Low Chilled-Water Delta-T in a Primary-Secondary Plant
A 400-ton water-cooled centrifugal chiller plant serves a university building through primary-secondary pumping. Design evaporator flow is 800 gpm, with 54°F return and 44°F supply. During a design-week commissioning test, the operating chiller shows about 42% load on its onboard display. A portable ultrasonic flow meter confirms 800 gpm, and calibrated immersion sensors read 49°F return and 44°F supply. The chilled-water pumps are constant-speed, both primary pumps run, and secondary differential pressure is at setpoint. The AHUs have three-way valves at their coils, and several coil leaving-air temperatures are 52-54°F rather than 55°F. Last year's balancing report showed design flows at all AHUs. The plant operator wants to add a second chiller to improve cooling, but no space is uncomfortably warm and the operating chiller is not fully loaded. The building automation system trend shows the secondary loop differential-pressure control valve bypassing water most of the afternoon.
1. Calculate the measured evaporator ΔT and capacity. What does the result say about whether the plant is capacity-limited?
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Measured ΔT = 49°F − 44°F = 5°F. Chilled-water capacity = 500 × gpm × ΔT = 500 × 800 × 5 = 2,000,000 Btu/h, or 2,000,000 ÷ 12,000 ≈ 167 tons. That is about 42% of the 400-ton nominal capacity, which matches the chiller's onboard load indication.
At design flow and design ΔT of 10°F, the same 800 gpm would carry about 333 tons. The chiller is not at its capacity limit, and the plant is not short of nominal chiller tonnage. The low ΔT means the loop is returning water that is too cold relative to the supply temperature. Adding a second chiller would not correct that; it would add capacity that the loop cannot use and could make staging and efficiency worse. The immediate issue is a low-ΔT syndrome, not a lack of chillers.
2. Explain how three-way valves and the differential-pressure bypass produce low ΔT. Why can the coils still deliver cold air while the plant performance is poor?
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With three-way valves, each coil receives essentially constant water flow. At part load, the valve bypasses some supply water around the coil and mixes it back into the return. The coil gets less heat transfer, while the return water is diluted with cold supply water. The result is a smaller return-to-supply temperature difference at the plant even though total flow remains at or near design. The differential-pressure control valve does something similar at the loop level: when zone valves close, it bypasses supply water to the return to protect the pumps and maintain ΔP, further lowering the return temperature.
The coils can still produce 52–54°F leaving air because they are being over-supplied with cold water relative to the actual load. The spaces are not warm, so occupants do not complain. However, the chiller sees a low return temperature, unloads, and may operate inefficiently; the plant cannot use its full capacity even though water flow is at design. The low ΔT is a distribution and control problem, not an evaporator capacity problem.
3. Outline the diagnostic and corrective steps you would take before recommending any plant expansion.
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First, verify the measurements: check the immersion sensors in an ice bath or against a calibrated reference, confirm the ultrasonic flow meter with a second method, and inspect sensor locations for stratification or conduction errors. Survey every AHU coil for water-side ΔT, valve position, and leaving-air temperature. Confirm whether the three-way valves are actually bypassing and whether the loop ΔP bypass is open when it should be closed. Check for air in the loop, fouled coils, closed isolation valves, and pump curves that do not match design.
Then correct the distribution: replace or reconfigure three-way valves as two-way where appropriate, use differential-pressure control at the most remote coil, rebalance the secondary loop, and reset supply-water temperature only after the ΔT is restored. Trend the plant for a week at part load and verify that the chiller can load and that the ΔT approaches design before considering a second chiller. Document sensor calibration, valve positions, coil ΔT, flow, and the before-and-after plant capacity. If the building later adds load and the corrected plant still cannot meet it, then a capacity expansion can be justified.