Siemens SIPART DR21, Manual

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3 Functional description of the structure switches
3.4 Controller types (S1, S42 to S45)
Manual
78
SIPART DR21
C73000-B7476-C143-08
The ratio controller behaves like the slave controller S1 = 1 with respect to switching of the set-
point ratio wv so that the information and tables there apply accordingly. The variables wi and
w
E
must be replaced by wvi and wv
E
. This controller type can also be used as a ratio controller
with fixed ratio (manually adjustable) or with commanded ratio factor.
A fixed ratio factor is used for example in simple combustion rules, (see example) where the
ratio factor is reset manually if necessary for varying fuels. If it is possible to measure the
effects of the ratio factor (combustion quality, pollutants in the flue gas) a commanded ratio
controller is used. Here a master controller adjusts the ratio factor (ratio cascade) with the
combustion quality as a control variable.
Another application for ratio cascades are concentration controls, e.g. pH-value-controls. The
pH-value is the controlled variable of the command controller, the flow of alkali and acid the
commanded process variable and the following (controlled) process variable of the ratio
controller.
D
Example of a ratio control
+
+
x
1
GAS
Q
G
Q
L
K
G
K
L
x
2
v
x
y
x
d
- -
w

c
+
AIR
Figure 3-13 Control diagram ratio control
In a combustion control the air-/gas flow should be in a constant ratio. The command vari-
able (commanding process variable) is the air flow Q
L
which is preset in the range 0 to
12000 m
3
/h as a signal 4 to 20 mA. The controlled variable (following process variable) is
the gas flow Q
G
with a measuring range 0 to 3000 m
3
/h which is also available as a 4 to
20 mA signal. In an ideal combustion the air-/gas ratio is
L
ideal
= = 4.
Q
L
Q
G
= L
⋅ λ
Q
L
Q
G
The air factor
λ
is then 1 and should be adjustable in the range from
0.75 to 1.25 on the controller.