GB2354071A

GB2354071A - Gas detector

Info

Publication number: GB2354071A
Application number: GB9921586A
Authority: GB
Inventors: Roger Dennis Payne; Roger Barrett
Original assignee: Apollo Fire Detectors Ltd
Current assignee: Apollo Fire Detectors Ltd
Priority date: 1999-09-13
Filing date: 1999-09-13
Publication date: 2001-03-14
Also published as: GB9921586D0

Abstract

A gas detector has a sensor S for, e.g. detecting CO resulting from a fire, and dust removal means comprising a diffusion path 7 where the dust in the gas precipitates to form accretions which fall from the path 7 under the influence of gravity. The gas detector typically forms part of a fire alarm. A gas diffusion and dust precipitation cover located in a gas sensor housing is also claimed.

Description

2354071 GAS DETECTOR WrrH DUST REMOVAL DEVICE This invention relates to a.

gas detector which includes means for removing dust particles from a gas to be detected.

Gas detectors are placed in a variety of different locations for signalling a change in a parameter relating to the composition of gas surrounding the detector. For example, they can be used to signal an alarm when the presence of one or more gasses exceeds a predetermined level. However, when the environment of the detector contains dust particles, these can settle on the detector, in the course of time, and thereby interfere with its normal operation. Some detectors could therefore be triggered to give false alarms, whilst others may be made less sensitive, or even inoperative. In the field of fire detection, low cost "point" fire detectors are widely used to monitor the ambient for an abnormally high concentration of combustion products (in the form of gas or smoke), but they are restricted to areas that are substantially free of dust. The presence of high concentrations of dust can give rise to false alarms in ionisation, photoelectric and optical beam smoke detectors. Moreover, when dust settles, it can quickly obstruct apertures and pathways leading to sensors in gas detectors (such as CO fire detectors) rendering them inoperative. Therefore, despite the wide choice of low cost point smoke and gas fire detectors, none are suitable for locations where they would be exposed to high concentrations of dust, for example, the inside of grain silos, the holds of bulk carriers, and over conveyors.

In the past, this problem was addressed by regular maintenance and cleaning, but this is a time consuming and labour intensive process. If dust filters are used, these can also become quickly clogged and must therefore be frequently changed.

The invention seeks to solve this problem in a simple and efficient manner so that gas detectors can be used for long periods without the need to be cleaned frequently, 2 especially in environments where high concentrations of dust particles are present.

Whilst the invention has particular utility in the field of fire detection, it can be applied for example, to devices that detect gases other than CO, and for purposes other than fire detection. (Embodiments of the invention are described with reference to fire detection, but references to such use are not to be construed as limiting.

Moreover, references herein to "dust" are to be construed broadly in the sense of any particulate material that is mobile in a gas, not necessarily airborne, but light enough to be carried by gas.) In its broadest aspect, the invention provides a gas detector comprising a gas sensor for detecting the presence of a gas, and dust precipitating means for allowing a dust laden gas to enter the gas detector by diffusion and causing dust particles in the dust laden gas to precipitate before the gas is detected by the gas sensor.

In an embodiment of the invention, the dust precipitating means decreases the concentration of dust particles as the gas diffuses, along a pathway, towards the gas sensor. For example, the dust precipitating means comprises a substantially closed chamber, which is open to admit the dust-laden gas by diffusion and which allows the dust to settle out from the gas, the precipitation occurring naturally and without the need for any power supply. This is clearly a distinct advantage over means which may need a power supply in order to cause precipitation. In an embodiment of the invention, the gas detector includes a wall or walls defining the substantially closed chamber with at least one entrance aperture to admit the dust laden gas by diffusion.

The wall or walls define a gas diffusion pathway extending, after the entrance aperture, far enough to allow the dust to settle, whilst the entrance aperture is large enough to prevent it from becoming clogged by dust accretions.

Preferably, the dust precipitating means causes a build-up of precipitated dust to fall, due to gravity, before the gas diffusion pathway becomes blocked. Whilst the dust 3 could be allowed to settle out on to a wall surface and/or be collected within the detector, the gas detector is preferably arranged so that the settled dust falls out of the detector. For example, the wall or walls of the above mentioned substantially closed chamber can also define a reduced aperture, which is downstream of the entrance aperture on the gas diffusion pathway (i.e. it is along the direction of diffusion of the gas into the detector). The reduced aperture is smaller than the entrance aperture, so that when dust settles out, it then falls (due to gravity), out of the entrance aperture, without being obstructed by the sides of the entrance aperture. Preferably, means are provided to fit the gas detector to the ceiling, walls, or floor of a room, so that the entrance aperture faces downwardly away from the diffusion pathway, i.e. below the reduced aperture, to ensure that dust particles (forming accretions) fall out of the chamber. This self-cleaning action is clearly a distinct advantage. In the case of wall mounting, a tubular shape having a bend (e.g. through 90') can be used to cause the entrance aperture to face downwards. In the case of floor mounting, the gas detector is used to sense the presence of.gases heavier than air and the gas sensor (which is contained in a housing) can communicate with the entrance aperture through a tube, which is generally U-shaped, so that the entrance aperture faces downwardly (these arrangements are described in more detail below). The reduced aperture can be defined by a narrowed or waisted section e.g. where the wall or walls define a tube.

A reduced aperture is not, however, essential, since dust will settle out in (say) a straight tubular section. The reduced aperture could alternatively be defined by the space between the internal wall surface of (say) a tubular member and a baffle of smaller cross-section located within the tubular member. An example of this would be a circular disc of comparatively small diameter located in a tube having a larger internal diameter so that there is an annular space for the gas to diffuse through. (As indicated above, the gas diffusion pathway can include bands so as to position the entrance aperture so that it is facing downwardly in use).

Preferably, the aspect ratio defined as the length of the pathway to the width (e.g.

4 diameter) of the entrance aperture is at least 2:1, and preferably in the range of 5:1 to 20: 1. For example, the maximum aspect ratio of the pathway can be determined by the required response time for the gas detector, in so far as the aspect ratio controls the rate of difftision of gas molecules from the inlet aperture to the gas sensor. With gases with very mobile molecules, such as CO, the aspect ratio may be 10:1, or greater for the realisation of response times acceptable for normal fire protection applications. The minimum aspect ratio of the pathway is that which substantially prevents dust particles from reaching the end of the pathway and hence the gas sensor. This is determined by factors including the size and weight of the dust particles and the wind velocity, in so far as it produces turbulence in the entrance aperture of the pathway, and orientation in relation to the force of gravity. With a gas diffusion pathway which is effectively dead-ended, the net gas flow in the pathway is zero. Beyond the turbulent region, upward movement of dust is by Brownian motion, constrained by gravity. In relatively still conditions, an aspect ratio of 2:1 can impede the movement of larger dust particles as the gas diffuses from the entrance aperture towards the gas sensor. For general purpose applications, however, an aspect ratio of between 5:1 and 20:1 is appropriate.

In the preferred embodiment of the invention, dust becoming attached to the wall or walls of the pathway is subject to the force of gravity and will break away, when a significant amount has accumulated. Such accumulations fall completely away from the walls defining the chamber, thereby maintaining the pathway open. Hence, a pathway which widens towards the entrance aperture is advantageous, because larger dust particles tend to accumulate in the wider section of the pathway, so reducing the possibility of the pathway being blocked, when an accumulation of large dust particles breaks off. Vibration, such as may be experienced in a ship's hold, will further encourage dust accumulations to break off.

In preferred embodiments of the invention, the width or diameter of the entrance aperture is 5 millimetres or more so that sufficient dust can accumulate which will then break off before blocking the pathway.

Preferably, the walls of the chamber are either made of transparent material, or include at least one window, so that the distance over which the dust accumulates, in the pathway, can be directly observed. A transparent cover, or one with a window, is preferable where the gas detector incorporates an indicator. In another embodiment, the indicator is located at a gas sensor end of the pathway, so that it may be viewed through the entrance aperture. Such an indicator may be an LED, for example, which flashes to indicate that a gas has been sensed.

Preferably, the gas detector includes means, such as a base, which facilitates fitment to a ceiling, wall or floor of a room (or of some other enclosure) so that the entrance aperture faces downwardly when the detector is installed. This causes dust particles settling on the wall or walls to accrete and then fall out of the chamber through the aperture. This is self-cleaning action is highly advantageous in very dusty atmospheres and has the advantage of simple and low cost construction, and ease of fitment to existing detectors.

In some embodiments of the invention, the dust precipitating means can cause the dust particles to clump together, whilst the dust laden gas is diffusing towards the gas sensor.

The invention may be put into practice by making a cover to fit over or around a conventional gas detector, for example a fire detector which responds to CO or other gas entering the sensing area.

The invention therefore extends to providing a gas diffusion and dust precipitating cover comprising means which enable it to be fitted to or around the housing of a gas 6 detector, the cover being open at one end to define a gas diffusion entrance aperture, forming the entrance of a substantially vertical pathway leading to a reduced aperture, the aspect ratio of the pathway (defined as the length of the pathway to the width of the entrance aperture) being at least 2:1 and preferably in the range of 5:1 to 20:1; the width of the entrance aperture, in a horizontal plane, being at least 5 millimetres, the internal wall or walls of the cover being such that the pathway narrows with increasing distance from the inlet aperture to the reduced aperture, the pathway enabling communication with a gas sensor located in the housing of the gas or smoke detector.

The cover preferably has a circular cross-section and includes a lower conical section and an upper bowl or dish shaped section.

In the case of applying the invention to conventional fire detectors, the housing of a "point" CO fire detector is usually very similar to, if not the same, as that of a smoke detector (i.e. as produced by the same manufacturer). Such housings are designed to allow free ingress of smoke and combustion gases and usually feature at least a number of circumferential apertures that allow smoke travelling across a ceiling to flow freely into and through the fire detector. In practice, the pathway through the fire detector is indirect and via a gauze in order to afford mechanical protection to the sensor and to exclude light and insects from a sensing chamber. Such housings are used not only for smoke detection, but also for CO detection in a substantially dust free atmosphere. However, the invention can be simply applied to modify these conventional or standard fire detectors just by making a suitable "cover" which can be demountably fitted, for example, by screw or bayonet fitting, or by a push fit.

Embodiments of the invention will now be described with reference to some of the accompanying drawings, in which:

7 Fig. I shows in part section a typical prior art point fire detector; and

Figs. 2-7 show different embodiments of the invention; Fig. I shows in part section a typical prior art point fire detector D that features a head H (including the external housing with circumferential apertures A) which is demountable from a base B that is fixed to the ceiling. The detector further includes mechanical protection baffle P, insect resistant gauze G and photo electric, ionisation or CO sensor S. A disadvantage of this design is that it has many apertures pathways 10 that can be obstructed or blocked by dust deposits. Fig. 2 shows a first embodiment of the invention where a gas detector D comprises a base B and demountable head H having a CO gas sensor S. A cover or housing 2, having a conical section and a bowl-shaped section, is a push or twist fit on the base 15 B. The cover has a circular cross- section which varies along its length, and a circular inlet aperture 4 (in the horizontal plane) which admits dust-laden gas by diffusion. The aperture 4 forms the entrance of a substantially vertical pathway 7 having an aspect ratio, maximum vertical dimension (height) to maximum inlet aperture dimension (diameter), of at least 2:1. The width of the aperture 4 in the 20 horizontal plane is at least 5 millimetres, the internal wall surface of the pathway 7 being vertical or inclined to the vertical (as shown). Preferably, the pathway narrows with increasing distance away from the inlet aperture 4. The pathway 7 communicates with a static volume containing a gas sensor S, i.e. the volume is substantially sealed against gas ingress except by aperture 4 and pathway 7. 25 As the dust-laden gas diffuses along the pathway 7, the dust precipitates and forms accretions which build up in size and then break- off. These then fall out of the cover or housing and provide a self- cleaning action. The build up of dust is thereby prevented or inhibited by natural means, without the need for any power supply.

8 In the embodiment of Fig.3, a conventional gas or smoke detecting device D is enclosed by a cover 2 and the pathway 7 of the cover 2 has a similar aspect ratio. The cover is attached to a ceiling pate 10 by a screw or bayonet fitting 9.

Fig. 4 shows a similar embodiment to Fig. 3, but the conical section is longer so the aspect ratio is 5:1. The maximum aspect ratio of the pathway 7 can be determined by the required response time required of the detecting device in so far as the aspect ratio controls 10 the rate of diffusion of gas molecules from the inlet aperture to the gas sensor. In the case of gases with very mobile molecules such as CO, the aspect ratio may be 10:1 or greater for the realisation of response times acceptable for normal fire protection applications. The minimum aspect ratio of the pathway 7 is that which substantially prevents dust particles from reaching the end of the pathway. This is determined by 15 the size and weight of the dust particles, the wind velocity in so far as it produces turbulence in the entrance end of the pathway, and orientation in relation to the force of gravity. Because the pathway is effectively dead- ended the net gas flow in the pathway is zero. Beyond the turbulent region, the upward movement of dust is by Brownian motion constrained by gravity. In relatively still conditions, the aspect ratio 20 of 2:1 can impede the movement of large dust particles from the entrance aperture to the gas sensor. For general-purpose applications, however, an aspect ratio of between 5:1 and 20:1 is appropriate. Dust that becomes attached to the internal wall surfaces of the pathway 7 is subject 25 to gravity and will break away when a significant amount has accumulated, thereby falling out of the cover 2 and maintaining the pathway 7 open. The inlet aperture is therefore advantageously wider than the waist of the cover (which forms a reduced aperture).

9 In preferred embodiments of the invention the minimum size of the inlet aperture (e.g. diameter) is greater than 5 millimetres so that sufficient dust can accumulate to cause it to break-off before the pathway 7 becomes blocked.

The cover or housing may be made of a transparent material so that the distance to which dust accumulates in the pathway may be directly observed. A transparent cover or one with a window is preferable where the gas detecting device incorporates an indicator. In another preferred embodiment the indicator is located at the gas sensor end of the pathway such that it may be viewed through the gas inlet aperture. 10 The indicator can be, for example, a flashing LED which is triggered by exceeding a predetermined threshold of CO concentration, as determined by the sensor and the detection circuitry (which can be of known construction). It could also be a device for indicating the level of gas present. In place of, or in addition to an on-board indicator, the gas detector could send a signal to a remote indicator or control system, 15 e.g. by wire or radio. Fig. 5 illustrates an embodiment similar to Fig. 4, but where the cover or housing (2) has an extra tubular section (2a) which includes a right-angled bend, so that when detector D is secured to a vertical wall surface, the entrance aperture (4) faces 20 downwardly. The embodiment shown in Fig. 6 is a further development, where the additional tubular section (2b) is generally U-shaped, so that the detector D can be floor mounted for detecting gases which are heavier than air. The entrance aperture (4) 25 again faces downwardly.

Fig. 7 is a cross-section through a gas detector assembly similar to those shown in Figs. 3 and 4, but in this case including a wire cage (12) to provide protection. Such a cage (12) is advantageous when the pathway (7) is long compared to the width of the entrance aperture (4), i.e. when there is a high aspect ratio (and/or small crosssection).

11

Claims

1. A gas detector comprising a gas sensor (1) for detecting the presence of a gas and dust precipitating means for allowing a dust laden gas to enter the gas detector by diffusion and for causing dust particles in the dust laden gas to precipitate before the gas is detected by the gas sensor (1). 10 2. A gas detector according to claim 1, wherein the dust precipitating means decreases the concentration of dust particles as the gas diffuses along a pathway towards the gas sensor (1). 3. A gas detector according to any preceding claim, wherein the dust precipitating 15 means results in dust failing due to gravity. 4. A gas detector according to any preceding claim, wherein the dust precipitating means comprises a substantially closed chamber, which is open to admit the dust laden gas by diffusion and which allows the dust to settle out from the gas, the 20 precipitation occurring naturally and without the need for any power supply. 5. A gas detector according to claim 4, which includes a wall or walls (2) defining: (a) the substantially closed chamber (3) with at least one entrance aperture (4) to 25 admit the dust laden gas by diffusion; and (b) a gas diffusion pathway which extends, after the entrance aperture (4), far enough to allow the dust to settle, the entrance aperture (4) being large enough to prevent it from becoming clogged by dust accretions.

12 6. A gas detector according to claim 5, wherein the wall or walls (2) also define a reduced aperture (5) downstream of the entrance aperture (4) on the gas diffusion pathway, the reduced aperture (5) being smaller than the entrance aperture to ensure that when dust settles out and falls due to gravity, the falling dust does not obstruct the entrance aperture (4).

7. A gas detector according to claim 6, including means (6) to fit the gas detector to the walls, ceiling or floor of a room (or some other enclosure), so that the entrance aperture (4) faces downwardly away from the diffusion pathway, whereby dust particles (forming accretions) fall out of the chamber (3) due to gravity.

8. A gas detector according to claim 5, 6 or 7, wherein the aspect ratio, defined as the length of the pathway to the width of the entrance aperture (4) is at least 2: 1, and preferably in the range of 5:1 to 20: 1.

9. A gas detector according to any preceding claim, which includes a housing (2) open at one end to define an entrance aperture (4) in a horizontal plane, the entrance aperture (4) forming the entrance of a substantially vertical pathway (7) which leads to a reduced aperture (5), said pathway (7) providing a gas diffusion path to a gas sensor (1) contained in a volume which is sealed against gas ingress except by the gas entrance aperture (4); the aspect ratio of the width of the entrance aperture (4) to the pathway length being at least 2:1 and the maximum width of the entrance aperture (5), in a horizontal plane, being at least 5 millimetres; the internal walls of the housing (2), providing said pathway (7), narrowing smoothly with increasing distance from the entrance aperture (4) to the reduced aperture (5); the reduced aperture (5) being located directly over the entrance aperture (4).

10. A gas detector according to any of claims 5-9, wherein the walls (2) are transparent to enable the presence of dust to be seen.

13 11. A gas detector according to any preceding claim, wherein the dust precipitating means causes the dust particles to chunp together whilst the dust laden gas is diffusing towards the gas sensor (1).

12. A gas detector according to any preceding claims, wherein the gas detector includes a fire detector (8) (used for detecting gas) and a cover (2) which allows (a) the gas to enter by diffusion, and (b) the dust to precipitate. 13. A gas detector according to any of claims 5-9, wherein the pathway 10 communicates with sections having bends to enable the entrance aperture to face downwardly when the gas detector is fitted to, or placed on a surface. 14. A gas detector according to any of claims 5-9 or 13, wherein at least the wall or walls of the gas diff-usion pathway, or the whole gas detector is protected by a 15 cage. 15. A gas detector substantially as herein described with reference to any of Figs. 2-4 of the accompanying drawings. 20 16. A gas diffusion and dust precipitating cover, comprising means (9) which enable it to be fitted to or around the housing of a gas or fire detector (8), the cover (2) being open at one end to define an entrance aperture (4), forming the entrance of a substantially vertical pathway (7) leading to a reduced aperture (5), the aspect ratio, i.e. length of the pathway (7) to the width of the entrance aperture (4) being at least 25 2: 1 and preferably in the range of 5: 1 to 20: 1; the maximum width of the entrance aperture (4), in a horizontal plane, being at least 5 millimetres, the internal wall or walls of the cover (2) being such that the pathway (7) narrows with increasing distance from the entrance aperture (4) to the reduced aperture (5), the pathway (7) enabling communication with a gas sensor (1) located in the housing of the gas or 14 smoke detector which is substantially closed to prevent gas ingress except by said entrance aperture (4).