HEAT EXCHANGER FOR FOG GENERATOR

Abstract

 The invention concerns a heat exchanger (1) for gasifying a fog fluid in a fog generator, the heat exchanger comprising a plurality of concentric layers (2) with the same longitudinal axis (12) and an intermediate space (3) between the layers, constructed such that the fog fluid flows through the intermediate space in the direction of the longitudinal axis.

Description

BACKGROUND TO THE INVENTION

[0001] A fog generator for security applications is normally based technically on the principle of evaporation of a glycol (the fog fluid). An evaporated fog fluid is expelled by an output channel and nozzle into the "area to be fogged", and left to condense there under atmospheric pressure and room temperature directly into a sprayed aerosol-like fog. This fog obstructs vision and disorients the criminal.

[0002] To raise the temperature of the fog fluid from room temperature to evaporation temperature (∼250°C), 0.8 to 1 kJ is required per ml (depending on the formulation of the fog fluid, among others the water content). The heat flow to the transfer surfaces of the evaporation channels/passages is mainly provided by thermal conduction. The inlet to a heat exchanger is coupled to a fog fluid reservoir, whereby at the desired moment (fog emission) this fog fluid is injected by overpressure into the inlet of the heat exchanger. The overpressure may be generated by:

a) a mechanical pump and/or potential elastic energy (tensioned spring against a piston)

b) drive pressure from compressed or liquid propellant gas (vapour tension of propellant), and/or

c) drive pressure from gas generated as a result of a chemical reaction or chain reaction.

[0003] A heat exchanger for heating fluids (e.g. for kettles, washing machines and coffee machines) is described for example in WO2007/037694. The fluid to be heated flows through a spiral channel (and hence not in the axial direction of the spiral windings). Another embodiment in WO2007/037694, presented in figures 4a and 4b, contains a heating element with layers. The layers do not however have the same longitudinal axis. There is a need for a heat exchanger which is easy to produce, for heating fluids, in particular for heating fog fluid in a fog generator.

[0004] A heat exchanger in a fog generator for security applications is characterised by:

• A component in which heat (Joules) is stored through its thermal capacity C (e.g. steel: -0.46 J/°C per g) and/or any latent solidification heat from a phase change medium (see for example the heat exchanger described in EP2259004)
• The temperature of the heat exchanger, at least at the outlet level, is higher than the boiling point of the fog fluid to be evaporated.
• The heat exchanger is usually heated to the desired temperature via the transfer of Joules from an electrical resistance wire.
• The heat is transferred intensively between the internal channels and/or free passages of the heat exchanger and the fog fluid flowing through.
• All evaporated fog fluid is injected via an outlet channel and nozzle into the "area to be fogged", in order to condense directly there under atmospheric pressure and room temperature into a sprayed aerosol-like fog.

[0005] The fog generation capacity (flow in ml/sec) of a heat exchanger depends greatly on the feed pressure of the fog fluid presented to its inlet, and its design. In fog generators according to the prior art, the heat exchanger is provided with a channel or channels which are held at a high temperature (Fig. 1). The fog fluid is evaporated by driving it through the hot channel. Evidently, the rate of fog formation is crucial for fog generators for security applications. Current innovations in the field are aimed at increasing the rate with which fog is generated (both the speed of the start of fog formation and the volume of fog which is emitted per second). Thus PCT/EP2013/078112 discloses a fog generator in which the fog fluid is expelled by means of gas generation from a pyrotechnic substance. The fog fluid may also be expelled by a compressed/liquid propellant gas under high pressure (e.g. 80 bar). It has however been established that heat exchangers according to the prior art do not function optimally for such quasi-explosive injection of the fog fluid. Because the flow of fog fluid is up to 10x faster than in present devices, such heat exchangers cannot completely evaporate the fluid, usually because during the through-flow period of the fog fluid, not enough optimally transmittable Joules are available in the heat exchanger at the level of a heat transfer surface. Consequently, not only gas but also fog fluid is expelled via the outlet. Also, the heat flow via conduction has scarcely time to cover a distance (thermal gradient) of several millimetres due to the rapid through-flow of the fog fluid.

[0006] PCT/EP2013/078112 offers a solution thereto by proposing a plate heat exchanger with labyrinth design (Fig. 2); this development allows a rapid transfer of heat but also forms a relatively high dynamic resistance (because of the relatively long distance to be covered by the fluid to be evaporated). A pressure fall of 50 bar between the inlet and outlet of the heat exchanger is to be expected for a flow of 100 ml fog fluid per second. Although this pressure fall in itself is not problematic, due to the high initial pressure (80 bar or higher), this heat exchanger still has a number of disadvantages. Such a heat exchanger is complex to produce. The plates must be preformed and welded to each other individually.

[0007] An even greater problem however is that the plates distort due to the accumulation of very small deformations during and after crimping of the welded components. Even under an axial press, the sum of all undesirable deformations is difficult to keep under control; this reinforces, due to the uncontrollable deformation during the rapid transition from warm to cold of the "first plates nearest the inlet" during injection of the cold fluid, unpredictable clicking. In addition, it is particularly costly and difficult to produce the heat exchanger in a corrosion-resistant fashion. This is of great importance for a heat exchanger in a fog generator, in view of the high temperatures and the oxygen entering from the atmospheric environment (normally entering from the nozzle or as a result of the available oxygen from the thermal end reaction), with thermal decomposition products as a result of "corrosive" acidity level of the fluids used. Consequently there is a need for a heat exchanger for a fog generator which can fully evaporate a large flow of fog fluid, which is resistant to a high operating pressure, is simple to produce at low cost, and designed with the appropriate corrosion resistance.

DESCRIPTION OF THE INVENTION

[0008] The heat exchanger for gasification of a fog fluid in a fog generator according to the invention comprises several layers with a common axis and an intermediate space between the layers, i.e. constructed such that the fog fluid flows axially through the intermediate space. The term "common axis" as used here means in particular that several layers have the same longitudinal axis. The layers are more specifically concentric layers. The term "axial", as used here, means in the direction of the longitudinal axis. Preferably, the thickness of the layers is greater than the thickness of the intermediate space. In a particular embodiment, the ratio of the thickness of the intermediate space and the thickness of the layers is between 1:2 and 1:50. In a particular embodiment, the layers have a thickness between 0.1 mm and 5 mm.

[0009] The layers are constructed from a heat-retentive material. In a particular embodiment, the layers comprise a metal core, such as steel, iron, copper, aluminium or metal alloys. In a further embodiment, the layers consist at least partly of a corrosion-resistant material. Thus corrosion can be avoided for example by applying a corrosion-resistant layer on a steel or copper layer, or the layers may consist partially or completely of stainless steel or ceramic or carbon-containing materials, in particular stainless steel. The layers are preferably not hollow. The heat exchanger according to the invention preferably contains an inlet for the fog fluid at a first axial (longitudinal) end of the layers, and an outlet for gasified fog fluid at a second, opposite, axial (longitudinal) end.

[0010] There are various ways of obtaining layers with an intermediate space. One practical embodiment uses spacers which determine the intermediate space. Preferably, the spacers are attached to or form part of the layers.

[0011] In another embodiment, the layers are situated in a container and the volume of the container is filled by the layers to more than 50%, in particular to more than 70%, more particularly to more than 80%. According to the invention the heat exchanger preferably also contains a distribution means. The distribution means distributes/spreads the fog fluid over the section close to the inlet to the heat exchanger. Any type of distribution means may be used. The inlet to the heat exchanger may thus be designed such that the incoming fluid is spread over several channels and/or a distribution plate may be provided in which holes ensure a uniform distribution. It is also possible e.g. to provide a layer of grains between which the fog fluid is distributed, and thus flows more homogeneously between the layers.

[0012] In the same way as the distribution means situated in the vicinity of the inlet to the heat exchanger, it is also possible to provide collection means in the vicinity of the outlet. The collection means may help collect the formed gas in e.g. one outlet channel in the heat exchanger.

[0013] In another preferred embodiment, the heat exchanger according to the invention also comprises (inert) beads. The (inert) beads may be made of any material as long as they are compatible with the pressure and temperature in the heat exchanger and with the contact with the fog fluid. They may for example be made of thermoresistant plastic, or ceramic or carbon -containing materials, or from materials which contribute further to the heat capacity of the heat exchanger such as e.g. metal. In a preferred embodiment, they consist of corrosion-resistant metal, such as stainless steel. In a preferred embodiment, the mean diameter of the beads is greater than the thickness of the intermediate space. The beads limit or prevent the passage of fog fluid through spaces other than the intermediate space, and thus ensure a more uniform heating of the fog fluid.

[0014] In a particular embodiment, the layers consist of a plurality of tubes with a different internal volume. Thus the layers may consist of a plurality of cylindrical tubes with differing diameter. In addition it is also possible to use e.g. triangular, rectangular or polygonal tubes with differing internal volume. In this way the (concentric) walls of the tubes form the plurality of layers with a common axis and an intermediate space originates between the tubes, between which the fluid can flow axially.

[0015] Preferably, the layers consist of spiral windings of a rolled (spiral) plate. This embodiment also has the advantage that it is very simple to produce. The present invention also concerns a method for production of such a heat exchanger, the method comprising:

• the rolling of the plate in a spiral shape, forming an intermediate space between the spiral windings, and
• insertion of the spiral-shaped plate in a housing,

so that fog fluid can flow in the direction of the longitudinal axis (axially) through the intermediate space.

[0016] Preferably, the method also comprises the application of spacers to the plate before rolling in spiral shape.

[0017] In addition, the present invention provides a method for generating a dense opaque fog, the method comprising the following steps:

• heating of the heat exchanger according to the invention;
• introduction of fog fluid into the heat exchanger via the inlet to the heat exchanger, whereby the fog fluid flows through the intermediate space between the layers in the direction of the longitudinal axis (axially) and is transformed into its gaseous form; and
• allowing the resulting gas to flow out via an outlet from the heat exchanger, whereby a dense, opaque fog is generated as soon as it enters the environment.

[0018] The present invention also provides a fog generator comprising a heat exchanger as described here. Furthermore, the present invention also provides a fog generator comprising a reservoir which contains a fog-generating fluid, and a heat exchanger according to one of the embodiments of the present invention. The reservoir for the fog-generating fluid may be incorporated in the fog generator, both replaceably and non-replaceably.

BRIEF DESCRIPTION OF THE FIGURES

[0019] 

Fig. 1: Fog generator according to the prior art (described in EP1985962)

Fig. 2: Improved fog generator described in PCT/EP2013/078112 (not prior art)

Fig. 3: Fog generator according to the invention: Cross-section through the heat exchanger along the common, longitudinal axis (12) of the layers.

Fig. 4: Fog generator according to the invention: A. Cross-section through the heat exchanger transversely to the longitudinal axis of the layers (2). B. Enlargement of the indicated part of A.

Fig. 5: Layers with spacers. Side view of a layer (2) on which spacers (8) are applied in a pattern. The flow direction of the fog fluid is indicated with an arrow.

Figs. 6, 7 and 8: Layers with spacers. Cross-section of layers (2) of the heat exchanger according to the invention with spacers (8) which determine the intermediate space (3) between the layers. Different embodiments of the spacers (8) are shown in figures 6, 7 and 8.

[0020] As described herein, a fog generator according to the prior art (Fig. 1) comprises a reservoir (A) with fog-generating fluid (B) therein. This fluid is propelled e.g. by a pump or a propellant gas (C) to a heat exchanger (D) which consists of a channel(s) (E) surrounded by a heat-retaining material which is heated by a heating element (F). On flowing through the channel(s), the fluid is converted into its gaseous phase. In this way a dense fog is formed on expulsion of the gas, due to subsequent condensation thereof in the atmosphere.

[0021] An improved heat exchanger which can better handle the high flows necessary for a higher rate of fog fluid gasification is presented in Fig. 2 (PCT/EP2013/078112). This also contains a reservoir (A) with a fog-generating fluid (B). This is propelled by gas generated after ignition of a pyrotechnic substance (H). The heat exchanger (D) consists of a plurality of stacked plates (G). The plates have a passage (I). Due to the staggered arrangement of these passages, the fog fluid must pass through a "labyrinth". Thus the fluid comes into extended contact with practically the entire surface area of the hot plates, and is transformed into its gaseous form. The heat exchanger from PCT/EP2013/078112 is characterised by the following data: around 70% of the internal space is filled by the plates (193 ml plates to 82 ml free volume) and a contact area between the plates and the through-flowing fluid is around 11 dm² (area available for heat exchange).

[0022] Figures 3 and 4 show a specific embodiment of the heat exchanger according to the invention (1). The heat exchanger (1) comprises several layers (2) with the same longitudinal axis (12) and an intermediate space (3) between the layers. The heat exchanger comprises an inlet (4) for the fog fluid and an outlet (5) for the gasified fog fluid. The layers consist of a temperature-retaining material, e.g. metal. By bringing these to a high temperature (above the boiling point of the fog fluid, e.g. 250 °C), the fog fluid will gasify when it flows from the inlet (4) in the direction of the longitudinal axis (axially) through the intermediate space to the outlet (5).

[0023] Preferably, the thickness of the layers is greater than the thickness of the intermediate space. In a particular embodiment, the ratio between the thickness of the intermediate space and the thickness of the layers is between 1:2 and 1:50, in particular between 1:3 and 1:30, more particularly between 1:4 and 1:20. In another particular embodiment, the ratio between the thickness of the intermediate space and thickness of the layers is around 1:10. The layers preferably have a thickness of less than 5 mm, in particular less than 3 mm, more particularly less than 2 mm. In a particular embodiment, the thickness of the layer is 0.1 mm or greater, in particular greater than 0.2 mm. Preferably, the layers have a thickness between 0.1 mm and 5 mm. The thickness of the intermediate space is preferably less than 1 mm, in particular less than 0.5 mm, more particularly less than 0.3 mm. In a particular embodiment, the thickness of the intermediate space is around 0.05 mm. In a particular embodiment, the heat exchanger according to the invention comprises at least 5 layers, in particular at least 10 layers, more particularly at least 15 layers. Preferably, the heat exchanger according to the invention comprises at least 20 layers.

[0024] One of the advantages of the heat exchanger according to the invention is a high filling ratio (large quantity of heat-retaining material per internal volume of the container) coupled with a very large surface area for heat exchange, and at the same time a low dynamic pressure fall. An embodiment as shown in Fig. 2 has a central, solid metal rod (6) of 16 mm diameter, a container (7) with an internal diameter of 54 mm and a length of 140 mm. If the container were completely filled with steel with a specific weight of 7.8 kg/l, then the weight of this solid rod is: π * (54 mm / 2)² * 140 mm = 0.320 l => 2.5 kg steel

[0025] In the embodiment shown in Fig. 2,
the weight of the central rod is:

π * (16 mm/2)² * 140 mm = 0.028 I => 0.218 kg steel

and a spiral plate of 3,843 mm with the thickness of 0.5 mm:

3,843 mm * 140 mm * 0.5 mm = 0.269 l => 2.1 kg steel

[0026] Hence the fill ratio of an embodiment as presented in fig. 2 in comparison with a solid rod is:

(0.218 kg + 2.1 kg) in relation to 2.5 kg = 92 %

[0027] In the example above, the clearance section of the intermediate space is 182.15 mm² (3,843 mm * 0.5 mm), which corresponds to an (empty) tube of 15.6 mm. As the intermediate space is narrow, the viscous through-flow resistance will be greater but the dynamic pressure fall at a flow of 70 ml/sec remains negligible.

[0028] For comparison with the heat exchanger presented in PCT/EP2013/078112 (see fig. 2 of the present application):

	Practical maximum fill ratio	Area for heat exchange
PCT/EP2013/078112	70%	10.5 dm2
Present invention	92%	107.6 dm2

[0029] It is thus clear that the heat exchanger according to the invention allows, in a simple manner, a very high fill ratio and a particularly large area for heat exchange.

[0030] As an example, a spiral plate of 3,843 mm may be considered with a width of 110 mm and thickness of 0.5 mm. If this is made of stainless steel (e.g. AISI 430), this has a thermal conductivity of 25 W/m.K with a delta(T) of 100 °C. For one side of the plate (half thickness) the available capacity is thus:

[0031] For the two sides together, it is thus 8,454,600 W. As around 900 Joules (W.s) is required to evaporate 1 ml fog fluid, this means that such a heat exchanger is theoretically able to evaporate 150 ml fog fluid in less than one-tenth of a second. Such a quantity of fog fluid is more than sufficient to fill a volume of 150 m³ completely with an opaque fog.

[0032] There are various production methods for achieving a heat exchanger according to the invention. In a specific embodiment, the layers consist of a plurality of tubes with differing internal volume. Thus for example round, triangular, rectangular etc. tubes with a differing internal volume may be placed concentrically over each other and fixed (e.g. by using metal fixing lips or by welding to a fixing means at one or both axial ends of the tubes). In another embodiment, the layers consist of the spiral windings of a spiral plate. In this case for example a plate can be rolled into a spiral shape and inserted in a housing. In view of the ease of production of such a heat exchanger, the heat exchanger with a spiral-shaped plate is the preferred embodiment. The simplest method of production is to choose a metal plate with a high elasticity value (e.g. spring steel) as a material for rolling. The plate is rolled up tightly and inserted in the container (7) which has a slightly larger diameter than the diameter of the rolled plate. After insertion, the spiral will unwind slightly, whereby an even intermediate space is produced between the spiral windings of the plate. It is possible to fix the plate additionally by e.g. welding this to a fixing means at one or both longitudinal (axial) ends of the spiral plate, or by filling with a porous (liquid- and gas-permeable) permanent filling means such as small beads.

[0033] In a particular embodiment, the thickness of the intermediate space is determined by spacers (8). Preferably, the spacers are attached to or form part of the layers. Thus spacers may be fitted to the pipes or to a plate which is wound into a spiral shape. The spacers allow the thickness of the intermediate space to be determined very precisely and evenly, and may serve to fix the layers relative to each other. It is possible to use removable spacers to position the layers relative to each other during production, and then remove the spacers. So spacers may be attached e.g. with a solvent-soluble glue. After the layers have been fixed relative to each other with a fixing means at one or both axial ends of the layers, the spacers may be removed using a solvent which dissolves the glue. Preferably, the spacers however remain in the heat exchanger.

[0034] Any method may be used for applying spacers in or on the layers, as long as the spacers leave sufficient clear intermediate space to allow the axial through-flow of the fog fluid. Thus for example particles (e.g. a single layer of beads) may be glued to the layers, whereby the thickness of the intermediate layers is determined by the thickness of the beads (and glue). The particles may e.g. be glued into a pattern with sufficient space for through-flow, or the particles may be applied over the entire layer and then removed or dissolved e.g. after fixing of the layers.

[0035] A preferred embodiment in terms of simplicity, accuracy and cost, is the application of spacers in a pattern on the layers (see fig. 5). In a particular embodiment, the pattern is formed so that no rectilinear (axial) fluid flow is possible in the direction of the longitudinal axis. The fluid flow through the intermediate space is then disrupted into a divided and/or meandering through-flow, which benefits the exchange of heat.

[0036] As specified above, the spacers may be applied in various ways. Spacers may for example consists of metal, for example by the use of (e.g. self-adhesive) metal film. It is beneficial for production in this case to use prepunched self-adhesive metal film which itself adheres to a film carrier. By pressing the film carrier with the adhesive metal film onto the layers (e.g. tubes or plate), the punched spacers remain attached to the layers in the desired pattern. For this application for example, a stainless steel film may be used with the desired thickness (e.g. 0.05 mm). This production process can easily be automated, in particular if a plate is used which is rolled into a spiral shape. In this case, before or during rolling, the spacers may be applied to the plate by passing the metal film on its carrier together with the plate through a pressure roller. In this way the spacers are applied in the desired pattern before or during the rolling of the plate into the spiral shape. The thickness of the spacers determines the thickness of the intermediate space between the layers (in this case, the spiral windings of the plate). Fig. 6 shows a cross-section through a number of layers (2) on which spacers (8) are applied which determine the thickness of the intermediate space (3).

[0037] In another embodiment, the spacers form part of the layers. The layers may for example be formed to create the spacers. Protrusions or ribs created in the surface prevent the layers from lying too closely together. The thickness of the protrusions (spacers) determines the thickness of the intermediate space. The layers may for example be pressed through at various places to create such spacers, as shown for example in fig. 7. An alternative possibility is to emboss the layers to create spacers, as shown in fig. 8. Embossing can be achieved by passing the plate for example between two rollers, of which at least one roller has an embossing profile, whereby the material is pressed away from the layer following the profile of the roller.

[0038] One of the advantages of the present invention is that the heat exchanger can be produced simply with nonetheless a very precise and uniform thickness of the intermediate space. A small thickness of the intermediate space ensures that the fog fluid transforms well into its gaseous form. Nonetheless, during production, non-optimum spaces (9) may occur which have a greater thickness than the thickness of the (optimum) intermediate space. For example, when a plate is rolled into a spiral shape, at the start and end of the plate, this plate may e.g. not connect perfectly to the container and/or an internal rod. In another example, with tubes pushed over each other, for example the tube with the smallest internal volume may still have a greater section than the intermediate space between two tubes. Also, if the heat exchanger is not carefully produced (e.g. uneven thickness of spacers), greater and thinner intermediate spaces may result. If greater (non-optimum) spaces occur, these have a negative effect on the fog formation since a 'cold channel' can occur wherein the fog fluid is not completely transformed into its gaseous phase. This may even lead to 'sputtering'.

[0039] The inventors have however found that the heat exchanger according to the invention can be improved further very simply and cheaply, so that even when produced with imperfections, the heat exchanger may function perfectly well without the formation of cold channels. After the layers of heat-retaining material have been placed in the container, (inert) beads (11) may be introduced. Preferably, these have a diameter which is so large that they cannot enter the (perfectly formed) intermediate space but can enter the larger spaces (so-called "non-optimum spaces"). The beats constrict the non-optimum spaces and prevent these from forming channels with a different large flow, known as "cold channels", while keeping the (perfectly formed) intermediate spaces totally clear for the passage of fog fluid. In the context of this application, "optimum spaces" are intermediate spaces with the desired thickness (e.g. the thickness of uniform spacers). "Non-optimum spaces" are spaces with a greater thickness.

[0040] A particularly practical method for producing the heat exchanger according to the invention is that, after insertion of the layers in the container (e.g. a cylindrical tube (7) as shown in Fig. 3 and 4), small beads are scattered on the top. By vibrating the assembly for example, the beads fall into all spaces in which they fit. In particular, the diameter of the beats is selected such that they are larger than the thickness of the (optimum) intermediate spaces. In other words, a relatively minimum diameter of the filling beads is taken into account in the design choice concerning the intermediate spaces. The invention therefore allows, in a simple manner, very precise setting of channel parameters. If the desired thickness of the intermediate space is e.g. 0.05 mm (e.g. by use of spacers with this thickness), beads of e.g. 0.06 mm can be used. In this way, all spaces with the thickness of 0.06 mm or greater are filled by the beads, and only spaces with the desired thickness of less than 0.06 mm remain.

[0041] By scattering a surplus of beads onto the rods, in addition a layer of beads is created above the layers. These may be removed but may also serve as a distribution means. In a preferred embodiment, the heat exchanger according to the invention also comprises a filtering means to prevent the outflow of beads from the container. Such filtering means may be located in the vicinity of the outlet and/or inlet. The filtering means may be the same as or different from the distribution means. One example is the use of a mesh gauze (10a and 10b) above and/or below the container.

[0042] The beads (11) may be selected from a material which may or may not contribute to the heat capacity of the heat exchanger. Preferably the material of the beads is a material which contributes to the heat capacity, such as metal beads. The beads may take any form, but in a particular embodiment they are substantially spherical. The beads preferably comprise at least partially a corrosion-resistant material. In a particular embodiment, the beads consist of stainless steel. In another embodiment, the beads have a metal core which is surrounded by a corrosion-resistant layer.

[0043] The longitudinal axis (12) of the heat exchanger according to the invention may also comprise a heat-resistant material, but this is not essential. For a heat exchanger with a spiral-shaped plate, the spiral windings may extend from the longitudinal axial line (12) radially towards the outside, or they may extend from a distance from this longitudinal axis radially towards the outside. The latter is shown for example in figs. 3 and 4, wherein a central rod is shown. The spiral windings start from the peripheral diameter of the central rod and run radially towards the outside. The same applies to a heat exchanger with tubes with differing internal volume. Thus the innermost tube may have such a small diameter that the through-flow is the same as in the mean intermediate space. If the internal volume is greater, this may also be filled with the (inert) beads described above. The central rod may be hollow or solid. If it is solid, it preferably consists of a heat-retaining material such as metal, and/or a phase-changing material as described in EP2259004. If it is hollow, it may be filled with (inert) beads or the heat exchanger may be constructed such that no fog fluid can flow through the hollow space. In a particular embodiment, the heat exchanger according to the invention comprises a heating element on the longitudinal axis of the layers. In a further embodiment, the heat exchanger according to the invention comprises a heating element surrounded by the plurality of layers. This heating element is preferably located centrally. This embodiment ensures a good distribution of heat to the surrounding layers and a simple production process. The present invention thus also provides a heat exchanger (1) for the gasification of fog fluid in a fog generator, the heat exchanger comprising a heating element surrounded by a plurality of layers (2) with the same longitudinal axis (12) and an intermediate space (3) between the layers, constructed such that the fog fluid flows through the intermediate space in the direction of the longitudinal axis (axially).

[0044] The present invention also provides a method for the production of a heat exchanger as described here, the method comprising:

• the rolling of a plate into spiral shape with the formation of an intermediate space between spiral windings, and
• the insertion of the spiral-shaped plate into a housing,

so that fog fluid can flow through the intermediate space in the direction of the longitudinal axis (axially).

[0045] The method may furthermore comprise the application of spacers to the plate before the plate is rolled. In addition, the method may comprise the addition of beads after the spiral-shaped plate has been inserted in the housing.

[0046] The heat exchanger according to the invention is particularly simple to produce and requires no welding of the material which ensures the heat storage and transfer. In addition, it may be produced in a cheap manner with good corrosion resistance. For creating the layers, for example stainless steel plate material may be used. This material is easy to use and cheap. If beads are used, particularly little material is required (a few grammes per heat exchanger). Furthermore, stainless steel beads of e.g. 0.06 mm are particularly cheap to purchase. The heat exchanger also allows particularly rapid gasification of a quantity of fog fluid injected under a very high pressure, thanks to its large heat exchange surface area relative to its weight and structural volume.

Claims

1. A heat exchanger (1) for gasifying a fog fluid in a fog generator, the heat exchanger comprising a plurality of layers (2) with the same longitudinal axis (12) and an intermediate space (3) between the layers, constructed such that the fog fluid flows through the intermediate space in the direction of the longitudinal axis.

2. Heat exchanger according to claim 1, wherein the thickness of the layers (2) is greater than the thickness of the intermediate space (3).

3. Heat exchanger according to claim 2, wherein the ratio between the thickness of the intermediate space and the thickness of the layers is between 1:2 and 1:50.

4. Heat exchanger according to any of the preceding claims, wherein the layers have a thickness between 0.1 mm and 5 mm.

5. Heat exchanger according to any of the preceding claims, wherein the layers comprise a metal core and optionally consist at least partly of a corrosion-resistant material.

6. Heat exchanger according to any of the preceding claims, wherein the heat exchanger has an inlet (4) for the fog fluid at a first longitudinal end of the layers, and an outlet (5) for gasified fog fluid at a second opposite longitudinal end.

7. Heat exchanger according to any of the preceding claims, wherein the thickness of the intermediate space is determined by spacers (8) which are attached to or form part of the layers (2).

8. Heat exchanger according to any of the preceding claims, furthermore comprising beads (11) with a diameter which is greater than the mean thickness of the intermediate space (3), wherein the beads (11) limit or prevent the passage of fog fluid through spaces other than the intermediate space.

9. Heat exchanger according to any of the preceding claims, wherein the layers are situated in a container (7) and wherein the internal volume of the container is filled to more than 70% by the layers (2).

10. Heat exchanger according to any of the preceding claims, wherein the layers consist of spiral windings of a spiral-shaped plate.

11. Heat exchanger according to any of claims 1 to 9, wherein the layers consist of a plurality of tubes with a differt internal volume.

12. A method for producing a heat exchanger according to claim 10, the method comprising:

- the rolling of a plate into a spiral shape, forming an intermediate space (3) between the spiral windings, and

- insertion of the spiral-shaped plate in a housing (7),
such that fog fluid can flow longitudinally through the intermediate space (3).

13. Method according to claim 12, furthermore comprising the application of spacers (8) onto the plate before the rolling into a spiral shape.

14. Method for generating a dense, opaque fog, the method comprising the following steps:

- heating of the heat exchanger (1) according to any of claims 1 to 11;

- introduction of fog fluid into the heat exchanger via an inlet (4) of the heat exchanger, whereby the fog fluid flows through the intermediate space (3) between the layers (2) in the direction of the longitudinal axis and is transformed into its gaseous form; and

- allowing the resulting gas to flow out via an outlet (5) from the heat exchanger, whereby a dense, opaque fog is generated as soon as it enters the environment.

15. A fog generator comprising a heat exchanger according to any of claims 1 to 11.

Drawing