US 3592261 A
Description (OCR text may contain errors)
United States Patent [561 Relenences Cited UNITED STATES PATENTS 2,620,830 12/ 1952 VSchultz 285/213 X 1,894,956 111933 Kerr 1 165/178 X 2,195,403 4/1940 Bay 165/178 3,001,766 9/1961 Last A165/178 X 3.123.133 311964 Pels etal 165/178 3.330.337 H1967 Heth 165/178 Primary Examiner-Frederick L. Matteson Assistant Examiner-Theophil W. Streule Attorney-Mam Jangarathis ABSTRACT: 1n combination with a heat exchanger, insulating tube inserts positioned in the inlet portion of thc heat exchanger tubes which are at least coextensive with the portion ofthe tubeuetained in the inlet tube sheet to reduce both thermal stresses and the overall temperature ofthe tube sheet.
YPATENED JULI 3 sen INVENTOR,
Gory M. Block AT TORNE YS HEAT EXCHANGER This invention relates to a new and improved heat exchanger and more particularly to a heat exchanger and tube insert especially suitable for high temperature operation.
ln many processes a heat exchanger is employed for rapidly cooling a fluid which is at a very high temperature. Thus, for example, in the production of olefins from normally gaseous hydrocarbons by thermal cracking or in the steam reforming of naphtha and the like, the reaction effluent must be rapidly cooled to a temperature below the point where secondary reactions which decrease overall yield and cause coking occur. ln some processes, such rapid cooling is effected by passing the reaction effluent through the tubes of a transfer line heat exchanger wherein the effluent is rapidly cooled by indirect heat transfer with a suitable coolant. The high temperatures involved in effecting cooling in such transfer line exchangers cause design problems; particularly at the inlet tube sheet, in that the tube sheet must have a minimum thickness, which may be in the order of several inches, in order to withstand the operating pressures and such thickness requirements are further increased by the themial stresses caused by the high heat input to the tube sheet caused by heat transfer from the process fluid through the tube walls. The thermal problems; Le., thermal stresses, are further increased at higher operating pressures as a result of the requirement for a thicker tube sheet to withstand such higher pressures. ln addition to the problems caused by thermal stresses, the high temperatures of the tube sheet reduce the strength of the material from which the tube sheet is formed, thereby making the tube sheet material more prone to attack by process fluids.
There have been several proposals for alleviating the hereinabove noted problems, but such proposals have met with little success. One proposal involves increasing the total number of tubes to lower the heat transfer coefficient, but
such an expedient results in a unit with a greater total surface, which significantly increases overall costs. Moreover, this approach is limited by the requirements for rapid cooling and minimum velocity and therefore, does not pennit-reducing heat input to a satisfactory level.
Another proposal involves the use of a better class of metal; e.g., a high alloy, for the tube sheet; however, such metals have low thermal conductivities which would actually result in` an increase in thermal stresses thereby offsetting they advantages derived from the improved physical properties of the metal. A further proposal involves increasing the tube pitch to obtain more cooling surface at the backface of the tube sheet, but such an approach requires an increase in the thickness of the sheet as the spacing increases.
Accordingly, ari object of the invention is to provide a new and improved heat exchanger.
Another object of the invention is to provide a heat exchanger especially suitable for a high temperature operation.
A further object of this invention is to reduce thermal stresses on the tube sheet ofa heat exchanger.
Yet another object of this invention is to provide a new and improved heat exchanger which may be easily fabricated.
A still further object of the invention is to provide an insulating heat exchanger tube insertA These and other objects of the invention shouldA be more readily apparent from the following detailed description thereof when read with reference to the accompanying drawing wherein like numerals designate like parts and wherein:
FIG. 1 depicts an embodiment of the heat exchangeritube insert of the invention.
FIG. 2 is a partial view of the inlet end of a heat exchanger including another embodiment of the heat exchanger tube in sert ofthe invention.
The objects of this invention are broadly accomplished by providing the inlet.er id of heat exchanger tubes with an openended tubular insert which extends into the tubes for a length which is at least about coextensive with the portion of the tube retained in the tube sheet; i.e., the length of the insert is at -least equal to the thickness ofthe tube sheet. The hot material which is to be introduced into the tubes flows from the heat exchanger inlet chamber through the insert and into the tubes. The insert functions to insulate the tube sheet from hot material flowing through the tubes, thereby reducing the overall temperature of the tube sheet and thermal gradients across the tube sheet.
The invention will now be described with reference to cmbodiments thereof illustrated in the accompanying drawing but it is to be understood that thi.` scope of the invention is not to be limited thereby.
Referring now to FIG. l of the drawing, which illustrates an embodiment of the tube insert of the invention, a hollow openended metal tube'insert l0, formed ofa suitable metal, such as stainless steel, is comprised of a generally cylindrical center portion 1l which outwardly flares into an enlarged cylindrical end portion l2 and further contains an outwardly extending annular lip portion 13 at the other end. The center portion 1l is covered with an insulating jacket 14 which may be either a liber-type ofinsulation; e.g., the fiber insulation sold by Johns- Manville under the mark CERAFELT, or a molded insulation casted directly on the liner; e.g., the moldable material sold by .lohns-Manville under the mark MlN-K-2000. The exterior of the insulating jacket 14 is covered and protected by a thin piece of metal foil l5 which is generally maintained in position by overlapping a portion of the foil and applying a spot weld to the overlapped portion. The particular type of insulating material employed is dependent upon: the conditions employed in the exchanger, such as, temperature-and flow conditions; the tube sheet thickness; the properties of the fluid to be cooled; and the like. ln general, the molded insulation, which has a very high resistance to heat transfer, is employed when the exchanger is operated at a higher pressure and temperature, necessitating a thick tube sh; et, and the fiber insulation is employed at lower operating temperatures and pressures.
The center portion l1 of the tube insert l0, including the insulating jacket 14 has a length that is about equal to the thickness of the tube sheet so that proper insulation is provided over the entire thickness of the tube sheet. lt is to be understood, however, that the insert may extend into the tubes some distance beyond the tube sheet, but in general such excess distance is kept at a minimum in order to prevent a lowering of the heat transfer efficiency of the exchanger. The enlarged end portion 12 has a diameter which is sufficient to provide an interference fit between the end portion l2 and the in- Vside of the heat exchanger tube, thereby maintaining the insert within the heat exchanger tube. The lip portion 13 may also be suitably fastened to the inlet tube sheet, for example, by a tack weld.
Referring now to FlG. 2 of the drawing, there is illustrated another embodiment of the tube insert of the invention, shown in relationship to a heat exchanger and tube sheet, and which is particularly adapted for use with tube sheets provided vwith flexible insulation pads. The tube insert illustrated in FIG. 2 is similar to the embodiment of the insert illustrated in FIG. l and like parts are designated by like prime numerals.
The tube insert l0' contains an enlarged end portion 12, a lip portion 13' and is provided with suitable insulation 14 covered by foil l5'. A stop ring 101 for positioning the tube insert 10 ina heat exchanger tube is rigidly secured to the foil 15',.for example, by a spot weld, at a position such that the tube insert l0' extends into the heat exchanger tube a distance llziting cover 1'12 .is comprised of a flexible pad llyformed-of a suitable insulation material, which is enclosedin a` metal casing 114. A soft insulation fill 115 is placed between the metal casing 114 and the tube sheet 111 and the insulating cover is fixedly mounted to the tube sheet by a suitable connection, such as a bolt generally indicated as 116.
The tube insert 10' is maintained within heat exchanger tube 117 by an interference fit between theenlarged end portion 12' of the insert 10 and the inner wall of tube 117. A fluid to be cooled flows into the tube insert l' at the lip portion 13' thereofand flows from the tube insert, at the enlarged end l2', into heat exchanger tube 117. As a result ofthe insulating effect of the tube insert, both overall temperature and thermal stresses on the tube sheet are significantly decreased.
The invention is not limited by the embodiments thereof illustrated in the drawing and numerous modifications and variations are possible within the spirit and scope of the invention. Thus, for example, a tube insert may be provided which is similar to the one illustrated in FlG. 2, except that the insulating jacket and foil cover are only employed over the portion of the insert which extends into the heat exchanger tubes. ln accordance with this modification, the stop ring is attached directly to the metal of the tube insert and the portion of the insert which extends in to the tube inlet chamber is free of an insulating jacket and foil cover. This modification is particularly suitable for heat exchangers wherein the inlet tube sheet is provided with a castable refractory insulation.
As another modification, the metal tube insert may be entirely free of insulation whereby gas trapped in the annular space between the center portion of the tube insert and the inner wall of the heat exchanger tube provides the required insulation. This modification may be employed when the heat exchanger is operated under less severe conditions; e.g., lower temperatures and pressures.
As a further modification, the tube insert may be supported in the tube without effecting an interference fit between the enlarged end portion of the insert and the inner wall of the heat exchanger tube; e.g., the tube insert may be supported in the heat exchanger tube by a tack weid between the lip portion of the insert and the end of the heat exchanger tube.
As still another modification, the enlarged end portion of the tube insert may have a shape other than a cylindrical shape; e.g., the enlarged end portion may have a frustoconical shape.
The above modifications and various other modifications and combinations of insert configurations and insulations should be readily apparent to those skilled in the art from the teachings herein.
The invention is further illustrated by representative dimensions for a tube insert, for example, the tube insert of FIG. 1. lt is to be understood that the scope of the invention is not limited by such dimensions.
Length of insert-2.5
Outside diameter of center portion-,875
Outside diameter of center portion-including insulation and foil-.950
Outside diameter of enlarged portion- .954H
Heat exchangers incorporating the tube insert ofthe invention provide numerous advantages when employed in heat exchangers used for rapid cooling of high temperature streams. Thus, the tube insert reduces heat transfer between hot fluid in the heat exchanger tubes and the inlet tube sheet which would normally occur through the tube wall, thereby reducing the overall temperature of the tube sheet and thermal stresses therein. As a result of the reduction of thermal stresses, thickness requirements for the tube sheet are reduced, and in addition, the lower temperatures in the tube sheet reduces the susceptibility of the tube sheet to attack by process fluids. As another feature of the tube insert ofthe invention, the lip portion and the enlarged end portion reduce fluid turbulance.
Numerous modifications of the invention are possible and therefore the invention may be practiced otherwise than as particularly described.
l. ln combination with a heat exchanger employed for the rapid cooling of a high temperature tiuid having at least one heat exchange tube supported by a tube sheet and wherein the high temperature fluid is passed through said heat exchange tube` a heat insulating insert positioned in the inlet end of the tube having a length which is at least coextensive with the portion of thc heat exchange tube retained in the tube sheet whereby high temperature fluid to be introduced into the heat exchange tube flows through the insert, said insert comprising a hollow open-ended metal tube having a high temperature heat resistant insulating jacket means wrapped around the exterior thereof, said insulating jacket means insulating the tube sheet from the high temperature fluid flowing through the tube to reduce the overall temperature of the tube sheet and the thermal gradients across the tube sheet; and a thin piece of metal foil covering the heat-insulatingjacket.
2. The heat exchanger as defined in claim 1 wherein the heat exchanger includes a plurality of tubes, each tube being provided with an insert.
3. The heat exchanger of claim 2 wherein the inlet tube sheet is covered with insulating material and the tubular insert extends beyond the heat exchanger tubes and through the insulating material for a length which is about equal to the thickness of the insulating material.
4. The heat exchanger of claim 3 wherein said insulating jacket of the insert also covers the portion ofthe insert extending through the insulating material of the tube sheet.
5, The heat exchanger of claim 3 wherein the portion of the tube insert extending through the insulating material of the tube sheet is free of an insulating jacket.
6. The heat exchanger of claim 3 wherein the insert includes a stop ring at the portion of the insert to be located at the end of the heat exchanger tubes for positioning the insert in the tubes.
7. The heat exchanger as defined in claim 1 wherein the metal tube includes an enlarged end portion, said enlarged end portion being free ofthe heat-insulating jacket, said enlarged end portion being positioned within the portion of the heat exchange tube beyond the portion ofthe heat exchange tube retained in the tube sheet.
8. The heat exchanger as defined in claim 7 wherein the enlarged end portion of the insert defines an interference fit with the inner wall of the heat exchange tube.