About Multiple-Effect Evaporators - Introduction, Working Principles, Advantages, and How to Choose

Multiple-effect evaporators have been an important component in modern industrial processes. As industries constantly seek ways to optimize energy use and enhance operational efficiency, multiple-effect evaporators become more crucial.

This guide aims to provide a comprehensive understanding of how these systems work, their advantages, and the critical considerations for their effective implementation. We'll explore the ingenious design principles behind these evaporators and how they significantly changed energy efficiency and cost-effectiveness in various industrial applications.

Multiple-Effect Evaporators Introduction

The "Multiple-Effect Evaporator", also called MEE Evaporator, is a system that enhances energy utilization by reusing the secondary steam generated from one effect as the heat source for the subsequent effect. This principle, when applied with technologies like thermal compression and mechanical compression, significantly reduces energy consumption in multi-effect evaporators, making their efficiency far superior to that of single-effect evaporators.

Multiple-effect evaporation is extensively used in numerous industries, including food, chemical, dairy, fermentation, gelatin production, and environmental protection.

Multiple-Effect Evaporator Working Principles - MEE Tech

In multiple-effect falling film evaporators, the steam generated by heating the first effect is not sent to the condenser but is reused as a heating medium for the second effect. This approach can reduce fresh steam consumption by about 50%. Utilizing this principle repeatedly can further decrease fresh steam consumption. The total temperature difference between the highest heating temperature in the first effect and the lowest boiling point in the last effect is distributed across each effect, with the temperature difference per effect decreasing as the number of effects increases. Therefore, to achieve a specified evaporation rate, the heating area must be expanded.

Preliminary estimates suggest that the heating area required for all effects increases proportionally with the number of effects. As a result, while steam savings gradually decrease, investment costs significantly increase.

Key Components of Multiple-Effect Evaporator

A standard multiple-effect evaporator setup comprises preheaters, evaporators, condensers, separators, vacuum systems, pumps, pipe fittings and valves, and control systems. Due to their high energy efficiency and stable operation, these systems are increasingly accepted by a growing number of users and industries. However, the performance of multiple-effect evaporation systems can be affected by various factors, including the physical properties of the material, altitude, process parameters, and operational practices.

Advantages of Multiple-Effect Evaporation

1. This technology employs multiple effects under vacuum conditions for evaporation. Its main advantages include:

2. Lowering the boiling point of solutions under vacuum;

3. Greater heat transfer driving force compared to atmospheric pressure, which results in reduced heat transfer area;

4. Suitability for processing thermally sensitive materials that might decompose, polymerize, or deteriorate at higher temperatures;

5. Lower evaporation temperatures, leading to minimal material corrosion and thermal loss;

6. Use of low-pressure or low-grade steam as a heating source, improving energy utilization.

How to Choose an Effective Multiple-Effect Evaporator

1. Number of Effects

Consider the size of the treatment capacity, the boiling point elevation of the material being evaporated, and the number of equipment units. Larger capacities favor multiple-effect operations; more effects mean less steam consumption but higher equipment investment; a larger boiling point elevation suggests a smaller effective temperature difference, thus reducing the number of effects needed.

2. Selection of Barometric Condensers

When recovering exhaust steam (secondary steam from the last effect), indirect condensers like shell and tube or spiral plate heat exchangers should be used. If exhaust steam recovery is not needed, direct condensers like atmospheric condensers or water jet pump condensers are suitable. When the last effect operates at atmospheric pressure, exhaust steam can be directly vented without condensing.

3. Process Flow Selection

Co-current operation reduces the pressure in subsequent effects compared to earlier effects, reducing the power required for pumps between effects. This also causes flash evaporation when moving from a higher to a lower temperature effect, thus reducing fresh steam consumption. However, co-current flow results in higher concentration, lower temperatures, increased viscosity, and reduced heat transfer rates in later effects. The counter-current operation has the opposite effect, requiring more power for inter-effect pumps and increasing steam consumption. It is not suitable for thermally sensitive materials or materials whose corrosiveness increases with temperature and concentration. Crossflow and mixed flow can also be considered based on specific material properties.

4. Heating Area Selection

The determination of the heating area for evaporators is based on a comprehensive consideration of material balance, heat balance, heat transfer calculations, and the chosen process flow.

Multiple-effect evaporators can be categorized into standard systems and forced circulation systems, with the latter incorporating multiple external circulation evaporators and additional external circulation pumps.

Generally, a double-effect evaporator system with a heat pump has similar energy consumption to a triple-effect system, allowing for increased flow velocity in the evaporator tubes, enhanced heat transfer, and increased evaporation intensity. Forced circulation evaporators are suitable for scenarios involving crystallization during the evaporation process and non-thermally sensitive materials.

The implementation of multiple-effect evaporators proves the ingenuity and progress in industrial technology, offering a pathway to sustainable and efficient production processes. As we have mentioned above, they significantly reduce energy consumption while maintaining high operational standards. MEE evaporators help enterprises to stay competitive and environmentally responsible.

If you are interested in further information or want to make informed decisions, you could contact us. Vnor has on providing Evaporation and Crystallization solutions since 2003. Our expert team is always ready to help.