www.DaikinApplied.com 69 IOM 1322 • WATER-COOLED SCROLL COMPRESSOR
Description Low Evap Press Time Requirement to continue
Check #1 15 seconds after start Evap Press >(0.48*Low Evap Press SP)
Check #2 30 seconds after start Evap Press >(0.66*Low Evap Press SP)
Check #3 45 seconds after start Evap Press >(0.83*Low Evap Press SP)
Check #4 60 seconds after start Evap Press > Low Evap Press SP
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 oset value
which is calculated based on
1. Type refrigerant
2. Number of compressors
3. Condenser conguration
4. How many high pressure unload occurrences since the
previous midnight. See below for initial oset value and
step-increase for each additional occurrence.
Hold Clear Reset Value =
High Pressure Unload set point – [“Initial Oset” + [“Step-
Increase” x [“number of occurrence”-1]]]
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 o 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 rst 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-increase10psi). 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.
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.
NOTE: Any signal less than 4 mA does not limit the chiller
capacity
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