24/69
Siemens Building Technologies
Electronic Air / Fuel Ratio Control System
CC1P7873.1en
HVAC Products
5 Description of functions
15.10.2002
5.8 Application of a correcting variable
Application of a correcting variable represents a possibility to interfere in the operation
of the RVW26…, aimed at
optimizing the combustion process
.
For instance, using the correcting variable input of the RVW26..., an O
2
trim control
system RPO25… can be employed in conjunction with the air / fuel ratio control sys-
tem.
Using the oxygen value as a reference, practically all disturbance factors that affect the
combustion process will be taken into account.
An analog DC 0…10 V signal can be fed to the correcting variable input «X2».
This correcting variable signal acts either only on the auxiliary actuator (AUX1)
or, optionally, on both auxiliary actuators (AUX1 and AUX2) (refer to «Program-
ming»).
A correcting variable signal
reduces
the position of the auxiliary actuators that deter-
mine the amount of air, and thus always the amount of excess air for the combustion
process.
When using operation with an impact from both auxiliary actuators, the readjustment of
the O
2
valve is made in parallel.
This readjustment of the O
2
value is made proportionally to the current burner otuput
and to the influencing factor «DISTURB»:
Setload´ (%) = Setload (%) -
Correcting variable X2 (V) x DISTURB (%)
10 (V) x 100 (%)
x Setload (%)
Setload
Load signal without the correcting variable
Setload´
Load signal with influence of the correcting variable
X2
Correcting variable
DISTURB
Programmable influencing factor (maximum correction)
The relative effect on the amount of air ensures that a certain correcting signal has
nearly the same impact on the amount of excess air, irrespective of burner output. This
is true only if the load / position curves are programmed such that the LOAD graduation
is proportional to the actual output of the burner (refer to «Commissioning notes for
burners»).
When used in connection with the RPO25... and RVW25..., the impact of the correcting
variable must be set to the maximum value, since the RPO25... provides O
2
trim control
in this application.
If, for some reason, the impact of the correcting variable must be restricted, the follow-
ing table can be used:
D
O
2
DISTURB
»
1 %
5 %
»
2 %
10 %
»
4 %
20 %
»
5 %
30 %
»
8 %
50 %
General
Function
Legend
