exhaust and from neutral to exhaust positions repeatedly until a stable position of the feed water valve is reached or until the feed water valve has reached its wide-open position, in which case the pressure in housing Eli) and in sending line 59 will be at its minimum value. It will thus be apparent that the value of sending impulse transmitted from the sending head 3 l may vary in infinitely small steps over the range between minimum and maximum pressures, and vice versa.

Regulator 21 (see Figures 4 and 5) The uptake draft damper regulator 2! comprises a cylinder 73 within which a piston i4 is disposed for movement in either direction under the control of a pilot valve (5 which when moved upwardly from its neutral position causes power medium to be introduced at the upper end of the cylinder on the top side of the piston, causing it to move downwardly, or when moved downwardly from its neutral position permits the power mediurn entrained in the upper end of the cylinder to be exhausted and power medium to be admitted to the lower end of the cylinder so that the piston is caused to move upwardly. A compensating or cut-off mechanism 16 disposed for operation in response to movement of the piston, is so connected with pilot valve 15 that when the piston has moved a predetermined amount in either direction, pilot valve 55 is returned to neutral position, whereby further movement of the pilot to one or the other of its on positions is required before the piston moves farther one direction or the other.

Piston M has a piston rod 'l'l which extends through one end of the cylinder and it carries a frame comprising upper and lower crossheads l8 and iii and side rods 88 and 81 which are secured to the crossheads. The compensating mechanism 16 includes an angling bar 82 carried by side rod and is disposed to operate a lever 83 which is pivotally supported from the cylinder. The upper end of lever 33 carries a roller 84 which rides on the inner face of angling bar 82 and the other end of the lever is connected to a spring 85 which is under tension and urges lever 83 in a direction in which roller 84 will be in contact with the angling bar. A lever 86 is connected "to an arm 81 extending at right angles from lever '88. One end of lever 86 is connected to stem 86 of pilot valve 15 and the other end thereof is connected to a link 89 which is operatively connested to pressure-sensitive elements 98 and 9| by means of a floating lever 92. Link 89 may be connected at any point between the ends of float- 'i'ng lever 92, depending upon the relative amount of control which one or the other of the pressuresensitive elements are to have over the full op= erating range of the regulator 24.

The pressure-sensitive elements 90 and 9] which respond to the sending impulses sent out from steam header master sender 22 and sending head 3| associated with feed water valve 3| may comprise resilient bellows which are urged in a direction tending to collapse or compress them by means of springs 94 and 95, and these springs may be adjusted manually as to total compressure required. When the pressures in these bellows are zero or at minimum value the compression springs urge them downwardly to compress them, pulling floating lever 92 and link 89 downwardly. When moving downwardly, lever 85 turns counterclockwise and moves pilot valve stem 88 upwardiy to admit pressure from pressure supply source 91 to the upper end of the cylinder. Piston M will then move downwardly but as it moves downwardly, compensating lever 83 will be turned clockwise by angling bar 82, whereby lever 86 is also turned clockwise. As lever 86 turns clockwise valve stem 88 is returned to neutral position and the piston comes to rest. Thus, when pressures in the bellows 90 and 9| are at minimum values piston 14 of the regulator will come to rest at the bottom of the cylinder. When in this position the uptake draft dampers will be in their maximum open position and this position of the uptake dampers corresponds to the maximum firing rate on the boiler that will be required when the steam demand on the boiler is at a maximum.

As the pressures in bellows 90 and 9| begin to increase floating lever 92 will move upwardly and move pilot valve stem 88 downwardly to a position at which the upper end of the cylinder is connected to exhaust port 98 and the lower end of the cylinder is connected to the power supply medium causing the piston to move upwardly. As the piston moves upwardly angling bar 82 turns compensating lever 83 counterclockwise in which case lever 86 will turn clockwise and move pilot valve stem 88 towards or to neutral position at which time the piston will come to rest and remain so until the pilot valve is again moved to its position in which power medium is introduced in the lower end of the cylinder. The piston will continue to move upwardly so long as the pressure in either one or both of the pressureresponsive elements 98 and 91 increases.

If one of these bellows 98, 9| expands a given amount and the other contracts by the same amount, the net movement of the floating lever 92 will be zero, when link 89 is connected to the midpoint of lever 92, and the pilot valve stem 88 will not be shifted out of neutral position. By connecting the pilot valve operating link 89 to floating lever 92 at any predetermined point on either side of its midpoint, it will be apparent that the operation of the regulator may be placed more predominately under the sending impulses of master sender 22 than under the impulses from sending head 3|, or vice versa. In this system, however, it is preferred that regulator 2| be predominently controlled by master sender 22.

As a general rule, when a boiler is operating at full rating and the maximum rate of feed water is being delivered to the boiler, about 10% of the total fuel burned is needed to raise the temperature of the incoming feed water to the temperature of the water in the boiler at the pressure existing in the boiler. Therefore, the amount or control necessary to be exerted by the sending head 3] on regulator 2| need be about only 10% of the total control over regulator 2 I.

With these assumed conditions, the pilot valve link 89 should be connected to the floating beam in such position that the control force sent out by the sending head 3| to element 9| will increase th draft in the uptake of the furnace about 10% over that provided by the pressure impulse sent out from the steam header master sender 22 to element 99. For the same reason th impulse sent out by sending head 3| should also be such that the firing rate of the boiler can be decreased by about 10% when necessary as when the rate of supply of feed Water to the boiler is decreased suddenly or reduced to a value sufficient to balance the amount of steam withdrawn from the boiler where the load has steadied down to a constant value or has decreased slightly.

Primary draft regulator 18 (see Figures 4 and 6) Regulator I8 is provided with an operating mechanism for its pilot valve similar to that just described for the uptake draft damper except that these elements are connected in reverse sense to the floating lever as shown in Fig. 6. Bellows 98a and em are connected in reverse sense because as the air flow through the boiler increases, the pressure impulses from the air-flow regulator 26 increase when increasing these control impulses should cause regulator [8 to move downwardly to open damper 17, whereas the pressure impulses transmitted from the steam drum master sender 21 decrease with increasing boiler load. Therefore, since the pressure impulses from air-flow regulator 26 and from steam drum master sender 21 vary in opposite sense, the pressure-sensitive elements 90a and 9! a should be reversely connected so that the movement of either one or both will be additive in so far as movement of pilot valve stem 88 is concerned.

When the air-flow impulse delivered to element am from air-flow regulator 26 is at a minimum corresponding to minimum air flow through the boiler minimum combustion rate in the boiler, th control impulse issuing from the steam drum master sender will be at a maximum value. Under these conditions primary air damper i! should be near its minimum closed position. Under these conditions it will be apparent from 6 that floating lever 92 will be moved upwardly in a direction to more pilot valve stem 88 downwardly to admit pressure power medium to the lower end of the cylinder and cause the piston to travel upwardly to the highest position required under these conditions. If the pressure impulse from steam drum master sender remains at a ximurn but the air flow through the boiler begins to increase, then the pressure impuls sent out by this regulator will increase, bellows 9la operated in response to this pressure will commence to ex pand, and floating lever 232 will tend to turn counterclockwise about its pivotal connection to bellows Thus, pilot valve stem 83 is moved in an upward direction to permit power medium to flow into the u per end of cylinder 7'53 cause piston to in e downwardly. The piston will mo e downwardly until the com ensating mechanism described in connection witn the uptake draft damper regulator 2i returned the pilot valve to neutral position.

The control force or impulse transmitted by air-flow regulator 26 to bellows 9 la of primary air regulator i8 is, as described above, directly proportional to the rate of air flow. The value of the control force transmitted by drum master sender 2; to bellows Sim of the regulator it will be inversely proportional to the pressure of the steam in steam drum 2 under conditions where the air flow has acquired a steady value. If the load on the boiler is constant and the air flow through the boiler is constant but for some reason or the other the quality of the fuel changes, i. e. the B. t. u. value of the fuel decreases, the pressure in steam drum 2 will drop. This drop in pressure is immediately responded to by drum master sender 2'! and the impulse transmitted thereby to the primary air draft regulator bellows Silia is changed so that the primary air draft damper is opened wider to increase the rate of delivery of fuel to the furnace. This will raise the combustion rate, that is increase the B. t. u. input to the furnace, to a value at which the steam drum pressure will be restored to the value required to maintain the pressure in the steam header constant and at the desired value above the steam header pressure.

If the load on the boiler increases from a value which has been steady for a while or for a time long enough for the control system to come to equilibrium, the first adjustment made is in the position of the draft dampers I9. These are opened to increase the draft called for by control force established by master sender 22 in response to the drop in steam pressure in the steam header caused by increased steam flow to the steam-consuming units. As soon as draft dampers 19 are opened to a wider position, the air flow through the boiler increases and the air-flow regulator transmits an impulse immediately to bellows Sla of the primary air regulator [8. The rate of delivery of fuel is therefore increased and the combustion rate is increased. As soon as the pressure within the combustion chamber of the furnace drops as the result of a wider opening of draft dampers 19, the furnace pressure regulator 25 transmits an impulse through sending line $9 to forced draft damper regulator 24, causing this regulator to open forced draft damper 23 until the pressure within the combustion chamber of the furnace is restored to the constant value desired. While this readjustment in the position of forced draft damper 23 is taking place, the pressure impulse transmitted by air-flow regulator 26 to volume tank 31 has been building up until it has reached a value sufficient to unbalance the fuel-air ratio relay 29. This unbalancing of fuel-air ratio relay 29 changes the loading on bellows 54 of the drum master sender El, causing it to modify its control impulse to a value which would have been established had the steam drum pressure dropped. Therefore, the fuel-air ratio relay causes drum master sender 2! to send out an impulse to element 953a of regulator 18 corresponding to the value that would have been sent out for an equivalent lower steam pressure and this impulse is established before the steam pressure in the steam drum has decreased to a value that would correspond to this new impulse. The primary air regulator l8 opens damper ll to a position of wider opening and the combustion rate is increased sufficiently to raise the steam pressure in drum 2 to the value called for by the air-flow loading on bellows When the steam output of the boiler increases the rate of delivery of feed water to the boiler must also be increased, which requires the open ing of feed water valve H to a higher flow position. This valve is operated by a device that responds to the level of the water in the boiler but is not illustrated in the drawings because it is so well known in this art. Vfhen the feed water valve opens sending head 3i associated therewith is actuated so that an impulse is transmitted to bellows 95] of draft damper regulator 2i, causing this regulator to move dampers l9 to a wider open position and increase the draft, as described previously herein.

Thus, if we assume the load on the boiler as increasing from stand-by values at a steady rate of increase, the position of draft dampers l9 will be changed or moved towards wide-open position at a rate corresponding substantially to the rate of increase of load on the boiler and this position will be under the control of both the rate of delivery of feed water to the boiler and to the rate of increase of steam required as refiected by slightly decreasing steam pressures in steam header Q. As the draft dampers are opened the air flow through the boiler increases and this increase in air flow is responded to by the air-flow regulator 26 to increase the rate of delivery of fuel to the furnace.

Regulator 24 (see Figure 7 Regulator 24 is similar in construction to regulators l3 and El and differs merely in the mechanism employed for operating its pilot valve too. The supply of power medium which is connected to inlet ill! of the valve is introduced into the upper end of power cylinder )2 when the pilot valve stem W3 is moved downwardly and into the lower end of the cylinder when the valve stem is moved upwardly. The valve is returned to neutral position by angling bar Hi4 and a bell crank Hi5.

Pilot valve stem [63 is moved downwardly by a diaphragm lilii against the resistance of a compression spring iii! in response to pressure impulses delivered thereto by regulator 25. Spring lhl is disposed between diaphragm Eat and a support socket N38 carried by bell crank HES. Thus, if, when the piston of cylinder N32 is in'its lowermost position, a pressure is applied to diaphragm and valve stem W3 is moved downwardly, the piston will rise, and as it rises bar hi4 turn bell crank i515 clockwise, and moves spring socket M8 upwardly, whereby spring liii is compressed and diaphragm I85 and valve stem Hi3 are returned to their neutral positions. The piston the comes to rest. Further upward movement the piston will take place step by step only the result of ever increasing pressures be" delivered thereto by regulator 25 through line 953. If the pressures in sending line 99 decrease from a high value to successively lower and lower values, pilot valve stem m3 will be moved upwardly under the preponderance of the force of spring IE7, but through the action of hell crank I05 and bar I04 the compression of the spring is decreased to balance the pressure force on diaphragm I06 resulting in a return of the pilot valve to neutral position. Thus, the regulator piston will move step by step in the downward direction and each position of rest will be determined by each lower value of pressure impulse supplied to diaphragm I06 by regulator 25.

Regulator 26 Regulator 26 comprises a pressure-tight housing I I in which a flexible diaphragm III is disposed dividing the housing into pressure-tight chambers H2 and H3. Chamber H2 is connected by a pipe II4 to the interior of the furnace but on the upstream or high-pressure side of the preheater 1 and chamber H3 is connected by pipe I I5 to the interior of the furnace but on the downstream or low-pressure side of the preheater. Thus the diaphragm is subjected to the pressure drop produced by the flow of combustion gases through air preheater 1 and, therefore, to the total air flow to the furnace. The upper side of the diaphragm is connected by a link I I6 to a pilot valve I II similar in construction and mode of operation to valve 45. A lever I II! which is supported at one end on a knife edge H9 and suspended from an adjustable tension spring I at the other end, is connected to link Il6 by means of a knife edge I2I that bears on the top side of the beam. Spring I20 is so ad justed that it balances the weight of all parts connected to diaphragm III and still holds the diaphragm in a position in which exhaust port I22 is closed and inlet port I23 is wide open when minimum furnace air flow and minimum pressure drop on diaphragm III are established. Thus, for each increase in furnaceair flow and corresponding pressure drop across preheater 1, diaphragm III will be moved downwardly to a different but definite position. Temporary resistance to downward movement of the diaphragm is offered by a dash pot piston I23a and a spring I24 interposed between the diaphragm and the piston, and this temporarry resistance prevents over-response of diaphragm III to a change in air flow through the boiler and thereby prevents the establishment of an excessive pressure impulse in line 35.

The dash pot mechanism comprises a cylinder I25 in which piston I23 is disposed a reservoir I26 at the upper end of the cylinder and a variable volume reservoir I21, such as a bellows, at the lower end of the cylinder and a by-pass I28, having a needle valve I29 therein which controls the rate of liquid such as oil between reservoirs I25 and I21. Bellows I21 is disposed in a pressuretight chamber I30 to which pressures are transmitted from sending line 36 through a pipe I3I. The pressure in chamber I30 is therefore equal to the control impulse sent out by valve II 1. The bellows is urged to its expanded position by a gradient spring I32 so that the total change in contraction of the bellows will be proportionate to the total change in pressure in pipe 36; in other words, the volume in the space below piston I25 will be proportionate to the pressure in pipe 36.

When diaphragm I I I starts moving downward 1y from its uppermost position, in which position exhaust port I2?! is open and inlet port I23 is closed, the pressure in sending line 30 increases. This increase in pressure acting on bellows I32 contracts the bellows, displacing oil and moving piston I23a upwardly. This results in a temporary restraint to further increase in pressure being sent to line 36, the restraint being nullified at a rate controlled by the flow of fluid through needle valve I 29 into reservoir I23. If during the time Which elapsed for the restraining effect of the dash pot to be wiped out. the air flow through the furnace had come to a steady value, diaphragm I I I will have come to rest in a position in which the force exerted by spring I20 balances the total force on the diaphragm. Thus, for each higher value of air flow through the furnace, a correspondingly higher value of pressure drop acts on diaphragm I I I. For each higher value of pressure drop, diaphragm III will be moved downwardly to a position corresponding to such pressure drop, and valve II1 will transmit a correspondingly higher value of pressure to pipe 36, bellows chamber I30 and to bellows 9Ia or regulator I8. When maximum air flow through the boiler occurs, maximum pressure is transmitted by valve I I I to pipe 36.

When the air flow through the boiler decreases diaphragm III will move upwardly, but under the temporary restraint of the dash pot, and decrease the pressure impulses transmitted to pipe 35 to a value corresponding to the values of air flow throughout the range of maximum to minimum flow.

Regulator 25 (for details of construction see Figure 8) Regulator 25 is like regulator 26 just described. In operation, pipe H4 is connected to the combustion chamber of the furnace so that chamber I I2 responds to the pressure therein and pipe I I5 is open to the atmosphere. Thus, diaphragm III is positioned in accordance with the difference between furnace chamber pressure and atmospheric pressure, and the impulses sent out by valve I I1 to pipe 99 and thence to diaphragm I00 of regulator 24 will be of such value that damper 23 is adjusted to a position that will maintain the pressure in the combustion cham ber of the furnace substantially constant.

i-tatio relay 29 (see Figure 9) Ratio relay 29 comprises a pressure-receiving chamber I35 to which pressures from pipe 36 connected with valve N1 of regulator 25, are transmitted, and a pressure-sending chamber I36 to which pipe 55 is connected and from which loading pressures are delivered to chamber 55 associated with master sender 21.

The upper end of chamber I35 is closed by means of a flexible diaphragm I31 and the upper end of chamber I36 is closed by means of a diaphragm I38. Diaphragms I31 and I38 are urged downwardly by means of tension springs I39 and I40, respectively. The tension in springs I39 and I40 may be adjusted to any predetermined value by means of screw-threaded shafts MI and I42, respectively. Screw MI has threaded engagement with traveling nut I43 to which the lower end of spring I39 is secured, and shaft I42 has screw-threaded engagement with a travclin nut I44 to which the lower end of spring I40 is secured.

Springs I39 and I40 are adjusted so that each exerts a downward pull on its diaphragm of a predetermined value, say for example, 60 pounds.

Diaphragm I31 is connected by a bracket I45 to one end of a beam I41 and diaphragm I30 is connected by bracket I48 to the other end of beam I41. A fulcrum roller I49 is positioned on the under side of beam I41 and may be adjusted lengthwise thereof 'to any redetermined position I by means of a screw I58 and a traveling nut IEI connected with the bearing pedestal 152 for roller I45.

The end of beam 14:! to which bracket I 38 is connected is provided 'with'an extension I53 that is operatively connected to a pilot valve EM. This pilot valve includes a .valve stem 155 which may be .moved upwardly or rdownwardly by beam extension 53 to control the "admission of pressure from a pressure :supply pipe Ia-"E6 in which a medium, such as compressed air, is maintained at constant pressure, through an inlet port I51 of the valve to an outlet port I 58 which has communication with the interior of chamber IE6; or when the valve is in another position, inlet port I! is closed and the interior :of' chamber I36 is placed in communication with an exhaust port I59=which has common communication with outlet port I53. Thus, pilot valve I54 when operated in one direction will serve to "increase: the pressure "in chamber I36 or to decrease it when operated in the opposite direction, depending upon the value of pressure transmitted to the interior of housing I3'5 by pipe 36..

So longas beam Ml i-s in alevel position, pilot valve stem I55 willbe in its neutralposition; i. -e., exhaust port I59 and inlet port Iii-I will both be closed. If the beam is out of level position and is tilted clockwise, inlet port I5! is opened and exhaust port IE9 is closed so that pressure medium may flow from pipe P55 into chamber H6 and thence to pipe '55. If the beam is "out of level and tilted in a "counterclockwise direction,

pilot valve stem I55 will be moved upwardly,

whereby the inlet port I5? is closed and exhaust port 1.59 uncovered to (allow pressure medium within chamber I36 and pipe ES to escape 'to atmosphere until the pressure in chamber I has been reduced to a value at which the total force exerted -by bracket M8 on beam I4! balances the total force exerted by bracketzMS on beam I47.

It will be observed that since springs E39 and I are initially adjusted to a predetermined tension, say 60 pounds, then under these conditions the maximum downward pull exerted. on the ends of beam M will be 60 pounds. Therefore, if the pressure 'in oharnbers I35 and Mt are raised to say 10 pounds per square inch gauge, this pressure will exert an upward force on diaphragms I37 and I33,dependi-ng'on th'e area :of the diaphragms, while the spring exert :a force in the opposite direction of 60 pounds. Thus the total force exerted on the ends of beam 141 will be the difference between the downward forces exerted by springs I38 and I40 and the total forces exerted upwardly on 'diaphra'gms II Sl and I38 by the pressures in c'lfiambers I35 and I35.

Therefore, from the above, it will be apparent tha b when the total force exerted by the pressures in chambers i555 and I36 on diaphragms I 3! and I33 are equal to the opposite forces exerted by springs 139 and M0, the net forces acting on the ends of beam I l-i will be zero and the beam will be in a .level position. Furthermore, if the pressure in chamber 35 is decreased from the values :above assumed, beam I41 will tend to turn counterclockwise, whereby exhaust port IE9 is placed .in communication with the interior of chamber 535., allowing pressure medium to escape from this chamber and from pipe 56 until the forces acting on the opposite ends of beam I41 :are again in balance.

By moving fulcrum roller I49 to one side or the tained substantially constant.

other of the midpoint of brackets M6 and I48 the pressures transmitted from pipe I may be caused to bear a definite ratio to the pressures received by-chamber I35 from pipe 36.

For further andmore complete statement'of the principles of operation of the ratio relay 2%, see United States Patent 2,016,824, ,grantedOctober 8, 1935, to'George l/V. Smith.

An advantage of utilizing relay 2% in the control system, such as described, is that where two or more boilers are operating in parallel and supplying steam to steam header 4, this relay may be so adjusted that each boiler will maintain a steam pressure in steam drum 2 at a value sufficiently high or above the value of steam prespressure in steam header 6 to-compensate for the pressure drop through the superheater of each boiler. Thus in this manner-each boiler maybe caused to carry its share of the load even though the pressure drops through the superheaters of each boiler may be different at full load and intermediate ratings.

Summary of operation Thus from the above it will be seen that the draft on the furnace is controlled jointly from steam pressure in steam header i and from the rate of delivery of feed water to the boiler by means of sending head iii and feed water valve 3!; that the rate of air flow-through the boiler is measured by regulator 26 which in turn transmits an impulse to regulator is that controls the rate of delivery of fuel to the boiler in accordance with air flow; that the furnace pressure regulator 25 transmits a pressure impulse to the forced draft damper regulator 24 so that the pressure within the combustion chamber is maintained substantially constant. It will also be apparent that the steam drum master sender 21 responds to the pressure of steam in steam drum 2 to so modify the operation of fuel regulator E8 in accordance with steam drum pressures and as modified by loading from the air flow impulse established by air -fiow regulator 26, that the fuel delivery rate is adjusted to cause the pressures in steam drum 2 to increase as the load on the boiler increases to such extent that the pressure in the steam drum exceeds the pressure in the steam header by the amount of the pressure drop through the superheater at any boiler rating above stand-by rating. The forced dra t damper regulator 24 opens the forced draft damper at such rate that the pressure within the combustion chamber of the furnace is main- As the combustion rate of the furnace is increased the delaying action of the needle valve which controisthe building up of pressure in the volume tank 3'1 is gradually wiped out so that the ratio relay finally becomes unbalanced and causes the drum master regulator to be loaded in such direction that a higher steam pressure is required in the steam drum to put this master regulator in balance. This results in the master regulator changin the control force supplied to the pilotvalve-operating mechanism of the primary draft regulator it so that this regulator opens the damper ll wider and increases the rate of delivery of fuel to the furnace until the pressure in the steam drum has reached a value higher than the pressure in the steam header by the amount of the drop through the superheater.

t will also be apparent that the time-delay device 3 illustrated as a needle valve delays the action of the control force transmitted by the air-flow regulator 26 to the loading bellows 54 of steam drum master sender 21 so that the delivery of combustion air to the furnace can be brought up to the proper value before the combustion rate is further increased. The fuel-air ratio relay 29 acts merely as a relay between the air-flow regulator 25 and the steam drum master 2?, and is so constructed that it is possible to vary the ratio of the loading pressure on loading bellows 54 to the value of the impulse transmitted by the air-flow regulator as required. For example the range of pressures transmitted by the air-flow regulator may vary from atmospheric to say 65 pounds per square inch, whereas by means of the ratio regulator the loading pressures delivered to the loading bellows of the steam drum master may be caused to vary from atmospheric value to say 20, 30, 40, or 45 pounds per square inch.

If the boiler is operating at maximum rating or at any rating above stand-by the uptake draft dampers 19 are moved by regulator 2| towards closed position. This decreases the air flow through the boiler and this decrease in air flow is responded to by air-flow regulator 26 to cause the primary draft regulator 18 to move damper ll towards closed position to reduce the firing rate. When the impulse sent out by the air-flow regulator 26 decreases, the pressure in the volume tank 31 immediately decreases because check valve 35 operates to quickly reduce the pressure in the volume tank. This quick action quickly unbalances ratio relay 29 in a direction that reduces the loading imposed on steam drum master 21 so that the impulse transmitted by it to the primary air control damper regulator is changed to a value calling for a greater reduction in the firing rate in the boiler. While this adjustment in the firing rate is being made there will be a slight excess of air supplied to the boiler and this will increase the pressure in the combustion chamber of the furnace, but this increase in pressure is responded to by the combustion chamber pressure regulator 25 and an impulse is transmitted to the forced draft damper regulator 24, causing it to move the forced draft damper towards closed position until the pressure in the combustion chamber of the furnace is restored to the desired value.

Having thus described the invention and disclosed what now appears to be a preferred emfor developing a control force for varying the responsive to the control force developed by said air-flow-responsive means and to steam pressure in the boiler for so modifying the rate of delivery of fuel that the steam pressure in the boiler will, at loads above stand-by load, vary directly in proportion to the pressure drop through the superheater.

2. A system according to claim 1 characterized by the fact that means responsive to the rate of delivery of feed water to the boiler are provided for supplementally controlling the draft adjusting means so that the total draft is in part proportional to the rate at which feed water is delivered to the boiler.

3. A system according to claim 1 characterized by the fact that delivery of feed water to the boiler is controlled by a valve and that means adapted to be actuated by and in accordance with the flow control position are provided for so modifying the draft controlling means that the draft is adjusted in part in proportion to the rate of delivery of feed water to the boiler.

4. A system according to claim 1 characterized by the fact that uptake dampers are positioned by a regulator having more than one means of controlling the position of the regulator and the draft dampers operated thereby, one of said controlling means being actuated by and in accordance with the rate of flow of feed water to the boiler and the other of said controlling means being actuated by and in accordance with the steam output from the distribution header.

5. A system according to claim 1 characterized by the fact that the means which are responsive to boiler steam pressure and to air flow through the boiler furnace comprises a steam-pressure actuated member, a control-force sending member, means operated by and in accordance with movement of the steam-pressure actuated member, and a member responsive to the air-flow responsive means for loading the steam-pressure actuated member in a direction that will result in an increase in the rate of delivery of fuel to the furnace of such magnitude that the steam pressure in the boiler will rise as the load on the boiler is increased.

6. A system according to claim 1 characterized by the fact that means are provided for delaying for a predetermined time interval the rate at which the fuel delivery rate is increased by the effect of said air-flow control force on the means which responds both to boiler steam pressure and to said air-flow control force.

7. A system according to claim 1 characterized by the fact that means are provided for delaying for a predetermined time interval the rate at which the fuel delivery rate is increased by the effect of said air-flow control force on the means which responds both to boiler steam pressure and to said air-flow control force, and that means are provided for rapidly diminishing the effect of said air-flow control force on said boiler steam pressure and air flow impulse responsive means where the rate of air flow through the boiler is decreasing.

GEORGE W. SAA'I'HOFF.

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