Water/Brine Reset —
Three types of chilled water/
brine reset are available, Reset Type 1, Reset Type 2, and
Reset Type 3. They can be viewed or modified on the CON-
FIG screen (accessed from the EQUIPMENT CONFIGU-
RATION table). See Table 2, Example 6.
The LID default screen status message indicates when a
reset is active. The WATER/BRINE CONTROL POINT tem-
perature on the STATUS01 table indicates the chiller’s cur-
rent reset temperature.
To configure a reset type, input all configuration informa-
tion for that reset type on the CONFIG screen. Then activate
the reset type by entering the reset type number in the SELECT/
ENABLE RESET TYPE input line.
RESET TYPE 1 (Requires an optional 8-input module) —
Reset Type 1 is an automatic chilled water temperature reset
based on a 4 to 20 mA input signal. The value for Rest Type
1 is user configurable (DEGREES RESET AT 20 mA). It is
a temperature that corresponds to a 20 mA signal. (4 mA
corresponds to 0° F [0° C]; 20 mA corresponds to the tem-
perature entered by the operator.)
This reset type permits up to ±30° F (±16° C) of auto-
matic reset to the chilled water/brine temperature set point,
based on the input from a 4 to 20 mA signal. The signal is
hardwired into the No. 1 eight-input module.
If the 4 to 20 mA signal is externally powered from the
8-input module, the signal is wired to terminals J1-5(+) and
J1-6(–). If the signal is powered internally by the 8-input
module (for example, when using variable resistance), the
signal is wired to J1-7(+) and J1-6(–). The PIC must be
configured on the SERVICE2 screen to ensure that the
appropriate power source is identified. See Table 2,
Example 9, 20 mA POWER CONFIGURATION.
RESET TYPE 2 (Requires an optional 8-input module) —
Reset Type 2 is an automatic chilled water temperature reset
based on a remote temperature sensor input.
This reset type permits ±30° F (±16° C) of automatic re-
set to the set point based on a temperature sensor wired to
the No. 1 eight-input module (see wiring diagrams or cer-
tified drawings). The temperature sensor must be wired to
terminal J1-19 and J1-20.
Configure Reset Type 2 on the CONFIG screen (Table 2,
Example 6). Enter the temperature of the remote sensor at
the point where no temperature reset will occur (REMOTE
TEMP [NO RESET]). Next, enter the temperature at which
the full amount of reset will occur (REMOTE TEMP [FULL
RESET]). Then, enter the maximum amount of reset re-
quired to operate the chiller (DEGREES RESET). Reset
Type 2 can now be activated.
RESET TYPE 3 — Reset Type 3 is an automatic chilled wa-
ter temperature reset based on cooler temperature differ-
ence. This reset adds ±30° F (±16° C) based on the tempera-
ture difference between entering and leaving chilled water.
Reset Type 3 is the only reset available without the need for
a No. 1 eight-input module. No wiring is required for Reset
Type 3, because it already uses the cooler water sensors.
Configure Reset Type 3 on the CONFIG screen (Table 2,
Example 6). Enter the chilled water temperature difference
(the difference between entering and leaving chilled water)
at which no temperature reset occurs (CHW DELTA T [NO
RESET]). This chilled water temperature difference is usu-
ally the full design load temperature difference. Enter the
difference in chilled water temperature at which the full
amount of reset occurs (CHW DELTA T [FULL RESET]).
Next, enter the amount of reset (DEGREES RESET). Reset
Type 3 can now be activated.
Demand
Limit
Control
Option
(Requires
Optional 8-Input Module) —
The demand limit may
be externally controlled with a 4 to 20 mA signal from an
Energy Management System (EMS). The option (20 mA DE-
MAND LIMIT OPTION) is enabled or disabled on the CON-
FIG screen (Table 2, Example 6). When enabled, the control
is set for 100% demand with 4 mA and an operator config-
ured minimum demand set point at 20 mA (DEMAND LIMIT
AT 20 mA) .
The EMS demand reset input is hardwired into the No. 1
8-input module. The signal may be internally powered by
the module or externally powered. If the signal is externally
powered, the signal is wired to terminals J1-1(+) and
J1-2(–). If the signal is internally powered, the signal is wired
to terminals J1-3(+) and J1-2(–). When enabled, the control
is set for 100% demand with 4 mA and an operator config-
ured minimum demand set point at 20 mA (DEMAND LIMIT
AT 20 mA).
Surge Prevention Algorithm —
Surge occurs when
lift conditions become so high that the gas flow across the
impeller reverses. This condition can eventually cause chiller
damage. Lift is defined as the difference between the pres-
sure at the impeller eye and the impeller discharge. The maxi-
mum lift that a particular impeller wheel can produce varies
with the gas flow across the impeller and the size of the
wheel.
The surge prevention algorithm is operator configurable
and can determine if lift conditions are too high for the com-
pressor. If they are, the PIC takes corrective action. The al-
gorithm also notifies the operator, via the LID, that chiller
operating conditions are marginal.
The surge prevention algorithm first determines if correc-
tive action is necessary. This is done by checking 2 sets of
operator configured data points: the minimum load points
(MIN. LOAD POINTS [T1/P1]) and the maximum load
points (FULL LOAD POINTS [T2/P2]). See the SERVICE1
screen or Table 2, Example 8. These points have default set-
tings. Information on how to modifiy the default minimum
and maximum load points can be found in the Input Service
Configurations section on page 54.
Figures 18 and 19 graphically display these settings and
the algorithm function. The 2 sets of load points (default set-
tings) describe a line that the algorithm uses to determine
the maximum lift of the compressor. Whenever the actual
differential pressure between the cooler and condenser and
the temperature difference between the entering and leaving
chilled water are above the line on the graph (as defined by
the minimum and maximum load points) the algorithm goes
into a corrective action mode. If the actual values are below
the line, the algorithm takes no action.
Corrective action can be taken by making one of 2 choices.
If the optional hot gas bypass line is present, and the op-
erator selects the hot gas bypass option on the SERVICE1
screen (selects 1 for the SURGE LIMIT/HGBP OPTION),
then the hot gas bypass valve can be energized. If the hot gas
bypass option is not present, then the SURGE LIMIT/HGBP
OPTION is on the default setting (0), and the guide vanes
are held. (Also see Table 4, Capacity Overrides.) Both cor-
rective actions reduce the lift experienced by the compressor
and help to prevent a surge condition.
37
