US2581316A - High rating fire-tube boiler and method of boiler operation - Google Patents

Description

1952 J. J. WOLFERSPERGER 5 3 HIGH RATING FIRE TUBE BOILER AND METHOD OF BOILER OPERATION Filed Oct. 1, 1945 IN VEN TOR. Jam J. wufifies seae 14 rrae/ve Y5 Egg- Patented Jan. 1, 1952 OFFICE HIGH RATING FIRE-TUBEv BOILER AND METHOD OF BOILER OPERATION John J. Wolfersperger, Milwaukee, Wis. Application October 1, 1945, SerialNo. 619,671

This invention relates to improvements in high rating fire tube boilers and methods of boiler operation.

It is the object of the invention to develop increased horse power relative to the size of the installation without substantial sacrifice of efiiciency. In terms of rating, ten square feet of heat transfer surface per boiler horse power is taken as standard or 100%; a boiler horsepower represents 33,475 B. t. u. per hour, or the heat required to evaporate 34%; lbs. of water per hour from and at 212 F. Merely decreasing the surface area, as by reducing the length of the fire 12 Claims. o1.'12 2-134) tube, will increase the rating but result in a corresponding increase of flue gas temperature at the discharge end of the fire tube and consequent loss of energy and efficiency. These considerations have heretofore limited fire tube boiler ratings in commercial practice to a maximum of about 200 more or less. In contrast, the present invention has provided boilers so compact as to be rated up to 2000% without substantial efliciency reduction and with even higher ratings apparently possible.

The present application is a companion to my Patent No. 2,499,207, and to my application No. 543,878, filed July 7, 1944, now abandoned. Further objects of the present invention will appear more particularly in the following disclosure.

In the drawings:

Fig. 1 is a plan view of apparatus embodying the invention.

Fig. 2 is an enlarged detail view of the boiler and burner organization in vertical axial section.

Fig. 3 is a view taken in section on the line 33 of Fig. 2.

The boiler comprises a small tank 5 carried on any suitable support 4. Fire tube 6 is coiled within tank 5 in a helix having an axially disposed inlet 1 near one end and a connection through the opposite end of the tank to flue 8. Taps are provided in the tank at 9 and I for inlet and outlet of water or other fluid heated.

The mixing and ignition chamber H is, for convenience, compactness, and heat economy, preferably disposed inside of the coiled fire tube with its admission end exposed outside of the end of tank 5. To this chamber fuel and air are supplied under substantial pressure from fuel pump l2 and blower or air compressor l3, driven by any suitable means, as by motor I4.

Casing extension l and sleeve l6 are disposed at the end of chamber H. The compressed air pipe I! enters the casing extension l5 and the air thence passes through the sleeve l6 into chamber H. The fuel pipe l8 enters laterally through the casing extension and the sleeve and leads axially into chamber H to the fuel nozzle l9. For the purposesofthis invention the fuel is preferably oil and the nozzle I9 is any suitable atomizing nozzle. However, other fuels may be used.

The nozzle I9 is supported centrally in chamber II by spiderarms 20 (Fig. 3) which carry the nozzle, from an inner sleeve 2|. Sleeve 2| is, in turn, supported by spider means 22 and 23 from chamber and from intermediate sleeve 24. Intermediate sleeve 24 is anchored at 25 to the wall of chamber H and spaced therewithin by peripheral projections 21, 28. This latter sleeve is axially offset from sleeve 2| and more closely approaches the diameter of the chamber than that of sleeve 2|. Both sleeves arepreferably bevelled inwardly at their discharge ends.

Mounted on nozzle I9 is a disk 30 which is peripherally spaced from the inner surface of inner sleeve 2| but materially restricts and controls air fiow through such sleeve. Through this disk extend the ignition electrodes 3|, 32 which are mounted in insulating tubes 33 supported by holder 29 and connected by spring conductors 34 to respective terminals 35, 36.

Through a nipple 38 at the end of chamber projects a tube 31 constituting a flow-restricting throat. This tube opens into a combustion chamber 40, affording communication between the mixing chamber II and the admission end 1 of fire tube 6. The combustion chamber may, in some circumstances, be omitted, but in all but the lowest ratings is not only desirable but essential.

The tubular throat .31 is preferably part of a casting 4| which, instead of being made of refractory material, is preferably-of steel or alloyed heat-resisting material substantially filling the end of chamber and provided with a conically tapering surface 42 leading to the throat. Welded joints are provided between casting 4| and chamber I and between nipple 38 and combustion chamber 40, and between chamber 4|! and fire tube 6.

The fact that the tapering wall surface 42 leading to the throat is within the end of chamber protects it from such cooling by the water of the boiler as might otherwise condense fuel on its surface. It, in turn, is protected from excessive heat by the sheath of air which surrounds the flame until the throat 31 is reached.

The throat is preferably of such dimensions as to have cross sectional area 5 to A that of the burner tube II and a length at least equal to, and preferably about one and one-half times its own diameter. The combustion chamber is of somewhat greater diameter than the throat diameter or the fire tube diameter and is preferably of a length approximately three times its diameter. Since the relative proportions shown are significant, some actual dimensions of a practicable device of the construction illustrated will be given. In that device, the throat diameter is two inches. The diameter of combustion chamber 40 is three inches and its length 8 inches. In the particular device shown, the length of the 2% fire tube is about 30 feet and its diameter about 2%" I. D. but these dimensions may be varied according to design.

Heat transfer per unit of surface has long been known to increase in almost direct proportion to velocity of flow but practical advantageof-this fact has not heretofore; been realized in boiler equipment of this type; I- proportion the section and length of the passage through my: boiler and use relatively high .airxandifueli inlet: pressures to achieve flow at so high a velocity that heat de veloped is transferred very rapidly to. the water through ashort fire tube. of: limited. area with no excessiveheatt at any point: in the boiler. Despite the short length' of tubing used, flue gas temperature has been found in. many embodiments' of this invention to be itery'low andto increase onlyabout 10 to degrees'F. for each 100% increase in rating.

llnmy boiler, it ismy purpose that the use of refractory be eliminated or reduced to a mini mum, and that excessively high temperatures in a To eliminate these difiiculties, it will be noted that'I providea single source of air under pressure and hermetically seal the entire passage from the air source to the boiler outlet to the flue. Taking full advantage of the hermetically sealed cond'uit,.I use from one to two pounds to a severalpounds of air pressure and sufli'ciently highfuel pressures: to'deliver the fuel into the air, thereby developing a rate of gas flow at the entrance T to the fire tube from approximately 450-to approximately 2000-feet per second; a rat-e of flowunheard of in fire tube boilers.

The amount of air passing through the inner tube-2l, subject tothe control of disk 30; is just adequate to support stable ignition. The relativelylarge cross section of chamber ll keeps velocities-relatively low-at all points therein. An annular body'of additional air reaches the burning fuel through tube 24' and another annulanflow of air passes about the outside of tube 24' and envelope the flame to shield chamber: ii and'cas-ting M from the flame. However, pending= complete-mixture ofthe added air with the over-rich mixture ignited at the burner, noexcessive amount of heat is'developed. The tapered surface 42' and throat 3; to which it leadsareof great service in promoting perfect admixture of the diluting air with the over-rich mixture in which the flame is being propagated. The relative flow of fuel and air will, of course; be-controlled: to achieve as nearly perfect combustion as possible. The velocity of flow in the throat is sorapid that-the mixing in the throat and com.- plete combustion are eflected almost instantaneously;

In a perfect mixture of air and fueL'fl'ame; is

said to propagate at the rate of 1000 feet per second. In some embodiments of this invention I find velocities at the entrance to the fire tube of almost double this value. Moreover, since the gases delivered by the throat already contain some products of combustion, the flame propagation therein cannot occur at the optimum rate of 1000 feet/sec. Thus the importance of the enlarged combustion chamber 40 following the short constricted throat 31 is that is prevents such excess velocities as would exceed the rate of flame propagation and hence pull the flame from the unburned fuel, while maintaining a velocity adequate to achieve full combustion.

Actual examples are as follows:

An embodiment of this invention was operated as a. water heater at 1500% rating with 20 sq. ft. of heating surface and an output of 30 boiler horse power. The air pressure used was 4 pounds per square inch. Gas; Velocities at the throat outlet wereapproximately 257feet/sec. and. at the fire tube inletapproximately, SOO feet/sec. The observed flue gas temperature was only 510 F. The-averageboilenoftheprior art hasa fire tube inlet velocity of 40-to ft./sec. and-operates at -200 rating with flue gas temperatures comparable to those of my boiler. at 1.500% rating.

Another embodiment of this invention was operated as,a steam v boiler at over 2000% rating with 28.5 sq. ft. or" heating surface and60 boiler horsepower output. Air pressure was-l2 lb. Gas velocities-at the-throat outlet were approximately 218 ft./sec., and. at the firetube, inlet approximately; 19.60;it./sec..; Fluegas;;temperature at thishigh1200.0%;ratingwas only 390 F.

The following general particulars: are given:

The air pressure. will; ordinarily exceed onethirdpound per square inch for ar300% rating and willrunfrom: two to twelve pounds in boilers up to 2000% rating. The oil or other fuel. pressure must. exceed air pressure in each case and, toachieve: oil atomization, commonly amounts to 100 pounds/sq. in.

Burner tube H has an area usually amounting to 6 to .7- square inches per gallon per hour of fuel oil consumed.

The cross section of the throat 3'! is .43 sq. inch to 1.5 sq. in. per gallon per hour of fuel consumed.

The air flowing around disk 30 through the inner sleeve may typically approximate 1 to 3% of the total. volume; air. flowing between the inner and intermediate sleeves 64 to 72%; air outside the intermediate sleeve 26 to 34%.

Fire tube internal cross sectional areas range from less than 1 sq. inch per horse power at the lowest ratings (actually .64 to .74 sq. in. at 300% rating) down to dimensions of. the order of .06 sq. in. per horse power at 2000%' rating. Per gallon of fuel oil consumed per hour these sectionsrange from less than 2.5 sq. in. maximum down to about .20 sq. in.

In all embodiments, the size of the combustion chamber, bothas to cross section and length, isso chosen with reference to air pressures used, fuel consumed, rating desired, and throat dimensions that the major part of all combustion occurs in the combustion chamber rather than in the chamber 1 l or the fire tube, the velocities in the combustion chamber being substantially as high aswill permit full flame propagation. In actual practise, only about two per cent of the fuel is consumedin burner tube H and a negligible amount in the coil of the fire tube, almost the entire combustion being effected in chamber 40. Thus all heat in excess of that required to rating boiler herein disclosed is valuable. There is no apparent limit on the ratings for which embodiments of the invention may be designed, provided air and fuel under appropriate pressures are made available. The higher the air pressure, the higher the velocity at which the products of combustion can be forced through one or more fire tubes of restricted cross sectional area. Incorporation of the mixing and combustion chambers within tank 5 not only saves space but controlsthe temperature of the walls of the immersed chambers.

Since my invention has been described particularly with reference to the structure used, the

'for a brief interval (in the throat 31) the velocity may approach the rate of flame propagation. Up to this point the flame has been contained within an envelope of air which, in the throat, is in process of being introduced by the turbulence in the throat into the rich mixture in which combustion is being supported. Under these conditions there is no difficulty, despite the high velocity, in maintaining combustion during the brief interval of passage through the throat. A desirable perfection of mixture being there achieved, the consequent expansion and relative retardation of flow in the combustion chamber is followed by substantially instantaneous combustion. Immediately thereafter, flow is again accelerated, using the abrupt expansion of the gases to assure their high velocity fiow through the restricted cross section of the immersed fire tube at a rate and for a distance such as to deliver all their heat, in excess of about 350 to 500 F., to the boiler fluid. Success depends, of

course, quite largely upon the immersion of the 'fire tube, for the heated fluid must be capable of receiving heat at the extremely high rate at which it is rejected by the fiue .gases at the high velocity maintained.

The invention has particular application to boilers wherein the fire tube cross section area ranges from a maximum of about .9 square inch per boiler H. P. at a rating of about 225% down to .056 sq. in. or less per boiler H. P. at a rating of 2000% or more. The smaller the tube section in relation to flow, the higher will be the fuel and air pressure required at the inlet of the hermetically sealed system. Apart from mere increase in pressure differentials as between the system inlet and outlet, a device operating at substantial super-atmospheric pressures has advantages in efliciency due to greater density of the gases involved.

I claim:

1. In combination, a fire tube boiler operable at a rating in excess of about 225% and comprising a boiler tank, fire tube means having a total cross section no larger than i; square inch per boiler H. P. and at least partially submerged therein and having a flue gas outlet, separate I '6 mixing and combustion chambers and an intervening throat smaller in cross section than either of said chambers and shorter than either, and through which the chambers communicate with each other, said chambers and throat being in series with said tube means, an igniter in the mixing chamber, and means for supplying all required air and fuel under super-atmospheric pressure to said mixing chamber, said throat,

combustion chamber and fire tube means constituting a continuous conduit completely closed from the mixing chamber to said outlet.

2. The device of claim 1 in which the means for supplying air comprises a compressor having an output air pressure of at least one-third pound per square inch and the means for supplying fuel comprises a pump having an output pressure in excess of that of the compressor.

3. A fire tube boiler comprising in a hermetically sealed system for continuous flow, a mixing chamber, a throat of smaller cross section than the mixing chamber and disposed at the outlet thereof, a combustion chamber of larger cross section than the throat and into which the throat opens, and a fire tube leading from the combustion chamber and provided with a fiue gas outlet; together with a tank of liquid to be heated and in which at least a major portion of said tube is contained in a position immersed in such liquid; the mixing chamber having a central nozzle, an igniter, means for supplying fuel to the mixing chamber through the nozzle at substantial super-atmospheric pressure, means for supplying air to the mixing chamber about said nozzle at substantial super-atmospheric pressure but less than that of the fuel, baffle means for diverting and segregating a predominantly major portion of the air from the immediate vicinity of the nozzle, said air supply means delivering a sufficient quantity of air to the immediate vicinity of the nozzle to support combustion, said latter quantity of air being immediately mixed with said fuel topermit the burning of the fuel to be initiated by the igniter, said bafile' means terminating between the nozzle and the throat for the release and exposure of the segregated air to the burning fuel mixture, and the throat being adapted to promote abrupt and substantially complete admixture of the burning fuel with said segregated air for immediate combustion in the combustion chamber of the remaining fuel.

4. The combination set forth in claim 3 in which the fuel supplying means supplies liquid fuel at a predetermined rate, and the cross section of said throat is no greater than 1.5 square inches per gallon of fuel per hour.

5. The combination set forth in claim 3 in which the mixing chamber is provided with means for supplying, under pressure, liquid fuel at a predetermined rate, and the fire tube has a cross section less than three square inches per gallon of fuel per hour.

6. The combination set forth in claim .3 in which the fire tube comprises a helical coil in the tank, and at least the combustion chamber is disposed in the tank inside such coil, the cross section of said throat being no larger than 1.5 sq. in. and the fire tube no larger than 3 sq. in. per gallon of fuel per hour, the cross section of the combustion chamber being not substantially greater than required to effect complete combustion.

7. A method of burning fuel in a fire tube boiler which comprises mixing the fuel at subthe rate of flow from exceeding the rate of flame propagation after thorough admixture of air and fuel is achieved, abruptly completing combustion at the relatively retarded rate of flow,

and delivering heat from the products of combustion while flowing them at relatively increased velocity through the boiler fire tube.

-8.- A method of burning fuel in a boiler having a fire tube, which method comprises compressing air toa pressure'of at least one-third pound per square inch, delivering fuel to a portion'of'such air, igniting the fuel, mixing with said portion and the burning fuel all the air required for complete combustion of said remaining fuel, the addition of such air constituting said fuel afully combustible mixture and the air being added during continuous initial flame acceleration and while abruptly accelerating flow prior to substantial combustion, suddenly expanding the mixture and relatively retarding its flow during substantial completion of combustion of. the fuel,. and thereafter relatively reaccelerating said flow in the fire tube.

9. The method. set. forth in claim 8 in which not to exceed about. 2% of the fuel is burned prior to said abrupt acceleration.

10; In combination in a high rating fire tube boiler, a boiler tank, fire tube means extending through such tank and ending in a flue vent, a combustion chamber at least partly within the boiler tank. and opening into the fire tube means, an annular wall of. tapering, section providing a restricted throat opening into the combustion chamber and much smaller in cross sectionthan the combustion chamber, amixing chamber ten to fifteen times larger in section than the throat and for which the saidv wall provides an end, the mixing chamber communicating through the throat with said combustion chamber, means for supplying air to. the mixing chamber at a pressure in excess ofone-third pound per square inch, meansfor supplying fuel to the mixing chamber at a. pressure exceeding that of the air, and. means for igniting. the fuel in the mixing chamber, said air supplying means. including 'separateprimaryair and secondary air delivery passages, the. primary air passage opening into said chamber adjacent. said fuel supplying means and having an air capacity only sufiicient to support combustion. of a small percentage of such fuel, the secondary air delivery passage opening into the mixing chamber between said fuel supplying means andsaid throat, for delivconsumption of more than-a few per. cent-of the fuel to abruptly mix-thev remainder ofthe airin the throat and to assure abrupt combustion in said combustion chamber and to develop high gas velocity in said fire tube means for ahigh rate of heat delivery thereto.

11. The device of claim 10 in which the fuel supplying means comprises an atomizing nozzle and means for delivering a liquid fuel thereto, the throat section being between .43 square inch and 1.5 square inches pergallon of fuel per hour and the fire tube section between .20 square inch and 2.5 square inches per gallonv of fuel per hour.

12. The combination'with' a fire tube having a flue gas outlet, of a fuel and air admission chamber, a throat and a combustion chamber in series connection and in communication with said fire tube for the delivery of products of combustion thereto, an igniter in the admission chamber, means for supplying fuel to the admission chamber at a pressure in excess-of one-third pound per square inch, means for supplying to the admission chamber all of the air required for the combustion of such fuel at a pressure of at least one-third pound per square inchand less than that at-which fuel is supplied, said admission chamber, throat, combustion chamher and fire tube constituting a continuous and hermetically closed passage from said air-supplying means to saidoutlet, the ratio-of the cross section of the throat to the fuel supplied by the fuel supplying means being no greater than -a ratio of 1.5 square inches per gallon of liquid fuel per hour and the throat being smaller in cross section and shorter in length than either of said chambers, and the ratio of the cross section of the fire tube to the rate of fuel supply being no greater than a ratio of 3- square inches per gallon of liquid fuel per hour, the air supplying means comprising a baflie for withholding nearly all of the air unmixedwiththe fuel until the air and fuel in the-admission chamber approach the throat, the throat being located in the pathof the burning fuel and: unmixed air, and abrupt mixture of such air with the burning fuel taking place in the courseof the acceleration required to traverse the throat, said acceleration being produced by thereduction in section between the mixing chamber and the throat, and abrupt and substantial completion of combustion taking. place inv the combustion chamber for effecting a high flue-gas velocity and rate of heattransfer in the fire tube.

JOHN J. WOLFERSPERGER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 974,166 Maxim Nov. 1, 1910 1,097,698 Whitcomb May 26,1914 1,948,537 Noack Feb. 27, 1934 1,948,538 Noack Feb. 27, 1934 1,986,561 Davis Jan. 1, 1935 2,097,255 Saha Oct. 26, 1937 2,173,115 Hutto Sept. 19, 1939 2,264,226 Toner Nov. 25, 1941 2,321,109 Sellers June 8, 1943 2,404,335 Whittle July 16, 1946 OTHER REFERENCES Marks, Mechanical Engineering Handbook, 4th edition, pp. 1162, 3.

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