A FOG GENERATING DEVICE

Abstract: This invention relates to a fog-generating device comprising a container (1) with an alcohol containing mixture (4) for generating the fog and means (5) for driving the alcohol mixture from the container (1) to a heat exchanger (2) for evaporating the alcohol mixture (4). The heat exchanger (2) is connected to an outlet orifice (6) for expelling the fog (7) under pressure. The means for driving the alcohol mixture (5) comprise an amount of a propellant gas, the propellant gas having a pressure approaching its vapour pressure. The propellant gas is at least partly soluble in at least one component of the alcohol containing mixture and is preferably selected from the group of partly fluorinated hydrocarbons. The container (1) is closed off by a valve (8), which is provided to be closed after it has been opened for a pre-set period of time, to seal the container (1) from the environment.

Description: A fog generating device. Background of the invention. The present invention relates to a device for generating fog as disclosed in the preamble of the first claim. Fog generating devices find large interest in the field of alarm devices. In EP-A-0.730.771 a fog-generating device is disclosed from which, upon activation by an alarm device, approximately instantaneously a volume of a hardly transparent vapour is expelled. Expelling the vapour reduces the visibility in the neighbourhood of the device to a minimum and complicates the sense of direction of unauthorised persons trying to get access to the room in which the fog has been expelled. This is of special interest in case for shops or warehouses where often significant amounts of goods are withdrawn in case of burglary. The device disclosed in EP-A-0.730.771 comprises a first container in which a fog generating mixture of propylene glycol and triethylene glycol is stored together with an amount of water as a driving agent. The mixture is stored at high temperature, below the boiling point of the alcohol mixture, but above the boiling point of water. As a consequence the water is at least partly evaporated thus ensuring an over pressure in the first container. The first container is connected to a heat exchanger, the latter being equipped with a heat exchanger for heating and evaporating the alcohol mixture to generate the fog. The heat exchanger is maintained at a second temperature above the first temperature, sufficiently high to evaporate the alcohol mixture. The connection between the first container and heat exchangers is ensured by a valve, which in normal circumstances is closed, but is opened upon activation by the alarm device. Upon activation by the alarm device, the valve between the first container and heat exchanger is opened. Because of the over pressure in the first container the alcohol mixture is forced to flow from the first to the heat exchanger upon opening of the valve. In the heat exchanger the mixture is further heated, evaporated and finally expelled from the outlet to generate the fog. However, as the hot glycol/water mixture is corrosive, the parts through which this mixture is displaced need to be made of expensive non- corrosion sensitive materials. There is thus a need to a device that is less subject to corrosion. From GB-A-1.039.729 a smoke generator is known for generating smoke for testing ventilation systems, wind tunnel testing, air flow visualisation etc. The smoke generator of GB-A-1.039.729 comprises a reservoir with liquid carbon dioxide. Upon opening of the reservoir, a pre-determined amount of liquid carbon dioxide passes to a pressure gauge and further through a pressure- reducing valve towards an oil-containing reservoir. Simultaneously with forcing the oil to flow from the reservoir trough a draw off pipe, the carbon dioxide gas passes into a continuous stream of fine bubbles, which mixes with the oil to form a foam. From the oil reservoir, the oil is forced to flow through a heat exchanger and further to valve from which, when opened a non-explosive, dense white smoke is expelled for a period of two minutes approximately. Carbon dioxide is preferred, as this gives a dense white smoke. Brief description of the invention. It is an object of this invention to provide a fog- generating device, which is less subject to corrosion. A fog-generating device, which is less sensitive to corrosion, may be obtained with the technical features of the characterising part of the first claim. Detailed description of the invention. In the device of this invention, the means for driving the alcohol mixture from the container in which it is stored to the heat exchanger, comprise an amount of a propellant gas, at least part of which is dissolved in the fog-generating alcohol mixture. The propellant gas is selected from the group of partly halogenated hydrocarbons, preferably from the group of partly fluorinated hydrocarbons. The amount of propellant gas is chosen such that the pressure in the alcohol/propellant container approaches the vapour pressure of the propellant gas. The composition of the fog-generating alcohol mixture is chosen such that it contains at least one component capable of at least partly dissolving the propellant gas. The container thus contains the liquid alcohol mixture with dissolved propellant gas, the latter being in equilibrium with propellant gas in the gas phase above the liquid level. The use of a propellant gas for driving the fog generating mixture from the container in which it is stored to the heat exchanger, has the advantage that corrosion problems may be minimised. At the temperatures remaining in the heat exchanger glycol compounds get oxidised, decompose and entail corrosion problems. The inventor has observed that after fog has been generated and expelled and the container with the fog generating mixture is closed off again, still some of the fog generating alcohol mixture remains in the connection between the container and the heat exchanger, as well as in the heat exchanger itself. The inventor has now found that by using a propellant gas as a driving means the heat exchanger is freed of remaining alcohol mixture, which he explains as follows. After the fog has been expelled, the pressure within the heat exchanger drops to a value approaching atmospheric pressure. At atmospheric pressure, the propellant gas dissolved in the alcohol mixture expands and escapes, and forces flowing of any fog generating mixture remaining in the heat exchanger towards the outlet of the heat exchanger. As the heat exchanger is freed of remaining alcohol mixture, the risk to corrosion may be minimised. The use of propellant gas as means for driving the fog generating alcohol mixture has the advantage that an improved expansion of the fog generating mixture may be obtained and a fog of a prolonged life time, with a reduced tendency to raise. Without wanting to be limited thereto, the inventor believes that this effect may be attributed to three phenomena that are related to each other. First of all partly halogenated hydrocarbons have a high vapour pressure at room temperature, mostly above 8 or 10 bar. Upon opening of the container with the fog generating mixture, the propellant gas produces a high expansion coefficient. The expansion of the alcohol mixture when expelled from the outlet of the heat exchanger appeared to be more intensive than obtainable with the prior art device, and to involve an improved dispersion of the hot alcohol mixture to a fog with droplets with a smaller average size. The smaller average size of the droplets results in a fog the properties of which approach the properties of an aerosol, with particles with an improved tendency to floating and a diminished tendency to condensation. Secondly it has been found that the fog cloud takes a temperature not too far from the environment temperature. This may be explained as follows. Evaporation of the alcohol mixture while it flows through the heat exchanger involves an increase of the pressure and over-heating of the propellant gas. As a result, the propellant gas dissolved in the alcohol mixture as well as the part present in the gas phase show an intensive expansion and can be said to explode as it were, when expelled from the outlet of the heat exchanger. The energy required for this explosion of the propellant is withdrawn from the hot alcohol fog, which in turn gets cooled. The result is a fog cloud with a temperature approaching the environment temperature, with a limited tendency to move towards the ceiling. The better the expansion of the propellant gas, the better cooling may be obtained, the better the aerosol properties, quality and life time of the fog will be. Thirdly, the fog obtainable with the device of this invention has a lower water content than the prior art fog, with a reduced tendency to condensate and a longer condensation lag. In the prior art device the presence of water in the fog generating mixture is required to amplify the break-up effect of the fog generating mixture into small droplets when ejecting the fog. In the present device the propellant gas produces this effect, so that the use of water to this end can be largely dispensed with. The inventor has further observed that the fog cloud takes and remains at the relevant position approximately between the ground floor and 2 or 3 meter above it. This is the relevant position approximating the position of somewhat more than the length of a burglar. In the device of this invention the volume ratio of the amount of propellant with respect to the amount of alcohol preferably ranges from 2- 30 %, preferably about 10-25 %. Varying the amount of propellant allows controlling the temperature of the fog cloud and adapting its temperature to the temperature of the environment to improve the quality and lifetime of the fog cloud. Below 2 % the break up and dispersion become unsatisfactory, above 30 % the density of the fog becomes insufficient. It is preferred to have the container with the fog generating mixture and the propellant gas mounted into a housing, which is removably connectable to the heat exchanger. The use of a removable unit allows re-use of the fog-generating device after it has been put to work and improving safety of the device. With former device the fog generating alcohol mixture was stored in the container at elevated temperature. Before refilling care needed to be taken to cool the first container sufficiently down before adding new alcohol or before removing it and replacing it by a new one. As with the present invention, the fog generating alcohol mixture may be stored at room temperature, the precautions required with the known device are rendered superfluous. Description of the figures. The invention is further elucidated in the appending figures and the description of the figures. Figure 1 shows a cross section of a preferred embodiment of the device of this invention. As can be seen from figure 1, the device of this invention comprises a container 1 connected to a heat exchanger 2 through a pipe 3. The container 1 contains a mixture 4 for generating the fog. The container 1 is preferably mounted in a housing, which is removably connectable to the heat exchanger 2, through a coupling 9. The heat exchanger 2 is provided for evaporating the fog generating mixture 4 that is contained in the container 1. The heat exchanger 2 gives access to an outlet orifice 6 from which fog 7 may be expelled into the environment. Container 1 is closed off by means of a valve 8. Valve 8 is provided to be opened upon actuation, to allow flowing of the fog generating mixture 4 from the container 1 to the heat exchanger 2. Thereto valve 8 may for example be coupled to an alarm device and be opened upon actuation by the alarm device. Valve 8 is provided to be closed off after a pre-set period of time, preferably after a pre-set volume of the fog generating mixture has left the container 1 and a predetermined volume of fog has been generated. A timer connected to valve 8 may for example effect the closing of valve 8 after a pre-set period of time. This allows re-using of the device without involving the necessity of each time replenishing the fog-generating mixture. The amount of fog released will be determined by the opening time of valve 8, the total resistance of the valves and pipes and the pressure of the propellant. To force flowing of the fog generating mixture 4 from the container 1 to the heat exchanger 2 when opened, the container 1 contains driving means 5 for driving the fog generating mixture 4. The driving means 5 comprise an amount of a propellant gas, at least part of which is dissolved in at least one component of the alcohol mixture responsible for the fog generation. The pressure of the propellant gas is adjusted to a value, which may be somewhat lower than, but preferably approaches the vapour pressure of the propellant gas. With this vapour pressure an optimum expansion and break up of the evaporated fog generating mixture may be obtained. The fog generation is optimised by selecting the nature of the propellant gas such that at least part of it is soluble in at least one component of the fog generating mixture and by optimising the amount of propellant in the fog generating mixture. Suitable propellant gases for use with this invention include the group of partly halogenated, preferably partly fluorinated hydrocarbons as these have a relatively high vapour pressure at room temperature, i.e. a vapour pressure above 4, preferably 8, more preferably 10 bar. Below 4 bar the expansion coefficient of the propellant is getting low, above 10 bar an optimum fog generation has been found. The amount of propellant gas present in the container 1 may be varied within wide ranges. In general however care will be taken to dissolve as much propellant gas in the alcohol mixture as possible and to have dissolved propellant in equilibrium with propellant in the gas phase. Preferred propellant gasses are the so-called partly fluorinated hydrocarbon HFC gasses, or mixtures containing of one or more HFC gasses. Preferred HFC gasses include difluoromethane, trifluoroethane, CH3-CF3, 1 ,1 ,1 ,2-tetrafluoroethane, pentafluoroethane CHF2-CF3, 1 ,1 ,1 ,2,3,3,3- heptafluoropropane and mixtures of two or more of these compounds. These mixtures are generally commercially available as for example R125, or from Solvay as so-called Solkanes®, e.g. Solkane 410A, Solkane 507 etc. As the fog generating mixture 4, use can be made of the mixtures generally known to the man skilled in the art. The fog generating mixture suitable for use with the device of this invention will usually contain water, at least one alcohol and at least one glycol. If so desired, mixtures of two or more alcohols, and mixtures of two or more glycols may be used. However, to optimise the quality of the fog, the fog generating mixture preferably contains approximately 5-15 vol. % of water, 7.5-25 vol. % of ethanol, 50-80 vol % of glycol. The glycol may be a mixture of 10-25 vol. % of triethylene glycol, the remainder being dipropylene glycol, but other glycols and glycol mixtures may also be used. An example of a very suitable fog generating alcohol mixture is: 10 vol. % of water, 16.5 vol. % of ethanol, 10 vol. % of triethylene glycol, 63 vol. % of dipropylene glycol. An example of another suitable fog generating mixture is 14.3 vol. % of ethanol, 54.8 % of dipropylene glycol, 8.8 % of triethylene glycol, 8.8 % of water and 13.3 % of HFC R- 125. Ethanol is added as it is a solvent for the propellant gas. However, other alcohols may be used as well. The presence of dipropylene glycol is preferred over the lower glycols such as for example diethylene glycol, as the former hardly reacts with water at the temperatures remaining in the device, whereas the latter is reactive with water to produce corrosive compounds. The presence of triethylene glycol is preferred as this allows increasing the density of the fog. The presence of dipropylene glycol is preferred as the transformation of this compound to an aerosol is easier, because of its lower boiling point and viscosity. The presence of dipropylene glycol involves the formation of droplets with a relatively smaller size as compared to triethylene glycol, thus reducing the transparency of the fog. The result of using this mixture is a dense, hardly transparent fog with a retarded tendency to condensation. The presence of water is preferred as this renders the mixture inflammable. However, it is preferred to limit the amount of water in the fog generating mixture to a value below 15 vol. %, preferably below 10 or 5 vol. % to limit decomposition of the glycol when heated and to prolong the life time of the fog cloud. The fog generating alcohol mixture may further comprise additional compounds, if so desired. Suitable examples include menthol, tearing gas etc. It is important that when removing the container 1 , escape of any of the fog-generating mixture remaining in the container 1 into the environment is limited as much as possible. To achieve this, the container comprises a valve 8, which remains closed as long as the device is not actuated to generate fog and is closed again after it has been opened for a pre-set period of time to allow fog generation. This can for example be achieved by providing an electric current towards the valve 8 upon actuation of the device. As long as there is no electric current towards the valve 8, the valve remains closed and closes off the container 1. As soon as it receives electric current, valve 8 is opened to allow a flow from the container 1. The time valve 8 remains open is preferably controlled, e.g. by a timer, to prevent that the whole fog generating mixture is consumed in one go. The same effect can however be obtained in a purely mechanical manner, with a valve operable from the outside of the container 1. The removable connection of the container 1 to the heat exchanger 2 facilitates replenishing of the fog generating mixture after it has been consumed and improves the safety of the device. It is intended that the container 1 is replaced as a whole, to avoid replenishing of the alcohol mixture as such. Thus, there is no need to take care that the alcohol mixture has cooled down to a sufficient extent, before replenishing it as was the case with the prior art device. The use of a propellant gas as a means for forcing the fog generating mixture to flow from the container 1 to and through the heat exchanger 2 has several advantages. The device of this invention is energy saving as compared to the prior art device, mainly for two reasons. The device of this invention is thus capable of generating a sufficiently high pressure from its own components. The use of a propellant gas as a means for driving the fog generating mixture involves that the fog generating mixture is forced to leave the container upon simple opening of the container 1. The propellant gas plays the key role in increasing the pressure when opening the container 1 , in the heat exchanger, as well as in breaking up the liquid alcohol mixture to form the fog. The use of steam to these ends may thus be dispensed with. Secondly, by using a propellant gas as a driving means, there is no need anymore to store the fog generating alcohol mixture in the first container at high temperature. In the absence of the need to generate steam, the dimensions of the device may be decreased. The inventor has further found that the propellant gas dissolved in the fog generating mixture has a significantly higher break up capacity as compared to steam, which means that a pre-set volume of fog may now be generated with a smaller volume of the fog generating mixture. The advantage is entailed that the amount of alcohol mixture stored in the container may be reduced, as may the the dimensions of the device, without this going at the expense of the amount of fog created. Also, is no need anymore to store the fog generating alcohol mixture at elevated temperature to shorten the response time, which renders the device economically favourable. The use of propellant gas as a means for driving the alcohol mixture from the first container 1 to the heat exchanger 2 allows reducing the residence time of the fog generating mixture in the heat exchanger as the propellant gas has a larger expansion coefficient. The reduced residence time together wit the reduced water content allows minimising the risk to unwanted side reactions involving decomposition of the fog generating alcohol mixture in the heat exchanger. Moreover, by minimising the risk to the presence of remaining alcohol mixture when not in use, the risk to corrosion is minimised. Thus, the heat exchanger may be made of the cheaper usual materials, e.g. normal steel, and does no longer necessitate the use of high quality stainless steel. The use of propellant gas as a means for driving the alcohol mixture from the first container 1 to the heat exchanger 2 result in an improved dispersion of the fog generating mixture when expelled. The simultaneously reduced water adds to this that the droplet size when expelling the fog, may be further reduced. The use of a propellant gas as a driving means allows improving the safety of the device. The propellant gases used with the present invention are inflammable, have fire-extinguishing properties and reduce the risk to explosions. The inventor has further observed that after the fog has been expelled and the valve 8 is closed off, still some of the fog generating alcohol mixture remains in the connection between the container and the heat exchanger as well as in the heat exchanger itself. In the prior art device this alcohol mixture gets decomposed in the heat exchanger 2 as this one is still at high temperature. With the present invention, this problem can be overcome as the majority of the fog generating mixture which would remain in the heat exchanger is forced to leave the heat exchanger by the propellant. The inventor namely arrived at the insight that after the fog has been expelled, the pressure within the heat exchanger drops to a value approaching atmospheric pressure. At atmospheric pressure, the propellant gas dissolved in the alcohol mixture expands and escapes, and forces flowing of any fog generating mixture remaining in the heat exchanger 2 between the valve 8 and the outlet 6 towards the outlet 6 of the heat exchanger 2. The operation of the device of this invention may be described as follows. The container 1 contains an amount of an alcohol mixture as described above, and an amount of propellant gas with a vapour pressure of above 10 bar at room temperature. The propellant gas above the liquid phase is in equilibrium with the propellant gas dissolved in the alcohol mixture. Usually a layer of liquid propellant gas resides at the bottom of the container 1. Upon actuation by an alarm device, valve 8 closing of the interior of the container 1 , is opened. This involves a sudden expansion of the propellant present in the gas phase and the liquid phase at once, and an instantaneous displacement of an amount of the complete alcohol mixture 4 from the container 1 through valve 10 and connection 3, towards the heat exchanger 2. In the heat exchanger 2 mostly first the ethanol will be evaporated, followed by the water and the glycol - although these phenomena take place almost simultaneously, giving rise to the formation of a fine mist. Because of the high temperature remaining therein, pressure increases as a consequence of which the vapour is expelled through the outlet orifice 6 at high velocity as an aerosol of fine liquid particles. The expansion of the propellant gas when leaving the outlet orifice involves a cooling of the alcohol/glycol vapour to a temperature approaching environment temperature. The expelled fog is a dense white fog, which takes a position between approximately the floor and 2-3m above it to remain there for a prolonged period of time. When the predetermined amount of alcohol mixture has been forced to leave the container, the device is automatically cleaned from any remaining alcohol mixture by the propellant gas. As soon as the predetermined amount of alcohol has left the container and has been expelled, heating of the heat exchanger is interrupted. The pressure within the heat exchanger drops to a value approaching atmospheric pressure. The propellant gas dissolved in the alcohol mixture remaining in pipe 3 expands and escapes and forces any remaining fog generating mixture to flow towards the outlet 6 of the heat exchanger 2.

Claims: CLAIMS 1. A fog-generating device comprising a container (1) with an alcohol containing mixture (4) for generating the fog and means (5) for driving the alcohol mixture from the container (1) to a heat exchanger (2) for evaporating the alcohol mixture (4), the heat exchanger (2) being connected to an outlet orifice (6) for expelling the fog (7) under pressure, characterised in that the means for driving the alcohol mixture (5) comprise an amount of a propellant gas, the propellant gas having a pressure approaching its vapour pressure, the propellant gas being at least partly soluble in at least one component of the alcohol containing mixture and being selected from the group of partly halogenated hydrocarbons. 2. A fog-generating device as claimed in claim 1 , characterised in that the volume ratio of the amount of propellant with respect to the amount of alcohol mixture ranges from 2-30 vol. %. 3. A fog-generating device as claimed in claim 1 or 2, characterised in that the propellant gas is selected from the group of partly halogenated hydrocarbons and mixtures of one or more partly halogenated hydrocarbons which have a vapour pressure at room temperature of at least approximately 4 bar, preferably at least approximately 10 bar. 4. A fog generating device as claimed in claim 3, characterised in that the propellant gas is selected from the group of partly fluorinated hydrocarbons and mixtures thereof. 5. A fog-generating device as claimed in any one of claims 1 -4, characterised in that the propellant gas is selected from the group of partly fluorinated ethane or methane. 6. A fog-generating device as claimed in any one of claims 1-5, characterised in that the container is mounted into a housing, which is removably connectable to the heat exchanger. 7. A fog-generating device as claimed in any one of claims 1-6, characterised in that the alcohol mixture comprises at least one alcohol and at least one glycol. 8. A fog-generating device as claimed in any one of claims 1-7, characterised in that the fog generating mixture (4) comprises approximately 5-15 vol. % of water, 15-25 vol. % of ethanol, 10-20 vol. % of triethylene glycol, the remainder being diethylene glycol. 9. A fog-generating device as claimed in any one of claims 1-8, characterised in that the container (1) is closed off by a valve (8), which is provided to be closed after a pre-set period of time to seal the container (1 ) from the environment.

Inventors: VANDONINCK ALFONS, VANDONINCK, ALFONS
Applicants: BANDIT, VANDONINCK ALFONS, VANDONINCK, ALFONS
Application year: 2001
Main classification: F41H9
IPC codes: F41H9