Orion System RNE Modular Controller - Manual (Page 55)

RNE Modular Controller Field Technical Guide
SEQUENCE OF OPERATION
55
Economizer Mode and Supply Air Tempering
4 Compressor Units (RNE 120-140)
The 1 st two VFD Compressors will both be driven out of the
Analog Output (AO) #1, and the second two VFD Compressors
will be driven out of the AO #2. All four compressor relays will
be enabled separately as needed. The 1
st VFD Compressor will
modulate to maintain the Active Supply Air Cooling Setpoint.
If the compressor signal reaches 80%, the Stage Up Timer will
begin. If the Stage Up Delay period elapses and the compressor
signal is still at our above 80%, the 2
nd VFD Compressor relay
will be enabled. The VFD Compressor signal from AO #1 will
then be cut to half of its current value and these fi rst two VFD
Compressors will modulate together as needed to maintain the
Supply Air Setpoint.
If the VFD Compressor signal on AO #1 again reaches 80%, the
Stage Up Timer will begin. If the Stage Up Delay period elapses
and the compressor signal is still at or above 80%, the 3
rd VFD
Compressor will be enabled and will modulate from AO #2. The
signals from AO #1 and AO #2 will then go to half of the current
value of the signal from AO #1 and modulate as needed. If the
VFD Compressor signal on AO #2 reaches 80%, the Stage Up
Timer will begin. If the Stage Up Delay period elapses and the
compressor signal is still at or above 80%, the 4
th VFD Compres-
sor will be enabled. The compressor signals will again be cut to
half their current value and will then modulate together as neces-
sary to maintain the Supply Air Setpoint.
If more than one compressor is active and the VFD Compressor
signal(s) falls to 30% for the Stage Down Delay period, the last
compressor staged on will then stage off. The VFD Compres-
sor signal(s) will then modulate as needed. This will repeat as
compressors stage off. If the last compressor remaining on then
reaches 0% for the Stage Down Delay period, it will be deacti-
vated.
Economizer Operation
This section assumes you have confi gured your HVAC unit to control
the Outdoor Air Dampers in an Economizer Mode of operation.
The Economizer is used as the fi rst stage of Cooling if the Outdoor Air
or Wetbulb Temperature is below the Economizer Enable Setpoint. For
Wetbulb control of the Economizer, an Outdoor Air Humidity Sensor
must be installed. If the unit is equipped with a Return Air Temperature
Sensor, the Outdoor Air or Wetbulb temperature must be at least 5°F
colder than the Return Air Temperature to allow the Economizer to
modulate. By using the Return Air Temperature reference, it allows the
RNE to calculate whether the Outdoor Air Temperature will assist in
Free Cooling. The Return Air Temperature and Space Temperature must
be above 50°F for the Economizer operation to occur.
The RNE Controller can monitor an Outdoor Air Humidity Sensor and
combine that reading with the Outdoor Air Temperature reading to calcu-
late a Wetbulb Temperature. If this Wetbulb Temperature is not available,
only the Outdoor Air Temperature will be used. Whichever temperature
is available, it must be below the Economizer Enable Setpoint by 1°F
to enable the Economizer during the Cooling Mode of operation. When
the temperature rises 1°F above the Economizer Enable Setpoint, the
Economizer will be disabled and return to the Minimum Position.
As soon as the Cooling Mode is started, the Economizer will calculate a
starting damper position based on the Outdoor Air Temperature and the
differential between the Supply Air Temperature and the Active Supply
Air Temperature Setpoint. After it moves to this initial Setpoint, further
adjustments will be made in small increments to fi ne tune the damper
position to maintain the Active Supply Air Temperature Setpoint. If the
Economizer reaches 100% open and the Supply Air Temperature is still
too warm, the Mechanical Cooling will be enabled to operate to provide
additional stages of Cooling. Once a Mechanical Cooling Stage has been
activated, the Economizer will remain full open until the Mechanical
Cooling Stages are off or until the Outdoor Air Temperature or Wetbulb
Temperature causes the Economizer to be disabled.
If the Economizer is not enabled to provide Cooling during the Occu-
pied Mode, it will still maintain the Minimum Position programmed to
provide minimum fresh air into the building. During the Unoccupied
Mode, the Economizer will be closed. If during the Unoccupied Mode
there is a call for Cooling and the Economizer is enabled by Outdoor Air
Drybulb or Wetbulb temperature, it will modulate between full closed
and full open to provide Free Cooling.
The Supply Air Temperature must be above the Supply Air Temperature
Setpoint before the Mechanical Cooling can be activated.
During Dehumidifi cation, the Economizer will remain at its minimum
position.
Supply Air Tempering
WARNING: Because of the lower compressor lockouts used for
Water Source Heat Pump Units, the Supply Air Tempering sequence
will not function in those applications.
For units confi gured as VAV units (Supply Air Temperature Sensor is
confi gured as the controlling sensor), unit heat can be used to maintain
a leaving air temperature near the Cooling Leaving Air Setpoint. This
may be necessary in conditions very cold outdoor air is being introduced
into the unit preventing it from maintaining the Supply Air Temperature
(SAT) Cooling Setpoint of 55º (Adj). There are four conditions that must
be met before the heating can be activated in this sequence:
1. The outdoor air temperature must fall below the Low Ambient
Protection Setpoint (the Low Ambient Protection Setpoint must
always be below the Mechanical Cooling Lockout Setpoint).
2. The SAT must fall below a special Tempering SAT Heating Setting
(used only in this sequence) which is 2º below the SAT Cooling
Setpoint (not adjustable).
3. The economizer must be at its minimum position. This prevents
heating from coming on before the controller has had a chance
to bring the SAT under control by closing the economizer to its
minimum position.
4. The VFD must be operating above the Heating Minimum VFD
Setpoint.