Runnable simulator field guide
Chiller sequencing PLC scenario: test checklist
Direct answer
The learner can state every transition in a two-chiller sequence and prove demand rise, demand fall, unavailable equipment, failed proof and restart cases without short cycling.
Written for building-automation and controls learners implementing plant enable, chilled-water demand, equipment availability, lead rotation, staging, proof, alarms and orderly shutdown.
Scenario setup
Load metric, enable conditions, chilled-water setpoint, deadband, minimum on and off times, equipment availability, lead rotation, pump and flow proof, staging delay and alarm policy.
I/O and state map
Temperature and demand through plant state, pump request, flow proof, selected chiller command, run proof, delivered capacity, staging decision and operator indication.
Normal run
Lead pump and chiller start in order, obtain proof, control load, stage lag only when required and stop in the declared reverse sequence.
Boundary run
Low load, rapid demand change, failed flow proof, unavailable lead, lag failure, sensor bias, manual mode, alarm reset and power return.
Injected fault
A demand, availability, selection, command, timing, proof, capacity, sensor, alarm or restart mismatch.
Target check
The sequence reviewed against equipment limits, building controls requirements, target code, trend data and witnessed functional-performance tests.
How should two chillers be sequenced?
Define plant enable, pump and flow proof, lead selection, chiller start, capacity confirmation, lag staging, unloading, minimum timing and failed-equipment response.
What prevents chiller short cycling?
Use manufacturer-approved minimum on and off times, stable demand logic, appropriate deadbands and proof conditions, then test changing-load boundaries.