US 3242983 A
Description (OCR text may contain errors)
'its lengthwise traverse of the shell.
'U-tubes or by a floating return header.
flow results in much lower thermal efliciency.
United States Patent 3,242,983 HEAT EXCHAN GER APPARATUS Noel H. de Nevers, Salt Lake City, Utah, assignor to Chevron Research Company, a corporation of Delaware Filed Jan. 14, 1965, Ser. No. 426,481
3 Claims. (Cl. 165-159) This is a continuation-in-part application of Heat Exchanger Apparatus, N. H. de Nevers, filed December 13, 1961, Serial No. 159,063, now abandoned.
This invention relates to an improved type of shell and tube heat exchanger.
The invention solves certain prior art problems that exist to a greater extent in large heat exchangers, i.e., those exchangers having shells longer than about 5 to feet than in smaller heat exchangers of the same type.
In small shell and tube heat exchangers, one fluid passes through many tubes while another fluid flows through a cylindrical shell surrounding the tubes. The tubes are fixed (by rolling or welding) to tube sheets which form the end closures of the cylindrical shell. These end closures prevent any mixing of the two fluids. Baflies generally are located transversely in the shell to force the shell fluid to flow back and forth across the tubes during In these small exchangers a very close approach to true countercurrent flow is achieved. True countercurrent flow is desirable because it gives the greatest heat transfer efficiency (maximum heat transferred per unit area of heat transfer surface).
In large heat exchangers (shells longer than about 5 to 10 feet), the aforesaid method of construction is not feasible. The difference in a thermal expansion of the shell and tube is conducive to the rupture of the tube sheet or the tubes. The flexibility of the material is adequate to take up this differential expansion in the small units, but not in the larger units, for which some other means must be found to prevent tube and/ or sheet rupture from thermally-induced stresses.
The customary solution for this thermal expansion problem has been to provide for the fluid in the tubes to be returned to the inlet end of the exchanger either by However, this solution of the thermal expansion problem is achieved at the cost of a significant reduction in thermal efliciency of the exchanger. This reduction is a result of passing the tube side fluid countercurrent to the shell side fluid in only half of the exchanger and passing it cocurrent to the shell side fluid in the other half of the exchanger. Cocurrent As a result to obtain adequate heat transfer, the designer must alter design variables of the exchanger to compensate for the lower heat transfer efiiciency as for example by increasing the length of the exchanger. With such changes, however, there is a corresponding increase in fabrication and repair costs of the exchanger.
The object of the invention is the provision of an improved multipass heat exchanger and method of fabrication in which space within the exchanger is economically allocated to maximize the heat transfer between the shell and tube side fluids and simultaneously reduce the size of the shell cover, headers, tubes, etc. heretofore required to provide such transfer.
Conventional two-tube-pass one-shell-pass heat exchangers normally are designed with an efliciency factor of as low as 80% (i.e., having a thermal efficiency only 80% of that of a true countercurrent exchanger). The exchanger of the present invention has a thermal efiivciency much more closely approaching that of a true "ice the design reduces the size of the exchanger over those of prior art exchangers that provide similar inlet and outlet temperatures for the shell side and tube side fluids.
The invention will best be understood, and. further objects and advantages thereof will be apparent, from the following description when read in connection with the accompanying drawing in which:
FIGURE 1 is a sectional elevation view illustrating a conventional two-tube-pass one-shell-pass heat exchanger of the aforesaid type, namely one in which tubes for one fluid medium are longitudinally disposed within the shell, and baffles are provided for elongating the path of the flow of the other fluid through the apparatus;
FIGURE 2 is a sectional elevation view illustrating an embodiment of the present invention, also comprising tubes for one fluid longitudinally disposed within the shell and battles for elongating the path of the flow of the other fluid through the apparatus;
FIGURE 3 is a transverse sectional view taken along the lines 3-3 in FIGURE 2.
In accordance with the present invention, there is provided a compact multipass heat exchanger for maximizing heat transfer between shell side and tube side fluids in which the tube side fluid passes through a multiplicity of small diameter tubes in countercurrent flow with the path of the shell side fluid, and returning therethr-ough by a single large conduit in concurrent flow with the shell side fluid. The small tubes are transversely located across the exchanger to substantially fill the interior of the shell. In that manner, the heat transfer area of the tubes in the countercurrent flow direction is increased over that of conventional exchangers. In addition, the economical spacing of the small tubes and return conduit within the shell, provide an over-all decrease in the size of the exchanger in that the transverse spacing required to provide mechanical support for the single return conduit occurs only over one radial section of the exchanger, not over the entire interior region, as in the conventional exchangers.
Further in accordance with the present invention, it will be understood that the invention as set forth in the foregoing paragraph may be modified by reversing operation thereof, for example by passing said second fluid first through said conduit and thence through said tubes, and by reversing the flow of said first, or shell, fluid through said apparatus to maintain tube fluid flow countercurrent to said shell fluid flow and to maintain the flow of tube fluid in said conduit in cocurrent flow with the shell fluid.
The invention is applicable to shell and tube heat exchanger apparatus having at least two tube passes and at least one shell pass. It is of the essence of the invention to provide, in any such type of apparatus, for passing tube fluid longitudinally through the apparatus countercurrent to the shell fluid in at least eight times as many uniformly disposed fluid streams as there are tube fluid streams flowing cocurrent to the shell fluid. Preferably, there will be 25 to 600, more preferably to 500 uniformly disposed tube fluid streams flowing through the apparatus countercurrent to the shell fluid. These streams will be disposed within the apparatus throughout the interior of the shell including the space transverse to the axis of symmetry and the shell. An exemplary conventional two-tube-pass one-shell-pass shell and tube heat exchanger might contain 500 tubes, each onehalf inch in diameter, in which tube fluid would flow countercurrent in 250 tubes and cocurrent in 250 tubes. An exemplary apparatus of the present invention, which would be used in lieu of said conventional apparatus, would comprise 450 transversely disposed tubes, each onehalf inch in diameter, and one tube about six inches in diameter. The tube fluid in the latter apparatus would flow countercurrent to the shell fluid in the 450 one-halfinch tubes and cocurrent in the single six-inch tube. The large tube or conduit in the apparatus of the present invention is preferably longitudinally disposed within the heat exchanger shell toward the periphery thereof and has sufficient area to pass the tube side fluid after the latter exists from the small tubes without change in flow pattern. The large conduit also reduces the transverse dimensions of the exchanger thereby reducing fabrication costs in that the spacing required for mechanical support of the single return conduit occurs only over one radial section of the exchanger, not over the entire central region as in conventional exchangers. It will be understood that more than one large tube or conduit may be used, so long as the aforesaid conditions are fulfilled.
It has been found that a higher mean temperature difference may be obtained with the aforesaid apparatus, enabling more heat transfer to take place than in a conventional apparatus with the same heat transfer area, i.e., in an exchanger having tubes of equal size in both the cocurrent and countercurrent flow directions.
It has been found that the apparatus accomplishes a closer approach to true countercurrent flow by providing for tube fluid flows countercurrent to the shell current in many small tubes and cocurrent to the shell fluid in one large conduit. The resulting heat transfer per unit area of exchanger provides a higher thermal efficiency for the apparatus enabling a reduction in the all-over dimensions of the exchanger over those of conventional exchangers.
Referring now to FIGURE 1, there shown is a sectional elevation view of a conventional type of shell and tube heat exchanger. Tubes 1, for carrying a first fluid medium are longitudinally disposed in elongated pressure vessel or shell 2, as shown. Tube sheet 3 and separator 4 divide one end of shell 2 into an inlet chamber 5 and an outlet chamber 6. A first fluid medium is passed through inlet 7 into inlet chamber 5, thence through tubes 1 to outlet chamber 6, and thence through outlet 8. A second fluid medium is passed through inlet 9 into shell 2, thence longitudinally through shell 2 by an elongated pathway created by baffles 10, and thence through outlet 11. In such a heat exchanger, either fluid may be at a higher temperature and will convey heat to the other fluid through the walls of tubes 1, acting as intermediary heat exchange materials. It will be noted that in this conventional type of heat exchanger expansion of the tubes introduces no stress on the shell.
Referring now to FIGURE 2, there shown is a sectional elevation view illustrating an embodiment of the present invention, comprising small tubes distributed throughout the interior of the shell including the space transverse thereto, and a large conduit longitudinally disposed Within the shell for passage of tube fluid through the apparatus and baffles for elongating the path for the flow of the shell fluid through the apparatus. Tubes 21, for carrying a first fluid medium are disposed in elongated pressure vessel or shell 22 as shown. Tube sheet 23 and separator 24 divide one end of shell 22 into an inlet chamber 25 and an outlet chamber 26. Tube sheet 27 and cover 35 form a floating head fluid transfer chamber 28, as shown.
A large conduit 29 is disposed in shell 22, as shown, coextensive with tubes 21. It has a cross-sectional area that may be substantially less than the sum of the crosssectional areas of tubes 21.
There is an economic balance to be struck in the design of the size of the conduit 29 between low pressure drop through the conduit and minimum surface area contained in the conduit. It can be shown that the minimum crosssectional area of the conduit 29 need be only small large 21 to maintain an equal pressure drop in each direction of travel of the tube side fluid, where dsman is the diameter of tubes 21 and dlarge is the diameter of conduit 29 Of course the linear velocity of the fluid in conduit 29 relates to velocity of the fluid in tubes 21 in the same manner as that which exists for their respective areas. Consequently, it is desirable that 'care be taken in the selection of the respective sizes of these members to maintain a uniform flow pattern of the tube side fluid under operating conditions.
A first fluid medium is passed through inlet 30 into inlet chamber 25, thence through tubes 21 to fluid transfer chamber 28, thence through large conduit 29 into outlet chamber 26, and thence through outlet 31. A second fluid medium is passed through inlet 32 into shell 22, thence longitudinally through shell 22 by an elongated pathway created by baflies 33, and thence through outlet 34. Either the tube fluid or the shell fluid may be at a higher temperature and will convey heat to the other fluid through the Walls of tubes 21, acting as intermediary heat exchange materials. It will be seen that, in operation of the embodiment shown, a number of separate tube fluid streams, flowing therethrough countercurrent to the flow of shell fluid at locations radially distributed throughout the interior of the shell can be made substantially greater than the number of fluid streams flowing through the apparatus cocurrent to the shell fluid. Therefore more heat transfer can be caused to take place in the apparatus with the same heat transfer area than in the case with conventional shell and tube heat exchanger apparatus. In addition, the one fluid stream flowing cocurrent with the shell fluid can be directed therealong by a single conduit that economically utilizes space within the exchanger.
Referring now to FIGURE 3, there shown is a transverse sectional view taken along the lines. 33 in FIG- URE 2. Tubes 21 are shown disposed throughout the interior of the shell in spaced locations for passing the tube fluid through the apparatus in countercurrent flow to the shell fluid. The centers of these locations may define a series of circles having a common origin located at the axis of symmetry of the exchanger, but preferably the tube arrangement involves positioning the tubes across the entire interior of the shell in either (1) equilateral, (2) staggered square, or (3) in line square tube arrays as conventionally understood in the art. The spacing between each tube in any of the above-mentioned arrays is preferably equal to prevent hot spots from developing during operation of the exchanger and to equalize the relative pressure drop per unit section of the exchanger.
Large conduit or tube 29 is shown disposed within shell 22 for tube fluid flow through the apparatus in cocurrent flow with the shell fluid. The conduit 29 is preferably disposed close to the periphery of shell 22 as shown and attaches at its ends to the tube sheet, one of which permanently attaches to the sidewall of the shell 22. The strength of the tube sheets, of course, is directly related to the type of material used and inversely related to the number of openings formed therein to receive the tubes 21 and the conduit 29. Consequently by using only a single conduit, the amount of material required for support can be substantially reduced, if desired. The design thus provides that the conduit need be supported in only one radial location of the tube sheet, not across their entire surfaces as in conventional exchangers. The saving in space can be used to reduce the size of the exchanger or increase the number of tubes passing the tube fluid in countercurrent flow with the shell side fluid. However, the flow capacity of the conduit 29 must be designed so as to return the tube fluid through the exchanger without a change in flow pattern.
It will be seen that the apparatus of the present invention inherently results in distributing the total cross-sectional area of countercurrent flow tubes 21 over a much greater portion of the cross-sectional area of shell 22 than in conventional shell and tube heat exchanger apparatus. Consequently the apparatus of the present invention accomplishes greater heat transfer than in conventional similar apparatus having the same total heat transfer area so that the heat transfer efficiency therefore is superior. Furthermore, there is provided a constuction that allows a reduction in the size of the exchanger but with no corresponding reduction in performance.
In the design and fabrication of the exchanger in accordance with this invention, there will necessarily be involved a re-examination of the heat balance equations heretofore used to describe the operation of multipass heat exchangers. In previous designs the cross-sectional dimensions of the tubes used to convey the tube side fluid have always been assumed to be equal. See for example, pages 340 et seq., Principles of Engineering Heat Transfer, Warren H. Geidt, D. Van Nostrand, 1957. In the optimization of the exchanger in accordance with the invention, there are involved tubes of unequal dimensions. Therefore the solution of these heat balance equations will be of increased complexity, and for this purpose the assistance of acomputer may be desirable.
In one modified design approachcalled the logarithmic-mean temperature correction -approaohit can be shown for a single-shell-pass, two-tube-pass exchanger with unequal tube sizes in the two tube passes, that the temperature of the shell side fluid, t at any location along the length of the exchanger can be expressed by d tt (a-l-b) U ab dac dz 0., C, 0
where: U is the average over-all conductance of the entire exchanger,
In the above equation, it is assumed that the average over-all coeflicient, U, is constant throughout the exchangers, the fluid capacity rates are constant, and the fluid mixing is suflicient to make the temperature uniform across any cross section of the exchan er.
In order to achieve a workable methematical model of the heat transfer using the above equation, the above equation is first solved; then the solution is differentiated to relate the heat transfer in the exchanger to the temperature of the shell side fluid, t along the length thereof, by considering the rate of change of t with the length of the exchanger.
Mean temperature difference between the tube side and shell side fluids, Ar is then determined in terms of the inlet and outlet temperatures of the fluids using Newtons basic cooling laws in conjunction with the last-mentioned equation, i.e., the equation describing the rate of change of the temperature of the shell side fluid t with exchanger lengths.
If desired, nomographs can be prepared using a dimensionless correction factor where:
Ar is the mean temperature difference between the tube and shell fluids, and
At is the log mean temperature difference between these fluids calculated for countercurrent flow.
In calculating optimum cross-sectional dimensions for the exchanger, it will be necessary in some cases to assume a series of different temperature difference between 6 the fluids before optimum dimensions can be found as is presently practiced in the art.
Another approach which eliminates the trial-and-error calculations noted above is the effectiveness-number of heat transfer units approach. It is based on the fact that:
(1) the average over-all conductance of the entire exchanger,
(2) the heat transfer area of the exchanger,
(3) the fluid capacity rate of the shell side fluid, and
(4) the fluid capacity rate of the tube side fluid,
are functions of the terminal temperatures of the fluids. This approach, however, again must take into consideration the change in tube dimensions, and basically involves optimizing the cross-sectional dimensions by comparing the actual heat transfer rate of the exchanger with the thermodynamically limited maximum heat transfer rate that could possibly be realized.
For fabrication purposes, it may also be desirable to use a plurality of large tubes or conduits 29; but this is not a preferred form of the apparatus of the present invention and in any event the invention would require at least eight and preferably 25600 and still more preferably -50 times as many small tubes as large conduits.
It will become more apparent to those skilled in the art that various modifications and variations may be made in the apparatus of the present invention as described above. Consequently all modifications that are within the spirit of the invention are intended to be included within the scope of the appended claims.
1. In shell and tube heat exchanger apparatus comprising an elongated heat exchanger shell having an axis and a sidewall, a fluid inlet for a first fluid at one end thereof and a fluid outlet for said first fluid at the other end thereof, the improvement which comprises a bundle of elongated tubes longitudinally disposed in said shell for passage of a second fluid within said tubes in two passes through said shell, inlet means at one end of said shell for conveying said second fluid into said tubes without physical contact with said first fluid and outlet means at the other end of said shell for conveying said second fluid from said tubes without physical contact with said first fluid, said bundle including the combination of a multiplicity of tubes of relatively small circular cross section for said first pass of said second fluid through said apparatus in countercurrent flow with the pass of said first fluid through said apparatus and a single large conduit of constant circular cross-section for said second pass of said second fluid through said apparatus in cocurrent flow with the pass of said first fluid through said apparatus thereby forming a compact heat exchanger apparatus having maximum thermal efficiency by decreasing the relative heat transfer of said first and second fluids in said cocurrent flow direction and increasing the relative heat transfer in said countercurrent flow direction, said large conduit having a minimum cross-sectional area equal to:
fi X Asmall lar e where dsman is the diameter of each of said multiplicity of tubes, dlarge is the diameter of said large conduit and Asman is the total cross-sectional area of said multiplicity of tubes, said multiplicity of tubes being transversely and symmetrically disposed within said exchanger across the interior of said shell unoccupied by said large conduit to provide maximum heat transfer area between said first and second fluids in the countercurrent flow direction, said shell fluid forming a moving column of fluid immediately adjacent to the exterior surface of said large conduit as said first fluid travels between said fluid inlet and said fluid outlet therefor.
2. Heat exchanger apparatus comprising an elongated shell having an axis and a sidewall, a plurality of circular cross-sectional tubes transversely disposed within said shell across the interior thereof between said axis and said sidewall for passing tube fluid through said apparatus generally countercurrent to shell fluid at a desired volumetric rate, a single conduit of circular cross-section disposedwithin said shell coextensive with said plurality of tubes, said conduit having a minimum circular crosssectional area equal to:
large where d is the diameter of each of said multiplicity of tubes, d is the diameter of said large conduit and Asman is the total cross-sectional area of said multiplicity of tubes to return said tube fluid through said apparatus at said rate in a return pass cocurrent to said shell fluid thereby to maximize thermal efliciency of said apparatus by decreasing relative heat transfer of said tube fluid and said shell fluid in said cocurrent flow direction and increasing heat transfer in said countercurrent flow direction, means for introducing said tube fluid into said tubes, means for directing said tube fluid from said tubes through said conduit, means for withdrawing said tube fluid from said conduit, and means for passing said shell fluid in a sinusoidal path through said apparatus countercurrent to said tube fluid in said tubes and cocurrent to said tube fluid in said conduit, said shell fluid forming a moving column of fluid in the region immediately adjacent'to the exterior surface of said single conduit as said shell fluid passes through said apparatus.
3. Heat exchanger apparatus comprising an elongated shell, a single large conduit of circular cross-section longitudinally disposed within said shell for passing tube fluid through said apparatus generally cocurrent with shell fluid at a desired volumetric rate, a plurality of tubes longitudinally transversely disposed within said shell in radially spaced locations across the entire interior of said shell, said plurality of tubes having a total cross-sectional area equal to:
small large where A is the cross-sectional area of said single large conduit, dsman is the diameter of each of said multiplicity of tubes and d is the diameter of said large conduit to return said tube fluid through said apparatus at said rate in a return pass countercurrent to said shell fluid thereby decreasing relative heat transfer of said tube fluid and said shell fluid in said cocurrent flow direction and increasing heat transfer in said countercurrent flow direction to maximize thermal efliciency of said apparatus, means for introducing said tube fluid into said conduit, means for directing said tube fluid from said conduit through said tubes, means for withdrawing said tube fluid from said tubes, and means for passing said shell fluid in a sinusoidal path through said apparatus countercurrent to said tube fluid in said tubes and cocurrent to said tube fluid in said conduit.
References Cited by the Examiner UNITED STATES PATENTS 167,182 7/1875 Mallory l,-l46 1,497,491 6/1924 Elliott -146 X 1,856,771 5/1932 Loefller 165-155 X 2,468,903 5/1949 Villiger 165155 2,942,855 6/1960 Wellensiek 165159 X FOREIGN PATENTS 596,021 7/ 1959 Italy.
267,189 6/ 1950 Switzerland.
KENNETH W. SPRAGUE, Primary Examiner.
FREDERICK L. MATTESON, JR., Examiner.