www.DaikinApplied.com 49
IOM 1210-7 • MAGNITUDE
®
MODEL WMC CHILLERS
o
peraTIon
1. Cool LWT setpoint
2. Max Reset Delta T setpoint
3. Start Reset Delta T setpoint
For example, a reset is accomplished by changing the Active
Leaving Water variable from the (Cool LWT setpoint) to the
(Cool LWT se Max Reset Delta T setpoint) when the
evaporator (return – leaving) water temperature delta varies
from the (Start Reset Delta T setpoint) to 0.
To prevent a possible surge at startup, the compressor will not
start if the evaporator LWT is lower than Templifier No Start set
point.
Reset Type – 4-20mA
(Cool Mode)
The Active Leaving Water variable is set equal to the Cool LWT
setpoint if the reset signal is less than or equal to 4 mA. It is set
equal to (Cool LWT se Max Reset Delta T setpoint) if
the reset signal equals or exceeds 20 mA. The Active Leaving
Water variable will vary linearly between these extremes if the
reset signal is between 4 mA and 20 mA. An example of this
action is shown in
the figure below
.
Figure 50: LWT Reset
NOTE:
Temperatures in above figures are examples only.
Reset Type – 4-20mA (Heat Mode)
The Active LWT Target variable is set equal to the Heat LWT
set point if the reset signal is less than or equal to 4 mA. It is
set equal to (Heat LWT [BAS] set point - Max Reset Delta T set
point) if the reset signal equals or exceeds 20 mA. The Active
LWT Target variable will vary linearly between these extremes
if the reset signal is between 4 mA and 20 mA. An example of
this action is shown below.
Figure 51: LWT Reset
NOTE:
Temperatures in above figures are examples only.
Reset Type – RETURN (Heat Mode)
The Active Leaving Water variable is adjusted by the return
water temperature. When the chiller mode = HEAT, the active
Leaving Water variable is reset using the following parameters:
1. Heat LWT setpoint
2. Max Reset Delta T setpoint
3. Start Reset Delta T setpoint
Reset is accomplished by changing the Active Leaving Water
variable from the (Heat LWT Setpoint) to the (Heat LWT
setpoint – Max Reset Delta T setpoint) when the condenser
(leaving – return) water temperature delta varies from the
(Start Reset Delta T setpoint) to 0. Note, it will in reality it
only approaches 0.0 therefore you only approach Max Reset.
The reset is filtered to not allow sudden changes in the Active
setpoint, which may cause sudden stops in leaving water
satisfied. Choosing this option can also in effect give you return
water control. An example of this action is shown below.
Figure 52: LWT Return Reset
NOTE:
Temperatures in above figures are examples only.
4 ma
20 ma
Cool LWT Set-Point
(44.0°F)
Max Reset Delta T
(10.0°F)
(54.0°F)
0 ma
