US2870729A - Control systems - Google Patents

Description

Jan. 27, 1959 J. F. SHANNON ETA.

CONTROL SYSTEMS Filed Jan. 1o, 1952 3 Sheets-Sheet 1 19.1.27, 1959 ...'F. '...HANNQN E. 2,810,729

*CONTROL SYSTEMS 3 Sheets-Sheet 2 yFiieol Jan. 10, 1952 5 Sheets-Sheet 3 HH IHII J. F. SHANNON ET AL CONTROL SYSTEMS m .mi

Filed Jan. l0, 1952 m SNAM RONIA mNRH NAC NABS w.. EH n V H Ns o M 1:..W-D

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ILLIM- l Unite States Patent O CONTROL SYSTEMS Jack F. Shannon, Euclid, vClayton H. Barnard, South Euclid, and Howard C. Schink, Cleveland Heights, Ohio, assignors to Bailey Meter Company, a corporation of Delaware Application January 10, 1952, Serial No. 265,897 l 8 Claims. (Cl. 114-16) This invention relates to control systems and apparatus for steam-powered submarine propulsion plants equipped with boilers having pressurized furnaces. Oil is the fuel to be burned preferably, although any fuel burned insuspension, such as gas or pulverzed coal might equally as well be utilized with our invention. Under surface and snorkel operating conditions, air for combustion is to be furnished by means of centrifugal compressors, while under submerged operation, liquid oxygen will be pressurized, heated to normal temperature, vaporized under pressure and supplied to the furnace for combustion.

It will be understood that a snorkel is a breathing device used by a submarine andmay include a tube or a pair of tubes housing air intake as well as air and gas exhaust pipes that can be extended above the surface of the water for allowing operation with furnace or engine when the submarine issubmerged. The arrangement allows discharge of exhaust air and gases at all times without entry of water when waves submerge thej35 exhaust port. The inlet port allows air entrance when not covered by a wave but closes against water entry when the port is submerged by a wave.

When using gaseous oxygen for supporting combustion exit products of combustion will be recirculated to dilute the gaseous oxygen and thus limit furnace temperature.

During submerged operation, furnace pressure is necessarily held high enough to permit the exit gases to be discharged overboard against the submergence pressure then existing.

A principal object of our present invention is to provide method and apparatus for controlling the operation of such a steam generating plant operating under surface, snorkel, or submerged conditions.

Another object is to provide an automatic control system of the fluid pressure actuated type for steampowered submarine propulsion plant.

Other objects will become evident from a study of the drawing and of the description thereof as well as the claims appended hereto.

In the drawings:

Fig. l is a diagrammatic showing of a steam-powered submarine propulsion plant to which our invention has been applied.

' Fig. 2 is a schematic showing of the measuring and controlling instrumentalities of Fig. 1.

Figs. 3 and 3A illustrate a submarine, in side elevation, with a portion of the hull broken awayto disclose a diagrammatic showing of the vapor generator, propulsion equiment, auxiliaries, control panel, and principal interconnecting piping. i

The showing of the drawing is quite diagrammatic and not to any scale. The same numbers are used to designatek like parts on the sheets.

A pressurized boiler furnace 1 is shown as being supplied with fuel oil through a conduit 2, by means vof a pump 3 which is driven by a constant speed electric motor 4. Regulation of the rate of supply of fuel oil is had through the agency of a diaphragm actuated control valve 5 located in the conduit 2; while its rate of ow is continuously measured by a flow meter 6.

Air to support combustion is supplied to the furnace 1 by way of a conduit 7 to which is connected a cruising compressor 8 driven (when operating) by a constant speed electric motor 9. Also connected to the air supply conduit 7 is a main compressor 10 which is driven by an exhaust gas expander 11 and a steam turbine 12. When the main compressor 10 is in operation approximately 30 percent of its driving power is supplied by the expander 11 and the remaining 70 percent by the steam turbine 12.

Under certain conditions of operation, to vbe explained hereinafter, oxygen to support combustion is supplied to the furnace 1 through a conduit 13 by an oxygen pump 14 driven by a variable speed drive electric motor 15. The showing is quite diagrammatic and omits the equipmentl for gasification of liquid oxygen and two stages of gaseous oxygen pressure control. -The rate of gaseous oxygen supplied to the furnace through the conduit 13 is measured by an oxygen rate of flow meter 16 and is controlled by a diaphragm actuated control valve 17 located in the conduit 13.

Under certain conditions of operation all of the exhaust products of combustion from the furnace 1 pass through a conduit 20 to the expander 11 whereas, under other conditions of operation, a portion of the products of combustion are diverted through a branch conduit 21 which joins the oxygen supply conduit 13 on the furnace side of the oxygen regulating valve 1 7.

A separation drum 22 is connected to the Huid circuits of the boiler 1 and is supplied with feed water through a conduit 23 by means of a feed water pump 24 driven by a motor 25. Feed water supply rate through the conduit 23 is under the control of a diaphragm actuated valve 26 while the rate of water ow through the conduit 23 is continuously measured by a rate of ow meter 27. A water level measuring device 28 is continuously sensitive to liquid level within the drum 22 for a purpose to be explained hereinafter.l

Superheated steam leaves the boiler 1 through a conduit 30 which branches as at 31 to supply the steam turbine 12 when it is operating. The principal output of the power system, namely superheated steam, leaves through a conduit 32 and is measured by a rate of ow meter 33. The conduit 32 supplies superheated steam to one or more main propulsionturbines 110.

There are certain basic requirements to be met in the operation of such a power plant. The rate of combustion must be controlled to satisfy steam demand. Fuel should be proportioned to oxidant when using air during surface or snorkel operation, or when using oxygen during submergence. A proportioningcontrol is provided capable of supplying sufficient oxygen for eflicient combustion of the fuel oil. Contol of feed water supply will proportion the water inow in accordance with requirements, and a spill-over valve from the drum to the main `condenser provides against excessively high drum levels during maneuvering or rapid start operations. When using gaseous oxygen for combustion, exit gases will be recirculated to limit furnace temperature. During submerged operation, furnace pressure will be maintained high enough to permit the exit gases to be discharged overboard against the submergence pressure then vexisting. n

The boiler plant will function under threel phases of operation; surface, snorkel and submerged. The rst two modes of operation will utilize air from either the cruising or the main compressor, depending upon load requirements. During submerged operation, liquid oxy- Patented Jan. 27, l1959 i asap-,pee

ee will be evaneretesiaiiri `suppl iesi to the `iii gaseous form. l

Surface operation- Light load The cruising cor npresser` 8 willbeusedjto supply' air forl, `combustion during lperiods ofv light loadoperation. Since this unitis driven by a constant speed dirtcurrent motor 9i, control oftheamount. of combustion, air supplied is obtainedby throttling a compressor spill-over valve 3 5 which wastes (tothe hull),v the;air no t required for combustion. CQIltQl ofk thequantity of the fuel oil fired is accomplished by adiustinaa Strekeeperaier-On pump 3. and regulating one .or more burner, control valves A steam pressure, measuring-transmitter including Bourdon tube T26` and pneumatic pilot valve 37 measures steam pressure inthe superheater outlet header 3i)y and d eyelops a proportional air pressure `signal in pipe 3 8 joiningthe A chamber of an ambient pressurecut-back relay 39 which may be of the general type described and claimedin Dickey Patent 2,098,913. The output preSSllIe of relay` 39, availablein chamber D, is subjected through apipe l0-upon the A chamber o f a ratiostandardizing relay 41 which is of the type described and claimed in Gerrie Re. 21,804. Relay 41 provides a lproportional control with reset characteristics. `It provides for the demand index (nal steam pressure), a floating control of high sensitivity superimposed upon a positioning control of Vrelatively low sensitivity. A function ofl the adjustable bleed 4. between the C and D chambers ofthe relay, istosupplement the primary control vof the pressure etective in pipe 4d vwith asecondary control of the same or of diierent magnitude as a follow-up or supplemental action to prevent over-travel and hunting. The relay` 4- 1 is also providedr with ratio adjusting means .illustrated as ahandknob 43.

4When we speak of ambient pressure or temperature, We, mean the pressure or temperature ambient to. vor adjacent the particular apparatus under discussion and within the hull or other confined space in which ,the apparatus is located. It is intended to mean the pressure or temperature within the submarine hull, lairplane body, or other conned space containing the controlling instrumentalities and/,or the controlled devices. In the present submarine embodiment it `might be termed hull interior pressure or temperature.

The output signal of relay 41, available in apipe 44, is passed through a hand-automatic selector valve station 45 of thetype described and claimed in the Fitch Patent 2,202,485 providing a possiblity of selective remote hand or automatic operation.

The output signal from selector station 45, Whether by automatic means or hand adjustment, is available in a pipe 46 and acts through the various relaysuto adjust the supply rates of the elementsof combustion tosatisfy the demand for steam. Under cruising operationl (light load)v this is through a control ofthe compressor spillover valve 35 and the fuel oil burner valve S.

The master signal in pipe 46. is subjected upon thee cheese in Steam preseureis.representedby echange in.

leading pressure v'vhiehv turisticas. through relayed?, :41, 4.7 endet., te vary the degree Qfjthretiiilis QLSpilI-Over valve 35, the resultanty change, in combustion air s upplied to the furnace through conduit 7` is measured by an` air ow rate meter 51 and its effect is balanced againstthe master air pressuresignal.

vThe air ow meter 51 is connected'l-across anorifice 52` located in conduit 7 and actuates the movableelement of .a-pneumatio pilot 53 to=therebyil establishdnftha A ehamber.- Qi: a computer;releyfisanair pressure eqn-1.

.anxiously proportional@ the. amount of air, being furnished the furnace. The output of relay 54, available in a pipe 55, is subjected upon the A chamber of relay 47 to balance against the master steam pressure signal of the B chamber. Thus the relay 47 continuously checks the master demand for airdlowkagainst the actual measured air ow and providesareadjusfting effect upon valve 35 if necessary.

Control o f .fuel o il v supply vrate is accomplished through a lcomputenrelay 56vvhich balances the master signal (pipe ,46 to A chamber of relay 56) against a Vsignal' (in B chamber of relay 56) originating from a-fuelioil ow meter 6 previously mentioned.

The meter 6 is sensitive to the pressure differential produced by fuel oil flow through an orifice 60 located in oil supply conduit 2; and positions the movable ele' ment of a pilot 61 thereby establishing in the B chamber of a ratio standardizing relay 62 an airpressure signal continuously representative of fuel oil supply'rate'. 'The outputof relay 62 vis `subjected upon the B chamber -ofcomputer relay 56 in tie-,back opposition or balance to the ymastersteam lpressure signal. If the pressures acting upon the, relay 56 yare not in balance, indicative ofa need foichange yin fuel oil supply rate, the unbalance-is represented by va change incontrol pressure inV pipe 63 which acts vthrough asclector station 64 to readjust thev throttling position., ofj fuel oil burner `valves 5v and/or` the Oiipumnsireke mechanism.

In order to provide the possibility of manual adjust,-V mentof-theratio of -fuel vto air flow required, an Aoxidant ratiorelay 67fis installed in thevcontrol circuit for this: purpose. The input pressure to this relay, subjected` upon the-relay A chamber from pipe 55, `is proportionall to` combustion air being supplied and byirneansnf a knob-68 adjustment, the ratio ofthe output to .input pressures may be adjusted. The output pressure (in pipe 69;) in-turn acts upon the A* chamberfof relay62 where; it isvr balanced lagainst the B chamber-pressurev propor. tional to-oil iiovv. The output of relay 62 toy computer, relay 56 is thereforeiproportional.to anyy deviation from the desired ratiovof oil to combustion oxidant. While the primary changes von both the combustion air and they oililow are made from the steam pressurecontroller 3 6, 37, deviationsn the desired proportion of fuel-,oilito combustion air are accomplished by variations in thel air pressure signal from the ratiorrelay 62.

During this type of surface operation a hand isolating` valve 78 is closed and all of the exhaust `products of combustion from the furnace are discharged over-board by Way of a-pipe 79 through a co-ntrol valve 8 0.

`Overboard valve 80 is under direct automatic control of controller 8.1 ink order to balance the submergence pressure against that of furnace 1 to insure continuous f discharge, overboard of the .products of combustion-dur- Surface operation-Heaverloads` For operation at greater thanrcruising loade conditions, the main air compressor 1t) -is used as the source of -comf bustion air. Control of the' compressor 10 isiv accom-y plished by thefturbneA Athrottle,- valvef-l'insteam condnitg31. The.; master steam pressure,- signal is impressed uma-the: A .chamber of -a- .reiter standardizing; relayr. e

whose output signal (in pipe 73) acts through a selector station 74 in pipe 75 to position the steam throttle 70. Relay 72 serves as an air flow tie-back by having its B chamber loaded from aid flow through a branch 76 of the pipe 55. A change in air pressure signal from the master selector 45 will cause the main compressor 10 to continually change speed in one direction until the air pressure signal from the air ilow relay 54 (in pipes 55,

-76) indicates that the required change in combustion air flow rate has been accomplished. During this phase of operation, the compressor spill-over valve 35 remains in a closed position with all changes in combustion air rate being effected by varying the speed of the main air compressor. The transition from the cruising to the main compressor is made a semi-manual operation as it occurs rather infrequently and under preselected desire by the operator. p

During this somewhat heavier load operation the exhause products of combustion are utilized by way of expander 11 to provide about 30 percent of the power requirements of main compressor 10. vThe hand isolating valve 78 is opened and the controller 81 is hand adjusted to maintain valve 80 normally closed so that all of the products of combustion will pass through the expander.

Exhaust gases from the expander 11 leave through a conduit 111 to the snorkel exhaust (Fig. 3A). Under surface operation the snorkel exhaust freely passes gases to the atmosphere. Under snorkel operation the intermittent or occasional wave submergence of the snorkel exhaust does not introduce suicient back pressure,"rel ative to expander discharge pressure, to disturb or pulsate air supply to conduit 7 from the main compressor 10.

We have indicated in Fig. 2 that the air flow meter 51 is4 provided with automatic compensators sensitive to ambient air pressure and temperature.

Submerged operation During submerged operation it becomes necessary to use oxygen as the oxidant of combustion, and liquid oxygen is vaporized, pressure controlled, and diluted with recirculated products of combustion. In addition to providing automatic control of steam pressureythe correct fuel-oxidant ratio, and drum liquid level; the system maintains the correct ratio of recycled exhaust gases to gaseous oxygen and the desired furnace combustion pressure.

The main compressor is not used and hand isolating valve 78 is closed. Selector station 74 is turned to hand and throttle valve 70 is remotely manually closed. Selector 74 thereafter isolates valve 70 from pressure signalsy of pipe 73.

The control of gaseous oxygen flow, through selector station 85 from steam pressure and total oxidant is accomplished in the same manner as when firing with the main compressor with the exception that the control signal in pipe 73 is used to throttle'valve 17.

Gaseous oxygen flow rate is measured by a rate meter 16 arranged to position the movable element of a pilot 91 thereby establishing in a pipe 92 a loading pressure continuously representative of oxygen supply. The computer relay 54, previously sensitive to measured air How, is now sensitive to measured oxygen ilow (pipe 92) and its output in pipe 55 controls the oxygen valve 17 and fuel oil valve 5.

The oxygen ratio relay 90 determines the proportions of recirculated gas mixed with gaseous oxygen to dilute the latter. The input air pressure signal to the A chamber of relay 90 from pipe 92) is proportional to gaseous oxygen flow and the knob adjustment of relay 90 serves as a means of varying the ratio of the input to the output y ,y pressure in a pipe 94 and thus provide selection of the proportions desired. A measuring device sensitive to furnace temperature indicates the same 0n ,the control gases overboard against the depth of submergence.

panel 112 (Fig. 3A) available to the operator who may adjustthe ratio knob of'relay 90 to regulate the ratio of. recirculated gas owy to gaseous oxygen iow and thus maintain furnace temperature below an excessive value.

Recirculated gas ow through a conduit 21, is regulated by a control valve 86 and is measured by a rate meter 87. Meter 87 is arranged to position the movable element of a pilot 88, establishing in a pipe 89 a fluid loading pressure continuously representative of recirculated gas flow rate.

The output pressures from the oxygen'ratio relay 90 and the gas flow controller 87, 88 then act through a ratio standardizing relay 96 which produces an air loading pressure proportional to deviation from the desired ratio of gas to 'oxygen ilow, and through selector station 97 to adjust the positioning of recycled gas ow valve 86.

Under this mode of operation, with a portion of the products of combustion recirculated to dilute gaseous oxygen and thus limit furnace temperature, the remainder of the exit gases are discharged overboard through valve 80 and pipe 79. Combustion chamber pressure must be maintained sufliciently high to discharge the Oxygen gaseous supply is used to aspirate the recycled gas to the furnace. Combustion pressure control is accomplished by throttling valve 80 in overboard conduit 79.

Both themain compressor and the cruising compressor have characteristics such that flow and pressure rise and fall together. When it is desired to transfer from air burning (particularly from the main compressor) to` oxygen tiring, it will be seen that, as we begin to open oxygen valve 17 through hand manipulation of selector station 85, the oxygen flow meter 16 will act through pipe 92, relay 54, pipe 76 and relay 72 to indicate an excess of oxidant and tend to cut back on valve 70. To prevent this and maintain combustion chamber pressure, we tie the output of selector to relay 48 (through pipe 99) which controls the spill-over valve 35 lto dump air without changing compressor speed.

For all three modes of operation the feed water control supplies the boiler in accordance with load demand. This is accomplished by metering superheated steam ilo-w in the outlet header 32 with flow rate meter 33 and feed water inflow rate through conduit 23 by flow meter 27. Air pressures proportional to steam outflow and water inow are developed by these measuring controllers, and through a relay develop an air pressure which positions the vfeed water control valve 26 to maintain the same rate of water inow to the boiler as steam ow therefrom. Deviations in separator drum (22) water level are sensed by measuring controller (28 which develops an air pressure proportional to water level and acts upon the relay to modifyvthe steam flow-water flow control of valve 26 as necessary to maintain the correct separator drum level. During rapid load swings, excessively high dmm levels are prevented by opening the drum spill-over valve 113 by the air pressure developed by controller 28.

Snorkel operation When firing the unit under snorkel conditions, changes in ambient pressure within the hull are expected although the depth of submergence is relatively constant.

The ambient pressure cut-back relay 39 has been included main.Y air'icompressor, .vv-hem the airtforfcombustongis: drawn, from,thet-hullainterior. t Were ;the firing. rate "not reduced( and the `-air withdrawalV ffrtom' the hull :undiminr ished) during snorkel closure of some 8 seconds-fthel hulltairl; woulde be ,exhausted .tof suchV anrextent' fas. to `-be dangerous tothex personnel; Additionally;I la trip j switch 114 y-measuring hull -pressure has. been. included \for fthe 1 purpose :ofttt'ippingg out. the -:lires byjvalve lsrfupon reaching a dangerously low .'hull pressure;

Referring specically to Fig. 2, relays 41', 72, 47, 62 and `96 :have been designated asvv standardizing I.type relays. PorVV use under ambient-pressure-changing.= Icon ditions, such Aas .might bey encountered-'during snorkel'- operation, or even ,under varying :depth -r'of submergence, itwas found'thatf-thisfrelay Wouldrtendtovhold a constant absolute .pressure `.following-a isuddcnzchangerather'; than 'a constant 'gagezrpressure :which :is Jrequireditoffthe system, and vwhichthewremainderjof the= componentsv produce.v To compensate for fthis, aniadditionalsbellows was; added, together with ,ableed Vvalvermwhichil would compensate forr the Vreset'rate bleed adjustment in such:- away that the outputduring'variablerambient pressureL conditions e will remain on a constant gage lpressure basis.` Detailsl of thiscompensation Wasnotlshown2in1l the present i drawing- 'and t reference should. beffnad tothe copending'application ofv Howard C.. Sohink, ySerial,v No.-318,308, filed November '1, 1952, nowPatent,1 2,860,- 650-whichf is directed particularly to such. anfarnbient pressure or -barometric pressure compensated-"relay It will be seen that we -have providedfapparatusiandi a-methodfof operation g to handle a; steamv .boileiilplantt for-a. submarine which. willfunotionfunder three definite phasesl of operation; surface; snorkel andzsubmerged.` Theirsttwo conditions of operation take ainr from athe :hull interior 'by way of either acruising or' maini compressor; depending uponA load requirements. Duringifsubmerged' operation, liquid oxygen lis "evaporated andfsupplied lto theV boiler furnace in lgaseouszform. Regardless -of fthe method of operation,- the 1automatic :combustion .'Lcontrol:

performs the following basic functions; maintaining steam pressure t. at: the :correct -value, ,maintaining combustion pressure, maintaining the correct ratio`of fuel tolvoxida'nt,v and".v maintaining feed' wa'ter supplyirate. asneededi. Change-over from one-.method `offfirirrgsto.anothertis manuale-,through remotelyk actuated'` instrumentalitie's and immediately following lsuch change-over, theautomatic control system' functions under the new mode of operation.

Whilewe havedescribed certain' automatic methods and systems of` operationpunder diierent'conditions.of service, it will beapparent. that ourl methods imayl be manually performed'through iobservation'of the various. measuring instrumentalitie's` and remote manual controli'of the .controllablefactors In Fig. BAW/e show'vaI central' con-trol panel 112 :upon which Ymay'bemountedtvarious measuring'devic'esoit-variables in the operation. ofzthe plant,'for.example, meters 36,151; 16,6, 37,31, 33, 28,127. etapas-'well as manualfautomatic selector-,stations 45; v74, j 85, 49, 64, 97, and 125, Some, or all of the relays may also be mounted on the panel 112, as well as controllers 101 and 114. The operator may observe .the measuring instrumentalities' and remotely manuallyoperate the selectorstations to perform the claimedmethods'.

It'will be evident that, while` we have chosen to illus-- trate and describe one preferred embodiment of our invert-z tionin connection withla submarine as exempli'f'ying an enclosed'space, wel contemplate that our invention is equallyf-as well adapted't-o enclosed spacesas, for 'example, the 'pressurized hulls of airplanes, the pressurizedl or"evacuated containersV of 1auto-maticpiles,v or the like.

What we claim as new, and'desire to secure 'by'L'etters Patent ofthe United States-is:

l. The method 'of operatingn4 the steaml generating processv of a submarine propulsion unit having apressurized combustion'furnace supplied 'a iluentfuel and fluid: oxidant including, supplying the oxidant from the submarinei-nterionn controllingthe fuel antioxidant supplya rate '.toesatisfy steam :demand 'ruponethe vrunitydischargin L the products :of 'combustion to,v the: submarine texterionrby? maintainingithe pressurel within 'the .furnace Substantially@ constantande-suticiently high to maintain-discharge'.fronr; thef-furnaceat variousdepths ofsubrnergence;andlimi'ting1 the1ftrnacefuel and 'oxidant supply 4Jfrorn'thefai'rpressuref within the submarine.

2.-The method; offclaim- 1 whereiny thenair is the-com-- bustion oxidant withdrawn from' thesubmarine interior at: -a relatively-'constante rate :and vany-excess air .-is confV tinua-lly; returned tto. the .submarine interior.

3. A systemfor controlling the steam generation; processl of-,a\ submarinepropulsion; unit subject -to sur,- face.snorkel -and submerged.- operating conditions-ina cluding in combination, a steam generating furnace f com.-` bustiongchamber in thefsubmarineinter-ior., a-fuelfsupply conduit-fior the-furnace,- an-.oxidant-supply;conduit'tfor'i theffurnaceya rstmeter forfthe fuel conduit,` afsecondt.

meter for the A oxidant conduit, regulating valves fin eachv offthe conduits; amaster fsteam` pressure controller posi.- tioning'the conduitY valves simultaneously; a ratio. deter mining means .l sensitives to the-rstand. second meters modifying the regulation of the fuel supply conduityalve,

aw constant supply :of `fuel -for the `fuel supply conduitof theturnace,` ,a compressor oftconstantflspeedfy connectedl` tothe oxidant. supplyV conduit for supplying@ air ffrom' the submarine interior,` .andta valve y'i112 the oxidant conduit l between the compressonand furnace'under the-conjoint control of `the master steam-- pressure controller'and .ther oxidant. meter.- f

4.- `The combination. of 'claim 3 in lwhichnthecom-f pressor is driven by a steam turbine and gas expander@ andcthere` is fincluded, a duct Iforfconveyiligithe :gaseous products of combustion from' the-furnace `to .the-expander; a control valve for the steam duct coming V'to-the turbine,l

and means sensitive .to the master steam-pressure controller and the oxidant 'meter "toconjointly position the control' valve.

5.A Theccombinationrof c1airn:4 lwherein meansrespon-w sive .tothe interior lair pressure of the-submarineopposesf thetsmaster steam apressurecontroller tondecrease :the-

. rate of'fuelandoxidant` supply .when theinterior airpres-.

` supply conduit is connected to a source `of oxygen. andf thereis included invthecombination; a'meterffor-irthe oxygeni in f; the oxidant isupply f conduit, afvalve` inuthe oxidant. supply conduitpositioned'byr the master steam: pressure controller fand* the: oxygen meter;y and a system=v for mixing with .the 'oxygen'. a portion of the'produc'ts'zof` combustion. l

8;. Thef'combination ofv claim7 in whichythe'mixing` i systemfincludesta conduit from=fthe"furnace :exhaust tov the-furnace burners, a meter for :the products "of 6Com-, busti'onainthe conduit; andva'valve'in the conduitv for the' products-.of combustion.positioned;bythey oxygen-meter andthe Vproducts of Acombustion meter.l

References- Cited in the lileofthis patent4 UNITED STATES PATENTS 1,126,6161 Cage' Jan. 26,;r 19I5 1,345,757' Emmet.l Julyff, 1920 1,380,304v Norton May`31, 1921 1,695,472 Roucka .1. De :..18,` 1928 1,721,800 Wunsch July' 23',LV 1929' Beslerlv Iulyl; 1941 (Gther references on following page) 9 UNITED STATES PATENTS Rasor July 14, 1942 Roberston June 15, 1943 Holthouse May 14, 1946 McCracken Dec. 17, 1946 Fellows Aug. 15, 1950 Dickey Dec. 30, 1952 Bristol June 1, 1954 10 Bristol June 15, 1954 Hoke Oct. 18, 1955 FOREIGN PATENTS Great Britain Apr. 19, 1917 Great Britain July 24, 1946 OTHER REFERENCES

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