Heat Exchanger

Abstract

A baffle is provided adjacent to the outlet side of the tube sheet of a multiple tube pass heat exchanger. A portion of the input fluid is passed between the baffle and the tube sheet, rather than through the tubes, so that the tube sheet is maintained at a substantially uniform temperature. Ferrules pass the outlet gas portions from the tubes to the outlet chamber of the channel.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to shell and tube heat exchangers, for indirect heat exchange between fluid streams, one of which is passed through the tubes while the other circulates through the shell of the unit external to the tubes. A typical application is the preheating of the tail gas from a high pressure nitric acid process, prior to passing the tail gas through an expander for power recovery, by passing the cold tail gas through the tubes of a heat exchanger. The tail gas is heated by heat exchange with a hot process gas stream formed by the catalytic oxidation of ammonia vapor with air to form nitrogen oxides, for subsequent conversion to nitric acid.

2. Description of the Prior Art

Apparatus for fluid heat exchange involving the provision of thermal shields or the like for the formation of stagnant regions of reduced fluid flow is shown in U.S. Pat. Nos. 1,651,875; 2,203,357; 2,252,069; 3,132,691 and 3,279,532. The provision of heat exchange between the tail gas produced by a nitric acid process and the hot effluent gas stream formed by catalytic ammonia oxidation is described in U.S. Pat. application No. 409,507 filed Nov. 6, 1964 and now U.S. Pat. No. 3,467,492, granted Sept. 16, 1969.

In multiple pass heat exchangers heating or cooling a fluid through a large temperature range, great difficulty is experienced in maintaining a suitable seal between the tube sheet and the shell side fluid due to the differences in expansion resulting from the different temperatures at the inlet and the outlet tubes. For a typical shell and tube heat exchanger design, the tube sheet temperature tends to approach the temperature of the tubes in the pierced section since, unless boiling or condensing occurs on the shell side, heat transfer coefficients inside the tubes will be higher than on the relatively stagnant shell side face of the tube sheet, and heat transfer surface areas contacting the tube side fluid inside the tube sheet is normally substantially greater than that contacting the shell side fluid on its face. In a multiple pass unit therefore, the tube sheet tends to have substantially different temperatures at the sections where the tube side fluid is respectively admitted and removed. When these temperatures are substantially different, the thermal expansion of each part will be correspondingly different, resulting in different forces being exerted at the opposite sides of the joint between the tube sheet and shell. Such differing stresses will lead to a relative loosening of part of the gasket seal at these points, or cracking of a welded seal when provided, requiring at least the tightening of the bolts after each heating or cooling of the unit, and ultimately requiring replacement of the gasket after a few cycles.

SUMMARY OF THE INVENTION

In the present invention, the difference in temperatures between, for example, the top and bottom of the tube sheet is minimized by providing a baffle and tube ferrules on the downstream half of the tube sheet so as to permit some inlet fluid to bypass to the outlet fluid side, thus making the latter half of the tube sheet temperature more nearly approach that of the inlet half. Internal insulation can also be provided in the channel to minimize differences between top and bottom temperature, or these differences relative to the new uniform tube sheet temperature could become a problem.

The ferrules provided should be tapered or be a nonheat conductive material, not only to facilitate assembly but also to insure against their contacting the tube wall, which could permit transfer of heat from the tube side fluid to the tube sheet. Alternatively, spacers could be provided on each ferrule, to permit some of the inlet gas to directly cool the tube wall before mixing with the outlet gas, thus further reducing temperature variations on the tube sheet.

The baffle or shield is supported and spaced from the outlet half of the tube sheet on ferrules rolled lightly into each outlet tube. Holes provided in the channel partition plate permit the bypassing of sufficient inlet fluid to flow behind the shield and ferrules, and generally between the shield and the tube sheet, to hold the entire sheet at close to the inlet fluid temperature. When the tube sheet is held to a more nearly constant temperature by the shield, the external leakage problem may be transferred to some extent to the channel flange joint. In order to further obviate such transfer of the temperature differential and leakage problem, internal insulation may be provided in at least the top half of the head in order to further minimize temperature differences. In an alternative embodiment, the outer edge of the shield is flanged in order to direct sufficient cooler fluid across the body flange from the space between the shield and the tube sheet, in order to minimize the temperature differences on the body flange without providing internal insulation.

The primary advantage of the invention is that leakage problems due to differential thermal expansion of the tube sheet in shell and tube heat exchangers are prevented. Another advantage is that the apparatus is relatively simple to fabricate and install. A further advantage is that temperature differences between the shell and head or channel are effectively prevented.

It is an object of the present invention to provide an improvement in fluid heat exchangers.

Another object is to provide an improved shell and tube heat exchanger.

A further object is to provide a shell and a tube heat exchanger in which a portion of the input tube side fluid is bypassed and utilized to minimize temperature differentials in the tube sheet.

An additional object is to prevent differential thermal expansion of members in a shell and tube heat exchanger.

Still another object is to contact the tube outlet side of the tube sheet of a shell and tube heat exchanger with a portion of the inlet fluid to prevent differential thermal expansion and leakage.

These and other objects and advantages of the present invention will become evident from the description which follows.

DESCRIPTION OF THE DRAWINGS AND PREFERRED EMBODIMENTS

Referring now to the drawings, FIG. 1 is a sectional elevation view of one embodiment of the invention, as applied to a U-tube heat exchanger,

FIG. 3 is an enlarged view of a portion of the apparatus of FIG. 1, showing structural details,

FIG. 3 is a sectional elevation view of FIG. 2, taken on section 3-3,

FIG. 4 is a sectional elevation view of an alternative embodiment of the invention, as applied to one conventional type of shell and tube heat exchanger,

FIG. 5 is an enlarged view of a portion of the apparatus of FIG. 4, showing structural details,

FIG. 6 is a sectional elevation view of FIG. 5, taken on section 6-6, and

FIG. 7 is a sectional elevation view of another alternative embodiment of the invention.

Referring now to FIG. 1, stream 1 is typically the cold tail gas from a high pressure nitric acid process, however stream 1 may in practice consist of any cold fluid which is to be heated, or any hot fluid which is to be cooled. For purposes of clarity, stream 1 will be described as a cold nitric acid process tail gas. The cold gas stream 1 passes via nozzle 2 into the channel 3, which consists of a heat exchanger head. An internal insulation layer 4 is provided over the entire inner surface of channel 3, and a partition 5 divides the channel 3 into a lower cold gas inlet section and an upper heated gas outlet section. The cold gas flows from the lower section of channel 3 below partition 5 into the inlet section 6 of heat exchange U-tube which are mounted in tube sheet 7. One tube is shown in the FIG. The tube sheet 7 is mounted between channel 3 and the heat exchanger shell or body 8. A hot fluid stream 9, which in the case of a nitric acid production facility may consist of the hot gas mixture formed by catalytic ammonia oxidation to nitrogen oxides, is passed via nozzle 10 into shell 8, and stream 9 flows downwards external to the heat exchanger U-tube and thereby becomes cooled by indirect heat exchange with the cold tail gas flowing within the U-tube. The resulting cooled gas stream is removed from shell 8 via lower nozzle 11 as stream 12, which is now passed to further heat exchange or process usage.

The heat exchanger U-tube is defined by the straight inlet section 6, the curved or semicircular return section 13, and the straight outlet section 14. A tapered ferrule 15 is lightly rolled into the outlet end of tube 14, and the ferrule 15 is mounted on the baffle or shield 16 which extends upwards from partition 5 adjacent to and spaced from the tube sheet 7. The baffle 16 is generally disposed substantially parallel with the tube sheet 7. The heated tail gas flowing through tube section 14 passes through ferrule 15 and is discharged into the upper heated gas outlet section of channel 3 beyond baffle 16, so that the heated tail gas does not contact or heat the tube sheet 7.

In accordance with the present invention, one or a series of openings 17 is provided in the section of the partition 5 between tube sheet 7 and shield 16. A portion of the cold inlet gas stream 1 flows from the lower cold gas inlet section of channel 3 through the opening 17, and thereafter the cold gas portion flows upwards between tube sheet 7 and shield 16, thus serving to cool the tube sheet 7 and maintain tube sheet 7 at a substantially uniform temperature. The cold gas portion next flows, around the upper end of shield 16, which is spaced from channel 3, and the cold gas portion then joins the main heated gas stream in the upper heated gas outlet portion of channel 3. The heated tail gas is removed from channel 3 via nozzle 18 as stream 19, which is not at high pressure and elevated temperature and is suitable for passage to expansion and power recovery in a suitable gas turbine, expander or the like.

Referring now to FIG. 2, a modified version of a portion of the structure of FIG. 1 is shown in enlarged sectional detail. The ferrule 15 extends between shield 16 and tube sheet 7 and into tube section 14, and only the portion of ferrule 15 within tube 14 is tapered. Ferrule 15 is joined to shield 16 by outer continuous weld 20, and tube 14 is joined to tube sheet 7 by outer continuous weld 21. The welds 20 and 21 provide a sealing effect against gas flow at the respective joints. The tapered portion of ferrule 15 is centrally oriented within and spaced from tube 14 by the provision of a plurality of projecting shoulders or nubs 22, which are staggered about the inner periphery of tube 14, or the outer periphery of ferrule 15, so as to permit a portion of the cold gas flowing between shield 16 and tube 7 to flow into tube 14 through the annular passage between ferrule 15 and tube 14, and thereby provide a further cooling effect with respect to tube sheet 7. Tube 14 may be rolled onto tube sheet 7 by the provision of spaced ridges 23.

FIG. 3 is a sectional view of FIG. 2, and shows the central ferrule 15, the tube 14 and the weld 21 in concentric coaxial alignment. The four spacers or nubs 22 extending inwards from tube 14 to contact with ferrule 15 are also shown.

Referring now to FIG. 4, an alternative embodiment of the invention is illustrated in sectional elevation view. Cold tail gas stream 24 flows via nozzle 25 into the lower cold gas inlet section of channel 26, which is divided into a lower section and an upper heated gas outlet section by partition 27. The lower section of channel 26 is not provided with an internal layer of insulation. The cold gas flows from the lower section of channel 26 into the plurality of tubes 28, the inlet ends of tubes 28 being mounted in tube sheet 29. The gas outlet ends of the linear tubes 28 are mounted in tube sheet 30, which is provided with a floating head 31 for gas return to the plurality of linear tubes 32, with the tubes 28 and 32 being generally parallel and disposed within heat exchange shell 33 provided with head 34.

A hot fluid stream 35, which is typically a hot nitric acid process gas stream similar to stream 9 described supra, is passed via nozzle 36 into shell 33, and stream 35 flows upwards between tube sheet 29 and central baffle or partition 37 and external to tubes 28 and 32, thus heating the cold tail gas within tubes 28 and 32. The hot gas next flows above baffle 37 and downwards between baffle 37 and tube sheet 30, and external to tubes 32 and 28, thus further heating the cold gas within tubes 32 and 28. The resulting cooled gas is removed from shell 33 via nozzle 38 as stream 39, which is passed to process usage.

The heated tail gas flowing from tubes 32 next flows through ferrules 40, which are mounted in baffle or shield 41 and extend into tubes 32. The baffle 41 extends upwards from partition 27 and is generally substantially parallel with tube sheet 29, and is spaced from tube sheet 29. In accordance with the present invention, one or a plurality of openings 42 is provided in partition 27, in the section of partition 27 between baffle 41 and tube sheet 29. A portion of the cold gas flows from the lower inlet section of channel 26 through opening 42 and upwards between baffle 41 and tube sheet 29, and external to the ferrules 40, thus cooling the upper portion of the tube sheet 29 and maintaining element 29 at a substantially uniform temperature. The cold gas portion next flows upwards and around the upper end of baffle 41, and joins the main stream of warmed gas discharged into the upper outlet section of channel 26 via ferrules 40. The upper warmed gas outlet section of channel or head 26 is provided with an internal layer of insulation 43. The warmed tail gas is removed from the upper section of channel 26 via nozzle 44 as stream 45, which is now passed to a suitable gas turbine, expander or the like.

FIG. 5 is an enlarged view of a portion of the apparatus FIG. 4, and shows an embodiment of the invention in which ferrule 4 is spaced from the tube 32 and within tube 32 by the provision of outer linear wires or rods on the outer surface of ferrule 40. The ferrule 40 is attached to baffle 41 by outer seal weld 46, and the tube 32 is rolled into tube sheet 29 by ridges 47 and attached to tube sheet 29 by outer seal weld 48. The longitudinal or linear wires 49 are attached to the outer surface of the portion of ferrule 40 within tube 32, and wires 49 are usually parallel to the axis of ferrule 40. Elements 49 may consist of bars or rods in practice, and one or a plurality of elements 49 may be provided in suitable instances. A portion of the cold gas stream, which is flowing upwards between baffle 41 and tube sheet 29, flows into tubes 32 via the annular passage between ferrule 40 and tube 32 maintained by the spacer wires 49. The inward flow of cold gas between ferrule 40 and tube 32 further serves to maintain the tube sheet 29 at a substantially uniform temperature, by cooling the end of tube 32 which is in contact with tube sheet 29.

FIG. 6 is a sectional view of FIG. 5, taken on section 6-6, and shows the concentric and coaxial arrangement of ferrule 40, tube 32 and weld 48, as well as three spacer wires 49 disposed between ferrule 40 and tube 32.

Referring now to FIG. 7, an alternative embodiment of the invention is shown, which obviates any need or requirement for the provision of an internal layer of insulation on the inner surface of the channel. Cold gas stream 50 flows via nozzle 51 into the lower cold gas inlet section of the channel or head 52, which is divided by partition 53 into a lower inlet section and an upper warmed gas outlet section. The cold gas flows from the lower section of channels 52 into tubes 54 which are mounted in tube sheets 55, which is mounted between head or channel 52 and shell 56. A hot fluid is circulated in shell 56 external to tubes 54 and return tubes 57, which receive warmed gas from the discharge end of tubes 54 by suitable gas return means, not shown. The tubes 57 discharge the warmed gas into ferrules 58, which are mounted in baffle 59. In accordance with the present invention, the baffle 59 is spaced adjacent to and usually substantially parallel with tube sheet 55, and baffle 59 is disposed in the upper warmed gas outlet section of channel 52. One or a plurality of openings 60 is provided in partition 53, at the section of partition 53 between baffle 59 and tube sheet 55. A portion of the cold inlet gas flows from the lower inlet section of channel 52 through opening 60 and upwards between baffle 59 and tube sheet 55 and external to ferrules 58, thus cooling the tube sheet 55 and maintaining all of tube sheet 55 at a substantially uniform temperature. A flange 61 extends inwards and into the warmed gas outlet section of channel 52 from the upper end of baffle 59, and flange 61 is disposed adjacent to channel 52, so that the cold gas portion discharged upwards from between baffle 59 and tube sheet 55 is diverted laterally and adjacent to channel 52 by flange 61. Thus, the portion of channel 52 adjacent to and connected with the upper portions of tube sheet 55 and shell 56 is also cooled by the cold gas portion admitted via opening 60. The warmed gas discharged from ferrules 58 into the upper warmed gas outlet section of channel 52 combines adjacent to the terminus of flange 61 with the cold gas portion employed for cooling, and the warmed gas is then removed from the upper section of channel 52 via nozzle 62 as stream 63.

Numerous alternatives within the scope of the present invention, besides those mentioned supra, will occur to those skilled in the art. The invention is generally applicable to any type of shell and tube heat exchanger, and to the indirect heat exchange between any two fluids, either of which may be gaseous or liquid, and either of which may be warmed or cooled. Thus, the streams 1, 24 or 50 may be a hot fluid which is to be cooled in the heat exchanger apparatus, in which case the improvement of the present invention will serve to maintain the respective tube sheet 7, 29 or 55 at a substantially uniform elevated temperature. The portions of this ferrules within the tubes will generally be spaced from the tube walls as described supra, in order to allow for inwards flow of a cold gas portion in the annular passage between the ferrule and the tube end as described supra. However, the ferrules may alternatively be lightly rolled into the tubes, as shown in FIG. 1, in which case such gas flow would be restricted. The heat exchanger may be operated in practice in a vertical, horizontal or inclined position, depending on process conditions, and the apparatus of the present invention is generally applicable to any process or facility requiring indirect heat exchange between a cold fluid and a hot fluid. In instances when one of the fluids is a gas or vapor, partial or total condensation of this fluid to the liquid state may occur in the apparatus. In instances when one of the fluids is a liquid, partial or total vaporization of this fluid to the gaseous or vapor state may occur in the apparatus. The apparatus is also applicable to instances when one or both of the fluids is a gas-liquid mixture, such as a gas stream containing entrained liquid droplets.

Claims

We claim:

1. An apparatus for exchanging heat between a first fluid and a second fluid by indirect heat exchange which comprises a heat exchanger shell, a tube sheet extending across one end of said shell, a channel disposed about the outer side of said tube sheet and connected to said shell, a partition in said channel, said partition extending to said tube sheet and dividing said channel into a fluid inlet chamber and a fluid outlet chamber, at least one fluid inlet tube, said inlet tube extending into said shell from the portion of said tube sheet adjacent to said fluid inlet chamber, at least one fluid outlet tube, said outlet tube extending from said shell to an opening in the portion of said tube sheet adjacent to said fluid outlet chamber, means to transfer fluid from the discharge end of said fluid inlet tube to the fluid inlet end of said outlet tube, means to pass said first fluid into said inlet chamber, whereby a first portion of said first fluid flows through said inlet tube, said fluid transfer means and said outlet tube, a shield disposed in said fluid outlet chamber, said shield being substantially parallel with and spaced from the portion of said tube sheet adjacent to said fluid outlet chamber, a ferrule, said ferrule being mounted in said shield and extending from said shield into said fluid outlet tube and spaced from the inner tube wall, whereby the first portion of said first fluid discharged from said outlet tube flows through said ferrule and into said fluid outlet chamber, at least one opening provided in said partition between said shield and said tube sheet, whereby a second portion of said first fluid flows through said opening and between said shield and said tube sheet, thereby maintaining the temperature level of the portion of said tube sheet adjacent to said fluid outlet chamber at a temperature level approximately that of the valance of said tube sheet, said second portion of said first fluid thereafter flowing into said fluid outlet chamber to mix with said first portion of said first fluid discharged from said ferrule, means to remove said first fluid from the fluid outlet chamber of said channel, and means to circulate said second fluid through said shell external to said fluid inlet tube and said fluid outlet tube, whereby heat is exchanged between said second fluid and said first portion of said first fluid flowing through said tubes.

2. The apparatus of claim 1, in which the extension of said ferrule within said fluid outlet tube is tapered.

3. The apparatus of claim 1, in which at least one spacer is provided between said ferrule and said fluid outlet tube, whereby part of said second portion of said first fluid flows between said ferrule and said outlet tube.

4. The apparatus of claim 1, in which internal insulation is provided on the inner surface of the portion of said channel defining said fluid outlet chamber.

5. The apparatus of claim 1, in which internal insulation is provided on the inner surface of all of said channel.

6. The apparatus of claim 1, in which a flange is provided on the outer edge of said shield, said flange extending into said fluid outlet chamber, whereby said second portion of said first fluid is directed parallel to said channel adjacent to the outer joint between said channel and said shell.

7. The apparatus of claim 1, in which said means to transfer fluid from the discharge end of said fluid inlet tube to the fluid inlet end of said outlet tube is a second channel mounted on the opposed end of said shell.

8. The apparatus of claim 1, in which said means to transfer fluid from the discharge end of said fluid inlet tube to the fluid inlet end of said outlet tube is a curved tube section.

9. The apparatus of claim 8, in which said inlet tube, said curved tube section and said outlet tube define a U-tube for heat exchange.

10. The apparatus of claim 1, in which said first fluid is initially cooler than said second fluid, and said first fluid is heated by indirect heat exchange with said second fluid while flowing through said tubes.

11. The apparatus of claim 1, in which said first fluid is a gas.

12. The apparatus of claim 11, in which said gas is the cold tail gas from a nitric acid production process, and said second fluid is a hot gas stream generated by the catalytic combustion of ammonia vapor with air to form a nitrogen oxides-rich gas stream.