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The controller automatically starts the compressors when the unit is in operation and there are no active alarms
affecting either the entire unit or the compressors that are to be started. Compressors are started in sequence
so as to minimise inrush currents.
After a settable delay to unit start-up, the air flow switch input must get closed so that the controller can enable
the compressors. If the input of the air flow switch opens during unit operation, the air flow alarm triggers after
a settable delay, which causes the unit and the compressors to stop.
The compressor controller described above is purely proportional, so it has an intrinsic defect: the error, i.e.
the distance between the setpoint and the ambient temperature, tends to zero, but never reaches it. If you wish
to have a more accurate temperature control, the PI (proportional and integral) controller can be enabled so
as to successfully achieve the setpoint to the detriment of a possible under elongation, i.e. the temperature
may drop below the stored setpoint for a span of time. This option is designed for the controller to override
activation of the next step (compressor) after the integral time has elapsed in cases when the activation of
some compressors leads to a deadlock and the ambient temperature setpoint is not achieved.
6.4.1.2 Free cooling
In units equipped with an external air damper the controller can use the external air conditions, where these
are favourable in terms of temperature or enthalpy depending on the set control mode, to cool ambient air by
properly managing opening of the external air damper.
Temperatures conditions requested to activate free cooling and damper management are reported in the first
picture.
The figure graphically shows the conditions required for free cooling activation and damper management.
If the temperature of the external air is lower than the ambient temperature minus the differential for free
cooling/free heating enabling (DenFCH), the unit is enabled for free cooling operation.
Damper modulation occurs within the free cooling differential (DFC). Dampers control logic during free cooing
differential (DFC) are reported in the second picture. Starting from its min. value, damper opening begins at
the summer setpoint (SpC) minus the free cooling offset (OFC): it reaches its max. value at the summer
setpoint (SpC) minus the free cooling offset (OFC) plus the modulation differential of the free cooling damper
(DSFC).
Opening of the external air damper can be limited by the evaporation pressure value during winter operation.
This function is illustrated in a dedicated section.