Chiller Staging and Sequencing Using Measured Load, Flow and Lift
Two chillers running at half load are not automatically more efficient than one carrying the duty. The answer changes with compressor design, operating lift and the pumps and towers brought online. Good staging selects a workable equipment combination, then checks its whole-plant performance.
Measure the cooling duty
Place flow and supply/return temperature measurements around the same hydraulic boundary. In a primary-secondary plant, bypass mixing can make plant flow different from building flow. Check sensors, signal timing and units before using calculated load to make staging decisions.
For water near normal chilled-water conditions, with density ≈ 1,000 kg/m³ and specific heat ≈ 4.18 kJ/(kg·K), ΔT is return minus supply temperature:
Q (kW) ≈ 4.18 × V̇ (L/s) × ΔT (K)
A measured 25 L/s and 4.8 K difference gives 501.6 kW. Against a 600 kW nominal rating, one machine would carry 83.6% of nominal capacity; two equal machines sharing evenly would each carry 41.8%. These percentages describe an example, not preferred staging points.
Use available capacity and lift
Part-load ratio is load divided by the applicable capacity. The nameplate example above must be replaced by capacity available at actual water temperatures and operating limits. Condenser conditions and chilled-water setpoint affect both capacity and power.
Compressor lift relates to the refrigerant pressure rise between evaporator and condenser. Saturation-temperature difference is a useful corresponding measure; water-temperature differences are only proxies. Use manufacturer performance maps or validated plant measurements rather than a universal “best” part-load percentage.
Reject combinations that cannot carry the flow
Check minimum and maximum evaporator flow for every active chiller, plus the permitted rate of flow change. Opening a second parallel path can reduce flow through the running machine before the new one is ready. Coordinate pumps, isolation valves, bypass control and flow proof.
At low load, two chillers may require more minimum flow than the distribution system needs. In primary-secondary layouts, monitor bypass direction and supply-temperature mixing. Trane’s system-design guidance explains why hydraulic configuration changes the staging problem.
Illustrative nominal loading only. Select staging from actual capacity, flow limits and whole-plant performance.
Build in time and hysteresis
Use separate stage-up and stage-down criteria, persistence delays, minimum on/off times and a stabilization period after transitions. Set these from the equipment requirements and system response, never from an arbitrary universal timer. Safety interlocks retain priority.
Stage up when the approved capacity, temperature or efficiency criteria persist; stage down only when the remaining combination can meet duty and flow limits. Prove pumps and valves before enabling a chiller, retain required pump overrun, and provide an alarmed fallback for unavailable equipment. Johnson Controls’ chiller-timer guidance illustrates these separate timing functions.
Test the whole sequence
Compare total chiller, chilled-water pump, condenser-water pump and tower electrical power at similar duty and conditions. Preserve required redundancy and service limits. Trend starts, load, flow, temperatures and alarms through increasing and decreasing load, equipment failure and lead-lag rotation. Balance operating hours without forcing unnecessary changeovers.
Develop a coordinated plant design
Nexora’s Chilled Water System Design Handbook connects plant staging, hydraulic design and BMS sequences. Explore the handbook when documenting how the equipment and controls should work together.
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