WO2017088924A1

WO2017088924A1 - Method and apparatus for preventing fouling of a heat exchanger element

Info

Publication number: WO2017088924A1
Application number: PCT/EP2015/077771
Authority: WO
Inventors: Stefano GANASSIN; Quirijn EPPINGA
Original assignee: Tri-O-Gen Group B.V.
Priority date: 2015-11-26
Filing date: 2015-11-26
Publication date: 2017-06-01
Also published as: EP3380803A1

Abstract

The invention relates to a method for preventing fouling of an exterior surface of a heat exchanger element, in particular an evaporator in an ORC energy converter, by the build-up of soot or dust, comprising blowing a cleaning gas flow over said exterior surface of the heat exchanger element. The cleaning gas flow may be blown over the heat exchanger element in a direction that is substantially perpendicular to a direction of flow of a heat exchanger fluid through said element. The invention further relates to an apparatus for preventing fouling of an exterior surface of a heat exchanger element, in particular an evaporator in an ORC energy converter, comprising blowing means for blowing a cleaning gas flow over said exterior surface of the heat exchanger element. The blowing means may comprise one or more orifices arranged adjacent the plane defined by the mutually parallel heat exchanger elements, e.g. in a sidewall of a chamber in which the heat exchanger element is arranged. And finally the invention relates to a heat exchanger comprising one or more heat exchanger elements and a fouling prevention apparatus as described above.

Description

Method and apparatus for preventing fouling of a heat exchanger element

The present invention relates to a method and apparatus for preventing fouling of an exterior surface of a heat exchanger element, in particular for a heat exchanger that is operative in a dust or soot laden environment.

For a heat exchanger element to function properly, the heat transfer must be efficient. This requires that a wall of the heat exchanger element separating the fluids that are to exchange heat should have minimum heat resistance. Heat exchangers are used e.g. in energy converters or generators based on the Rankine cycle. In such an energy converter a phase changing fluid, e.g. water is evaporated by external heat, e.g. from a burner. The vapour (steam) is used to drive a turbine which is coupled to an electrical generator. Then the vapour (steam) condensates and the condensate (water) is recirculated to be heated and evaporated again. Instead of water another phase changing fluid, like e.g. an organic fluid can be used in the primary cycle. Moreover, thermal oil or pressurized water can be used as intermediate heat transfer medium in the heat exchanger, but without phase change. A special class of energy converters as manufactured by the applicant is based on the so-called organic Rankine cycle (ORC). Such energy converters allow Rankine cycle heat recovery from relatively low temperature sources, such as biomass combustion, industrial waste heat, geothermal heat, solar ponds, etc. An ORC energy converter relies on the use of an organic, high molecular mass fluid with a liquid-vapour phase change or boiling point that occurs at a lower temperature than the water-steam phase change.

Heat transfer efficiency dramatically decreases when a surface of a heat exchanger element is fouled or soiled, e.g. by dust or soot collecting on the surface. Dust or soot deposition is a serious problem in wood-fired energy converters and in ORC energy converters which use heat generated by combustion of biomass. As the composition of biomass is usually less stringently controlled and more variable than that of e.g. fossil fuels, burning biomass often leads to the formation of soot and ashes, which are carried along with the flue gasses into the heat exchanger or evaporator of an ORC energy converter. As biomass always contains a certain amount of un- combustible substances, these substances will appear as dust or ash in the flue gas. Soot and dust particles will adhere to the surfaces they encounter, including the exterior surface of a heat exchanger element. As soot and dust particles accumulate on the exterior surface of the heat exchanger element, its heat transfer efficiency will drop accordingly. This problem is currently observed in wood processing plants, where solid biomass is combusted for heating and drying purposes, , but where the heat is first used to drive an ORC energy converter. The evaporators of these ORC energy converters typically have to be cleaned on a weekly basis unless a proper on line cleaning system is installed.. In order to restore the heat transfer efficiency, heat exchanger elements have to be cleaned regularly. Conventionally, this involves long stops of the ORC energy converter, letting it cool down, accessing the casing where the heat exchanger element(s) is/are arranged, and then cleaning the heat exchanger element(s). The heat exchanger element(s) may be cleaned by means of shot blasting, in which an abrasive medium is directed at the exterior surface of the heat exchanger element at high speed, so as to remove the accumulated dust or soot. This is tedious and time-consuming.

Several techniques have been developed for on-line cleaning of the exterior surfaces of heat exchanger elements, i.e. cleaning these surfaces without accessing the chamber where they are arranged, and without stopping the ORC energy converter. One known technique is acoustic, and involves periodically generating 140 dB sound waves and directing these at the heat exchanger elements. This method is not suitable for use in or near residential areas. Another known technique involves oscillating the heat exchanger element(s). As a result of these oscillations any material which has adhered to the exterior surface may be dislodged. However, oscillating the heat exchanger element(s) is complex and may eventually lead to damage or failure, e.g. due to fatigue. And finally, a known technique involves the use of an air canon, by which high pressure air is directed from the combustion room into the chamber in which the heat exchanger element(s) is/are arranged. However, this technique does not work satisfactorily, since back pressure in the exhaust or filter may prevent the pressurized air from flowing through the evaporator, so that pressurized air may actually re-enter the combustion chamber. Moreover, in situations where there are many heat exchanger elements at relatively close pitch the airflow cannot reach all parts of the exterior surfaces of all the heat exchanger elements.

There is thus a need for an improved method of preventing the build-up of soot and dust on an exterior surface of heat exchanger elements. In accordance with the invention, a method of preventing fouling is provided which comprises blowing a cleaning gas over the exterior surface of the heat exchanger element. By actually directing the cleaning gas flow to the exterior surface of the specific heat exchanger element, rather than merely introducing pressurized air into an evaporator from the side of the combustion chamber, effective cleaning of the exterior surface can be achieved. It should be noted that the term "cleaning gas" as used herein also covers a vapour used for cleaning purposes, like e.g. steam.

In a preferred embodiment, the cleaning gas flow is blown over the heat exchanger element in a direction that is substantially perpendicular to a direction of flow of a heat exchanger fluid through said element. In this way accumulation of the dust and soot at one of the ends of the heat exchanger element is prevented and uniform cleaning is obtained.

When a plurality of substantially parallel heat exchanger elements are cleaned simultaneously, and the cleaning gas flow is blown over the heat exchanger elements in a direction that is substantially parallel to a plane defined by the mutually parallel heat exchanger elements, multiple heat exchanger elements can effectively be cleaned by a single cleaning gas flow.

In order to achieve an optimum cleaning effect, without the cleaning gas flow being obstructed by the heat exchanger elements, it is advantageous that the cleaning gas flow is blown from at least one orifice that is arranged adjacent the plane defined by the mutually parallel heat exchanger elements.

When cleaning gas flow is blown from a plurality of orifices forming a corresponding plurality of sub-flows at different angles to the direction of flow of the heat exchanger fluid, a large area may be covered by the cleaning gas flows.

In order to generate a sufficiently forceful cleaning gas flow, it is advantageous if the or each orifice is formed as a nozzle configured to accelerate the cleaning gas flow to sonic or supersonic speed before being blown over the heat exchanger elements.

When the heat exchanger elements are arranged in substantially parallel layers at different levels, and a plurality of cleaning gas flows are blown over the heat exchanger elements at said different levels, the entire evaporator or at least a large part thereof can be cleaned at once.

In order for the cleaning gas flow to reach all parts of the heat exchanger elements, it may be advantageous if two cleaning gas flows are blown over the heat exchanger element(s) from substantially opposite sides.

When the cleaning gas flow is periodically blown over the heat exchanger element(s), the energy required for cleaning the heat exchanger elements is reduced. In this way an optimum balance may be achieved between loss of heat transfer efficiency due to fouling of the heat exchanger elements and the energy required for cleaning the heat exchanger elements.

An energy-efficient cleaning operation is achieved when the cleaning gas flow is generated by discharging a pressurized cleaning gas from a container.

The invention further relates to an apparatus for performing the method discussed above. To that end, the invention provides an apparatus for preventing fouling of an exterior surface of a heat exchanger element, in particular an evaporator in an ORC energy converter, comprising blowing means for blowing a cleaning gas flow over said exterior surface of the heat exchanger element.

Some preferred embodiments of the fouling prevention apparatus are defined in dependent claims 12-14.

The heat exchanger elements may be arranged in a chamber having at least one sidewall and the at least one orifice may be formed in said at least one sidewall, so that the desired orientation of the gas flow can be achieved in a structurally simple manner. In one embodiment the at least one orifice comprises a slot extending substantially parallel to the plane defined by the mutually parallel heat exchanger elements. Such a slot allows a relatively large amount of cleaning gas to be blown over the heat exchanger elements.

In an alternative embodiment, the at least one orifice comprises a row of orifices, said row running substantially parallel to the plane defined by the mutually parallel heat exchanger elements and adjacent orifices in said row being oriented at different angles to the direction of flow of the heat exchanger fluid. This arrangement allows a relatively large area to be covered by blowing relatively small amounts of cleaning gas through the orifices.

Further preferred embodiments are defined in dependent claims 18 and 19.

Generating cleaning gas flows at different levels can be done in a structurally simple manner when the apparatus comprises a plenum extending along the different levels and connecting the orifices at or near the different levels.

When the plenum is tubular and protrudes slightly into the chamber, the cleaning gas flows can over a large area of the heat exchanger elements.

In a structurally simple embodiment of the apparatus, which is moreover easy to maintain, the plenum is arranged in a door in one of the sidewalls of the chamber. Arranging the plenum in a door allows for easy retrofitting of the cleaning apparatus to an existing energy converter.

Further preferred embodiments of the fouling prevention apparatus are defined in dependent claims 22 to 24.

In order to empty the container as fast as possible so as to achieve high gas speeds, it is advantageous if the container has an outlet opening in fluid communication with the at least one orifice, and said outlet opening is controlled by a rapid action valve.

And finally, the invention further provides a heat exchanger which comprises at least one heat exchanger element and a fouling prevention apparatus of the type described above.

The invention is now further illustrated by way of a number of exemplary embodiments thereof, with reference being made to the annexed drawings, in which:

Figure 1 is a diagrammatic representation of an ORC energy converter in which a fouling prevention apparatus in accordance with the invention can be used,

Figure 2 is a cross-section along the lines II-II of the heat exchanger shown in

Figure 3,

Figure 3 is a cross-section along the lines III-III of the heat exchanger shown in figure 2,

Figure 4 is a schematic illustration of an apparatus in accordance with the invention, Figure 5 is a side view of a door closing off a compartment of the heat exchanger of figures 2 and 3, including a tubular plenum with orifices forming part of the apparatus,

Figure 6 is a front view of the tubular plenum with orifices of Figure 5,

Figure 7 is a cross-section along the lines VII-VII in Figure 6,

Figure 8 is a view corresponding with Figure 5 but showing another embodiment of the plenum having orifices,

Figure 9 is a cross-section along the lines IX-IX in Figure 8,

Figure 10 is a front view of a sidewall provided with orifices of an apparatus in accordance with another embodiment of the invention,

Figure 11 is a rear view of the apparatus as shown in Figure 10,

Figure 12 is a side view of the part of the apparatus shown in Figures 10 and 11, and

Figure 13 is a top view of the part of the apparatus shown in Figures 10-12.

The main components of a conventional ORC energy converter 8 as illustrated in Fig. 1 are a combined preheater and evaporator 1, a turbine 2, a condenser 6 and a feed pump 5 all connected by a circuit C for circulation of an organic working fluid. Also a recuperator 4 and a pre- feed pump 7 can be used in the ORC energy converter. The liquid organic working fluid is pressurized by the feed pump 5 to a high pressure and then enters the combined preheater and evaporator 1. The working fluid is preheated in the preheater part PH, then evaporated in the evaporator part EV and superheated in the superheater SH, by a heat source HS with which the working fluid is brought into heat transferring contact. In this embodiment the heat source HS is formed by flue cleaning gasses from incineration of biomass, e.g. wood residues and/or chips, which carry a significant amount of dust and soot.

The slightly superheated working fluid enters the turbine 2 and expands, causing the turbine 2 to rotate. Rotation of the turbine 2 is converted into electric power by a generator 3. The working fluid exiting the turbine 2 is commonly dry vapor at high temperature and the working fluid heat can be utilized in the recuperator 4 for an initial preheating of the liquid working fluid before it enters the combined preheater and evaporator 1. Low temperature vapor is then condensed in the condenser 6 and pressurized again in one or two steps. In the case of two steps, as illustrated in this embodiment, this is realized by the pre-feed pump 7 and the feed pump 5 - which may be driven by the turbine 2. The pre-feed pump might be necessary to provide the feed pump with sufficient initial pressure, and/or to provide pressure for lubrication of the bearings.

Since the heat source HS is laden with dust and soot particles, which will adhere to heat exchanger elements 11 of the evaporator (or combined preheater/evaporator/superheater) 1 , thus reducing heat transfer efficiency, the ORC energy converter 8 is provided with an apparatus 10 for preventing fouling of the heat exchanger elements 11 as will be discussed below. This fouling prevention apparatus 10 is designed for cleaning the evaporator 1 by blowing high energy cleaning gas flows along the exterior surfaces of the heat exchanger elements 11.

The evaporator 1 includes a chamber 12 in which a plurality of the heat exchanger elements 11 are arranged (Fig. 2, 3). The chamber 12 is defined by two opposite sidewalls 9, a front wall 17 and a rear wall 18. The front wall 17 may form a door providing access to the interior of the chamber 12. The heat exchanger elements 11 are arranged in arrays, defining both horizontal planes 13 and vertical planes 14.

Each vertical plane 14 is formed by a plurality of tubes 19 constituting the heat exchanger elements 11. These tubes 19 are arranged above each other and extend between the sidewalls 9 of the chamber 12 of the evaporator 1. The tubes 19 are all connected to form a continuous conduit 21 for a heat exchanger fluid.

Horizontally adjacent tubes 19 are arranged in layers which extend between the front wall 17 and the rear wall 18. Each of these layers defines a horizontal plane 13.

In the illustrated embodiment, flue gasses F enter the evaporator 1 from the top 25 and exit at the bottom 26. They flow past the heat exchanger elements 11 in vertical direction, i.e. at right angles to the tubes 19 and the flow direction of the heat exchanger fluid. When the flue gasses F carry along particles, like dust and soot, part of these will adhere to the heat exchanger elements 11 , thus increasing the virtual wall thickness and the distance between the flue gas F and the heat exchanger fluid flowing through the heat exchanger elements 11. As stated above, this deposit of soot and dust leads to a reduction of the heat transfer efficiency. In order to restore the heat transfer efficiency, the ORC energy converter 8 is provided with a fouling prevention apparatus 10 in accordance with the invention.

This apparatus 10 includes a source of pressurized cleaning gas, e.g. a container or vessel 27 filled with pressurized air, which is in fluid communication with a plurality of orifices 28 (Fig. 6,7) arranged in one or more of the front and rear walls 17, 18 of the chamber 12 (Fig. 4). In the illustrated embodiment the orifices 28 are arranged at different positions, so that each set of orifices 28 allows an air flow to be generated over a layer of heat exchanger elements 11. The orifices 28 may be oriented at an acute angle to the horizontal plane 13 that is defined by the mutually parallel heat exchanger elements 11, so as to direct the air flow towards that plane 13.

The orifices 28 may be connected to a plenum 29, which in turn may be connected to the container or vessel 27 by means of a conduit 30, e.g. a flexible hose. The air in the container or vessel 27 may be pressurised by a compressor (not shown), which may compress the air to a pressure of 5 to 8 bar. The vessel or container 27 has a series of outlets 31A-D, each of which is closed off by a valve 32A-D. Each valve 32A-D may be a rapid action valve. In this way a high flow rate is achieved, which in turn leads to high air flow velocities inside the chamber 12. These high air flow velocities are required for dislodging the accumulated dust or soot from the heat exchanger elements 11.

In one exemplary embodiment the orifices 28 are shaped as slots which extend substantially parallel to the horizontal plane 13 defined by the adjacent heat exchanger elements 11 (Fig. 10). Each slot-like orifice 28 extends substantially over the width of the associated plenum 29. In this embodiment there are multiple plenums 29 on the illustrated part of the front wall 17, each extending vertically along the various levels of the heat exchanger elements 11. The slot-like orifices 28 are substantially evenly spaced in vertical direction, so that each slot-like orifice 28 is arranged between two adjacent layers of heat exchanger elements 11. The plenums 29 have branches 40 which are connected by a manifold 33 (Fig. 11, 13), which in turn is connected by a duct or hose 30 to the container or vessel 27 filled with pressurized air. Each slot-like orifice 28 may be shaped such that it may function as an aerodynamic throat and that the airflow may be accelerated to a supersonic speed in this throat before being blown out along the heat exchanger elements 11. Alternatively, the slot-like orifices 28 may have a straight cross-section, a divergent cross-section or a convergent cross-section, depending on the pressure of the air that is supplied to the slot-like orifices 28. The slot-like orifices 28 provide for a relatively widely distributed airflow covering large part of the surface area of the heat exchanger elements 11. However, they also cause a relatively large airflow, thus requiring relatively large vessels or containers 27 to be used.

In an alternative embodiment, which is currently the preferred embodiment, a plurality of rows 34 of relatively small circular orifices 28A-E are formed at various levels in a wall of a tubular plenum 29 (Fig. 6, 7). Here again, the vertical spacing between the rows 34 corresponds with that of the layers of heat exchanger elements 11. The orifices 28 A-E in each row 34 are arranged at different angles al, a2 with respect to the sidewalls of the chamber 12 and with respect to the orientation of the heat exchanger elements 11. In this way the orifices 28 A-E allow various sub-flows of pressurized air to be discharged in different directions, thus forming a diverging or conical flow field. In the illustrated embodiment two spaced apart vertical plenums 29 having associated rows 34 of orifices 28A-E are mounted in the sidewall segment 17, thus generating two flow fields. These diverging and partially overlapping flow fields will reach most parts of the heat exchanger elements 11 in the adjacent layers, so that excellent cleaning is achieved.

In the illustrated embodiment each row 34 of orifices includes five orifices 28A-E which have an angular spacing of around 25 degrees each. The central orifice 28C has an axis A which is substantially perpendicular to the front wall 17 of the chamber 12. The orifices 28 A-E are formed in a part 35 of the tubular plenum 29 which protrudes from the front wall 17 of the chamber 12 so that a relatively wide cone of pressurized air may be formed without interference from the front wall 17 of the chamber 12. In order to reduce the amount by which the plenum 29 protrudes into the chamber 12, which leads to interference with the heat exchanger elements 11, the plenum 29 may be arranged in a recessed part 36 of the front wall 17. To this end the edges 37 of an opening 38 receiving the plenum 29 may be set back with respect to the plane of the front wall 17, so that the plenum 29 does not protrude into the chamber 12 to any significant extent, while still allowing unimpeded airflow out of the diverging orifices 28A-E (Fig. 8, 9).

In the illustrated embodiment the evaporator 1 has four doors which together form the front wall 17 and each of which provides access to a part of the chamber 12. Each door is provided with several rows 34 of orifices 28A-E at different levels, each row 34 between two adjacent layers of heat exchanger elements 11. The orifices 28 in each door are connected by a plenum 29, and each plenum 29 is individually connected to the container or vessel 27 by a separate conduit (duct or hose) coupled to a dedicated rapid action valve 32A-D (Fig. 4).

By this arrangement the sets of orifices 28 in each of the doors can be provided with pressurized air sequentially, so that each blow-down of the container or vessel 27 results in a quadrant of the evaporator 1 being cleaned. After the container or vessel 27 has been emptied, all valves 32A-D are closed and it is refilled with pressurized air by a compressor (not shown). Thus, whenever it is detected that a part of the evaporator 1 is soiled by dust or soot, the relevant valve 32A, B, C or D is opened and the pressurized air is supplied to the relevant plenum(s) 29 and discharged from the associated orifices 28 to clean the heat exchanger elements 11 in that part of the evaporator 1. The sequence in which the pressurized air is supplied to the various segments or doors and the intervals between cleaning actions can be pre-programmed, e.g. based on a timer, or can be determined by a controller on the basis of detection of the level of soiling.

In this way the invention allows a heat exchanger, in particular an evaporator in an ORC energy converter, to be regularly cleaned at relatively low cost by using a relatively simple system of orifices from which a cleaning gas flow, in particular a pressurized airflow is blown over the exterior surfaces of heat exchanger elements. Such cleaning leads to an increase in average heat transfer efficiency and thus to a higher yield of the ORC energy converter.

Although the invention has been illustrated by reference to a number of embodiments, it will be clear that it is not limited to these embodiments and can instead be adapted and modified in various ways. For instance, the number of orifices and their distribution over the sidewall or sidewalls of the chamber could be selected differently. Instead of arranging orifices between each pair of layers of heat exchanger elements, it might be possible to use fewer orifices, e.g. one row of orifices for two pairs of layers, if the orifices and air supply would support higher strength cleaning gas flows. Moreover, orifices could be arranged not just in the front wall, but also in the rear wall, and even in the sidewalls, if oriented at an acute angle to those sidewalls. And the cross-sectional shape of the chamber could be different from the one shown here, e.g. round, in which case there would be no front or rear walls, and the orifices would be arranged simply in the sidewall or peripheral wall. The shape and dimensions of the orifices could be varied as well. And other sources of pressurized air or other cleaning gasses could be used to generate the cleaning gas flows.

The fouling prevention method and apparatus are not limited to use in combination with evaporators, let alone evaporators in ORC energy converters. Instead, the method and apparatus can be used for any so-called "shell-and-tube" heat exchanger where the flue gases flowing past the heat exchanger elements are fouled and will lead to deposition of e.g. soot and/or ashes on the exterior surface of the heat exchanger elements. The method and apparatus can be used both for aligned and staggered heat exchanger configurations, regardless of the size and material of the heat exchanger elements or tubes, the size of the bundle of heat exchanger elements or tubes, the mechanical construction of the bundle or number of passages.

The scope of the invention is defined solely by the appended claims.

Claims

1. Method for preventing fouling of an exterior surface of a heat exchanger element, in particular an evaporator in an ORC energy converter, comprising blowing a cleaning gas flow over said exterior surface of the heat exchanger element to prevent build-up of soot or dust.

2. Method according to claim 1 , characterized in that the cleaning gas flow is blown over the heat exchanger element in a direction that is substantially perpendicular to a direction of flow of a heat exchanger fluid through said element.

3. Method according to claim 1 or 2, characterized in that a plurality of substantially parallel heat exchanger elements are cleaned simultaneously, wherein the cleaning gas flow is blown over the heat exchanger elements in a direction that is substantially parallel to a plane defined by the mutually parallel heat exchanger elements.

4. Method according to claim 3, characterized in that the cleaning gas flow is blown from at least one orifice that is arranged adjacent the plane defined by the mutually parallel heat exchanger elements.

5. Method according to claim 4, characterized in that the cleaning gas flow is blown from a plurality of orifices forming a corresponding plurality of sub-flows at different angles to the direction of flow of the heat exchanger fluid.

6. Method according to claim 4 or 5, characterized in that the or each orifice is formed as a nozzle configured to accelerate the cleaning gas flow to sonic or supersonic speed before being blown over the heat exchanger elements.

7. Method according to any of the preceding claims, characterized in that the heat exchanger elements are arranged in substantially parallel layers at different levels, and a plurality of cleaning gas flows are blown over the heat exchanger elements at said different levels.

8. Method according to any of the preceding claims, characterized in that two cleaning gas flows are blown over the heat exchanger element(s) from substantially opposite sides.

9. Method according to any of the preceding claims, characterized in that the cleaning gas flow is periodically blown over the heat exchanger element(s).

10. Method according to claim 9, characterized in that the cleaning gas flow is generated by discharging a pressurized cleaning gas from a container.

11. Apparatus for preventing fouling of an exterior surface of a heat exchanger element, in particular an evaporator in an ORC energy converter, comprising blowing means for blowing a cleaning gas flow over said exterior surface of the heat exchanger element to prevent build-up of soot or dust.

12. Apparatus according to claim 11, characterized in that the blowing means are arranged for blowing the cleaning gas flow over the heat exchanger element in a direction that is substantially perpendicular to a direction of flow of a heat exchanger fluid through said element.

13. Apparatus according to claim 11 or 12, characterized in that the blowing means are arranged for simultaneously cleaning a plurality of substantially parallel heat exchanger elements, the blowing means being arranged to blow the cleaning gas flow over the heat exchanger elements in a direction that is substantially parallel to a plane defined by the mutually parallel heat exchanger elements.

14. Apparatus according to claim 13, characterized in that the blowing means comprise at least one orifice that is arranged adjacent the plane defined by the mutually parallel heat exchanger elements.

15. Apparatus according to claim 14, characterized in that the heat exchanger elements are arranged in a chamber having at least one sidewall and the at least one orifice is formed in said at least one sidewall.

16. Apparatus according to claim 14 or 15, characterized in that the at least one orifice comprises a slot extending substantially parallel to the plane defined by the mutually parallel heat exchanger elements.

17. Apparatus according to claim 14 or 15, characterized in that the at least one orifice comprises a row of orifices, said row running substantially parallel to the plane defined by the mutually parallel heat exchanger elements and adjacent orifices in said row being oriented at different angles to the direction of flow of the heat exchanger fluid.

18. Apparatus according to any one of claims 14-17, characterized in that each orifice is formed as a nozzle which is adapted to accelerate the cleaning gas flow to sonic or supersonic speed before being blown over the heat exchanger elements.

19. Apparatus according to any one of claims 11-18, characterized in that the heat exchanger elements are arranged in substantially parallel layers at different levels, and a plurality of orifices are formed at or near said different levels.

20. Apparatus according to claim 19, characterized by a plenum extending along said different levels and connecting the orifices at or near said different levels.

21. Apparatus according to claim 20, characterized in that said plenum is tubular and protrudes slightly into the chamber.

22. Apparatus according to claim 20 or 21, characterized in that said plenum is arranged in a door in said at least one sidewalls of the chamber.

23. Apparatus according to any one of claims 15-22, characterized in that orifices are formed in opposite sidewalls of the chamber.

24. Apparatus according to any one of claims 11-23, characterized in that the blowing means are arranged for periodically blowing a cleaning gas flow over the heat exchanger element(s).

25. Apparatus according to claim 24, characterized in that the blowing means comprise a container that is at least partially filled with a pressurized cleaning gas and that is in fluid communication with the at least one orifice, the blowing means being arranged for discharging the pressurized cleaning gas from said container.

26. Apparatus according to claim 25, characterized in that the container has an outlet opening in fluid communication with the at least one orifice, said outlet opening being controlled by a rapid action valve.

27. Heat exchanger, in particular an evaporator in an ORC energy converter, comprising at least one heat exchanger element and a fouling prevention apparatus according to any one of claims 11-26.