Low Delta-T and Humidity in a Variable-Flow Chilled-Water Plant
A 12-storey office building has a 1,200 TR chilled-water plant with two equally sized variable-speed centrifugal chillers in a primary-secondary configuration. Design chilled-water supply/return is 6.5/13.0 C. During summer, BMS trends show supply 6.4 C, return 11.1 C, and one chiller loaded to 92% while the second is off. AHU chilled-water valves hunt between 15% and 100% every few minutes. Space conditions are 24.5 C and 64% RH against a design of 24 C and 50% RH. The building has 15% outdoor air and variable-frequency fans. A recent test shows the three-row cooling coils have a design face velocity of 2.8 m/s and 2.0 mm fin spacing; several coils have visible dust but are not blocked. Differential pressure across the worst coil is 180 Pa versus 120 Pa design. The owner wants to raise supply-water temperature to improve chiller efficiency and add a bypass to fix low delta-T.
1. Explain how one chiller can be at 92% while the plant delta-T is only 4.7 K, and valve hunting is occurring. Distinguish among coil fouling, valve authority/control, and a hydraulic bypass.
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At 6.4 C supply and 11.1 C return, the plant delta-T is 4.7 K versus 6.5 K design. The chiller is loaded because it is controlling supply temperature, but the returning water is too cold, so it must reject the same load with more flow. That is a low-delta-T syndrome, not proof of excess load. Valve hunting suggests the control loop is unstable: a valve with poor authority, an oversized valve, or a faulty differential-pressure sensor can cycle between 15% and 100%, sending slugs of cold water back. Coil fouling would normally reduce water-side heat transfer, giving a lower delta-T and a high coil approach; the measured 180 Pa air-side differential pressure versus 120 Pa design indicates air-side restriction, which reduces airflow and coil capacity. A hydraulic bypass or decoupler passing flow would also return cold water. Distinguish by trending valve position with flow, measuring coil entering/leaving water temperatures and air temperatures, checking valve authority (valve pressure drop versus circuit pressure drop), and verifying that the decoupler is not flowing.
2. Evaluate the owner’s proposal to raise chilled-water supply temperature and add a bypass. Recommend a sequence of corrective actions, with reasons.
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Raising supply temperature improves chiller efficiency by reducing lift, but it raises the coil apparatus dew point and makes 50% RH harder to meet; at 64% RH the building already has a latent problem. Adding a bypass to 'fix' low delta-T is counterproductive: it returns cold water directly, lowering plant delta-T further and forcing more chiller flow. Do not do either yet. Sequence: (1) clean coils and verify airflow/fan curves; the 180 Pa versus 120 Pa differential pressure indicates fouling. (2) Re-commission AHU valves: check authority, stroke, actuator, and differential-pressure sensor; restore stable modulation. (3) Verify the decoupler/bypass is closed and balance primary/secondary flows. (4) Measure coil performance at design flow and entering conditions. (5) Only then apply a small supply-temperature reset, with a humidity limit, and only if coil capacity and dehumidification are adequate. If a bypass is needed for minimum chiller flow, it should be a controlled minimum-flow bypass, not a fixed bypass.
3. The room is 24.5 C/64% RH. What field measurements would separate an air-side cause from a water-side cause, and what immediate operating change would you make?
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Measure at the worst AHU: entering and leaving air dry-bulb and wet-bulb, air flow (or fan speed and static), entering and leaving water temperature, water flow, valve position, and condensate rate. If the coil is wet and the leaving-air wet-bulb is close to the coil surface temperature but airflow is low, the cause is air-side (fouling, fan, filter). If water flow is low or the coil approach is large at design flow, the cause is water-side (air lock, fouling, valve). If the valve is nearly closed, it is control or hydraulics. Immediate change: clean the worst coils, replace filters, restore design airflow, and hold supply-water temperature at 6.5 C; do not raise it. If humidity remains high, reduce outdoor-air latent load or add dedicated dehumidification rather than over-cooling.