GB2472782A

GB2472782A - Cleaning of finned heat exchangers where the cleaning fluid flows from the interior to the exterior

Info

Publication number: GB2472782A
Application number: GB0914294A
Authority: GB
Inventors: Michael Watson
Original assignee: Tube Tech International Ltd
Current assignee: Tube Tech International Ltd
Priority date: 2009-08-17
Filing date: 2009-08-17
Publication date: 2011-02-23
Also published as: GB0914294D0

Abstract

A method of cleaning a heat exchanger which has a stack of finned tubes 16 extending between two headers 12, (14, fig 1) wherein cleaning fluid is introduced between the fins 22 to flow from the interior of the stack to the exterior. In one embodiment a jetting lance is passed through guide tubes 30, which are located in one of the headers, into passage ways between the heat exchanger tubes. The passageways may be formed by the spaces between disc shaped fins or by apertures within the cooling fins, alternatively the passage ways may be defined by a tube with a perforated or reticulated wall 32 and the lance may have seals ahead and behind its jets. In another embodiment spray bars that are connected to a source of cleaning fluid are located between and parallel to the finned cooling tubes and are used to introduce cleaning fluid between the fins.

Description

CLEANING OF FINNED HEAT EXCHANGERS

Field of the invention

The present invention relates to the cleaning of finned heat exchangers, such as the air cooled heat exchangers and condensers used in large chemical processing plants, for example oil refineries and heat exchangers heated by furnace gases.

Background of the invention

At certain stages when refining crude oil, it is necessary to cool a fluid by rejecting its heat to the ambient atmosphere. For this purpose, heat exchangers are used which are in essence a much larger version of the radiators used in motor vehicles to cool the engine water.

In particular, the heat exchangers comprise a stack of finned cooling tubes extending between two headers, over which air is blown by a fan. The fan blows air into a plenum, which serves the same purpose as the cowling of a vehicle radiator, namely to guide all the blown air to flow over the finned tubes of the heat exchanger. In the case of a chemical processing plant, the heat exchanger is mounted with its cooling tubes extending horizontally or in apex or tent formation, while the fan and the plenum are arranged above or below the heat exchanger to blow air vertically through the fins.

Finned heat exchangers are used for heating as well as cooling of a fluid. In the latter case, they may be arranged in a furnace so as to be heated by the furnaces gases.

The efficiency of such a heat exchanger gradually diminishes with time for two reasons. First, dirt collects between the fins of the heat exchanger tubes, interfering with the heat transfer to or from the air flowing over the heat exchanger. Second, a deposit can build up lining the inner walls of the heat exchanger tubes, thereby restricting the fluid flow through tubes as well as reducing the thermal transfer through the walls of the tubes.

The conventional way of cleaning the fins of a finned heat exchanger is to resort to high pressure jetting. This well known process involves directing fine high pressure (several hundred bar) jets of water from a lance at the space between the fins.

One way is to jet the fins of the heat exchanger from above. In the case of an air cooled heat exchanger, this could be carried out while the heat exchanger is in use but that is not satisfactory for several reasons. First, the dirt that collects between the fins is blown into the heat exchanger by the fan. Therefore the highest density of deposit to be removed is on the fan side of the heat exchanger. As the heat exchanger will have a stack of several tiers of tubes, typically six such tiers, there is no direct line of sight for the high pressure water jets to penetrate the lower tiers of fins, which are only cleaned by water that trickles down through the heat exchanger stack.

Aside from not being very effective, such cleaning is often carried out by personnel standing on top of the heat exchanger and this can cause damage to the fins. Increasing the jetting pressure to improve jet penetration can also cause damage to the fragile fins. Last, the spray created by the water jets, is heated by contact with the cooling tubes and the fins and if the exchanger is in use it is deflected back towards the person operating the lance, making the task both unpleasant and dangerous.

The better alternative for an air cooled finned heat exchanger is to jet the fins from the fan side but that requires the fan to be switched off and isolated. In this case, the lance operator has to standing within the plenum and to contend with all the heated dirty water that bounces back towards him on impact with the stack or falls on him under the action of gravity if it succeeds in penetrating into the stack.

In the case of a heat exchanger heated by furnace gases, it is clearly not possible to use high pressure jetting while the heat exchanger is on line.

Because proper cleaning of finned heat exchangers requires them to be taken off line and possible closure of the entire chemical processing plant, a plant operator will normally tolerate a gradual reduction in performance over a period of typically one year before the thermal efficiency drops to such an extent that servicing of the heat exchanger becomes unavoidable.

Object of the invention The present invention is concerned with the cleaning of the fin side, of a finned heat exchanger and aims to permit such cleaning to be performed while the heat exchanger is still in use, thereby avoiding down time and enabling the heat exchanger to operate at all times near its peak efficiency.

Summary of the invention

According to a first aspect of the present invention, in order to clean a heat exchanger having a stack of parallel tubes interconnected at their ends and fins on sections of the tubes lying between the two ends to assist in transferring heat between the interior of the tubes and the gas flowing over the fins, a cleaning fluid is introduced between the fins, to flow from the interior towards the exterior of the stack.

The manner in which the tubes are interconnected at their ends is not of primary importance to the invention.

Thus the heat exchanger may be a single pass heat exchanger in which case two unpartitioned headers may be arranged one at each end of the heat exchanger stack. Alternatively, it may be a multi-pass heat exchanger in which one or both of the headers are partitioned. For a maximum number of passes each finned tube may be connected at each end to only one other finned tube by means of a U-bend.

The invention calls for a modification of the heat exchanger stack so that it incorporates passaqes dedicated to the cleaning of the fins. In some embodiments of the invention, the passages are themselves spray bars connectible to a source of pressurised cleaning medium, such as water, so that large sections (or possibly the whole of the heat exchanger) are sprayed with the cleaning fluid at the same time.

In alternative embodiments, the passages are designed to receive and guide a jetting lance that can be advanced through the heat exchanger parallel to the tubes to clean smaller sections of the fins at a time.

In accordance with a second aspect of the invention, there is provided a method of cleaning a finned heat exchanger having a heat exchanger having a stack of parallel tubes interconnected at their ends and fins on sections of the tubes lying between the two ends to assist in transferring heat between the interior of the tubes and the gas flowing over the fins, which method comprises providing clear passageways extending between and parallel to the tubes within the finned sections of the heat exchanger, providing in alignment with the passageways guide tubes that pass through and are sealed relative to a header and introducing a jetting lance through the guide tubes and passageways for cleaning the fins by directing jets of cleaning fluid from the interior towards the exterior of the heat exchanger.

The passageways within the finned sections of the heat exchanger may simply be formed by spaces between disc shaped fins that are arranged on the finned tubes or apertures within the cooling fins.

In this case, the jetting lance may be introduced through a guide tube into each passageway and advanced over the length of the heat exchanger parallel to the axis of the finned tubes.

Alternatively, each passageway may be defined by a tube with a perforated or reticulated wall to allow the jets of the jetting lance to be introduced into the interior of the heat exchanger.

The lance in the latter case may be provided with seals ahead of and behind the jets so that the fluid emitted from the lance only pressurises a small section of the perforated tube at any one time.

In accordance with another aspect of the invention, there is provided a method of cleaning a finned heat exchanger having a stack of parallel tubes interconnected at their ends and fins on sections of the tubes lying between the two ends to assist in transferring heat between the interior of the tubes and the gas flowing over the fins, which method comprises providing spray bars within the stack extending between and parallel to the tubes within the finned sections of the heat exchanger, and connecting the spray tubes to a source of cleaning fluid to spray jets of the cleaning fluid that flow between the fins from the interior towards the exterior of the heat exchanger.

In this aspect of the invention, the spray bars that remain permanently within the themselves produce the jets of cleaning fluid instead of acting as a guide for a lance that produces the jets.

A spray bar cannot produce jets with as much power as a lance and cannot therefore be used as effectively when a heavy deposit has been allowed to build up on the fins. On the other hand, however, cleaning the heat exchanger by means of spray bars is much less labour intensive than jetting as it only involves connecting a source of cleaning fluid to the spray bars. As such it can be carried out more frequently, thereby avoiding severe fouling altogether. If for any reason the jets of a spray bar should become blocked, it is of course possible to use the spray bar as a lance guiding passageway, if the spray bars include guide tubes passing through and sealed relative to the header. In this case, a lance may be used at less frequent intervals to clean the spray bar itself at the same time as pressure jetting the fins of the heat exchanger.

It is not however essential for the spray bars to be accessed by guide tubes passing through a header and for their regular use it would suffice to provide a connector behind a header.

According to a further aspect of the present invention, there is provided a heat exchanger having a stack of parallel tubes interconnected by headers at their ends and fins on sections of the tubes lying between the two ends to assist in transferring heat between the interior of the tubes and the gas flowing over the fins, characterised by clear passageways extending between and parallel to the finned tubes within the finned sections of the heat exchanger, and guide tubes arranged in alignment with the passageways which guide tubes pass through and are sealed relative to a header, the guide tubes enabling a lance to be introduced into the passageways for cleaning the fins while flowing outwards from the interior towards the exterior of the heat exchanger.

In one embodiment of the invention, the passageways within the finned sections of the heat exchanger are formed by spaces between disc shaped fins that are arranged on the finned tubes or apertures within the cooling fins.

In an alternative embodiment of the invention, each passageway is defined by a tube with a perforated or reticulated wall.

Instead of jets of water or cleaning medium being aimed at the fins from the outside of the heat exchanger, in the present invention, water jetting is carried out from within the heat exchanger, the cleaning fluid flowing from the inside to the outside. By having numerous guide tubes built into the heat exchanger, the jets of cleaning fluid can be positioned immediately adjacent the fins to be cleaned, thus making the cleaning highly effective. The cleaning can be performed while the heat exchanger is on line without danger to the lance operator.

If the passageways are defined by perforated tubes, it is possible that it may be blocked by the dirt blown into the heat exchanger by the fan. The deposit is however relatively soft and can readily be dislodged by a jet of water if a lance is used or by the cleaning medium. The operator will therefore meet with little resistance permitting the use of a flexible or semi-flexible lance.

To feed the lance into a guide tube, the lance operator needs to stand by the header, not by the finned tubes. The entrance to the guide tube will be on the same side as the header bolts. Access is gained to the guide tubes by removing a collar or plug that blanks off the guide tube.

According to a still further aspect of the invention, there is provided a heat exchanger having a stack of parallel tubes interconnected at their ends and fins on sections of the tubes lying between the two ends to assist in transferring heat between the interior of the tubes and the gas flowing over the fins, characterised by a plurality of spray bars extending between and parallel to the finned tubes within the finned sections of the heat exchanger, and means for connecting the spray bars individually, in groups or simultaneously to a supply of cleaning medium to produce jets of cleaning medium for cleaning the fins while flowing outwards from the interior towards the exterior of the heat exchanger.

The operator can pressurise the perforated tube from the one end access point using any form of suitable cleaning fluid whether or not under pressure. Alternatively, the operator may insert a pressurised lance into the perforated tube through the guide tube. The lance may have at its end a double seal arrangement between which pressure jetting nozzles are arranged. As the seal arrangement is pushed along the perforated tube, it restricts the flow to a few apertures within the perforated tube thereby allowing the cleaning medium to be applied at greater pressure if required at certain locations along the finned length. More than one operator can perform cleaning at the same time due to its safety.

The operator will be at some distance from the heated water spray created by the lance and will not be subjected to a hot and dirty shower. The small amount of water will either be carried away as a spray by the air blown by the fan or will fall into the plenum from which it can be collected for disposal. There is thus nothing to preclude the cleaning operation from being performed while the heat exchanger is on line. In the case of a heated heat exchanger, the water would simply evaporate.

The ability to clean the fin side of the heat exchanger while it is on line permits the operation to be carried out at more regular intervals. This ensures that the heat exchanger remains near its peak efficiency at all times.

Furthermore, the time and cost involved in cleaning the heat exchanger is reduced because there is less of a deposit to be removed from between the fins during each cleaning operation on account of the reduced service interval.

Furthermore, frequent cleaning substantially reduces corrosion and degradation of the finned sections as they are kept in a constantly clean state.

Brief description of the drawings

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which Fig. 1 is a plan view from above of a conventional air cooled heat exchanger as used in an oil refiner, Fig. 2 is a side view of a header of the heat exchanger in Fig. 1, Fig. 3 is a similar view to that of Fig. 2 but showing a modified heat exchanger embodying the present invention, and Fig. 4 is a section in the section plane designated A-A in Fig. 3.

Detailed description of the preferred embodiment(s) Fig. 1 shows a conventional air cooled heat exchanger having headers 12 and 14 and finned tubes 16 extending between the headers 12, 14. Inlet and outlet connectors 18 and 20 are provided for the fluid to be cooled, which in the case of an oil refinery may be hot crude oil. The connectors 18 and 20 are shown as being on different headers but they may be mounted on the same header. This will depend on the -10 -manner in which the headers are partitioned and the number of passes that the fluid makes through different finned tubes of the heat exchanger. The invention is not restricted to any particular design of fluid flow path.

The sections of the finned tubes 16 between the headers 12, 14 have fins 22 to increase the surface area over which heat transfer can occur. In the illustrated embodiment, the fins 22 are shown as circular discs fitted to individual finned tubes. Touching hexagonal spacers 26 are also fitted to the finned tubes 16 along their length so that they support one another.

To allow the finned tubes 16 to be cleaned, threaded holes normally plugged by means of bolts 24 are formed in at least one of the headers 12, 14. The bolts 24 are shown in Fig. 2 which is an end view of the header 12. It will also be seen from Fig. 2 that the finned tubes 16 are arranged in a matrix such that in each group of three adjacent finned tubes 16, the centres of the tubes lie at the apices of an equilateral triangle. As the circular fins are dimensioned not to overlap, this leaves an empty space at the centre of each equilateral triangle into which a high pressure jetting lance can be introduced.

The present invention in one of its embodiments proposes introducing a jetting lance into these spaces from one end of the heat exchanger. To enable a lance to be introduced in this manner, it is necessary to modify at least one of the headers to provide holes aligned with these spaces.

A header of a modified heat exchanger is shown in Fig. 3 and in section in Fig. 4. At each location where it is desired to feed a lance into the heat exchanger 10, a guide tube 30 is provided. Each guide tube 30 is suitably sealed -11 -relative to the front and rear plates 12a and 12b of the header 12, as shown in Fig. 4.

In the illustrated embodiment of the invention, the guide tubes 30 continue into the finned section of the finned tubes 16 and extend over the length of the heat exchanger. The lance is therefore guided over the entire length of the heat exchanger 10. To permit cleaning of the fins, holes 32 are formed in the guide tubes 30 where they lie next to a finned section of the finned tubes 16.

It will be appreciated that as the fins themselves define an empty space for the lance, it would be possible to dispense with the reticulated sections of the guide tubes and to rely on the fins for guidance.

Instead of individual discs on the finned tubes 16, it would alternatively be possible to provide fins in the form of thin plates extending across the width of the heat exchanger with several finned tubes passing through the same fin. In latter case, additional holes may be provided in the fins for passage of a lance.

It is nevertheless preferred to use reticulated guide tubes to guide the lance throughout the length of the heat exchanger as this avoids possible damage to fins in the event of a serious misalignment. Also, if a rotating lance is used, a guide tube is desirable to prevent both the fins and the lance from being damaged.

In use, when it is desired to clean the fin side of the heat exchangers, a high pressure jetting lance is introduced into one of the guide tubes 30 and advanced through the header until it reaches the finned section of the finned tubes. It is also possible to use several lances at the same time. Each lance has a forward facing jet to clear a path for itself in any deposit that may have collected within the -12 -reticulated section of the guide tube 30 or between the fins if the guide tube 30 terminates at the header. The lance is then advanced so that laterally jets clean between the fins.

If desired, the lance may also have rearwardly facing jets to help advance the lance further into the heat exchanger.

Throughout the cleaning operation, the jetting of the fins is taking place at some distance from the lance operator, who is not exposed to the mist of hot and dirty water produced the jetting. The operation can therefore be carried out safely and does not require the fan to be stopped nor the heat exchanqer to be taken off line.

Instead of feeding a lance down each guide tube 30, it is possible to size the holes 32 such that they acts as jets and each tube 30 can then act as a spray bar. In such an embodiment of the invention, one can connect each spray bar to a supply of cleaning medium and clean the entire length of the heat exchanger at the same time. If desired, the spray bars may also be connected to each other so that several spray bars are pressurised at the same time. It is preferred for the spray bars to be designed such that a lance may be inserted into each spray bar from at least one end to enable it to be cleaned should its jets become blocked. This however is not essential as a connector may be mounted on the spray bars behind a header.

Though described above by reference to an air cooled heat exchanger, it will be clear that the method of cleaning between the fins is equally applicable to a heat exchanger heated by a furnace.

Claims

-13 -CLAIMS1. A method of cleaning a finned heat exchanger having a stack of tubes extending between two headers and fins on sections of the tubes lying between the two headers to assist in transferring heat between the interior of the tubes and the gas flowing over the fins, in which method a cleaning fluid is introduced between the fins, to flow from the interior towards the exterior of the stack.
2. A method of cleaning a finned heat exchanger having a stack of tubes extendinq between two headers and fins on sections of the tubes lying between the two headers for assisting in the transfer of heat between the interior of tubes and a gas flowing over the fins, which method comprises providing clear passageways extending between and parallel to the tubes within the finned sections of the heat exchanger, providing in alignment with the passageways guide tubes that pass through and are sealed relative to a header and introducing a jetting lance through the guide tubes and passageways for cleaning the fins by directing jets of cleaning fluid from the interior towards the exterior of the heat exchanger.
3. A method as claimed in claim 2, wherein the passageways within the finned sections of the heat exchanger are formed by spaces between disc shaped fins that are arranged on the finned tubes or apertures within the cooling fins, the jetting lance being introduced through a guide tube into each passageway and advanced over the length of the heat exchanger parallel to the axis of the finned tubes.
4. A method as claimed in claim 2, wherein each passageway is defined by a tube with a perforated or reticulated wall to allow the jets of the jetting lance to be introduced into the interior of the heat exchanger.

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5. A method as claimed in claim 4, wherein the lance is provided with seals ahead of and behind the jets so that the fluid emitted from the lance only pressurises a small section of the perforated tube at any one time.
6. A method of cleaning a finned heat exchanger having a stack of tubes extending between two headers and fins on sections of the tubes lying between the two headers for assisting in the transfer of heat between the interior of tubes and a gas flowing over the fins, which method comprises providing spray bars within the stack extending between and parallel to the finned tubes within the finned sections of the heat exchanger, and connecting the spray tubes to a source of cleaning fluid to spray jets of the cleaning fluid that flow between the fins from the interior towards the exterior of the heat exchanger.
7. A heat exchanger having a stack of tubes extending between two headers and fins on sections of the tubes lying between the two headers to assist in transferring heat between the interior of the tubes and the gas flowing over the fins, characterised by clear passageways extending between and parallel to the finned tubes within the finned sections of the heat exchanger, and guide tubes arranged in alignment with the passageways which guide tubes pass through and are sealed relative to a header, the guide tubes enabling a lance to be introduced into the passageways for cleaning the fins while flowing outwards from the interior towards the exterior of the heat exchanger.
8. A heat exchanger as claimed in claim 7, wherein the passageways within the finned sections of the heat exchanger are formed by spaces between disc shaped fins that are arranged on the finned tubes or apertures within the cooling fins.

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9. A heat exchanger as claimed in claim 7, wherein each passageway is defined by a tube with a perforated or reticulated wall.
10. A heat exchanger having a stack of parallel tubes interconnected at their ends and fins on sections of the tubes lying between the two ends to assist in transferring heat between the interior of the tubes and the gas flowing over the fins, characterised by a plurality of spray bars extending between and parallel to the finned tubes within the finned sections of the heat exchanger, and means for connecting the spray bars individually, in groups or simultaneously to a supply of cleaning medium to produce jets of cleaning medium for cleaning the fins while flowing outwards from the interior towards the exterior of the heat exchanger.
11. A exchanger constructed substantially as hereinbefore described with reference to and as illustrated in Figures 3 and 4 of the accompanying drawings.