Daikin WGZ030DW Installation, Operation And Maintenance Manual page 68

Stage Down Now
The Stage Down Now flag is set based on the following tests:
If Unit Mode = Cool AND
LWT error < -0.5 * Control band AND
More than one compressor running AND
Stage down timer expired THEN
Stage Down Now = True
If Unit Mode = Cool AND
LWT error < (-0.5 * Control band - stop delta) AND
One compressor running AND
Stage down timer expired THEN
Stage Down Now = True
If Unit Mode = Cool AND
Number of compressors running > Demand limit AND
Stage down timer expired THEN
Stage Down Now = True
If Unit Mode = Heat AND
LWT error < -0.5 * Control band AND
More than one compressor running AND
Stage down timer expired THEN
Stage Down Now = True
If Unit Mode = Heat AND
LWT error < (-0.5 * Control band - stop delta) AND
One compressor running AND
Stage down timer expired THEN
Stage Down Now = True
Compressor Sequencing
Compressor staging is based primarily on compressor run
hours and starts. Compressors that have less starts will
normally start before those with more starts. Compressors that
have more run hours will normally shut off before those with
less run hours. In the event of a tie on number of starts, the
lower numbered compressor starts first. In the event of a tie on
run hours, the lower numbered compressor shuts off first. Run
hours are compared in terms of tens of hours.
If possible, only one compressor per circuit will start before
starting the second compressor on any circuit. If a circuit is
unavailable for any reason, the other circuit shall be allowed
to stage the second compressor on. When staging down, one
compressor on each circuit shall be left on until each circuit has
only one compressor running.
Low Ambient Start (WaterCooled = NO)
In order to avoid low evaporator pressure alarms at startup, low
OAT start logic allows for running at low evaporator pressures
for a longer time than normal as well allowing multiple
compressor restart attempts before locking out the circuit.
Low Ambient Start Procedure
A low OAT start is initiated if the condenser refrigerant
saturated temperature is less than 85.0°F when the first
compressor starts. Once the compressor starts the circuit
IOM 1322 • WATER-COOLED SCROLL COMPRESSOR
Unit Controller Operation
is in a low OAT start state for a time equal to the Low OAT
Start Time set point. During Low OAT Starts, the freezestat
logic and low evaporator pressure events are disabled. The
absolute limit for low evaporator pressure is enforced and the
compressor will shutdown if the evaporator pressure gets down
to 5.0 psi. For R410A, the absolute limit for low evaporator
pressure is 20.0 psi.
When the Low OAT Start Timer has expired, if the evaporator
pressure is greater than or equal to the Low Evaporator
Pressure Unload set point, the start is considered successful
and normal alarm and event logic is reinstated. If the
evaporator pressure is less than the Low Evaporator Pressure
Unload set point when the Low OAT Start Timer expires, the
start is unsuccessful and the compressor will shutdown.
Three compressor restarts per circuit are allowed when a
circuit fails to start a compressor in a Low Ambient Start
attempt. On the third failed Low Ambient Start attempt the
Restart Alarm is triggered and the circuit will not attempt to
restart a compressor until the Restart alarm has been cleared.
Circuit Capacity Overrides
The following conditions shall override the automatic capacity
control when the chiller is in cool mode only. These overrides
keep a circuit from entering a condition in which it is not
designed to run.
Low Evaporator Pressure
If a compressor in a circuit is running and the evaporator
pressure drops below the Low Evaporator Pressure Hold set
point, no more compressors will be allowed to start on that
circuit. The limit shall be active until the evaporator pressure
reaches the hold Low Evaporator Hold set point plus 8.0 psi for
R134a or 13.0 psi for R410A. On that circuit's VIEW CIRCUITn
(1) screen is displayed the "Hold Clear @" value which is the
limit by which the evaporator pressure must reach to allow for
additional loading. A Low Evaporator Pressure Hold event will
be recorded in the Event Log.
If two or more compressors are running in a circuit and the
evaporator pressure drops below the Low Evaporator Pressure
Unload set point, the circuit will begin reducing capacity. If
two compressors are running, one of the compressors will be
stopped after a time delay has expired which is one-half the
calculated freeze time (lower pressure then shorter time). If
three compressors are running, one compressor will stop after
a this time delay which is one-half the calculated freeze time
and, ten seconds later, if the pressure has not risen above the
unload set point an additional compressor will be stopped.
The last compressor on a circuit will not stop due to the unload
condition. The low evaporator pressure unload event will clear
when the evaporator pressure rises either 8.0 psi for R134a or
13.0 psi for R410A above the Low Evaporator Pressure Hold
set point. A Low Evaporator Pressure Unload event will be
recorded in the Event Log.
If the evaporator pressure drops below the Low Evaporator
Pressure Unload set point and one compressor on the circuit is
running then the following table applies.
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