WO2012160491A1

WO2012160491A1 - Self-cleaning heat exchanger

Info

Publication number: WO2012160491A1
Application number: PCT/IB2012/052495
Authority: WO
Inventors: Michael SUMISON
Original assignee: Tube Tech International Limited
Priority date: 2011-05-20
Filing date: 2012-05-17
Publication date: 2012-11-29
Also published as: GB2491116A; GB201108503D0

Abstract

A self-cleaning heat exchanger 10 comprises a set of tubes 12 extending between two headers 14, an insertion unit for introducing tube scouring projectiles into one header and a filter for separating the projectiles from the liquid being processed in the heat exchanger at the other header after they have passed through the tubes. In the invention, the insertion unit comprises a movable injector assembly 16 disposed within one of the headers (14) for firing a projectile into the end of a tube (12) and an indexing mechanism disposed within the header (14) to align the injector assembly (16) sequentially with the ends of the tubes (12). Figure 2.

Description

SELF-CLEANING HEAT EXCHANGER

Field of the Invention

The present invention relates to a self-cleaning heat exchanger of the type having a set of tubes extending between two headers .

Background of the invention

A self-cleaning heat exchanger is already known, for example from US 3,021,117, in which the fluid flowing through the tubes is water. To prevent a build-up of fur within the tubes, projectiles in the form of "sponge balls" of an appropriate size and made of a suitable material are introduced into the header at one end of the heat exchanger. These projectiles gently scour the surface of the tubes during their single pass from one header to the other. On leaving the second header, the water is passed through a filter which separates the projectiles and recycles them back to the first header.

It would be desirable to adopt a similar approach to prevent scale formation on the tubes of a heat exchanger in which the liquid flowing through the tubes is not water but a more viscous fluid such as crude oil. Whereas the

temperature of water in a heat exchanger may typically be around 150°C, the temperature of a heavy oil may be 450°C. This naturally has a bearing on the material from which the projectiles can be made. More importantly for the present invention, even at such elevated temperatures the viscosity of the heavy oil is much greater than that of water. As a result, the flow velocity of the liquid is reduced and this in turn creates a risk that the projectiles will come to a standstill on meeting a resistance, resulting in blockage of the tubes. Object of the Invention

The present invention seeks therefore to provide a self-cleaning heat exchanger that is suitable for processing viscous liquids such as crude oil

Summary of the invention

According to the present invention, there is provided a heat exchanger having two headers and a bundle of tubes having inner and outer walls and mouths at their opposite ends opening into the two headers, heat being exchanged in use between a process fluid flowing through the headers and the bundle of tubes and a second fluid contacting the outer walls of the tubes, the heat exchanger further comprising a cleaning unit disposed within one of the headers for cleaning the tubes, the cleaning unit being operative to clean each tube of the heat exchanger individually and comprising a movable injector assembly which, during use, injects into each tube a jet of a cleaning liquid of lower viscosity than the process liquid, and an indexing

mechanism that aligns the injector assembly sequentially with the mouths of the tubes, wherein the injector assembly is additionally operative to position a scouring projectile at the mouth of a tube and to propel the projectile down the tube using the jet of a cleaning liquid, the injector assembly comprising a coupling body having a through bore with an axis parallel to the axes of the tubes, a nozzle operative to direct the jet of cleaning liquid axially down the through bore and a lateral opening connected to a flexible tubing for admission of projectiles into the through bore of the coupling body.

As taught in US 3,021,117, on leaving the second header, the process liquid may be passed through a filter to separate the projectiles for recycling back to the first header . If the process liquid in the heat exchanger is crude oil, then the jet of cleaning liquid may be of diesel fuel, petroleum or paraffin, that can subsequently be separated from the remaining constituents of the crude oil.

The indexing mechanism of an embodiment of the

invention includes a carriage supporting the injector assembly that is slidably mounted for movement along an arm that is itself supported within the header for movement relative to the tube plate, the latter being a transverse plate that is sealed relative to the ends of the tubes and defines one wall of the header. In this way, the carriage can be moved along orthogonal coordinates to align with any one of the tubes of the head exchanger.

The carriage may be moved along Cartesian coordinates (X,Y) by the indexing mechanism but as tube bundles are more commonly cylindrical, they are more suited to an indexing mechanism employing polar coordinates (r,9) . Thus it is more convenient for the arm supporting the carriage to be

rotatably mounted in the header.

The motor used to move the arm supporting the carriage relative to the tube bundle is preferably mounted outside the header. For example, the arm may be connected to a ring gear that is journalled within the header and connected to a worm gear or bevel gear that is driven by a shaft passing through a seal in the wall of the header.

It is easier to move the carriage along the rotatable arm by a motor mounted within header. Motors mounted within sealed insulation can function within that environment reliably over extended periods. It is however alternatively possible to provide a drive train that uses a motor mounted outside the header to move the carriage along the arm. The latter drive train need only be able to couple to the carriage in one angular position of the rotating arm. This is because it is sufficient to be able to move the carriage along the arm only after completion of each revolution.

Thus, the carriage would be set to one radius and the arm swept through 360° to align the injector assembly with all the tubes on that radius. The radius can then be reset for the next sweep and the cycle repeated until all the tubes have been cleaned.

It is furthermore possible to use the externally mounted motor to drive the carriage along the arm and to rotate the arm if it is connected to two drive trains through selectable clutches.

Brief description of the drawings

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which :

Figure 1 is a section through one end of a self- cleaning heat exchanger embodying the invention, and

Figure 2 is schematic end view of the tube bundle of the heat exchanger of Figure 1 showing the indexing mechanism for the projectile injector assembly.

Detailed description of an embodiment

Figure 1 is section through one end of a self-cleaning heat exchanger 10. The heat exchanger 10 comprises a bundle of tubes 12 sealed at each end relative to a respective tube plate 13 which forms one a wall of a header 14. Each header acts as a closed manifold supplying the process liquid under pressure to all the tubes 12.

Figure 1 shows only the header 14 at one end of the tube bundle. In use, a process liquid, such as crude oil, that is to be heated or cooled flows from one header to the other through the heat exchanger tubes 10. The outer surfaces of the tubes 12 are in contact with a second fluid at a different temperature that is used to cool or heat the process liquid flowing through the tubes 12 by heat transfer through the walls of the tubes 12.

The process liquid flowing through the tubes 12 tends with time to build a deposit on the inner walls of the tubes 12. Such a deposit interferes with heat transfer and reduces the efficiency of the heat exchanger. If not removed, the deposit risks blocking the heat exchanger completely. If the heat exchanger is not self-cleaning, then it requires cleaning at regular intervals and this would entail shutting down an entire chemical plant, such a refinery. It is to avoid such expensive down time, that it is desirable to employ a heat exchanger that is self cleaning.

Self-cleaning can achieved by injecting a lower

viscosity cleaning liquid into individual tubes while the heat exchanger is in normal operation. The lower viscosity liquid may by virtue of its physical and/or chemical properties alone act to remove any deposit beginning to adhere to the inner walls of the tubes. More preferably, the jet of cleaning liquid is used to propel a projectile down the tube 12 to scour the inner wall of the tube. The

projectiles may be balls that travel through the tubes with the process liquid and wipe the inner surfaces of the tubes during their passage. If the tubes are wiped frequently, no heavy deposit will be built up and there is no need for the projectiles to apply a severe scouring action.

It has been proposed previously to introduce

projectiles into one header and to rely exclusively on the flow of the process liquid to transport them through the tubes 12 of the heat exchanger. One reaching a similar header at the other end of the heat exchanger (not shown in Figure 1), the projectiles are suitably filtered out, cleaned if necessary, and returned to the first header for recycling through though the heat exchanger.

The problem with relying on the flow of the process liquid alone to transport the projectiles through the tubes 12 is that it difficult to ensure that all the tubes will be cleaning at regular intervals as the flow is not necessarily uniform over the cross section of the bundle. Furthermore, if one tube should present a slight obstruction a projectile may become lodged within it thereby permanently blocking the tube .

The illustrated embodiment of the invention overcomes these problem by firing a projectile down all the tubes in sequence using an injector assembly movable across the tube plate by an indexing mechanism. The forced injection ensures that the projectile can overcome the resistance offered by any deposit that has started to build up since the last passage of a projectile through the tube and the indexing mechanism ensures that all the tubes are cleaned

periodically .

To fire a projectile down each tube, the illustrated embodiment of the invention uses an injector assembly 16 which essentially comprises a coupling body 18 having an axial through bore 20 and a blind transverse bore 22

intersecting the axial bore.

The axial bore 20 extends parallel to the axes of the tubes 12 and is connected by way of a flexible pipe 24 and a valve 26 to a pressurised supply of cleaning liquid, such as diesel fuel. The transverse bore 22 is connected to a flexible pipe 28 that is filled with projectiles through a connector 30. As each projectile is introduced into the connector 30 it pushes against the ones ahead of it, thus making sure that a projectile is positioned in the through bore 20. When the injector assembly is correctly aligned in the manner to be described below with one of the tubes 12, the valve 26 is opened and a jet of the cleaning liquid, which is less viscous than the crude oil, is injected behind the projectile lying within the through bore 20. This jet fires the projectile into the tube 12. Once any initial resistance has been overcome in this manner, the projectile is transported through the tube with the crude oil processed and wipes the inner surface of the tube 12 clean. The cleaning liquid following the projectile further assists in cleaning any deposit lining the inner wall of the tube 12.

Diesel fuel, paraffin or petroleum can be used as the cleaning liquid when the process liquid is crude oil but the invention is not restricted to the processing of crude oil. When a different liquid is being processed, a different liquid may be used on its own or for firing projectiles.

To align the injector assembly 16 sequentially with the tubes 12, it is mounted on an indexing mechanism that is permanently mounted within the header 14. The indexing mechanism comprises an arm 40 fixed to a ring gear 42 that is permanently mounted in the header. The ring gear 42 and the arm 40 are rotatable, as represented by an arrow 50, by a motor 44 mounted outside the header 14 and coupled to the ring gear 42 by means of a shaft 46 and bevel gear 48. In place of the bevel gear 48, the ring gear 42 could be driven by a worm wheel. The shaft 46 passes through a suitable seal or packing to avoid leakage of crude oil.

The injector assembly 16 is mounted on a carriage 52 that is movable along the length of the arm 40. In the illustrated embodiment, a second motor 54 mounted on the arm 40 is used to move the carriage 52. As the carriage 52 can be moved to any radius and the arm 40 can be rotated to any angle, the injector assembly 16 can be indexed into

alignment with any one of the tubes 12. In use, the indexing mechanism is programmed to align the injector assembly with all the tubes 12 sequentially and to repeat the process continuously so that each and every one of the tubes 12 is cleaned periodically by passage of a projectile. The duration of a cleaning cycle is such that insufficient time passes between cleaning cycles for any serious deposit to build up on the inner wall of any of the tubes .

It will be appreciated by the person skilled in the art that many modifications may be made to the illustrated embodiment without departing from the scope of the invention as set out in the appended claims. For example, though the described indexing mechanism relies on polar coordinates, this being best suited to the shape of the illustrated tube bundle, it would be possible to use a mechanism employing Cartesian coordinates.

Furthermore, though the carriage 52 has been described as being movable by a motor 54 mounted within the header, it is alternatively possible to use a motor mounted outside the header. In the latter case, the motor may either be

permanently coupled to the carriage 52 or the coupling may only take place in one angular position of the ring gear 42.

As a further example, the drive trains connecting the motors 44, 54 to the ring gear 42 and the carriage 52 need not be shafts and gears but may use cables or chains.

Claims

1. heat exchanger having two headers and a bundle of tubes having inner and outer walls and mouths at their opposite ends opening into the two headers, heat being exchanged in use between a process fluid flowing through the headers and the bundle of tubes and a second fluid

contacting the outer walls of the tubes, the heat exchanger further comprising a cleaning unit disposed within one of the headers for cleaning the tubes, the cleaning unit being operative to clean each tube of the heat exchanger

individually and comprising a movable injector assembly which, during use, injects into each tube a jet of a

cleaning liquid of lower viscosity than the process liquid, and an indexing mechanism that aligns the injector assembly sequentially with the mouths of the tubes, wherein the injector assembly is additionally operative to position a scouring projectile at the mouth of a tube and to propel the projectile down the tube using the jet of a cleaning liquid, the injector assembly comprising a coupling body having a through bore with an axis parallel to the axes of the tubes, a nozzle operative to direct the jet of cleaning liquid axially down the through bore and a lateral opening

connected to a flexible tubing for admission of projectiles into the through bore of the coupling body.

2. A heat exchanger as claimed in claim 1, wherein the indexing mechanism includes a carriage that supports the injector assembly and is slidably mounted for movement along an arm that is itself supported within the header for movement relative to a tube plate, that forms part of the header and seals around the tubes.

3. A heat exchanger as claimed in Claim 2, wherein the arm supporting the carriage is rotatably mounted in the header .

4. A heat exchanger as claimed in claim 2 or 3, wherein a motor serving to move the arm supporting the carriage relative to the tube bundle is mounted outside the header .

5. A heat exchanger as claimed in claim 4, wherein the arm is connected to a ring gear that is journalled within the header and connected to a gear that is driven by an output shaft of the motor that passes through a seal in a wall of the header.

6. A heat exchanger as claimed in any one of claims 2 to 4, wherein the carriage is movable along the arm by a motor mounted within header for movement with the arm.

7. A heat exchanger as claimed in any one of claims 2 to 5, wherein the carriage is movable along the arm by the motor serving to move the arm supporting the carriage relative to the tube bundle or by a further motor mounted outside the header.