WO2004005823A1

WO2004005823A1 - Heat recovery tower, method for constructing same and unit for use therein

Info

Publication number: WO2004005823A1
Application number: PCT/FI2003/000491
Authority: WO
Inventors: Kalle Helenius
Original assignee: Metso Paper, Inc.
Priority date: 2002-07-05
Filing date: 2003-06-18
Publication date: 2004-01-15
Also published as: AU2003239628A1; EP1552234A1; ATE408111T1; EP1552234B1; FI118826B; FI20021332A0; FI20021332A; DE60323523D1; NO20050634L

Abstract

The invention relates to a heat recovery tower for recovering heat energy from the gas generated in an industrial plant. The-heat recovery tower according to the invention is characterized in that it comprises a number of units that are arranged one on top of the other and detachably connected to each other, with at least two of them having been selected from: bottom basin, air/air-heat exchanger unit, scrubber unit, and air/water-heat exchanger unit. The invention also relates to a unit designated to be operating as a part of a heat recovery tower as well as a method for constructing a heat recovery tower for recovering the heat energy from the gas generated in an industrial plant.

Description

Heat recovery tower, method for constructing same and unit for use therein.

The invention relates to a heat recovery tower for recovering heat energy from the gas generated in an industrial plant. The invention also relates to a unit designated to be operating as a part of the heat recovery tower, and to a method for constructing a heat recovery tower for recovering the heat energy from the gas generated in an industrial plant.

Many industrial processes produce great quantities of heat energy, which, for economic purposes, should be recovered and used for other processes or be utilized in some other manner, whenever possible. In many processes, for example in the wood processing industry, wet and hot gases and steam, with significant quantities of water and often other substances, are generated. Recovering heat from such gases and quantities of gas requires large equipment, which take much of the factory floor space. For example, in dryers used in paper manufacturing, the warm, moist air is typically lead through pipes with a fan to heat exchangers arranged sequentially, and only at the end of the process the intermediate agent to be processed is lead to a tower-like part, from which the treated gas is lead to the roof of the building to be released in the open air.

Thus, it is the object of the invention to provide a structure that does not have the above-mentioned disadvantages, i.e. that can be applied when renovating old factories, thereby increasing the heat recovery efficiency and/or heat recovery capacity, respectively, with the same structural volume, when compared to the previous equipment. The object is also to provide a heat recovery tower with reduced gross expenditure, and which could be manufactured industrially in series production.

A further object of the invention is to provide a module used in such heat recovery tower, and a method for constructing the heat recovery tower.

The objects of the invention are reached as it is shown in the appended independent claims.

Accordingly, the invention relates to a heat recovery tower for recovering the heat energy generated in an industrial plant. The heat recovery tower according to the invention is characterized in that it comprises a number of units that are arranged one on top of the other and detachably connected to each other, with at least two of them having been selected from:

- a bottom basin,

- an air/air-heat exchanger unit

- a scrubber unit, and

- an air/water-heat exchanger unit.

Therefore, the heat recovery tower according to the invention comprises units that are freely stackable one on top of the other. The overall dimensions and shapes of the units are preferably substantially identical enabling very easy stacking. Therefore, all the units in the heat recovery tower according to the invention are preferably substantially joint so, that the floor area of the tower is the same as the square area of the bottom unit.

Consequently, the heat recovery tower according to the invention can be used when, for example, renovating old factories, thereby increasing the heat recovery efficiency and/or heat recovery capacity, respectively, with the same structural volume, when compared to the previous equipment. Accordingly, it also provides a heat recovery tower with reduced gross expenditure, which can be manufactured industrially in series production.

The heat recovery tower according to the invention also has preferably such a small area of base, that several such towers can be located near the source of the air or gas, whereupon transfer pipes that are long, large in diameter, expensive and inconvenient in many ways, are avoided. With the heat recovery tower according to the invention, it is also possible to attain short assembly time, whereupon, e.g. the new building projects of old factories have short downtime in this respect. Further, with the aid of a heat recovery tower according to the invention it possible to simplify the making of the foundation, since such tower requires only a relatively small, although solid foundation due to the weight being distributed on a relatively small area. With the aid of a heat recovery tower according to the invention also transportations can be simplified. The units are preferably designed so that they can be transported in normal trucks and containers. The assembling in the factory can also be performed without so-called haulage i.e. the units can be lifted to the place of assembly, one on top of the other with a relatively big mobile crane, through a hole in the factory ceiling. Moreover, savings are made in the manufacturing stage in the factory by making e.g. a welding jig for the assembly of the outer casings of the units, provided that in their horizontal cross-sections, all the units in a same tower are identical in shape and size.

According to a preferred embodiment of the invention, the heat recovery tower consists at least of the following units, arranged one on top of the other, seen from the bottom upwards

- a bottom basin,

- one or more air/air-heat exchanger units,

- one or more first scrubber units,

- one or more air/water-heat exchanger units, and

- one or more second scrubber units.

A person skilled in the art knows that, in order to obtain the required result, there can be a desired number of above-mentioned units. For a person skilled in the art, defining the number of units is a routine calculation. As an example it can be mentioned, that there can be 1 to 5 of the said air/air-heat exchangers, and 1 to 5 of the said air/water-heat exchangers. Furthermore, there is preferably one of said first scrubber units and preferably one of said second scrubber units. Naturally, there can also be some other number of each of the scrubber units, such as two, three, four, five or six, and their number can be freely selected independently. Additionally, it is possible to arrange one or more (e.g. 2, 3, 4 or 5) separation units in the heat recovery tower according to the invention for separating a liquid, such as water, from a gas.

A heat recovery tower according to the invention can be arranged so, that it extends at least from the factory floor to the ceiling. In this case, besides the said units, one or more extension units, that are empty, can be used in the tower. Obviously, the said extension units can also be used in other cases. When designing the heat recovery tower according to the invention, it is preferred that these extension units are available in different heights.

As mentioned above, in their horizontal cross-section, the units of the heat recovery tower according to the invention are identical in shape and size. According to an embodiment of the invention, said unit has substantially the form of a rectangular prism, comprising a first, second, third and fourth outer surface, with the first and third outer surface being substantially of the same size and parallel, and the second and fourth being substantially of the same size and parallel, and where

- on the first and second edges of the outer surfaces there has been arranged a sealing element with

- a sealing surface having a first length and being aligned perpendicular to the outer surface direction, and

- mounting surface having a second length and being substantially parallel to the outer surface direction,

so that the sealing surface and, accordingly, the mounting surface of the first edge of the first unit have been adjusted to be aligned against the sealing surface and, accordingly, against the mounting surface of the second edge of the second unit when arranging two units one on top of the other, and furthermore

- first guiding means have been arranged on the first edges of the outer surfaces, and

- second guiding means, arranged to be fitted to the said first guiding means, have been arranged on the second edges of the outer surfaces.

A person skilled in the art knows, that it is possible to use also other solutions to facilitate the stacking of the units and improving the sealing. It is also known, that the unit can contain the said guiding means, but the sealing has been arranged in a different way or vice versa. Similarly, it is not commented here whether the said means and surfaces are on the top or bottom edge of the unit seen in the completed assembled heat recovery tower.

The above-described guiding means facilitate the assembling of the units quickly and with planned accuracy. Opening this kind of structure is also easy, if such operation should sometimes be carried out. This kind of structure is also economical to manufacture.

If the configuration of the horizontal cross-section of the heat recovery tower is at least nearly rectangular, units with different capacities can be created especially by selecting the longer side of the rectangular in high capacity units to be fairly large in comparison with the other side. If the shorter side is e.g. about 2.45m long, which still can fit in a marine container, the longer side can be less than 6m, whereupon it can fit in a 20ft marine container. Larger units fit in 30 or 40ft marine containers. If, additionally, the outer wall panels of the unit forming a layer have been connected to each other only with welded joints, bolt joints and their problems - problems with fitting the seal, with tightness and with appearance in case of leaking - can entirely be avoided. A welded joint is also strong and simple in comparison with the double-flanged butt joints. In this way, also a fairly strong structure can be achieved by creating a continuous, closed circle. Sufficient dimensional accuracy of the structure can also be easily obtained.

A seal, such as a sealing strip, can also be arranged on the sealing surface. A tight and long lasting joint, with no previously common disadvantages, such as using screws and nuts through the seal, can be obtained with such sealing structure. If between two units a seal is used that compresses only along a chosen distance when the upper unit is lowered on top of the lower unit, a sealing solution is obtained with ideal seal functioning. Due to the design and manufacturing, compressing takes place to a suitable extent, and no excess compressing can occur. In such cases there is no need for so-called sealing glue or additives, such as, e.g. silicone paste, the use of which has many known disadvantages.

At least one service platform at a distance from the factory floor can also be arranged in the heat recovery tower according to the invention. Service and repairs can thus be carried out safely and quickly from the service platform without any separate structures or e.g. elevating platform equipment. At least one service platform of the heat recovery tower can be supported with the supports attached to the braces on the outer surface of the tower. Such service platform structural solution is simple, and at the same time a strong and solid securing is obtained. The service platform support can be secured with at least and preferably with only one bolt or similar, to the said brace, whereupon the assembly, and respectively, the detachment of the service platform can be carried out very quickly.

Adjustable bearer feet can be arranged to the bottom unit of the heat recovery tower, with which the heat recovery tower can be adjusted as well as possible to a desired position, usually in a horizontal position. In this way a bottom unit of the tower structure can be obtained, which is simple and quick to assemble and easy to arrange to a horizontal position in both directions with e.g. a spirit level. In that case, even the base can be at least somewhat uneven. It is also possible to arrange braces on the outer surfaces of the heat recovery tower, on which the guiding means of the upper unit are resting. In this way a structure can be obtained where the braces function not only as traditional reinforcing structural elements, but also as guiding means between the layers, greatly facilitating and speeding up the assembly.

If the heat recovery tower according to the invention extends above the roof, through a hole at the ceiling of the room, the floor space can be used efficiently, since most of the units are located at the upper part of the room, which space is not usually used to its full potential. It is often important, that the discharge outlet is on the roof and thus outside, without any special arrangements.

The object of the invention is also a unit arranged to be operating as a part of the heat recovery tower, and having substantially the form of a rectangular prism, comprising a first, second, third and fourth outer surface, the first and third of which being substantially of the same size and parallel, and the second and fourth being substantially of the same size and parallel, and a first and second at least partially open end, and inside which unit it has been arranged at least one passage for the gas to be led through the unit, said passage extending from the first end to the second end, to which passage at least one of the following devices has been operationally connected

- an air/air-heat exchanger,

- a scrubber,

- an air/water-heat exchanger.

According to one embodiment of the invention

- a mounting surface having a second length and being substantially parallel to the outer surface direction,

so that the sealing surface and, accordingly, the mounting surface of the first edge of the first unit have been adjusted to be aligned against the sealing surface and, accordingly, against the mounting surface of the second edge of the second unit when arranging two units one on top of the other,

It is possible to arrange, on the mounting surface of the unit according to the invention, mounting means known as such by a person skilled in the art, such as a hole and a screw and a nut fitting in the hole, or a rivet.

According to a preferred embodiment of the invention, the first guiding means is a guiding surface and the second guiding means is a guiding spline.

Naturally, the features described above in connection with the heat recovery tower also apply to the unit according to the invention.

Furthermore, the invention relates to a method for constructing a heat recovery tower for recovering the heat energy from the gas generated in an industrial plant, which method is characterized in that it consists of at least the following steps:

- designing a limited number of individual standard units that can be fixed together one on top of the other, whereupon designing at least: one or more bottom basin units, one or more air/air-heat exchanger units with a required operational capacity, one or more scrubber units and one or more air/water-heat exchanger units with a required operational capacity,

- defining the target characteristics of the heat recovery tower to be built, such as for example the heat energy recovery capacity, to be obtained by the heat recovery tower to be built,

- constructing the heat recovery tower by joining together a such number of pre- designed standard units that is required for to obtain the defined target characteristics.

Thus the invention provides for use a method with which the delivery-specific designing is reduced, the delivery time becomes shorter and the manufacturing costs become lower. Additionally, with the method according to the invention, a heat recovery tower can be constructed which is modular in the respect that a desired number of units can be arranged one on top of the other in order to obtain the desired operation, efficiency and height for each operating situation.

Naturally, the features shown above in connection with the heat recovery tower and the unit also apply to the method according to the invention.

The invention is described in more detail below with reference to the enclosed drawing in which

- Figure 1 shows as a schematic perspective view the heat recovery equipment according to the prior art,

- Figure 2 shows schematically the heat recovery tower according to an embodiment of the invention,

- Figure 3 shows as a schematic exploded perspective view the heat recovery tower and the exhaust air fan of Figure 2,

- Figure 4 shows as a schematic perspective view the elements standing on the floor according to an embodiment of the invention,

- Figure 5 a shows as a schematic perspective view and a partial cross-section a jointing structure between the units according to an embodiment of the invention,

- Figure 5b shows schematically a structure according to Figure 5a connected to each other and with seals,

- Figure 6 shows schematically a sealing structure in the corner of the upper part of the lower unit,

- Figure 7 shows schematically a so-called AHR-heat recovery unit when the honeycomb board is being changed and

- Figure 8 shows schematically the fixing of the service platform to the heat recovery tower.

In Figure 1 of the drawing, the reference number 1 denotes a known heat recovery equipment with which energy is recovered from the air or from some other gas, which is processed in such equipment usually at least several cubic meters per second, and in large units usually tens of cubic meters per second. The gas to be processed is brought through a large pipe indicated by an arrow 2, to a preprocessing equipment 3 in which there are usually filters, de-watering equipments and at least one heat exchanger, where a warm intermediate agent heats some intermediate agent flow, for example in order to heat the paper machine room during a cold season. Reference number 4 denotes an electric motor operated fan blowing the gas out of the equipment. A heat exchanger 5 recovers the heat and finally the gas is blown through a pipe up to the factory roof, the pipe extending high enough in the direction of an arrow 6. The equipment has been built on steel pipe frames 7 and 8 standing on the floor. Also the fan 4 and large pipes 2 and 3 are supported on the floor. Thus, the floor has several points of base on a wide area, usually more than ten, for the heat recovery equipment 1. In some other known solutions, the fan is located on the factory roof to suck air through the equipment.

A heat recovery tower 9 according to an embodiment of the invention and shown in Figure 2, has been arranged on the factory floor 10 and the tower 9 extends up to the factory ceiling 11. Usually the distance between the floor 10 and the ceiling 11 is 5 to 10 m, but it can be more, even over 12 m. The heat recovery tower 9 has a rectangular horizontal cross-section and it has been constructed of module-type units made e.g. of steel plate, usually by welding stainless or acid-proof steel plates. There are units for different uses and of different sizes, to obtain the desired performance. Towers can be for example 1,70 m, 2,05 m and 2,45 m wide, whereupon even the widest of them can be transported by the normal means of transport, for example by road transport or marine container. Length of the modules can be standard, for example 4,20 m, whereupon no problems should rise during transportation, and 4,20 m is anyhow relatively large for several applications. The height can also vary, usually between 0,50 to 1,50 m. If there is enough space for transportation, two, even three units can be transported one on top of the other. It is clear, that the capacity of the unit is at least almost directly proportional to the section's horizontal cross-sectional area - and also to the height. For clarity, no unit lines have been drawn in Figure 2. The heat recovery tower occupies a maximum of 2,45 m x 4,20 m of floor space, if keeping to the above-mentioned dimensions. Naturally, other dimensions can also be used if necessary. On the factory floor 11, there is also the fan 4 run by an electric motor 12 elevated on a stand 13. A seal collar 14 is used in the roof through hole. Figure 2 also shows service platforms needed in the service and maintenance, and the connecting steps with railings. The units and their structures will be explained in more detail below.

Figure 3 shows an exploded view of the heat recovery tower 9 and the exhaust air fan 4 of Figure 2, showing the layout of the units in the tower and partly, the layout of the equipment by the tower, which are the exhaust air fan 4 with its electric motor 12 and stand 13. The lowermost unit of the heat recovery tower 9 is a bottom basin 17, located on the factory floor 10 on the feet 18, arranged preferably and substantially horizontally. It should be noted, that the mass of the heat recovery tower 9 is usually tens of tons. A reactive attenuator can be used as a spacer unit between the bottom basin 17 and the fan 4. Reference numbers 19a and 19b denote so-called CHR-cells i.e. air/air-heat exchangers. Their number can vary, usually at least one, typically two, even three or more can be chosen if so required. The next unit is a so-called scrubber unit 20 followed usually by two of so-called AHR-cells 21a, 21b, i.e. air/water -heat exchangers. Also their number may vary within the range of 1 to 3, even though even more can be chosen, if required. The next unit upwards in the tower 9 is preferably a scrubber unit 22, above which empty extension units 23a, 23b are arranged, with which the factory room ceiling 11 is reached. Above the roof a seal collar 14 is used. On top of this, can next be arranged a sound attenuator 24, above which there still is a reducer cone 15 and a pipe 16, through which the processed gas gets freely out to the atmosphere.

The effect/tower of these cells is typically for the CHR-cells usually 300 to 2000 kW, preferably 400 to 1500 kW, more preferably 500 to 1000 kW, and for the AHR-cells, usually 300 to 15000 kW, preferably 1000 to 10000 kW, more preferably 2000 to 5000 kW. These effects can usually be reached with one or more cells.

Figure 4 shows as a perspective view the elements arranged on the factory floor, which are the fan 4, the electric motor 12 on the stand 13, and the bottom basin 17 with its feet 18. In this embodiment, a reactive attenuator 25 has been located between the fan 4 and the bottom basin 17, the object of which is to muffle the noise generated by the fan. The reactive attenuator 25 has a damping chamber, separated with a perforated plate from the flow port in the middle. Usually a rather short reactive attenuator 25 is used, for example a device of about 0,7 m long in the direction of flow.

Figure 5 a shows as a perspective cut view a connecting structure between the units according to an embodiment of the invention, before the top unit 27 is lowered to its place. Reference number 26 denotes the lower unit with the wall plate bent outward horizontally by a distance, for example 5 cm, forming a sealing surface, and from there upwards by a distance, for example also 5 cm, forming a mounting surface. The lower part of the upper unit 27 is first bent outwards almost horizontally, for example less than 5 cm, forming a sealing surface, and from there upwards, for example 4 cm, forming a mounting surface. Bottom part of the upper unit has guiding splines 38, functioning as guiding means during the assembly of the upper and lower blocks. The guiding spline 38 rests on the guiding surfaces of the side shoring profile 33. When all the vertical joints of the unit are welded, there is no need at all for vertical flanged joints. Thus, also the crossing flanged joints can be avoided, the sealing of which has been known to be problematic. Previously there has been need to use sealing compound, for example silicone mixture on such places, the use of which is no longer necessary in structures according to the invention.

In Figure 5b, the upper unit and the lower unit are already connected to each other, that is they are in place. The weight of the upper unit ensures that the seal 28 is pressed so, that the seal 28 is pressed to become about 20 to 35% thinner than the original thickness, whereupon the seal 28 functions as a sealing means in a proper way. Further pressing is prevented with a structural solution so, that the ends of the side shoring profiles 33 meet when the pressure of the seal 28 is suitable. In the solution according to this embodiment of the invention, the said seal remains well in place during assembly, because during assembly the upper unit is lowered on top of the lower unit and the seal is not subjected to any lateral load. Since there are no fixing screws on the sealing surface or near it, the fixing screws will not press the seal, as often was the case in previous structures, meaning that the structure is hardly at all deformed, and thus, there is no increase of possibilities for leaking.

Finally, a cotter bolt (not shown) or similar, is passed through holes 31a and 31b to lock the two units together.

Figure 6 shows the corner structure and seals 28 of the upper part of the lower unit 26. The salient corner of the unit has a vertical welded corner shoring profile 32. The corners of the seal 28 have been cut to an angle of 45 degrees and they are cured together to an angle of 90 degrees. It is recommended, that the seal be taped in place with, for example, a two-sided tape during manufacturing, whereupon the seal is in place and secured both during transportation and assembly, since the flange prevents the seal from damaging. It should be noted, that the flange forms also a rather stiff structure, whereby the flange has a clearly stiffening effect on the unit.

Figure 7 shows a so-called AHR-unit i.e. the air/water-heat exchanger 21a during the changing of cells. The unit 21a has the corner shoring profiles 32 according to the invention on the outer surface of all four corners, and also all sides have vertical side shoring profiles 33. Energy is recovered from the heat energy contained in the gas flowing upwards in the AHR-unit by heating the water, that is circulated in the heat exchange plate of the heat exchanger. The AHR-unit has at its end a bypass manifold 34 for introducing the water to the heat exchanger plates, of which one plate 35 has been taken out for replacement through a service opening 36b at the end of the AHR-unit. The number of plates is usually several dozens in a single AHR-unit. Other service openings 36a and 36c are located at the areas between the side shoring profiles of the end. For the start up, service hatches 37a to 37c are fitted to tightly cover the service openings 36a to 36c. It should be noted, that the servicing can be carried out through the openings 36a to 36c at the end of the unit without having to dissemble the entire heat recovery tower. Figure 7 shows also the guiding splines 38, provided at the end of each corner shoring profile 32 and side shoring profile 33. In Figure 7, they are located at the bottom, but they can also be installed on the top. The object of the guiding splines 38 is to help during assembly, since when the spline 38 meets the corresponding hole of the shoring profile of the lower unit, the upper unit will be nicely guided in its place and also the seal areas fall correctly in their place. Since the unit is several meters long, supporting bars 39 are used for keeping the sidewalls at the right distance from each other. Also crossing support means, such as draft bars (not shown) can be used when required, in order to keep the rectangular horizontal cross-section.

Figure 8 shows the fixing of the service platform support to the side shoring profile 33. Usually it is necessary to provide some sides and some levels of the heat recovery tower 9 with service platforms, which facilitate inspection, repairing and servicing work. The side shoring profiles 33 are preferably strong enough to bear the stresses caused by the service platforms and service men. The service platform support 40 is made, for example of steel plate by cutting and bending and thus of only one piece. An upper boom 41 can by definition be of a desired length, for example 0,7 m to bear a sufficiently wide service platform. The upper boom 41 is, from its vertical parts, for example bolted with nuts through a hole 42 to the side shoring profile 33 and a lower boom 43 rests on the same side shoring profile 33, somewhat, for example 0,3 m, lower. The lower boom 43 is slanting upwards away from the unit so that at the outer end of the support 40 the vertical distance between the lower boom 43 and upper boom 41 is about 0,15 m. Near the said outer end, the upper boom 41 preferably has a hole 44 for fixing the railing post. The presented railing structure is very easy to manufacture and erect as well as solid and strong. It is also very quick to dismantle if so required. The service platform itself can be made of grille plate or the like, known as such. The heat recovery tower 9 according to this embodiment of the invention operates in such a way that the fan 4 moves the gas through the pipes to the recovery process and in the process. The fan 4 noise is reduced by using an attenuator 25. Into the heat recovery tower 9, all the desired units have been assembled one on top of the other, including for example heat exchangers, scrubbers, water collecting plates, entrainment separators, and, if required, extension pieces. In addition, a sound attenuator and a reducer cone with its pipe to open air can optionally be arranged on top of the tower. The heat recovery tower can also consist of at least one separation equipment for separating water from the moist air or steam led through the heat recovery tower. With the above-mentioned tower dimensions, the amount of passing air is usually 35 to 50 m /s and the exhaust air mass flow rate is usually 20 to 40 kg/s of dry air. The mass of the tower is typically 15 to 50 tons. The solution according to the invention can thus save in the gross expenditure of a factory project and in the manufacturing, transporting and assembly costs of the tower, typically 20 to 50% compared with the prior art. The heat recovery tower 9 is constructed of standard components, whose testing has already been carried out in the factory.

The invention is not limited to the accompanying embodiment, but several modifications can be thought of in the scope of the appended claims. The reference numbers should not be regarded as restrictive to the claims.

Claims

1. A heat recovery tower for recovering heat energy from gas generated in an industrial plant, characterized in that the heat recovery tower (9) comprises a number of units arranged one on top of the other and detachably connected to each other, with at least two of them having been selected from the group:

- a bottom basin (17),

- an air/air-heat exchanger unit (19a, 19b),

- a scrubber unit (20, 22), and

- an air/water-heat exchanger unit (21a, 21b).

2. A heat recovery tower according to claim 1, characterized in that the heat recovery tower (9) comprises at least the following units, arranged one on top of the other, seen from the bottom upwards

- a bottom basin (17),

- one or more air/air-heat exchanger units (19a, 19b),

- one or more first scrubber units (20),

- one or more air/water-heat exchanger units (21a, 21b), and

- one or more second scrubber units (22).

3. A heat recovery tower according to claims 1 or 2, characterized in that there are 1 to 5 pieces of the said air/air-heat exchangers (19a, 19b).

4. A heat recovery tower according to any of the preceding claims, characterized in that there are 1 to 5 pieces of the said air/water-heat exchangers (21a, 21b).

5. A heat recovery tower according to any of the preceding claims, characterized in that there is one of said first scrubber units (20) and that there is one of said second scrubber units (22).

6. A heat recovery tower according to any of the preceding claims, characterized in that in addition, it has one or more extension units (23a, 23b).

7. A heat recovery tower according to any of the preceding claims, characterized in that in their horizontal cross-section the said units are substantially identical in shape and size.

8. A heat recovery tower according to any of the preceding claims, characterized in that a unit is substantially in the shape of a rectangular prism, comprising a first, second, third and fourth outer surface, with first and third outer surface being substantially of the same size and parallel, and second and fourth outer surface being substantially of the same size and parallel, and where

- a sealing surface having a first length and being aligned perpendicular to the outer surface, and

- a mounting surface having a second length and being substantially parallel to the outer surface,

so, that the sealing surface and, accordingly, the mounting surface of the first edge of the first unit have been adjusted to be aligned against the sealing surface and, accordingly, against the mounting surface of the second edge of the second unit, when arranging two units one on top of the other, and where

- second guiding means, arranged to be fitted to the first guiding means, have been arranged on the second edges of the outer surfaces.

9. A heat recovery tower according to claim 8, characterized in that in addition a sealing (28), such as a sealing strip, has been arranged on the sealing surface.

10. A heat recovery tower according to any of the preceding claims, characterized in that the heat recovery tower extends at least from the floor (10) to the ceiling (11) of an industrial plant.

11. A heat recovery tower according to any of the preceding claims, characterized in that in the heat recovery tower (9) at least one service platform has been arranged at a distance from the floor (10) of the industrial plant.

12. A unit arranged to be operating as a part of a heat recovery tower, characterized in that the unit has substantially the shape of a rectangular prism, comprising a first, second, third and fourth outer surface, the first and third of which being substantially of the same size and parallel and the second and fourth being substantially of the same size and parallel, and a first and a second, at least partially open end, and that inside the unit it has been arranged at least one passage for the gas led through the unit, the passage extending from the first end to the second end, to which passage at least one of the following devices has been operationally connected

- an air/air-heat exchanger,

- a scrubber,

- an air/water-heat exchanger.

13. A unit according to claim 12, characterized in that

14. A unit according to claim 13, characterized in that mounting means have been arranged on the mounting surface.

15. A unit according to claim 13 or 14, characterized in that the first guiding means is a guiding surface.

16. A unit according to any of the claims 13 to 15, characterized in that the second guiding means is a guiding spline

17. A method for constructing a heat recovery tower for recovering the heat energy from the gas generated in an industrial plant, characterized in that the method consists of at least the following steps:

- designing a limited number of individual standard units that can be fixed together one on top of the other, whereby designing at least: one or more bottom basin units (17), one or more air/air-heat exchanger units (19a, 19b) with a required operational capacity, one or more scrubber units (20) and one or more air/water-heat exchanger units with a required operational capacity,

- defining the target characteristics of the heat recovery tower to be built, such as for example the heat energy recovery capacity to be obtained by the heat recovery tower to be built,

- constructing the heat recovery tower by connecting a such number of pre-designed standard units that is required for to obtain the defined target characteristics.