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.)
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.
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.
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 LambdaPro 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).