Particle precipitator

Abstract

A particle separation system for separating particles in an airflow upstream of a detection chamber in an aspirating smoke detector is disclosed. The particle separation system includes an airflow path for directing the airflow from an inlet to an outlet. The airflow path includes a first airflow path section in a first direction and a second airflow path section in a second direction, the first and second directions being different relative to each other. The airflow path also includes at least one electrically charged surface such that the airflow undergoes electrostatic precipitation as it traverses the airflow path. A method of separating particles in an airflow upstream of a detection chamber in an aspirating smoke detector is also disclosed.

Claims

1. A particle separation system for separating particles from an airflow in which they are entrained, at a position upstream of a detection chamber in an aspirating smoke detector, the particle separation system including an airflow path for directing the airflow from an inlet to an outlet, the airflow path including: a first airflow path section in which the airflow passes in a first direction and a second airflow path section in which the airflow passes in a second direction, wherein the first and second directions are different relative to each other; the intersection between the first airflow path section and the second airflow path section includes a transition region between the first and second directions; and the transition region includes a bend or curve where the two sections meet, and the airflow is caused to travel around the bend or curve as it moves from the inlet to the outlet; and at least one passively electrically charged surface located on the outside of said bend or curve downstream and adjacent the transition region and exposed to the airflow such that some particles in the airflow electrostatically adhere to the passively electrically charged surface as the airflow traverses the airflow path, whereby said adhered particles are separated from the airflow.

2. The particle separation system according to claim 1, wherein the electrically charged surface may be an electret material (polarized polymer), or the charge may be intrinsic in the surface itself.

3. A method for separating particles from an airflow in which they are entrained, at a position upstream of a detection chamber in an aspirating smoke detector, the method including: using a particle separation system located upstream of the particle detection chamber of the aspirating smoke detector, directing the airflow along a first airflow path section in a first direction and then along a second airflow path section in a second direction, wherein the first direction and second direction are different relative to each other; the intersection between the first airflow path section and the second airflow path section includes a transition region between the first and second directions; the transition region includes a bend or curve where the two sections meet, and the airflow is caused to travel around the bend or curve as it moves from the inlet to the outlet; and a passively electrically charged surface is located downstream and adjacent the transition region between the first and second directions on the outside of said bend or curve; and exposing the passively electrically charged surface to the airflow path such that some particles in the airflow electrostatically adhere to the passively electrically charged surface as the airflow traverses the airflow path, whereby said adhered particles are separated from the airflow.

4. A particle separation system for separating particles from an airflow in which they are entrained, at a position upstream of a detection chamber in an aspirating smoke detector, the particle separation system including: a particle separation chamber having a first cross section; an inlet airflow path for introducing the airflow into the particle separation chamber; and an outlet airflow path for exiting the airflow from the particle separation chamber; wherein the inlet and outlet airflow paths have cross sections smaller than the particle separation chamber such that the airflow introduced into the particle separation chamber is caused to rapidly expand, wherein said rapid expansion causes particles entrained in the airflow to reduce their speed; wherein the particle separation chamber includes one or more passively electrically charged surfaces such that some particles in the airflow electrostatically adhere to the one or more passively electrically charged surfaces as the airflow traverses the airflow path, whereby said adhered particles are separated from the airflow; wherein the passively electrically charged surface is folded, corrugated or textured in order to present a greater surface area to the airflow so as to attract more particles; and wherein within the particle separation chamber there is located a barrier or wall located in front of and near the inlet airflow path such that, as the airflow is introduced into the chamber, it is caused to diverge around the barrier or wall towards said at least one passively electrically charged surface.

5. The particle separation system according to claim 4, wherein the passively electrically charged surface may be an electret material (polarized polymer), or the charge may be intrinsic in the surface itself.

6. A method for separating particles from an airflow in which they are entrained, at a position upstream of a detection chamber in an aspirating smoke detector, the method including: introducing the airflow into a particle separation chamber; causing the airflow to undergo a rapid expansion within the particle separation chamber in a first region, wherein said rapid expansion causes the airflow to reduce its speed; providing a barrier or wall within the first region of the particle separation chamber near an inlet to the particle separation chamber thereby causing divergence of the airflow around the barrier or wall; exposing one or more passively electrically charged surfaces, located adjacent the first region, thereby attracting particles out of the airflow, such that some particles in the airflow electrostatically adhere to the passively electrically charged surface as the airflow traverses the airflow path, whereby said adhered particles are separated from the airflow, wherein the passively electrically charged surface is folded, corrugated or textured in order to present a greater surface area to the airflow so as to attract more particles; and exiting the airflow from the particle separation chamber.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Exemplary embodiments of the present invention will now be described by way of non-limiting example only, with reference to the accompanying drawings, in which:

(2) FIG. 1 illustrates an electrostatic precipitation chamber of an aspirating smoke detector according to a first preferred embodiment of the invention;

(3) FIG. 2 illustrates a particle separation system according to second preferred embodiment of the present invention;

(4) FIG. 3 illustrates an alternative embodiment of the particle separation system of FIG. 2;

(5) FIG. 4 illustrates a particle separation system according to a third preferred embodiment of the present invention;

(6) FIG. 5 illustrates an alternative embodiment of the particle separation system of FIG. 4;

(7) FIG. 6 illustrates a further alternative embodiment of the particle separation system of FIG. 4;

(8) FIG. 7 illustrates a detection system according to a fourth preferred embodiment of the present invention;

(9) FIG. 8 illustrates an alternative embodiment of the detection system of FIG. 7; and

(10) FIG. 9 illustrates a detection system according to a further embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

(11) FIG. 1 illustrates the operation of an electrostatic precipitation chamber located upstream of a detection chamber (not shown) of an aspirating smoke detector (not shown). The airflow entering the electrostatic precipitation chamber generally contains a range of particleslarger particles, generally associated with dust rather than smoke, and smaller particles which are generally associated with smoke. In an aspirating smoke detector, the object of the electrostatic precipitation chamber is to preferentially attract the larger particles out of the airflow leaving the smaller particles unaffected so that they can travel to the detection chamber of the smoke detector.

(12) Referring to FIG. 1, the air flow A containing suspended particles enters a pipe region D. Electrically charged surfaces C attract particulates B out of the flow resulting in an outflow E with a reduced particle content. The electrostatic field may be passively or actively generated and may be adjusted such that larger particles are preferentially drawn out of the airflow leaving relatively smaller particles to continue in outflow E.

(13) In alternative, more complex embodiments the inventors have realised that inertial separation techniques can be combined with electrostatic filtration techniques to enhance performance of the system. In this regard, the airflow changed or path can be shaped, disturbed in such a way that heavy and light particles take different paths. In combination with this the electrostatic precipitator can be placed such that the heavy particles are more likely to be located closer to it, thus increasing the relative capture of the heavy particles. FIG. 2 illustrates a particle separation system according to a second embodiment of the invention. The particle separation system includes an airflow path defined by pipe D which further includes a first airflow section G and second airflow section H that are arranged at 90 to each other, thereby introducing a sharp bend K in the pipe D. The bend K forms a transition region between the first airflow section G and second airflow section H.

(14) An electrically charged surface C is provided downstream and adjacent the transition region, and on the outside of the bend as illustrated. The electrically charged surface is a smooth surface and may form a wall of the pipe D.

(15) Air flow with suspended particles A enters the pipe D at the inlet end, travels through pipe D and around the bend K. As indicated in FIG. 2, as the fluid flow A passes around the bend, the larger, heavier particles F tend to take a wider path around the bend as they are carried by their momentum and approach the electrically charged surface C and adhere to it while the lighter, smaller particles B follow the air stream more closely and exit the pipe D as part of the remaining flow E.

(16) It will be appreciated that other forms of bends or curves may be utilised to alter the direction of travel of the airflow such that larger particles within the airflow come into close proximity to the electrically charged surface by virtue of their own momentum as the airflow travels around the bend or curve.

(17) FIG. 3 illustrates an alternate form of electrically charged surface C. In this embodiment the electrically charged surface C is folded in order to present a larger surface area to the air flow A. By increasing the surface area of the electrically charged surface C by folding, corrugating, or similarly texturing the surface, the number of particles intercepted by the charged surface C is increased.

(18) A further embodiment of a particle separation system is illustrated in FIG. 4. Air flow with suspended particles A enters via a relatively small dimension inlet into chamber J where the air flow undergoes a rapid expansion in region H before contracting again to leave the chamber via an outlet which is also of relatively small dimension. Electrically charged surfaces C are located within the chamber and preferably line the perimeter walls. Most preferably the electrically charged surfaces C are located adjacent the region of rapid expansion and towards the outlet.

(19) Smaller particles B follow the streamlines more readily and leave the chamber J as part of the exit stream E. Larger, heavier particles F lag the air stream motion and as a result are more likely to approach the electrically charged surfaces C to which they will adhere.

(20) Further variations to the system of FIG. 4 are illustrated in FIGS. 5 and 6. In FIG. 5 a barrier G is placed in front of the entering air flow A. The air stream diverges to move around the barrier causing eddies to develop. This brings the air stream, and consequently the particles suspended therein, to cycle into close contact with the electrically charged surfaces C several times, thus increasing the likelihood of suspended particles adhering to the charged surfaces C. The larger heavier particles are more likely to be at the periphery of the eddies due to centrifugal acceleration and are therefore preferentially removed from the air stream.

(21) Referring to FIG. 6, the capture of larger, heavier particles is enhanced by increasing the area of the charged surface C that is exposed to the air stream. In the illustrated configuration, the electrically charged surface C is constructed as a folded arrangement. Eddies in the air stream cycle while in close proximity to the extended surface area of C and the charged surface is therefore more effective in removing suspended particles. Due to the cyclic nature of the eddies and the centrifugal acceleration arising therefrom, larger, heavier particles are preferentially removed during the traversal of the electrically charged surface C.

(22) It will be appreciated that there are variations on the configurations illustrated, involving more elaborate structures designed to direct the heavier, larger particles onto or near the electrically charged surface.

(23) In a further preferred embodiment the electrically charged surface is an electret material (polarised polymer), or an actively charged surface relying on a high-voltage source to maintain the surface potential. Alternatively, the electric charge may be intrinsic in the manufacture of the pipe or duct through which the air flows. Electrically charged regions can develop during the plastic moulding process and the appropriate choice of material and moulding technique may be sufficient to produce the desired electrostatic field avoiding the need for a separate charged surface.

(24) It has been observed, as is expected, that the efficiency of removal of particles from the air flow diminishes over time as the charged surface accumulates particulate material. It is therefore advantageous to incorporate a detection system that can determine the amount of material accumulated in order to signal the need for replacement or cleaning of the charged surface.

(25) In a further embodiment of the invention, the charged surfaces are made of a transparent or partially transparent material, and the detection system employs parameters such as the reflection of light from or transmission of light through the charged surface. Such parameters vary over time and are used to indicate the need for replacement or other maintenance.

(26) A detection system as envisaged above is shown in FIGS. 7 and 8. In FIG. 7 a light source K transmits light M though a transparent charged surface C to receiver L. As the surface C becomes contaminated the received light intensity at receiver L is reduced.

(27) Similarly in FIG. 8. A light source K transmits light onto the transparent charged surface C. Some of the light M is received by the receiver L. The amount of light so reflected depends on the amount of particulate on the surface C and so this can be used to indicate that maintenance is required.

(28) A similar method of detecting accumulated material on the charged surfaces involves using an acoustic transducer to excite the charged surface acoustically and measure parameters such as resonance and damping.

(29) As shown in FIG. 9, acoustic transducer N is used to mechanically excite vibration in the charged surface C. Measurement of damping or resonance indicate the extent to which mass is adhering to the charged surface C.

(30) In some embodiments the transducer used may be the charged surface itself. For example, an electrostatic loudspeaker may be made by placing a charged surface between electrodes and varying the voltage between the parts of the surface or electrode structure. In another variation, the charged material may be made of a piezoelectric material, such as PVDF which will allow for both maintaining a charged surface and for exciting the surface acoustically.

(31) It will be understood that any of the optical or acoustic parameters described above, either singly or in combination may be useful in determining the extent of material adhering to the charged surface and therefore the need for maintenance.

(32) It will be appreciated that the preferred embodiments of the present invention provide improved particle separation systems resulting in improved particle separation and therefore dust rejection. Particle separation systems as described incorporated in smoke detection systems reduce the likelihood of false alarms triggered by exposure to dust for a given alarm level setting.

(33) In a preferred embodiment, a particle separation system is incorporated in an aspirated particle detection system (not illustrated). The particle detection system includes an inlet into which air is drawn from the volume being monitored. The airflow is drawn up the upper sample inlet to an electrostatic precipitation chamber. The electrostatic precipitation chamber may be of any of the types previously described in the specification. After traversing the electrostatic precipitation chamber, the air sample passes into a detection chamber. The detection chamber can, for example, be an optical particle detection chamber such as the one used in a Vesda air sampling smoke detector as produced by Xtralis Proprietary Limited. In such systems, a beam of light is shone across the air sample and scattered light is monitored by a light receiver to detect light scattered from particles in the airflow. The output of this detector is then processed by a smoke detection controller and alarm logic applied to determine if particles exist and whether an action needs to be taken in response to their detection. The particle detection system also includes an aspirator in the form of a fan which is used to draw air through the particle detection system. After traversing the particle detection system, the air is passed back to the atmosphere via an exhaust.

(34) A further embodiment of a particle detection system is typically referred to as a sub-sampling particle detection system (not illustrated). In this regard, the sub-sampling particle detection system includes a primary airflow path in which an aspirator draws air from an inlet and out to an outlet. The airflow will typically be an air sample from a volume being monitored for the presence of particles. From this main airflow, a sub-sample is drawn via a sub-sampling path. The sub-sampling path is passed through an electrostatic precipitation chamber and then to a particle detection chamber. The particle detection chamber and electrostatic precipitation chamber can be the same as that described in the previous embodiment. Air from the sub-sampling path then rejoins the airflow main path.

(35) Either of the aspirated or sub-sampling particle detection systems described above can be implemented in aspirated smoke detection systems (not illustrated). In one embodiment, the aspirated smoke detection system is coupled to a sampling pipe network which includes a plurality of sampling pipes arranged in a branched configuration. In each of the branches, there are a plurality of sample inlets or sampling holes. Air is drawn into the sampling holes through the pipe network to the particle detection system where the air samples are analysed to determine whether particles exist. As will be appreciated by those skilled in the art the various branches of the particle detection network may be used to monitor different air volumes (such as rooms) within a premises.

(36) The foregoing describes preferred embodiments of the present invention and modifications may be made thereto without departing from the scope of the invention. It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.