OMM TGZ-1 TGZ 040A through TGZ 190A 49
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
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.
Hold Clear Reset Value =
High Pressure Unload set point – [“Initial Offset” + [“Step-Increase” x [“number of
occurrence”-1]]]
Initial Offset & Step Increase
Condenser
Configuration
No. of
Compressors
R134a
Step
Increase
4 30 psi 10 psi
Water-Cooled
6 20 psi 10 psi
4 N/A 15 psi
Air-Cooled
6 N/A 15 psi
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-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.
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