When to use: Simulate a commercial chiller plant with two 300-ton chillers, a cooling tower, and primary/secondary pumping. Vary building load from 0–100% and watch the sequencing logic stage chillers on/off to optimize efficiency. Track kW/ton plant efficiency in real time.
This simulator models a two-chiller central plant with cooling tower, primary/secondary pumping, and automated staging logic — tracking plant efficiency in kW/ton as building load varies from 0 to 100%. Engineers use it to optimize chiller staging sequences, evaluate part-load performance, and demonstrate integrated plant design per ASHRAE 90.1.
A chiller plant sequences equipment to maintain the lowest achievable kW/ton at any given load. At low loads, operating a single chiller at moderate part-load ratio (PLR) is more efficient than splitting load between two chillers at very low PLR, because chillers are typically most efficient between 60–80% PLR. The plant stages the second chiller on when the first approaches 85% of capacity.
Primary-secondary pumping decouples chiller flow (constant primary) from distribution flow (variable secondary), allowing secondary pumps to modulate with VFDs while chillers maintain design flow. Variable primary pumping eliminates the bypass pipe but requires careful minimum-flow controls on chillers. Condenser water temperature reset — lowering the setpoint on mild days — can improve chiller COP by 1–2% per degree of reduction.
ARI Standard 550/590 defines chiller efficiency metrics. IPLV (Integrated Part-Load Value) represents weighted average efficiency at 100%, 75%, 50%, and 25% load. NPLV (Non-Standard Part-Load Value) allows adjustment for site conditions. ASHRAE 90.1 Table 6.8.1 specifies minimum IPLV/NPLV for chillers by type and capacity.
ASHRAE Standard 90.1 establishes minimum chiller efficiency in kW/ton and IPLV. Water-cooled centrifugal chillers must meet approximately 0.55 kW/ton full load and 0.45 kW/ton IPLV for large units. Air-cooled chillers have higher thresholds (0.9–1.0 kW/ton) due to less efficient heat rejection. ASHRAE 90.1 also requires waterside economizers when feasible.
ASHRAE Guideline 22 provides measurement and verification protocols for chiller plant performance. AHRI 550/590 governs chiller performance certification. Local jurisdictions may require Title 24 (California) or other state energy codes with additional requirements for chiller plant controls and fault detection.
Chiller plant design must consider both full-load and part-load efficiency, since buildings spend most operating hours at partial load. A plant with two equal-sized chillers is flexible but may be less efficient at very low loads than a plant with one large and one small chiller (lead-lag configuration). Free cooling economizers using a plate heat exchanger can provide chilled water when outdoor wet-bulb temperature is sufficiently low, eliminating compressor energy entirely.
Pumping system design significantly affects plant efficiency. The cooling tower fan and condenser water pumps consume 5–15% of total plant kW. Variable speed cooling tower fans and variable condenser water flow (on compatible chillers) can reduce this substantially. Total plant kW/ton including all auxiliaries is the critical metric for energy code compliance.
Drag the building cooling load slider from 0–100% and observe which chillers stage on. Plant kW/ton updates in real time, showing how efficiency changes with load. Click Run Simulation to record kW/ton history on the chart — this reveals the efficiency curve over a simulated operating period.
Note the staging threshold at 85% of Chiller #1 capacity, where Chiller #2 starts. Observe that kW/ton improves as load increases on a single chiller (better PLR) but then worsens slightly when the second chiller starts at low load. This illustrates why optimal staging logic is important to minimize energy waste.
Full-load plant efficiency (including cooling tower and pumps) of 0.6–0.8 kW/ton is typical for a well-designed water-cooled chiller plant. IPLV-based plant efficiency of 0.5–0.65 kW/ton is achievable with modern centrifugal chillers, VFD pumps, and variable speed cooling towers. Air-cooled plants run 1.0–1.2 kW/ton full load.
Primary-secondary pumping uses constant-speed primary pumps to maintain design flow through each chiller, and variable-speed secondary pumps to distribute chilled water to the building at varying flow rates. A bypass (decoupler) pipe between primary and secondary loops allows flow to recirculate when building demand is low. This protects chillers from low-flow conditions while allowing the secondary system to save pump energy.
The second chiller should stage on when the first is operating near its full-load capacity — typically 85–90%. Staging too early wastes energy by running two machines at low load; staging too late risks insufficient cooling capacity. Modern BAS systems calculate the optimal stage point based on chiller curve data and current load trends.
Lowering condenser water supply temperature (CWST) on mild days allows chillers to operate at a lower condensing pressure, reducing compressor work and improving COP. As a rule of thumb, each 1°F reduction in CWST improves chiller efficiency by about 0.5–1.5%. ASHRAE 90.1 requires CWST reset on systems with cooling towers.
A waterside free cooling economizer uses a plate-and-frame heat exchanger to transfer heat from the chilled water loop directly to the condenser water loop, bypassing the chiller compressor entirely when outdoor wet-bulb temperature is low enough. At an entering wet-bulb of 45°F or lower, free cooling can meet the entire building cooling load, saving 100% of chiller compressor energy.
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