Working Principle of Evaporators

An evaporator is a heat exchange device widely used in various industrial fields such as chemical engineering, food processing, pharmaceuticals, and seawater desalination. Its working principle is based on the physical phenomenon of liquid evaporation, where liquids transform into gases under specific conditions. This article explains the working principle of evaporators in detail.

Basic Principle

The core principle of an evaporator is to use heat to convert liquid into vapor, thereby achieving liquid concentration, drying, or separation. This process primarily involves two key aspects: heat transfer and phase change.

Typically, an evaporator consists of a heating chamber and an evaporation chamber. The heating chamber provides heat to raise the temperature of the liquid to be evaporated. When the liquid absorbs sufficient heat and reaches its boiling point, it begins to vaporize in the evaporation chamber, forming steam. After separation from the liquid, the steam is discharged from the evaporator, while the concentrated liquid (or residue) is discharged through the corresponding outlet according to process requirements.

Heat Transfer Modes

• Heat Conduction

This is a common heat transfer method in evaporators. For example, in shell-and-tube evaporators, the heating medium (such as steam) flows in the shell side, transferring heat to the liquid inside the tubes through the tube wall. The tube wall acts as a medium for heat conduction, raising the temperature of the liquid side and triggering evaporation. This method relies on the thermal conductivity of the tube wall material and the temperature difference between the heating medium and the liquid.

• Convective Heat Transfer

Convection also plays an important role. When heated, natural or forced convection occurs within the liquid. In natural convection, density differences due to uneven temperatures create a natural circulation, distributing heat more evenly to promote evaporation. Forced convection, usually driven by external devices like pumps or fans, accelerates heat transfer. For instance, in forced circulation evaporators, pumps circulate liquid at high speed inside the evaporator, enhancing contact and heat exchange between the liquid and the heating surface.

• Radiative Heat Transfer

Radiation may influence evaporation in specific types of evaporators, such as solar-powered ones, where solar radiation transfers heat to the liquid. However, it plays a minor role in most conventional evaporators.

Factors Affecting Evaporation Efficiency

• Temperature

Higher liquid temperatures accelerate molecular motion, reducing the heat required for boiling and increasing evaporation speed. Methods like raising the heating medium’s temperature or increasing the heating area can enhance efficiency.

• Pressure

Lowering pressure on the liquid surface reduces its boiling point, enabling evaporation at lower temperatures. Vacuum evaporators use this principle to save energy and protect heat-sensitive substances from high temperatures.

• Liquid Properties

Viscosity, surface tension, and specific heat capacity affect evaporation. High-viscosity liquids have poor fluidity and higher heat transfer resistance, slowing evaporation, while low surface tension liquids form uniform films, aiding heat transfer.

• Structure and Type

Evaporators with larger heat exchange areas or special designs (e.g., plate evaporators) promote turbulence and heat transfer, improving efficiency.

Working Principles and Characteristics of Different Evaporators

• Natural Circulation Evaporators

Rely on natural convection from density differences. They have simple structures, easy maintenance, and low operating costs but lower heat transfer efficiency, suitable for small-scale, low-speed applications.

• Forced Circulation Evaporators

Use pumps for forced liquid circulation, increasing flow rate and heat transfer coefficient. They offer high efficiency for large-scale, high-viscosity, or easily crystallized materials but have higher investment and energy consumption.

• Thin-Film Evaporators

Form liquid films on heating surfaces, flowing and evaporating quickly via gravity, centrifugal force, or scrapers. They boast high efficiency, prevent scaling, and suit heat-sensitive, high-viscosity materials but have complex structures and high maintenance costs.

• Multi-Effect Evaporators

Consist of series-connected evaporators, where secondary steam from the previous effect heats the next. This reuses heat, improving energy efficiency significantly.

Evaporators, through rational design and heat transfer optimization, play a vital role in industrial production by enabling efficient liquid processing and resource utilization.