1. Field of the Invention
This invention relates to a flame detecting apparatus, more paraticulary to a novel flame detecting apparatus in which a flame detecting electrode and an auxiliary electrode are disposed in a flame.
2. Description of the Prior Art
In many application, domestic, commercial and industrial, flame combustion of a fuel is used as a source of heat. It is essential, in the interests of safety, that there should at no time be an accumulation of unburnt gases in the combustion chamber (such as will occur on flame failure) which may be accidentally ignited and cause an explosion. Therefore, it is necessary to have some means for detecting and giving an indication of flame failure, the means preferably stopping the supply of fuel to the combustion chamber. Many types of flame detector are available to indicate flame failure and prevent the build-up of this potentially hazardous condition. These flame detectors are usually based on one of the following principles:
(1) THERMOSTAT EFFECT;
(2) ACTION OF LIGHT SENSITIVE THERMIONIC TUBES; AND
(3) ELECTRICAL PROPERTIES OF FLAME GASES.
The flame detectors based on either of the first principles are subject to fundamental problems. Thermostats have a slow response time because of the finite time between flame failure and the detection of cooling which gives an indication of the flame failure. On the other hand, light sensitive thermionic tubes require delicate and expensive amplifying means. These devices also require accompanying fault detection equipment to ensure that they are operating properly.
The third principle on which flame detectors have been based involves making use of the electrical properties inherent in flame gases, for example electrical conductivity or rectification. A flame detector using the electrical conductivity or rectification action of a flame has a voltage of several hundred volts AC applied between a flame detecting electrode and a burner, and a minute current which is caused to flow through the flame between the detecting electrode and the burner is amplified by an amplifier circuit of high input impedance which employs a field-effect transistor or the like. Alternatively, a light emitting device, such as a neon tube, is caused to emit light by the use of the minute current, and a photo-conductive device is operated by the emitted light. The applied voltage may, for example, be 250 volts AC, with the detected current only being some 4 microamps.
In another type of flame detector using the third principle, only the detecting electrode inserted into the flame is used. This type uses the fact that some of the atoms or molecules in a flame are thermally ionized by the high temperature, that is, there are many positive ions of H.sub.3 O.sup.+ in the top region of the flame and many negative ions of Ho.sup.- in the bottom region of the flame. Also many electrons produced by the thermal ionization are present in the middle region of the flame. It may be said, therefore that the flame is an electrical conductor, although with a very large impedance. When an electrode is disposed in the flame, the electrons are caught by the electrode one by one, so that a current flows through the flame, the electrode being charged to negative potential. This negative potential is used as a detecting signal. However, generally, the detecting voltage is very small, for example, 0.6 to 0.8 volts, and the current is also very small, for example 50 to 120 nanoamps.
With the known flame detectors therefore a quick and reliable response cannot be obtained, because the detecting signals are so small.