I. Underlying Mechanism: Redistribution of Total Cooling Water Flow and Sudden Change in Heat Pump Cooling Capacity

When multiple units share a parallel cooling water main with a fixed total pump flow rate:

Single-unit startup: A new branch line is added, reducing the cooling water supply to each unit; heat exchange in the condenser deteriorates, refrigerant condensing pressure rises, heat pump heating output increases, and steam generation in the evaporator chamber suddenly surges; A vacuum pump operating at a constant pumping speed cannot evacuate the surge of steam fast enough, causing the vacuum to drop instantly and oscillate continuously.

When a single unit is shut down: One branch is closed, and all excess cooling water is supplied to the operating units. This results in excessive cooling of the condenser, a decrease in the heat pump’s heating capacity, and a sharp reduction in steam generation; the amount of vapor in the gas phase decreases, the proportion of non-condensable gases increases, and the negative pressure surges and drifts.

These sudden fluctuations in steam generation are the primary source of vacuum fluctuations.

II. Secondary Superimposed Disturbances Amplify the Magnitude of Vacuum Fluctuations

Pipeline Water Hammer

The opening and closing of valves generates water pressure surges, causing drastic fluctuations in the main pipeline pressure. This leads to brief disruptions in the cooling water flow to each condenser, resulting in persistent instability in heat exchange efficiency and erratic fluctuations in evaporation temperature, which further disrupts the steam production rhythm.

Boiling Point Shift Due to Imbalanced Heat Dissipation Across Units

Insufficient cooling → intense heat generation, high evaporation temperature; under the same negative pressure, foaming and carryover are more likely to occur. Foam entrains salt mist, clogging the demister screen, which alters the flow resistance in the vapor phase and causes secondary vacuum fluctuations.

Passive changes in vacuum system load

When steam production surges, the vacuum pump load increases sharply; when steam production drops abruptly, the proportion of non-condensable air drawn into the pump increases. Both scenarios prevent the negative pressure from stabilizing within the set range.

III. Comparison of the Two Configuration Differences

No branch flow-limiting/pressure-stabilizing valves (most common in the field)

After a single unit is started or stopped, the vacuum of the remaining units fluctuates visibly. These fluctuations are more severe during the late stages of concentration when the mother liquor viscosity is high and steam production is low, leading to foaming and condensate contamination.

Each unit’s cooling water branch equipped with a constant flow valve

Starting or stopping a single unit does not compete with other units for water flow; heat dissipation conditions remain stable, steam production is steady, and vacuum remains largely unaffected.

IV. Derived Chain Reaction Process Failures

Persistent vacuum fluctuations can lead to: increased foam carrying material, clogging of demister screens, drift in conductivity probe readings, misjudgment of staged concentration programs, and amplification of the effects of minor gas seepage through manholes.