Sequence of Operation
30MP Multi-chiller CARRIER CORPORATION ©2018
Installation and Start-up Guide All rights reserved
22
The 30MP Multi-chiller sequences chillers in order to maintain the desired water temperature setpoint. The control
can be configured to maintain supply or return water temperature.
Whenever the controller is in an occupied mode, the water temperature sensor is compared to the desired
setpoint. If the water temperature sensor value is above the setpoint, then a PID will calculate how much
additional capacity is required. The minimum amount of chillers is defined by the parameter
Minimum # of
Chillers to Run.
If the current number of chillers is less than the minimum number, then the chillers are started
every 30 seconds until the minimum number is reached. Thereafter, additional stages are added subject to the
configured
delay time.
As each chiller is started, it is limited to its first stage of capacity until all available chillers are operating. Once all
machines are operating at minimum capacity, if additional capacity is required, any 3-stage machine is loaded to
2 stages next, until all 3-stage machines are loaded to that capacity. If that level is reached and still further
capacity is needed, then all machines are sequentially loaded until every machine is operating at 100% capacity.
This sequence provides the most efficient operation of multiple chillers.
As the load decreases and the required capacity is reduced, the reverse cycle occurs. All machines are reduced by
1 stage to either 50% or 67%, depending on the number of machine stages. Next, as a further capacity reduction
is needed, the second stage of any 3-stage machine is dropped until only 1 stage is operating per machine. As
further reduction is needed, the first stage is dropped on each machine in the opposite order of the chiller
sequence. Dropping any stage is subject to the
delay time between stages. Additionally, if only 1 stage
is operating per machine and any machines are equipped with hot gas bypass, the hot gas is used to maintain the
desired water temperature, while waiting for the RCR time delay to expire, in order to further reduce capacity. Hot
gas bypass provides improved water temperature control under light load conditions and prevents excessive
chiller start/stop cycles.
The controlling setpoint for chilled water is calculated based on the following settings:
• Configure
for the design water temperature that you want.
• Use
Cooling Setpoint OA Reset
to increase the water temperature setpoint as a function of the outdoor air
temperature.
You must have an OA sensor to use this feature.
• Use
RH Cooling Setpoint Reset
to reduce any calculated OA reset if a high humidity condition occurs.This
usually happens under light load at lower OA temperatures.
You must have an RH sensor to use this feature.
To use an occupancy schedule, you must configure time periods to schedule the transitions from occupied to
unoccupied operation. The control maintains the desired chilled water setpoint when occupied and disables the
chiller when unoccupied. The controller defaults to using the
point, which is factory set to
by default. The unit will stay unoccupied until you either configure a Time Schedule or a third party control system
the
point. You must set the local time and date for these functions to operate
properly.
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