WO2014138784A1

WO2014138784A1 - Shell and tube heat exchanger arrangement

Info

Publication number: WO2014138784A1
Application number: PCT/AU2014/000239
Authority: WO
Inventors: Brady Haneman; Alan Wang
Original assignee: Hatch Pty Ltd
Priority date: 2013-03-11
Filing date: 2014-03-11
Publication date: 2014-09-18
Also published as: CN105164486A; BR112015022304A2; EP3017259A1; AU2014231761A1; EP3017259A4

Abstract

A shell and tube heat exchanger arrangement comprising at least two interconnected heater cells (30), the heater cells (30) each in turn comprising a plurality of inner tubes (36) defining a tube side of the heat exchanger and being housed within an outer shell (34) defining a shell side of the heat exchanger, characterised in that the inner tubes (36) each project beyond the outer shell (34) at one end thereof at which point they are each connected to an intermediate pipe member (44) by way of which that heater cell (30) may be connected to another heater cell.

Description

"Shell and Tube Heat Exchanger Arrangement"

TECHNICAL FIELD

[0001] The present invention relates to a shell and tube heat exchanger arrangement. More particularly, the shell and tube heat exchanger arrangement of the present invention is intended to simplify the interconnection of adjacent heater units, relative to the prior art, and to facilitate the cleaning of the shell side of such heater units.

BACKGROUND ART

[0002] The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an

acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.

[0003] The Applicants are the registered proprietors of several prior patents directed to multi-cell heating systems, including Australian Patents 676920, 697381 and

2006201746. The heater systems of these prior patents had been primarily developed for flash evaporation equipment utilised in the alumina refining industry. However, the useful application of the inventions described in those patents is not limited to that particular industry and may be extended to all branches of industry that encounter similar or identical processing problems to those mentioned below.

[0004] In order to understand the relevance of the advantages provided by the

Applicant's prior inventions, the subject of the abovementioned patents, and the further advantages provided by the present invention, it is helpful to explain some of the problems encountered. To this end, a brief description of the processes involved will now be provided.

[0005] Alumina, or aluminium oxide, is chemically designated as Al₂0₃. It is an important mineral used industrially to manufacture a wide range of products from abrasives to aluminium metal. Its occurrence in commercial quantities is mainly as bauxite ore, in which alumina is present in the form of hydrates and silicates. Of these, the hydrates, which occur as both alumina monohydrate and alumina trihydrate, are the only compounds that are extracted and these must be separated from the remainder of the ore. A typical commercial bauxite ore ranges from about 30% to about 60% extractable alumina.

[0006] Industrially, the extraction of alumina is accomplished by the Bayer process, so called after the Austrian chemist K J Bayer who developed the process in 1888. In this process, finely ground bauxite is mixed with aqueous caustic soda solution and heated. This causes the alumina hydrates to go into solution, which allows them to be separated from the residual solids. Commercially, this latter step is carried out by a combination of sedimentation and filtration.

[0007] Cooling of the filtered liquid reverses the effects of the heating process and the dissolved alumina hydrates are precipitated while the remaining liquid reverts to its initial state and can be reused to repeat the process. The temperature range of the process depends upon the quality of the bauxite. In this respect, the digestion of trihydrate ores normally requires temperatures less than 150°C, whereas the digestion of monohydrate ores requires temperatures ranging to as high as 300°C.

[0008] In commercial plants the extraction of alumina is usually carried out in a continuous process and on a large scale. The caustic liquor stream is continuously recirculated and alternately heated and cooled in accordance with the requirements of the Bayer process. However, because of the large scale of operation the energy content of the liquor stream is very high, especially in high temperature refineries, and efficient energy management is essential to the economy and viability of the plant. A large proportion of the equipment installed in alumina refineries is therefore dedicated to heat recovery.

[0009] The main area for heat recovery is in the digestion section of the refinery where heat is transferred from the outgoing hot Bayer solution to the incoming cold bauxite slurry. This is accomplished in tubular heaters, wherein cold slurry flows through the inside of the tubes and hot flash vapour flows on the outside of the tubes. The vapour condenses on the cold(er) tubes, which causes it to release its heat of evaporation, which is then absorbed by the cold slurry stream.

[0010] This heating process is usually carried out in a number of stages.

Thermodynamic theory shows that the efficiency of this heat recovery process increases with the number of heating stages that are employed. In practice, the number of stages is limited by economic considerations. In particular, each additional stage requires more equipment and there is thus a point beyond which the marginal increase in efficiency does not warrant the additional investment.

[0011] It is in the tubular heaters of the digestion section of a Bayer process plant, particularly those in high temperature Bayer plants, that a number of problems arise that add considerably to the capital cost for equipment, as well as to the cost for operating and maintaining the plant.

[0012] In particular, to operate the Bayer process on a bauxite ore that contains both alumina monohydrate and alumina trihydrate, the slurry of bauxite and aqueous caustic soda must be heated to temperatures as high as 300°C in order to successfully dissolve the monohydrates. However, and as will be described in more detail below, this requires the slurry to be taken to temperatures where the deposition of scale is extreme, compared with most processes in which heat exchangers are normally employed.

[0013] Even when the slurry only has to be heated to temperatures of approximately 150°C for trihydrate digestion, the deposit of scale is significant.

[0014] In this respect, as the slurry temperature rises, the trihydrate goes readily into solution, but a portion of the trihydrate is converted to monohydrate. The monohydrate is not readily soluble until it reaches a higher temperature and thus monohydrate scale tends to precipitate and deposit on the walls of the heaters. Similarly, other types of scales, for example silicates and titanates, are also deposited.

[0015] The major problem with the buildup of scale on any heater unit is that it seriously effects the heat transfer coefficient. The scale deposits also increase resistance to fluid flow and thus add considerably to the hydraulic gradient necessary to maintain the required flow through the apparatus. In order to provide at least a minimum acceptable time span between heater down time for de-fouling (the cleaning cycle), generous safety factors are usually provided during the initial design. Consequently, the heaters are provided with surface areas several times higher than comparable heaters would be in industries where the deposition of scale is not a significant factor. This adds considerably to the total length of heater tubing. Pumping heads are therefore high which in turn requires the equipment and its connecting pipe work to withstand considerably higher pressures than would otherwise be the case. [0016] Clearly, these design allowances have a cumulative effect and have a large impact on the capital and operating costs of the plant equipment.

[0017] Furthermore, despite the significant safety factors built into the design and the expense of the equipment, its service life remains limited. It still requires a considerable period of down time to de-scale tubing and carry out associated maintenance, such as the replacement of blocked tubes and the like. It is therefore standard practice in the industry to provide ample spare equipment, so that cleaning can be carried out on a rotating basis without effecting plant production or the continuity of operations. Indeed, in large conventional refineries it is not unusual to have from 30% to 50% spare equipment in the heat recovery section.

[0018] Finally, the thermal performance of the heaters is directly related to the rate of flash steam generation in the evaporator vessels and this has an important bearing on the quality and utility of the condensate that is collected from the heater train.

[0019] Not surprisingly, the industry has been engaged for many years in actively developing improvements in the digestion plant. These improvements have been in the way of process improvements, equipment improvements, operating improvements, or a combination thereof.

[0020] One such improvement utilised a system of tubes provided with heating jackets, instead of conventional shells, where the tubes are jacketed in small groups. The tubes are large in diameter compared with standard heat exchanger tubes and essentially continue uninterrupted throughout the length of the entire heater system. The jackets are not continuous, but are applied intermittently in accordance with the number of evaporators and to suit dismantling of heater elements for cleaning and maintenance. Regarding thermal design, there is essentially no difference between such tube heaters and conventional heaters; but mechanically the differences are significant.

[0021] However, this design was primarily aimed at eliminating the old fashioned autoclave type digester in which the build-up of scale was quite out of proportion with what is considered bad scale build-up in modern alumina refineries. It did not therefore specifically address the problems that now remain in modern large scale plants. [0022] Following that, a somewhat similar design was developed with a similar aim of finding a suitable alternative to the old style autoclave. Since scaling-up of tube digesters remained a problem, a heater design was adopted that contained three tubes within each jacket. Two of these tubes conveyed bauxite slurry, the third conveyed spent liquor. At the end of the heater system, and prior to digestion, the three flows were combined to provide a digestion slurry of the requisite consistency. The flow through the tubes was periodically switched, so that each of three lines are in turn subjected to spent liquor flow, with the aim of dissolving the scale. This procedure was carried out in-situ under operating conditions (i.e. at temperature), and removed at least a portion of the scale. However, the equipment required periodic cleaning with acid, to remove the components of the scale that were insoluble in spent liquor or that remained undissolved, and this cleaning process could not be carried out under operating conditions.

[0023] The metallurgical implications were more serious, as the spent liquor generated by this process was above 140°C and could not be contained in carbon steel. This required alloys that are either extremely expensive, or, if only moderately expensive, such as ferritic stainless steels, were difficult to weld.

[0024] Another development has been to retain conventional shell and tube heat exchangers and install them with the tube bundles vertically instead of horizontally. This has proved to be reasonably effective in limiting the growth of scale within the tubes and demonstrates the effect of gravity on precipitation and

sedimentation/cementation on the tube wall.

[0025] The Applicant's prior inventions, those being the subject of the abovementioned patents, have been directed to providing a multi-cell heating system that allows individual heater cell trains to be cleaned while the remainder of the equipment remains in operation. For example, Australian Patent of Addition 2006201746 provides a multi- cell heating system for increasing the temperature of a tri-hydrate bauxite ore slurry to a temperature of up to 160°C through a number of heater cell trains, the heating system comprising an array of heater cells, the array comprising a plurality of heater cell trains and a plurality of heater cell stacks, each stack being associated with and being in fluid communication with a respective heat source, and each train being defined by aligned individual heater cells in adjacent stacks such that the slurry may be split to flow through two or more of the trains in order to be heated thereby, the array being configured such that there is an inlet temperature at one side thereof and an outlet temperature at the other side thereof, wherein the interconnection of trains, stacks, cells and heat sources is such that each heater cell train, and each heater cell, is able to be isolated from the heating medium. In a preferred form, the heat sources are evaporators and the heat medium is vapour or steam. The heat source may also be condensed vapour (condensate).

[0026] The isolation referred to above provides two significant ways in which the deposition of scale within the heater cells and the performance of the heating system as a whole can be controlled: i. isolation of a single train of heater cells allows that single train to be cooled, drained and cleaned while the remaining trains stay in operation. By thus cleaning each train in turn and at regular intervals, the overall deposition of scale can be maintained at an average level; and ii. isolation of individual heater cells allows evaporator vapour flows to be controlled and distributed so as to maintain the desired pressure/temperature profile through the evaporators. Such isolation or, if necessary, modulation of the vapour flows to individual heater cells, also provides a means of controlling the rate at which the temperature increases along the heater tube, ie. the heat flux. This is an advantageous feature of the invention, in that the rate of deposition of scale is a function of both temperature and heat flux.

[0027] Thus, the manner in which the heater cells are arranged and connected provides a useful measure of control over the relative thermal performance between heater cell trains as well as over the thermal gradient within heater cell trains. It allows the surface area required for heat transfer to be minimised and plant performance to be optimised.

[0028] Furthermore, the multi-cell system allows the deposition of scale to be distributed in a manner that will substantially reduce the amount of equipment that would otherwise be required. The system also provides a large measure of control over the pressure/temperature profile through the heating system. This is particularly advantageous in process plants in which the heating system forms part of a heat recovery system in which the heating medium consists of process vapour extracted from flash evaporating vessels. In such plants, the quality of the vapour and condensate are highly dependent on a relatively steady pressure/temperature profile. A steady pressure/temperature profile is also required to maintain the driving force between evaporators. This is the force or pressure required between adjacent evaporators to ensure that the designated evaporating liquid flow can be maintained. This force or pressure is directly related to temperature and should therefore be maintained steady throughout the operational cycle of the plant, irrespective of the increasing build-up of scale and the concomitant decline of the capacity of individual heater cells to condense/extract vapour from the evaporator vessels.

[0029] In this respect, in prior art techniques the heater units that subject slurry to an increase in temperature each contain a number of tubes that run parallel through a single line of heater shells. Thus, individual tubes cannot be isolated from the vapour flows. This prevents individual tubes from being descaled while the heating plant remains in operation and thereby prevents the scale deposition within a single heating unit from being maintained at average conditions. Descaling in such prior art techniques, in which a number of tubes run through a single line of shells, takes place when the operational heating unit is fully scaled and requires the heating unit to be taken out of operation. In order to maintain the required pressure temperature/profile between when all tubes are clean and all tubes are fully scaled, such prior art provides excess heat transfer area, which is initially flooded with condensate to render the tubes temporally ineffective. As tubes scale, the condensate level is reduced in order to expose more tube area to vapour or other heating medium.

[0030] The multi-cell heater system of the Applicant's prior patents referred to above do not require such excess heat transfer area, and control their operation by maintaining an average scale deposition at a steady state.

[0031] Indeed, in prior art systems the localisation of heater scale is particularly troublesome. It causes the thermal performance of heaters in the high temperature range to decline much more rapidly than the performance of the heaters outside that range. As the performance of the most affected heaters declines, the

pressure/temperature profile through the heat recovery system changes, and the thermal duty is gradually transferred to the lesser scaled heaters. The temperature intervals between these cleaner heaters increase and effectively this reduces the number of heater stages that are actually utilised. This in turn reduces the overall efficiency of the heat recovery process and limits full utilisation of the heat transfer area in those heaters in which the deposition of scale is basically still within acceptable limits; in other words a point is reached where the total available heater surface area would still be serviceable if it would be more evenly divided over the number of stages, but where, because the number of useful stages has effectively been reduced, the heater system as a whole can no longer perform and rapidly declines in efficiency.

[0032] Furthermore, the performance of the evaporation vessels is directly related to the troublesome localisation of heater scale. In a conventional heat recovery system, evaporator duty is always equal to the condensing capacity of its corresponding heaters. The evaporators are sized to keep the upward vapour velocity below a certain limit. This is in order to prevent any caustic or solid or other contaminating matter, contained in the evaporator fluid, from being carried along with the vapour flow. When heaters foul at different rates and the pressure/temperature profile and thermal capacities change, the vapour generating rates of the evaporator vessels must change accordingly. The vapour rates in the cleaner stages will gradually increase and their upward vapour velocities may eventually exceed allowable values. Impurities will then be carried along by the vapour stream and effect the process in two ways: i. impurities will be deposited on the outside of the heater tubes and this will further diminish heat transfer capability. Moreover, scale on the outside of the tubes is difficult to detect and is also much more difficult to remove than scale deposited on the inside of the tubes; and ii. the impurities will pollute the condensate which is then no longer fit to be returned to the steam plant. The condensate must then be used for secondary purposes and this generally results in the loss of much of the energy it contains. There may also be an increase in plant water consumption to compensate for the loss of boiler make up water.

[0033] Depending on the degree and exact location of scale deposition, unequal heater fouling will ultimately limit the capacity of a heat recovery system in one of three possible ways: i. because the heater train has reached the limits of its thermal capacity, while a significant proportion of the total heat transfer area may still only be moderately fouled; ii. because it has reached the limit of its hydraulic (pressure) capacity due to localised constrictions of the flow area; and iii. because it produces bad condensate.

[0034] These limitations are addressed by the multi-cell heating system of the

Applicant's prior patents noted above. This multi-cell heating system allows the available heat transfer area to be more efficiently utilised. It enables the

pressure/temperature profile through the evaporators to be controlled, while minimising the required heat transfer area. Vapour generation rates therefore remain uniform and the likelihood of impurities contaminating the condensate is lessened. More importantly, because the temperature intervals between stages remain constant, there is no reduction in the number of effective stages, when heaters become scaled, and there is therefore no concomitant reduction in the overall efficiency of the heat recovery process.

[0035] In a preferred form of the Applicant's prior invention, the heater cells are multiple pass heaters. However, each pass preferably comprises only a single tube. Preferably, such single tube heaters resemble conventional heaters in that they have a shell side, a tube side, a tube plate and tube passes, although unlike a conventional heater, they have no channel section.

[0036] While each heater cell may contain one or more passes, again each pass preferably comprises only a single tube. In its most convenient arrangement, which does not require provisions to be made for differential expansion between tubes and shell, the tubes are arranged with return bends within the shell. Thus, externally to the tube plate, individual passes may be affected by means of return bends to provide a single continuous tube within each shell.

[0037] The number of passes to be employed is a design consideration, in which tube size, shell diameter, heater length and required heat transfer area are weighed up to provide the most cost-effective unit. [0038] At the tube plate, all tubes may be flanged to provide access for manual descaling. However, cleaning flanges generally need not be provided for internal bends, which preferably have sufficient radius to allow standard cleaning equipment to be effective. At the opposite end to the tube plate the shell may be provided with a flanged cap to allow for inspection of the internal bends.

[0039] The shell diameter is generally determinable by tube bundle geometry. The shell preferably also has sufficient volume, clear from the heat transfer area it provides, to act as condensate receiver. In this respect, condensate collection inside the heater shell eliminates the requirement for a condensate receiver for each individual cell or stack of cells. It also simplifies the condensate pipe work connecting the cells and helps equalising vapour flow rates from evaporators.

[0040] The number of cells per stack is generally determined by economic and operational considerations. Consistent with the principles set out in the foregoing, to accommodate two different vapour streams, at least two heater cells per stack are required. However, an additional cell should be provided for descaling. In practice, the most suitable number of cells per heater stack ranges from three to seven.

[0041] However, a heater stack of five smaller cells - being four operating cells and one spare cell - provides a more suitable arrangement. Four operating cells allow the heat transfer area per stage to be redistributed at 25% increments, the spare cell then amounting to only 25% stand by equipment which is reasonably representative of the ratio between the length of the operating cycle and the length of the descaling cycle.

[0042] As described above, a stack of heater cells is preferably installed opposite each flash evaporator, and each heater cell stack is preferably capable of having vapour or steam fed to it from its associated flash evaporator. Thus, each heater cell in a stack is preferably individually valved and can have its steam supply individually connected, varied or isolated as required.

[0043] To heat slurry to about 160°C, the number of stacks is generally 3 to 6 and preferably 4 to 5.

[0044] Preferably, the flow of slurry is at right angles to the flow of steam or vapour. If each stack is considered to be arranged vertically, the steam or vapour flow to the heater cells is also distributed vertically, in which case the slurry flow runs horizontally, arranged in tiers or trains. Thus, all of the uppermost units in each stack are connected on the tube side to form one single continuous slurry stream - similarly for the second unit in each stack, all being connected to provide a single uninterrupted stream from the slurry inlet to the end of the respective heater cell train.

[0045] With regard to the manner in which the array of heater cells are connected, there are various aspects to be considered. In particular, scale growth has a significant effect on hydraulic resistance. It may also seriously effect flow distribution between parallel streams propelled from a common pressure source. In particular, such streams foul at different rates, flows being distributed in accordance with the hydraulic resistance of each individual stream. However, this distribution does not necessarily coincide with the comparative thermal capacity of each stream and overall thermal performance will thus be impaired. Therefore, heater cell trains are preferably individually controlled, and this may be achieved in two ways.

[0046] Firstly, each stream may be pumped individually. However, this generally only is practical in large plants in which the flow through each individual train of heater cells is large enough to warrant a dedicated pump, or, in the case of multiple chamber positive displacement pumps, to warrant a set of dedicated pump chambers. Secondly, all streams may be pumped from a common source and a flow control valve may be installed in each line of heaters. Scale growth is a gradual process and while the rates of growth may vary, there are no sudden fluctuations in the way scale growth effects the heaters. Manual control, by means of throttling valves, is therefore quite satisfactory. Furthermore, both manual control and automatic control may be activated by the outlet temperature of the slurry heater streams.

[0047] With regard to the collection and transmission of condensate, individual heater cells may operate at different temperatures. Their condensates should then preferably not be collected in a common receiver vessel installed at each stack as in conventional plants, but each train of heater cells should have its condensate collected and transmitted separately. Effectively, this divides the condensate system into a number of parallel streams running from the high pressure end to the low pressure end of the plant. In this respect, individual streams are small and this allows condensate to be collected inside each heater cell. Separate condensate pots at each heater stack are therefore not required, except where accumulated condensate flows through the heater shells are likely to be detrimental to effective tube areas. The requirement for condensate pots, reflux, vapour lines and steam traps, or flow control valves, may therefore be minimised or eliminated provided the condensate piping is arranged in the preferred manner.

[0048] The preferred arrangement relies on orifice plates and allows steam to by-pass when the condensate flow rate declines. This may occur when plant throughput is low and/or when condensing capacity declines due to scale deposition. Such by-pass steam is not detrimental to heater train operation provided it reaches the next heater shell in a saturated state at the downstream pressure condition. Within the range of plant operating conditions that can be expected in practice, such will always be the case. Moreover, in the case of reduced condensate flow due to heater tube scaling, bypass of steam actually enhances thermal performance. Indeed, it distributes vapour to a downstream heater stage without a concomitant increase in upstream evaporator pressure and thus helps to maintain the desired pressure/temperature profile.

[0049] The resulting condensate system is relatively simple and provides additional advantages over systems utilised in conventional plants. Its advantages are not confined to high temperature plants, but are equally applicable to low temperature plants.

[0050] With regard to the collection of the non-condensable gases entrained in the vapour stream, these are separated in the condensation process and are collected in the heaters. However, these gases are detrimental to heater performance and thus should preferably be removed. Each cell in the heater train is therefore preferably connected to a non- condensable vent system. While the individual vent streams are small, each stream is saturated with water vapour and collectively they represent a significant amount of energy, as well as condensate. Water vapour can be separated from non-condensable gases by cooling, and this is preferably carried out in two low pressure heater cells. Energy is thus retrieved by the incoming slurry stream. The heat transfer area required for this operation should be taken into account in the thermal design.

[0051] There are significant design, constructional, operational and maintenance advantages to making all heater cells identical. Thermal design should therefore preferably be arranged to suit heater cells of equal transfer area, except for the first two heater stacks. The first two stacks are preferably designed so that a smaller area is dedicated to condensing evaporator steam, wherein the difference in area is preferably to allow one entire cell in each of the first two heater stacks to be dissociated from the evaporator vessels and to be utilised for non-condensable cooling.

[0052] The heating system of the Applicant's prior art results in an extremely compact and flexible physical arrangement in which each heater cell is effectively a node on a network formed by the slurry and vapour distribution systems. In particular, the arrangement facilitates the descaling operation while the plant is in operation. This considerably reduces the amount of equipment that has to be taken out of service for descaling at any one time and provides large savings in the amount of spare equipment that needs to be installed. The arrangement also provides a great deal of operator control over the deposition of scale, both within slurry streams as well as between slurry streams. It also provides control over the distribution of vapour flow, both between evaporators and between heater cells, and over the quality of condensate.

[0053] The Applicant's prior art thus advantageously provides a heating system that allows individual heater cell trains to be cleaned, while the remainder of the equipment remains in operation. This provides a means of controlling the overall scale growth within a single operating heating system in such a way that the total of accumulated scale deposition within this single operating heating system is always maintained at an average value (steady state). In effect, this means that heat transfer areas may then be determined for average conditions of scale formation, rather than for maximum conditions. Since the buildup of scale is the most significant factor in determining the amount of heat transfer area that needs to installed, the multi-cell heating system will allow large savings in capital expenditure to be made. By providing means of controlling vapour flows and thereby allow the designated pressure/temperature profile of the plant to be maintained, the multi-cell heating system also has significant operational advantages.

[0054] The heater system of this invention has been primarily developed for flash evaporation equipment in the alumina refining industry. However, the useful application of the invention is not limited to that industry and may be extended to all branches of industry that encounter similar or identical processing problems to those mentioned below.

[0055] The original design for the heater cells, as described above, makes no direct accommodation for cleaning the shell-side of the heaters. Consequently, a significant reduction in useable heat transfer area may result from scale accretion in the heater shell which cannot be readily accessed for cleaning.

[0056] The mechanical design of heaters of the prior art incorporated a method of connection of the heaters via a 'tube-sheet' to lube-sheet' connection. The specifically manufactured tube-sheets require careful handling during heater cleaning to avoid damage to the flange face. Damage to the flange face interferes with the tight tolerances required to seal the tube-sheet to tube-sheet joints resulting in flange joint leakage. Any small scratch caused during handling may be gouged out in use, ultimately causing slurry breakthrough and leakage. Further, thermal and pressure related stresses experienced by the flange joint in use further contribute to leakage in prior art arrangements. For example, the various components (the inner tubes, the gaskets and flanges) may be caused to move out of alignment under the influence of such stresses.

[0057] Furthermore, the specifically manufactured flanges utilise expensive special ring joint gaskets which require replacement on each heater cleaning cycle.

[0058] The shell and tube heat exchanger arrangement of the present invention seeks to eliminate the use of the specifically manufactured flanges and preferably utilises more readily available flanges and standard gaskets. These flanges are connected to individual interconnecting and removable pipe spools. Each interconnecting pipe spool preferably has a standard weldneck flange provided at each end thereof and which is in turn connected to the flanges provided on the respective heaters. This arrangement is intended to substantially eliminate the need for the specifically manufactured flange (tube-sheet) with its associated machining of three tube holes and associated mating of two tube-sheets face to face.

[0059] This arrangement provided by the apparatus of the present invention allows for largely unencumbered access for hydrocleaning hose used for high pressure water jet cleaning of the internal tube surfaces. [0060] The apparatus of the present invention in one form further incorporates shell side hydro-blasting access nozzles on the heater return bend (vapour inlet) for shell side cleaning, such as may be provided by way of high pressure waterjet cleaning. The removable pipe spools are preferably provided in both the vapour and condensate piping systems to facilitate scale removal.

[0061] Throughout this specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

SUMMARY OF INVENTION

[0062] In accordance with the present invention there is provided a shell and tube heat exchanger arrangement comprising at least two interconnected heater cells, the heater cells each in turn comprising a plurality of inner tubes defining a tube side of the heat exchanger and being housed within an outer shell defining a shell side of the heat exchanger, characterised in that the inner tubes each project beyond the outer shell at one end thereof at which point they are each connected to an intermediate pipe member by way of which that heater cell may be connected to another heater cell.

[0063] Preferably, each intermediate pipe member is removable from its respective inner tube.

[0064] Still preferably, each inner tube terminates beyond the outer shell at a flange. Each intermediate pipe member further preferably has a flange provided at each end thereof, thereby facilitating releasable connection between inner tubes.

[0065] In one form of the present invention each inner tube has an elbow provided substantially adjacent the flange provided thereon.

[0066] Preferably, the inner tubes are received through a tube sheet at the end of the outer shell. [0067] Still preferably, the or each heater cell has at least one shell side cleaning nozzle provided therein. The cleaning nozzle is preferably provided in a return bend of the heater cell.

[0068] In accordance with the present invention there is further provided a heating system comprising at least two interconnected heater cells, the heater cells each in turn comprising a plurality of inner tubes defining a tube side of the heat exchanger and being housed within an outer shell defining a shell side of the heat exchanger, characterised in that the inner tubes each project beyond the outer shell at one end thereof at which point they are each connected to an intermediate pipe member by way of which that heater cell may be connected to another heater cell.

[0069] Preferably, each intermediate pipe member is removable from its respective inner tube.

[0070] Still preferably, each inner tube terminates at a flange. Each intermediate pipe member further preferably has a flange provided at each end thereof, facilitating releasable connection between inner tubes.

[0071] In one form of the present invention each inner tube has an elbow provided substantially adjacent the flange provided thereon.

[0072] Preferably, the inner tubes are received through a tube sheet at the end of the outer shell.

[0073] Still preferably, the or each heater cell has at least one shell side cleaning nozzle provided therein. The cleaning nozzle is preferably provided in a return bend of the heater cell.

BRIEF DESCRIPTION OF THE DRAWINGS

[0074] Further features of the present invention are more fully described in the following description of a non-limiting embodiment thereof. This description is included solely for the purposes of exemplifying the present invention. It should not be understood as a restriction on the broad summary, disclosure or description of the invention as set out above. The description will be made with reference to the accompanying drawings in which: -

Figure 1 is a partial top plan view of a shell and tube heat exchanger apparatus in accordance with the prior art, showing adjacent heater cells connected by way of a removable bend, the removable bend having tube-sheet to tube-sheet joins provided at each end thereof;

Figure 2 is a partial top plan view of a shell and tube heat exchanger apparatus in accordance with the present invention, showing adjacent heater cells connected by way of a removable intermediate pipe member positioned between the flanged ends of the inner tubes of the adjacent heater cells;

Figure 3 is a partial top plan view of a single heater cell of the apparatus of Figure 2, showing the inner tubes positioned within the outer shell and how the inner tubes project beyond the end of the outer shell and connect to an intermediate pipe member; and

Figure 4 is a partially cut away top plan view of two interconnected heater cells of the apparatus of Figure 2, showing how the inner tubes are accommodated within the outer shell, how the inner tubes project beyond the end of the outer shell of one heater cell, and how the intermediate pipe members facilitate the interconnection with the adjacent heater cell.

DESCRIPTION OF EMBODIMENTS

[0075] In Figure 1 there is shown a portion of a heater cell train 10 in accordance with the prior art, for example Australian Patent of Addition 2006201746. The heater cell train 10 of the prior art comprises a plurality of aligned heater cells 12, each in turn comprising an outer shell 14 within which is housed a plurality of inner tubes (not shown). The inner tubes terminate at a tube-sheet or flange 16, as does the outer shell 14. A complimentary gasket (not shown) is provided between the flange 16 and a complimentary flange 18 provided on a removable bend 20. The removable bend 20 is provided between adjacent heater cells 12.

[0076] The tube-sheets or flanges 16 and 18 are a specifically manufactured item and require careful handling during heater cleaning to avoid damage to the tube-sheet or flange face. Damage to the tube-sheet or flange face interferes with the tight tolerances required to seal the tube-sheet to tube-sheet joints that may result in flange joint leakage, thereby inhibiting the heating efficiency of the heater cell 12. Furthermore, the specifically manufactured flanges incorporate expensive ring joint gaskets which require replacement on each heater cleaning cycle.

[0077] In Figure 2 there is shown a plurality of aligned shell and tube heater arrangements or heater cells 30 in accordance with the present invention. The heater cells 30 are arranged to provide a heater cell train 32. Each heater cell 30 comprises an outer shell 34 and a plurality of inner tubes 36, for example three inner tubes 36, as best seen in Figures 3 and 4. The inner tubes 36 are received through a tube-sheet 37 provided at an end 38 of the outer shell 34 and have an elbow 40 provided therein adjacent a terminal flange, for example a weldneck flange 42. The manner in which the inner tubes 36 are received through the tube-sheet 37 avoids the joins between inner tubes and tube sheets typical of prior art arrangements, and thereby acts to minimise leaking of slurry into the shell-side of the heater cell 30.

[0078] An intermediate pipe member 44, having a flange, for example a weldneck flange 46, provided at each end thereof, is connected to the inner tubes 36 of adjacent heater cells 30.

[0079] Prior art heat exchanger apparatus have provided no direct accommodation for cleaning the shell-side thereof. Consequently, many heat exchanger apparatus of the prior art suffer a deterioration in useable heat transfer area as a result of scale accretion in the heater shell which cannot be readily accessed for cleanout.

[0080] The apparatus of the present invention further incorporates shell side hydro- blasting access nozzles 48 on a heater return bend (vapour inlet) 50 for shell side cleaning, such as may be provided by way of high pressure waterjet cleaning.

[0081] The removable intermediate pipe members are preferably provided in both the vapour and condensate piping systems to facilitate scale removal.

[0082] As noted above, the mechanical design of heaters of the prior art incorporated a method of connection of the heaters via a 'tube-sheet' to 'tube-sheet' connection. The specifically manufactured tube-sheets require careful handling during heater cleaning to avoid damage to the flange face. Damage to the flange face interferes with the tight tolerances required to seal the tube-sheet to tube-sheet joints resulting in flange joint leakage. Any small scratch caused during handling may be gouged out in use, ultimately causing slurry breakthrough and leakage. Further, thermal and pressure related stresses experienced by the flange joint in use further contribute to leakage in prior art arrangements. For example, the various components (the inner tubes, the gaskets and flanges) may be caused to move out of alignment under the influence of such stresses.

[0083] Furthermore, the specifically manufactured flanges utilise expensive special ring joint gaskets which require replacement on each heater cleaning cycle.

[0084] The shell and tube heat exchanger arrangement of the present invention seeks to eliminate the use of specifically manufactured flanges and utilises more readily available flanges and standard gaskets. These flanges are connected to individual interconnecting and removable intermediate pipe members, or pipe spools. As described above, each interconnecting pipe spool preferably has a standard weldneck flange provided at each end thereof and which is in turn connected to the flanges provided on the respective heaters. This arrangement is intended to substantially eliminate the need for the specially designed flange (tube-sheet) with its associated machining of three tube holes and associated mating of two tube-sheets face to face.

[0085] This arrangement provided by the apparatus of the present invention allows for largely unencumbered access for hydrocleaning hose used for high pressure water jet cleaning of the internal tube surfaces.

[0086] The incorporation of shell side hydro-blasting access nozzles on the heater return bend (vapour inlet) of the heart exchange apparatus of the present invention allows for shell-side cleaning. Removable pipe spools are provided in both the vapour and condensate piping systems to facilitate scale removal.

[0087] Modifications and variations such as would be apparent to the skilled addressee are considered to fall within the scope of the present invention.

Claims

CLAIMS:

1. A shell and tube heat exchanger arrangement comprising at least two

interconnected heater cells, the heater cells each in turn comprising a plurality of inner tubes defining a tube side of the heat exchanger and being housed within an outer shell defining a shell side of the heat exchanger, characterised in that the inner tubes each project beyond the outer shell at one end thereof at which point they are each connected to an intermediate pipe member by way of which that heater cell may be connected to another heater cell.

2. The heat exchanger arrangement of claim 1 , wherein each intermediate pipe member is removable from its respective inner tube.

3. The heat exchanger arrangement of claim 1 or 2, wherein each inner tube

terminates beyond the outer shell at a flange.

4. The heat exchanger arrangement of any one of claims 1 to 3, wherein each

intermediate pipe member has a flange provided at each end thereof, thereby facilitating releasable connection between inner tubes.

5. The heat exchanger arrangement of claims 3 or 4, wherein each inner tube has an elbow provided substantially adjacent the flange provided thereon.

6. The heat exchanger arrangement of any one of the preceding claims, wherein the inner tubes are received through a tube sheet at the end of the outer shell.

7. The heat exchanger arrangement of any one of the preceding claims, wherein the or each heater cell has at least one shell side cleaning nozzle provided therein.

8. The heat exchanger arrangement of claim 7, wherein the cleaning nozzle is

provided in a return bend of the heater cell.

9. A heating system comprising at least two interconnected heater cells, the heater cells each in turn comprising a plurality of inner tubes defining a tube side of the heat exchanger and being housed within an outer shell defining a shell side of the heat exchanger, characterised in that the inner tubes each project beyond the outer shell at one end thereof at which point they are each connected to an intermediate pipe member by way of which that heater cell may be connected to another heater cell.

10. A heating system according to claim 9, wherein each intermediate pipe member is removable from its respective inner tube.

11. A heating system according to claim 9 or 10, wherein each inner tube terminates at a flange.

12. A heating system according to any one of claims 9 to 11 , wherein each

intermediate pipe member has a flange provided at each end thereof, facilitating releasable connection between inner tubes.

13. A heating system according to claim 11 or 12, wherein each inner tube has an elbow provided substantially adjacent the flange provided thereon.

14. A heating system according to any one of claims 9 to 13, wherein the inner tubes are received through a tube sheet at the end of the outer shell.

15. A heating system according to any one of claims 9 to 14, wherein the or each heater cell has at least one shell side cleaning nozzle provided therein.

16. A heating system according to claim 15, wherein the cleaning nozzle is provided in a return bend of the heater cell.