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Building Technologies
Boiler Sequence Controller RMK770
CE1P3132en
HVAC Products
5 General functions, fundamentals
01.03.2005
The degree of difficulty
λ
is calculated as follows:
Tu
λ
=
Tg
For the degree of difficulty of a controlled system, the following guide values can be
used:
λ
<0.1
= easy
λ
0.1…
λ
0.3 = medium
λ
>0.3
= difficult
P-band Xp = 2 × Tu / Tg ×
∆
x /
∆
y × 100 % ˜
2 × Tu / Tg × Ksmax
Integral action time Tn = 3 × Tu
Change of valve position
∆
y = 40 %
Change of flow temperature
∆
x = 18 K
Tu = 6 s
Tg = 18 s
P-band Xp = 2 × 6 s / 18 s × 18 K / 40 % × 100 % = 30 K
Integral action time Tn = 3 × 6 s = 18 s
The maximum system gain Ksmax can be estimated from the differential of maximum
flow temperature upstream of the mixing valve and the minimum return temperature, for
example. The value of Ksmax may have to be increased to give consideration to a
nonlinear valve characteristic. TVmax = 80 °C and TRmin = 20 °C => Ksmax = 60 K.
To obtain a reliable step response, it is important during the measurement to keep the
temperature upstream of the valve and the return temperature (mixing) as constant as
possible.
During the measurement, the boiler and return temperatures should reflect winter con-
ditions at relatively low outside temperatures.
On actual plant, it is not always possible to get a reliable step response.
Without a step response, or in the case of unsatisfactory control action after entry of the
calculated parameters, the on / off pulses after a setpoint step give hints for setting the
parameters.
A distinction is to be made between 2 cases:
A
B
313
1D
12
Setpoint
Open pulse
Close pulse
Open pulse
Close pulse
Flow
temp.
A The control pulses are too long:
Measure the effective valve running time (0…100 % stroke) and enter. If the pulses
are still too long, increase P-band Xp
B Several successive relatively short on or off pulses: Increase integral action time Tn
Degree of difficulty
Setting rules
Example
Maximum system
gain Ksmax
Note
Setting without step re-
sponse
The flow temperature
fluctuates about the set-
point