Guardian SCC 25, Manual

Page: 39 / 79

Page 39
GUARDIAN SCC-40 Issue (Auto VI) & V3.6t 07/12/00
RUNNING
For all modes of operation status inputs and cutout trip limits are continuously
checked for fault conditions. If any are found then the compressor is
immediately stopped and the appropriate trip mode indicated.
Over current and high discharge temperature capacity unloading operate in all
in all modes.
MANUAL MODE.
In MANUAL mode, provided CAPACITY is selected, the capacity may be
adjusted by operation of the RAISE/LOWER buttons. A raise/lower request is
actioned at one pulse every five seconds, the pulse duration dependant on the
RAISE/LOWER pulse width settings.
LOCAL AUTOMATIC.
In LOCAL AUTOMATIC mode the capacity is adjusted dependant on the
suction pressure being above or below the control band selected.
In REMOTE AUTOMATIC mode up to six compressors are available to run. One
compressor always assumes overall control and is referred to as the Master.
The Master compressor is selected on least runtime hours and Ready status.
If runtime hours are equal then priority is based on the compressors' unit
number, compressor 1 having the highest priority.
When a compressor has established its' self as Master it uses the
communications link to request status information from the other compressors.
The action of transmitting by the Master will prevent any other REMOTE mode
compressors from starting, regardless of suction pressure.
The status request command also contains the status information of
all other compressors. This status information consists of:-
i) Runtime hours
ii) Slide position
iii) Compressor mode requires REMOTE automatic for control.
iv) Compressor State - Ready, Waiting, At minimum, At maximum, Master,
Load sharing, Over current, High Discharge.
When the suction pressure is above the control setpoint then the Master
compressor will start. It will then attempt to maintain control by automatic
adjustment of its capacity. If the suction pressure remains above the control
band (+0.1 bar) such that the slide has reached 100% or the compressor has
had an over current or high discharge back-off, then the stage up timer will be
started. The timer will be reset if the capacity falls below 100% or the suction
pressure goes into or below the control band after a back-off.
Once the stage up timer elapses the Master compressor will stop transmitting,
releasing control to the compressor with the next highest priority. As all
compressors have runtime hours etc., each compressor waits for its priority time
out period before becoming Master.
The next compressor to establish its self as Master goes through the starting
procedure. Whilst the suction pressure is above the control band the Master
continues to raise its slide. When the Master brings the suction pressure into or
below the control band; then, provided it slide is less than Capacity Low Limit,
it will optimise capacity to improve efficiency. The Master will select a compressor