Specifications

43
are also stopped. If the configured lead chiller stops for an
alarm condition, the configured lag chiller takes the lead
chiller’s place as the lead chiller, and the standby chiller serves
as the lag chiller.
If the configured lead chiller does not complete the start-up
before the PRESTART FAULT TIMER (configured in LEAD-
LAG screen) elapses, then the lag chiller starts and the lead
chiller shuts down. The lead chiller then monitors the start
request from the acting lead chiller. The PRESTART FAULT
TIMER is initiated at the time of a start request. The
PRESTART FAULT TIMER provides a timeout if there is a
prestart alert condition that prevents the acting lead chiller from
starting in a timely manner. The PRESTART FAULT TIMER
parameter is on the LEAD-LAG screen, which is accessed
from the EQUIPMENT SERVICE table of the SERVICE
menu.
If the lag chiller does not achieve start-up before the
PRESTART FAULT TIMER elapses, the lag chiller stops, and
the standby chiller is requested to start, if configured and ready.
Standby Chiller Configuration and Operation
— A chiller is
designated as a standby chiller when its LEAD/LAG CONFIG-
URATION value on the LEADLAG screen is set to “3.” The
standby chiller can operate as a replacement for the lag chiller
only if one of the other two chillers is in an alarm (*) condition
(as shown on the ICVC panel). If both lead and lag chillers are
in an alarm (*) condition, the standby chiller defaults to operate
in CCN mode, based on its configured occupancy schedule and
remote contacts input.
Lag Chiller Start-Up Requirements
— Before the lag chiller
can be started, the following conditions must be met:
1. The lag chiller status indicates it is in CCN mode and
is not in an alarm condition. If the current lag chiller is
in an alarm condition, the standby chiller becomes the
active lag chiller, if it is configured and available.
2. Lead chiller ramp loading must be complete.
3. The configured LAG START TIMER entry has elapsed.
The LAG START TIMER starts when the lead chiller
ramp loading is completed or when a lag chiller stops.
The LAG START TIMER entry is on the LEADLAG
screen, which is accessed from the EQUIPMENT
SERVICE table of the SERVICE menu.
4. Lead chiller ACTIVE DEMAND LIMIT (see the
MAINSTAT screen) value must be greater than 95% of
full load amps.
5. Lead CHILLED LIQUID TEMP must be greater than
the CONTROL POINT temperature (see the MAIN-
STAT screen) plus
1
/
2
the CHILLED LIQUID DEAD-
BAND temperature (see the SETUP1 screen).
NOTE: The chilled liquid temperature sensor may be
the leaving chilled liquid sensor, the return liquid sen-
sor, the common supply liquid sensor, or the common
return liquid sensor, depending on which options are
configured and enabled.
6. Lead chiller temperature pulldown rate (TEMP PULL-
DOWN DEG/MIN on the TEMP_CTL screen) of the
chilled liquid temperature is less than 0.5º F (0.27º C)
per minute for a cumulative duration greater than the
PULLDOWN TIMER setting in the LEADLAG
screen.
When all the above requirements have been met, the lag
chiller is commanded to a STARTUP mode (SUPVSR flashing
next to the point value on the STATUS table). The PIC III
control then monitors the lag chiller for a successful start. If the
lag chiller fails to start, the standby chiller, if configured is
started.
Lead/Lag Pulldown Timer Operation
— Some lead/lag chill-
er applications with large chilled liquid loop volumes must
accommodate intermittent slugs of warm ENTERING
CHILLED LIQUID for short time periods. This type of tran-
sient condition can result when a control valve rapidly opens to
allow flow through a previously isolated branch or zone within
the chilled liquid system. A PULLDOWN TIMER can be con-
figured to delay starting the lag chiller so it does not excessive-
ly cycle on and off for short time periods when intermittent
slugs of warm ENTERING CHILLED LIQUID pass through
the chillers. A larger PULLDOWN TIMER entry gives the
warm slug of water more time to pass through the chillers
before the lag chiller will start.
The chiller CONTROL POINT can be configured to either
LEAVING CHILLED LIQUID or ENTERING CHILLED
LIQUID temperature. The PIC Controls monitor the tempera-
ture pulldown rate of the CHILLED LIQUID and display the
result as CHILL LIQ PULLDOWN/MIN in the HEAT_EX
screen. Samples of the CHILLED LIQUID temperature are
taken once every 10 seconds and compared against the previ-
ous CHILLED LIQUID sample. A positive value of CHILL
LIQ PULLDOWN/MIN indicates that the CHILLED LIQUID
temperature is decreasing between successive samples. If
CHILL LIQ PULLDOWN/MIN rate is a minimum of
0.5 degrees F per minute then the PULLDOWN: SATISFIED
parameter in the LL_MAINT screen displays “YES”, other-
wise, the PULLDOWN: SATISFIED parameter displays
“NO”.
If the lead chiller is unable to achieve the CONTROL
POINT, the lag chiller will not start unless the lead chiller is un-
able to maintain a CHILL LIQ PULLDOWN/MIN rate of
0.5 degrees F per minute for a time period equal to the number
of minutes entered in the PULLDOWN TIMER parameter.
PULLDOWN TIME in the LL_MAINT screen displays the
remaining delay left before the lag chiller is allowed to start
based on the pulldown timer. PULLDOWN TIME will count
down starting at the value entered in PULLDOWN TIMER
when Ramp Loading is complete and when PULLDOWN:
SATISIFIED = NO.
The lag chiller pulldown start condition is met when PULL-
DOWN TIME lapses to 0.0 min.
If PULLDOWN: SATISFIED changes to “YES” as the
PULLDOWN TIME is counting down to zero, the PULL-
DOWN TIME will start to count back up provided that the
CHILLED LIQUID temperature has not fallen to less than the
CONTROL POINT plus one half of the CHILLED LIQUID
DEADBAND. The PULLDOWN TIME will start to count back
down again should PULLDOWN: SATISFIED change back to
“NO”. The PULLDOWN TIME will be immediately reset to
the value entered in the PULLDOWN TIMER parameter if the
CHILLED LIQUID temperature decreases to less than the
CONTROL POINT plus one half of the CHILLED LIQUID
DEADBAND.
Lag Chiller Shutdown Requirements
— The following con-
ditions must be met in order for the lag chiller to be stopped.
1. Lead chiller AVERAGE LINE CURRENT or PERCENT
MOTOR KILOWATTS load value (on the MAINSTAT
screen) is less than the Lead Chiller Percent Capacity.
NOTE: Lead Chiller Percent Capacity = 105 – LAG %
CAPACITY. The LAG % CAPACITY parameter is on
the LEADLAG screen, which is accessed from the
EQUIPMENT SERVICE table on the SERVICE
menu.
2. The lead chiller CHILLED LIQUID TEMP is less than
the CONTROL POINT temperature (see the MAINSTAT
screen) plus on half of the CHILLED LIQUID DEAD-
BAND temperature (see the SETUP1 screen).
3. The configured LAG STOP TIMER entry has elapsed.
The LAG STOP TIMER starts when the lead chiller
CHILLED LIQUID TEMP is less than the chilled liquid
CONTROL POINT plus
1
/
2
of the CHILLED LIQUID
DEADBAND and the lead chiller compressor motor load