Flame detectors

Flame monitoring is necessary for the safe operation of an oil burner. During the pre-ventilation time and operation, it monitors whether the function sequence is correct. Depending on the type of flame, a distinction is made between photoelectric flame monitoring and an infrared flickering light detector. Ionisation monitoring and a UV flame detector are also used in special applications. Photoresistors are semiconductor devices that reduce their resistance in light (usually cadmium compounds). Accordingly, the current that occurs changes when a constant voltage is applied. The monitoring current strength (photocurrent) is 6–160 mA. The response sensitivity is ~ 5–30 Lux. Nowadays, photoresistors are used exclusively for yellow burners. Low-impedance resistors actuate a relay without an amplifier, high-impedance resistors actuate a relay with an amplifier. The response time is ~ 0.5 s. The photoresistor is mounted in a position on the burner housing in which the light of the flame is noticeable and the thermal load is low at the same time. Photoresistors cannot be used to monitor blue flames, as their sensitivity to the radiation of these flames (UV range) is too low. Photocells consist of an airless glass bulb with two electrodes. The applied voltage is approx. 100 V. The cathode is made e.g. of caesium and emits an electron current (photocurrent) on exposure to light. More recent designs use photocells that respond to ultraviolet radiation in a very tight wavelength range (UV detector). They are insensitive to longer-wave radiation. When a selenium-coated plate is exposed to light, photoelements generate a DC voltage of approx. 0.1–0.2 V, as a result of which a control current in the 8–25 mA range flows. Photoelements are hardly used any longer nowadays.