US1623773A - Oil still - Google Patents

Description

J. E. BELL OIL STILL Filed A112. 10 1925 2 Sheets-Sheet l @i www SNNSK a l lNVENTOR y KNN .SY

ATTORNEY:

April 5 .1927.

J. E. BELLI OIL STILL Filed Aug@ 1o, 192:5

. SQ /A L Sukkx .Sw/

MINVENTOR ATTORNEY;

Patented Apr. 5, 1927.

UNITED STATES PATENT OFFICE. l

vJOI-IN E. BELL, OF BROOKLYN, NEW YORK. ASSIGNOB TO SINCLAIR REFINING M- PANY, OF CHICAGO, ILLINOIS, A CORPORATION OF MAINE.

OIL STILL.

Application mea August 1o, 1923. serial No. 656,671.

This invention relates to improvements in the construction and operatiton ot oil stills, and more particularly of oil cracking stills,

'whereby the efficiency aud capacity of the still is increased.

The invention is ot special value for cracking petroleum hvdrocarbrms of higlrboiling point, such as kgas oil and the like. 1n pressure stills to form morn` volatile or lighter hydrocarbons therefrom. such as those. which constitute` commercial l`Q'asoline.

Pressure stills and other stills for cracking oil are heated by t'urnaces in which coal,

.gas or oil is burnedand. for economy df fuel consumption. it is desirable that therduel be burned in-the furnace with 'the leasts'amount of excess air and the maximum furnace temperature. Excessive temperatures are, however, objectionable in the heating chambers of oil cracking stills. Carbon is formed during the cracking operation and tends' to deposit on the side of the tubes or heating surfaces of the still, and such a layer of carbon or of heavy tar is a poor conductor of beat and insulates thel metal surfaces from the oil on the inside, With the result that the temperature of the metal is increased. and this increase of temperature becomes excessive with increased thickness of the carbon deposit. The overheating of the still or of the tubes or heating elements of a cracking still, and the resulting increased tendency of carbon formation, tend to give hot spots, and involve a considerable element of danger or hazard to life and property due to the possible bursting of a tubewhichhas become overheatedand burned. Inasmuch as the pressure in cracking stills is frequently maintained at pressures around 10() -pounds per square inch and may be maintained considerably higher. for example, around 60o-700 pounds, any break or rupture is followed bv a disastrousl fire.

ln an oil cracking still the tubes or other heating surfaces most exposed to the heat of the furnace will be the tiret to suffer. The danger ot' overheating such tubes or surfaces can be decreased b v reducing the fuel consumption, but this in turn reduces the capacity ofthe still, while. if the still is driven hard to increase its capacitv by increasing the tuel consumption. the danger 0f overheating is greatly increased,

According to the present invent-ion, danger of overheating is avoided or minimized, and a heating action ot increased uniformity is obtained, while the invention nevertheless enables the still to be operated with increased eticiency and increased capacity ot' output.

According to the present invention. I return and admix-with the products ot combustion in the Firebox or between the.firebox and the heating chamber of the still,`a part of the hot flue gases from near the outlet of the heating chamber. In addition I preheat air by heat interchange with the flue gases tures and combustion. By regulating the amount of the flue gases returned and ad- Vmixed with the products of combustion, the

temperature of the heating gases can be reduced from the highesttemperatures of the irebox to a predetermined lower temperature before the gases come in contact Withthe heating surfaces of the still. The flue gases which are returned are at approxi- 4mately the temperature of the waste gases leaving the heating chamber, and. 'while these gases serve to reduce the high temperatures obtained with the preheated air in the irebox, they reduceit only a part ofi/the Way down to the temperature ot' the't returned gases. Accordingly. the dilution of the. products of combustion is notaccompanied by Va reduction in eiiiciency-such as accompanics the burningot .fueljwith an excessive amount of cold air. or sueltas-would accompany the introduction ""o'f lcold gases..\0n

the contrary, the reduction in efficiency commonly incidentl to the use of cold air in the tireboxv is avoided by the use of preheated air, and the eiiiciency of the heating operation thereby improved; AWhile the operation '1 -atures is thus taken I enable an economy in fuel l CI'ERSES is further improved and danger of overheating avoided or minimized, and a more uniform heating effect obtained, bythe return of the flue gases for admixture with the products of combustion. f

In my prior application Ser. No. 532,432, filed January 28, 1922, and my companion application, Ser. No. (356,672 filed August 10, 1923, I have described and claimed the improved method and apparatus in which a part of the flue gases are returned for admixture with the products of combustion to lower the temperature of the products of combustion before they come in contact withf-fy the heated surfaces of the still. The present invention includes the improvement of said prior applications and further improves the operation of the stills b v increasing their efficiency and capacity. In addition to returning a partI of the tiue `ases the present invention provides for utilizing the flue gases after they have served their purpose in heating the still, 'by preheating air therewith and supplying the preheated air for promoting theA combustion and improving the heating operation. This-'improved heating operation which would otherwise be objectionable because of the increased temperadvantage of and its objections overcome. The increased intensity ofcombustion and the increased temperatures obtained by the use of the preheated air consumption to be ,obtained with like heating effect, or, with the same fuel consumption, cause an increased heating effect; while the moderating of the high temperatures by ,the admixed hot flue ga's gives a more uniform heating effect. de-

the temperature of the gases first .coming in contact with the still, inc-reales the velocity of the gases passing over the heating surfaces, and gives an increased 'heating effect at parts of the still more remote from the fircbox.

The `invention as above pointed out is of more or less general application to oil stills where overheating isV objectionable and where increased efficiency of operation and increased output, without objectionable overheating. are desired The invention is of special value, however,'in conjunction with oil cracking stills, for reasons above` pointed out.

The oil cracking stills `to which the inven' 3 be either stills of the tion is applicable may shell type, or tube stillszand, in the case of tube/stills, the stills maybe circulatory stills "peratures in the lire box which,

where the oil is circulated from-a large sup; ply tank through the heating tubes and back to the supply tank, or it may be a tubular still in which they oil is passed in a single pass through the coil withoutrecirculation. The invention is particularly advantageous in connection with such tubular cracking stills where the heated gases come in Contact with a part of the tubes and subsequently pass over other tubes or other portions of the same tubes before escaping to the stack.

The charging stock employed in the cracking stills can be varied and pressure maintained during the crackingy operation may similarly be varied. In cracking heavier oils such as gas oil, pressures of around 90-125 pounds, for example, may be used, while in cracking lighter oils 7 of the nature of kerosene, a considerably increased pressure may be employed, for example, around 300-325 pounds or much higher, e. g., around GOO-T00` pounds. The cracking stills may be operated either to produce pressure distillate, or a so-called synthetic crude and, in the case of a tubular cracking still, the oil may be recirculated through the tubes or may pass therethrough in a single pass.

In the case of a tubular cracking still the tubes are advantageously ar'ranfred in a vertical heating flue and the heating gases introduced near the top of the flue and discharged near the bottom. By arranging the firebox on a level with the bottom of the tubes andby providing a bridge wall separating the firehox from the heating chamber, the tubes are protected from direct radiation and the flue gases may advantageously be introduced in the vertical flue between the firebox and the heating iue. The heat interchanger for preheating the air by contact with the flue gases may also be arranged in a vertical flue. By providing a sufficicntl ly intimate Contact of the flue gases with the entering air, the air can be preheated to a temperature approximating that of the flue gases escaping from the heating chamber and the flue gases in turn may be cooled to 1 a relatively low temperature. In case' the natural draft of the flue gases is not sufficient, this may be supplemented by forced draft, for example, by using a blower. The. introduction of air may similarly be effected 1 by the suction of the burner or the natural draft. of the furnace` or it may be suppleinentcd by forcing means such as a blower. The return of the flue gases from near the outlet of the heating chamber to the fircbox. or to the fiue intermediate the firehox and still. may similarly be effected bv suitable circulating means such as a blower or .-tcam et.. f

The relative proportion of returned flue 1 gas to fresh products of conibusion can be varied and regulated as desired. The temwithout the prcheatingr of the air` might be around 2400o F., may, with the preheating of the. air. reach a much higher temperature', but the cooling effect of the ,tine gases returned. for example, at a temperature around 800 F., will serve to reduce such excessive temperatures to a temperature which will be suitable for' the and the temperature l creased volume of heating-glises passes ing effect in the heating of the still, without .objectionable overheating; while the heating operation i.' made/more uniform, and an inthrough the heating chamber and in contact with the heating surfaces of the still `with an increased velocity7 which increases the rate ana elliciencv of heat absorption and has the further desirable result or tendency of carrying soot and slag through the heating chamber rather than depositingit upon .v the heating surfaces.

The invention requlres but little modi,- fication of furnacev structures such-"as are 'A -commonly used in heating oil cracking stills.

The ue gases may, for example, be'returned through openings or llues in the walls, floors or lining ofthe' furnace-and a sim le blower or steam ljet may used for regu ating the amount offlue gases returned. The prein heater for the `air may similarly be arranged in the outlet ue or stack and connections made between the preheater and the burner or rebox in such a way 'as to supply the preheated air in the amount required for theI combustion. The returned flue gas may also be returned and introduced around the walls of the tirebox and bridge wall so as to protect or assist in protecting thearch or other parts of the furnace from the intense heat of combustion.

The increase in the rate of gas flow over the heating surfaces, while it increases the `rate and eliiciencv of heat absorption, also increases the friction loss, while the passing .of the flue gases and air through the preheater introduces additional friction, and, to maintain the same furnace draft it is necessary to have a somewhat increased draft at the still outlet. When this is provided, however, the capacity of the still will be increased and its efficiency will also be increased. while the still will nevertheless be protected from objectionable overheating.

The invention will be further described in connection with the accompanying drawings showing two types of oil stills with which the invention is combined. Both of these stills are tubular stills. The still illustrated in. Fig. .1 is of the general construction and operation set forth in Patent No. 1.285.200,1granted to the Sinclair Refining Co. November 19, 1918,-011 the application of Edward W. Ison. The invention of particular advantage, as above noted, in conjunction with this and other tubularl cracking stills; but it will le understood that the invention is of more general application in its broader aspects.

In the accompanying drawings, Fig. 1 shows a still of the general character above mentioned, partly in section and partly in elevation, the illustration being somewhat conventional and diagrammatic; F1g. 2 1s a vertical view partly in section and partlyin elevation showing the furnace i and cracking coil of a modified form of cracklng st1ll;

- Fig. 3 is a sectional view'taken on the line 3--3 of Fig. 2; and

Fig. 4 is an enlarged sectional view showing one of the elements of the heat interchanger.

Referring `first to the construction illustrated in Fig. 1, the still illustrated is a tubular cracking still of the general construction illustrated in. said Patent No. 1,285,200. The 7still comprises the bulk supply tank or drum 1', the vertical heating tubes 2, the pipe connections `3, 4 and\5 connecting .the ulk supply tank with the top and bottom headers or manifolds of the heating tubes, and the pump 6 for circulating the oil from the su ply tank or drum through the heattu es and back to the supply tank.

he still is provided with a reflux tower V7 connected with the vapor dome of the still by a vapor line 8 and arranged to refurn the condensed vapors through the pipe 9.

The irebox 10 is shown as having one or more oil burners 11, althou h theinvention is not hmited to the use o oil as the fuel.

A vertical flue 12 connects thevirebox with the upper4 end of the heating flue 16 in which the vertical heating 'tubes l2 are located.

The bridge wall 13 is relatively high and .separates the vertical flue 12 from the heating flue 16, while also-protecting thel heatmg tubes., from direct radiation from the iirebox. Arranged Within the heating iue 16 are battles 14 and 15 which cause the heating gases to pass over the heating tubes in three successive passes before they escape' through the outlet connection 17. Balles 18 l protect the lower headers from direct contact with the heating gases.

A vertical 'flue 19 is connected with the outlet 17 .at its lower end and with-the L9 mii stack connection 20 at its'upper end. Ar' y ranged within this 'verti'catflue are a series of tubes 2l which may be of the construction illustrated in Fig. 3, Air enters the chamber 22 at the top of these tubes.` and flows downwardly therethrough countercurrent tothe upward ilow of ases from the furnace. A fiue 23 connects e lower ends of these air heatingtubes' and leads the pre- .heated air to the connecting flue or dues 24 13 and supplies the preheated air to the burners. lVith oil burners 11 such as those illustrated, these burners will have an entraining effect and will serve to dravv7 the preheated air through the openings 25, and this induction of air may be supplemented by the stack draft or by supplemental forcing means.

Side connections, best shown in Fig. 3, serve for the return of flue gases from near the outlet of the heating chamber to a point 'between the firebox and the first contact of the heating gases with the tubes. Two side connecting pipes are shown-leaving the heating chamber at 26 near the outlet therefrom. Steam injectors 28 are shown for permitting the flow of these gases through the connecting pipes 27. These gases are discharged into the flue 29 and escape through the openinu or openings 30 at the bottom of the. bridge wall and tend to rise along the front of the bridge Wall and serve to protect the bridge Wall, while they are gradually admixed with the products of combustion 'and serve to cool them before they come into contact with the heating tubes 2. v

In Fig. 2 the same or corresponding parts are indicated by the same reference numerals as in Fig. 1. The general arrangement of the rebox, heating flue and preheater is the same as in the construction of Fig. 1. The heating tubes, however, are horizontally' arranged and mav be the tubes of a single pass cracking stil in which the oil enters at an inlet 31 at the bottom and passes up in a series of passes backand forth from one side of the still to the other through the heating tubes 3:2 until the cracked oil escapes at 33. In the arrangement shown, the top and bottom headers 34 and 35 are each connected with ten tubes which are connected by end connections 36 with the intermediate tubes so that a series of ten separate circulating streams is provided between the bottom and top headers. When the heating coil is thus used as a single pass cracking coil, the oil heated therein may discharge into a. chamber lnaintained at the saine pressure and in which further cracking takes place. or the cracked oil may discharge into a vaporizing chamber or into a cooler or condenser maintained either at the same. pressure or at atmospheric pressure or at a predeteimined intermediate pressure.

rl`he heat intcrchauger for preheating the air by contact with the flue gases should be ot' a construction which permits effective heat transfer. 'lhe construction of heating tube shown in l"ig. 3 enables such an intimate heat transfer to be obtained. These tubes are made of sheet metal and have a minimum thickness. for example, of .i3/Gt of an inch. vWith tubes having suoli exceedingly thin metal Walls it is possible to arrange a large number of tubes and a corresponding number of spaces therebetween so that a very large surface of heat interchange is provided between the down-flowing air and the up-lovving flue gas. This enables lthe air to be preheated effectively to a temperature approximating that of the escaping flue gases. By returning such preheated air and utilizing it for the combustion, the efficiency and temperature of the combustion is increased, but this increased combustion is neutralized by the returned flue gases so that the heating gases that come in contact with the tubes of the still are at a regulated temperature which Will avoid objectionable overheating of the tubes.

It Will be noted that the general direction of the flow of the heating gases and oil is opposed so that the relatively cooler oil enteringat the bottom of the tubes extracts the heat from the partially cooled gases and the oil which has thus been heated is exposed to the hottest gases in the upper portion of tbetubes, or in the upper tubes of Fig. :2. The temperature of these gases 1s,` however. tempered so that objectionable overheating is avoided, and the increased velocity of the gases will cause an increased amount of heat to be absorbed in the lower passes of the heating fiue.

In practice there may be a small leakage of air through the furnace structure and into the heating chamber, which results in the introduction of a certain amount of free oxygen into the furnace gases. This oxygen will be heated 'by the gases with which it admixes and may serve to bring about a slight additional combustion, but this is independent of the main preheating effect obtained in the tirebox by preheating the air required for combustion and supplying this preheated air to the lirebox.

It will thus be seen that the present invention provides both for increasing and for decreasing the temperature in the furnace and that this increase and decrease are successive and are so combined that full advantage can be taken both of the increased temperature of combustion made possible by preheating the air, and of the lower temperature and increased gas volume and velocity made possible by admixing hot flue gases with the products of combustion before the heating gases are employed for heating the still. An increased heating etfect. but neverthele--s a mild heating effect. can thus be obtained, and the etliciency of the heating operation improved and the capacity o the still increased; and can be obtained Without increased fuel consumption and Without objectionable overheating of the still, as \vell as without the employment of supplemental equipment other than the preheater for preheating the air by contact with the flue gases and such adthese improved results lOfi ditional means as may be required to give the necessary draft or circulation of the air and gases lthrough the apparatus.

I claim:

l. The improvement in the heating of oil cracking stills in which oil is heated to the cracking temperature by hot products of combustion from a firebox passing externall over the oil heating surfaces of the stil, which comprises admixing a portion of the hot waste heating gases which have previously passed over the heating surfaces with the products of combustion from the firebox before passing them over the heating surfaces of the still, 4preheating air by heat interchange with waste heating gases not to be returned as a diluent, and supplying the air so reheated to the lirebox for combustion of uel therein.

2. The. improvement in the heating of oil cracking stills in which oil is heated to i the cracking temperature by hot products of combustion 'from a irebox passing externally over the oil heating surfaces of the still, which comprises diluting the hot products of combustion from the firebox with recirculated waste heating gases prior to bringing the heating gases into contact with the oil heating surfaces, preheating air by heat interchange with the waste heating gases and supplying the reheated air to the rebox for increasing t e temperature of the roducts of combustion with which the recirculated waste gases are admixed.

3. The improvement in the heating of oil cracking stills in which oil is heated to the cracking temperature by hot products of combustion froml a lirebox passing externally over the oil heating surfaces of the still, which comprises supplying preheated air to the lirebox for combustion of fuel therein, reducing the temperature of the resulting products of combustion from thev firebox by admixing hot waste heating gases therewith, passing the resulting gaseous mixture over the oil heating surfaces of the still, and subsequently passing the Waste heating gases not to be4 againv admixed. with the products of combustion from the firebox in heat interchangiiig relations with air to be supplied to the firebox for combustionof fuel therein.

4. The improvement `in the heating of oil crackino` stills in which'oil is heated to the cracking temperature by hot products of combustion from, a firebcx assing externall over the oil heating sur aces of thev stil, which comprises recirculating al part of the hot waste heating gases which have previously passed over the heat-ing surfaces and admixing them with the hot products of .com-

bustion from the irebox to give a gaseous,l

mixture of reduced temperature, assing the resulting gaseous mixture over t e heating surfaces of the still, preheating air by heat interchange with hot waste heating gases which have previously passed over the heating surfaces of the still, and supplying the preheatedy air to the firebox for improving the .combustion therein, whereby the teinperature of combustion is increased by the preheated air and the increased temperature is reduced before the heating gases contact with the oil heating surfaces by the admixture of the hot waste heating gases.

5. I`he improvement in the heatin of oil cracking stills in which oil is heate to ther cracking temperature b hot products of combustion from a fire ox passing 'exter-v nally over the oil heating surfaces of the still, which comprises circulatin the oil and the heating gases from the lirebo'x lin heat exchanging relation through the heating surfaces of the still and in generally countercurrent directions, preheating airby heat interchange with hot waste heating gases which have passed over the oil heating surfaces and -supplying the air `so preheated to the firebox for the combustion therein, dilutin the het products of combustion from tie lirebox with hot .waste heatingl gases at a lower temperature, and circulating the resultingv gaseous mixture over the heating surfaces of the still.

6. lhe improvement in the heating of oil cracking stills in which oil is heated to the cracking temperature by hot products of combustion from a iirebox passing externally over the oil heating surfaces of the still, which comprises diluting the hot products of combustion from the iirebox with het gases at a temperature approximately .equal to the temperature of the waste heating gases leaving the oil heating surfaces, preheatmg air by heat interchange with the iot waste heating gases whichvhave previously passed over the heating surfaces and supplying the preheated air to the lirebox the iirebox by supplying preheated air to the' firebox, then decreasing'the temperature of the roducts of combustion from the firebox y admixing recirculated waste heating.

gases therewith, passing the resulting gaseous mixture over thev oil heating surfaces,

and preheating the air supplied to the lirebox,by heat interchange with the hot waste 'iis heating gases escaping from the still.I the recirculated hot waste heatinggases being returned immediately after passing over the oil heating surfaces and being at approxi- 'mately the temperature of the heating gases 'for admixture with the hot products of combustion Jfrom the firebox before introducing them into the first pass, preheating air by heat interchange with Waste heating gases leaving the last pass. and supplying the preheated air to the tirebox for increasing the temperature of the products of combustion with which the returned gases ai'e admixed.

9. The improvement in the heating of oil cracking stills in which oil is circulated from a bulk supply through heating tubes and back to the bulk supply and in which the oil circulating through the heating tubes is heated to the cracking temperature by hot products of combustion from a firebox passing externally thereover, which comprises passing the heating gases from the iirebox over the heating tubes, returning part of the hot Waste heating gases which have passed over the heating tubes and ad mixing them with the hot products of combustion from the rebox to cool the latter prior to passing the admixture over theheating tubes, preheating air by heat interchange with the hot waste heating gases and supplying the preheated air to the firebox for the combustion of fuel therein.

10. The improvement in the heating of oil cracking stills in which the oil is circulated in a single pass through heating tubes and in which the oil is heated to the cracking temperature in the heating tubes by hot roducts of combustion from a iirebox passing externally thereover, which comprises passing the heating gases over tubes in a generally countercurrent direction to the iow of oil, reeirculating a part of the hot waste heating gases which have previously passed over the heating tubes and admixing' it with the hot products of combustion from the irebox to give heating gases of lower temperature before passing the heating gases over the heating tubes, and increasing the temperature of the hot products of combustion from the firebox with which the returned gases are admixed by preheating air by heat interchange with waste. heating gases not to be again recirculate'd over the oil heat-ing surfaces and supplying the preheated air to thelirebox for the combustion therein.

ll. The improvement in theheating of oil cracking stills in which the oil is circulated the heating in a single pass through heating tubes and tubes in a generally countercurrent direction to the lflow of oil, recirculating a part of the hot waste heating gases which have previou'sl passed over the heating tubes and ad- 'mixing it with the hot products ot' combustion from the firebox to give heating gases of lower temperature before passing the heating gases over the heating tubes, and increasing the temperature of the hot. produetsof combustion from the tirebox with which the returned gases are admixed by preheating air and supplying the preheated air to the firebox for the combustion therein, the preheating of the air being effected by heat interchange with hot waste heating gases which have passed over the heating tubes.

12. An oil cracking still having a heating chamber, oil heating surfaces adapted to be externally heated therein, a irebox. means for conducting hot products of combustion from the firebox to the heating chamber. means for diluting and cooling the hot products of combustion from the lirebox with hot Waste heating gases from the heating chamber before entrance into the heating chamber, and means including a heat interchanger for preheating air by heat interchange with hot waste heating gases not returned to the heating chamber and for suppl 'ing the air so preheated to the firebox tior combustion.

13. An oil cracking still having a heating chamber, oil heating surfaces adapted to be externally heated therein, an inlet and an outlet to the heating chamber, a firebox communicating with the heating chamber at the inlet, a ue communicating with the outlet of the heating chamber, a fiue connecting the heating chamber near the outlet with the firebox and means for causing a return of a ortion of the waste heating gases from the eating chamber through said flue to the firebox, and means including a heat interchanger arranged for preheating by heat interchange with Waste heating gases in the first named flue and for supplying the air so preheated to the tirebox, whereby the temperature of combustion in the firebox is increased and this temperature is decreased by the admixture of returned Waste heating gases.

14. An oil cracking still having a heating chamber, oil heating surfaces adapted to be externally heated therein, an inlet and an outlet to the heating chamber, a firebox communicating with the heating chamber at the inlet, means for returning hot Waste heating gases from near the outlet of the heating chamber and admixing them with the hot products of combustion from the firebox before the heating gases come in contact with the heating surfaces of the still Ain theheating chamber, a heat interchanger arranged for preheating air by heat interchange with the waste heating gases from the outlet of the heating chamber, and meansy for introducingthe preheated air into the irebox.

15. A tubular oil cracking still having a heating chamber, oil heating tubes adapted 4to be externally heated therein, a firebox communicating with the heating chamber,

. means for admixing hot- Waste heating gases from the heating chamber with the products of combustion from the firebox at a point between the firebox and the heating chamber, and means including a heat inter-changer for'preheating air-by heat interchange with the remainder of the hot Waste heating gases from the heating chamber and for supplying the air so preheated to the, firebox fork combustion.

16. A tubular oil cracking still having a heating chamber, oil heating tubes adapted to be externally heated therein, a firebox communicating with the heating chamber vmeans for supplying preheated air to the rebox for the combustion of fuel-therein,

and means for admixing hot waste heating i gases from the heating chamber with the products of combustion from .the lirebox at ranged in the heating chamber, a firebox communicating with the upper end of the heating chamber, an outlet near the lower lend of the heating chamber, a heat intery changer arranged for preheating air by heatexchange with the hot waste heating gases from the outlet, means for supplying the preheated air to the rebox for the combustion ofvfuel therein, and means for returning hot waste heating gases from near the outlet of the heating chamber and admixing them with the pro-ducts of combustion from the firebox at a point between lthe firebox and the heating chamber.

--fJoHN n nr Ln) In .testimony whereof I ailx mysi atu-re. Y.

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