Carrier 19XRV Start-Up, Operation And Maintenance Instructions Manual page 48

Lead/Lag Chiller Configuration and Operation
• A chiller is designated the lead chiller when its
LEADLAG: CONFIGURATION value on the LEAD-
LAG screen is set to "1."
• A chiller is designated the lag chiller when its
LEADLAG: CONFIGURATION value is set to "2."
• A chiller is designated as a standby chiller when its
LEADLAG: CONFIGURATION value is set to "3."
• A value of "0" disables the lead/lag designation of a
chiller. This setting should also be used when "normal"
operation without regard to lead/lag rules is desired (in
LOCAL or CCN mode).
When configuring the LAG ADDRESS value on the
LEADLAG screen of chiller "A" enter the address of the
chiller on the system which will serve as lag when/if chiller
"A" is configured as lead. For example, if you are configuring
chiller A, enter the address for chiller B as the lag address. If
you are configuring chiller B, enter the address for chiller A as
the lag address. This makes it easier to rotate the lead and lag
chillers. Note that only the lag and standby chiller addresses
specified in the configured lead chiller's table are relevant at a
given time.
If the address assignments in the LAG ADDRESS
and STANDBY ADDRESS parameters conflict, the lead/lag
function is disabled and an alert (!) message displays. For
example, if the LAG ADDRESS matches the lead chiller's
address, the lead/lag will be disabled and an alert (!) message
displayed. The lead/lag maintenance screen (LL_MAINT) dis-
plays the message 'INVALID CONFIG' in the LEADLAG:
CONFIGURATION and CURRENT MODE fields.
The lead chiller responds to normal start/stop controls such
as the occupancy schedule, a forced start or stop, and remote
start contact inputs. After completing start-up and ramp
loading, the PIC III evaluates the need for additional capacity.
If additional capacity is needed, the PIC III initiates the start-up
of the chiller configured at the LAG ADDRESS. If the lag
chiller is faulted (in alarm) or is in the OFF or LOCAL modes,
the chiller at the STANDBY ADDRESS (if configured) is
requested to start. After the second chiller is started and is
running, the lead chiller monitors conditions and evaluates
whether the capacity has been reduced enough for the lead
chiller to sustain the system alone. If the capacity is reduced
enough for the lead chiller to sustain the CONTROL POINT
temperatures alone, then the operating lag chiller is stopped.
If the lead chiller is stopped in CCN mode for any reason
other than an alarm (*) condition, the lag and standby chillers
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.
The PRESTART FAULT TIMER provides a timeout if
there is a prestart alert condition that prevents a chiller from
starting in a timely manner. If the configured lead chiller does
not complete its start-up before the PRESTART FAULT
TIMER (a user-configured value) elapses, then the lag chiller
starts, and the lead chiller shuts down. The lead chiller then
monitors the lag, acting as the lead, for a start request. The
PRESTART FAULT TIMER parameter is on the LEADLAG
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 LEADLAG: 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 and will operate based on its configured
CCN 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. Lead chiller ramp loading must be complete.
2. Lead chilled water temperature must be greater than the
CONTROL POINT temperature (see the MAINSTAT
screen) plus 1/2 the CHILLED WATER DEADBAND
temperature (see the SETUP1 screen).
NOTE: The chilled water temperature sensor may be the
leaving chilled water sensor, the return water sensor, the
common supply water sensor, or the common return
water sensor, depending on which options are configured
and enabled.
3. Lead chiller ACTIVE DEMAND LIMIT (see the MAIN-
STAT screen) value must be greater than 95% of full load
amps.
4. Lead chiller temperature pulldown rate (TEMP PULL-
DOWN DEG/MIN on the TEMP_CTL screen) of the
chilled water temperature is less than 0.5° F (0.27° C) per
minute for a sustained period of 100 seconds.
5. 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.
6. The configured LAG START TIMER entry has elapsed.
The LAG START TIMER starts when the lead chiller ramp
loading is completed. The LAG START TIMER entry is
on the LEADLAG screen, which is accessed from the
EQUIPMENT SERVICE table of the SERVICE menu.
When all the above requirements have been met, the lag
chiller is commanded to a STARTUP mode (indicated by
"CONTRL" flashing next to the CHILLER START/STOP
parameter in the MAINSTAT screen). 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.
Lag Chiller Shutdown Requirements — The following condi-
tions must be met in order for the lag chiller to be stopped.
1. Lead chiller AVERAGE LINE CURRENT or MOTOR
PERCENT KILOWATTS (on the MAINSTAT screen) is
less than the lead chiller percent capacity.
NOTE: Lead Chiller Percent Capacity = 115 – LAG %
CAPACITY. The LAG % CAPACITY parameter is on the
LEADLAG screen, which is accessed from the EQUIP-
MENT SERVICE table on the SERVICE menu.
2. The lead chiller chilled water temperature is less than the
CONTROL POINT temperature (see the MAINSTAT
screen) plus /
1
2
temperature (see the SETUP1 screen).
3. The configured LAG STOP TIMER entry has elapsed.
The LAG STOP TIMER starts when the lead chiller
chilled water temperature is less than the chilled water
CONTROL POINT plus 1/2 of the CHILLED WATER
DEADBAND and the lead chiller compressor motor load
(MOTOR PERCENT KILOWATT or AVERAGE LINE
CURRENT on the MAINSTAT screen) is less than the
Lead Chiller Percent Capacity.
NOTE: Lead Chiller Percent Capacity = 115 – LAG %
CAPACITY. The LAG % CAPACITY parameter is on the
LEADLAG screen, which is accessed from the EQUIPMENT
SERVICE table on the SERVICE menu.
48
the CHILLED WATER DEADBAND