Working Principles of Different Types of Evaporators

Central Circulation Tube Evaporator

Principle: Natural circulation is driven by the density difference between the heating tubes and the central circulation pipe. The solution inside the heating tubes (small tubes) vaporizes when heated, reducing its density. The non-boiling liquid in the central circulation pipe (large tube) has higher density, creating a descending-ascending circulation flow.

Features: Compact structure, stable operation, but low circulation speed (0.4–0.5 m/s). Suitable for low-viscosity, non-strongly corrosive solutions.

Suspended Basket Evaporator

Principle: An improvement over the central circulation tube design, featuring a suspended heating chamber. The solution descends through the annular gap channel while circulating upward through the heating tubes. The enlarged annular gap channel area increases circulation velocity to 1–1.5 m/s.

Features: Removable heating chamber facilitates cleaning; suitable for solutions containing crystalline precipitates; however, material consumption is high.

Dry Shell-and-Tube Evaporator

Principle: The refrigerant flows through the tubes and evaporates, achieving multi-pass heat exchange via end cover partitions to absorb heat from the shell-side secondary coolant (such as water).

Features: Requires minimal refrigerant charge, suitable for small refrigeration systems, but demands precise flow control. Flooded shell-and-tube evaporator

Principle: Liquid refrigerant fills the shell side and directly exchanges heat with the secondary coolant (such as water) in the tube side. The resulting gas enters the compressor directly, achieving high heat transfer area utilization.

Features: High efficiency, but requires a large refrigerant charge and is susceptible to performance degradation due to residual lubricant.

Fall Film Evaporator

Principle: Liquid flows down the tube wall as a uniform film formed by the distributor. Evaporation occurs through contact with the heated surface under gravity, and the vapor-liquid mixture enters the separation chamber.

Features: Rapid evaporation rate, low energy consumption, suitable for high-viscosity or easily fouling liquids (such as fruit juices and chemical solutions).

Rising Film Evaporator

Principle: Liquid enters the heating tube from the bottom, rapidly vaporizes upon heating to form an ascending vapor stream, and carries the unevaporated liquid upward as a thin film.

Features: Suitable for low-viscosity, highly heat-sensitive liquids (such as milk). Requires control of the length-to-diameter ratio (L/d = 100–250) to maintain a stable liquid film.

Forced-Circulation Evaporator

Principle: Forced circulation of the solution via an external pump overcomes the density gradient limitations of natural circulation, making it suitable for high-viscosity, crystallization-prone, or high-boiling-point solutions.

Features: Controllable circulation speed (up to 2–3 m/s), but relatively high energy consumption and requires regular maintenance of power components.