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Demand Limit Control, Option — (Requires Op-
tional 8-Input Module) —
externally controlled with a 4 to 20 mA signal from an en-
ergy management system (EMS). The option is set up on the
Config table. When enabled, the control is set for 100% de-
mand with 4 mA and an operator configured minimum de-
mand set point at 20 mA.
The Demand Reset input from an energy management sys-
tem is hardwired into the number one, 8-input module. The
signal may be internally powered by the module or exter-
nally powered. If the signal is externally powered, the signal
is wired to terminals J1-1(+) and J1-2(–). If the signal is in-
ternally 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 configured minimum demand set
point at 20 mA.
Surge Prevention Algorithm —
configurable feature which can determine if lift conditions
are too high for the compressor and then take corrective ac-
tion. Lift is defined as the difference between the pressure at
the impeller eye and the impeller discharge. The maximum
lift that a particular impeller wheel can perform varies with
the gas flow across the impeller, and the size of the wheel.
The algorithm first determines if corrective action is nec-
essary. This is done by checking 2 sets of operator config-
ured data points, which are the MINIMUM and the MAXI-
MUM Load Points, (T1/P1;T2/P2). These points have default
settings as defined on the Service1 table, or on Table 4. These
settings and the algorithm function are graphically dis-
played in Fig. 18 and 19. The two sets of load points on this
graph (default settings are shown) describe a line which the
algorithm uses to determine the maximum lift of the com-
pressor. Whenever the actual differential pressure between
the cooler and condenser, and the temperature difference be-
tween the entering and leaving chilled water are above the
line on the graph (as defined by the MINIMUM and MAXI-
MUM Load Points) the algorithm will go into a corrective
action mode. If the actual values are below the line, the al-
gorithm takes no action. Modification of the default set points
of the MINIMUM and MAXIMUM load points is described
in Before Initial Start-Up, Input Service Configurations sec-
tion, page 42.
Corrective action can be taken by making one of two choices.
If a hot gas bypass line is present, and the hot gas is con-
figured on the Service1 table, then the hot gas bypass valve
can be energized. If a hot gas bypass if not present, then the
action taken is to hold the guide vanes. See Table 4, Capac-
ity Overrides. Both of these corrective actions will reduce
the lift experienced by the compressor and help to prevent a
surge condition. Surge is a condition when the lift becomes
so high that the gas flow across the impeller reverses. This
condition can eventually cause machine damage. The surge
prevention algorithm is intended to notify the operator that
machine operating conditions are marginal, and to take ac-
tion, such as lowering entering condenser water tempera-
ture, to help prevent machine damage.
Surge Protection —
Surging of the compressor can be
determined by the PIC through operator configured settings.
Surge will cause amperage fluctuations of the compressor
motor. The PIC monitors these amperage swings, and if the
swing is greater than the configurable setting in one second,
then one surge count has occurred. The SURGE DELTA PER-
CENT AMPS setting is displayed and configured on the Serv-
ice1 table. It has a default setting of 25% amps, SURGE
PROTECTION COUNTS can be monitored on the Maint03
table.
The demand limit may be
This is an operator
A surge protection shutdown of the machine will occur
whenever the surge protection counter reaches 12 counts within
an operator specified time, known as the SURGE TIME PE-
RIOD. The SURGE TIME PERIOD is displayed and con-
figured on the Service1 table. It has a default of 2 minutes.
LEGEND
P = (Condenser psi) − (Cooler psi)
T = (ECW) − (LCW)
ECW
— Entering Chilled Water
HGBP — Hot Gas Bypass
LCW
— Leaving Chilled Water
Fig. 18 — 19EF Hot Gas Bypass/Surge Prevention
LEGEND
P = (Condenser kPa) − (Cooler kPa)
T = (ECW) − (LCW)
ECW
— Entering Chilled Water
HGBP — Hot Gas Bypass
LCW
— Leaving Chilled Water
Fig. 19 — 19EF With Default Metric Settings
29
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