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The algorithm first determines if corrective action is nec-
essary. This is done by checking 2 sets of operator con-
figured data points, which are the MINIMUM and the
MAXIMUM Load Points, (T1/P1;T2/P2). These points have
default settings for each type of refrigerant, HCFC-22 or
HFC-134a, as defined on the Service1 table, or on Table 4.
These settings and the algorithm function are graphically
displayed in Fig. 20 and 21. 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
between 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
algorithm takes no action. Modification ofthe defaultset points
of the MINIMUM and MAXIMUM load points is described
in the Input Service Configuration section on page 50.
Corrective action can be taken by making one of 2 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 —
Capacity 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 chiller dam-
age. The surge prevention algorithm is intended to notify the
operator that chiller operating conditions are marginal, and
to take action to help prevent chiller damage such as low-
ering entering condenser water temperature.
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 sec-
ond, then one surge count has occurred. The SURGE DELTA
PERCENTAMPS setting is displayed and configured on the
Service1 screen. It has a default setting of 25% amps, SURGE
PROTECTION COUNTS can be monitored on the Maint03
table.
Asurge protection shutdown of the chiller will occur when-
ever the surge protection counter reaches 12 counts with-
in an operator specified time, known as the SURGE TIME
PERIOD. The SURGE TIME PERIOD is displayed and
configured on the Service1 screen. It has a default of
2 minutes.
Lead/Lag Control
NOTE: Lead/lag control is only available on chillers with
PSIO Software Version 09 or higher.
Lead/lag is a control system process that automatically starts
and stops a lag or second chiller in a 2-chiller water system.
Refer to Fig. 16 and 17 for menu, table, and screen selection
information. On chillers that have PSIO software with Lead/
Lag capability, it is possible to utilize the PIC controls to
perform the lead/lag function on 2 chillers. A third chiller
can be added to the lead/lag system as a standby chiller to
start up in case the lead or lag chiller in the system has shut
down during an alarm condition and additional cooling is
required.
NOTE: Lead/lag configuration is viewed and edited under
Lead/Lag in the Equipment Configuration table (located in
the Service menu). Lead/lag status during chiller operation
is viewed in the MAINT04 table in the Control Algorithm
Status table. See Table 2.
Lead/Lag System Requirements:
• all chillers must have PSIO software capable of perform-
ing the lead/lag function
• water pumps MUST be energized from the PIC controls
• water flows should be constant
• CCN Time Schedules for all chillers must be identical
LEGEND
ECW — Entering Chilled Water
HGBP — Hot Gas Bypass
LCW — Leaving Chilled Water
⌬P = (Condenser Psi) — (Cooler Psi)
⌬T = (ECW) − (LCW)
Fig. 20 — 19XL Hot Gas Bypass/Surge
Prevention
LEGEND
ECW — Entering Chilled Water
HGBP — Hot Gas Bypass
LCW — Leaving Chilled Water
⌬P = (Condenser kPa) — (Cooler kPa)
⌬T = (ECW) — (LCW)
Fig. 21 — 19XL with Default Metric Settings
34