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Cascade control
The controller is specially designed to control two compressor
groups with cascade.
The coordination between the two compressor groups and the
injection function is performed internally, inside the controller.
Coordination
In the "Select plant type" menu, set the MT-LT coordination to the
MT-LT release/start to coordinate the compressor groups.
In the "Cascade control" menu, select the LT compressor request
to start the injection.
Injection signal
The coordination of the injection ensures that liquid is injected
into the cascade exchanger when the LT compressors are running,
or when the Tc-LT pressure becomes too high.
Example
MT Compressor
LT Compressor
Injec. heat exchanger
T1: The load on the LT circuit requires compressor capacity.
LT starts compressor and activates injection signal
T2: The first MT compressor starts when the MT registers the
increase in pressure.
T3: Last LT compressor stops , injection stops and the MT require-
ments is removed. The last MT compressor stops..
Depending on the plant/design type, the injection can be defined
in the following ways:
- Always ON. Follows the main switch on the controller.
- Follows the LT circuit
- Follows a remote signal on a DI input.
102
Capacity controller RS8GN202 © Danfoss 11-2013
EVR in liquid line
The signal follows the injection signal, but features a time delay
and safety functions.
Extra cascade
The controller can send an injection signal to an external cascade
control. The signal will follow the injection signal.
Control sensors
The following are used as control sensors: Po-MT, Pc-LT, Scasc2
and Scasc3.
In plants in which nothing is mounted between Sd-LT and Scasc3,
the Sd-LT signal can be used instead of Scasc3.
Superheat
The function contains an adaptive algorithm that independently
adjusts the valve's opening degree, so that the evaporator con-
stantly delivers optimum refrigeration at best possible superheat.
The superheat reference will be limited by the settings for min.
and max. superheat.
If the superheat is very low, the valve may be closed very quickly
using the "SH closed" setting.
When the superheat has dropped to under the "SH closed" limit,
this function will immediately reduce the degree of opening of
the valve.
To ensure that the close function does not generate the general
superheat regulation, the "SH closed" setting must be at least 3-5
K lower than "SH min".
Optimised superheat regulation is used when Pc-LT is above the
set value.
If the condensing pressure falls below the set value, the optimised
superheat regulation ceases and the ETS valve closes gradually
until the pressure rises above the value.
MOP control
(MOP = Max. Operating Pressure)
The MOP function limits the valve's degree of opening as long as
the evaporating temperature is higher than the set MOP tempera-
ture.
AK-PC 783
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