12
RSMD-CM Technical Guide
SEQUENCE OF OPERATIONS
Cooling Mode, Dehumidication, and Head Pressure Control
Cooling Mode Operation
In the Cooling Mode, as Supply Air Temperature (SAT) rises
above the Active SAT Cooling Setpoint, the compressors will
stage on and modulate to maintain the Active Evaporator Coil
Suction (Saturation) Temperature Setpoint. Two compressors are
controlled per RSMD-CM. Multiple RSMD-CM Modules are
needed when there are more than two compressors.
Modules with One Digital and One Fixed Compressor
If the digital compressor modulates to 100% and the SAT is
still above the SAT Cooling Setpoint for the Cooling Stage
Up Delay, then the xed compressor will stage on. The digital
compressor will then be allowed to modulate as necessary
to maintain the Active Evaporator Coil Suction (Saturation)
Temperature Setpoint. Minimum o times must also be met
before compressors can stage on.
Modules with Multiple Digital Compressors
If the rst digital compressor modulates to 100% and the SAT is
still above the SAT Cooling Setpoint for the Cooling Stage Up
Delay, then the second digital compressors will enable and the
two digital Compressors will then modulate together to maintain
the Active Evaporator Coil Suction (Saturation) Temperature
Setpoint.
Staging Down Compressors
If the digital compressor(s) have modulated down to 30% for
the Stage Down Delay period and the SAT has fallen below the
SAT Cooling Setpoint minus the Stage Control Window, then the
last compressor to have staged on (digital or xed) will stage o
assuming its Minimum Run Time has been met. Any remaining
digital compressors are then allowed to modulate as needed. If
the last remaining digital compressor reaches 0% for the Stage
Down Delay, it will stage o.
Dehumidication Operation
The RSMD-CM activates the Cooling Stages based on the
Evaporator Coil Temperature compared to the Evaporator Coil
Suction (Saturation) Temperature Setpoint. The Evaporator
Coil Suction (Saturation) Temperature is calculated by using
the Suction Pressure Transducer and converting the pressure to
temperature.
For Copeland™ Digital Scroll Compressor units, the RSMD-CM
will modulate the Copeland Digital Scroll™ Compressor to
maintain the Evaporator Coil Suction (Saturation) Temperature
Setpoint and activate the compressors as necessary.
On units that have one digital and one xed capacity compressor,
if the xed capacity compressor is activated, the Copeland™
Digital Scroll Compressor will only be allowed to modulate
within the range of 70-100% in order to prevent the loss of reheat
capacity during low load conditions. If, with both compressors
on, the rst digital compressor has modulated down to its 70%
minimum and the Coil Suction Temperature falls below the Coil
Temperature Setpoint minus the Cooling Stage Control Window,
then the second compressor will stage o once its Compressor
Minimum Run Time and the Stage Down Delay Times have been
met. At that point, the Copeland™ Digital Scroll Compressor
can modulate down as needed to maintain the Coil Temperature
Setpoint.
If the RSMD-CM has two digital compressors, the first
compressor will be locked at 100% and the second compressor
will modulate.
Head Pressure Control
The RSMD-CM can monitor a Head Pressure Transducer and
control a condenser fan or water valve to maintain a Head
Pressure Setpoint.
A condenser relay is commanded on when the rst compressor
is enabled (except if the unit Heat Pump is in Defrost Mode).
On an air cooled unit, the condenser fan will be controlled with
0-10 VDC output signal. On a water cooled unit, the water valve
will be controlled with a 2-10 VDC signal.
When the Condenser Signal rst activates, it maintains at 100%
for 10 seconds.
In the Cooling Mode, the Condenser Signal will modulate to
maintain the Cooling Head Pressure Setpoint. On units with a
condenser fan, the signal can modulate between 15% and 100%.
On a unit with a condenser water valve, the signal can modulate
between 25% and 100%. If the Head Pressure exceeds 550 psig,
the Condenser Control Signal will immediately go to 100% and
a High Head Pressure Alarm will be generated. The alarm will
be deactivated when the Head Pressure drops below 540 psig.
In the Dehumidication Mode, the Condenser Output Signal
controls to the Reheat Head Pressure Setpoint. High Head
Pressure conditions produce the same eects as in the Cooling
Mode.
If no Head Pressure Transducer is detected, the Condenser Output
Signal will be maintained at 100%.
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