GB2467942A

GB2467942A - Self cleaning heat exchanger

Info

Publication number: GB2467942A
Application number: GB0902905A
Authority: GB
Inventors: Michael Watson
Original assignee: Tube Tech International Ltd
Current assignee: Tube Tech International Ltd
Priority date: 2009-02-23
Filing date: 2009-02-23
Publication date: 2010-08-25
Also published as: WO2010095110A2; GB0902905D0; WO2010095110A3

Abstract

A self cleaning heat exchanger 10 having tubes 14 extending between two headers 12, 16 comprises introducing 20 tube scouring projectiles 18 into a first header 12 and separating the projectiles from a fluid in the heat exchanger 10 after passing through the tubes 14 into the second header 16. The fluid flow is periodically accelerated through the tubes 14 to dislodge projectiles 18 that have come to a standstill (i.e. become stuck) on encountering excessive resistance while passing through a tube 14. The heat exchanger 10 may be a multi-pass type whereby the headers 12, 16 are partitioned 17. The means for periodically accelerating fluid flow may comprise of an auxiliary positive displacement pump 30 applying pulses of high pressure to the first header 12 so that fluid flows through all the tubes 14 in unison/together. The fluid may flow through only some of the tubes 14 whereby an obturating plate (40, figs 2 & 3) moves axially and rotates thereby obstructing some of the tubes 14 while those not obstructed are cleaned. A separate housing (50, figs 4 & 5) may be formed on the first header 12 and lances (54, figs 4 & 5) mounted in the first header 12 connected to a high fluid pressure source may be moveable towards and away from the ends of the tubes 14 so that jets of fluid penetrate into some of the tubes.

Description

SELF-CLEANING HEAT EXCHANGER

Field of the Invention

The present invention relates to a self-cleaning heat exchanger or condenser having a set of tubes extending between two headers and comprising means for introducing tube scouring projectiles into one header and means for separating the projectiles from the fluid after passing through the tubes at the other header.

Background of the invention

A self-cleaning heat exchanger as described above is already known 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 wsponge 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 with a multiple of passes. 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 of the type described above in which the risk of blockage of tubes by projectiles is reduced even when a more viscous liquid is passed through the tubes of the heat exchanger.

Summary of the Invention

According to the present invention, there is provided a self-cleaning heat exchanger having a set of tubes extending between two headers and comprising means for introducing tube scouring projectiles into a first header and means for separating the projectiles from the fluid after passing through the tubes into the second header, characterised by means for periodically accelerating the fluid flow through the tubes of the heat exchanger in order to assist in dislodging projectiles that come to a standstill on encountering excessive resistance while passing through a tube.

The heat exchanger may be a single pass or multi-pass exchanger. In the latter case, the header may be partitioned such after passing through a first set of tubes in a first direction, the fluid passes through a second set of tubes in the opposite direction.

The means for periodically accelerating the fluid flow through the tubes of the heat exchanger may act to accelerate the flow in all the tubes of the heat exchangers in unison.

In this case, the means for periodically accelerating the fluid flow through the tubes of the heat exchanger may comprise an auxiliary positive displacement pump operative to apply pulses of high pressure to the first header and/or subsequent headers. High pressure pulses applied to the opposite ends of the same tube need to be staggered in time.

Alternatively, the means for periodically accelerating the fluid flow through the tubes of the heat exchanger may act to accelerate the flow in only some of the tubes of the heat exchangers.

The means for accelerating the flow in some of the tubes of the heat exchangers may comprise an obturating plate for obstructing the flow through the remaining tubes of the heat exchanger, the plate being movable relative to the tubes so as to allow the flow through all the tubes to be accelerated at different times.

In the latter embodiment of the invention, no separate pump is required but the flow through the remaining tubes is interrupted, which may be undesirable.

It is alternatively possible for the means for periodically accelerating the fluid flow through the tubes of the heat exchanger to act to accelerate the flow in some of the tubes of the heat exchangers while allowing normal flow to take place through the remaining tubes. The means for periodically accelerating the fluid flow through the tubes of the heat exchanger may in this case comprise lances mounted in the first header and possibly in subsequent headers and connected to a higher pressure source.

Groups of lances may be mounted in the first or any header in such a manner as to be movable between a first position in which their ends are spaced from the mouths of associated tubes and a second position in which their ends obstruct the mouths of the associated tubes.

Alternatively, the lances may be mounted in a fixed position in the first or any header with their ends spaced sufficiently from the mouths of associated tubes to permit projectiles to enter the associated tubes, the lances having nozzles for creating jets capable of penetrating into the tubes, to launch the projectiles and accelerate them during their passage

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 schematic representation of a self-cleaning heat exchanger in accordance with a first embodiment of the invention, Figure 2 is a schematic representation of a self-cleaning heat exchanger in accordance with a second embodiment of the invention, Figure 3 is a section along the line 111-111 in Figure 2, Figure 4 is a schematic representation of a self-cleaning heat exchanger in accordance with a third embodiment of the invention, and Figure 5 shows the embodiment of Figure 4 with the lances in an operative position.

Detailed Description of the Preferred Embodiment(s) Figure 1 shows the core 10 of a heat exchanger for use in an oil refinery. The core has a first header 12, a set of tubes 14 and a second header 16. Oil is introduced from a pressurised supply from a previous refining stage into the header 12. The oil flows through the tubes 14 where heat exchange takes place with a second fluid flowing through the shell (not shown) of the heat exchanger through the walls of the tubes 14. The oil then enters the second header 16 from which it is then passes to the next oil refining stage. The construction of the shell and the fluid circulating in the shell are not of particular concern to the present invention and will not therefore be described in detail.

Figure 1 also shows that by optionally placing partitions 17, shown in broken lines, in the headers, the fluid may pass several times (three in the illustrated example) throuqh different sets of tubes of the heat exchanger. This has the effect of connecting the sets of tubes in series instead of being in parallel. This increases flow resistance but also increases the time that the two fluids in the heat exchanger remain in thermal contact.

As so far described, the heat exchanger is not self- cleaning. In order to render the heat exchanger self-cleaning during normal operation, projectiles 18 are introduced into the oil entering the header 12 through an insertion unit 20. These projectiles are balls of a material that can withstand the temperature of the hot oil and they are designed to gently scour the inner surface of the tubes 14 as they are entrained through them by the oil flow. After leaving the second header and on some heat exchangers continuing through another set of tubes into subsequent headers 16, the projectiles 18 are at the end of their "through tube" cleaning cycle separated from the oil by means of a filter 22 and recycled back to the insertion unit 20.

Systems in which projectiles are recirculated in this manner are already known but have hitherto only been used in heat exchangers or condensers in which the liquid flowing through the tubes 14 is water. In such heat exchangers, the fast rate at which the water flows through the tube is sufficient to ensure that the projectiles do not come to a standstill on meeting resistance. When used for crude oil, there is a risk that the more slowly moving liquid will not suffice to prevent the projectiles from becoming jammed within the tubes and causing a blockage.

To avoid this problem in the embodiment of Figure 1, and dependent on the number of passes, a reciprocating piston or other positive displacement pump 30 is connected by a first one way valve 32 to the filter 22 and by a second one-way valve 34 to the header 12. As the working chamber of the piston increases in volume, oil is drawn from the filter 22 and as the volume of the working chamber decreases, the displaced oil enters the header 12 in addition to the oil entering the header 12 from the preceding oil refining stage. This creates a pulse of high pressure that acts on all the tubes 14 in the same manner as when a plunger is used to clear a blocked drain. Repeated high pressure pulses will act to dislodge any early deposit formation or projectiles blocking a tube 14 and ensure that the projectiles reach the next header 16.

The pump 30 may be operated constantly or for a few minutes every hour. The frequency of piston reciprocation as well as the cycle repetition frequency when the pump is not operated continuously can be set by trial and error to minimise the energy consumption of the pump 32 while ensuring that the tubes 14 remain clear at all times.

While the valves 32 and 34 have been shown as external one-way valves, they may be internal to the pump 30 and they may be constructed for example as poppet valves that are opened and closed in synchronism with the reciprocation of the piston and at each separate header point.

The second embodiment of the invention, shown in Figures 3 and 4, also operates by periodically accelerating the oil flow through the tubes 14 but it does so without the need for a separate pump. In Figure 3, an obturating plate is mounted within each header 12 on the end of a shaft 42 that can rotate about its own axis and translate parallel to its own axis to move the plate 40 towards and away from the mouths of the tubes 14.

As can be seen from the section of Figure 4, the plate 14 is a solid circular plate with a missing portion, the portion being a quadrant in the illustrated embodiment. In the position shown in Figure 3, the plate 40 is retracted away from the mouths of the tubes 14 and does not interfere with the normal flow of oil through the heat exchangers.

Periodically, however, the plate 40 is moved to cover the mouths of some of the tubes 14 while leaving only the tubes in alignment with the missing quadrant exposed to the oil in the header 12. As the same oil flow is now forced through a reduced number of tubes, the pressure in the header 12 is increased along with the flow rate through the tubes 14 that are still in operation, resulting in any early deposit formation or blockages of these tubes being cleared. The shaft 40 is now rotated to force the oil flow to pass through the tubes 14 in a different quadrant and this process is repeated until all the tubes are cleared.

The proportion of the tubes 14 left exposed by the plate 40 and the frequency of the cleaning cycles can be set to ensure that the tubes 14 remain clear at all times while interfering as little as possible with the flow of oil through the heat exchanger.

In the embodiment of Figures 4 and 5, a separate housing 50 is formed on the outer face of the first header 12. The separate housing is connected to a high pressure oil supply. Within the housing 50 there is mounted a plate 52 carrying a set of lances 54 through which oil from the housing header 50 can be injected into the tubes 14.

Under normal operating conditions, the plate 52 is retracted, as shown in Figure 4, and the high pressure oil supply is turned off. Oil from the first header 12 thus flows through the tubes 14 in the normal way.

Periodically, a cleaning cycle is effected which involves operating the high pressure oil supply and advancing the plate 52 and the lances 54 into the position shown in Figure 5. Oil is now forced to flow under higher pressure through the tubes 14 to clear any blockage.

The sections of Figures 4 and 5 show only one plane of the heat exchanger and there are in practice several such planes forming a rectangular array when in the direction of the axes of the tubes 14. The high pressure oil supply is only required to supply enough oil for one plane of tubes not all of them. While oil under high pressure is being fed through one plane of tubes, oil under normal operating pressure will continue to pass through the remaining tubes.

The cleaning cycle does not therefore interrupt the operation of the heat exchanger.

Each plane of tubes has an associated set of lances 54 mounted on a respective plate 52 in the separate housing 50 and the different plates 52 are brought into operation at different times so that only one plane of tubes 14 is unblocked at any one time.

As an alternative to the lances 54 obstructing the mouths of the tubes 14, it is possible for the ends of the lances to remain spaced the mouths of the tube and to aim a high pressure jet of oil down the tubes. In this case, it is not necessary to mount the lances on a movable plate and they may instead project permanently into the first header 12, while leaving enough space between their ends and the mouths of the tubes 14 for the projectiles 18 to enter into the tubes.

Claims

CLAIMS1. A self-cleaning heat exchanger having a set of tubes extending between two headers and comprising means for introducing tube scouring projectiles into a first header and means for separating the projectiles from the fluid after passing through the tubes into the second header, characterised by means for periodically accelerating the fluid flow through the tubes of the heat exchanger in order to assist in dislodging projectiles that come to a standstill on encountering excessive resistance while passing throuqh a tube.
2. A heat exchanger as claimed in claim 1, constructed as a multi-pass heat exchanger.
3. A heat exchanger as claimed in claim 2, wherein at least one of the headers is partitioned such after passing through a first set of tubes in a first direction, the fluid passes through a second set of tubes in the opposite direction.
4. A self-cleaning heat exchanger as claimed in any preceding claim, wherein the means for periodically accelerating the fluid flow through the tubes of the heat exchanger are operative to accelerate the flow in all the tubes of the heat exchangers in unison.
5. A self-cleaning heat exchanger as claimed in any preceding claim, wherein the means for periodically accelerating the fluid flow through the tubes of the heat exchanger comprise an auxiliary positive displacement pump operative to apply pulses of high pressure to the first header.
6. A self-cleaning heat exchanger as claimed in any one of claims 1 to 3, wherein the means for periodically -10 -accelerating the fluid flow through the tubes of the heat exchanger are operative to accelerate the flow in only some of the tubes of the heat exchangers.
7. A self-cleaning heat exchanger as claimed in claim 6, wherein the means for accelerating the flow in some of the tubes of the heat exchangers, comprise an obturating plate for obstructing the flow through the remaining tubes of the heat exchanger, the plate being movable relative to the tubes so as to allow the flow through all the tubes to be accelerated at different times.
8. A self-cleaning heat exchanger as claimed in any one of claims 1 to 3, wherein the means for periodically accelerating the fluid flow through the tubes of the heat exchanger are operative to accelerate the flow in only some of the tubes of the heat exchangers while allowing normal flow to take place through the remaining tubes.
9. A self-cleaning heat exchanger as claimed in claim 8, wherein the means for periodically accelerating the fluid flow through the tubes of the heat exchanger comprise lances mounted in the first header and connected to a higher pressure source.
10. A self-cleaning heat exchanger as claimed in claim 9, wherein groups of lances are mounted in a header in such a manner as to be movable between a first position in which their ends are spaced from the mouths of associated tubes and a second position in which their ends obstruct the mouths of the associated tubes.
11. A self-cleaning heat exchanger as claimed in claim 9, wherein the lances are mounted in a fixed position in a header with their ends spaced sufficiently from the mouths of associated tubes to permit projectiles to enter -11 -the associated, the lances having nozzles to direct jet capable of penetrating into the tubes.