With the wide availability of Liquid Petroleum Gas (LPG) and Compressed Natural Gas (CNG), most manufacturers of industrial heating systems in India have started using gas burner systems in preference to oil burner systems. Gas-based systems are much cheaper to operate, and hence, these are becoming very popular in various applications such as, boilers furnaces, bakery ovens, industrial dryers, etc. Obviously, such varied applications would involve various burner designs, which are appropriate for the applications in question. For example, boilers and furnaces would have high capacity burners with very stable flames. However, bakery ovens and dryers may have low capacity burners with small and often unstable flames.
Traditionally, Light Detecting Resistors (LDR) have been extensively used for detecting oil flames. They are cheap and reliable. However, they are not suitable for detecting gas flames. The gas flame is inherently blue in color and emits very little visible light. There are two methods of detecting gas flames:
1. Specially designed UV flame sensors, which detect the UV radiations emitted by a naked flame. The radiations are typically in a narrow band of 190 to 260 nano-meters. The UV sensor is, in fact suitable for all type of flames, such as, oil, gas coal and even a candle flame!
2. Other alternative is to use a "Flame Rod" immersed in a flame, which is used to detect the flow of small current through the rod. The small current flow is due to ionization air around the flame. Hence, flame rod is also called "Ionization" or "Flame" electrode.
The UV sensors are a good choice and are very reliable. However, they are very expensive. Hence, it is often uneconomical to justify their usage in medium and low cost gas heating systems. In such cases, flame rods are popularly used. One of the main problems with flame rod based controllers is that of nuisance tripping, which is irritating to the user. The flame rod based burner controllers operate reliably, provided the user is aware of the principles and limitations of flame rod based flame sensing.
Principles of Flame Detection
Any process of combustion involves release of heat energy, which also heats up the air associated with the flame. The combustion process also results in knocking out the electrons from the surrounding atoms, resulting in free electrons and positively charged atoms (ions). An ion is an atom, which has lost an electron. Such a process is referred to as "ionization". The presence of charged particles enables current flow in flame rods - similar to flow of current in valves.
The flame detection principle can be best illustrated by a model. A gas burner has typical distribution of electrons and positive ions in a gas flame. The free electrons being lighter and more mobile will concentrate at the periphery of the flame. In practice, this area roughly corresponds to the blue outer zone of the gas flame. The positive ions being heavier tend to concentrate at the bottom of the flame close to the burner metal surface. In practice, this region corresponds to yellow inner core of the flame.
The current flow through the flame rod is essentially determined by the flow of electrons through the flame rod. However, the positive ions, which are concentrated close to burner surface also play an important role in the flow of electrons! During combustion, the hot burner metal surface gives up a few electrons - just like a hot cathode in a valve. The positive ions located near the burner immediately absorb such electrons to become inert atoms again. The electrons emitted by the burner surface are compensated by absorption of electrons by the flame rod, which are present at the periphery of the flame. It is this phenomenon that results in the flow of ionization current through the flame rod. Hence, for a good current flow to occur, the burner surface must be large and hot enough to generate sufficient electrons.
In practice, factors such as, the burner design, flame size, flame rod location, etc., and play an important role in the determining the magnitude of the ionization current flow. Typically, one can expect currents in excess of 15 to 20 micro-Amps in most installations. However, in some installations, the currents could be lower than 2 micro-Amps. Such installations may typically have small flames operating at low temperatures, which can often be unstable.
Even though, it is possible to design burner controllers to detect currents as small as 0.5 micro-Amps, however, for reliable operation, it is advisable to achieve current flows of at least 2 to 4 micro-Amps under all conditions. Hence, it important to be aware of the factors mentioned above to avoid nuisance tripping by the burner controller.
Factors Affecting Ionization Current Flow
It has been observed that the magnitude of current flow through flame rod is affected by the following factors:
1. Optimal air-fuel ratio results in good ionization. Excess of gas results in poor ionization, whereas excess of air has marginal effect on the ionization current. In fact, the ionization current measured by the ammeter can be used to adjust the air fuel ratio! The airflow can be adjusted, so that, the current flow is at maximum.
2. Intensity of ionization increases with gas calorific value, volume and flame temperature. Obviously, this factor is application dependent. Hence, the designer may need to give loser attention to other factors given below.
3. The burner surface area touching the flame must be sufficiently large. In practice, it must be at least 4 to 5 times the surface area of the flame rod touching the flame. If the area of the burner surface touching the flame is insufficient, then additional plates can be welded to burner metal frame to increase the area.
4. The flame rod and the burner surface must always be in contact with the flame. Or else, it may result in loss of flame signal and consequent burner controller trip! This is an important issue when the flame has a tendency to move around. In such cases, flames rod mounting will play a crucial role for obtaining reliable flame signal.
5. As mentioned above, the flame rod must be mounted so that it is in contact with the flame under all conditions. However, the flame rod must also be mounted so that it is in the blue zone (outer zone) of the flame, where there is a good availability of electrons. The current flow will be less if the flame rod is immersed in the yellow zone (inner core), which is rich in positive ions. Since, in practice, the zones may not be well defined - the flame rod mounting would require special care and adjustment.
6. The best way to adjust the flame rod is to actually connect a micro-ammeter (which is available as a part of most multi-meters) and check the ionization current levels.
Burner Controller Design Features
The flame rod based burner controllers for gas burners are designed with safety as the primary criteria. In other words, the main objective will be to trip the burner as quickly as
possible, when the flame signal is not present - typically within one second. Most safety standards specify such a requirement. In fact, electronic controllers have replaced the older mechanical gas safety devices mainly because they were slow to trip. In addition, a
retrial (which is common for small oil burners) is generally not advisable in the case of gas burners. A retrial may result in an explosion due to concentration of gas-air mixture.
The above feature may result in nuisance tripping in the case of small capacity burners with low temperature and unstable flame - which are common in applications, such as, small capacity dryers and bakery ovens. In such cases, a careful evaluation of the retrial feature may be done and incorporated in the burner controller specifications.
The flame sensing circuits used in the burner controllers provide AC voltage for ionization current detection. The flame rod is connected to "Line" terminal of the AC source provided by the controller. The burner frame is connected to "Ground" terminal. The flame rod design essentially behaves like a rectifier. The controller is designed to detect the current flow only in one direction. This design feature provides two important advantages. The controller can be designed to be insensitive to flow of AC currents - which generally are due to insulation leakage. Hence, it provides for a more reliable detection of flame signal. More importantly, it will also enable detection of short circuits in flame rod circuit, which results in the absence of rectifier action and a flow of AC current. It is a very important safety feature in the case of flame rod based flame detection. Such a situation may occur due to insulation failure in the flame rod circuit or due to flame rod coming in contact with burner body.