GB2251067A - Smoke detector - Google Patents

Abstract

A smoke detector comprises a smoke chamber 35 which is laterally bounded by a system of baffles 36 which permit air to enter the chamber from the surrounding atmosphere. The baffle system comprises a plurality of substantially continuous baffles around the periphery of the chamber including at least a first such baffle 53 which extends downwardly from the roof of the chamber partially towards the floor of the chamber, and a second such baffle 54 which extends upwardly from the floor of the chamber partially towards the roof of the chamber. <IMAGE>

Description

SMOKE ALARM This invention relates to a smoke detector.

A smoke detector, for the purpose of this specification, is a device which is normally fixed on or close to a ceiling, and contains a smoke chamber which is open to the atmosphere so that smoke in the atmosphere can enter the chamber. The device has means for detecting smoke in the chamber, and raising an alarm. The detection means typically includes a light source for projecting light into the chamber and a light detector arranged to detect light scattered from smoke in the chamber, and the alarm may be raised by flashing a light and/or sounding a horn. However, other kinds of detection means and alarms may be used.

Such smoke detectors are well known, and typical examples are described in U.S. Patent Specifications 4,199,755, 4,249,082, 4,321,466 and 4,374,329.

The design of smoke chambers in smoke detectors requires adequate consideration to be given to the following four requirements: (i) The smoke must easily enter the chamber and not be excessively impeded by baffles; or labyrinths.

Smoke detectors rely on the natural draughts in a room or area which can sometimes be very slow and sluggish and can come from any direction. This requires that a chamber be as open as possible.

(ii) Ambient stray light entering the chamber and impinging on the light sensor (usually a photodiode) can cause false alarms or a degradation in the sensitivity. Ideally to prevent this, the chamber should have labyrinthine/baffled smoke entry apertures.

This is obviously in direct conflict with the first requirement.

(iii) Smoke must cross the "active" volume of the chamber. This volume is the intersection of the light beam with the field of view of the light detector. The chamber should direct the smoke so it passes through this volume.

(iv) Dust and contamination depositing in the chamber will increase the light being scattered back off the walls such that it appears as "smoke" to the detector. This causes the unit to become excessively sensitive and eventually to cause it to false alarm.

Figure 1 is a schematic plan view, and figure 2 a schematic side view, of a typical prior art smoke detector which is on the market.

The prior art smoke detector includes a housing 10 which is fixed on or close to a ceiling 11, the housing 10 containing an LED light source 12 and a photodiode light detector 13. These are shielded at 14 from direct view of one another, and are angled downwardly towards a circular smoke chamber 15 below the housing 10.

The smoke chamber 15 is laterally bounded by a system of baffles 16, the baffles 16 being part helical as shown. The smoke chamber is closed at the top by a roof 17 (into which the LED 12 and photodiode 13 are set) and at the bottom by a floor 18, so that substantially the only way in for air is via the lateral system of baffles 16. The floor, roof and baffles of the smoke chamber are typically moulded of a black plastics material.

The beam of light 19 from the LED 12 and the field of view 20 of the photodiode 13 intersect in a central volume 21 of the smoke chamber, the so-called active volume. As is well known, circuitry (not shown) is provided within the housing 10 which operates to trigger an alarm, such as by flashing a light and/or sounding a horn, when the degree of light scattered to the photodiode 13 by smoke in the chamber 15 exceeds some pre-determined amount.

This prior art detector has certain disadvantages.

In figure 1 typical air streamlines are illustrated at 22 and it can be seen that they are somewhat tortuous. Some of the air has a tendency to go around the outside rather than through the chamber.

This is not important when there is plenty of airflow, but it can become critical at very low airspeeds as the chamber might be by-passed entirely.

The baffles 16 stop direct light impinging on the light receiver. However, direct light can get into the chamber area and it is possible that at some angles it could scatter off the walls into the light detector, see for example the ray. The light is obviously absorbed by the black plastic at every "reflection" but only an extremely small amount of light is needed to cause problems. (Only about one part in a million of the light from the source needs to be scattered onto the detector to cause an alarm).

The air travels directly across the bottom surface of the smoke alarm chamber. Normally at very low airspeeds (i.e. little draughts in the room) any dust in the air will have settled under gravity so it is below the smoke entry areas of a unit (typically they are 0.5 to 2.0 inches below the ceiling). Under these conditions very little dust will sediment on to the bottom inner chamber surface as very little dust is actually travelling through the unit in the first place. However, when the airspeed is increased in a room (e.g. due to severe draughts from open windows, the room being vacuum cleaned, blankets being shaken, etc.) dust is entrained up into the air and there is now a possibility of it being blown into the unit and depositing itself on the inner surfaces. The turbulence in the air as it flows and "tumbles" across the surface, as shown at 221 in figure 2, can deposit the dust.Furthermore, electrostatic attraction from charges embedded in the plastic can enhance deposition.

It is an object of the invention to provide a construction of smoke detector which mitigates these disadvantages.

According to the present invention there is provided a smoke detector comprising a smoke chamber which is laterally bounded by a system of baffles which permit air to enter the chamber from the surrounding atmosphere, the baffle system comprising a plurality of substantially continuous baffles around the periphery of the chamber including at least a first such baffle which extends downwardly from the roof of the chamber partially towards the floor of the chamber, and a second such baffle which extends upwardly from the floor of the chamber partially towards the roof of the chamber, the said second baffle being the innermost baffle of the said plurality of baffles.

Preferably, the baffle system has an upward-facing opening to the atmosphere surrounding the chamber.

Preferably, also, the baffle system further includes a plurality of further baffles which extend generally radially outwardly of the chamber so that adjacent such baffles converge towards the chamber.

An embodiment of the invention will now be described with reference to the accompanying drawings, wherein: Figures 1 and 2, previously described, are schematic views of a prior art smoke detector.

Figure 3 is a vertical cross-section through an embodiment of the present invention taken on the line 3-3 of figure 4, figure 3A being an enlarged view of part of the smoke chamber floor of figure 4, and Figure 4 is a horizontal cross-section through the embodiment taken on the line 4-4 of figure 3.

Referring to figures 3 and 4, the smoke detector according to the embodiment of the invention includes a housing 30 which in use is fixed on or close to a ceiling 31, the housing 10 supporting a sub-housing 301 containing an LED light source 32 and a photodiode light detector 33. These are shielded at 34 from direct view of one another, and are angled downwardly towards a generally rectangular smoke chamber 35 below and supported by the housing 30.

The smoke chamber 35 is laterally bounded by a system of baffles 36, and is closed at the top by a roof 37 (into which the sub-housing 301 is set) and at the bottom by a floor 38, so that substantially the only way in for air is laterally via the system of baffles 36. The floor, roof and baffles of the smoke chamber are moulded from a black plastics material.

In use the beam of light 39 from the LED 32 and the field of view 40 of the photodiode 33 intersect in a central volume 41 of the smoke chamber 35, the so-called active volume referred to previously, which is located above the floor 38 of the smoke chamber.

Circuitry (not shown) is provided within the housing 30, in an area 45 (figure 4) to one side of the smoke chamber 35 and sub-housing 301, which circuitry operates to trigger an alarm, such as by flashing a light and/or sounding a horn, when the degree of light scattered to the photodiode 33 by smoke in the chamber 35 exceeds some pre-determined amount. Since this invention is not concerned with the details of such circuitry, nor indeed with the specific detection technique, further details of these aspects will not be further given.

The smoke chamber 35 and detection circuitry are protected by a plastics cover 46 which is attached to one side of the housing by a "snap-on" hinge 48 and to the other side by a "snap-onw latch 49. The cover has all around its outside a large number of parallel vertical grill apertures 47 which are as open as possible to impede as little as possible the flow of air into the baffle system 36.

The baffle system 36 comprises a plurality of substantially continuous circumferential baffles 52, 53 and 54 which extend around the periphery of the smoke chamber 35. The baffle 53 extends downwardly from the roof 37 of the chamber partially towards the floor 38 of the chamber, while the innermost baffle 54 extends upwardly from the floor of the chamber partially towards the roof of the chamber. The top end of the baffle 54 is slightly above the level of the lower end of the baffle 53.

The baffle system 36 further comprises a plurality of substantially vertical baffles 51 (not shown in figure 3) which extend generally radially outwardly of the smoke chamber 35 and over substantially the full height thereof between the roof 37 and floor 38, so that adjacent pairs of such baffles converge towards the chamber 35.

The baffle 52, which is an upwardly and outwardly flared extension of the chamber floor 38, defines with the baffle 53 an upward-facing opening 56 to the atmosphere surrounding the chamber.

The baffle system 36 may be moulded in two parts, one part being the roof 37 and circumferential baffle 53, and the other part the floor 38 and circumferential baffles 52 and 54. The radial baffles 51 may be moulded to either part, and either they and/or the circumferential baffles 52 to 54 will have appropriate slots to accommodate the thickness of the other at the "intersection" points.

The airflow characteristics of the above detector will now be described.

From figure 4 it can be seen that the air 42 travels essentially horizontally and that the generally radial baffles 51 enhance the airflow through the chamber. These baffles concentrate the airflow and increase the velocity to make the most of the prevailing draught.

Figure 3 shows how the air 42 flows in the other plane, normal to the ceiling. The three baffles 52 to 54 deflect the air with the minimum of interference to the airflow, and direct the air upwards into the active volume 42 of the chamber 35. This is a much more effective method of sampling the air than that used previously for two reasons.

Firstly, all smoke alarms inevitably deflect the airflow downwards, as they are obstacles protruding from the ceiling (this is illustrated in figure l(b)).

The air streamlines 42 through the present detector, up to almost the baffle 53, mirror the natural movement of the air around the outside of the detector, so the flow travels easily into the chamber 35, with little hindrance. Unlike in the prior art example there is less tendency for the air to go either side of the unit so the maximum amount of air flows into the chamber 35.

Secondly, the wide entrance 56 all the way around the chamber 35 (typically the apertures are 7mm in width and 20mm in length between baffles 51) ensures there is very little drag caused by the walls. This contrasts with the narrow curving channels in the prior art example (typically the apertures are 3mm in width and lOmm in length). The air in these narrow channels will have a tendency to slow down due to wall friction - and with very low velocities the flow may actually by-pass the chamber entirely thus rendering the unit ineffective.

The present baffle system is much more efficient at blocking ambient stray light from the chamber 35. In the prior art example it is possible that about two "reflections" from the black plastic walls could be sufficient for light to be able to impinge onto the detector. In the present case it would require light coming from behind the unit (essentially impossible for direct light when mounted on a ceiling - reflections from the ceiling surface are the only possibility) to undergo at least three "reflections" (and associated absorptions) before it could impinge on the detector. This makes the present detector far less sensitive to the effects of stray light.

Furthermore, the present detector is very effective in preventing dust and other airborne contamination from depositing in the chamber where it could scatter light on to the detector. The area where dust could be a problem is the interior of the baffle 54 and in particular the floor 38. These are two features which help to reduce dust deposition. The airflow is deflected away from the floor 38 by the baffle 54 so turbulent deposition from eddies will not occur. The air is stagnant in the vicinity of the floor 38 so the swirling eddies are too far away to be able to "throw" their dust particles on to this surface. Similarly, the dust in the airflow is kept too far away from this surface for electrostatic attraction to have any effect.

A further enhancement is to add small riblets to the floor 38, as shown at 60 in figure 3A. The light impinges on these vertical riblets and dust is deposited out of the light path.

As mentioned earlier, the main concern as regards dust deposition is when there is high airflow, because more and larger dust is likely to be entrained into the air. The acceleration of the air around baffles 53 and 54 as shown in figure 3 will tend to deposit dust on the outer edges of these baffles as marked by xxx. This is due to the inertia of the dust particles - this causes them to cross the streamlines when the air is going around a bend. Dust which lacks sufficient inertia to deposit on these bends will most likely be carried across and out of the chamber 35, as its sedimentation velocity will be low and it will be little affected by turbulent deposition. Baffle 54 is particularly effective as it "skims" off the dust from the lower streamlines. It is the dust in these that would be most likely to deposit as these travel nearest to the surfaces where dust could cause problems.

Various modifications of the baffle system described above are possible. For example, there may be more than three circumferential baffles such as the baffles 53 and 54, provided that the innermost baffle is an upwardly extending baffle such as the baffle 54, in order to keep the main airflow in the chamber 35 away from the floor 38. In such case alternate baffles will extend alternately downward from the chamber roof and upward from the chamber floor. However, too many such baffles will, of course, slow down the airflow speed.

Claims (6)

CLAIMS:

1. A smoke detector comprising a smoke chamber which is laterally bounded by a system of baffles which permit air to enter the chamber from the surrounding atmosphere, the baffle system comprising a plurality of substantially continuous baffles around the periphery of the chamber including at least a first such baffle which extends downwardly from the roof of the chamber partially towards the floor of the chamber, and a second such baffle which extends upwardly from the floor of the chamber partially towards the roof of the chamber, the said second baffle being the innermost baffle of the said plurality of baffles.

2. A smoke detector as claimed in Claim 1, wherein the baffle system has an upward-facing opening to the atmosphere surrounding the chamber.

3. A smoke detector as claimed in Claim 1 or 2, wherein the baffle system further includes a plurality of further baffles which extend generally radially outwardly of the chamber so that adjacent such baffles converge towards the chamber.

4. A smoke detector as claimed in any preceding claim, wherein the floor of the chamber is ribbed.

5. A smoke detector as claimed in any preceding claim, wherein a light source and a light detector are set in the roof of the chamber, the light beam from the source and the field of view of the detector intersecting in a central volume of the chamber above the floor.

6. A smoke detector substantially as described with reference to the figures 3 and 4 of the accompanying drawings.