Danfoss EKC 316A Reference Manual page 9

Double Loop (address o56 Reg.type = 3)
The controller can regulate the superheat using a double loop
system. The so called outer loop is really the same as in the single
loop system except output of PI block is the reference for the inner
loop.
The inner loop also has also PI block with Proportional Gain factor
is KpT0 and the Integration Time is TnT0.
The double loop control system is shown in the diagram below.
The feedback of the inner loop is temperature difference between
media temperature S3 and S1. This value represents the load on
the evaporator and large values will tend to increase the opening
degree OD% of the valve.
The tuning of the double loop is more complicated than the
single loop and it is advisable not change to many parameters at
the same time. The starting point should be to use the following
settings.
Parameter Value
Kp factor 0.7
Tn sec 120
KpT0 3
TnT0 sec 30
If the superheat is unstable, the KpT0 parameter should be slightly
reduced. The value parameter Kp factor is not large so little is gain
by reducing this parameter.
EKC 316A
Kp factor and Tn settings
Superheat Ref. SHref
+
+
PI Control
-
-
Superheat
(S2 – S1)
-
+
Refrigerant Temp. S2
Address
n04
n05
n20
n44
Reference manual RS8ED102 © Danfoss
PI Control
ETS Valve
Evaporator Temp. S1
03-2006
KpT0 and Tn T0 settings
Evaporator
+
-
(S3 – S1)
9