Pressure Relief via Emptying vs. Shutdown with Pressure Maintenance: Differences in Fouling Rate

Shutting down with pressure relief and emptying leads to a significantly faster fouling rate, and the resulting scale is harder and more difficult to clean; fouling is much slower during shutdown with pressure maintenance, and the scale layer is also looser.

The core difference lies in: emptying causes the heat exchange surface to dry and crystallize, while pressure maintenance keeps the equipment continuously wet and submerged, preventing dry-wall fouling.

## Shutdown with Pressure Relief and Emptying (Breaking Vacuum, Open Discharging)

After shutdown, the vacuum is broken and the feed liquid is emptied, allowing the interior of the equipment to communicate with the atmosphere and the temperature to drop gradually.

A thin film of residual feed liquid forms on the heat exchange tubes and the walls of the evaporation chamber. As air enters, moisture evaporates rapidly, the residual liquid is continuously dried and highly concentrated, and substances such as salts, colloids, calcium, magnesium, and silicon directly crystallize and harden on the metal surface, forming dense, strongly adherent hard scale.

Moreover, this aged scale acts as crystal seeds; upon the next startup, new fouling preferentially grows on it, accelerating the fouling rate and causing cumulative fouling.

Fouling is particularly severe in areas above the liquid level, such as tube walls, the top of the evaporation chamber, and demisters.

Additionally, since the feedstock contains surfactants and colloids, emptying and drying tend to form composite hard scale consisting of oil, scale, and colloids, which is extremely difficult to remove even with high-pressure water.

## Shutdown with Pressure Maintenance (Maintaining Vacuum, Sealing, and Filling with Feed Liquid)

During shutdown with pressure maintenance, the system remains sealed and maintains a certain vacuum level, isolated from the atmosphere, preventing moisture loss and evaporation.

The heat exchange surface is constantly immersed in liquid with no dry areas. The concentration of residual feed liquid remains largely unchanged without excessive concentration, thus avoiding massive precipitation of crystalline hard scale.

Only a small amount of loose deposits form without adhering firmly to the equipment surface. These deposits disperse easily during circulation upon the next startup. The fouling rate is much lower than that of the emptying method, significantly reducing overall fouling and cleaning frequency.

## Straightforward Summary

Shutdown with emptying is equivalent to "baking a layer of crystalline hard shell" on the inner wall of the equipment during each shutdown, resulting in fast-growing and stubborn fouling.

Shutdown with pressure maintenance keeps the equipment submerged in liquid at all times, preventing drying and hardening, leading to slow fouling with soft deposits.

For high-salinity, easily fouling feedstocks containing surfactants, pressure maintenance is strongly preferred over emptying, as the difference in fouling will be highly significant.