System Management (Field Installed)
Chiller Plant Control
Chillervisor System Manager / Sequence of Operations (Controls)
Note: The following is a sample of a typical sequence of operation that may be adapted for each
particular project. This is provided as a suggestion on how to write a sequence of operation. This
sample sequence assumes a chilled water plant with centrifugal chillers and one smaller
reciprocating chiller. The sequence of the chillers is to change based on calendar date. The ACR
sensor is located in the common chilled water supply line. Differential pressure control is also
required.
4.1 Chiller System Manager (CSM)
A. Description:
1. General:
The Chiller System Manager, CSM, shall issue enable/disable commands to each chiller PIC. A
running status from the PIC to the CSM shall only be validated when the PIC has confirmed that
the chiller auxiliary equipment (chilled water pump, cooling tower, oil pump, etc.) is and has
operated correctly.
Each time a chiller is commanded on the CSM shall fully open the differential pressure valve(s).
Whenever a chiller is running, the CSM shall modulate the differential pressure valve to maintain
its adjustable differential pressure setpoint. The valve(s) opening rate shall be unrestricted to
open the valve(s) quickly when a chiller starts. The closing rate shall be restricted to help prevent
unstable operation.
2. Occupancy:
The CSM shall through its internal time schedule, an operator initiated timed override or a network
command enable the chilled water plant control unless the operator has disabled the program
through the toggle enable/disable point.
3. Summer/Winter Selection Mode:
The CSM shall automatically select one of two different chiller sequences based on operator
adjustable calendar dates of Oct. 1st (Winter mode) and April 1st (Summer mode). This sequence
assumes that no chillers are running when the season changeover occurs and does not use the
soft start demand limiting program. If the operator requests, the sequence shall be expanded to
include both.
4. Summer Mode (with feathering chiller):
After the CSM has determined that it is in occupied mode, it shall start the feathering chiller and
allow its local PIC to control its capacity by sending the PIC a local Chilled Water setpoint to
maintain.
Each PIC shall maintain its own ramping and loading rates.
5. Additional Cooling Required:
The first lead chiller shall be enabled to start if the inhibit timers are below the unoccupied time,
the pulldown rate is less than .5°F/ minute, the running capacity is above a calculated value, and
the supply water temperature is above the current supply water temperature setpoint plus an
adjustable delta temperature value. After the CSM has proven that the first lead chiller was started
successfully, it shall disable the feathering chiller. If the start inhibit timers are clear and the ACR
routine determines that another stage of cooling is required, the feathering chiller shall once again
be enabled. This sequence will continue to assume that load continues to increase until all chillers
are enabled to run ending with the feathering chiller.
6. Reduced Cooling Required:
The feathering chiller shall be disabled when the CSM determines through a load target program
that the average %kW of all running chillers is at or below the corrected RCR kW setpoint, the
control sensor is below its setpoint (plus the delta temperature entered), and the stop requirements
for the delay timers have cleared. If the load continues to decrease and all of the requirements
detailed above are true, the last lag chiller started shall be stopped and the feathering chiller shall
be enabled. If the load continues to drop and all the RCR load conditions detailed above are still
true, the feathering chiller shall be stopped. If the load continues to drop and all RCR requirements
are true, the CSM shall start the feathering chiller and after proving its run status the CSM shall
stop the next lag chiller.
This sequence will continue to assume that load conditions decrease until all chillers except for the
feathering chiller are off.
7. Summer Mode — Faulting:
If the feathering chiller is faulted by the CSM, it shall automatically send a stop command and
start the next chiller in the sequence until the feathering chiller is available again. If the CSM faults
any of the other chillers in the sequence, it shall automatically send a stop command to the faulted
chiller, and shall start the next available chiller(s) until the on-line capacity is greater than or equal
to the current load capacity (tons) prior to the fault. If the total on-line capacity is greater than or
equal to the current load capacity prior to the chiller fault an additional chiller shall not be started.
8. Winter Mode (starting lead chiller):
After the CSM has determined that it is in occupied mode it shall start the reciprocating chiller and
allow its local PIC to control its capacity by sending the PIC a local chilled water setpoint to
maintain. Each PIC shall maintain its own ramping and loading rates.
9. Additional Cooling Required:
The next chiller in the sequence shall be enabled to start if the inhibit timers are below the
unoccupied time, the pulldown rate is less than .5°F/minute, the running capacity is above an
operator entered value, and the supply water temperature is above the current supply water
temperature setpoint plus an adjustable delta temperature value. After the CSM has proven that
the first lead chiller was started successfully, it shall disable the reciprocating chiller and it shall be
moved to the end of the start sequence. If the start inhibit timers are clear and the ACR routine
determines that another stage of cooling is required, the second lead chiller shall be enabled. This
sequence will continue to assume that load continues to increase until all chillers are enabled to
run ending with the reciprocating chiller.
10. Reduced Cooling Required:
The last lag chiller started shall be disabled when the CSM determines through a load target
program that the average %kW of all running chillers is at or below the corrected RCR kW
setpoint, the control sensor is below its setpoint (plus the delta temperature entered), and the stop
requirements for the delay timers have cleared. If the load continues to decrease and all of the
requirements detailed above are true, the next lag chiller started shall be stopped. When the load
has reached a point that the CSM is ready to disable the last lead chiller, the CSM shall start the
reciprocating chiller and after it has proven its running status, it shall disable the last centrifugal
chiller.
11. Winter Mode — Faulting:
If the reciprocating chiller is faulted by the CSM it shall automatically send a stop command and
start the next lead chiller in the sequence. The reciprocating chiller shall be removed from the
sequence until the chiller becomes available again.
If the CSM faults any of the other chillers in the sequence, it shall automatically send a stop
command to the faulted chiller, and shall start the next available chiller if the on-line capacity is
greater than or equal to the current load capacity (tons) prior to the fault. If the total on-line
capacity is greater than or equal to the current load capacity prior to the chiller fault, an additional
chiller shall not be started.
12. Unoccupied:
The CSM shall automatically index the chilled water plant to an occupied mode based on its
internal time schedules. When the chilled water plant becomes unoccupied, all chillers shall be
stopped at one minute intervals.