Heat accumulator for fog generator

Abstract

The invention provides a heat accumulator (1) for vaporizing fog liquid in a fog generator, the heat accumulator comprising multiple closely contiguous, parallel oriented rods (2) with a diameter of between 0.2 mm and 15 mm.

Claims

1. A heat accumulator suitable for vaporizing a liquid, the heat accumulator comprising: multiple closely contiguous, parallel oriented, non-hollow rods with a diameter between 0.2 mm and 15 mm, comprising a metal core; inert beads around and/or between the multiple closely contiguous, parallel oriented non-hollow rods, wherein an average diameter of the inert beads is larger than 0.16 times a diameter of the multiple closely contiguous, parallel oriented, non-hollow rods.

2. The heat accumulator according to claim 1, wherein the multiple closely contiguous, parallel oriented, non-hollow rods have a diameter of between 0.5 mm and 5 mm.

3. The heat accumulator according to claim 1, wherein the multiple closely contiguous, parallel oriented, non-hollow rods at least partially comprise of corrosion-resistant material.

4. The heat accumulator according to claim 1, wherein the multiple closely contiguous, parallel oriented, non-hollow rods are located in a container, said container having an internal volume filled for more than 70% by the multiple closely contiguous, parallel oriented, non-hollow rods.

5. The heat accumulator according to claim 4, wherein the internal volume of the container, measured at the multiple closely contiguous, parallel oriented, non-hollow rods, is filled for more than 75% by the multiple closely contiguous, parallel oriented, non-hollow rods.

6. The heat accumulator according to claim 1, wherein the multiple closely contiguous, parallel oriented, non-hollow rods are stacked hexagonally.

7. The heat accumulator according to claim 1 comprising at least 7 of the multiple closely contiguous, parallel oriented, non-hollow rods.

8. The heat accumulator according to claim 1, further comprising a distribution agent.

9. A method for vaporizing a liquid, the method comprising: heating the heat accumulator according to claim 1; introducing a liquid via an inlet into the heat accumulator, whereby the liquid is converted into a gaseous form; and letting the gas obtained flow out via an outlet of the heat accumulator.

10. The method of claim 9, wherein the liquid is a fog generating liquid, and wherein the gas generates a fog as soon as it gets in the atmospheric environment.

11. A fog generator comprising a reservoir that comprises a fog generating liquid and a heat accumulator according to claim 1.

12. The fog generator according to claim 11, wherein the reservoir comprising the fog generating liquid comprises a movable wall with the fog generating liquid on a first side of the movable wall and a propellant on a second side of the movable wall.

Description

SHORT DESCRIPTION OF THE DRAWINGS

(1) FIG. 1: Prior art fog generator (described in EP1985962)

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

(3) FIG. 3: Fog generator according to the invention: cross-section parallel to the rods

(4) FIG. 4: Fog generator according to the invention: cross-section perpendicular to the rods

(5) FIG. 5: Fog generator according to the invention: detail of cross-section perpendicular to the rods

(6) FIG. 6: Detail of cross-section of optimally stacked rods

(7) As has already been described herein, a prior art fog generator comprises (FIG. 1) a reservoir (A) comprising the fog-generating liquid (B). This liquid is driven, for example by a pump or propellant (C), to a heat accumulator (D) that comprises (a) channel(s) (E) surrounded by thermal retention material heated by a heating element (F). This liquid is converted into its gaseous phase when flowing through the channel(s). When the gas is ejected, a dense fog is formed due to its subsequent condensation in the atmosphere.

(8) An improved heat accumulator, which can better deal with the higher debit in fog liquid vaporization, is represented in FIG. 2 (PCT/EP2013/078112). This also comprises a reservoir (A) with fog generating liquid (B). This is driven by gas generated after the ignition of a pyrotechnic substance (H). The heat accumulator (D) comprises multiple stacked plates (G). The plates have a passage (I). The connected stacking of these passages makes the fog liquid follow a labyrinth path. As such, the liquid comes extensively into contact with practically the entire surface of the hot plates and, in this way, is converted into its gaseous form. The heat accumulator from PCT/EP2013/078112 is characterised by the following data: approximately 70% of the internal space is filled with the plates (193 ml plates in respect of 82 ml free volume) and there is a touching surface between the plates and the liquid flowing through of approximately 11 dm.sup.2 (surface available for heat exchange).

(9) FIGS. 3 and 4 show a certain embodiment of the heat accumulator according to the invention (1). The heat accumulator comprises multiple closely contiguous, parallel oriented rods (2). The fog liquid enters the heat accumulator via the inlet (3) and flows through the rods, due to which it is heated and converted into the gaseous phase. The gas leaves the heat accumulator via the outlet (4). There is a distribution agent (5) at the inlet, in this case a terminal plate in the form of braided mesh (5a) (woven mesh). Moreover, there is a layer of inert beads (5b) at the top that facilitates further distribution. There are also collection means (6) at the outlet, here comprising a layer of braided mesh (6a) and a collection plate (6b), which combines multiple channels into a single outlet channel.

(10) In a practical embodiment with 1100 rods of 1.4 mm in diameter and 146 mm in length, manufactured from stainless steel (AISI 430), the outer surface of the rods is approximately 71 dm.sup.2 (surface available for heat exchange).

(11) The container with an internal volume of 288 ml, is then filled up 247 ml (83.5%) with rods and there is remaining free volume of 41 ml (16.5%). The total weight of the heat accumulator can, in this way, be limited, inclusive of rods (1925 g), bottom (270 g), cover and disks (252 g) and container (850 g) to only about three kilogram and this with a minimal total volume. The heat accumulator is preferably cylindrical, as this form is optimal in respect of thermal isolation and pressure resistance. The rods are preferably hexagonally stacked. More in particular, the rods are straight rods in a parallel orientation. A least 7 rods are required for hexagonal stacking, but at least 20 rods are preferably used. These quantities are needed to obtain a high density (herein also referred to as stacking density or filling percentage). In a particular embodiment, at least 100, more particularly 200 and in especially at least 500 rods are used.

(12) Although a theoretical stacking density of pi/(12^0.5)=0.9 can be obtained in case of optimal circle stacking (hexagonal stacking or hexagonal circle packing), it is lower in practice. As FIG. 4 shows, there is always a space into which no further rod fits (7), which will reduce the density. This disorder in the stacking cannot be avoided in practice and may result in cold channels throughout the heat accumulator. After all, liquid that flows through non-optimal channels, relatively seen, has a too large debit and cannot be fully converted into its gaseous form. However, it should be stressed that this cold channel formation and discharge of non-vaporised liquid is much more restricted than in case of a prior art heat accumulator as in FIG. 1. The heat accumulator described above can, without further modification, perform adequately and is suitable to vaporize liquid under high pressure and with a high debit.

(13) A solution against non-optimal channels is filling up these non-optimal channels by inserting rods with a suitable diameter (Apollonian packing). However, this is difficult to perform in practice because the locations, form and section size of the non-optimal channels in the production environment are difficult to predict, and it is cumbersome and error-prone to try and detect these via vision or optical sensors. Another way is to shape the inner wall of the cylinder (container) along the longitudinal direction (eg. extruded tube) in such a way that the hexagonally stacked rods fit with their stacking pattern to this wall. For example, longitudinal protuberances, cavities or polygon ribs may be provided to which to rods can closely connect. In this case, the wall is preferably implemented as such that the section of a channel that is formed between the wall and the adjacent stacked rods is always smaller than or equal to the section A (FIG. 7) of an optimal channel (a channel formed between 3 perfectly stacked rods). However, the inventor has established that the heat accumulator according to this invention can be improved further very simply and cheaply. Inert beads can be introduced after the rods have been introduced, as compactly as possible, into the container in the heat accumulator. They preferably have a diameter that is so large that they cannot end up between perfectly stacked rods (with optimal channels between them), but can in the areas where there is no perfect stacking (the so-called non-optimal channels, 7). The beads constrict the non-optimal channels and prevent these from still forming channels with an abnormally high flow cold channels, while keeping the optimal channels between the perfectly stacked rods (8) completely free for the passage of the fog liquid. Optimal channels, in this application refers to channels that are formed by three rods. Non-perfect channels are formed by at least four rods or are partly formed by the inner wall of the cylinder (wall); these are described as non-optimal channels in this application.

(14) An especially practical method for producing a heat accumulator according to the invention is to disseminate beads on top of the rods after introducing them in the container (e.g. a cylindrical tube (9) as shown in FIGS. 3 and 4). By, for example, vibrating it entirely, the beads will fall into all the spaces where they fit in (the inscribed circle within the non-optimal channels). It was established that only about six grams of beads with a diameter of 0.3 mm are required for a kilogram of rods with a diameter 1.4 mm. Moreover, by disseminating an abundance of beads, a layer of beads is created on top of the rods (5b). These can be removed, but can also be used as distribution agent. A preferred embodiment of the heat accumulator according to the invention also comprises a filter agent; this to prevent the beads from flowing out of the container. Such filter agent can be located in close proximity of the inlet and/or the outlet. The filter agent can be the same as or different to the distribution agent. An example is using braided mesh (5a and 6a) at the top and bottom of the container.

(15) The diameter of the inscribed circle (10) between the three perfectly stacked rods can be calculated as follows. The sum of the radius of the inscribed circle (r2) and the radius of the rod (r1) forms the hypotenuse (c) in a rectangular triangle with a rectangular side that is the radius of the rod (FIG. 6). The angle between this hypotenuse (c) and the rectangular side (b), within a hexagonal stacking, is always 30. The hypotenuse (c) then has a length of b/cos(30. Thus, r1/(r1+r2)=)cos(30), or r2 is r1*(1/cos(30)1). Therefore, the ratio between the radius of the rods (r1) and the radius of the inscribed circle (r2) is approximately 1 to 0.1547; this ratio of course also applies to the diameters and the inscribed circle. Beads with a minimum diameter of more than 0.16 times the diameter of the rods are therefore used in a preferred embodiment. Thereby, the optimal channels (spaces between the optimally stacked rods) are not filled with the beads, but the beads actually occupy the non-optimal channels (channels with an inscribed circle that is larger than the diameter of the beads).

(16) In other words, the design choice with regard to the diameter of the rods corresponds with a proportional minimal diameter of the filler beads. The invention therefore allows for setting the channel parameters accurately in a very simple way. In a further embodiment, beads are used with a diameter between 0.16 and 0.7 mm, in particular between 0.16 and 0.5, and more in particular between 0.16 and 0.3 times the diameter of the rods.

(17) The section of an optimal channel, located between the three rods with the same diameter, can be calculated by reducing the area of the triangle from FIG. 6 with half of the area of the section of the rods. Therefore, the section A is (see FIG. 7):

(18) $\frac{D*\left(D*\sqrt{\frac{3}{2}}\right)}{2}-\frac{*\left(\frac{D^2}{4}\right)}{2}$
with D being the diameter of the rods. It is of course also possible to use rods with different diameters, although the section of optimal channels (formed by only three rods) then no longer complies with the formula above. Rods with the same diameter are used in a preferred embodiment.

(19) The beads can be made from a material that contributes or doesn't contribute to the heat capacity of the heat accumulator. The material of the beads is preferably a material that contributes to the heat capacity, such a metal beads. The beads can be of any shape, but are substantially spherical in a particular embodiment. The beads preferably comprise, at least partially, a corrosion-resistant material. The beads comprise stainless steel in a particular embodiment. In another embodiment, the beads comprise a metal core surrounded by a corrosion-resistant layer.

(20) The heat accumulator according to this invention is very simple to produce and does not require any welding of the material that takes care of the heat storage and transfer. Moreover, it can be produced cheaply with a good corrosion resistance. Stainless steel coil material can, for example, be used for producing the rods. This material is easy to use and cheap and it can simply be cut to the desired length. Very little material is required (a few gram per heat accumulator) if beads are used. Moreover, stainless steel beads of 0.3 mm are very cheap to procure. Moreover, the heat accumulator allows for a particularly fast vaporization of an injected quantity of fog liquid under very high pressure thanks to its large heat exchange surface in relation to its weight and occupied volume.