Riello 20155654, Installation, Use And Maintenance Instructions

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20155699 54 GB
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8.1.10 Control mode
The firing rate controller is attempting to adjust actual value to
set-point value.
A direct correlation is assumed between internal firing rate and
boiler temperature, i.e. the higher the internal firing rate, the fast-
er boiler the rise of temperature.
If curves are programmed in a different way the firing rate control-
ler will not operate. Four parameters define the control character-
istics:
•
Adjustment time
Adjustment time defines the intervals of deviation is checked
and a new adjustment is determined.
• P term
The proportional term affects directly on deviation defined as
difference between set-point value and actual value.
P >

higher step response
• I term
The integral term is calculated from present deviation and
previous deviation to set-point value
I >

faster approximation to set-point

danger of over-
shooting!
• D term
The difference term is calculated from variation of actual val-
ues. This may result in accelerating, respectively retarding ef-
fects.
In practice adjustments of PID-controller is orientated by given
controlled system. Out of characteristics of the controlled system
data can be deduced, i.e. by experimental determination.
–
P share, I share, D share are added up and serve as adjust-
ments to the firing rate default of the fuel/air ratio control.
– As long as the actual value is below set-point, P term and I
term are positive, that is to say both of these terms will in-
crease the firing rate default.
– In such a case only D term has a negative value (assuming
that boiler temperature is rising). Use D term carefully be-
cause it leads to a higher burden for the actuating elements.
– In order to avoid excessive overshoot during burner start-up
adjust parameters to achieve a suitably large D term.
– If despite a large set-point deviation the burner is not run at
full or base firing rate you should increase the P term.
– The longer you select the adjustment time the calmer the fuel/
air ratio control. However, this also increases the actual val-
ues’ deviation from set-point value and leads to slower ad-
justment.
Tab. N
Adjustment of the values according to the controlled system is
highly recommended.
Hot water
installations
Steam boiler
installations
P term 120 280 600
I term 60 360 300
D term 20 50 25
Adjustment
time
15 2 20