When multiple low-temperature evaporators share the same cooling water circulation piping, starting or stopping a single unit can disrupt the vacuum stability of the other units in operation. The root cause lies in fluctuations in the flow rate and pressure within the cooling water circuit, which are then transmitted to the entire vacuum system—including the condenser and vacuum pump—and is not due to a malfunction of the vacuum pump itself.

When one unit is started up, the cooling water valve opens instantly, causing the water flow within the piping network to redistribute. This results in a momentary drop in local water pressure, leading to a brief shortage of cooling water inflow to the other running units. Since the cooling water is responsible for condensing the secondary vapor discharged by the vacuum pumps, any fluctuation in water volume will degrade the condensation efficiency. As a result, the vapor cannot be fully liquefied, the concentration of non-condensable gases within the system increases, and the actual vacuum level in the chamber drops. Conversely, when a unit is shut down and the cooling water valve closes, the flow in the piping suddenly decreases, causing the system pressure to spike momentarily. This results in a brief excess of cooling water flow to the other units, leading to excessive condensation and, similarly, causing short-term vacuum fluctuations.

In a shared cooling water system without pressure-stabilizing buffers, differences in piping resistance can amplify these disturbances. The cooling water inlet valves for each evaporator are typically electric on/off valves or proportional control valves. The opening and closing of these valves instantly generate water hammer effects, with pressure surges propagating along the main pipe to all branches. Even if the valve openings of other equipment remain unchanged, the actual water flow reaching the heat exchanger still fluctuates, causing the condensation temperature to swing back and forth, which directly results in fluctuations in the vacuum reading.

The vacuum system is highly sensitive to cooling water temperature and flow rate. Low-temperature evaporation relies on condensation to liquefy large amounts of water vapor, thereby reducing the load on the vacuum pump. Even slight fluctuations in cooling water flow can cause changes in condensation capacity, altering the partial pressure of water vapor inside the chamber and causing the vacuum to drift. Units operating under higher loads and with higher concentration ratios will exhibit a more pronounced response to cooling water fluctuations. Since the system’s resistance to disturbances is inherently weak during the late stages of concentration, the vacuum fluctuations caused by such start-stop cycles become even more pronounced.

In addition to direct flow shocks, secondary effects may also occur. Once vacuum fluctuations occur, the saturated temperature in the evaporation chamber changes accordingly, causing the unit’s heat pump heating output to adjust in response. This leads to oscillations in the evaporation temperature; in severe cases, it can trigger foaming and result in intermittent material carryover. During on-site troubleshooting of unstable vacuum, technicians often repeatedly repair vacuum pumps or replace sensors, overlooking the mutual interference caused by the shared cooling water distribution network.

It is also important to distinguish between different scenarios. If the main pipeline has sufficient diameter margin, is equipped with a pressure stabilizing tank, and each branch is fitted with an independent flow control valve, the impact caused by the start-up or shutdown of a single unit will be significantly reduced, and the interference will be so minimal as to be almost imperceptible. Conversely, if the cooling water main pipeline is undersized, lacks a pressure stabilizing device, and the branches do not have independent flow control, the mutual interference between units will be very pronounced, constituting a typical system design flaw.

A common phenomenon observed in the field is that when several units are operating simultaneously, everything runs smoothly; however, as soon as one unit is started or shut down, the vacuum of the remaining units in production will experience a brief drop in pressure and fluctuate, gradually returning to stability after several tens of seconds to one or two minutes. To avoid such issues, increase the diameter of the main cooling water pipe, install a pressure-stabilizing buffer on the main line, configure independent flow control for each unit’s cooling water branch, and optimize the control logic to prevent multiple units’ valves from opening or closing simultaneously, thereby reducing pressure surges in the piping network caused by start-up and shutdown.