115
The electronic combustion controller utilises the physi-
cal correlation between the level of the ionisation cur-
rent and the air ratio
λ
. The maximum ionisation cur-
rent is achieved at an air ratio of 1 for all gas qualities.
The ionisation signal is evaluated by the combustion
controller. The air ratio is regulated to a value between
λ
=1.24 to 1.44. This range provides for an optimum
combustion quality. Thereafter, the electronic gas train
regulates the required gas volume subject to the pre-
vailing gas quality.
To check the combustion quality, the CO
2
content or
the O
2
content of the flue gas is measured. The pre-
vailing air ratio is determined using the actual values.
The correlation between CO
2
or O
2
content and air
ratio
λ
is shown in the following table.
Air ratio
λ–
CO
2
/O
2
content
Air ratio
λ
O
2
content (%)
CO
2
content (%) for nat-
ural gas E
CO
2
content (%) for natu-
ral gas LL
1.20
3.8
9.6
9.2
1.24
4.4
9.2
9.1
1.27
4.9
9.0
8.9
1.30
5.3
8.7
8.6
1.34
5.7
8.5
8.4
1.37
6.1
8.3
8.2
1.40
6.5
8.1
8.0
1.44
6.9
7.8
7.7
1.48
7.3
7.6
7.5
To achieve optimum combustion control, the system
regularly carries out an automatic self-calibration; also
after power failures (shutdown). For this, the combus-
tion is briefly regulated to maximum ionisation current
(corresponding to air ratio
λ
=1). Self-calibration takes
place shortly after the burner start and lasts for about
5 s. During calibration, higher than normal CO emis-
sions may occur briefly.
Function description
Electronic combustion control unit
5792 925 GB
Functions
Summary of Contents for Vitocrossal 100 CI1
Page 138: ...138 5792 925 GB ...
Page 139: ...139 5792 925 GB ...
