Evaporators, as core equipment in evaporation processes, directly affect evaporation efficiency through their structure and performance. Typically composed of a heating chamber and a separation chamber, evaporators have undergone technological evolutions, from early jacketed or coiled types to modern vertical long-tube liquid film evaporators and scraped film evaporators, each representing technical progress.

Circulation-type Evaporators

Based on solution flow characteristics, indirectly heated evaporators are categorized into circulation-type and single-pass types. Circulation-type evaporators rely on natural density differences or external power for solution circulation, including natural and forced circulation subtypes.

Central Circulation Tube Evaporators

A key circulation-type evaporator, it features a central circulation tube in the heating chamber. The cross-sectional area of this tube accounts for 40–100% of the total heating tube area. When heated, the liquid in heating tubes, with larger heat-receiving area, has lower density, creating a density difference that drives natural circulation: liquid descends through the central tube and rises through heating tubes. With compact structure and easy operation, it is widely used but has low circulation speed (<0.5 m/s) and reduced effective temperature difference due to constant solution concentration near the finished product.

Basket-type Evaporators

An improved version of central circulation tube evaporators, their heating chamber hangs like a basket in the shell, facilitating top cleaning. The annular gap between the heating chamber and shell acts as a circulation channel (100–150% of heating tube area), enabling higher circulation speeds (1–1.5 m/s). However, they still face issues like limited speed and difficult maintenance.

Forced Circulation Evaporators

These use pumps to drive solution circulation, achieving much higher speeds (1.5–3.5 m/s) than natural circulation. This enhances heat transfer and reduces crystal precipitation or scaling by minimizing temperature gradients in heating tubes. Despite higher investment and pump maintenance needs, they excel in handling large-scale or high-viscosity materials.

Single-pass Evaporators

Single-pass evaporators form a liquid film on heating tube walls, achieving desired concentration in one pass with short residence times (seconds to over ten seconds), ideal for heat-sensitive materials.

Rising Film Evaporators

Composed of vertical long tubes (25–50 mm diameter, length-diameter ratio 100–150), they use external steam condensation to drive liquid upward in a film, suitable for high-viscosity solutions.

Falling Film Evaporators

Raw liquid is added from the top of heating tubes, flowing downward as a film by gravity while being concentrated. Vapor-liquid mixtures gather at the bottom and enter the separation chamber, suitable for concentrated solutions.

Rising-Falling Film Evaporators

Combining rising and falling film designs, raw liquid first rises in the rising film chamber, then descends in the falling film section, adapting to scenarios with significant viscosity changes.

Scraped Film Evaporators

Rotating scrapers distribute liquid evenly on tube walls, accommodating high-viscosity, heat-sensitive, easily crystallized, or scaled materials. They can even fully dry solutions to produce solid products at the bottom.

Other Evaporator Types

Direct Contact Heat Transfer Evaporators

Fuel (e.g., gas, heavy oil) burns with air to generate high-temperature flue gas, which is directly injected into the solution for efficient heat transfer, suitable for corrosive materials.

Submerged Combustion Evaporators

The combustion chamber is submerged in the solution, with high-temperature flue gas injected to rapidly boil and vaporize the liquid, ensuring efficient heat transfer.

Evaporator technologies continue to evolve, with each type offering unique advantages to meet diverse industrial needs, from handling heat-sensitive materials to corrosive or high-viscosity solutions.