Daikin WGZ030DW Operating And Maintenance Manual page 46

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, R22, and R407C 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, R22, and R407C 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
Check #1
Check #2
Check #3
Check #4
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 CIRCUITn (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, and 4) how many high pressure unload occurrences since the previous mid-
night. See below for initial offset value and step-increase for each additional occurrence.
46
Low Evap Press Time
15 seconds after start
30 seconds after start
45 seconds after start
60 seconds after start
WGZ030D through WGZ200D
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
OMM 1130-2