Proportional Balancing of a Direct-Return Chilled-Water Loop
A secondary chilled-water loop serves six AHUs. Design flows are AHU-1 120 gpm, AHU-2 100 gpm, AHU-3 80 gpm, AHU-4 60 gpm, AHU-5 60 gpm, and AHU-6 40 gpm, for 460 gpm total. The variable-speed pump is set to a constant 80-ft differential at the pump discharge. With all coil balancing valves wide open and all control valves 100% open, the pump reads 520 gpm at 65 ft. AHU-1 reads 145 gpm; AHU-6 reads 25 gpm. Design coil pressure drops range from 12 ft at AHU-1 to 18 ft at AHU-6. The loop is direct return. A differential-pressure sensor at the pump reads 65 ft; a second gauge at AHU-6 reads 40 ft. The engineer wants the pump trimmed rather than throttled.
1. Calculate total overflow and branch percentages, and decide whether pump trim alone can solve the imbalance.
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Total overflow = 520 − 460 = 60 gpm, or 13% above design. AHU-1 = 145/120 = 121% of design; AHU-6 = 25/40 = 62.5%. The direct-return layout and oversized pump explain the pattern: near coils receive excess flow and the far coil is starved. Pump trim alone cannot solve it. Trimming to 460 gpm would reduce total flow, but AHU-6 would still be below design because the near branches would continue to take more than their share. The balance valves must redistribute flow after the pump is trimmed or the VFD is reset.
2. Outline the proportional-balancing sequence, identifying the index circuit and the order of valve adjustment.
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Measure all coils at 100% open and record percentages. AHU-6 at 62.5% is the index circuit: farthest and lowest. Throttle the nearest oversupplied coils first-AHU-1, then AHU-2, then AHU-3-to raise pressure at AHU-6. Adjust AHU-6 to its design 40 gpm only after the near coils are throttled; otherwise its balance valve may be wide open and still short. Re-measure all branches and adjust each valve so all end at approximately the same percentage of design at the same time. Work from the index outward, not from the pump inward.
3. Specify the final pump and verification requirements, including what to do if AHU-6 remains low.
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After balancing, total flow should be 460 gpm. If the pump still runs at 520 gpm, trim the impeller or reduce the VFD speed-do not use a throttling valve as the primary control. Confirm AHU-6 gauge pressure and flow. If AHU-6 remains below 90% of design after near coils are throttled, look for a closed valve, pipe restriction, or undersized branch-not a balance-valve problem. Verify that no coil is below 90% or above 110% of design, and document pump speed, head, and all valve positions. Recheck at part load because a constant 80-ft setpoint may be excessive.