Low Chilled-Water Delta-T Caused by a Three-Way Valve Bypass
AHU-3 serves an 8,000-cfm office floor with a 6-row chilled-water coil. Design conditions are 55°F supply air, 76°F return air, 44°F entering water, 56°F leaving water, 30 gpm, and 15 ft coil pressure drop. The coil valve is a three-way modulating valve with a 0-10 V actuator. At full cooling load the valve is commanded to 10 V. Measured total water flow is 45 gpm, coil pressure drop is 9 ft, water is 44°F entering and 56°F leaving the coil, and loop return is 50°F. Air is 58°F leaving and 76°F returning. Pump differential is steady at 70 ft, the coil differential-pressure sensor is calibrated, and the coil balancing valve is wide open. Fan airflow is 8,000 cfm by duct traverse, and the filter is clean.
1. Calculate design and actual coil capacity, the flow through the coil, and the bypass flow.
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Design capacity = 30 gpm × 500 × 12°F = 180,000 Btu/h. Coil flow from pressure drop: Qcoil = 30 × sqrt(9/15) = 23.2 gpm. Bypass = 45 − 23.2 = 21.8 gpm. Coil capacity = 23.2 × 500 × (56 − 44) = 139,200 Btu/h. Loop return temperatures confirm mixing: (23.2 × 56 + 21.8 × 44)/45 = 50.2°F. Airside capacity = 8,000 × 1.08 × (76 − 58) = 155,520 Btu/h. Water-side and air-side capacities are within about 10%, so the coil is transferring heat, but only about 23 gpm is going through the coil while 45 gpm passes through the valve assembly.
2. Diagnose the low loop delta-T and explain why adjusting the balancing valve is the wrong first response.
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The coil pressure drop corresponds to 23 gpm, not 45 gpm. At a 100% cooling signal, the three-way valve is bypassing about 22 gpm directly from supply to return. That is a valve, actuator, or linkage fault-not a balancing problem. Check stroke versus signal at 0, 5, and 10 V; check the actuator linkage and stroke limit; and verify the bypass port is not leaking. The coil approach is 58 − 44 = 14°F versus a design approach of 11°F, consistent with reduced coil flow. Closing the balancing valve would reduce total flow but would not stop the bypass and could starve the coil further. Fix the valve before changing any balance setting.
3. List the tests you would perform before signing off, and the final verification criteria.
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Tests: (1) stroke the valve 0-50-100% and record coil pressure drop and temperatures at each point; (2) verify actuator voltage and actual stem travel; (3) isolate the coil and check the bypass port for leakage; (4) verify water temperature sensors in a bath; (5) check airside bypass and confirm 8,000-cfm airflow. After repair, rebalance the coil to 30 gpm and confirm approximately 12°F water delta-T at design load, 55°F supply air, and 180,000 Btu/h capacity. Recheck loop return temperature; it should approach 56°F, not 50°F. Document that the original 45 gpm was total valve flow, not coil flow.