GB2095389A - Shell and tube exchanger - Google Patents

Abstract

A shell and tube heat exchanger has an outer shell 9 and a series of heat exchange tubes 24 located therein, said tubes being secured at their open ends through a tube sheet 31, and a pair of end caps 27 located within the end cover space defined by a shell cover 19 forming said space with the tube sheet 31 and being adapted to form a flow chamber between the open ends of certain tubes in said heat exchanger in such a manner that fluid leaving the end of any of said certain tubes is redirected into the others of said certain tubes by substantially sealingly connecting each said end cap 27 to an end tube sheet 31 of said exchanger independently of the inlet or outlet of said tube side fluid, said fluid flowing from the inlet cover space through said tubes and flow chambers to the outlet cover space. The tubes may be straight and at least one end cap may be located in the end cover space at each end of the tubes. Or the tubes may be U-tubes and there may be only one end cover within which both end caps are located (Figure 14 not shown). <IMAGE>

Description

SPECIFICATION Shell and tube heat exchanger The present invention relates to heat exchangers of the type wherein a plurality of open-ended tubes extend between tube sheets which are located in an outer tubular shell, the ends of which are enclosed by means of shell covers. In apparatus of this type, one fluid, known as the tube side fluid, flows through the tubes, while a second fluid, known as the shell side fluid, flows over the outside of the tubes and heat exchange between the two fluids takes place through the walls of the tubes. The space within the shell covers being occupied by the tube side fluid.

Heat exchanger design is an imprecise art; many assumptions must be made about operating condition, fouling and corrosion effects for as new, normal and extreme conditions.

Even when good data is available, overall accuracy in sizing of exchangers is often no better than plus or minus 50%. As a consequence, exchanger design tends to be highly conservative and capital and generating costs are higherthan would be wished.

Ideally, it should be possible to adjust exchanger configuration to optimise the effects of heat transfer, fouling and power consumption to give the best operating efficiency under actual operating conditions; this is virtually impossible with a conventional fixed pass shell and tube heat exchanger.

To achieve the optimum heat exchange between the shell side and the tube side fluid at available tube side conditions an optimum velocity of flow through the tubes is required to promote good heat transfer from the inside of the tubes to the outside economically with minimum fouling.

To achieve variations of flow velocity in the tubes the fluid is directed through the tube bundle a multiplicity of times by dividing the total number of tubes in the tube sheets into the appropriate groupings to give the required cross sectional area of flow per pass and allowing the fluid to pass through these sections of tube groupings in turn, to give the required multiplicity of times that the fluid flows through the tube bundle.

A higher heat transfer efficiency is obtained by having a predominance of counterflow between the shell side and the tube side flow than for parallel flow.

To reduce the thermally produced stresses between the shell and tubes, expansion bellows may be positioned between the shell and tube sheet.

A standard tube shell layout for each size tube sheet which allows for a varying number of tube side passes and can be readily interchanged from one to the other, is advantageous for mass producing with variable duties.

In existing apparatus the division of the tube groupings to achieve the multiplicity of flow required is achieved by providing pass partitions which divide the tube sheet into the required sections to give the required number of tube side passes. The position of the inlet or outlet nozzles on the shell cover is dependent on the pattern the pass partitions make with the tube sheet in dividing the flow areas to give the required number of tube side passes, this dependency of the nozzle positions, to the position of the partition plates reduces the piping layout flexibility associated with the connection to the nozzles.

Another disadvantage with existing apparatus is that the number of tube side passes of existing units cannot be altered readily as required.

An object of the present invention is to substantially overcome the abovementioned disadvantages of conventional apparatus by providing apparatus which can be designed for a multiplicity of passes of tube side fluid to give a predominance of counter flow and provide the flexibility of changing the number of tube side fluid passes to a more appropriate number conveniently and to allow for suitable nozzle positions in the shell cover as desired, independently to the number of tube side passes used and the position of the pass partitions.

A further object of the invention is to reduce the axial forces between the tubes and tube sheets in apparatus by employing expansion bellows between the shell and tube sheets to remove the axial forces resulting from thermal differential expansion between the shell and tubes by positioning the expansion bellows in a more appropriate position than with conventional apparatus.

In one broad form the invention provides a shell and tube heat exchanger having a pair of end caps located within the end cover space and being adapted to form a flow chamber between the open ends of certain tubes in said heat exchanger in such a manner that fluid leaving the end of any of said certain tubes is redirected into the others of said certain tubes by substantially sealingly connecting each said end cap to an end tube sheet of said exchanger independently of the inlet or outlet of said tube side fluid, said fluid, said fluid flowing from the outlet cover space through said tubes and flow chambers to the outlet cover space.

The end channel caps are positioned so as to direct the tube side fluid to flow alternately from end to end through the tube bundle from the inlet shell cover space to the outlet shell cover space independent of the number of passes and pass partitions.

The sealing between the tube sheet outer face and the sealing face of the end caps may be accomplished by a sealing strip which is shaped by the protruding tube ends from the outer tube sheet face, the contour of the sealing strip being determined by the desired route which the strip takes to give the desired tube side pass area through the tube sheet.

The sealing strip is proportioned such that it fits between the tube ends and protrudes beyond them to seal with the end cap sealing edge spaced apart from the tube ends with the opposite edge of the sealing strip located on the tube sheet.

The preferred shape of the sealing strips on the outer face of the tube sheet to give the required tube side pass area is that of hexagonal curvilinear rings.

The invention thus provides the following advantages: Gasketing is superior. All pressure containing gaskets may be ring type. No bar gaskets are necessary.

Maintenance is simplified through standardisation.

Delivery is shortened through pre-approved designs using standard components.

Cost is low, because of simple design and standardisation.

Odd pass units have a higher efficiency over a wider range of process conditions than those of even tube pass design.

Flexibility in changing the number of tube side passes, such that standard annular flow exchangers may have a large number of passes, and readily up to 7 passes are available. Strip type end caps give up to 17 passes. Baffles may be horizontal or vertical.

T.E.M.A. inlet and outlet tube impingement requirements are easily met.

Piping may be connected to inlet and outlet end at any positioin around the circumference of the end channels independent of the number of tube side passes.

Installation is simple. Exchangers may be installed directly on a flat surface without the need of plinths.

The internal bellows design does not leave a heel of the shell fluid as does the external bellows design.

Fixed tube sheets are welded to shell and channel.

This avoids problem joints, especially those caused by alternating hot and cold cycles.

All exchangers are odd pass with standard tube sheet and baffle tube pattern.

There is a low tube to tube sheet joint load due to small differential temperature in tube passes. The flow pattern gives only one temperature drop for each pass. By contrast, the conventional bar even multipass design, gives a 4 pass temperature drop for4 pass and over designs.

The replacement of bellows is simplified by the internal bellows position.

Lagging is simplified. There are no flanged connections between the fixed tube sheets and the end channels.

Internally installed bellows are protected from external damage during transport, erection and from general wear and tear.

The internal bellows design imparts a partial hydraulic balance between the shell and tube side of the tube sheet.

There are no channels for bypass flow in the tube pattern in contrast with bar type even multipass tube patterns where dummy tubes have to be inserted to prevent bypass. Standard longitudinal edge strips between the tube bundle and the shell reduce bypassing.

Because the tubside flow is directed by a series of ring partition plates, the tube bundle thermal expansion is axial which allows for good gland sealing. In the case of bar type fluid flow pass partition plate design, thermal expansion tends to bend the tube sheet at an angle to the axis.

Conventional packed lantern ring design exchangers are only available in one ortwo pass units. 1,3, 5 and 7 pass units are available in the heat exchangers of this invention.

Weld failure between partition plates and channel shell cannot occur with the heat exchangers of the present invention. Weld failure between partition plates and the end channel shell, due to high thermal stresses, is common in conventional equipment.

Complete bypass of tube side fluid is not possible in odd pass heater exchangers; even pass heat exchangers allow for the complete bypassing of the tube bundle by the tube side fluid, should the partition plates fail due to corrosion.

No flange bending stress is transferred to the tube sheet. Because of the internal bellows design, the tube sheet thickness is kept to a minimum.

Exchangers with internal bellows are not subject to shear forces in the bellows. Conventional bellows of split shell design are subject to shear loads, the value of which is dependent on the relative position of the supports.

Removable tube bundle with internal bellows allows easy leak testing and gasket tightening by removing the end cover and end caps, and applying pressure to the shell side. In conventional floating head tube bundle type, leaks cannot be readily identified and required.

Good control of tube side velocity gives minimum fouling.

A complex cooling water system may be trimmed to benefit cooling water usage and power consumption.

Furthermore, the ability to readily change the number of passes, makes it possible to choose and exchanger which will give an optimum combination of shell and tube side flow rates and pressure drops.

There will also usually be significant savings in capital and operating costs compared with conventional exchangers. Tube lengths may be selected for a given number of passes which result in a virtually constant heat loss-velocity relationship. Flow conditions may then be varied as desired to match particular requirements of cooling water supply temperature and pressure and varying heat loads.

The invention in accordance with preferred embodiments, will now be described with reference to the accompanying drawings, in which: Figure 1 shows in longitudinal section through a heat exchanger, including a form of end cap, according to the invention; Figure 2 is an elevational view of the section 2-2 of Figure 1; Figure 3 is a section on line 3-3 of Figure 2; Figure 4 is an elevational view of the section 4-4 of Figure 1; Figure 5 is a view of the section 5-5 of Figure 4; Figure 6 is a sectional view of another form of the invention; Figure 7 is a sectional view showing another form of the invention; and Figure 8 is a sectioned view of another form of the invention; Figures 9 to 13 show an alternative embodiment of the invention; and Figures 14to 17 show a still further embodiment of the invention.

Figure 1 is a longitudinal section through a heat exchanger 100 of a type with both tube sheets 31,32 secured to the tubular shell 9 and the shell covers 13 and 19, secured to the tube sheets, adapted for an odd number of tube side fluid passes by use of one form of end cap.

Fluid enters the exchanger 100 through inlet nozzle 23, into the inlet shell cover space then passes along the tubes 24' to be captured within the chamber 27' of end cap 27. The fluid then passes from chamber 27' along tubes 24" in the opposite direction to the direction of flow of the fluid in tubes 24'. The fluid leaving tubes 24" is received in chamber 28' wherein it is then passed into central tubes 24"' to travel back along the length of the heat exchanger and to issue forth into end space 20 before being removed from the heat exchanger through outlet nozzle 21.

Fluid to be used on the shell side of the exchanger enters the shell 9 through inlet nozzle 25 and follows a torturous path across and along the tubes defined by baffles 50 before leaving the shell 9 through nozzle 26. End caps 27 and 28, as shown in Figures 2, 3, 4 and 5, are simply metallic caps which are affixed to the tube sheets 31 or 32 by suitable means such as threaded studs 51. The caps 27 and 28 can have appropriately shaped curvilinear walls covered by a removable flat plate, thus forming the flow chambers 27 and 28. The caps 27 and 28 can be made from any suitable material apart from metal, such as plastics.

In Figure 6 there is provided a heat exchanger having an outer tubular shell 9, a first tube sheet 10 fixedly attached to the cover 13 which in turn is clamped to flange 11 of shell 9. A second tube sheet 14 movably mounted within the shell 9 at the other end thereof. The sealing between the second tube sheet and shell being achieved by clamping packing rings 15 and lantern ring 16 in a conventional manner between the two flanges 17 and 18. Flange 17 being secured to shell cover 19 and flange 18 being secured to a second shell 9.

The first shell cover 13 and tube sheet 10 define an outlet space 20, containing the tube side fluid, which exhausts through outlet nozzle 23, attached to said shell cover 13. The second shell cover 19 and tube sheet 14 define an inlet space 22 containing tube side fluid supplied through the inlet nozzle 21 attached to the second shell cover 19.

A plurality of open-ended tubes 24 extending between and secured to the tube sheets 10 and 14 communicate with the spaces within the heat ex changer shell covers 13 and 19. Flow of the shell side fluid is over the outside of the tubes 24 and contained in shell 9. The shell side fluid is administered into the shell 9 through inlet nozzle 25 and exhausts through the outlet nozzle 26, having flowed over the tubes 24 as desired. Multipass flow of the tube side fluid is achieved by attachment of end caps 27 and 28 in accordance with one form of the invention, to the tube sheets 10 and 14, respectively.

Figure 7 shows a variation of the arrangement as set forth in Figure 1, wherein the bellows 53 are located between a floating tube sheet 54 and the shell 9. This allows for the longitudinal expansion of the tubes 24 to be totally independent of the expansion of the shell 9.

Similarly, as shown in Figure 8, the bellows 40 extend between a floating tube sheet 54 and an end plate 44 sealably attached 41 to an upstanding internal flange 43 on the inside of the shell 9. A simple series of bolts 42 provide the fixing means.

Figures 9to 13 show an embodiment which is similar to that shown in Figures 1 to 5. However the end caps 27 and 28 are seen to be a different shape when these two embodiments are compared. It can also be seen that these two embodiments, by using differently shaped end caps, provide a different distribution of the number of tubes used for the tube side flows in the various passes. Of course there are many other shaped end caps which can be used, and thus many distributions of the number of tubes used for various passes for the tube side flow.

Figures 14 to 17 show an embodiment of the present invention which uses U-shaped tubes and thus, overall, an even number of passes of the tube side flow. The outlet and inlet spaces, 20 and 21, are side by side at a common end of the shell 9. These spaces are separated by a conventional pass partition 80. The end caps 27 and 28, shown clearly in Figures 15 and 17 can of course be of varying shapes and have varying numbers of pass partitions therein.

The end caps 27 and 28 may be removably attached to the tube sheets. The end caps 27 and 28 may comprise an end cover being attached to a wall section adapted to extend between the tube sheet and the end cover.

Using this method of providing pass partitions the inlet and outlet tube side nozzles are independent of the number of tube size passes and the pass partitions of the end caps.

Claims (4)

1. A shell and tube heat exchanger having an outer shell and a series of heat exchange tubes located therein, said tubes being secured at their open ends through a tube sheet, and a pair of end caps located within the end cover space defined by an end cap forming said space with the tube sheet and being adapted to form a flow chamber between the open ends of certain tubes in said heat exchanger in such a manner that fluid leaving the end of any of said certain tubes is redirected into the others of said certain tubes by substantially sealingly connecting each said end cap to an end tube sheet of said exchanger independently of the inlet or outlet of said tube side fluid, said fluid, said fluid flowing from the inlet cover space through said tubes and flow chambers to the outlet cover space.

2. The heat exchanger of claim 1, wherein the tubes are straight and there is at least one end cap located in the end cover space located at each end of the tubes.

3. The heat exchanger of claim 1, wherein the tubes are U-tubes and there is only one end cover wherein both end caps are located within said end cover.

4. A shell and tube heat exchanger substantially as described herein with reference to, and as illustrated by the accompanying drawings.