Daikin WGZ030DW Installation, Operation And Maintenance Manual page 46

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.
Description Low Evap Press
Time
Check #1 15 seconds after
start
Check #2 30 seconds after
start
Check #3 45 seconds after
start
Check #4 60 seconds after
start
High Condenser Pressure – Unload
Logic
If the discharge pressure rises above the High Condenser
Pressure Unload set point and more than one compressor
on the circuit is running, the circuit will stage down. One
compressor will shut down as soon as the pressure rises
above the unload set point and if two remain running then one
more will shut down 10 seconds later if the pressure is still
above the unload set point. On that circuit's VIEW CIRCUIT n
(1) screen is displayed the "Hold Clear @" value which is the
limit by which the condenser pressure must drop to allow for
additional loading. A High Condenser Pressure Unload event
will be recorded in the Event Log.
No stage up will be allowed on the circuit until the condenser
pressure drops below the unload set point less an offset value
which is calculated based on
1. Type refrigerant
2. Number of compressors
3. Condenser configuration
4. How many high pressure unload occurrences since the
previous midnight. See below for initial offset value and
step-increase for each additional occurrence.
Hold Clear Reset Value =
High Pressure Unload set point – ["Initial Offset" + ["Step-
Increase" x ["number of occurrence"-1]]]
IOM 1322-1 • WATER-COOLED SCROLL COMPRESSOR
Requirement to continue
Evap Press >(0.48*Low Evap
Press SP)
Evap Press >(0.66*Low Evap
Press SP)
Evap Press >(0.83*Low Evap
Press SP)
Evap Press > Low Evap Press
SP
Initial Offset & Step Increase
Example of Operation: Assume a packaged water-cooled
WGZ unit with 4 compressors using R410A as the refrigerant.
Circuit number one is fully loaded (both compressors running)
when that circuit's condenser pressure exceeds the High
Condenser Pressure Unload set point. Circuit number one will
unload by turning one of the two compressors off immediately.
This circuit will not be allowed to load back up until its
condenser pressure decreases below the "Hold Clear @" value
which is displayed on the "VIEW CIRCUIT 1 (1)" screen. Since
this is the first High Condenser Pressure – Unload occurrence
for that circuit, the circuit will be allowed to load once the
condenser pressure drops below the High Condenser Pressure
Unload set point minus 40 psi. If using default value (405 psi)
this will be 365 psi. After some time, the condenser pressure is
allowed to decrease below this value and circuit number one is
again allowed to load up.
Again its condenser pressure exceeds the High Condenser
Pressure Unload set point and it unloads. Because this is
the second High Condenser Pressure – Unload occurrence
for that circuit, the circuit will not be allowed to load until the
condenser pressure drops below the High Condenser Pressure
Unload set point minus 50 psig (40 psi plus step-increase 10
psi). If using default values, this will be 355 psi. As you can
see, for each occurrence the discharge pressure must get
lower and lower (based on Step-Increase value) before the
circuit is allowed to again load. This logic is intended to prevent
excessive cycling of compressors.
Unit Capacity Overrides
The following conditions override the automatic capacity
control when the chiller is in cool mode only.
Demand Limit
The maximum unit capacity can be limited by a 4 to 20 mA
signal on the Demand Limit analog input. This function is
only enabled if the Demand Limit set point is set to ON. The
maximum unit capacity stage is determined as shown in the
following graph.
46
Unit Controller
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