US3409074A - Combined inlet channel and heat exchanger shell with heat recovery means - Google Patents

Description

Nov. 5, 1968 R w NE ETAL 3,409,074

K. COMBINED INLET CH EL A HEAT EXCHANGER SHELL WITH HEAT OVERY MEANS Filed Feb. 1966 gg g 26 2o 22 52 -50 GA Fig. 2.

' INVENTORS KENNETH R. WAGNER HENRY N. LA CROIX muiz. mm

ATTORNEY United States Patent 3,409,074 COMBINED INLET CHANNEL AND HEAT EX- CHANGER SHELL WITH HEAT RECOVERY MEANS Kenneth R. Wagner, Jersey City, and Henry N. Lacroix, Orange, N.J., assignors to Foster Wheeler Corporation, Livingston, N.J., a corporation of New York Filed Feb. 28, 1966, Ser. No. 530,581 8 Claims. (Cl. 165134) ABSTRACT OF THE DISCLOSURE A combined inlet channel and heat exchanger shell in which heat recovery means are disposed in the channel for reducing the gas temperature. A tube sheet is disposed between the inlet channel and the shell such that the tube sheet can withstand the gas temperature after heat has been recovered.

In typical process plants, for example, ammonia, methanol, ethylene or hydrogen producing plants, hot process gases are quenched by passing the gases through the tubes of a waste heat exchanger. Steam may be generated on the shell side of the waste heat exchanger in the cooling process to meet the steam requirements for other parts of the plant.

As a result of higher design temperatures (e.g., over 1700 F.) and pressures of the process gases entering the waste heat boiler inlet channel in the quench system, as well as present trends toward higher steam pressures (e.g., 1500 p.s.i.g.) and steam or equivalent production rates (e.g., 100,000 lb. per hour) on the shell side, the design of the hot end tube shet in the inlet channel of the boiler is frequently critical. The combination of high temperatures on both sides of the hot end tube sheet (steam, water or other coolant on the shell side of the tube sheet and hot process gases on the channel side) and high pressures results in an almost impossible design condition, especially where the hot tube sheet is desired to be as thin as possible and to be connected to the shell by a simple joint. There can be established in the tube sheet severe thermal stresses resulting in tube sheet failure and tube bundle rupture. Refractory material which frequently covers the hot channel side of the tube sheet to protect the metallic surfaces thereof from high temperature and corrosion, may crack due to the extreme thermal conditions, exposing the carbon steel surfaces of the tube sheet to the hot process gases and corrosion. Bowing of the tube sheet may occur lowering the structural integrity of the tube sheet. In addition, the hot inlet channel which frequently comprises a carbon steel pressure shell with a stainless steel inner shroud blast plate (or insulation guard) with a refractory layer therebetween, besides being relatively costly, is also subject to cracking.

Several methods havebeen suggested for protecting the tube sheet under extreme thermal conditions. However,

none of these methods have been entirely satisfactory. One manner of protecting the tube sheet was to cool the hot channel entrance gases by direct injection of a cooling stream into the hot gases. This contaminates the gases or dilutes them to an undesirable extent and reduces useful heat recovery. Another manner was to add a second tube sheet in spaced relationship close to the hot end tube sheet, between which feedwater or other coolant was circulated by means of a pump to cool the hot tube sheet.

Accordingly, it is an object of the presentinvention to provide an improved arrangement which provides protection for the hot tube sheet under the above temperature and pressure conditions.

It is another object of the present invention to provide Patented Nov. 5, 1968 a means for cooling the process gases in a hot channel and for providing useful recovery of heat obtained in the cooling.

It is still another object of the present invention to provide an improved high temperature channel design having a simplified channel pressure shell and affording effective protection of the hot end channel and tube sheet.

In accordance with the present invention there is pro-- vided a heat exchanger comprising a heat exchanger shell; a tube sheet disposed at opposite ends of the heat exchanger shell defining an interior region; an inlet channel and an outlet channel adjacent the tube sheets, the inlet channel having walls defining an inner surface; a tube bundle disposed in the interior shell region and supported at opposite ends by the tube sheets, said tube bundle in communication with the inlet and outlet channels; a tubular means disposed in the inlet channel; means for passing a high temperature fluid into the inlet channel at a position remote from the tube sheet adjacent said inlet channel; and means for passing a cooling fluid through the tubular means for cooling the high temperature fluid passing through the inlet channel. With this invention the high temperature gases entering the inlet channel are cooled to reasonable tube sheet temperatures before they reach the hot end tube sheet enabling conventional tube sheet design. Heat is removed from the gases and, recovered by the cooling fluid which may be used for subsequent process use.

In another aspect of the invention the cooling fluid may be the aforementioned shell side Water, in communication therewith, further providing heat recovery etficiencies.

Also provided in accordance with the invention, the tubular means comprises a helical coil which is disposed along the inner surface of the hot inlet channel removing heat from the hot gases primarily by radiation without imposing any appreciable pressure drop for the gases in the channel.

In still a further aspect of the invention the helical coil may be welded along its length to adjacent turns forming the pressure containment vessel for the hot channel and shielding the channel from the hot gases. In this manner the channel walls may be simplified in construction and constitution.

The above and other objects and advantages of the present invention will become apparent upon further consideration of the following detailed description with reference to the accompanying drawings, in which:

FIGURE 1 represents a heat exchanger in accordance with the invention in a process system;

FIGURE 2 is an enlarged view of the hot channel arrangement of the heat exchanger of FIG. 1; and

FIGURE 3 is a section view taken along the line 3-3 of FIG. 2.

Referring now to FIG. 1 representing a process plant in which the present invention may be used. A process fluid, e.g., methane, is heated and catalytically cracked in a fired heater reformer 12 in heater tubes 14 and passes therefrom through transfer line 16 into a hot entrance channel 18 of a waste heat boiler 20. The process gases are cooled in the waste heat boiler while passing through the heat exchanger tubes 22 leaving through exit channel 24 and line 26, the gases being cooled in the heat exchanger for subsequent process requirements downstream of line 26 (not shown).

Water circulates in the shell side of the heat exchanger between the hot entrance channel 18 and the exit channel 24 around the heat exchanger tubes 22 cooling the process gases and generating steam on the shell side .for other process uses. Water enters steam drum 28 disposed above the heat exchanger through feed line 30 and is preheated in the flue gas convection section 32 of the fired heater 12 in an economizer heat exchanger bank 34, flowing thereto from the drum via lines 36 and 38 through pump 40, and returning to the drum through return line 42. The pro-heated water enters the shell side of the heat exchanger through downcomer 44, and steam generated in the shell rises to the steam drum 28 via upcomer 46, comprising the natural circulation waste heat boiler loop. The steam leaves the drum 28 through steam outlet line 48 for use elsewhere in the process cycle.

The process gases entering the hot channel 18 from the reformer are at high temperatures, e.g., 1800 F. A radiant forced circulation steam generating loop 50 comprising a helical coil 52 located in the cylindrical hot entrance channel 18 cools the hot gases flowing through the channel reaching the hot end tube sheet 54 in the channel down to temperatures that the tube sheet 54 can safely withstand, e.g., 1200 F. For this purpose, water from the drum 28 exiting through line 36 is forced by pump 40 through pipe 56 into the helical coil 52 adjacent the hottest portion 58 of the entrance channel. The water flows through the coil 52 cooling the process gases (primarily by radiation), returning from the end of the coil adjacent the tube sheet 54 to the drum via pipe 60.

Reference is now made to FIG. 2 for an enlarged view of the hot entrance channel 18 and transfer line 16. Helical coil 52 comprises a radiant heat absorbing section consisting of a single coil, disposed in the channel 18 (comprising a widened portion of the transfer line 16). The coil 52 is of the tangent tube design (FIG. 3) with the coils welded together to form a gas tight element serving as the pressure containment vessel for the channel. With this arrangement heat losses through the channel wall 62 are minimized. The welded helical coil 52 forms a stagnation region 64 between the coil 52 and the channel wall 62 providing thermal insulation. Annular metal retainer gas seals 66 and 68 disposed at the entrance end 58 of the channel and at the exit end, seal the ends of the stagnation region for this purpose preventing the hot effluent gases from impingement on the channel wall 62 thereby preventing heating or corrosion of the channel walls.

It should now be apparent that with this invention the tube sheet can be constructed according to conventional design. As shown in FIG. 2 the tube sheet 54 is thin and made of carbon /2% molybdenum steel and contains a conventional refractory coating 70 to protect it from impingement by the process gases.

It should also be apparent that the entrance channel 18 does not require internal refractory lining and can be made of a simplified construction. As indicated in FIG. 2, the channel shell 62 is exclusively of thin carbon steel.

Although channel modifications would have to be made therein, the invention is particularly suited with a heat exchanger of the general type described in United States Patent No. 3,199,577, dated Aug. 10, 1965.

For purposes of this invention the inlet channel 18 includes the transfer line 16 between the reformer 12 and hot end tube sheet 54; accordingly, the coil 52 may be disposed anywhere therein to cool the gases reaching the tube sheet, and not necessarily adjacent the tube sheet.

In accordance with this invention the steam generating loop 50 may also comprise a once-through loop in which case pipe 56 would be connected to a separate feedwater source (not shown) instead of the steam drum. For preferred heat exchange characteristics, the cooling water in the helical coil 52 should exit therefrom substantially as water. Although not shown the water exiting from the helical coil may pass directly into the shell side of the heat exchanger about the tubes 22.

Although the invention has been described with reference to a specific embodiment, it is apparent that other variations may be made without departing from the scope and spirit of the invention as defined in the following claims.

What is claimed is:

1. A heat exchanger arrangement comprising:

a shell defined by a cylindrical wall and a pair of spaced-apart tube sheets,

means in flow communication with said shell defined by a cylindrical wall forming an entrance channel for receiving a high temperature gas and conveying said gas into said shell,

heat exchanger tubes in said shell supported between said tube sheets and said tubes communicating with said entrance channel,

continuous tubular means in said entrance channel for passing a relatively high pressure cooling fluid therethrough and said tubular means disposed in heat exchange relation with said high temperature gas in said entrance channel,

said tubular means including a forward coil, rearward coil, and at least one intermediate coil, with adjacent coils welded together to form a gas-tight enclosure,

a first circumferential gas-tight seal disposed between said forward coil and said entrance channel and a second circumferential gas-tight seal disposed between said rearward coil and said entrance channel adjacent to one of said tube sheets, whereby said high temperature gas is cooled with heat being recovered therefrom such that said gas enters the heat exchanger tubes and contacts the tube sheet adjacent the entrance channel at a reduced temperature.

2. A heat exchanger comprising:

a heat exchanger shell,

a tube sheet disposed at opposite ends of the heat exchanger shell defining an interior shell region,

an inlet channel and an outlet channel adjacent the tube sheets, the inlet channel having walls defining an inner surface,

a tube bundle disposed in the interior shell region and supported at opposite ends by the tube sheets, the tube bundle communicating the inlet and outlet channels therethrough,

a tubular means disposed in the inlet channel, said tubular means including a helical coil disposed along the inner surface of the inlet channel, said helical coil including a first helical turn at one end of the coil and a last helical turn at the other end of the coil, said helical coil being welded along its length to its adjacent helical turns so as to form a gas tight enclosure,

means for passing a high temperature fluid into the inlet channel at a position remote from the tube sheet adjacent said inlet channel,

means for passing a cooling fluid through the tubular means for cooling the high temperature fluid passing through the inlet channel,

a first circumferential gas-tight seal disposed between the inner surface and the first helical turn, and

a second circumferential gas-tight seal disposed between said inner surface and the last helical turn.

3. A heat exchanger according to claim 2 wherein said walls are cylindrical.

4. A heat exchanger according to claim 2 wherein said inlet channel walls consist of thin carbon steel.

5. A heat exchanger according to claim 4 wherein the tube sheet adjacent the inlet channel is lined with refractory.

6. A heat exchanger according to claim 5 further comprising:

means for passing a shell cooling fluid into the interior shell region about the tube bundle,

said last helical turn near the tube sheet adjacent the inlet channel, said first helical turn remote from said tube sheet adjacent the inlet channel, the cooling fluid passing into the helical coil at the first helical turn and exiting from the helical coil at the last helical turn,

said inlet channel walls being cylindrical and said 8. A heat exchanger-vapor generator system accordgas seals being annular. ing to claim 7, wherein said helical coil includes a first A heat exchanger-Vapor gancfatol y tem C mP i helical turn at one end of the coil and a last helical turn mg; at the other end of the coil, said coil welded along its a aste at 11 r g: a l; a tube, Sheet length to its adjacent helical turns so as to form a gas iilspqsed at OPPQSlte of h boner 'definmg an tight enclosure, the channels comprising cylindrical walls, Interior shell reglon; an Inlet channel and an outlet said tube sheet adjacent the inlet channel being lined channel adjacent the tube sheets, the inlet channel having walls defining an inner surface; a tube bundle disposed in the interior shell region and supported at opposite ends by the tube sheets, said tube bundle in fluid communication with the inlet and outlet channels; a tubular helical coil disposed along the inner surface of the inlet channel,

with refractory,

said heat exchanger further comprising a first annular gas-tight seal disposed between the inner surface and the first helical turn and a second annular gastight seal disposed between said inner surface and the last helical turn, said last helical turn near the tube sheet adjacent the a reformer for heating a process gas therein, 15 a transfer line for passing the heated process gas Inlet Channel,

from the reformer into the inlet channel at a posisaid first helical turn remote from said tube sheet adtion remote from the tube sheet adjacent said inlet jacent the inlet channel, the water passing into the channel, helical coil at the first helical turn and exiting from a steam generating system comprising: a steam drum the helical coil at the last helical turn.

disposed above the waste heat boiler; means for feeding water into said drum; a downcomer for References Cited passing Water from said drum into the interior shell UNITED STATES PATENTS region; an upcomer for passing steam generated in the interior shell region to the steam drum; a forced 2,900,168 8/ 1959 y 165144 X circulation loop comprising, means including a pump 2,951,685 9/ 9 0 Bliss et a1. 165134 X for passing water through the tubular helical coil for cooling the heated process gas passing through ROBERT O LEARY, Prlmary Emmmerthe inlet channel; upcomer means for passing the A W DAVIS Assistant Examiner. heated water in the coil into the steam drum.

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