CA1219254A - Close-coupled transfer line heat exchanger unit - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

In thermal cracking of hydrocarbons, especially steam cracking to light olefins, a transfer line heat exchanger unit is provided in which cracked gas flows from a furnace into heat exchange tubes, which comprises a distributor having an inlet for said gas and two or three diverging branches forming with said distributor a wye or tri-piece for passage of gas, each branch having along its length a substantially uniform cross-sectional area and being in fluid flow communication with a respective cooling tube. Unfired residence time and pressure drop are reduced, thereby improving selectivity to ethylene.

Description

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This inve~tion relates to a novel 2?pcratus for the close couplinc of furnace ~ubes, ~articul2rly radiant tu~es of 2 cr2cking furnace, to heat exchznge-s in a trznsfer line Steam cracking is a well-known process and is de-- scribed ~n U S Paten. 3,641,190 an~ Bri!ish Patent 1,077,918 In commercial practice, steam cracking is ccrried out by p2ssing a hydrocarbon feed mixed with 20-90 ~ol % ste2m th-ough me,al ~v-olysis tubes loc2ted in a fuel lire~ .urn2ce to r2ise the feed to crcckins tem?eratures, e g , about 1400 to 1700~ 2nd to supply the endothermic heat of re~ction, for the proàuction of ?roducts in_luZins unsaturated light hvdroc2rbons, pzr-ticul2rly C2-C4 oleCins and ciolefins, especi211y e~hylene, useful as chemicals and chemical intermediates.

BACRGROUND OF ,H~ INVENTION

The cracked ef luent may be coolea in a heat ex-changer connected to the furnace crac~ed gas outlet by a transfer line, which is thus termed 2 transfer line exchanger (TL~) Conventionally, the cr2cked gzs from many reaction tubes is manifolded, passed into the ex-pansion cone of a T1E, then throush a tube sheet ~nd lnto .he cooling tubes of 2 multitube shell and tube TLE
in order to cool the sas and cenerate steam In convention21 TL~'s the cracked g2s is dist~ibuted to the coolins tubes by the inlet chamber Since the cross sectional area o_ he TL~ tubesheet is 12~ge compare~ to ,he area of the inlet nozzle and outlet collection manifold, the cracked g2s mus, expznc when leaving the manifold 2nd con--ac. 2sain when entering the cooling tubes In a ~ ic21 exchange~, ~he veloci.y cro~s ,rom ~50 f~/sec a.

.~ ~

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l the inlet nozzle to 60 L^t~sec before entering the coolins

2 ~ubes. Once in the cooling tu~es, .he veloci~y is in-

3 creased asaln to approximately 300 ft/sec; this expansion ' anà contraction of the cracked g2s coupled with its low 5 velocity in the exchanger inlet chamber causes turbulence 6 and uncontrolled residence time. This uncontrolled 7 residence time causes a deterioration in the selecti~ity 8 to desirable olefins, and coking. The heavier components 9 and poly-nuclear aromatics in the cracked gas condense 10 and polymerize to form coke in the inlet chamber. During 11 process upsets or onstream decoking, this coke spalls and 12 plugs the exchanger tubes causing a drastic increase ln 13 ,he exchanger pressure drop. Als~ when hot gas strikes the 1~ dead flow zone caused by the tube sheet between the cool- !
15 ing tubes, heavier components and poly-nuclear aroma,ics 16 suspended in the cracked gas are knocked out of the gas 17 stream and condense and polymerize to for~ coke on th~ tube 18 sheet between the cooling tubes. This coke deposit grows 19 and gradually covers or blocks the entrance to the cool-20 ing tubes thus impeding heat transer and causing the ex-21 chan~er to lose its thermal er^ficiency. Furthermore such 22 expansion and contraction of the cracked gas caused by large 23 changes in velocity results in pressure loss, as discussedin 24 U.S. Patent 3,~57,485. According to the present inven-25 tion, these conditions are avoided and pressure loss is 26 reduced.
27 In the conventional design there is a dramatic in-28 crease in velocity (when the gas enters the c~oling,tubes) 29 which results in that the kinetic pressure loss is great 30 as compared with a small static pressure gain to give an 31 sverall much greater pressure loss, as contrasted with 32 the present invention in which there is no large or sud-33den increase in velocity so tha. the smaller loss in 34 kinetic pressure as compared with the gain in static 35 pressure aives an overall small pressure loss. Any de-36crease in velocity along the,path of flow is gradual and :::

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1 relatively small as ag~inst the standard expansion cone, ~ cr velocitv may be constant.
3 The ~l~ed expansion chamber is described in the fol-9 lowing U.S. patents:
3,357,~; 3,763,262 6 3,~49,212 3,910,347 7 3,456,719 4,078,292 ~ 3,5~2,487 4,097,544 9 3,574,781 4,151,217 In V.S. Patent 3,671,198 the outlet o~ each reaction 11 tube is connected to a respective quench tube which is 12 surrounded by a cooling jacket. This has the serious draw-13 back that with 2 single quench tube fitted to a single 14 reaction tube, in the event OL- plugging of the quench tube by coke, there will be loss oF flow and subsequent failure 16 of the reaction tube since the cracked gas will remain 17 therein, will reach excessively high temperature and cause 18 burnout. On the contrary, the subject heat exchange unit 19 has at least two flow paths for the gas and the probability 20 of both beco~ing plugged simultaneously is very low. This 21 is an excellent safety feature.
22 As residence time and hydrocarbon partial pressure 23 are decreased and cracking is carried out at higher radiant 24 coil outlet temperatures, the selectivity to desirable 25 olefins is improved. Accordingly, in recent years atten-26 tion has been directed to the use of pyrolysis tubes af-- 27 fo~ding short residence time, see for example an article 28 en.itled "Ethylene" in Che~ical W~ek, November 13, 1965.
29 To capi.alize on ~he benefits of very low residence 30 time cracking, it is necessary to quench the effluent 2s 3 quickly as possible in order to stop un~esirable cracking 32 reactions. To accomplish this, it is necessary to place 33 Ihe TLE as close as possible to the fired coil outlet to 34 reduce the unfired residence ti~e, i.e., the resid2nce , :

~ime ~easured fro~ when .he cracked process g2s leaves the 2 fi~ed zone of the furnace to when it enters the TLE cool-3 ing tubes. It is also desirable to minimize turbulence

4 and recirculation of the cracked gas between the fired

5 outlet and TLE cooling tubes as this uncontrolled resi-

6 dence time causes a deterioration in the selectivity to

7 desirable olefins and polymerization of the heavier

8 co~ponents to coke. That is, the uncooled transfer line

9 constitutes an adiabatic reaction zone ! n which reaction

10 can continue, see The Oil and Gas Journal, February 1,

11 1~71

12 It is hichlv desir2ble to reduce pressure build-u~

13 in the exchanger and loss of thermal efficiency. To ac- -

14 com~lish this the dead flow zones between individ~al cool-ing tubes must be eliminated to prevent the heavy compon-16 ents in the cracked g2s from condensing on these 2reas 17 and eventually ~estricting cracked gas flow to the cool=
18 ing tubes. These dead flow zones between the cooling 19 tubes are not entirely eliminated by the devices des-20 cribed in U.S. Patent 3,357,485.
21 ~rom a process point of view, not only the unfired 22 residence time needs to be minimized, but also the pres-23 sure drop in the transfer line and T~E outside of the 24 fire box must be reduced to improve the selectivity, be-25 cause large pressure drops result in increased pressure 26 and increased hydroczrbon partial pressure in the up-27 stream pyrolysis tubes connected thereto, which adversely 28 2ffects the pyrolysis reaction, as aforesaid. As dis-29 cussed above, pxessure drops are lower in the configura-30 tion of the subject invention than in a conventional 31 apparatus.
32 Another problem associated with the use o~ TLE's 33 concerns the temperature transition from the inlet 34 which receives hot gas from the furnace, to the cooler 35 exchan~e tubes, and the desirability of reducing the 36 therm21 stress on metal parts with such a steep thermal ;:~2~ S~

1 sracient. In U.S. Patent 3,8i3,476 a steam purged jacket 2 is employed in 'he inlet of the exchanser for this 3 purpose. Ap?licants achieve this objective Wi! hout the 4 use of ex?ensive steam by means of a novel str~cturing 5 o- the inlet of their heat exchanger unit.
6 SUM~RY OF THE Il`~lENTION
_ 7 In thermal cracking of hydrocarbons especially steam 8 cracking to light olefins, 2 transfer line heat exchanger g unit is provided in which cracked gas flows from a furnace lo into heat exchange tubes, which comprises a connectOr or 11 distributor havi,ng an inlet for said gas and two diverg- ;
12 ing branches forming with s2id connector a wye for passage 13 of gas, each branch having along its lengtn a substantially 14 uniform cross-sectional area and being in fluid flow com-

15 munication with a respective cooling tube. Thus, the

16 device can be close coupled to the radiant coils of the

17 furnace because the path of gas flow is short since each

18 branch of the wye leads directly into a cooling tube

19 whereas the expansion chamber of a conventional TLE-which

20 has to wide~ to accommodate a bundle of hezt exchange

21 tubes thus lengtheniny the path -- is eliminated. Unfired

22 residence time and pressure drop are reduced, thereby

23 improving selectivity to ethylene.

24 A wye or a tri-piece may be used, with a suitable,

25 relatively small angle of divergence between adjacent

26 branches. Each branch has a substantially uniform cross-

27 sectional area along its length preferably not varying

28 by more than about 10 percent, more preferably not vary-

29 ing by more thzn about 5 percent.
The large expansion of sas in a conventional TL~
31 inlet ch2mber with attendant large drop in velocity, is 32 avoided. In the present invention the ratio, R, of the 3~ -ombined cross-sectional areas of the branches of the 3~ wye or of the tri-piece to the cross-sectional area of 35 ,he connector may be expressed a~
36 ~ = abou. 1:1 to about 2:1, pre.erably about 37 1:1 to about 1.7:1.
.

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1 Generally, each branch has a smaller cross-section 1 area 2 than the connector. 3y contrast to the above values fo-3 R, for the conventional TL~ the ratio of the area at the 4 exDanded end of the cone to the area of the inlet will ~e much greater, about 10:1.
6 This con iguration àoes not permit recircula.ion of 7 ,he gas. Flow path of the g2s is streamline. It i5 8 also tube sheet-free, that is, gas flows from the radlant 9 ~ubes of the furnace into the wye or tri-piece, thence lQ directly into the cooling tubes without obs,ruction. 3y 11 zp?ropriate choice of dimensions the gas velocity can 12 be mainta~ned su~stantially constant from the furnace 13 outlet into the cooling tubes.
14 The unfired residence time is reduced from .C5 seconds for a conventional TLE to 0.010-0.015 seconds.
16 Very little coking occurs since the bulk residence time 17 in the unfired section is signiflcantly reduced and the 18 uncontrolled residence time due to recirculation of g2s 19 in the standard TLE inle. chamber is eliminated. Conse~
cuently the unit is well adapted for use with very short 21 residence time cracking tubes.
22 In order to minimize thermal stress, the wye or 23 tri-piece is enclosed and surrounded by a specially 24 designed jacket in ixed position with insulating material therebetween. The jacket or reducer has a 26 variable cross-sectional area and diameter with vari~ble 27 insulation thickness, the smaller diameter and less 28 insulation being at the hottestr inlet end of the 29 connector. The wye or tri-piece and the reducer may suitably be made of a Cr-Ni/Nb alloy such as Manaurite 31 900B manuactured by Acieries du Manoir-Pompey, or 32 Incoloy 800~. The insulating material may be, for 33 exa~ple, refractory material such as medium weight 34 cast2ble, VSL-50, manufactured by the A. P. Green Company 35 or Resco RS-SA manufactured by Resco Produc,s, Inc.

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1 B~I-7 D~SC~IPTIO~ OF THE DRAWI~GS

~ In the accompan~ing drawings, Fig. 1 is a schema~ic 3 view of a trans er line heat exchanger unit according to 4 ~he invention;
Fig. 2 is a cross-sectional view of a wye and ~igs.
6 2A, 23 and 2C are sections taken on lines A-A, B-3 and 7 C-C respectively, which sections are perpendicular to 3 the direction of gas flow;
9 ~is. 3 is a cross-sectional view o a tri-piece, ~nd ~ig. 4 is a cross-sectional view of one cooling tube 11 - the unit.

13 As shown in 7~ig. 1, the heat exchanger unit of ! his il invention may comprise, in general, a wye l-comprising a connector 2 and arms or branches 3 each of which leads 16 into its respective cooling tube 4. The direction of 7 g2s flow is shown by the arrow. The wve 1 is er.clos 18 in a jacket or reducer l0 A clean-out connection, not 19 shown, may be provided uPstream of the reducer.
Fig. 2 illustrates the wye in more detail. The con-21 nector 2 diverges, with a relatively small angle of 22 divergence, into the two branches 3. The angle is selected 23 to be small in order to avoid any abrupt changes in t~e 24 direction of flow of the gas which could cause a pressure 25 drop, and to make the structure compact. Suitably it 26 .~,ay be, as measured between the central axes of the 27 diverging brznches, see the arrows14, about 20 to about 28 ~0, preferably about 30. The branches straighten out 29 and become substantially parallel in their downstream

30 portions 5. This strai~htenina ic ~pl~v~ to confine

31 erosion to the branches of the wye where an erosion allow-

32 ance can be provided in the wall thickness. I- the

33 branches were not straightened prior to the gas entering 3~ the exchanger tubes, coke that miqht be contained in the~as 35 wvuld ~inge on the thin walls of ~e exchanger oooling t~7e an~ erode 36 a hole ~o_sh the tube in a rela~ively short ~e.

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1 Where the connector enlarges to acco~mod2te the b~anches, 2 a baffle 6,formed by the intersection of the branches of 3 he wve, is axially located to avoid or mlnimize expan-4 sion of the cross-sectional area of the flow path o the 5 g~s.
6 Thus, as shown in Fiss. 2A, 2B and 2C, in a preSerred 7 embodiment, the area at the line A-A is zbout the same as 8 at the line B-B, for example 1870 mm2, and at the line 9 C-C the connector has already divided into two branches 10 of roughly half said area each, for example 9?4 mm2. Thus I1 the ratio, R, of the sum o~ the cross-sectional areas of 12 .he branches to the cross-sectional area OL the connector 13 is roughly l:l, e.g., .988. This ratio achieves substan-14 tially constant gas velocity throughout the wye. Suit-15 ably the cooling tubes are sized to match or approximate 16 the areas of the respective wye branches, and in this il-17 lustration may be, for example, about 924 mm~. The bene-18 lits of the invention can also be obtained to 2 large ex-19 tent when R is greater than 1:1, up to ab~ut 2:1.
The cracked gas ~lows directly from the branches of 21 the wye to the respective cooling tubes. There is no 22 dead flow area such as a tube sheet in its flow path and 23 therefore heavy ends in the cracked gas will re~ain sus-24 pended and not 12y down as coke, blocking the flow area 25 to the cooling tubes.
26 The portions 5 of the wye, at their downstream ends, 27 are not attached to the respective cooling tubes 4 but 28 each is spaced from the cooling tube by an expansion gap 29 7 and held in position by a collar 8.
The temperature transition from the hot inlet 9 of 31 the distxibutor 2 which operates at approximately 1600-32 1900F to the cooler exchanger tube 4 which may opera~e, 33 e.g., at about 480F to about 612F, is accomplished in

34 a refractory filled alloy reducer 10. The reducer is

35 welded ~;o the distributor 2 and to the oval header 23 as

36 shown to prevent leakage of gasinto the atmos~here. The

37 use o~ a reducer minimizes the thermal gradient and there-3 .ore reduces the thermal stress. A reducer has a ~ariable 1 c-oss-section21 area and di2meter. The larger diameter 2 end 11 of the reducer has more insulation 12 between its 3 wall and the ho. internal "Y" fitting .han the small di-4 2me.e- end 13. Therelore, because of this vzriable in-~ sulation thickness, the small ciameter end which operates 6 at the hottest temperzture expands or grows ther~ally 7 approximately the same radial distance as the cooler, 8 lzrge diameter end. Since both ends of the reducer ther-9 mally grow approximately the same amount, ther~zl stresses 10 are minimized. The "Y" piece distributor 2 which conducts 11 the hot cracked g2s to the cold exchanger tubes operates 12 at the same temperature as the hot cracked gas. The "Y"
13 piece is not physically attached to the cold exchanger 14 tubes, and, therefore, there is no sharp temperature 15 gradient and no thermal stress at this point. Rather, 16 there is a thermal expansion gap 7 between the portions 5 17 of the "Y`' and-the exchanger cooling tubes 4 to permit 18 unrestricted expansion of the hot branches of the "Y".
19 Since there is a thermal expansion gap pxovided, the walls 20 of the reducer 10 act as the pressure-contai~ing member 21 rather than the "~" distribu,or.
22 Similar considerations as described above 2pply to 23 the tri-piece, illustrated in ~ig. 3.
24 Pig. 4 illustrates a single heat exchange tube which 25 is in fluid flow communication with one branch of a wye.
26 As shown, t.he downstream portion 5 OI the branch is 27 fitted to the cooling unit 20 so that gas can flow through 28 the inner tube 21 which is jacketed by the outer shell 22.
29 Water is passed via 2 header or plenum chamber 23 into 30 the annular enclosure 24 between the tube-in-tube 31 zrrangement 21-22, takes up heat from the hot cracked 32 gas and leaves as high pressure steam through header 25.
33 It will be understood that the furnace will be 3a e~uipped with a lzrge number of such transfer line heat 35 exchanger units. ~he units may be located at the top or 36 at the bottom o~ the furnace anà, in either case, gas fl9w 5~

1 ma~ be upflow or downflow.
2 The following examples ere inten~ed to illustrate, 3 without limiting, the invention.

In this illustration two 1.35 inch I.D. (internal 6 diameter) radiant tubes of a steam cracking furnace are 7 joined together by an inverted wye fitting at the arch 8 level of the furnacel flow of cracked gas with sas upflow g is then conducted at constant velocity to the wye fitting 13 of the heat exch2nger unit of this invention, immediately 11 upstream of the TLE cooli~g tubes. Gas flow is distributed 12 at constant velocity to two 1.35 inch I.D. exchanger 13 cooling tubes by this wye fitting. The ratio, R,- i5 14 equal to 1.
~ For naphtha cracking at a steam ~S) to hydrocarbon 16 (HC) weight/weight ratio, of 0 65S/HC, the unflred resi-17 dence time is about .012 seconds. Cooling tubes 27 feet 18 lon~ are required to cool the furnace effluent from 19 1573F (856C) to 662F ~350C). For heavy gas oil (end 20 boiling point above 600~) cracking, to avoid excessive 21 coking in the cooling tubes, the preferred outlet ~em-22 peratures are above 900F (43~C) which requires only 13-23 feet-long tubes. Por a ligh. gzs oil the same 27-feet-24 long exchanger tube may be used to cool the effluent to 2s 720~ (382C).
26 Table I summarizes comparative data as between a 27 conventional (e~pansion chamber) TLE and the present in-28 vention, for naphtha cracking. The total pressure drop 29 is given from the fired outlet to a point downstream of 30 the outlet collection manifold or outlet head of the TLE.
31 The unfired residence time is measured from just outside 32 the furnace fire box to the inlet of the cooling tubes.

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~Z~9'~54 1 T?.~L~ I
2 Conventiona~ Present 3 TLE Invention ~ mo.al ~ si S.l 2.0 5 ~nfired residence 5 time, sec. 0.0492 0.012 7 ~ Ethylene, wt. % -0.75 Base 8 It can thus be seen that if the present invention is used g ra.her than the conventional TLE, 0.75 wt.% more 10 e'hylene is produced.

, 12 In this unit the I.D. of the distributor was 50.8 mm 13 and of each branch of the wye was 43 mm. The angle of 1~ divergence was 30. Since area = ~D2 , the ratio, R, 15 e~uals 1.43. The total pressure drop is approximately 16 1.9 psi from the fired outlet to a point downstream o~
17 the outlet collection mani,old for the TLE cooling tubes.

19 In 2nother unit, the distributor is a tube of ~he 20 same diameter as the furnace radiant coil connected to it, 21 1.~5 inch I.D. The tube splits into two branches, each 22 having a 1.69 inch I.D. and each leading into a coolins 23 tube of the same diameter. The ratio, R, equals 1.67.
2~ For steam cracking of propane, the cracked gas e~fluent 25is cooled in this unit from 1600F to 998F in cooling 26tubes 10.~ feet long. Total pressure drop is approximately 271.6 psi ~rom the fired outlet to a poin. do~nstream of the 2~cooling tubes.
29 The present inven~ion therefore achieves close 'C ccu?ling of the TLr cooling tubes to the radiant coils 3:of the furnace. Elimination of the collection manifold ~2~

1 or nume~rous radiant coils an~ the TL~ inlet chamber of 2 the flared type, minimizes tur~ulence and reci~cula~ion 3 of cracked s2ses between fired outle. and TLE cooling ~ ~ubes. Thus, unfired residence time is reduced. These 'ac~ors reduce non-selective cracking and su~sequent 6 coking in the unit. Smaller pressure drop decreases 7 hydrocarbon partiai pressure in the radi~nt coils and 8 improves selectivity to ethylene. Operation without Dre-g quench upstream of the unit is permissible lor gas crack-ing at high conversions. The elimination of prequen~h 11 increases the furnace's therm~l efficiency by producing 12 more steam in the TLE due to higher TLE inlet temperature.
13 A prequench system has a 1200F inlet whereas the close-14 coupled ~LE system has about 2 1600F inlet. Thus, *he ~ invention has su~stantial therm21 e~ficiency advantages 16 and achieves valuable yield credits.

Claims (21)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR PRIVILEGE
IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A transfer line heat exchanger unit, close coupled to a furance, in which hot gas flows from a furnace outlet into heat exchange tubes for cooling said gas which comprises heat exchange tubes of tube-in-tube construction, a connector connected as its upstream end to a furnace outlet and having an inlet for said gas and two diverging branches forming with said connector a wye for passage of gas, the ratio, R, of the sum of the cross-sectional areas of the branches to the cross-sectional area of the connector being from about 1:1 to about 2:1 and each branch being in fluid flow communication with a respective cooling tube, said cooling tubes being tube sheet-free, the cross-sectional area of each branch being substantially the same as the cross-sectional area of the respective cooling tube.

2. A unit according to claim 1 in which R is equal to about 1:1 to about 1.7:1.

3. A transfer line heat exchanger unit, close coupled to a furnace, in which hot gas flows from a furnace outlet into heat exchange tubes for cooling said gas which comprises heat exchange tubes of tube-in-tube construction, a connector connected at its upstream end to a furnace outlet and having an inlet for said gas and two diverging branches forming with said connector a wye for passage of gas, said wye having along its length a substantially uniform total cross-sectional area and each branch being in fluid flow communication with a respective cooling tube, said cooling tubes being tube sheet-free, the cross-sectional area of each branch being substantially the same as the cross-sectional area of the respective cooling tube.

4. A modification of a unit according to claim 1 in which said connector has three said diverging branches forming with said connector a tri-piece, said tri-piece having along its length a substantially uniform total cross-sectional area.

5. A unit according to claim 3 in which the angle of divergence between the respective central axes of adjacant diverging branches is in the range of about 20° to 40°.

6. A unit according to claim 3 in which the cross-sectional areas of the branches are substantially equal to one another.

7. A unit according to claim 3 in which the cross-sectional area of a branch does not vary by more than about 10%.

8. A unit according to claim 3 in which the branches straighten out into substantially non-diverging parallel sections which are in direct fluid flow communication with the respective cooling tubes.

9. A unit according to claim 3 in which the gas flows from the furnace outlet into the cooling tubes essentially without expansion at constant velocity.

10. A unit according to claim 3 or 1 in which the furnace is a steam cracking furnace.

11. A transfer line heat exchanger unit, close coupled to a furnace, in which hot gas flows from a furnace outlet into heat exchange tubes for cooling said gas which comprises heat exchange tubes of tube-in-tube construction, a connector connected at its upstream end to a furnace outlet and having an inlet for said gas and three diverging branches forming with said connector a tri-piece for passage of gas, the ratio, R, of the sum of the cross-sectional areas of the branches to the cross-sectional area of the connector being from about 1:1 to about 2:1 and each branch being in fluid flow communication with a respective cooling tube, said cooling tubes being tube sheet-free, the cross-sectional area of each branch being substantially the same as the cross-sectional area of the respective cooling tube.

12. A unit according to claim 11 in which R is equal to about 1:1 to about 1.7:1.

13. A transfer line heat exchanger unit, close coupled to a furnace, in which hot gas flows from a furnace outlet into heat exchange tubes for cooling said gas which comprises heat exchange tubes of tube-in-tube construction, a connector connected at its upstream end to a furnace outlet and having an inlet for said gas and three diverging branches in the same plane forming with said connector a tri-piece for passage of gas, said tri-piece having along its length a substantially uniform total cross-sectional area and each branch being in fluid flow communication with a respective cooling tube, said cooling tubes being tube sheet-free, the cross-sectional area of each branch being substantially the same as the cross-sectional area of the respective cooling tube.

14. A unit according to claim 13 in which the angle of divergence between the respective central axes of adjacent diverging branches is in the range of about 20° to 40°.

15. A unit according to claim 13 in which the cross-sectional areas of the branches are substantially equal to one another.

16. A unit according to claim 13 in which the cross-sectional area of a branch does not vary by more than about 10%.

17. A unit according to claim 13 in which the branches straighten out into substantially non-diverging parallel sections which are indirect fluid flow communication with the respective cooling tubes.

18. A unit according to claim 13 in which the gas flows from the furnace outlet into the cooling tubes essentially without expansion at constant velocity.

19. A transfer line heat exchanger unit, close coupled to a steam cracking furnace, in which hot gas flows from a furnace outlet into heat exchange tubes for cooling said gas which comprises heat exchange tubes of tube-in-tube construction, a connector connected at its upstream end to a furnace outlet and having an inlet for said gas and two diverging branches forming with said connector a wye for passage of gas, said wye having along its length a substantially uniform total cross-sectional area and each branch being in fluid flow communication with a respective cooling tube, said cooling tubes being tube sheet-free; and in which a reducer is in fixed position enclosing the wye with insulation therebetween, the wye at its upstream end being affixed to the reducer, the diameter of the reducer and amount of insulation being smallest at the upstream end, a thermal expansion gap being provided between the branches of the wye and the respective cooling tubes.

20. A transfer line heat exchanger unit, close coupled to a steam cracking furnace, in which hot gas flows from a furnace outlet into heat exchange tubes for cooling said gas which comprises heat exchange tubes of tube-in-tube construction, a connector connected at its upstream end to a furnace, outlet and having an inlet for said gas and three diverging branches in the same plane forming with said connector a tri-piece for passage of gas, said tri-piece having along its length a substantially uniform total cross-sectional area and each branch being in fluid flow communication with a respective cooling tube, said cooling tubes being tube sheet-free, and in which a reducer is in fixed position enclosing the tri-piece with insulation therebetween, the tri-piece at its upstream end being affixed to the reducer, the diameter of the reducer and amount of insulation being smallest at the upstream end, a thermal expansion gap being provided between the branches of the tri-piece and the respective cooling tubes.

21. A transfer line heat exchanger unit, close coupled to a furnace, in which hot gas flows from a furnace outlet into heat exchange tubes for cooling said gas which comprises heat exchange tubes of tube in-tube construction, a connector connected at its upstream end to a furnace outlet and having an inlet for said gas and two or three diverging branches forming with said connector a wye of tri-piece for passage of gas, the ratio, R, of the sum of the cross-sectional areas of the branches to the cross-sectional area of the connector being from about 1:1 to about 2:1 and each branch being in fluid flow communication with a respective cooling tube, said cooling tubes being tube sheet-free, and in which a reducer is in fixed position enclosing the wye or tri-piece with insulation therebetween, the wye or tri-piece at its upstream end being affixed to the reducer, the diameter of the reducer and amount of insulation being smallest at the upstream end, a thermal expansion gap being provided between the branches of the wye or tri-piece and the respective cooling tubes.