The present invention relates to heat exchangers.

More specifically, the invention is intended to obtain spiral or helical heat exchangers that make use of enamelled steel.

The useful properties of enamelled steel are generally known, such as a high corrosion resistance, high resistance to wear and a high chemical resistance.

The use of enamelled steel in heat exchangers is also known on account of the above-mentioned qualities and also because such surfaces of enamelled steel are maintenance- friendly and resistant to high temperatures. Moreover, enamelled steel is thermally efficient for heat conduction due to the thinness of the ceramic layers .

The use of double-sided enamelled and corrugated steel plate is standard in air preheaters and gas-gas heat exchangers in industrial processes, such as in a desulphurisation installation for combustion gases.

These heat exchangers take on the form of large cages that are filled with corrugated double-sided enamelled steel with a large contact area with the gas with which it is brought into contact. The heat exchangers consist of a number of cages filled with enamelled sheet steel, which together yield a heat exchanging area of 30, 000 . In this application the enamelled steel is exposed to corrosion by the corrosive flue gases, and it must be chemically resistant but also a good thermal conductor.

These heat exchangers are of the regenerative type, which means that they will absorb heat for a certain time from a gas flow that is carried across half of the heat exchanger, after which this half is rotated away and cooled in another gas flow, until it has sufficiently cooled in order to be used again for the absorption of heat from the first gas flow, which is obtained by a subsequent rotation.

A typical example was described by A. Chelli et al. in XXI International Enamellers Congress, 18-22 May 2008 in Shanghai, p. 126-154. In this example two rotary heat exchangers with enamelled steel are applied as a heat exchanger in the same industrial desulphurisation process for flue gases.

A disadvantage of these heat exchangers with corrugated double-sided enamelled sheet steel in the current form is that they cannot be used as a counterflow heat exchanger in a continuous heat-exchanging process.

Another disadvantage of these heat exchangers is that they expose the corrugated double-sided enamelled sheet steel to frequent high temperature fluctuations on account of their regenerative function. Another disadvantage of these heat exchangers is that they are not static and thereby present a greater risk of mechanical failure and a lower thermal efficiency than static heat exchangers.

Among the static heat exchangers, the counterflow heat exchangers in particular are very thermally efficient.

In this application a hot fluid (gas or liquid) is guided through a heat exchanger in one direction and a cold fluid in the other direction _/ separated by a thermally conductive wall, through which the hot fluid transfers heat to the cold fluid.

These counterflow heat exchangers are even more thermally efficient if, instead of flat chambers that are separated by a flat wall, they consist of a first spiral or helical chamber through which a first fluid flows, which is surrounded along both sides by a second spiral or helical chamber through which a second fluid flows in the opposite direction, separated by spiral walls between the two flow directions .

For such applications, the known corrugated double-sided enamelled steel plate is not suitable for a partition wall, because it is not flat and moreover cannot be wound in a spiral or helix.

For such applications on the other hand thin flexible double-sided enamelled steel plate is indeed a suitable material, on account of its malleability, thermal conductivity and its corrosion-resistant surface.

The purpose of the present invention is to provide a solution to the aforementioned and other disadvantages, by providing a spiral or helical counterflow heat exchanger that makes use of flat thin double-sided enamelled steel plate .

To this end the invention concerns a spiral or helical counterflow heat exchanger consisting of two adjoining chambers, in which a fluid at a high temperature flows in one chamber in one direction, and in which a fluid at a lower temperature flows in the opposite direction in the other chamber, whereby both chambers are separated by one separating plate of monolithic double-sided enamelled flat steel annealed at temperatures above 500°C, and whereby the separating plate is held by its edges in a corrosion- resistant spacer that imposes a fixed distance to two other monolithic double-sided enamelled flat steel plates that each define one chamber at the side that is opposite the separating plate, and which prevents corrosion of the edges of the separating plate and of the two other enamelled steel plates.

An advantage of such a counterflow heat exchanger is that the thermally conductive wall between the two chambers is enamelled on both sides and is smooth, which protects the wall surface against corrosion, but also makes the wall maintenance-friendly because it is smooth and easy to clean. Another advantage is that such a thermally conductive wall is very thermally efficient and can also be produced at a low cost.

Another advantage of such a thermally conductive wall is that it can be very long, as the double-sided enamelled steel plate can be produced in long continuous bands, whereby a total length of approximately 150 metres is possible .

An additional advantage of such a heat exchanger is that the steel plate is already enamelled before assembly of the heat exchanger, such that no complex shapes such as spiral or helical heat exchangers have to be enamelled. The exceptional flexibility of the thin enamelled sheet steel enables the heat exchangers to be assembled after enamelling, which greatly simplifies their production.

A specific advantage of this type of counterflow heat exchanger is that the flow can proceed unimpeded because the surfaces of the double-sided enamelled partition walls between the chambers are completely flat and smooth and do not offer any resistance to a fast flow of the two fluids.

An advantage of such a spacer is that it not only protects the edges of the double-sided enamelled steel plate that are the most vulnerable to corrosion, but it also ensures that the two enamelled steel plates that define the chamber of the heat exchanger are at the same distance from one another everywhere. Another type of corrosion-resistant spacer with which a stack of flat double-sided enamelled steel plates can be separated consists of beam-shaped or round strips of Teflon or another chemically inert material/ which extend in the flow direction of the fluids between two flat double-sided enamelled steel plates stacked parallel to one another/ and are so arranged that the edges of the steel plates do not come into contact with the content of the flow chambers created, and such that the edges are not susceptible to corrosion from corrosive fluids. Only the inside of the chambers, which are defined by enamelled steel and Teflon or another chemically inert material, come into contact with the fluids.

In a preferred embodiment two chambers are formed by rolling up two monolithic flat double-sided enamelled but flexible steel plates into a spiral, so that two spiral chambers are created that are always separated by one single-walled but double-sided enamelled steel plate, whereby the first chamber opens out to a central pipe in the centre of the heat exchanger and the second chamber opens out to another central pipe and whereby a fluid at a lower temperature flows through the first chamber in the centripetal direction to then flow directly towards the outside again along one side of the heat exchanger via the first central pipe, and whereby a fluid at a higher temperature flows through the second chamber in the centrifugal direction and opposite to the flow direction in the first chamber via the other central pipe that is supplied by a supply of the hot fluid to be cooled along the other side of the spiral heat exchanger, and whereby the two flow directions are only separated by one single separating plate of double-sided enamelled steel through which the heat of the hotter fluid is transferred to the colder fluid.

An advantage of such a spiral counterflow heat exchanger is its compactness, whereby an elongated contact surface of the thermally conductive enamelled steel plate can nevertheless be built into a limited space.

Another advantage of such a spiral counterflow heat exchanger is that only one pair of long flexible double- sided enamelled steel plates is needed to enable one elongated chamber with hotter fluid flowing in one direction to exchange heat with one elongated chamber with the cooling fluid flowing in the opposite direction.

An additional advantage of such a spiral counterflow heat exchanger is that its operation is more thermally efficient due to its compactness, because there are fewer heat losses that cannot be absorbed by the counterflow.

Preferably the edges of the outermost wall and the innermost wall of the first chamber are held in a spiral corrosion-resistant spacer that prevents corrosion of the steel plates at the level of their edges.

An additional advantage of such a spacer is that it not only ensures a constant distance between the windings of the spirally wound double-sided enamelled steel plates, but also imposes the correct curve of the plates for the spiral shape.

The spiral heat exchanger can also be provided with another type of spacer that consists of beam-shaped or round strips of Teflon or another chemically inert material, that extend in the flow direction of the fluids between two spiral double-sided enamelled steel plates wound above one another and are arranged such that the edges of the steel plates do not come into contact with the content of the flow chambers 10, 11 created, which prevents corrosion of these edges.

Another preferred embodiment of the counterflow heat exchanger is the helical counterflow heat exchanger, constructed from three flexible double-sided enamelled steel plates that define two chambers and are wound helically around a central longitudinal axis. A first fluid is guided by the first chamber 10 and a second fluid is guided in the opposite direction by the second chamber 11. A helical spacer 15 imposes the mutual distance and the curve of the windings in the enamelled steel plates.

This helical counterflow heat exchanger can also be provided with an additional type of spacer that consists of beam-shaped or round strips 8' of Teflon or another chemically inert material, that extend in the flow direction of the fluids between the three helical double- sided enamelled steel plates wound around one other, and are arranged such that the edges of the steel plates do not come into contact with the content of the flow chambers 10, 11 defined by the beam-shaped or round strips 8' . An advantage of this helical counterflow heat exchanger is that it is of a compact form and can be built around a central cylindrical space, while the inside surface of the flow chambers remains seamless, and enables an unhindered flow of the fluids. The inert and smooth inside surface of the chambers also enables better maintenance, by regularly washing these spaces with cleansing agents suitable for this purpose.

With the intention of better showing the characteristics of the invention, a few preferred embodiments of counterflow heat exchangers according to the invention are described hereinafter by way of an example, without any limiting nature, with reference to the accompanying drawings, wherein:

Figure 1 schematically shows a cross-section of a set of corrugated double-sided enamelled steel plates in a regenerative heat exchanger according to the state of the art;

figure 2 schematically shows a cross-section of a set of flat double-sided enamelled steel plates in a counterflow heat exchanger according to the invention; figure 3 schematically shows a perspective view of a stack of flat double-sided enamelled steel plates held in spacers;

figure 4 shows a variant of figure 4 with a different type of spacer;

figure 5 schematically shows a cut-away perspective view of a spiral counterflow heat exchanger of flexible double-sided enamelled steel according to the invention;

figure 6 shows a variant of figure 5 with a different type of spacer;

figure 7 shows a helical counterflow heat exchanger consisting of three double-sided enamelled flexible plates, between which there are two chambers, that are wound helically around a central axis;

figure 8 shows a variant of figure 7 with a different type of spacer.

Figure 1 schematically shows a cross-section of a number of corrugated double-sided enamelled steel plates, as used in cages for regenerative heat exchangers in the current state of the art. In this case, a cold-rolled corrugated steel plate 1 that is enamelled on both sides is alternated with a flat double-sided enamelled steel plate 2.

Figure 2 schematically shows a cross-section of the simplest counterflow heat exchanger 3 according to the invention, consisting of two chambers A, 5 through which two fluids at different temperatures flow in opposite directions, that are separated from one another by a thin flat double-sided enamelled steel plate 6.

Figure 3 schematically shows a perspective view of a counterflow heat exchanger 7, consisting of a stack of flat double-sided enamelled steel plates 6, whose edges are held in corrosion-resistant spacers 8. Figure 4 shows a variant of figure 3 whereby a stack of flat double-sided enamelled steel plates 6 are separated by another type of spacer 8' that consists of beam-shaped or round strips of Teflon or another chemically inert material, that extend in the flow direction of the fluids between two flat double-sided enamelled steel plates 6 stacked above one another, and are arranged such that the edges of the steel plates do not come into contact with the flow chambers created and such that the edges are not subject to corrosion by corrosive fluids. Only the inside of the chambers that are defined by enamelled steel and Teflon come into contact with the fluids.

Figure 5 shows a spiral counterflow heat exchanger 9 that is formed from a set of two flat double-sided enamelled steel plates 6, 6' , that are flexible and which are wound at an equal distance from one another to form two spiral chambers 10, 11, whereby the first chamber 10 opens out to a central pipe ^" 12a in the centre of the heat exchanger and the second chamber 11 opens out to another central pipe 12b, whereby a fluid at a lower temperature flows through chamber 10 in the centripetal direction to then flow again directly to the outside along one side of the heat exchanger via the central pipe 12a, and whereby a fluid at a higher temperature flows through chamber 11 in the centrifugal direction and opposite to the flow direction of chamber 10 via another central pipe 12b that is supplied by a supply of the hot fluid to be cooled along the other side of the spiral heat exchanger, and whereby both flow directions are only separated by one double-sided enamelled steel plate 6 or 6' , through which the heat of the hotter fluid is transferred to the colder fluid.

Figure 6 shows a variant of figure 5 whereby the spiral heat exchanger is provided with spacers that consist of beam-shaped or round strips 8' of Teflon or another chemically inert material, that extend in the flow direction of the fluids between two spiral double-sided enamelled steel plates 6, 6' 6" wound above one another, and are arranged such that the edges of the steel plates do not come into contact with the content of the flow chambers 10, 11 created, which prevents corrosion of these edges.

Figure 7 shows a helical counterflow heat exchanger 9' made up of three flexible double-sided enamelled steel bands 6, 6' 6" that define two chambers 10, 11 and are wound helically around a central longitudinal axis 14. A first fluid is guided through the first chamber 10 and a second fluid is guided in the opposite direction through the second chamber 11. A helical spacer 15 imposes the mutual distance and the curve of the windings in the enamelled steel plates.

Figure 8 shows a variant of figure 7, whereby the same helical counterflow heat exchanger is shown, but is now provided with an additional type of spacer that consists of beam-shaped or round strips 8' of Teflon or another chemically inert material, that extends in the flow direction of the fluids between the three helical double- sided enamelled steel plates 6, 6' , 6" wound around one another, and are so arranged that the edges of the steel plates do not come into contact with the flow chambers 10, 11 defined by the beam-shaped strips 8' .

The operation of the counterflow heat exchanger according to the invention is very simple and as follows.

For the flat embodiment of the counterflow heat exchanger 7, constructed of long flat double-sided enamelled steel plates 6, a fluid (gas or liquid) is carried in one direction through chambers, while these chambers are separated from other chambers through which a fluid at a lower temperature is carried in the opposite direction.

The two chambers are only separated from one another by one common separating plate of flat double-sided enamelled steel, whose thermally conductive properties are so good that the heat of the fluid at a higher temperature is quickly emitted to the fluid at a lower temperature.

The length of the flat heat exchanger can be adapted to the desired temperature drop due to cooling or to the available space, because the bands of double-sided enamelled steel can be seamlessly produced in lengths of up to 150 m.

The hotter and colder fluid can consist of a gas and/or a liquid phase of the same substance or of two different substances. The high corrosion-resistance of the enamelled plates also enables chemically corrosive fluids to be sent through the heat exchanger. For the spiral type of counterflow heat exchanger 9 only two flexible double-sided enamelled steel plates 6, 6' are used, between which two chambers 10, 11 are created by both steel plates being held by the edges in a spacer (not shown in the drawing) , that not only ensures a constant distance between the two plates 6, 6' , but also keeps them in the right curve in order to wind up the two chambers 10, 11 such that they run into two central pipes 12a, 12b that carry the cooling fluid and the fluid to be cooled to the outside and to the inside respectively.

The cooled fluid is carried to the outside and the cooling fluid to the inside via the outermost surface of the spiral heat exchanger.

It goes without saying that the cooling fluid and the fluid to be cooled can be swapped over.

For the helical embodiment 9' of the counterflow heat exchanger, only three flexible double-sided enamelled steel plates 6, 6', 6" are used, between which two chambers 10, 11 are created by holding the steel plates by the edges in a corrosion-resistant spacer 14, that not only ensures a constant distance between the three plates 6, 6', 6", but also keeps them in the right helical shape in order to wind up the chambers 10, 11 such that the windings lie against the overlying windings and both chambers 10, 11 run into the other end of the helical counterflow heat exchanger.

The hotter fluid is guided through the first chamber 10 in a first flow direction, while the colder fluid is guided through the second chamber 11 in a flow direction opposite to the first flow direction of the hotter fluid. Both chambers 10 and 11 are only separated from one another by one single separating plate of flexible double-sided enamelled steel through which the hotter fluid transfers heat to the colder counterflow of the second fluid that flows into the counterflow heat exchanger at the opposite end of the helical heat exchanger to the first fluid, and flows out again at the same end where the first fluid flows in.

Due to its compact construction the helical counterflow heat exchanger 9 saves space, but nonetheless provides the possibility to exchange heat over a long and smooth enamelled steel band.

It goes without saying that the second fluid can also consist of the first fluid that has already been partially cooled at the bottom of the helix and flows out of the first chamber and is fed back through the second chamber to the top of the helix.

The present invention is by no means limited to the embodiments described as an example and shown in the drawings, but a counterflow heat exchanger according to the invention can be realised in all kinds of forms and dimensions, without departing from the scope of the invention as defined in the claims.