42
IOM 1206-1
Unit Controller Operation
Circuit Status
The displayed circuit status should be determined by the
conditions in the following table:
Compressor Control
Compressors should run only when the circuit is in a run or
pumpdown state. They should not be running when the circuit
is in any other state.
Starting a Compressor
A compressor should start if it receives a start command from
the unit capacity control logic.
Stopping a Compressor
A compressor should be turned off if any of the following
occur:
•
Unit capacity control logic commands it off
•
An unload alarm occurs and the sequencing requires this
compressor to be next off
•
Circuit state is pumpdown and sequencing requires this
compressor to be next off
Cycle Timers
A minimum time between starts of the compressor and a
minimum time between shutdown and start of the compressor
shall be enforced. The time values are determined by the
Start-start Timer and Stop-start Timer setpoints.
These cycle timers should not be enforced through cycling of
power to the chiller. This means that if power is cycled, the
cycle timers should not be active.
These timers may be cleared via a setting on the controller.
Condenser Fan Control
Condenser fan control should stage fans as needed any time
compressors are running on the circuit. All running fans
should turn off when the circuit goes to the off state.
Fan Staging
Fan staging shall accommodate anywhere from 2 to 7 fans on a
circuit using up to 5 outputs for control. The total number of
fans on shall be adjusted with changes of one fan at a time.
The tables below show the outputs energized for each stage.
Condenser Target
A condenser target should be selected from the setpoints based
on the number of compressors on the unit and the number of
compressors running. Each stage of capacity on a circuit will
use a different condensing target set point.
A minimum condenser target should be enforced. This
minimum will be calculated based on the evaporator LWT. As
the LWT varies from 7.2°C (45°F) to 32.2°C (90°F), the
minimum condenser target will vary from 23.9°C (75°F) to
48.9°C (120°F).
Staging Up
The first fan should turn on when the saturated condenser
temperature exceeds the condenser target. After this, the four
stage up dead bands shall be used. Stages one through four
#
Status
Conditions
1
Off:Ready
Circuit is ready to start when needed.
2
Off:Cycle Timers
Circuit is off and cannot start due to active cycle timer on all
compressors.
3
Off:All Comp Disable
Circuit is off and cannot start due to all compressors being
disabled.
4
Off:Keypad Disable
Circuit is off and cannot start due to circuit enable set point.
5
Off:Circuit Switch
Circuit is off and circuit switch is off.
6
Off:Alarm
Circuit is off and cannot start due to active circuit alarm.
7
Off:Test Mode
Circuit is in test mode.
8
Preopen
Circuit is in preopen state.
9
Run:Pumpdown
Circuit is in pumpdown state.
10
Run:Normal
Circuit is in run state and running normally.
11
Run:Evap Press Low
Circuit is running and cannot load due to low evaporator pressure.
12
Run:Cond Press High
Circuit is running and cannot load due to high condenser pressure.
13
Run:High OAT Limit
Circuit is running and cannot add more compressors due to the
high ambient limit on unit capacity. Applies only to circuit 2.
2 Through 4 Fans
Stage
Fan Outputs On
1
1
2
1,2
3
1,2,3
4
1,2,3,4
