Detection of combustion flame is achieved by using sensor to sense one of the following characteristics of flame
a. Visible bright yellow/white light
b. Infra Red radiations
c. Ultra-Violet radiations
d. Ionic Current
All combustion flame give out infra red and ultra violet radiations of different strength and also possess ions of varying number depending on flame temperature and type of fuel. Only oil/ pulverized coal flame gives out bright yellow/white visible light. Gas flame is blue and nearly invisible. The sensors used to sense the flame, are usually electronic type. These are usually delicate, costly and required to be operated below 50 deg C. Line of sight of flame and feasible direction of installing the sensor gave rise to two types of sensor enclosures; Front Viewing (FV) and Side Viewing (SV) types. Also enclosures are developed with facility to connect clean cool air supply for cooling the sensor and keeping the hot gases from reaching sensor. Enclosures are also developed for self-checking sensor.
1. Photo Resistor (LDR) type PR-EYE-FV and PR-EYE-SV
Diligent Micro Controls has developed different models of Photo Resistor type flame sensors. This sensor is characterized by resistance value that varies inversely with respect to the intensity of visible light falling on it. They differ mainly in size, direction of viewing, mounting arrangements etc.
2. Photo Voltaic type PV-EYE-SV
Diligent Photo Voltaic flame sensor develops dc voltage proportional to the intensity of visible light incident on the sensor. It is available in only side viewing type. This flame sensor serves the same purpose like photo resistor sensor but is costlier.
3. Infra Red type IR-EYE-FV
Diligent infra red sensor houses infra red element IRD1010 from M/s Satronics ( now Honeywell) in strong aluminum holder. This holder has quartz window to guard against the environmental condition in the furnace and also has cooling air adaptor to facilitate connection of cooling air supply. Sensing flame by detecting Infra Red emission from the flame is especially effective when the view of the flame is obscured by dust or smoke such as the in coal fired burner or blast gas burner.
4. Ultra Violet type UV-EYE-FV
Diligent Ultra Violet sensor houses infra red element QRA-2 from M/s Landis & Gyr( now Siemens) in strong aluminum holder. This holder has quartz window to guard against the environmental condition in the furnace and also has cooling air adaptor to facilitate connection of cooling air supply.
5. Flame Electrode for Ionisation Technique
Flame electrode is used to pick up ions present in the flame and decide the presence of flame. These are made in different sizes and shape to suit specific burners.