Ice Machine Control RTAC

Issue:

Ice Machine Control

Resolution:

The Ice Machine Control functions provide for:

• External initiation of ice building mode (either hardwired or communicated).
• Fully loaded chiller and chilled water pump operation during ice building mode and automatic ice building mode termination based on evaporator entering (glycol solution) temperature.
• External signaling of ice mode termination through output relay contacts.

When the Ice Building Option is installed, the UCM will accept either an isolated contact closure (External Ice Building command) or a Remote Communicated input (Tracer) to initiate and command the Ice Building mode.

The UCM provides a dry contact output for signaling of the chiller's ice building status. This output is known as the Ice Making Status Relay, and its NO contact is closed when ice building is in progress and opens when ice building has been normally terminated either through Ice Termination setpoint being reached or removal of the Ice Building command.

The UCM also supports a "Front Panel Ice Termination Setpoint", settable through the Service Tool, and adjustable from 20 to 31 degrees F (-6.7 to -0.5 C).  When in the Ice Building mode, and the evaporator entering water temperature drops below the ice termination setpoint, the chiller terminates the Ice Building mode and changes to the Ice Building Complete Mode.

Ice Machine Control is enabled/disabled through the UCM contact closure.  If Ice Building is enabled, and in the Auto mode:

• If not already running, the chiller, its circuits, and compressors will start as normal with the lead circuit's EXV prepositioned and condenser fans prestarted.  Once started, the chiller will be in the "Ice Building" Mode of Operation.  If the chiller is already running, it will continue to run, but will be brought to full load ice making control.  The Ice Making Status Relay is then energized to indicate the Ice Building Mode of operation has begun.
• All the available compressors/circuits will be started, staged and be brought to full load, in any order.  Circuits and compressors in lockout or diagnostic tripped states will not be started, though. Exception:  If the leaving water temperature is above 75F, the compressors will be inhibited from loading past their minimum female load step.  This is due to a capacity limit known as Hot Start Inhibit.
• The chiller's operational compressors remain fully loaded until the external ice building input command is removed (ice building contact is opened or Tracer removes ice building command) or the evaporator entering water temperature falls below the Ice Termination Setpoint.  At this point, the Ice Building mode is "normally terminated", the Ice Making Status relay is de-energized, and the current and condenser limit setpoints will return to their normal settings.
• Once Ice Building is normally terminated, the compressor(s) are shut down much the same as if the chiller was put into stop with all compressors stopping after a short run unload period.
• If the Ice Building mode was normally terminated on the satisfaction of the Ice Termination Setpoint, but before the Ice Building command was removed, the chiller status becomes "Ice Building Complete" and the Chilled Water Pump is turned off.  The UCM will not allow the chiller to restart in its normal Auto mode, until the Ice Building command is removed, at which time the Chilled Water Pump is restarted. The chiller returns to normal Auto mode of operation, and is allowed to restart after a 2 minute delay (called the "Ice to Normal Transition Timer").  After the delay, the chiller can restart again per the differential to start and the normal Chilled Water Setpoint.
• If the Ice Building command is terminated while the chiller is still running, but before the Ice Termination Setpoint has been reached, the chiller will shutdown as above, but the chilled water pump will not be stopped.  The chiller returns to normal Auto mode of operation, and wi.ll be allowed to restart after a 2 minute delay (called the "Ice to Normal Transition timer").  After the delay, the chiller can restart again per the differential to start and the normal Chilled Water Setpoint.
• Once normally terminated, the Ice Building mode cannot be re-entered unless the Ice Building Command is cycled.

Facts:

• RTAC