Chilled Water Valve Hunting in a Variable Flow Loop
A variable primary chilled water system serves 12 air handling units. The lead chiller is a 500 kW screw machine with a variable speed drive. The loop has a differential pressure sensor located two-thirds of the way down the main riser, controlling the lead pump VFD to hold 180 kPa. Since a new AHU was added at the far end of the loop, the chilled water control valves on several AHUs hunt, the pump speed swings between 55 and 85 percent, and the chiller occasionally trips on low evaporator temperature. The new AHU valve is a two-way valve sized for 30 percent more flow than its coil design. The differential pressure sensor reads steady when the pump is held at fixed speed.
1. Diagnose the cause of the hunting and chiller trips.
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The symptoms point to a control instability caused by an oversized terminal valve and a poorly located differential pressure sensor, not a chiller fault.
The new AHU valve is oversized by about 30 percent. Oversized valves have very high authority at design flow, so small changes in load cause large flow changes. When the valve modulates, the loop differential pressure changes sharply, the pump VFD responds, and the valve overcorrects. This produces the hunting cycle.
The differential pressure sensor is two-thirds down the riser. With the new AHU at the far end, the sensor does not see the pressure drop across the new branch. The controller holds 180 kPa at the sensor, but the far end may see too little pressure, so the new valve opens wide, then the pump speeds up, then the valve throttles. The pump speed swing of 55 to 85 percent is the loop chasing itself.
Low evaporator temperature trips occur because the hunting causes sudden flow reductions through the evaporator while the compressor is still loaded. Flow drops, evaporator temperature falls, and the chiller trips on its protection. The steady sensor reading at fixed pump speed confirms the sensor is not faulty; the problem is the control loop and valve sizing.
2. Recommend fixes and explain how you would tune the loop after the fixes.
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Fixes:
- Resize or replace the new AHU valve so it is sized for the coil design flow with proper authority, typically 0.3 to 0.5 for a two-way valve in a variable flow loop. A pressure-independent control valve is a good option here because it removes the effect of loop pressure changes.
- Relocate the differential pressure sensor to the hydraulically most remote branch, or add a second sensor and control on the higher of the two. This ensures the far end has enough pressure without overpressurizing the near end.
Tuning after fixes:
- Set the pump VFD to a fixed speed and confirm each AHU valve can achieve design flow at the worst-case branch.
- Re-enable differential pressure control with a slow integral time so the pump does not react to every valve movement. Verify the pump speed swing is under 10 percent at steady load.
- Confirm the chiller minimum flow limit is respected. Add a bypass valve if the loop can fall below minimum evaporator flow.
- Monitor evaporator temperature and pump speed for a week. If trips persist, review chiller minimum flow and loading rate settings.