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1 Technical Description
1.5 Functional description of the structure switches
1.5.4 Controller types (S1, S49 to S53)
-- Example of a ratio control
v
c
+
x
+
x
x
2
1
K
L
Q
Q
L
Figure 1-33
Control diagram ratio control
In a combustion control the air-/gas flow should be in a constant ratio. The command
variable (commanding process variable) is the air flow Q
3
12,000 m
/h as a signal 4 to 20 mA. The controlled variable (following process variable)
is the gas flow Q
with a measuring range 0 to 3,000 m
G
4 to 20 mA signal. In an ideal combustion the air/gas ratio is
Q
L
L
=
= 4.
« ideal
Q
G
Q
L
= L
The air factor λ is then 1 and should be adjustable in the range from 0.75
⋅
λ
Q
«
G
to 1.25 on the controller.
The ratio factor v (bei xd = 0) is determined partly by the transmission factors K of the trans-
mitter (measuring ranges).
x
= Q
K
with the values from the example
⋅
1
G
G
x
= Q
K
⋅
2
L
L
x
1
v =
=
x
2
1
v =
L
⋅ λ
«
With the values from the example
1
v = 1
⋅
4
λ
1
gives v =
i.e. the choice of the transmitter ranges has been made so that
λ
K
1
.
G
=
K
L
L
«
66
w
y
+
x
--
d
K
G
G
Q
K
G
G
⋅
with
Q
K
L
L
K
G
⋅
K
L
100 % ⋅ h ⋅ 12,000 m
⋅
3
3,000 m
⋅ 100 % ⋅ h
GAS
AIR
which is preset in the range 0 to
L
3
/h which is also available as a
100 %
K
=
G
3,000 m
100 %
K
=
L
12,000 m
Q
1
G
=
Q
L
⋅ λ
L
«
3
Manual
3
/h
3
/h
SIPART DR22 6DR2210
C79000-G7476-C154-03
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