IOM 1210-7 • MAGNITUDE
®
MODEL WMC CHILLERS
46 www.DaikinApplied.com
o
peraTIon
Compressor Capacity Control
Compressor capacity is determined by the status of the leaving
chilled water temperature (LWT), which is a direct indicator
of whether the chiller is producing enough cooling to satisfy
the cooling load. The LWT is compared to the active chilled
water setpoint, and compressor loading or unloading ensues,
considering any capacity overrides that may be in effect.
Capacity Overrides
The conditions described in the following subparagraphs
override normal capacity control when the chiller is in the
COOL mode. Of the following limits, the one creating the
lowest amp limit is in effect. The resulting present limit value
for compressor current is stored in the Active Demand Limit
variable.
Low Suction Pressure
If the suction pressure drops below the Low Suction pressure
– Inhibit setpoint, the unit will inhibit capacity increases. If
the suction pressure drops below the Low Suction pressure -
Unload setpoint, the unit will begin capacity decreases.
High Discharge Temperature - Load
If the discharge temperature rises above the High Discharge
Temperature - Load setpoint and the Suction SuperHeat is <
15.0 °F, the unit will begin capacity increases.
Maximum LWT Rate
The maximum rate at which the leaving water temperature
can drop (chiller mode = COOL) is limited at all times by the
Maximum Rate setpoint. If the rate exceeds this setpoint,
capacity increases are inhibited.
Demand Limit
The maximum amp draw of the compressor can be limited by
a 4 to 20 mA signal on the Demand Limit analog input. This
function is only enabled if the Demand Limit setpoint is set to
ON. The amp limit decreases linearly from the Maximum Amp
Limit setpoint (at 4 mA) to the Minimum Amp Limit setpoint (at
20mA). If the amp draw rises above the limit value, the unit will
inhibit capacity increases. If the amp draw rises to 3% or more
above this value, the unit will begin capacity decreases.
Network Limit
The maximum amp draw of the compressor can be limited by
a value sent through a BAS network connection and stored
in the Network Limit variable. If the amp draw rises above the
limit value, the unit will inhibit capacity increases. If the amp
draw rises to 3% or more above this value, the unit will begin
capacity decreases.
Minimum Amp Limit
The minimum amp draw of the compressor can be limited by
the Minimum Amps setpoint. If the amp draw drops below the
limit value, the unit will load capacity to maintain minimum
amps.
Maximum Amp Limit
The maximum amp draw of the compressor is always limited
by the Maximum Amps setpoint. This limit has priority over all
other functions including manual capacity control. If the amp
draw rises above the limit value, the unit will inhibit capacity
increases. If the amp draw rises to > 3% or more above this
value, the unit will begin capacity decreases.
