14
CT-600-2
Control and limitation
The control functions main objective is to regulate the operating mode and the capacity control of the
individual compressors. Different operating modes can be selected to define a requested capacity for the
ACP. A capacity distribution algorithm will find the optimum distribution of the requested capacity over the
available compressors, taking into account various parameters, such as compressor size, capacity regulation
type, running hours, start priority, etc. The starting and stopping of each compressor is handled through
separate control algorithms to assure correct unloaded start and unload after stop for the different
compressor types.
Limiter functions are used to limit the capacity for the ACP or individual compressors based on warnings
issued by the protection functions. A typical example would be low suction pressure or high motor current.
The limiter function will over-rule the request from the capacity regulation and limit the capacity to prevent an
alarm, that could stop the compressors. Some limiter functions will limit the total capacity of the ACP, other
limiters will limit the slide valve position of the individual compressors. The figure below shows the principle
of keeping the operating condition (To/Tc) within the dotted lines, i.e. when the suction pressure drops below
its low warning limit or the discharge pressure rises above its high limit, then the capacity will be reduced in
order to force the operating point back into the allowed area.
Besides the capacity control, different control functions are used to handle functions related to the external
inputs like start permission, alarms and digital feedback (outputs). The oil system and economizer control
functions are also handled.
Handling of alarms, warnings and timers
Each warning, alarm and timer is evaluated to decide how to react. Warnings will not stop the operation, but
may trigger a limitation of the capacity. Warnings can also become alarms, if they are time limited. Warnings
are to be regarded as an information to the user, that the operation is limited, but still without direct risk of
damage. An alarm will always stop operation in some kind – ranging from closing the economizer valve,
stopping a single compressor, to shutting down the complete ACP.
Safety chain function
The safety chain is essentially, not a part of the control system, but the wiring of the safety chain is part of the
wiring of the ACP LINK. The safety chain is individual for each compressor and directly connected to the CM-
SW-01 module and will cut the start-signal in case the chain is broken.
The safety chain is a hard wired serial connection of different components to assure safe operation. The
safety chain on ACP consist of, but is not limited to, below parts:
High pressure switch, connected to CM-SW-01 protects the system against high pressure.
Emergency switch, mounted on the side of the ACP LINK panel – allows anyone to stop the unit in
case of emergency, activating the emergency switch will stop all compressors. The emergency
switch terminals must be connected directly to the electrical panel safety circuit in serial connections
with the existing local emergency stop system.
