IOMM AGR-1 AGR 070A through 100A 63
4 - 20 mA (remote reset signal)
When selecting “4-20mA” as the reset mode, the controller will reset the leaving water temperature to
a higher value based on a percentage of the “Maximum Chilled Water Reset”. At 4mA or less, the
leaving water temperature is not reset. At 20mA the leaving water temperature is reset to the
maximum. Between 4 and 20 mA, the leaving water temperature is reset proportionally to the reset
input signal.
Outside Air
When selecting “Outside Air” as the reset mode, the controller will reset the leaving water
temperature to a higher value based on the temperature of the outside air (ambient). As the outside air
temperature decreases, the leaving water temperature is reset up until the maximum reset is obtained.
When the outside air temperature is above the outside air temperature reset setpoint the leaving water
temperature is not reset and is controlled to the leaving water temperature setpoint.
Ice (for lower ice setpoint requirements, consult McQuay Marketing Support)
When in “Ice” reset mode, and adequate amount of glycol must be added to the system to protect the
evaporator from freezing. McQuay recommends a minimum of 30% solution of ethylene glycol to
protect to 7°F with the minimum leaving water temperature at unit shut down of 21°F. At 21°F shut
down, the saturated refrigerant temperature can approach 10°F.
The leaving water setpoint must be calculated according to the following formula;
LvgEvapSpt = Desired Ice Temperature + 1/2 Control Band + Shut down Delta T
Lvg EvapSpt = 21°F + 1/2 (4)°F + 1.5°F
Lvg Evap Spt = 21°F + 2°F + 1.5°F
Lvg Evap Spt = 24.5°F
The remote 4-20mA input must be 4mA or less and the leaving water temperature will be controlled to
the calculated setpoint. When the remote reset input is above 4 mA, the value “MaxChwRst” is added
to the leaving water setpoint. This will be the day operating setpoint. This value cannot be reset
when in the day mode by any other reset options.
The alarm setpoints must be adjusted to allow operation of the chiller at the lower temperatures. The
following values should be adjusted and the example assumes 30% ethylene glycol solution with a
21°F shut down temperature:
1.
“FrzStat” Set to the saturated suction pressure that corresponds to a temperature several degrees
higher than the freeze point of the water / glycol solution. (10°F = 33psig.)
2.
“Frz H2O” Set at least 4° F below the shut down temperature, but not lower than the freezing
point of the water / glycol solution. (21°F - 4°F = 17°F)
3.
“LP cutout” Set 8 to 10 psi below the “Frz State” pressure but never below 20 psig. (33psi - 10
psi = 23 psi)
4.
“LP Cutin” Set to a saturated suction pressure equal to the shut down temperature plus 1/2 the
control band. (21°F + 2°F = 23°F; 23°F = 46psig)
Network
A network refers to a McQuay MicroTech control panel (CSC or RMS) that can provide controlling
functions to several McQuay chillers. One function is to control the chiller leaving water temperature
according to some control logic in the network control panel. The network control panel will control
the leaving water setpoint when the “Network” option in the chiller is selected. The network control
panel will send a signal that reflects 0 to 100% reset of the leaving water set point according to the
value entered into the “MaxChwRst” of the unit controller.
To Next Page
Loading...
Need help?
General
