US2507662A - Heating apparatus - Google Patents

Description

y 1950 J. M. CRIPPS 2,507,662

HEATING APPARATUS Filed m. 10, 1946 Irlcil ynvenlor v .A

M Clip 0s.

w a jam Patented May 16, 1950 HEATING APPABA'J-ZUS.

James M. Crisps, Wharton. I x., assignor 15g ere iwfie ea '10, .52 see N9- mess This invention relates to an improved heating apparatus, more particularly to an improved heating e011 structure.

In the arts there are many processes in which it is de s ired to heat fluids; such as gases, to high temperatures Her etofore; gases havebee'n heated in furnaces equipped with pipes or tubes'fcon nected with return bends to form a continuous coil. Such tubes have been fabricatedbf'iron and its alloys 'par'ti'cularly"th'e heat resistant chromium and nickel-containing alloys. "Aries sential limiting factor in'th'ermal processes utili'zi'ng such'tubes 'is' the temperature atwn ch the alloys softener deform. a 'gen'e'ralrule such limiting temperatureis "of the order of 1800 E. "When the relatively available ferrous alloys are used. When temperatures substa'n tially above this are invo ved other types joffniateri'al must'beu'sed.

f suitablerefractories.

In processes involved the heating of gases to temperatures substantially" above 1300 Cftub'e structures'of high" melting point metals, such'as molybdenum and tungsten have bee'nsug'g'ested and employed. However, these 'metals' arecuite' expensive not only becausebf" their relative scarcity but also because of the difiiculty in 'fab ricating these metalsinto' the ultimate desired shapes; furthermore, these metals also have properties that" prevent their economic usein continuous heating ofgasesto hightempera turesu The more commonly used structure for heat ing'gases to temperaturesabove abOut IBOO F. consist of regenerative furnaces constructed of checker-work or" refractory ceramics. High temperatures may be attained in'thi's type of furnace. The operation of, this type 'offurnace, as is known, is not particularly efficient. As is known,

these operate on a cyclic principle, 1. e., the flow of gas therethrough must be periodically stopped or diverted to permit heating up of the checker; work. Even when employed in batteriessuch furnaces are not too desirable because the diver sion and variation in flow and temperature is reflected in undesired variations in the products discharged fromthe furnace. Such variations usually complicate the treatment of the gas following the heating. Furthermore, furnaces of this type require a number of expensive valves that must operate efficiently at high tempera? ture in order properlyto control the alternate introduction'ciair and gas into the furnace" and their discharge from the furnace through'f'sepa rate channels.

Ithas been found that gases may be. highly stre sl we. .9 s q s i ed by bassin he a ff b h ternallyheated coil'fcompose' series of "tubes andfreturnfbends T con truc In order to enable a more ready comprehenion; o t e nv n on er is sh w n h s companying drawingfa typical physical embodiment. o t inrent on n hic F ei a n l inmati llu tr t on of a i de' n wh c e mp meld sentiment i i ricrei mw te li n Fis- S. nlar ed v t ca sec io al eta of the returnbendand associated tube'sections wh h. ru' f t orm o It has been found, as is described in the copendin g application of Otto Spring, Serial No. 540,385; filed June 15, 1944, now Patent No. 2,46;617, that gases may be heated to high'tem peratures of the order (of 2500 F! to 3000i F. by passing the gas through a specially constructed refractory'coil, theexterior of which is exposed to hot combustion gases. As there disclosed,

such 'a' continuous'coil may be mounted within a' fired furnace and such coil was established by associating tubesof suitable refractory material such as Carbofrax (silicon carbide) with return bends formed of the same material. The lower return bands were positioned or'ifthe fioor of the furnace andtheassociat'ed' tubes eittion of" the coil. In H mace thetubular units tended vertically from the lowerto the upper return bends. In this way the tubes were wholly supported on the furnace floor. Theupper return'bends were'spaeed somewhat belowthe roof of'the furnace so that the tubes were free to expand and contract ina'vertical direction with change in temperature in the furnace. In operation, gases to'be heated were passed through the coil while the exterior of the coil was exposed to hot'combustion gases in the furnace. With such a refractory coil gases may be heated to temperatures of 2 5Q 0 ,F. to 3000 F.

In such prior coil construction the return bends were each molded in two symmetrical portions and after baking the two sections were cemented together. This provided areturn bend in which the joint or seam between the sections was in the vertical axis or plane of the tube seca-lliilg the coil ill alfur inserted recesses in the return" bendsv ands itabl me t "It was learned that in commercial use of. such was found that after a certain period of operation, varying between one week and several months, gas began to leak from the coil. Upon examination it was ascertained that this was due to a slit or fissure which formed in the return bends. This crack or slit was generally coincident with the seams and was due to the breakdown of the cement joint. The fault could be temporarily remedied by applying cement to the seams. However, after some additional time of operation the seams again opened and the cementing had to be repeated. This, of course, involved a considerable loss in time for shut down and repair. Such repairing furthermore was not complete due to the fact that the portion of the slit contiguous the furnace floor could not be reached and thus remained an open leak.

After considerable experimentation and trials it was found that the difficulty could be practically obviated to insure marked longevity in the coil by forming the sections of the return bends so that the contiguous faces or the joint between the sections are in a horizontal plane, i. e., parallel to the floor of the furnace. This structure proved to be as efficacious as it was simple.

The improved coil structure and its operation can be more readily comprehended from a consideration of a simplified embodiment depicted in the accompanying drawings.

As there shown, the coil, designated generally by the letter C, is positioned in any suitable type of furnace. Such furnace may, for example comprise a floor I, end walls 2, side walls, not shown, roof 3 and the stack 4. The furnace may be fired by any suitable means such as the burner 5. A bridge wall 6 may or may not be utilized in the furnace structure.

The coil C preferably is constructed of a suitable refractory material, such as Carbofrax (silicon carbide) and is made up of the tubular units 7 and the return bends 8. The tubes 7 and return bends preferably are of standard shape and are therefore readily interchangeable and replaceable. The tubes may be of any desired size depending on the demands or requirements of a particular operation. Operations have been conducted utilizing tubes of 4 in. diameter and 52 in. in length.

The main coil may be connected to an inlet pipe 9 and to a discharge pipe or line If! through suitable connections H such as suitably conformed return bend units.

As shown more particularly in Fig. 2, the return bend comprises a lower section [2 and an upper section l3. The upper section is molded with the vertical cores I4 which communicate with the horizontal core portion l5. As shown, the upper section may be suitably conformed so as to provide the shoulders I6 upon which the terminal edges of the tubes 7 are supported.

The lower section l2 of the return bend is molded to provide a core portion 5?. As will be observed, when the two sections are assembled with the contiguous faces in registry a passageway provide adequate space for unrestricted thermal expansion of the tubes.

It will be observed that in this type of structure the entire weight of the tubes is wholly supported on the floor of the furnace through the lower return bends. Since the seams or joints of the return bends are in a plane normal to the axis of the tubes and to the plane of the compression load any tendency of the seams to open is resisted by the weight of the coil section positioned above such seam. In other words, the weight of a portion or all of the upper return bends and the full weight of the tubes associated with each lower return bend are advantageously utilized to apply pressure to and hence seal the horizontal seam. In the case of the upper return bends, the weight of the upper section acts to oppose any tendency of its seam to open.

It is to be noted also that by disposing the seam of the return bends in a horizontal plane such seams are spaced above the floor and below the roof and hence the entire area of the seam may be reached for inspection and repair. This is in marked contrast to the prior structure in which, as previousl noted, a portion of the vertical joint of the lower return bend was contiguous the floor and could not be completely cemented.

The marked efficacy and value of this improved structure can more readily be appreciated and evaluated by consideration of comparative operations utilizing the older type return bend with a vertical seam in one case and the novel structure of the invention in the other.

A test was conducted which exemplified the shortcomings of the older type structure. In this, a continuous coil, of the type described, was constructed of a series of refractory tubes and return bends and was disposed within a furnace. The return bends were made with vertical seams in accordance with earlier practice. Relatively cool air was continuously introduced into the inlet of the coil and during passage therethrough was heated by means of hot combustion gas passing through the furnace in contact with the exterior of the coil. The air was thus raised to a discharge temperature of 2800 F. About one month after commencement of the operation fine cracks appeared in the return bends coincident with the vertical seams. While the loss of air was definite and measurable it was initially slight. However, with continued operation of the furnace these cracks widened or expanded and finally became so large that considerable quantities of air was lost, greatly affecting the control of the process in which the heated air was utilized. The furnace was then shut down and the cracks repaired as far as possible with a refractory cement. However, a certain portion of the cracks contiguous the floor of the furnace could not be reached for cementing. While the leakage through such residual cracks was small it did require a compensating adjustment of the quantity of air admitted to the coil. The furnace was continued in use for several months when the cracks reappeared and leakage became so great as to again require repair. Such recurrent repair involved considerable loss of time in cooling down the furnace, repairing the leaks and. reheating to operating temperatures. Considering such loss of time the operating efficiency of the furnace was of the order of 70 to Similar tests were conducted employing the continuous coil structure of the present invention, i. e., a coil composed of vertically positioned refractory tubes and connected return bends the seams or joints of which return bends were in a plane normal to the axis of the tubes. Air was. passed continuously through the coil and heated to a discharge temperature of 2800" F. as in thecontrol test. It was found that such coil insured. the operation of the furnace for prolonged periods of time because the seams in the return bends did not open to form cracks. It was determined that this improved type of coil permitted operation of the furnace with an emciency of over 95%.

It will be understood that the particular embodiment of the invention shown and described is chosen to exemplify the essential principle of the invention, namely, of inhibiting leaks in the seams of return bends or equivalent units by disposing such seams in a non-vertical or substantially horizontal plane such that compressive loads of the coil are positively utilized to maintain an unbroken seam or joint. The invention, as will be appreciated, is susceptible of wide permissive variations in construction and specific design of return bend or equivalent units. For example, while the tube units have been shown as connected in series it is apparent that they may be connected in parallel, utilizing a refractory monolith, in the form of a manifold, in lieu of the separate return bends. Such a monolith manifold may be designed and constructed to present a horizontal duct or aperture extending substantially its entire length which duct is in communication with a plurality of refractory tubes. Similarly, it will be appreciated that the return bend seam does not necessarily have to be a truly flat surface; the essential feature of the invention, as noted, is the utilization of a substantial component of the weight of the tube and half the upper return bend to assist in sealing a joint in the lower return bend.

Again, it is apparent that several units may be employed in one furnace, each unit comprising a plurality of tubes in parallel communication with a single upper and single lower monolith manifold, which units may be interconnected either in series or parallel or both. Such monolith units may be fabricated in one piece or in sections such that the seams are a horizontal plane.

While the tubes have been shown disposed in an essentially vertical plane it will be appreciated that they may be positioned at some angle to the vertical; in such case the vertical component of forces operating to prevent opening of the seams would, of course, not be as great where the tubes are vertical but would be of suflicient magnitude to insure improved sealing of the return bends or ing it into a reaction chamber to react with hy- 6 drocarbons to produce acetylene. Obviously, instead =of air any other gas which does not react chemicall with the refractory material may be heated in the unit for subsequent reaction or other desired purpose. Similarly, it will be understood that this novel unit may be employed as a reaction chamber in processes involving gas or vapor phase reactions at highly elevated temperatures of the range described.

While preferred modifications of the invention have been described it is to be understood that these are given didactically to exemplify the underlying principles involved and not as limiting the useful scope of the invention to the specific illustrative embodiments.

I claim:

1. An apparatus for heating fluids which comprises, a furnace, a ceramic heating unit positioned within the furnace through which fluids to be heated are passed, the unit comprising a series of return bends of refractory ceramic material supported directly on the floor of the furnace, a series of vertically positioned tubes connected at one end with such return bends and at the other end with a series of refractory return bends, which latter return bends are spaced below the roof of the furnace, the said return bends being comprised of cemented sections, the joint between which lies in a plane substantially normal to the axis of the tubes.

2. An apparatus for heating fluids comprising, a furnace, a coil positioned within the furnace through which fluids to be heated are passed, said coil comprising a plurality of vertically positioned refractory ceramic tubes, a plurality of interchangeable return bends of refractory ceramic material connected in series with each end of the said tubes to form a continuous coil, the return bends being formed of cemented sections, the joint between which sections lies in a plane substantially normal to the axis of the tubes.

3. An apparatus for heating fluids which comprises, a furnace, a ceramic heating unit positioned within the furnace through which fluids to be heated are passed, the unit comprising a lower series of return bends of refractory ceramic material supported directly on the floor of the furnace, a series of vertically positioned tubes connected at one end with said return bends and at the upper end with an upper series of refractory return bends, interchangeable with said first return bends, which upper return bends are spaced below the roof of the furnace, the said return bends being comprised of cemented sections, the joint between which sections lies in a plane substantially normal to the axis of the tubes.

JAMES M. CRIPPS.

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

UNITED STATES PATENTS Number Name Date Re. 16,878 Leach Feb. 14, 1928 457,589 De Navarro Aug. 11, 1891 1,711,821 Abbott May '7, 1929 2,230,221 Fitch Feb. 4, 1941

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