High sensitivity and high false alarm immunity optical smoke detector

FIELD

0001The application pertains to smoke detectors having multiple sensing regions in combination with a particle separator. More particularly, the application pertains to optical-type detectors having multiple scatter angles.

BACKGROUND

0002Smoke sensors using the optical scatter principal are increasingly becoming the most common type of fire sensor on the market. Optical sensors however are very sensitive to non-fire aerosols like water vapor (condensed steam and mist), dust and ash, spores, cooking aerosols, insects and spiders.

0003Optical techniques are becoming common that attempt to differentiate between different types of smoke and non-smoke aerosols. Common techniques used in an optical scatter chamber are the use of different wavelength LEDS e.g. blue and near infra-red or different scatter angles e.g. 140 degrees and 70 degrees (or even a combination of both). In all these techniques a ratio is made between two different optical scatter paths in a common chamber. This ratio can then indicate the particle size of the aerosol in the chamber and therefore if the smoke is grey (larger particles) or black (smaller particles). That can be very difficult, is detecting non-fire aerosols, for example water vapor, as this can be generated at extremely high levels over a range of particle sizes very similar to the particle size of grey smoke. Therefore depending on the conditions under which the water vapor is generated, little or no difference can be detected in the optical ratio from that of grey smoke.

0004Note that this can also be true of other non-fire aerosols, so much so that manufactures usually resort to reducing false alarms by making the smoke sensor have a low sensitivity to grey smoke and by the use of spike detection (delaying detection if the aerosol profile changes too fast). It should be noted that repeated spike detection may also produce an excessive smoke detection delay. An alternative technique is to use a very fine filter material on the sensor and suck air thought it into the smoke chamber. Using such fine filters will require regular maintenance well before it starts to block the detection of larger smoke particles.

BRIEF DESCRIPTION OF THE DRAWING

0005<figref idref="f0001">Fig. 1</figref> illustrates an inverted and simplified view of an exemplary detector as mounted on a ceiling is a perspective view of a pontoon boat in accordance with the invention;

0006<figref idref="f0002">FIG.2</figref> is a block diagram of the external to internal optical ratio, smoke detection process;

0007<figref idref="f0003">Figs. 3A, 3B</figref>, and <figref idref="f0004">3C</figref> illustrate additional aspects of a detector as in <figref idref="f0001">Fig. 1</figref>;

0008<figref idref="f0005">Fig. 4</figref> illustrates aspects of the external air flow and external particulate induced scattering;

0009<figref idref="f0006">Figs. 5A, 5B</figref> illustrate additional aspects of the detector of <figref idref="f0001">Fig. 1</figref>;

0010<figref idref="f0007">Fig. 6</figref>, a side sectional view illustrates internal flow; and

0011<figref idref="f0008">Figs 7A, 7B</figref> illustrate various components of the detector of <figref idref="f0001">Fig. 1</figref>.

DETAILED DESCRIPTION

0012While disclosed embodiments can take many different forms, specific embodiments thereof are shown in the drawings and will be described herein in detail with the understanding that the present disclosure is to be considered as an exemplification of the principles thereof as well as the best mode of practicing same, and is not intended to limit the application or claims to the specific embodiment illustrated.

0013The present application relates to a ceiling mounted point fire detector that is designed, in one aspect, to be loop powered from an analogue, or digital, addressable fire alarm system. The detector includes an internal optical scatter chamber which samples the external environment via an output from a multi-stage cyclone particle separator. Air is returned to the external environment, via an exit point below which an open optical scatter chamber monitors the circulating air flow.

0014The multi-stage cyclone can be driven by a fan which is triggered-on after combustion products and/or aerosols are detected in the external environment. The cut diameter of the cyclone is set to remove almost all large (heavy) non-fire particles from the air flowing into the internal chamber, whilst smaller smoke aerosols are unaffected. This allows rapid and accurate smoke detection whilst being insensitive to massive quantities of non-fire aerosols.

0015So that the detector could detect the early phase of a fire, the internal scatter angle, wavelength and sensitivity are identical to the external scatter angle, which senses the external environment circulating above the exit flow. The ratio of both scatter paths is taken when the cyclone is active, giving a unity ratio for all smoke types. Accurate high sensitivity detection can now be applied to the internal scatter chamber for very early smoke detection. For non-fire aerosols, for example water vapor, the external to internal optical chamber sensitivity ratio will be far more than 100, enabling its easy identification and rejection as a false alarm.

0016Referring to <figref idref="f0001">FIG.1</figref> and <figref idref="f0002">FIG.2</figref>, a detector 10 includes a housing 12 which is releasibly attachable to a surface, such as a ceiling C by means of a ceiling plate 12a. The detector 10 can monitor ambient atmospheric conditions of an adjacent region R.

0017Detector 10 includes an internal, or closed, optical scattering chamber 14 and an external, or open optical scattering chamber 16. Ambient air A1, A2 is drawn into detector 10 via inflow ports in an air inlet ring 12b by the action of a particle separator 20. Separator 20 can include a fan or other type of air moving unit, without limitation.

0018Separator 20 could be implemented as a multi-element cyclone-type particle separator. It will be understood that a variety of separators come within the scope of the claims hereof. Exemplary separators have been disclosed in <patcit id="pcit0001" dnum="US20090025453A"><text>US Patent Application No 2009/0025453 published January 29, 2009</text></patcit>, entitled "Apparatus and Method of Smoke Detection". The published '453 application is assigned to the assignee hereof and incorporated herein by reference.

0019Water or water vapor is separated from ambient particulate matter by separator 20 and the remaining particulate matter flows, for example A3 into the internal optical scattering chamber 14. Outflow of A3 is from the chamber 14 through exit flow port 12c into the environment R.

0020While in the chamber 14 the airborne particulate matter scatters light from transmitter Tx. Scattered light is detected at receiver Rx. Both transmitter Tx and receiver Rx are coupled to control circuits 24. Circuits 24 can include analog/digital conversion circuitry as well as digital filter circuitry to implement the processing disclosed in <figref idref="f0002">Fig. 2</figref>.

0021Circuitry 24 can provide wired or wireless communications capability to an associated fire alarm monitoring system, not shown.

0022The external, or open, scattering chamber 16 includes first and second pairs of transmitter/receiver units Tx2/Rx2 and Tx3/Rx3. The two pairs of transmitter/receiver units are also coupled to control circuits 24. As those of skill will understand, two different scattering angles, one of which corresponds to the scattering angle of the chamber 14, can be provided.

0023The detector 10 advantageously presents a very low profile when viewed from the region R. The ceiling plate 12a can be substantially flat with the housing 12 extending away from the region R into the ceiling C to promote a very non-intrusive appearance.

0024The detector 10 monitors the ambient region R below that detector using two external near infra-red optical scatter angles. If relatively small levels of particulates move into this area, then a multi-stage cyclone, such as cyclone 20, is energized to draw the particulates in the ambient air, such as A1, A2, through the air intake ports in ring 12b, in the flat ceiling plate 12a. The multi-cyclone particle separator 20 removes almost all of any large non-fire aerosols that may be present, and then passes part of the sampled air, A3, into the internal optical scattering chamber 14 for smoke sensing.

0025The cyclone separator 20 can also be activated if low levels of CO or heat are detected or combined levels of any of the three monitored phenomena which could be indicative of the early phase of a fire. The rate or 'duty cycle' at which the multi-cyclone 20 will operate at, also can be increased with the levels of the monitored phenomena monitored.

0026Air drawn through the air inlet or ports in ring 12b, is passed via a mesh into the first cyclone stage, which is formed, for example, in a region of rotating air above a centrifugal fan with an area of exposed fan blades. This stage is primarily designed to remove large quantities of water vapor without clogging-up and minimizing the amount of water vapor passing to the centrifugal fan and final cyclone stage. The air flow through the inlet mesh is forced to be almost parallel to the mesh wires in order to maximize coalescent particle growth before the air flow enters the inlet holes of the cyclone. Liquid water is separated out on the side walls and allowed to drain back through the cyclone inlet holes.

0027The centrifugal fan drives the multi-cyclone 20 and actually forms the second stage of the particle separator. The fan is powered from a super-capacitor power supply, to average out the input current taken from the fire alarm loop. The fan speed and rotor blade radius determines the efficiency of this stage, with the first cyclone stage increasing the rotor speed due to the drop in air pressure. The outlet flow of the centrifugal fan is mostly returned to the external environment via an exit port 12c. However a small fraction of its output flow is fed into the final stage of the multi-cyclone 20. The aerosol density of the small faction of air flow at this point is representative of the entire aerosol density due to the mixing effect of the fan.

0028The final cyclone stage uses a tangential input, axial output reverse lift cyclone that is designed for a very sharp cut diameter of above 1 micron. This is achieved by the forced air flow into the tangential input and by feeding the axial output back into the fan input to provide suction in a small diameter vortex finder pipe. An additional cork-screw lift section is also used in the cyclone; while the conical exit section is reduced in length to fit into the sensor, this exit section also recombines with the main exit of the centrifugal fan. The filtered air flow from the axial output of the final cyclone stage is fed into an evaporation chamber before passing through the internal optical scatter chamber 14 and returned to the output 12c.

0029The main exit point 12c from the detector 10, allows the air flow to be passed back into the external protected area R, setting up a 'donut' shaped convection current, ensuring that fire products around and below the sensor can be sampled. If a real fire is present, then the sensed levels in the internal, or detection chamber 14 quickly, build up and the presence of a fire can be quickly and accurately detected.

0030After detecting a fire, the multi-cyclone 20 runs at a low'duty-cycle' to reduce power, whilst the levels in the detection chamber 14 can still be monitored to track any further build up of the fire products around the detector 10. This process also ensures that the chamber 14 can still be purged with clear air after a fire. If however, the sensed levels indicate that a non-fire aerosol triggered the cyclone 20, then it can be switched-off, until the monitored levels again indicate a possible fire. A constant re-triggering from a non-fire aerosol can also cause the cyclone 20 to enter the low'duty cycle' mode.

0031As the air flow in to the optical scatter chamber does not pass through a high filtration material filter, particles can not build-up on the filter and block it. Alternatively both large and small particles pass through the detector with the larger particles ejected at different point before recombining into a common exit point. As the multi-cyclone is event triggered, then the possibility of the detector being blocked by a build up of debris in a normal environment is effectively no more significant than a detector without a forced air flow and so requires no maintenance throughout its expected life.

0032One of the external optical scatter angles above the main exit point 12c has the same infra-red wavelength, sensitivity and scatter angle as the internal optical scatter angle in the chamber 14. This external scatter angle senses the external environment circulating above the exit point 12c when the cyclone is active. The analogue to digital conversion (ADC) outputs from both scatter paths have their background off-sets (clean-air readings) removed and are then digitally filtered with an update rate of between 5S to 20S, after this integration time a window comparator tests the ratio of both scatter paths. As the ratio must be unity for all smokes types, the window comparators ratio limits can be set quite wide for example 0.5 to 2.0. If the ratio is within the comparators limits and the signal is high enough for accurate calculations (a noise gate function) then the background readings are removed, before a high gain is applied to the ADC readings coming from the internal scattering chamber 14. A digital filter is then applied to this reading to before it is compared to a fire level, giving accurate and high sensitivity detection for very early smoke detection.

0033For non-fire aerosols, for example water vapor, the external to internal optical chamber sensitivity ratio will be far more than 100 i.e. well outside the window comparators limits, so the gain applied to the output of the internal scatter chamber will be only for normal smoke detection sensitivity. Alternatively the gain could be switched to a relatively low sensitivity, however this is not necessary as the cyclone removes nearly all the water vapor and there will be little or no response from the internal chamber, hence no false alarm is possible at any level of water vapor known to occur in practice. As the optical scatter ratio easily identifies the aerosol as a false alarm source, it can also indicate this to the fire alarm panel if this condition lasts for an excessive amount of time. Note that in the above description an enclosed external optical scatter chamber could be used instead of an open optical scatter angle with equal performance benefits.

0034A thermistor can also be positioned in the exit point 12c, just below the surface of the detector 10, so that if a small change in the ambient air temperature is detected by the thermistor, then the centrifugal fan can be tumed-on to sample the external air temperature and provide a fast heat detection response from the thermistor i.e. the buried thermistor can overcome the thermal inertia of the surrounding detector without having to protrude down from the ceiling in a protected molding feature.

0035<figref idref="f0003">Figs. 3A, 3B</figref>, and <figref idref="f0004">3C</figref> illustrate aspects of the detector in accordance herewith. <figref idref="f0003">Fig. 3A</figref> illustrates the detector 10 mounted into the ceiling C. <figref idref="f0003">Fig. 3B</figref> a side view of the detector 10 illustrates how the detector 10 extends behind the ceiling C, away from an external surface C1 of the ceiling C. <figref idref="f0004">Fig. 3C</figref> illustrates use of an installation/extraction tool 10-1 for use with the detector 10.

0036In <figref idref="f0005">Fig. 4</figref> illustrates external air flow, A1, A2, and A4 along with transmission and scattering associated with the external sampling region 16. In <figref idref="f0006">Figs. 5A, 5B</figref> further details of air flow and optical component placement for the external, open scattering region 16 are illustrated. <figref idref="f0007">Fig. 6</figref>, a side sectional view illustrates aspects of internal air flow in the detector 10. <figref idref="f0008">Figs 7A, 7B</figref> illustrate air flow as exiting the cyclone separator 20. The fan 20a implementable as a centrifugal fan, is illustrated in <figref idref="f0008">Fig. 7B</figref> coupled to the separator 20.

0037From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the invention. It is to be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims. Further, logic flows depicted in the figures do not require the particular order shown, or sequential order, to achieve desirable results. Other steps may be provided, or steps may be eliminated, from the described flows, and other components may be add to, or removed from the described embodiments.