US2111380A - Heating of fluids - Google Patents

Description

Mmh 15, 193s.

M. W. BARNES` HEATING OF FLUIDS Filed May 4, 1955 mwwmwwwwwww INVENTOR MARION W. BARNES Patented Mar. 15, 1938 f UNITED STATES HEATING or FLUrDs Marion W. Barnes, Chicago, Ill., assignor to Universal Oil Products Company, Chicago, Ill., a corporation-of Delaware Application May 4, 1933, Serial No. 669,325

11 Claims.

This invention particularly refers to an improved method and means of heating uids such as hydrocarbon oils to the high temperatures required for their conversion and more particularly refers to improvements in the type of furnace wherein separate combustion zones are employed, heat derived from one zone, primarily by radiation, being utilized to quickly heat the oil tothe desired conversion temperature while heat derived from the othercombustion zone, also primarily by radiation, is utilized to maintain the oil at a fairly constant conversion temperature for a predetermined time and combustion gases from both combustion zones being utilized to heat the incoming oil to a temperature below the desired conversion temperature attained by the oil in the rst mentionedcombusf tion zone.

One of the principal problems in the design of furnaces of the character above described. wherein the oil is first heated to the desired conversion temperature and then maintained at or near the attained temperature for a predetermined time, has been the'large drop in pressure due to friction through the iluid conduit,

which is necessarily of unusual length due to the time element required in the soaking section, i. e. that section wherein the oil is maintained at a. fairly constant temperature for a predetermined time. This necessitates exceptionally high pressures in the rst heating stages in order to maintain the desired conversion pressure in the soaking section and obviously greatly increases pumping costs as well aslnecessitating the use of thicker wall tubes and fittings to maintain the proper safety factor and obviate unusual fire and explosion hazard. An increase in the diameter of the fluid conduit for the purpose of reducing the pressure drop for a given volume of oil flowing therethrough, due to friction, greatly reduces the effectiveness of the heating surface measured over a given tube area, since the cross-sectional area, which determines the volume of the tube, increases out of proportion tothe increased circumference, which determines the heating surface. Therefore, the heating surface per unit volume of oil undergoing treatment is decreased by increasing the tube diameter. f

In the present invention in order to maintain the desired ratio of heating surface to tube volume and still minimize the pressure drop, due to friction through the iiuid conduit, the stream of oil during its passage through a portion of the fluid conduit is divided into two substantially equal streams, parallel rows of tubes of the usual size being employed-for the two streams. This necessitates the use ofboth exposed and shielded tubes in that portion of the fluid conduit where heat is absorbed principally by radiation.

Obviously, if one stream of hydrocarbon oil is passed through the shielded row of tubes and p the other stream through the exposed row of tubes, both streams will not be subjected to the same heating conditions, the stream in the exposed row being subjected to a higher rate of heating and consequently heated to a higher temperature than the stream in the shielded row.` Under these conditions a portion (one stream) of the oil would be subjected to a much more severe cracking condition than another portion (the other stream) of the same oil and the effects of such unequal heating are accumulative. The increased vaporization of the more highly heated stream increases the velocity and friction through that portion of the fluid conduit through which it passes thereby causing a larger proportion of the oil to pass through that portion .of the uid conduit which is subjected to less severe heating conditions and further decreasing the rate of heating and the temperature to which the oil is heated in this portion of the uid conduit.

This tends to bring about a condition wherein a predominant quantity\ of the total oil passes through the shielded row of tubes andA is heated therein vto a temperature substantially below that desired, while only a smalLquantity of the total oil passes through the exposed row of tubes in a highly Vaporous condition and is heated to a temperature greatly in excess of that desired.

.In the present invention in order to effect substantially equal heating in the Atwo streams and to maintain the quantities of oil in each stream substantially equal, a criss-cross flow is employed through the shielded and exposed rows of tubes, each of the two streams passing l row, substantially equal heating is obtained in both' streams and, due4 to equal heating and equal friction in the two streams, a substantially equal quantity of oil may be maintained in both streams.

In the preferred embodiment of the invention the oil tobe subjected to conversion is divided into two substantially equal streams which pass simultaneously and in series through an equal number of tubes. comprising similar portions of the fluid conduit located in the convection section of the furnace, wherein the two streams are subjected to equal heating by fluid heat derived from the combustion gases from both combustion zones of the furnace. The two streams of oil then join and the commingled stream passes in series through adjacent tubes comprising single rowsrof tubes adjacent the floor and adjacent the roof of one combustion zone,

whereby the oil is quickly brought to the desired conversion temperature primarily by `radiant heat derived from the hot combustion gases and the refractory walls of the furnace. il The oil is then again divided into two substantially equal streams, the flow of which forms a criss-cross path, as previously described, through adjacentY tubes comprising an exposed and a shielded row' of tubes adjacent the roof and adjacent the floor of the other combustion zone of the furnace,

whereby the two streams are subjected to substantially equal heating by means of primarily radiant heat derived from the combustion gases and refractory furnace walls of this portion of the furnace, the heating in this latter stage bel ing of sufficient intensity to maintain the oil at a substantially uniform conversion temperature during its passage therethrough,y said uniform Yconversion temperature being not appreciably (say 50 F.) aboveor below the maximum temperature attained in the preceding portion of th'e fluid conduit.

ess while split flow, such as described, is prelferred in the soaking section of the fluid conduit where such a high rate of heating is not required, due to the fact that the oil is simply maintained at a substantially constant temperature in this portion of the, heating stage, and split flow may also be employed in the so-called convection heating zone of the furnacewherein fluid heat from the combustion gases is imparted to the oil, due to the fact that most of the latent heat of vaporization and of cracking is added in subsequent portions of the heating coil, obviating the necessity of high rates of heating in the convection section.

One feature of the invention embodies, in conjunction with a uid conduit comprising shielded and exposed rows of tubes subjected to heating primarily by radiation, the `improved flow therethrough obtained by connecting diagonally adjacent tubes in the different rows and alternate tubes in the same row in series and passing therethrough substantially equal quantities of oil in separate streams in such a manner that the number of shielded tubes in each stream is equal to the numberof shielded tubes in the other stream and the number of exposed tubes in each stream is equal to the number of exposed tubes in the other stream so that each of the streams of substantially equal quantities of burners, (not shown) of oil are subjected to substantially equal heating.

The accompanying diagrammatic drawing is a sectional elevation in cross-section of one specific form of furnace embodying the various features of 'the present invention and in which the process of the invention and the desired results may be accomplished.

Referring to the drawing, the outer walls of the furnace are indicated at I and comprise the usual side walls, end walls, roof and floor of any desired standard construction. The interior of the furnace comprises two combustion zones designated, respectively, as 2 and 3 and a fluid heating or convection zone 4, which latter is separated from the combustion zones 2 ,and 3 by the respective bridge walls 5 and 6.

Fuel is supplied by means of any suitable form through one or a plurality of firing tunnels l to each of the combustion zones 2 and 3. Any suitablefuel such as oil, gas or pulverized solid fuel being supplied, together with a portion of the air required for combustion, through the central portions 8 of the tunnels while the desired auxiliary air or steam or both is supplied to the combustion zone through tunnels 9 and l0 adjacent the central tunnel ii.

and regulated by means of suitable dempers ll. The air and/or steam admitted through tunnels 9 and I0 commingles with the fuel and air undergoing combustion from tunnei 8, assisting in controlling the combustion conditions and particularly the character of the flame in combustion zones 2 and 3 of the furnace.

The materials undergoing combustion in zones 2 and 3 of the furnace give up a major portion of their radiant energy to the fluid conduit located along the floorand roof of the combustion zones, as will be later more fully described, by means of Ywhich radiant heat is transmitted to the oil passing through the fluid conduit. rThe combustion gases pass from combustion zones 2 and 3 over the bridge walls 5 and B, respectively, and pass thence downward through the so-called convection heating zone 4 of the furnace where vthey give up fluid heat largely by convection,

although to some extent by radiation, to the fluid conduit in this portion of the furnace and to the oil passing therethrough. The spent combustion gases pass from convection zone 4 through flue l2 toa stack (not shown)`.

The bridge walls 5 and 6, in the particular case here illustrated, are of the suspended Wall type with spaces 5 and 6', respectively, provided between their front and back Walls to accommodate supporting steel structure (not shown), and. when desired, air may be circulated through the openings 5 and 6', for the purpose of cooling the steel structure and, when desired, the air thus circulated Vand thereby heated may be utilized -as a portion or all of the air required for combustion in zones 2l and 3 of the furnace, although Well known means for circulating the air through spaces 5 and 6 and for supplying to zones 2 and 3 and/or to ring tunnels 'l are not shown in the drawing.

A bank I3 of tubes I4 is located in the ccnvection zone E of the furnace and consists of superimposed horizontally parallel rows of horizontal tubes. The tube bank i3 is divided into two similar sections I3 and I3" each containing the same number of tubes and each having the same number of tubes in each horizontal row. Adjacent tubes in each row and end tubes in adjacent rows of each of the two heating coils I 3 and I3f are connected in series at'their ends by means of suitable headers or` return bends (not shown), and located outside of the heating zone, The single lines LIE and I6 indicate the path ofv ow of the fluidlthrough the heating coil, solid portions of the linesindicating the connections between the tubes on the near side of the furnace and the dotted lines indicating the connections on the far side of the furnace. The arrows indicate the direction or flow of the fluid. The two streams of oil I5 and I6 of substantially vequal quantities pass through the respective banks i3 and 'I3" in zone 4 of the furnace where they are subjected/to substantially equal heating by fluid heat absorbed from the combustion gases from both of the combustion zones 2 and 3, the oil passing through this zone, in the particular case illustrated,` in a general direction counter-current to the flow of combustion gases therethrough.

A single row of horizontal tubes i4 comprises the floor bank I1 located adjacent the floor of combustion zone 2 and preferably, as here illustrated, depressed somewhat below the outlet from ring tunnel 'l and out of the direct path of the products undergoing combustion in this zone. A roof bank Ill located adjacent the roof of combustion zone 2 comprises a single horizontal row of horizontal tubes I 4. Adjacent tubes in rows .il and I8 are connectedat their ends in series 4 and the commingled streams of oil designated as l5 and i6 pass first through the iioor bank I'I and then through the roof bank I8, wherein the oil is brought to the desired conversion temperature derived, to some extent,'by convection but primarily by radiation from the combustion gases and the heated ,refractory furnace walls.

Two parallel horizontal rows i9 and 20 of tubes la comprise the roof bank of the soaking section of the uid conduit located adjacent the roof of combustion zone 3. The commingled streams of oil l5 and I6 flowing through tube I4' are again split into two substantially equal streams whichgare designated as I5' and l'.- Stream l5 enters the end'tube of the exposed row 20 and stream I6 enters the adjacent tube in the same row. The respective streams then pass in parallel though adjacent tubes in shielded row lil thence throughthe-next two adjacent tubes in exposed row 2li and s o on throughout this portion of the-fluidconduit. w

Parallel horizontal rows 2l and i2 of tubes ld comprise the remainder of the soaking-section of the fluid conduit located adjacent the floor of combustion zone 3 and, as in the case of iloor bank Ill, these tubes are preferably depressed somewhat below the outlet from the 'firing tunnel to remove them from the direct path of the materials undergoing combustion. `The two streams of oil l5 and I6 flow through the tubes in the shielded and exposed rows 2i and 22, -respectively, of the floor bank in a manner similar to that described in connection with rows I9 and 2li of the roof bank, the two streams commingling again in the nal shielded tube i4" of the floor bank from which the heated oil is discharged to any desired further stage of the cracking process.

It will be noted that the two substantially equal streams I 5' and I6 are subjected to substantially equal heating conditions in the soaking section of the fluid conduit, comprising shielded rows i9 and 2l and exposed rows 2l] and 22, in combustion zone 3. The ,two streams ow in parallel through adjacent tubes in both the shielded and exposed rows, insuring that any unequal heating conditions in different zones along the heating coil will not produce different heating conditions in the two streams; furthermore, the number of shielded tubes in each stream is equal to the number of shielded tubes in the other stream and the number .of exposed tubes in each stream is equal to the number of exposed tubes in the other stream, insuring no inequality in the'heating of the two streams, due to the different heating conditions in the shielded and exposed rows of tubes.

It will be understood that various modifications of the specific form of apparatus illustrated and the specific ow above described may be employed without departing from the scope of the invention. For example, the flow of oil through bank i3 may be Aconcurrent instead of countercurrent tothe flow of combustion gases through zone It ofthe furnace also, when desired, the 'direction of fl'ow of the oil through rows lll and it and/or the sequence of these two rows may be changed. It is also specifically within the scope of the invention lto omit the tubes in rows VI3, i9 and 2l] which are located above and opposed to the tubes in bank i3. When these tubes are omitted a' higher proportion of radiant heat will be irnparted to the tubes in the upper rows of bank I3 and consequently a somewhat higherv rate of heating will occur in this portion of the fluid conduit.

l claim as my invention:

n 1. A furnace for heating Huid which comprises, in combination, two combustion zones, means for independently controlling the heating conditions in each combustion zone, a convection zone located between the combustion zones, means for supplying combustion gases from both of the combustion zones to the convection zone, fluid conduits comprising a plurality of elongated tubular elements located Within the convection zone, means for heating said uid conduits by fluid heat from the combustion gases, fluid conduits comprising a plurality of elongated tubular elements located in each combustion zone, means for heating the latter fluid conduits predominantly by radiant heat from the respective comsingle stream through the fluid conduit in one combustion zone, and means for passing the fluid in a plurality of substantially equal streams through the fluid conduit in the other combustion zone, whereby each stream is subjected to substantially equal heating conditions in this zone.

2. A furnace for heating fluid which comprises, in combination, two combustion zones, means for independently controlling the heating conditions in each combustion zone, a convection zone l'ocated between the two combustion zones, means for supplying combustion gases from both of the combustion zones to the convection zone, fluid conduits `comprising a plurality of elongated tubular 'elements located within the convection zone, .means for heating said fluid conduits by vection zone d in such a manner that each stream is subjected to substantially equal heating conditions therein, means for passing' the heated fluid from the convection zone in a single stream through the fluid conduit in one combustion zone. and means for finally passing the heated fluid from the latter in a plurality of substantially equal streams through the fluid conduit in the other combustion zone, whereby each stream is subjected to substantially equal heating conditions in this zone.

3. A furnace for heating fluid, which com,- prises, in combination, two combustion zones, means for independently controlling the heating conditions in each combustion zone, a convection zone located between the combustion zones, means for supplying combustion gases from both of the combustion zones to the convection zone, fluid conduits comprising a plurality of elongated tubular elements located within the convection zone, means for heating said fluid conduits by fluid heat from the combustion gases, a fluid conduit comprising a single row of elongated tubular elements located adjacent the floor of one com,- bustion zone and a single row of elongated tubular elements located `adjacent the roof of the same combustion zone, a fluid conduit comprising a shielded row and an exposed row of elongated tubular elements located adjacent the roof of the other combustion zone and a shielded row and exposed row of elongated tubular elements located adjacent the floor of the latter combustion zone, means for passing the fluid to be heated in I a plurality of substantially equal streams through the fluid conduits in the convection zone whereby each stream is subjected to substantially equal heating conditions therein, means for passing the heated fluid from the convection zone in a single stream through the fluid conduit in the first ,mentioned combustion zone, and means for tubular elements located within the convection zone,y` means for heating said fluid conduits by fluid heat from the combustion gases, a fluid conduit comprising a single row of elongated tubular elements located adjacent the floor of one combustion zone and a single row of elongated tubular elements located adjacent the roof of the same combustion Vzone, a fluid conduit comprising a shielded row and an exposed row of elongated tubular elements located adjacent the roof of the other combustion zone and a shielded row and exposed row of elongated tubular elements located adjacent the floor of the latter combustion zone, means for passing the fluid to be heated in a plurality of substantially equal streams through the fluid conduits in the convection zone whereby each stream is subjected to substantially equal heating conditions therein, means for passing the heated fluid from the convection zone in a single stream through the fluid conduit in the first mentioned combustion zone, and means for finally passing the heated fluid fom the latter zone in two substantially equal streams through both the shielded and exposed tubes in the other combustion zone whereby each stream passes through an equal number of exposed tubes and an equal number of shielded .tubes-and the two streams flow in parallel through adjacent tubes in the shielded rows and adjacent tubes in the exposed rows. v

5. A process for the heating of hydrocarbon uids which comprises heating a flowing stream of the fluid to conversion 'temperature in a furnace, then dividing the stream into two substantially equal streams, passing the latter streams simultaneously through a fluid conduit located within the furnace, said fluid conduit comprising a shielded row and an exposed row of parallel elongated tubular elements subjected to heating primarily by radiation, passing each stream through diagonally adjacent tubes in the two rows and through alternate tubes in each row, the two streams passing in parallel through adjacent tubes in the same row in such a maner that each stream flows through the same number of exposed tubes and through the same number of shielded tubes, whereby a substantially equal quantity of fluid passes through each stream and each stream is subjected to substantially equal heating conditions.

6. In the heating of hydrocarbon fluid in furnaces of the type having a pair of independently fired combustion zones and a convection zone receiving combustion gases from both the combustion zones; the method which comprises first passing the hydrocarbon fluid through the convection zone and subjecting the same therein to convection heating from said gases, then passing the fluid in a stream through one of said combustion zones and further heating the same therein predominantly Iby radiant heat, raising the fluid to conversion temperature during 'its flow through the convection zone and the last named combustion zone, dividing said stream, after elevation of the fluid to conversion temperature, into a plurality of substantially equal streams and passing the latter through the other of said combustion zones wherein the fluid is maintained at conversion temperature predominantly by radiant heat, and subjecting the equal streams to substantially uniform heating conditions during their passage through the last named combustion zone.

'1. 1n the heating of hydrocarbon uuid in furnaces of the type having a pair of independentlyr fired combustion zones and a convection zone receiving combustion gases from both the combustion zones; the method which comprisesflrst passing the hydrocarbon fluid through the convection zone in a plurality of streams and subfjecting the same therein to convection heating from said gases, then uniting said streams and passingthe resultant composite stream through one of said combustion zones, heating the composite stream in the last named combustion zone to conversion temperature predominantly by radiant heat, then dividing the composite stream into a plurality of substantially equal streams and passing the latter through the other of said combustion zones wherein the iluidf is maintained at conversion temperature predominantly by radiant heat, and subjecting the equal/streams to substantially uniform heating conditions during their passage through the last named combustionzone.

8. In the heating and subjection of hydrocarl allinea bon huida to conversion temperatures in iurnaces oi the type having a pair of independently iired radiant heat zones and a convection heating zone receiving combustion gases from both the radiant heat zones, the method which comprises hrst passing the hydrocarbon Huid through said convection zone, then passing the huid in a single stream through one tube bank adjacent the door and through another adjacent the root oi one of said radiant heat Zones and heating the same in this zone predominantly by radiation, then dividing saidsingle stream into a plurality oij substantially equal streams and passing the latter through one tube bank adjacent the root and through another adjacent the Hoor of the other oi said radiant heat Zones, and subjecting the equal streams to substantially uniform radiant heating conditions during their ilovv through said other radiant heat zone.

9. lin the heating and subjection of hydrocarbon iiuids to conversion temperatures in furnaces of the type having a pair of independently fired radiant heat zones and a convection heating zone receiving combustion gases from both the radi ant heat zones, the method which comprises rst passing the hydrocarbon huid through said convection zone in a plurality oi streams, then combining said streams and passing the resultant single stream through one tube bank adjacent the door and through another adjacent the root ol" one oi said radiant heat zones and heating the same in this zone predominantly by radiation, then dividing said single stream into a plurality oi' substantially equal streams and passing the latter through one tube bant: adjacent the root and through another adjacent the door oi the other ot said radiant heat zones, and subjecting the equal streams to substantially unl form radiant heating conditions during their how through said other radiant heat zone.

10. ln the heating oi hydrocarbon uuid in iurnaces oi the type having a pair of independently fired combustion zones and a convection zone receiving combustion gases from both the combustion zones the method whicheomprisesheating the hydrocarbon fluid to conversion temperature by passage through the convection zone and one of said combustion zones, the fluid being heated predominantly by convection from said gases in the convection zone and predominantly by radiant heat in the combustion zone and the huid being passed through the latter none in a single stream, dividing said single stream, after elevation thereof to conversion temperature, into a plurality of substantially equal streams andpassing the latter through the other oi said combustion Zones wherein the Huid is maintained at conversion temperature predominantly by radiant heat, and subjecting the equal streams to substantially uniform heating conditions during their passage through the last named combustion ZODB.

characterized in that the hydrocarbon huid undergoing heating is passed through the convec- Y tion zone in a plurality oi streams.

ON W. BARNES.

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