Unit installation

20 OMRCPMICRO
Figure 7, Leaving water reset
Ice (remote reset signal)
When in "Ice" mode, all compressors will run 100% loaded (that is no unloaders energized) to make certain that
compressors cool appropriately. As a result, the number of stages in "Ice" mode will equal the number of
compressors. Use approved solutions in the chilled water loop to protect the system to at least 15°F lower
than the ice setpoint. When Leaving Evaporator Water (Lvg Evap") is set less than 34°F, "Ice" mode must be
used. Cylinder unloading of compressors when suction temperatures fall below 25°F will prevent the
compressor motor from cooling adequately. The lowest Leaving Water Ice set point for McQuay reciprocating
chillers is 21°F. With an Ice set point of 21°F, saturated suction temperatures of 12°F or 13°F are typical. This
is the lowest recommended saturated suction temperature for McQuay reciprocating products.
Most ice storage applications require dual reset control. Use terminals #134 and #135 on TB7 for field wiring 4-
20mA remote reset connections. See the field wiring diagrams at the end of this manual for more detail.
IMPORTANT
If a 4-20mA signal device is not available, a dual contact is required, then a dry contact
between terminals #215 and #135 may be used to initiate reset. When using a dry contact,
"open" will maintain the ice set point and "close" will initiate a full reset.
If the chiller is to be used for ice storage, select the "Ice" option in Menu 14. "LvgWater Spts", "ResetOpt=".
To calculate the leaving chilled water set point, the control band (Menu 14 "CntrlBand=") and shut down
delta-T (Menu 14 ShutDn D-T=") must first be selected. The equation to determine leaving chilled water
temperature:
Lvg Evap = Desired Ice Temp + 1/2"CntrlBand + " "ShutDn D-T"
Lvg Evap = 21°F + 1/2 (4) + 1.5°F
LvgEvap = 21 +2+ 1.5
Lvg Evap = 24.5
For day operation the maximum chilled water reset (Menu 14 "MaxChWRst=") is added to the "Lvg Evap" as
calculated above to reach the desired day operating leaving chilled water temperature.
When the Ice option is selected, the resetting of the leaving chilled water set point (during day operation) via
the 4-20mA input is not a functional option. To retain the option, the set points in Menu 14 must be changed
by a Building Automation System through our MicroTech Open Protocol Monitor Software.
When a 4-20mA signal is used to initiate ice reset, if a signal less than 4mA is received, nothing happens—the
ice mode temperature will be maintained. When a signal of 4mA or more is received, the unit changes to a non-
ice building mode temperature. The control signal will be displayed under "Reset Sig" on Menu #14.
The alarm setpoints (see Menu 22) will also need to be adjusted. The following is an example of alarm set
points for a typical ice operation: