Unit installation

52
TEMPERATURE CONTROL
Initially, the Control on Leaving or Return Water temperature is being decided and the required water
temperature set point (SP) for the plant has to be decided based on the requirement and entered in the
corresponding parameters. This set point becomes the reference point for the controller for the staging logic.
Now, when the system is started up, the software starts to check al the necessary inputs (Analog) to start the
unit. If the actual water temperature reading is more than the set point, the software generates an error signal.
Based on this error signal, the Algorithm generates the required chillers power to the plant. Based on this the
staging table, the actuator energizes the output relays to meet the load requirement.
During this process the software – also scan the Digital inputs. If any unwarranted interruption has occurred the
Alarm Signal is generated and the unit is shut-off for safety purposes.
The controller also takes care of the running hours of each compressor and the status of the compressor, fan,
solenoid, etc.
CONTROLLER STRUCTURE
Our method divides the problem in two tasks: the controller and the actuator.
The controller decides, given the error value and using a PID algorithm, the percentage value of the total chiller
power to be given to the plant. The actuator must set the output relays status (compressor, hot gas bypass and
head unloader) to best fit the amount of power required by the controller. This job is easier with a
multicompressor chiller.
Parameters that must be set are Proportional Band (error value in °F to have 100% of the chiller power) and
Integral Time (time required for the integral control to be equal to the proportional). Integral time, that takes
steady state error to zero, will be set on special application requirements.
STAGING LOGIC
The following assumptions are made:
1. The Hot Gas Bypass Solenoid is only on circuit 1, and it takes out 50% of the circuit power.
2. The Head Unloader is used on 1 circuit only.
3. If the leader is fixed then it is necessarily circuit #1.
4. The Head Unloader takes out or 50% of the power or 33% of the power, according to the set up parameter
M58.
5. When Hot Gas Bypass Solenoid is enabled, the leader circuit becomes always circuit 1 (CP1).
6. Hot Gas Bypass Solenoid is only used in the lower control step.
7. The control assumes the bigger stage configuration when the required power is equal to the bigger stage
power and the smaller stage configuration when the required power is equal to the smaller stage power.
Following an example for a 4 step staging.
-
CONTROLLER
PID
Control algorithm
ACTUATOR
Staging table
Hardware configuration
from
setup
Relay
configuration
Set point
given by
user
Actual temperature
read by controller
Error
Required chiller
power (0÷100%)