Low Delta-T in a Primary-Secondary Chilled Water Plant
Three 600 kW water-cooled chillers serve a 1,200 kW IT load through a primary-secondary pumping arrangement. Design secondary supply/return is 10/18°C (ΔT 8 K). At 80% load the BMS trends show 9.5°C supply, 14.5°C return (ΔT 5 K), secondary pumps at 95% speed, the bypass valve 40% open, and chillers staged at 90% and 70% with the third off. The plant cannot accept another chiller start without exceeding the electrical limit, and the site has already cleaned the primary strainers. Chilled-water differential pressure at the farthest CRAH is 35 kPa against a 50 kPa setpoint. No coil ΔT readings are available. The operator proposes adding a fourth chiller and raising pump speed to 100%. Design flow is 64 m³/h per chiller at 8 K. The load has been stable at 960 kW for three weeks.
1. Using the numbers, quantify the low delta-T and explain how it drives pump and chiller staging problems.
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At 960 kW and 5 K, flow = 960/(4.186×5) = 45.9 kg/s ≈ 165 m³/h. At design 8 K, the same load needs 960/(4.186×8) = 28.7 kg/s ≈ 103 m³/h. Actual flow is about 60% above design, which explains high pump speed, the 40% open bypass, and poor staging. Each chiller sees less than its design ΔT, so the plant must run more chillers and more flow for the same load. The low ΔT is a load-side or bypass symptom, not a reason to add a fourth chiller. Check CRAH coil ΔT, control-valve position and authority, bypass leakage, sensor calibration, and whether three-way valves are passing. The 35 kPa far-end differential versus a 50 kPa setpoint also suggests the DP sensor or reset schedule is not controlling the critical branch.
2. List the most likely causes and a diagnostic sequence that separates load-side problems from plant-side problems.
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Most likely causes are fouled or air-bound CRAH coils, wide-open or failed control valves, a leaking bypass, 3-way valves, and incorrect DP reset. Diagnose from the load backward: trend each CRAH valve position and coil ΔT at full load; isolate the bypass; verify supply/return sensors against a calibrated reference; check strainers and coil pressure drop; then review pump curves and DP sensor location. Plant-side checks include chiller evaporator approach and primary flow. Do not raise pump speed to 100% first-it increases bypass and may not improve critical-branch ΔT.
3. Recommend changes and state what result would show the plant no longer needs the fourth chiller.
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Restore load-side ΔT before adding capacity: clean coils, repair or replace valves with proper authority, close leaking bypass, recalibrate sensors, and reset DP to the critical CRAH. Add chiller staging on actual load and flow, not just return temperature. If ΔT returns toward 8 K, two chillers can carry 960 kW within the electrical limit and the fourth chiller is unnecessary. Track kW/ton and plant ΔT; expect pump energy to fall as flow drops. Only after load-side fixes should you evaluate a fourth chiller for growth.