If the vacuum line buffer tank is undersized, the pressure pulses caused by the vacuum pump’s periodic evacuation will directly disturb the vapor phase in the evaporation chamber, thereby reducing the demister’s actual separation efficiency. This type of failure is not due to damage to the demister itself but rather a performance degradation caused by fluctuations in vapor phase conditions; on-site personnel often misdiagnose it as a clogged demister screen or material failure.

The primary function of the buffer tank is to absorb the pumping pulses generated by the vacuum pump, converting periodic suction pulsations into a relatively stable vapor flow rate. If the tank is undersized and its volume is insufficient to offset the pulse energy, each suction cycle of the vacuum pump will transmit pressure oscillations along the vacuum piping into the evaporation chamber. As a result, the vapor pressure inside the chamber is not a constant negative pressure but rather undergoes small-amplitude, high-frequency fluctuations.

First, this disturbs the gas-liquid interface within the chamber, causing extensive bubble formation due to boiling. Since the gas-liquid interface is already in an unstable state, the superimposed pressure pulses cause the liquid surface to oscillate periodically up and down, altering the behavior of the collapsing boiling bubbles. When the pressure drops instantaneously, more fine microbubbles burst within the feed liquid, and a large number of tiny liquid droplets are ejected into the vapor phase, generating more mist than under steady-state conditions. Demisters are designed to intercept droplets through inertial collision based on a stable vapor flow velocity; if the amount of mist generated increases for no apparent reason and exceeds the demister’s processing capacity, entrained liquid will escape.

Second, the gas-phase flow velocity through the demister undergoes pulsating fluctuations, alternating between high and low values. Both mesh demisters and baffle-type demisters have an optimal operating velocity range: if the velocity is too low, large droplets tend to settle, but the capture efficiency for fine mist decreases; if the velocity is too high, droplets are penetrated and carried by the gas flow, resulting in secondary entrainment. Under pulsating conditions, the instantaneous peak velocity of the gas flow can far exceed the equipment’s rated design velocity. At the moment of the peak, high-velocity gas flows through the demister layer, causing the liquid film already captured and retained by the mesh to be instantly shattered by the high-speed flow. This generates finely atomized droplets that penetrate the demister components and enter the downstream piping. While the average flow velocity may appear to be within the design range, the instantaneous flow velocity exceeds the limit—this is the most typical issue with small buffer tanks.

Pulsating pressure also alters the drainage behavior of the liquid film inside the demister. The intercepted liquid relies on gravity to flow downward back into the evaporation chamber. Pressure fluctuations create a back-and-forth suction effect on the downward-flowing liquid film, preventing the liquid from flowing smoothly downward. The droplets are repeatedly pulled and suspended within the pores of the demister mesh, causing liquid to continuously accumulate on the mesh surface. This gradually forms liquid film bridges, blocking some flow channels. The gas phase is forced to pass through the remaining gaps at high speed, causing local flow velocities to surge further and creating a vicious cycle. Over time, localized liquid accumulation in the demister causes material carryover to progressively worsen.

In terms of operational performance, the fluctuations in the vacuum gauge reading may not appear significant to the naked eye because the instrument’s damping filter masks small, high-frequency pulses; however, the gas phase is actually undergoing continuous oscillation. This phenomenon typically manifests as intermittent spikes in the conductivity of the condensate. Even after thoroughly cleaning a brand-new demister screen, material carryover reoccurs shortly after operation resumes, despite no changes in the feed composition or concentration ratio. However, the material carryover issue improves significantly after increasing the buffer capacity at the vacuum pump outlet or in the piping.

This can also lead to other chain reactions: pressure pulses are transmitted to the evaporation chamber, causing slight fluctuations in the chamber’s vacuum level, which in turn disrupts the evaporation temperature and causes the heat pump load to fluctuate; when pulses are transmitted to the sensor, they can also cause the vacuum detection signal to fluctuate. If the demister is severely clogged with liquid, some droplets may be drawn into the vacuum pump, accelerating the contamination and degradation of the pump oil.

It is important to distinguish between vacuum fluctuations caused by pipeline resistance and pulsation fluctuations resulting from an insufficient buffer tank. Insufficient pipeline resistance leads to overall negative pressure deficiency, which is a steady-state deviation; an undersized buffer tank causes high-frequency periodic pulsations, which can disrupt demisting conditions even if the average vacuum level meets process requirements.

In practical terms, buffer tanks should not be selected based solely on pipe diameter; they must be matched to the vacuum pump’s pumping speed and provide sufficient buffer volume. The buffer tank should be installed as close as possible to the evaporation chamber and positioned before the vacuum pump. For equipment where replacing the tank with a larger one is not feasible, additional parallel buffer chambers can be added, or damping components can be installed in the vacuum line to reduce the amplitude of the pulses. At the same time, avoid liquid accumulation at the bottom of the buffer tank; condensate buildup inside the tank reduces effective volume and further exacerbates pulsation issues, so proper drainage must be ensured.