Field of Invention The present invention relates to a method and an apparatus for removing deposits from the walls of process equipment. Particularly the invention relates to a method and an apparatus for removing deposits collected on the walls of equipment used in the treatment of hot process gases. More specifically expressed, the present invention relates to a method described in the preamble of the first patent claim. By this kind of method surfaces which come in contact with various gases and are thus become fouled may be kept clean efficiently.

The present invention relates also to an apparatus defined more closely in the preamble of patent claim 8.

Background art Heat transfer is very common as a gas treatment stage; gases are either heated or cooled. For this purpose heat exhangers are used to effect transfer of heat between the gas and a heat transfer medium. Irrespective of the process in question, heat transfer surfaces are usually easily fouled since the gases are very seldom so clean that they do not foul the heat exchange surfaces at all while passing them. In the same way as the heat transfer surfaces of actual heat exchangers, also other surfaces such as the walls of flow channels which come in contact with gas may be fouled. This fouling of other surfaces may also be undesirable because they also may be constructed to serve as surfaces to be cooled or generally heat transfer surfaces. In some cases particularly the gas inlet conduit of the chamber in which the cooling of the gas is effected is very prone to collect deposits.

It is of particular importance in the devices mentioned above

to prevent clogging of gas passages and to keep clean the various parts, such as heat surfaces, inlet ducts and/or openings and walls, of different gas coolers and reactor chambers used for recovering heat from hot gases produced in different industrial processes. Particularly prone to foul are e.g. processes in which the hot process gases contain components such as vaporized or molten substances which during cooling of the gas turn sticky, for example by condensation or sintering. This kind of units are used for example in metal smelteries and municipal waste incineration plants.

Many kinds of apparatus and methods have been suggested for removing the undesired layer from the surfaces of gas cooling devices. For example U.S. patent 4,765,394 discloses a device for cleaning heat transfer surfaces in which the heat transfer surface is made of parallel cooling tubes. The supply of cleaning medium has been arranged so that the cleaning medium supplied at one wall is sprayed substantially onto the opposite wall. In this kind of arrangement remarkable amounts of cleaning agent must be used and also the feed must be arranged intensive enough to extend its cleaning effect onto the wall to be cleaned. Also U.S. patent 4,617,988 discloses a solution for keeping the surfaces clean. The essential feature in the methods disclosed in both the U.S. publications is that the cleaning medium is supplied explicitly onto the outer surface of the desposit on the wall substantially opposite to the supply nozzle; thus the cleaning medium is not brought directly to the boundary layer between the deposit and the carrying surface in which the adhering of the deposit has taken place but to the outer surface of the deposit.

EP 0 291 115 discloses a gas cooler in which hot product gas is cooled in a cooler having a porous wall and next to this inside the cooler a flexible wall. When cooling medium is supplied through the porous wall to the space between the porous wall and the flexible wall the medium vaporizes and

causes the flexible wall to bulge and the deposit to be detached from the flexible wall. The solution described here is, however, complicated and requires special materials in order to ensure reliable operation. Special materials and constructions are in most cases very expensive and may also be easily broken.

It has also been suggested to use various hammers to remove the deposits. Usually these require, however, a remarkable space outside the wall. In most cases, and in particular in case of a retrofit plant, the space is very limited and the positioning of hammer devices is thus very difficult. This kind of devices also impose certain strength and structural requirements on the process equipment and their effect cannot easily be limited very precisely.

For example CA patent 1,265,784, US patent 5,029,556 and FI patent 84841 dislose a few exemplary applications of the present invention; the above publications are here referred to by way of example in order to clarify the broad field in which the present invention is applicable.

Known methods and apparatus have several drawbacks and disadvantages which may be avoided by the method and apparatus of the present invention.

Disclosure of Invention

Thus the object of the invention is to provide a new method and apparatus distinguishing essentially from the prior art for detaching deposits from and for preventing the formation of deposits on the surfaces of process equipment.

It is also an object of the invention to provide a method and an apparatus for detaching deposits which is safer, more efficient, and more economical than known methods.

A further object of the invention is to provide a method and an apparatus for detaching deposits which is applicable in many kinds of processes and apparatus.

The present invention is based on the idea of first introducing detaching medium between the surface of the equipment and the boundary surface of the deposit and then detaching a deposit from the surface by providing a force pushing the surfaces apart, the force preferably being caused by an increase in the volume of the detaching medium, by a change in the temperature of the boundary surface or by a so- called cylinder force of the pressure of a liquid/gas, or by for example a hydraulic device.

Expressed more precisely, the method of the invention is mainly characterized by what is defined in the characterizing portion of the appended patent claim 1.

The apparatus according to the present invention is mainly characterized by what is defined in the characterizing portion of the appended patent claim 10.

The term boundary surface means in this context the area which is formed by a carrying surface such as the surface of the wall of for example a heat exchanger or some other equipment, and by the deposit collected thereon, and on which said carrying surface and the materials of the deposit contact or are in such close proximity with each other that the adhering mechanism of the deposit can be acted on by means of a medium and/or a mechanical force.

In this context the term medium means, if not otherwise defined, any substance or mixture performing the task assigned to it in this application; the substance or mixture may consist for example of several phases or substances, it may be liquid or gas or a mixture of these. The medium may be

inert or in the conditions of the boundary surface reactive, for example a combustible substance or mixture.

According to a first preferred embodiment of the invention the equipment surface which may comprise for example flow ducts such as tubes connected to each other by fins may be kept clean by introducing medium to the boundary surface of the surface of the equipment and the deposit so that the medium is in direct contact with both the surface and the deposit and that the behaviour of the medium in the conditions prevailing in the boundary surface results in conditions being created in the boundary surface which conditions produce a force pushing the surface of the equipment and the deposit apart from each other. This particularly applicable method is very effective especially in cases in which the deposit is porous whereby the medium penetrates deeper in the deposit. This preferred embodiment of the invention may be practiced by providing in the wall, which preferably consists of tubes connected to each other with fins to form a gas-tight structure, the fins connecting the tubes have been provided with means for transporting the medium through the fin. Naturally it is clear that the idea of the present invention may be applied also to other types of wall structures than to the one described above. In order to intensify the effect of the medium also mechanical or hydraulic devices may be used.

According to another preferred embodiment of the invention the force detaching the deposit and pushing the surface apart may be provided by introducing medium to the area to be cleaned in the boundary surface of the deposit and the surface of the equipment; the medium may be chosen such that it vaporizes in the conditions of the boundary surface and thereby increases in volume and consequently produces an effect resembling an explosion whereby the deposit is detached from the surface. The medium may also be a mixture which in the conditions prevailing in the boundary surface reacts either in an

endothermic or an exothermic way with the result that the sudden change in the pressure and/or temperature thereby caused produces an explosion-like effect detaching the deposit from the surface.

According to yet another preferred embodiment of the invention the force detaching the deposit and pushing the surface apart may be provided by introducing medium to the area to be cleaned in the boundary surface of the deposit and the surface of the equipment at such pressure that the pressure is able to create a force strong enough to detach the deposit. The detaching effect may also be produced by arranging by means of the medium the conditions in the boundary surface such that deformations take place in the boundary surface of the deposit which provide the required detaching force. This is preferably accomplished by a sudden change in the boundary surface temperature which may be brought about by evaporation of the medium.

According to the invention the medium required for the detaching is preferably introduced directly in contact with the boundary surface formed by the deposit and the surface of the equipment by a method allowing even distribution of the medium. Preferably the supply of the medium is effected so that the cross-sectional area of the medium flow increases in the flow direction thereof in the vicinity of the boundary surface. This may be effected for example with a feeding means the feeding conduit of which expands just before the boundary surface when approaching the deposit. Thereby the deposit possibly collected in this area during the periods between the cleaning cycles is easily detached at the beginning of or during the cleaning cycle. Also in this embodiment mechanical or hydraulic devices may be used with the medium.

According to another embodiment of the present invention the supply of the medium is effected via a nozzle, which permits

introduction of the medium into the boundary surface, but closes immediately after the supply has ceased. In other words, the nozzle operates as a back-flow barrier preventing the entry of the deposit material into the medium supply duct while the introduction of the medium is not activated. That way the present invention operates most reliably without a risk of clogging the supply nozzle and/or duct. With this aspect of the invention it is possible to operate the system as often, only, as needed without the risk of the nozzles getting clogged while not being used. This feature remarkably diminishes the amount of detaching medium used.

The nozzle in the introduction duct provides thus a unidirectional flow. The nozzle is preferably provided with means closing the nozzle when the nozzle is not being used. The nozzle functions so that it remains closed while the pressure in the duct does not exceed a certain level, above which the medium opens the nozzle. This is accomplished by providing the nozzle with a spring-loaded needle in which the spring functions as a counter force for the pressure of the medium. With this aspect of the invention it is ensured that operation of the apparatus is reliable even if deposits are accumulated on the nozzle itself.

According to yet another embodiment of the invention the force detaching the deposit may be provided by combining mechanical thrust and one of the detaching mechanisms produced by a detaching medium described above. In this case medium is supplied to the boundary surface and after, simultaneously with or before the feeding of the medium the surface of the deposit in the immediate vicinity of the boundary surface is subjected to a thrust whereby the combined effect of the forces is very strong. This kind of mechanical thrust may be provided for example by bringing a member transmitting power from a source of power through the wall directly against the surface of the deposit. The source of power may be for

example a device utilizing pressurized substance and yielding mechanical work such as a hydraulic or a pneumatic cylinder or a device driven by electricity. The power is transmitted to be directed to the deposit preferably by a pin or a rod.

The present invention provides for example the following advantages over the prior art: the cleaning effect may be directed directly to the boundary surface; the equipment required for carrying out the method are very simple, reliable in operation and their materials and constructions are economical and do not impose special requirements on the process; the method does not disturb the process; the apparatus according to the invention is easy to construct; the mediums used according to the method are easily available and safe to handle; the cleaning operation may easily be limited and adjusted to the area desired and carried out periodically; applicability to different processes and apparatus is very good; the method is very safe; the apparatus require little service and are simple to maintain.

Brief Description of Drawings

The invention will be described more below by way of example with reference to the accompanying drawings of which

Fig. 1 illustrates a waste heat boiler having an apparatus according to the invention installed therein for removal of fouling deposits; Fig. 2 illustrates section A - A of Fig. 1 which illustrates by way of example a preferred embodiment of the invention;

Fig. 3 illustrates detail D of Fig. 2; and

Fig. 4 illustrates another preferred embodiment of the invention. Fig. 5 illustrates an exemplary form of a nozzle for introducing detaching medium according to the present invention.

Best Modes for Carrying out the Invention

Fig. 1 illustrates a waste heat boiler 1 supplied with gas via inlet 2. Naturally the waste heat boiler has been provided with the required heat transfer surfaces by known methods but it is not necessary to illustrate them in this connection. While describing the invention here a waste heat boiler wall

3 has been chosen as an example to serve as the wall into which means 4 for transporting detaching medium to the boundary surface of the wall and the fouling deposit have been provided. The method and the apparatus of the invention may of course by applied also to cleaning and keeping clean surfaces in contact with gases of other kinds of plants. According to the invention the medium may be distributed to the area to be cleaned exactly as required by the cleaning operation; thereby the surfaces to be cleaned may be divided into zones referred to in the Figure with reference numbers

4 and 4' and each predetermined zone may thus be cleaned according to its individual need and also the number of points at which medium is supplied to the surface may be chosen as required. Thus, resources are not spent in the cleaning any more than is necessary; i.e. the cleaning is carried out optimally and no unnecessary amounts of the medium or its reaction products end up in the gases. This is a very important advantage provided by the present invention since for example in the waste heat boiler illustrated in Fig. 1 the first heat exchangers, the so-called radiation heat exchangers are remarkably more easily fouled than the subsequent heat surfaces.

The medium used for the detaching is supplied via distribution means 5 which preferably comprise means for raising the pressure of the medium and keeping the pressure at an adequately high level; in some applications for example water pumped from a tank may be used. Distribution of the medium into different zones may be effected with regulating members 6 controllable for example by signals from a process computer.

The gases cooled in the waste heat boiler are lead on via outlet 7 for further treatment.

Figure 2 illustrates section A - A of Fig. 1 depicting more closely a wall 20 of a waste heat boiler provided with a supply means 23 according to a preferred embodiment of the present invention. The wall structure illustrated in Fig. 2 comprises, for tranporting the heat transfer medium, ducts 21 connected with each other by fins 24 in such a way the they preferably form a gas-tight wall. Preferably, the fins between the ducts have been provided with means 23 for supplying the medium to the boundary surface between the deposit 24 and the wall 20. According to a preferred embodiment of the invention the medium to be used is selected from a group of substances comprising for example the following substances and mixtures thereof: water, steam, fuel or other oil, liquid gas, natural gas, methane or other hydrocarbon gas. Actually, the present ivention provides a possibility to use practically any desired liquid or gas as the detaching medium. The medium is brought to the supply means by a distribution duct 25 and supplied further to the boundary surface to a desired depth by means 23 and/or 23'.

The conditions in the embodiment of Fig. 2 may preferably be as follows. The gas temperature in the vicinity of the deposit is approx. 800 - 1000°C whereby the temperature of the heat transfer medium in the ducts 21 is preferably approx.

300°C. In this case it is advantageous to use for example water as the medium for detaching the deposit, the water being supplied via means 23 to the boundary surface at a pressure of 6 - 20 bar, preferably 10 - 20 bar, whereby the volume of water to be used is very small. The supply inlet provided in the surface 22 may in this embodiment have a diameter of 3 -

5 mm, i.e. very small, which is one indication of the small space requirement of the present invention. Any shape of the medium supply inlet is possible if it functionally serves the

purpose .

Figure 3 illustrates a preferred embodiment of the means 23, i.e. detail D of Figure 2. The medium is supplied through wall 34 by means of a duct 33 in such a way that the medium is efficiently distributed to a wide area of the boundary surface. Lead-through 35 in a wall 34 has preferably been formed to cause the cross-sectional diameter of the medium flow to increase towards the boundary surface. As an example of this kind of way of carrying out the method, Figure 3 illustrates a widening conical structure of a lead-through. Duct 33 has been disposed tightly to the surface of the wall 34 preferably so that the cross-sectional area of the cone at the connection point is the same as that of the duct 33 at the connection point but widens therefrom in the flow direction of the medium. A structure of this kind contributes, in addition to a more even distribution of the medium, due to its shape to easy removal of the desposit collected in the lead- through between the cleaning cycles. Figure 3 further illustrates a member 36 for directing mechanical force to the deposit. The member may be e.g.a rod or a bar or alike member. When the member 36 has been provided inside the duct 33 the medium may be supplied either via a slot between the duct 33 and the member 36 or alternatively the member 36 itself may have been provided with means 37 for supplying the medium. In an embodiment of this kind it is advantageous to start supplying the medium before activating the mechanical member whereby due to the effect of the medium the share of the mechanical force in the detaching of the deposit is reduced.

Figure 4 illustrates yet another embodiment of the invention in which ducts 41 transporting the heat transfer medium have been connected to each other with fins 44 so that the ducts and the fins form a preferably gas-tight wall. Preferably the fins between the ducts have been provided with means 43, 43',

preferaby nozzles, for supplying the detaching medium to a boundary surface between a deposit 46 and the wall 44. The medium is supplied to the supply means via distribution duct 45 and supplied further to the boundary surface via means 43 and/or 43'. In the embodiment of Figure 4 in addition to the detaching means, mechanical power transmission means 410 and 410' have been provided in the wall to detach the deposit, the power for the means 410 and 410' being provided from power sources 411 and 411', respectively. The power source may be any known apparatus capable of producing the movement and adequate force of the power transmission means to be directed on the deposit. A hydraulic or a pneumatic cylinder, an electric power engine etc, may be mentioned as examples. The supply of the detaching medium and the mechanical force may be arranged as separate systems 43 and 410 or as a combined system 43', 410' whereby the space requirement is remarkably smaller. Even if the supply of the detaching medium has been illustrated in the Figure as being outside the mechanical power transmission it may of course be arranged via the power transmission means via a duct provided therethrough and illustrated by a dash line 412, the duct being further connected to a distribution system 45' for the detaching medium.

Figure 5 illustrates a nozzle, which permits introduction of the medium into the boundary surface, but closes immediately after the supply has ceased. A nozzle 51 is provided with a duct 53 for supplying the detaching medium to an opening 61 controlled by a needle 59. The opening may be opened and closed by means of the needle 59. Since the opening is relatively small, even having a diameter of 1 - 3 mm, the entry of any deposits is efficiently prevented when the opening is closed. When the pressure of the detaching medium is not high enough the spring 56 keeps the needle at a position closing the opening 61. When the pressure of the medium is increased pressure is create towards a collar 65

compressing the spring being supported by a plate 57 in a space 55, thus releasing the needle from the opening 61. This allows the introduction of the medium via the opening 61 until the force caused by the pressure of the medium towards the collar 65 is smaller than the force of the spring. The nozzle may be designed to be activated at suitable pressure for each application; the activation pressure range may be very wide, e.g. 5 - 150 bar, the pressure being selected according to the properties and behaviour of the deposit of the process and the location in question. The activating pressure may be easily selected by providing the nozzle with a suitable spring. The nozzles may be attached to the means (23, 33, 43) for supplying the medium to the boundary surface as described in Figures 2, 3 or 4. Alternatively, due to the rigid construction of the nozzle, it may even be combined with the member (36, 410) for directing mechanical force to the deposit as described in connection with Figures 3 or 4. In that case the conduit 53 is connected to e.g. a duct 410 of Figure 4. In the embodiment of Figure 5 there is only one opening 61 shown, but it is possible to arrange the nozzle to be such that several openings are provided. The opening 61 has a form of a cone for the same reasons as disclosed in connection with Figure 3. The design of the nozzle contributes to the flow pattern of the spray produced; this is a useful feature if the nozzle is desired to be used e.g. for cleaning the heat exchangers when the deposit has been removed.

Industrial Applicability

The embodiments presented above as examples, only, and the accompanying drawing figures referring to apparatus for treatment of gas do not intend to limit the scope of protection of the invention but the scope must be interpreted to emcompass all possible variations thereof fulfilling the purpose of the invention and conceivable to a person skilled in the art based on the appended patent claims.