Chiller plant sequencing and optimization

A central chiller plant is a set of interacting machines: chillers, chilled water pumps, condenser water pumps and cooling towers. Each can be operated efficiently on its own and still produce an inefficient plant if they are not coordinated. Sequencing is the logic that decides which equipment runs, when to add or drop units, and what setpoints each should hold.

Good sequencing often reduces plant energy significantly without any new equipment. Poor sequencing, such as starting a second chiller too early or running towers at fixed temperatures, is one of the most common findings when existing plants are reviewed.

The goal: total plant efficiency

The useful measure is the energy of all plant equipment per unit of cooling delivered, often expressed as plant kW/ton or plant COP. Optimizing one component at the expense of another can make the total worse:

• Colder condenser water helps chillers but costs tower fan energy.
• Warmer chilled water helps chillers but can increase pump flow and air handler fan energy.
• Running more chillers at lower load can help or hurt depending on their part-load curves.

Optimization means finding combinations that minimize the total, at each load and weather condition.

Chiller staging

Staging up

The traditional approach adds a chiller when supply temperature rises above setpoint for some period. That is too slow and too blunt. Better staging uses one or more of:

• Chiller load or current: stage up when running chillers exceed a percentage of capacity (for example 85 to 95 percent) for a defined time.
• Calculated plant load: flow multiplied by delta-T compared to the capacity of running chillers.
• Chiller flow limits: stage up if evaporator flow approaches the maximum for running chillers, a common symptom of low delta-T syndrome.
• Supply temperature: as a backup when the chillers clearly cannot hold setpoint.

Staging down

Drop a chiller when the remaining chillers can carry the load comfortably, with enough margin to avoid immediate restaging. A deadband and minimum run and off times prevent short cycling.

Running chillers at efficient loads

Many modern variable speed chillers are most efficient well below full load, often in the range of roughly 40 to 70 percent, especially at low condenser water temperatures. For these plants, running two chillers at moderate load can use less energy than one chiller near full load, provided pump and tower energy is accounted for. Constant speed chillers generally peak nearer full load. The manufacturer's part-load data at varying condenser temperatures should drive the staging thresholds.

Unequal chiller sizes

Plants with a small "pony" chiller and larger units need staging logic that selects the best combination for the load, not just a fixed lead and lag order.

Pump sequencing

Pump arrangements depend on whether the plant is primary-secondary or variable primary. See primary-secondary vs variable primary flow.

• In primary-secondary plants, constant speed primary pumps are usually dedicated to chillers. Secondary variable speed pumps should be staged by speed or flow and run at equal speeds.
• In variable primary plants, pumps vary flow through chillers within the chiller's minimum and maximum flow limits. Changes in flow must be slow enough for chiller controls to follow. A bypass maintains minimum flow at low load.

Condenser pumps are typically staged one per chiller. Variable condenser flow can save energy but must respect chiller minimum flows and tower distribution limits.

Cooling tower sequencing

• Run as many tower cells as possible at low fan speed, within each cell's minimum water flow, since fan power drops steeply with speed.
• Isolate cells with valves when flow falls below the minimum distribution flow.
• Control fan speed to a condenser water setpoint that is reset rather than fixed.

Reset strategies

Chilled water supply temperature reset

Raising chilled water temperature reduces chiller lift. It is limited by air handler valve positions and space humidity. Common approaches:

• Reset based on the most-open cooling valve, using trim and respond logic.
• Reset based on outdoor air temperature or dew point, with humidity limits.

In variable flow systems, warmer chilled water increases pump flow, so pump energy should be weighed against chiller savings.

Condenser water temperature reset

Instead of a fixed 29.4 C (85 F) setpoint, set the tower leaving water temperature based on outdoor wet-bulb plus an approach, limited by the chiller's minimum entering condenser temperature. More advanced approaches calculate the setpoint that minimizes combined chiller and tower energy at the current load. These are covered in detail in chilled water and condenser water reset strategies.

Total plant optimization

Some plants use optimization software that models each component and continuously selects the combination of chillers, pumps, towers and setpoints that minimizes total power. Approaches include:

• Equipment-model based: uses manufacturer curves and measured data.
• Relational control: adjusts equipment speeds together based on load, using relationships derived from equipment efficiency curves.
• Rule-based with resets: well-tuned sequences following ASHRAE Guideline 36 style logic and good staging thresholds.

All of them need accurate instrumentation: flow meters, temperature sensors and power meters. An optimization program built on a drifting flow meter will make poor choices.

A practical optimization process

1. Instrument and trend. Confirm chilled water flow, supply and return temperatures, condenser temperatures, wet-bulb and power for each major component.
2. Calculate baseline plant efficiency at various loads and weather conditions.
3. Review staging thresholds against equipment part-load data.
4. Implement resets for chilled water, condenser water and pump differential pressure.
5. Fix distribution problems that limit the plant, especially low delta-T and overpumping.
6. Retest and trend after changes, comparing plant kW/ton at similar loads and weather.
7. Document the final sequence and setpoints for operators.

Good data is the foundation; see trend logging and using BAS data.

Common sequencing problems

• Chillers staged on temperature only, adding units late or early.
• Lead chiller never rotated, causing uneven wear.
• Towers fixed at 29.4 C (85 F) year round.
• Chilled water fixed at a low setpoint regardless of load.
• Pumps running at mismatched speeds.
• Minimum run times missing, causing short cycling.
• Operators overriding automatic control because sequences were never fully commissioned.

Frequently asked questions

How much can sequencing improvements save?

It varies widely with how the plant currently operates. Plants running fixed setpoints and simple temperature-based staging usually have the largest opportunity. Measurement before and after changes is the only reliable way to know.

Do I need optimization software?

Not always. Many plants achieve most of the available savings with well-written sequences, resets and good instrumentation. Software can add more, especially in large multi-chiller plants.

Key takeaways

• Optimize total plant energy, not individual components.
• Stage chillers on load and flow, using part-load data, not supply temperature alone.
• Run more tower cells at lower fan speed and reset condenser water temperature.
• Reset chilled water temperature while watching humidity and pump energy.
• Accurate instrumentation and commissioned sequences underpin everything, which is why plant reviews start with control system verification.