PATOL - Infra-red flame detection - Principles of operation

Infra-red emissions occur for all materials. The wave length spectrum and intensity of this IR depends on the material’s temperature, and for solid bodies is determined by the Laws of Physics formulated by Planck, Stefan, Boltzmann & Wien.
Figure 1 shows ‘black body’ IR emissions for passive material at various temperatures. Superimposed is the IR peak emission at 4.3um occurring on CO2 formation during the combustion of hydrocarbons.
Other conditions, such as arc welding, provide different spectra, however, only flame provides significant IR energy over a narrow band centred on 4.3um, with virtually zero accumulative energy occurring in the long pass band above 5um.
It is with an understanding of these criteria that the Patol Series 7000 infrared flame detection equipment has been designed.

The detectors (7010 & 7011) have sensors, configured in pairs as channels, which generally respond to modulated infra-red in the range 1Hz to 10Hz, as is produced by flame flicker. Static background black body emissions are automatically rejected. The unit’s narrow band (4.2-4.7um) and wide band (>5um) filters, together with electronic analysis in relation to the various energy level parameters, enable the discrete channels to reject spurious and transient emissions from the local environment. The detectors do not react to visible light from the sun or local luminaires.
It is possible for certain conditions to ‘dazzle’ any particular detector channel. For example hot machinery rotating at 300 rpm could produce 5Hz IR modulation at 4.3um. Whilst the channel’s analytical circuit associated with the long pass detector will identify this as an erroneous signal and override an Alarm being raised, that channel is in effect blind for the duration of the condition
The detector incorporates timer functions that check the ability of the unit to be able to detect flame by all channels for at least a proportion of a defined monitoring period. If the local environment has prevailing IR conditions at sufficient level to effectively “blind” any one channel’s flame detection function over all of this period, then a fault warning is signalled. The monitoring period is nominally set at 10s but can be adjusted to suit particular site conditions. The unit continuously checks itself. It will always provide a response to a flame condition, unless the site has problematical ambient conditions in which case an advanced fault warning will be registered.
The 7010 & 7011 units have eight pairs of detectors, each pair operating as a discrete channel (alarm path). With this arrangement (figure 2) the overall field of view is sub-divided such as to more readily analyse & compute background emissions. The optical viewing field of each detector pair is 30º x 90º. The overall arrangement is such that the eight channels are mounted on a 22.5º rotational pattern which provides a 360º coverage.