This causes interference; let’s explain the principles and symptoms in two parts.

1. Root Cause: Sudden changes in cooling water flow distribution cause corresponding fluctuations in the heat pump’s heat dissipation capacity.

The total output of the shared cooling water pump is fixed, and the water circuit is divided into parallel branches:

When a single unit starts up: A new branch is added, diverting cooling water from other units and reducing heat dissipation in their condensers. The high-pressure refrigerant in the heat pump rises, heating output decreases, vapor production in the evaporator chamber decreases, non-condensable gases accumulate, and the vacuum continues to drop;

When a single unit shuts down: The branch circuit closes, and all excess cooling water is supplied to the remaining units. Excessive cooling causes the heat pump’s heating power to increase, resulting in a sudden surge of vapor production. The vapor displaces the gas phase space, causing the vacuum to spike momentarily.

Since vapor production fluctuates between high and low levels while the vacuum pump’s pumping speed remains constant, the negative pressure inevitably oscillates back and forth.

2. Additional Disturbances: Water Hammer Amplifies Pressure Fluctuations

Opening and closing the cooling water valves during startup and shutdown causes water hammer, resulting in violent fluctuations in the main pipeline pressure. These fluctuations are transmitted to all condensers, causing turbulent heat exchange water flow and persistent instability in heat transfer efficiency. Evaporation temperature and boiling point fluctuate accordingly, further exacerbating vacuum drift.

3. Operational Differences

Without independent flow-limiting valves or pressure-stabilizing valves: After a single unit is started or stopped, the vacuum in the remaining units fluctuates significantly; these fluctuations are more severe during the late stages of concentration when steam production is low, making it easier for foam to carry material;

When each unit is equipped with an independent constant-flow valve: The water flow in each branch is not affected by diversion in the main pipeline, significantly reducing disturbances caused by unit start-up and shutdown, and maintaining a generally stable vacuum.