Chilled-Water Plant With Three-Way Valves: Low Delta-T and Warm Upper Floors
A 12-storey, 240-unit residential tower in Tehran has a central chilled-water plant with two 300 TR screw chillers, primary/secondary pumping, and variable-speed secondary pumps. Each apartment fan-coil unit is fitted with a three-way control valve; the differential-pressure sensor is at the top of the secondary riser. Design conditions are 7/12°C and 300 L/s. At peak summer load, upper-floor units report insufficient cooling, while each chiller loads only to about 55%. The common bypass is fully open, chilled-water supply/return is 7/9°C instead of 7/12°C, and the secondary pumps run at full speed. The contractor's remedy is to increase pump speed further and reset the chiller leaving-water temperature upward. The owner asks the mechanical supervisor to decide whether this is acceptable before handover.
1. Diagnose why the plant has a low chilled-water temperature difference while upper floors remain warm. Explain the interaction among the three-way valves, the bypass, the differential-pressure sensor location, and chiller loading.
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The key symptom is a 7/9°C temperature difference instead of 7/12°C, meaning the chiller is returning water that is too cold. That usually means a large portion of chilled water is bypassing useful load, not that the chiller is incapable.
With three-way valves, as apartment loads fall, the valve diverts water directly from supply to return to maintain coil flow. The plant therefore sees nearly constant secondary flow but reduced heat pickup, so return temperature stays close to supply. The open common bypass makes this worse: it provides a direct supply-to-return shortcut.
The differential-pressure sensor at the top of the riser can also mislead the control system. If the sensor sees a pressure condition influenced by the bypass rather than the true critical zone, the variable-speed pumps may run at full speed, increasing flow and further reducing the temperature difference. The chillers then load only to about 55% because the return water is cold and the evaporator is not being asked to remove the design heat.
Upper floors can still be warm because flow is not the same as cooling. Coils may be air-bound, fouled, or unbalanced; however, the plant data point first to a hydraulic distribution problem. The supervisor should require a root-cause check: valve positions, bypass position, strainers, air venting, coil differential pressure, and sensor calibration.
2. Propose a corrective scope and a commissioning verification plan. State what must be measured before the supervisor accepts the plant.
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A defensible corrective scope should address the distribution problem, not just raise pump speed:
- Replace or convert three-way fan-coil valves to two-way pressure-independent control valves, or at minimum re-pipe so the bypass is not permanently open.
- Re-commission the bypass: it should close under load and open only at minimum flow, with the correct minimum-flow setting.
- Verify the differential-pressure sensor location and calibration. The sensor must represent the critical zone, not a point dominated by the bypass.
- Rebalance coils and check for air, strainers, and fouled coils.
- Reset chiller leaving-water temperature only after hydraulic balance, not as a substitute for it.
Acceptance should require measured data at design and part-load conditions: supply/return temperature at the plant, secondary flow, pressure differential at the critical zone, valve positions, chiller load, and pump power. The plant should achieve the design temperature difference within an agreed tolerance at design flow, and the critical zone should hold its required differential pressure. A trend log over several days is more convincing than a single spot reading.
3. The contractor argues that increasing pump speed is cheaper and faster and asks the supervisor to approve handover with a temporary note. How should the supervisor respond?
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The supervisor should reject the remedy as technically unsound. Increasing pump speed on a system already showing 7/9°C will raise flow, increase pump energy, and usually make the low temperature difference worse. It can also increase noise and erosion and may overload the pumps. Resetting the chiller leaving-water temperature upward may mask the symptom while leaving upper floors warm.
Approving handover with a temporary note would transfer an unresolved design/commissioning defect to the owner and could be inconsistent with the supervisor's professional duty. The correct response is a written nonconformance: identify the hydraulic imbalance, require the contractor to submit a corrective method statement, and withhold acceptance of the affected system until measured performance is demonstrated.
The supervisor should distinguish between a genuine temporary commissioning measure and a permanent bypass of the design intent. A temporary measure may be allowed only with a defined test, a removal date, and owner acceptance; here the proposed change is neither bounded nor likely to work. The supervisor should document the decision and copy the owner.