68 AGZ 030A through 065A IOM 686
the "MaxChWRst". Any milliamp value between 4 and 20mA will cause a proportional value of the
Maximum Chilled Water Reset to be added to the Leaving Chilled Water Set Point. The external 4-20
milliamp control signal and current reset set point will be displayed under "Reset Sig" and "Active
Spt", respectively, on Menu 14.
By selecting Return as the reset mode, the leaving chilled water temperature will be reset by the
controller as required to maintain a constant return water temperature. The user must select the
"Return" option under "ResetOpt" and select the return set point to be maintained under "ResetSpt".
The current leaving chilled water reset set point will be displayed under "Active Spt".
If the chiller is to be used for ice storage, select the "Ice" option in Menu 14 "LvgWater Spt",
"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 is:
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.
The "Network" reset option allows a signal to be sent reflecting 0-100% reset of the "Lvg Evap"
set point (chillers) or "Lvg Cond" set point (THRs) based on the Maximum Chilled Water Reset set
point. This function acts much like the 4-20 milliamp reset option as described above.
The Maximum Pull Down Rate set point ("MaxPullDn") will prevent overshooting the active
water set point during initial start-up and normal operation. The controller will limit the rate at which
the chilled water loop temperature is reduced based on an adjustable set point (default of 0.5°F/min).
Every minute, the controller checks the leaving water temperature and compares it to the last reading.
If the pull rate has been exceeded, the controller will delay additional unit stages.
The Control Band ("CntrlBand") set point defines the temperature range on either side of the
active water set point that must be exceeded in order for a stage up or stage down to occur. The Start
Up Delta-T ("StartUpD-T") and Shut Down Delta-T ("ShutDn D-T") define when the unit will start
and shut down. The unit will cycle on from a shut down mode once the leaving chilled water
temperature has reached the Leaving Chilled Water Set Point plus half of the Control Band plus the
Start Up Delta-T. After the chiller starts, it will stage up and down according to the leaving chilled
water temperature (plus or minus) half the Control Band. Once the load is met and the chiller has
staged down to stage 1, the unit will shut completely off once the leaving chilled water temperature
has reached the Leaving Chilled Water Set Point minus half of the Control Band minus the Shut Down
Delta-T.
Lvg Evap = Desired Ice Temp + ½ "CntrlBand =" + " ShutDn D - T ="
LvgEvap F= ° + °21 ½(4) + 1.5 F
Lvg Evap = 21 + 2 + 1.5
Lvg Evap = 24.5
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