Abstract:
A corrosion-resistant alloy metal tube sheet used to construct a shell and tube heat exchanger ( 50 ) for cooling fluids with sea water passing through corrosion-resistant alloy tubes ( 25 ) contained in a horizontal carbon steel outer shell ( 1 ) that are supported and sealed at each end by passing them through holes ( 27, 30 ) in a carbon steel tube sheet ( 28, 31 ) and axially aligned holes ( 36, 37 ) in alloy tube sheets ( 34, 35 ) that cover and protect the adjacent interior carbon steel tube sheets from sea water corrosion. The walls of the holes ( 27, 30, 36, 37 ) have at least one annular groove ( 45, 46 ) and the ends of each tube are radially expanded to form circumferential ridges ( 40, 41 ) on the outside of each tube at a mating location with each of said annular grooves ( 45, 46 ) where they are forcibly driven into the grooves to form a circumferential joint having good mechanical strength and water tightness, thereby eliminating the need for welding the external joint between the alloy tube sheets and alloy tubes.

Description:
FIELD OF THE INVENTION 
   This invention relates to an improvement in the construction of shell and tube heat exchangers where sea water is the coolant for non-contact heat exchange with a gaseous or liquid fluid. 
   BACKGROUND OF THE INVENTION 
   Sea water heat exchangers are commonly utilized in the oil and gas processing industry and in refineries where fresh water supplies may be limited. Design details of shell and tube type heat exchangers are described in Perry&#39;s Chemical Engineers&#39; Handbook; 7th ed., McGraw-Hill. Reference is also made to the publications of the Tubular Exchanger Manufacturers Association (TEMA). 
   In chemical plant and refinery locations where sea water is plentiful and cheap, it is economically desirable to use sea water as the cooling medium in coolers for gases and liquids. However, because of its corrosivity, sea water has been used only as a coolant in coolers made from expensive corrosion-resistant alloys. 
   The alloy tube sheet protective cover duplicates the configuration and number and placement of the tube receiving holes in the carbon steel tube sheet. The alloy and carbon steel tube sheets are mechanically sealed at their periphery by means described below. 
   It is common practice to weld the extended end portion of the tube to the outside of the alloy tube sheet for sealing purposes. Welding is a time-consuming and costly manufacturing process for tube sheets with hundreds of tubes. Highly skilled and motivated welders are required to produce a quality product. Low quality welded joints can result in sea water leaks and the hidden corrosion of the carbon steel base plate. This problem is increased with passage of time when corrosive sea water coolant at high temperature is in contact with the carbon steel. Further, it is an expensive and time-consuming process to remove a tube with a welded end sealing joint from the alloy tube sheet. By eliminating welding, manufacturing and maintenance costs of such coolers would be reduced. 
   It is also known in the construction of shell and tube heat exchangers to insert the tubes into the holes in the tube carbon steel sheet and radially expand each of the tubes to secure it in place in a groove formed in the interior surface of the hole. There must be good mechanical bond strength and water tightness in the resulting joint between the tube sheet and each tube. 
   A method and apparatus for expanding a tube into a groove in the wall of a hole in a tube sheet is described in U.S. Pat. No. 4,142,581. However, this disclosure is not directed to the use of a sea water coolant and no sea water-resistant alloy tube sheet covering is present to protect the carbon steel tube sheet. There is also no corrosion-resistant alloy metal joint between the tubes and the tube sheet for sealing purposes and for corrosion protection of a carbon steel tube sheet. 
   The subject invention, produces a mechanically strong joint having chemical corrosion resistance to sea water. This joint permits the use of comparatively low cost method for protecting the carbon steel parts for the cooler, e.g., the shell and tube sheets. 
   Existing welding practices can now be replaced by the subject invention. In such case, there will be a savings in weld material, working time and speed up in the heat exchanger repair cycle. Also, the removal of tubes from the tube sheet will be easier in the absence of a welded seal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention and the manner for practicing its preferred embodiments will be further illustrated by the accompanying drawings wherein: 
       FIG. 1  is a vertical cross-sectional view taken along the central axis of the cylindrical heat exchanger constructed in accordance with the present invention; 
       FIG. 2  is an enlarged detail view of the heat exchanger of  FIG. 1  showing aligned holes in the tube sheets and the water tight joints between the tubing and the tube sheets; 
       FIG. 3  is a cross-sectional view along line  3 - 3  of  FIG. 1 , showing the symmetrical layout of the tubes passing through the right carbon steel tube sheet; 
       FIG. 4  is a cross-sectional view along line  4 - 4  of  FIG. 1 , showing a layout of tubes passing through a directional flow control tube sheet having a bottom passage for the free flow of 
       FIG. 5  is a view similar to  FIG. 2  showing another embodiment of the invention. 
       FIG. 6  is a view similar to  FIG. 2  showing yet another embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring now to  FIG. 1 , the shell and tube cooler  50  embodying the present invention comprises an elongated cylindrical closed shell having upstream end  2  and downstream end  3 , hot fluid inlet  4  and cooled fluid outlet  5 . Shell  1  is closed by flanged domed covers  6  and  7 . Ring gaskets  8  and  9 , that provide seals against leakage of the coolant, are placed respectively between left and right shell flanges  11  and  12  and left and right head cover flanges  13  and  14 . Any suitable gasket material may be used, e.g., Teflon, asbestos, synthetic rubber or fiberglass. Flanges  13 ,  11  and  14 ,  12 , respectively, are bolted with nuts and bolts  10 . Left and right domed covers can be expendable and made from carbon steel or alternatively from salt water-resistant alloy metal. Other conventional means (not shown) can be used to close the cooler, e.g., clamps, welding, etc. Cover  6  is provided with inlet pipe  15  for the introduction of cold sea water. Cover  7  is provided with outlet pipe  16  for the removal of the sea water after exchange in shell  1 . In the embodiment illustrated, inlet pipe  15  and outlet pipe  16  are positioned so that their central horizontal axes coincide with the central horizontal axis of shell  1 , but other configurations known to the art can be utilized in practicing the inventions. 
   Tube bundle  24  comprises a plurality of spaced horizontal tubes  25 . The left end  26  of each tube  25  in the tube bundle is passed through a separate corresponding hole  27  in carbon steel tube sheet  28 . All of the holes in the left (upstream) and right (downstream) carbon steel tube sheets  28  and  31  have the same reference numbers, respectively, i.e.,  27  for each of the holes in the left carbon steel tube sheet  28  and  30  for each of the holes in the right carbon steel tube sheet  31 . Similarly, each right end  29  of each tube  25  passes through a separate hole  30  in right round carbon steel tube sheet  31 . All of the holes in the left (upstream) and right (downstream) salt water-resistant alloy tube sheets  34  and  35  have the same reference numbers, respectively, i.e.  36  for each of the holes in the left alloy tube sheet and  37  for each of the holes in the right alloy tube sheet. The exterior faces  32  and  33  of carbon steel tube sheets  28  and  31  are covered or clad with a sea water-resistant alloy tube sheets  34  and  35 . All of the holes in the alloy tube sheets  34  and  35  have the same reference numbers, respectively, i.e.  36  for each of the holes in the left alloy tube sheet  34  and  37  for each of the holes in the right alloy tube sheet  35 . The central axis of each hole in each tube sheet is transverse to both faces of the tube sheets. All left and right tube ends  26  and  29  in tube bundle  24 , respectively, pass through holes  36  and  37  in alloy tube sheets  34  and  35 . 
   Corrosion-resistant alloy tubes  25  and alloy tube sheets  34  and  35  are made from a metal alloy selected from the group that includes Monel, Inconel, and stainless steel. 
   The opposing ends  26  and  29  of all tubes  25  in tube bundle  24  are provided with water tight joints where the tubes pass through each tube sheet. This is accomplished by radially expanding at least one circumferential ridges  40  and  41 , respectively, in the left and right ends of each tube. As the circumferential ridges are formed, they are simultaneously swaged and forcibly driven into mating circumferential annular grooves  45  in the surrounding walls of all of the holes  36  and  37  in left and right alloy tube sheets. In the preferred embodiment illustrated and described, the grooves have a rectangular cross-section. Circumferential ridges are also forcibly driven into all of the mating circumferential rectangular annular grooves in the surrounding walls of all of the holes  27  and  30  in carbon-steel tube sheets  28  and  31 . 
   When head cover flange  13  is bolted to shell flange  11 , the end portion  20  of head cover flange  13  compresses gasket  8  and a ring portion of the face of left alloy tube sheet  34 . Similarly, when right head cover flange  14  is bolted to right shell flange  12  the end portion  21  of right head cover flange  14  compresses gasket  9  and a ring portion of the face of right alloy tube sheet  35 . By this sealing means, coolant is prevented from entering into the shell side of the cooler. Corrosion-resistant alloy tube sheets  34  and  35  have a thickness in the range of about 1.0 to 1.5 cm. Carbon steel tube sheets  28  and  31  have a thickness in the range of about 2.54 to 25.4 cm. The outside diameter of tubes  25  can be the range of about 1.587 to 5.08 and have a wall thickness in the range of about 0.124 to 0.305 cm. 
   Fluid flow within shell  1  can optionally be controlled by a plurality of internal baffles  47  positioned transversely to the axis of shell  1 , as best shown in  FIG. 4 . 
   With reference now to  FIG. 4 , one of a plurality of conventional fluid directional flow control baffles  47  is shown for controlling the path that the gaseous or liquid fluid to be cooled takes in shell  1  from inlet to outlet. These baffles are made from carbon steel sheet and have a sectional opening in the bottom or top through which the fluid passes. The holes in the baffle are in alignment with the holes in the tube sheets so that the tubes are horizontal in the tube bundle. The use of directional flow control baffles in the heat exchanger is optional. 
   Referring now to  FIG. 2 , a portion of carbon steel tube sheet  28  is shown faced on its exterior surface with corrosion-resistant alloy tube sheet  34 . Also shown is the water tight joint made by simultaneously forming a circumferential ridge  41  on the surface of alloy tubing  25  and forcibly driving it into mating rectangular groove  45  in the surrounding wall of each hole  36  in alloy tube sheet  34 . For illustrative purposes, one rectangular shaped annular groove  46  and one rectangular shaped annular groove  45  are machined into the surrounding walls respectively of holes  27  in tube sheet  28  and in the walls of coaxially aligned holes  36  in alloy tube sheet  34 . However, there may be from 1 to 3 grooves, e.g., two parallel spaced annular grooves in the surrounding walls of each opening in the carbon steel tube sheets as well as in the surrounding walls of each aligned hole in the alloy metal tube sheets. In one embodiment, one annular groove is provided in the surrounding wall of each hole in tube sheets  34  and  35 , and two parallel spaced annular grooves in the surrounding walls of each of the holes in tube sheets  28  and  31 . 
   A tube expander of conventional design is inserted into each end of each tube in the tube bundle and expanded radially to form the circumferential ridges. For example, a conventional tube expander as shown and described in U.S. Pat. No. 4,142,581,can be used to make from one to three parallel circumferential ridges  40  and  41  on the outside surface of the tubes. Each circumferential ridge is transverse to the central axis of the tube on which it is formed. These circumferential ridges  40  and  41  are located at the end of each tube to mate with the annular grooves  46  and  45  in the walls of holes  27  and  36  in the tube sheets. As each ridge is formed, it is simultaneously forcibly pressed or driven radially into its corresponding mating annular groove  46  and  45  to provide a mechanically strong water tight joint. 
   The depth of the annular grooves  45  and  46  is in the range of about 0.25 to 1.0 mm, and the width is in the range of about 3 to 5 mm. 
   Optionally, the ends of tubes  25  can be flared outwardly and against the adjacent surface of the alloy tube sheet to improve its resistance to lateral movement. 
   Referring now to  FIG. 3 , the symmetrical arrangement of tubes  25  passing through the close fitting opening in round carbon steel tube sheet  31  is shown in Section  3 - 3  of  FIG. 1 . Clearance is shown between the close-fitting outside diameter of tube sheet  31  and the inside diameter of cylindrically shaped outer shell  1  to permit the tubes to be slidably introduced into outer shell  1  or removed therefrom for or repair or replacement. 
   Referring to  FIG. 5 , in a further preferred embodiment, the alloy tube sheet  35  can be provided with an opening  48  larger than the diameter of the alloy tube  25  and fitted with a liner or ring  60  that includes an interior radial groove  62 . This construction can be used where the alloy tube sheet  35  is relatively softer or more ductile than the alloy tube that is to be swaged into the tube sheet groove. The grooved lining ring  60  can be inserted by a press fitting alone or in combination with heating of the parts. The grooved lining ring can have a flange  64  on one or both sides to engage the surface of the alloy tube sheet to facilitate insertion of the alloy tubes and avoid having the lining ring dislodged by impact of an end of a tube during insertion. 
   As will be understood by one of ordinary skill in the art, the method of assembly and the finished construction of the invention will greatly facilitate the removal and replacement of the alloy tubes as compared to the prior art constrictions where the ends of the tubes were welded to the tube sheet. The flared end of a damaged or leaking tube can be removed by grading, an impact tool or other specialized cutting tool. The portion of the alloy tube forced into the grooves in the tube sheets can be cut away by the same type of tool used to cut the original grooves. The tube can then be withdrawn from the tube sheet. 
   Other modifications and variations of the invention as set forth above may be made without departing from the spirit and scope thereof, and therefore, only such limitations should be imposed in the invention as are indicated in the appended claims.