Siemens Building Technologies
Basic Documentation RVL470
CE1P2522E / 30.03.2000
Landis & Staefa Division
Function block "Boiler"
33/68
10.4.2. Control with a two-stage burner
10.4.2.1. Setting
parameters
For two-position control with a two-stage burner, the variables that can be set are the
switching differential and the minimum burner running time – which now apply to both
stages – plus the following variables:
•
The release limit (FGI) for the second stage. This is the variable generated from the
temperature (T) and the time (t). If the maximum limit is exceeded, the second burner
stage is released and can switch on, provided the minimum waiting time for the sec-
ond stage has elapsed
FGI =
∆
T dt
t
0
∫
where:
∆
T = ( w
−
0.5
*
SD
−
x ) > 0
•
The reset limit (RSI). This is the variable generated from the temperature (T) and the
time (t). If the maximum limit is exceeded, the burner will be locked and switches off
RSI =
∆
T dt
t
0
∫
where:
∆
T = ( x
−
w + 0.5
*
SD ) > 0
•
The minimum locking time for the second stage, that is, the period of time on comple-
tion of which the second stage can switch on at the earliest
10.4.2.2. Control
The controller compares the actual value of the flow temperature with the flow tem-
perature setpoint. If it falls below the setpoint by half the switching differential
(x < w – 0.5
*
SD), the first burner stage will be switched on. At the same time, the mini-
mum waiting time for the second burner stage is started and the release limit (integral)
is being generated. The controller ascertains for how long and by how much the flow
temperature remains below w – 0.5
*
SD. It continually generates the release limit based
on the time and the temperature.
If, on completion of the minimum locking time, the flow temperature lies below
w – 0.5
*
SD, and if the release limit reaches the set maximum limit, the second burner
stage will be released and switched on. The flow temperature starts rising.
When the flow temperature has exceeded the setpoint by half the switching differential
(x = w + 0.5
*
SD), the second burner stage is switched off again, but will remain re-
leased. The first stage continues to operate. If the flow temperature drops, the second
stage will be switched on again at x < w – 0.5
*
SD. The setpoint is now maintained by
the second burner stage.
If, however, the flow temperature continues to rise (x > w + 0.5
*
SD), the controller
starts generating the reset limit (integral). It determines for how long and to what extent
the flow temperature stays above the setpoint by half the switching differential. It con-
tinually generates the reset limit based on the time and the temperature. When the reset
limit reaches the set maximum limit, the second burner stage will be locked and the first
stage switched off.
The minimum waiting time and the calculation of the release limit at x < w – 0.5
*
SD are
started when the switch-on command for the first burner stage is given.
Due to the time-temperature integral, it is not only the duration of the deviation that is
considered, but also its extent, when deciding whether the second stage shall be
switched on or off.
SD
Switching differential
w
Boiler temperature setpoint
x
Actual value of boiler temperature
