Flame Conductivity VS Flame Rectification Systems

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1 Flame Conductivity VS Flame Rectification Systems There are two basic principles in flame rod detection systems-flame conductivity and flame rectification. Conductivity systems are, for the most part no longer used. Either type of system depends on the ability of the flame to conduct current when a voltage is applied across 2 electrodes in the flame. Heat from the flame causes molecules between the electrodes to collide with each other so forcibly as to knock some electrons out of the atoms, producing ions. This is called flame ionization. Positively charged ions flow to the negatively charged electrode; negatively charged electrons flow to the positively charged electrode. In a 60 Hz system, it changes its direction (polarity) 120 times a second. At one instant, one of the electrodes is positive, and 1/120 of a second later it is negative. As the voltage changes polarity, the flame current (ion flow) will change direction. For a conductivity system, the areas of the 2 electrodes (called flame and ground electrodes) are equal and the flame current between them is the same in both directions. This is the principle of a conductivity system. When an ac voltage is applied across the flame electrode and the ground electrode, alternating current proportional to the applied voltage flows through the flame. Because the flame current in a conductivity system is ac, this system cannot differentiate between a leakage current and an actual flame current. It is possible for the system to falsely indicate the presence of a flame (with possibly dangerous results) if the flame electrode is shorted to ground through a leakage circuit with about the same resistance as the impedance of a flame. A carbon deposit on the base of the flame electrode could form a very effective leakage path and cause a false flame indication. (A direct short of low impedance would, of course, make the system inoperative.)

2 The flame rectification system also uses 2 electrodes, but with 1 important difference-the ground electrode is always designed to be much larger than the flame electrode (flame rod). For effective operation, the area of the ground electrode must be at least 4 times that of the flame rod. Usually, the ground electrode will be the burner bead. Because of the difference in electrode size, more current flows in one direction than in the other.

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4 With the current in one direction so much larger than the current in the other direction, the resultant current is, effectively, a pulsating direct current which operates the electronic network. The flame relay pulls in, indicating the presence of a flame and allowing the burner sequence to continue. The larger the ratio of ground area to flame electrode area, the greater the flow of current in the proper direction-in other words, a rectified current. Only the ionized path through a flame and the different sized electrodes can provide the rectified current required for the operation of the electronic network in a rectification system. Should a high resistance leakage to ground occur in the flame circuit, it sends an ac signal into the network, and the system shuts down safely. The rectification system does recognize the difference between a high resistance leakage to ground and the presence of a flame. The flame current is measured in microamps. Normal microamps would be 2 to 10. Viessmann Vitodens 200 W, WB2B Lamda-Pro System The same principle that is applied to flame rectification is used with Lamda-Pro. The ionization of the atmosphere in and around the flame is used to create a microamp signal for the microprocessor to process and determine it is safe to allow the main gas valve to open allowing gas to flow and burners to light safely. The Ionization Rod inserted into the Matrix burner flame sends a microamp signal (flame signal) similar to the standard signal used for electric ignition gas systems. The Vitodens by establishing a Lamda relationship between the air and gas mixture it can in a fully automatic way control/manage the gas and air ratio. The Lambda-Pro System allows control of gas and air independently of each other to ensure optimum combustion with varying operating conditions and also fuel qualities. The Flame Ionization Electrode is used as the Lambda sensor (no other components required). The air (30% excess air) and the gas flow rates are independently controlled by the Electronic Boiler Control Unit.

5 The ionization sensing rod needs to be kept clean as is always the case. The microamp signal is used to electronically control the firing rate of the gas valve. This again ensures optimum performance at all times. The fuel flow is checked against the optimum flame signal and correct Lambda setting is determined for each current burner cycle. There is no derating required at higher altitudes. You would however have to have the correct coding card installed. The Vitotronic Control provides flame safety function plus the Lambda- Pro it is also the Main Programmable Temperature Controller. As an example of what might happen is the uncontaminated fresh air for combustion was somehow contaminated. An example with the coaxial venting arrangement a cross contamination of products of combustion getting into the air side of the co-axial vent. This would cause a poor reading on the ionization rod and would trigger an immediate reaction because you are beyond the parameters set within the control and the boiler would go off and you would have an E9 code. This is burner in fault. The troubleshooting for an E9 fault code states that the ionization current lies outside the permissible range during calibration. The corrective action is to check the rod and the cable. It also states to check for venting system leaks. In the initial start up steps it is required to check at the vent pipe adapter which has two measuring ports, one for combustion air-intake and one for flue gas measurement. CO2 level allowed in the air intake is no higher than 0.2% and O2 no lower than 20.6% (20.9% is normal O2 content in air).

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