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8 Control Mode
8 Control Mode
The firing rate controller tries to bring the actual value into line with the setpoint value. In doing
so, a direct correlation is assumed between internal firing- ate and boiler temperature, e.g. the
greater the internal firing rate, the faster the boiler temperature rises. Should the curves be
programmed differently, the firing rate controller will not function.
Four parameters determine the control action.
• Adjustment time
The adjustment time determines at what intervals the deviation is checked and the new
adjustment is determined.
• P term
The proportional term acts directly on the deviation, that is the difference between setpoint
value and actual value.
P > → higher step response
• I term
The integral term is calculated from the instantaneous deviation and the previous devia-
tion from the setpoint value.
I > → faster approximation to the setpoint
danger of overshooting!
• D term
The derivative term is calculated from the variation of the actual values. It has acceleration
and respectively slowing effects.
In practice the adjustment of the PID-controller is specified to each controlled system. With
the characteristics of the controlled system you try to deduce acceptable data, i.e. the values
are defined experimentally
• P term, I term, D term are added together and act as adjustment on the firing rate default
for the ratio control. The value is added up, starting from the instantaneous internal firing
rate.
• As long as the actual value is below the setpoint, P term and I term are positive, that is to
say both these term will increase the firing rate default.
• Only the D term is negative in such a case (assuming that the boiler temperature is rising).
Use the D factor sparingly because it leads to a higher burden of the actuators.
• In order to avoid excessive overshoot during burner start-up, the parameters should be
adjusted to achieve a suitably large D-term.
• If, despite a large setpoint deviation, the burner is not running at full firing rate or base firing
rate, the P term should be increased.
• The longer the adjustment time, the quieter the group. However, this also increases the
actual value's deviation from the setpoint value and leads to slower adjustment.
Values must be adjusted according to the controlled system.
* Default value
Hot Water Installations Steam Boiler Installations
P term 120* 600
I term 60* 300
D term 20* 25
Adjustment time 15* 20
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