When multiple cryogenic evaporators share the same cooling water circulation system, the start-up or shutdown of a single unit does indeed interfere with the vacuum stability of the other units in operation. This is a chain reaction in which hydraulic disturbances are transmitted to the condensation process and subsequently to the vacuum system. The fault typically manifests as a brief drop in vacuum or minor fluctuations, rather than a complete and immediate loss of vacuum; These fluctuations persist for tens of seconds to several minutes before partially stabilising, which can easily be misinterpreted as a leak in the vacuum pump or seals. Let us first clarify the underlying logic: the stability of the vacuum during low-temperature evaporation is highly dependent on the condenser’s condensation capacity. Water vapour generated in the evaporation chamber relies on cooling water to rapidly condense and liquefy; only then can the non-condensable gases within the system be successfully evacuated by the vacuum pump. Instantaneous fluctuations in cooling water flow rate or inlet temperature cause an immediate change in the condenser’s condensation efficiency, which directly alters the partial pressure of vapour in the chamber, leading to corresponding fluctuations in vacuum level. When one of the evaporators starts up and the cooling water valve opens, it instantly diverts a portion of the circulating water volume from the pipework network. This causes hydraulic shock on both the return and supply sides of the pipework network, momentarily diverting and reducing the cooling water flow rate of the other units in operation. Consequently, the cooling water supply to the condenser is temporarily insufficient, reducing its heat exchange capacity; water vapour is not fully condensed, and the partial pressure of non-condensed water vapour within the chamber rises. Even if the vacuum pump’s pumping capacity remains unchanged, the chamber vacuum will drop, and the evaporation temperature will rise slightly in tandem. Conversely, when a unit currently in operation is shut down and the cooling water valve is closed, the total return flow resistance in the piping network suddenly decreases, causing the network pressure to rise instantly, whilst the cooling water flow rate for the remaining units surges briefly. This brief excess cooling capacity causes rapid and extensive condensation of steam within the chamber, leading to a sharp drop in vapour pressure. The vacuum may experience an instantaneous surge, exceeding the set vacuum range, at which point the automatic control system intervenes to regulate it, triggering a new round of oscillations. If the diameter of the shared cooling water main is too small, the capacity of the circulation pumps is insufficient, or there is no independent pressure-stabilising buffer, these disturbances will be amplified. Even if the average flow rate in the main pipe is sufficient, the root cause of the disturbance lies in the hydraulic shock and flow redistribution that occur at the moment of start-up or shutdown, rather than an insufficient steady-state flow rate. There are also secondary, cumulative effects: following fluctuations in the cooling water flow rate, the outlet temperature of the condenser changes, and these temperature changes propagate along the piping system. As return water from multiple condensers converges into the main pipe, the temperature disturbances caused by start-up and shutdown flow back to the supply side, affecting the other units in operation and causing temporary fluctuations in inlet water temperature, which further exacerbates the instability of the condensation process. If the system incorporates plate-type water-cooled condensers, sudden changes in cooling water flow rate can also cause brief fluctuations in refrigerant-side pressure, leading to operating condition oscillations in the heat pump unit. This indirectly affects heat input to the evaporation chamber, resulting in more pronounced vacuum fluctuations. The actual phenomenon observed on site is not a sustained vacuum deficit, but rather pulsating fluctuations. Whenever other equipment is switched on or off, the unit’s vacuum level fluctuates briefly before returning to near-normal values; Units operating at higher loads and with higher concentration ratios are more sensitive to cooling water disturbances. During the later stages of concentration, the steam load is inherently high and the system’s resistance to disturbances is low, resulting in greater amplitude of vacuum fluctuations. During the low-load feedstock stage, steam production is low, so vacuum changes caused by disturbances are not prominent, and faults are easily masked. A common pitfall in troubleshooting is repeatedly checking the vacuum piping and gasket seals, or calibrating the vacuum pump performance, without identifying the problem; When a single unit is operated in isolation, the vacuum remains stable; however, vacuum oscillations occur as soon as multiple units are started or stopped in a staggered manner. Corresponding engineering improvement measures include: where possible, fitting each unit’s cooling water inlet and outlet pipes with independent pressure-regulating valves or flow-restricting valves to mitigate hydraulic shocks from the external network; increasing the diameter of the main pipeline and equipping the circulation pumps with pressure-stabilising tanks to reduce flow shocks during start-up and shutdown; and, where conditions permit, installing energy-storage buffers on the cooling water branch pipes of each unit; In terms of automatic control logic, implement slow-opening and slow-closing operations for the cooling water valves of newly added units to avoid severe hydraulic disturbances caused by instantaneous full opening or closing; during high-concentration operating conditions, avoid frequent start-up and shutdown of other evaporators within the same system as far as possible; it is also possible to configure independent small cooling water circuits for critical units, disconnecting them from the shared pipework network to completely eliminate mutual interference. Today 17:04