What is Electric Heating Evaporator and Why Do We Use Them?

In the industrial manufacturing and production sector, boilers and evaporators are two essential pieces of equipment for generating heat and steam to support various technological processes. However, many people often confuse the two due to their shared purpose of “producing steam.” In reality, boilers and evaporators differ significantly in operating principles, application scope, operating temperatures, and technical characteristics.

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Understanding the differences between a boiler and an evaporator is not only crucial for selecting the right equipment but also for ensuring optimal performance and system safety.

1. Core Differences Between Boilers and Evaporators

1.1. Operating Temperature

Operating temperature is one of the key factors distinguishing boilers and evaporators, as it clearly reflects their respective technical nature and functions.

Boilers typically operate at much higher temperatures, ranging from approximately 150°C to over 540°C, depending on design pressure and process requirements. In conventional boiler systems, water is heated to its boiling point under a specific pressure (typically 6–40 bar for medium-pressure boilers or 60–100 bar for high-pressure systems), producing saturated steam. If heating continues after full vaporization, superheated steam is generated—steam with a temperature above its boiling point—commonly used in thermal power plants or industrial applications requiring dry, stable, high-energy steam.

These high temperatures pose risks such as thermal expansion, high-temperature corrosion, or pressure explosions. Therefore, boiler design must strictly comply with standards like ASME Section I, EN , or TCVN . Safety accessories such as safety valves, pressure gauges, and automatic temperature control systems are mandatory.

By contrast, evaporators operate at significantly lower temperatures, usually between 40°C and 120°C. This is because the purpose of an evaporator is not to generate high-pressure steam, but rather to evaporate solvent or water from a solution to recover solids, concentrate liquids, or separate components. Many evaporator systems even operate under vacuum conditions, lowering the boiling point to 40–60°C—an important feature when processing heat-sensitive fluids like enzymes, milk, or biological extracts.

Lower temperatures help conserve energy and preserve the chemical integrity of materials, but also demand efficient heat transfer systems, such as plate heat exchangers, shell-and-tube, or falling film evaporators. Heat is typically supplied via hot water, low-pressure saturated steam, or thermal oil, depending on the application.

In short, boilers represent high-temperature, high-pressure, superheated environments, while evaporators offer energy-efficient, low-temperature solutions ideal for industries like food processing, cosmetics, pharmaceuticals, and wastewater treatment.

1.2. Operating Principle

Though both devices generate vapor, their operating principles differ fundamentally in terms of heat transfer mechanisms, energy sources, and end-use applications.

Boilers operate based on the direct or indirect transfer of heat from fuel combustion to water, producing saturated or superheated steam. The thermal energy generated from the combustion of fuels such as coal, FO/DO oil, natural gas, biomass, or electric resistance heating is transferred through tube walls or fire tubes, heating the water within the system to generate steam.

Throughout the process, both convective and radiative heat transfer mechanisms are employed to optimize combustion efficiency. The steam generated is commonly directed to steam turbines, industrial dryers, or heating systems. This process requires precise control of pressure, temperature, and feedwater flow to prevent dry firing, boiler explosion, or steam contamination.

Evaporators, on the other hand, function by partially evaporating a solution through external heat supply, transferring energy across a heat exchange surface. Their main goal is not to produce steam for energy transfer, but to separate water or solvent from a mixture, concentrate solutes, or recover valuable products.

Types of evaporators include:

• Natural Circulation Evaporator – uses density differences within tubes.
• Forced Circulation Evaporator – uses pumps to circulate liquid and enhance heat transfer.
• Falling Film Evaporator – utilizes a thin film over the heat surface, ideal for heat-sensitive products.
• Multiple Effect Evaporator (MEE) – reuses vapor from earlier stages to reduce energy consumption.

Evaporators do not burn fuel directly, but instead receive heat from indirect sources: low-pressure steam, hot water, thermal oil, or electric heating. This makes them safer, easier to control, and more energy-efficient than boilers—particularly in applications where precise temperature regulation is critical.

2. Practical Applications

Boilers are indispensable in industries requiring large, stable, and continuous heat supply. The most common application is generating saturated or superheated steam to drive processes or rotate steam turbines in power plants. In the textile and dyeing industry, steam is used to heat dye baths, press, and dry fabric. In food processing, it is used for sterilizing, cooking, steaming, and preservation of products like milk, juice, and canned goods. The chemical industry employs steam for thermal reactions such as cracking, concentration, and refining. Other sectors like mechanical engineering, rubber, paper, and pulp use boilers for drying systems, mold heating, and thermodynamic reactions.

In contrast, evaporators are used in industries focused on water or solvent removal without the need for high temperature or pressure. Typical applications include:

Food & beverage: Concentrating milk, juice, fish protein, fish sauce, syrups, etc., to reduce volume and enhance shelf life.

Pharmaceuticals & cosmetics: Removing solvents from extract mixtures or recovering active compounds under low-temperature conditions to preserve bioactivity.

Water treatment & environmental engineering: In industrial wastewater treatment, evaporators help concentrate hazardous effluent, recover clean water, and reduce disposal costs.

Chemicals and minerals: Used for salt crystallization or solvent recovery in polymer production or intermediate chemical processing.

3. Size and Scale

Boilers are complex, high-pressure, high-temperature systems, often large and heavy, requiring permanent installation and significant operating space. A water-tube industrial boiler with a steam output of 10–100 tons/hour can be 10–25 meters long, weigh several dozens of tons, and require a dedicated foundation. In addition to the main body, auxiliary components include feedwater tanks, pumps, water treatment systems, safety valves, economizers, chimneys, control panels, and maintenance platforms. Boilers are installed in place, hard to relocate, and must comply with stringent pressure vessel codes (ASME, EN , TCVN , etc.).

Evaporators, in contrast, are much more flexible in size and configuration. They are typically modular, organized by stages or effects to optimize concentration and energy savings. Sizes vary widely, from 1–2 meter-high units for laboratories to 5–10 meter-tall industrial systems, but they are generally lighter as they do not need to withstand high-pressure steam.

Thanks to low- or vacuum-pressure operation, evaporators are often built with thin stainless steel (SS304/316L), easily integrated into cleanroom production lines, especially suitable for food and pharmaceutical applications.

Moreover, evaporators can be portable or assembled as skid-mounted systems, making maintenance and upgrading easier. This makes them an ideal choice for energy-saving and resource recovery systems, with lower capital cost and smaller installation footprint than boilers.

An evaporator is a type of heat exchanger device that facilitates evaporation by utilizing conductive and convective heat transfer, which provides the necessary thermal energy for phase transition from liquid to vapour. Within evaporators, a circulating liquid is exposed to an atmospheric or reduced pressure environment causing it to boil at a lower temperature compared to normal atmospheric boiling.

The four main components of an evaporator assembly are:

Heat is transferred to the liquid inside the tube walls via conduction providing the thermal energy needed for evaporation. Convective currents inside it also contribute to heat transfer efficiency.

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There are various evaporator designs suitable for different applications including shell and tube, plate, and flooded evaporators, commonly used in industrial processes such as desalination, power generation and air conditioning. Plate-type evaporators offer compactness while multi-stage designs enable enhanced evaporation rates at lower heat duties. The overall performance of evaporators depends on factors such as the heat transfer coefficient, tube/plate material properties, flow regime, and achieved vapor quality.

Advanced control techniques, such as online fouling detection, help maintain evaporator thermal performance over time. Additionally, computational fluid dynamics (CFD) modeling and advancements in surface coating technologies continue to enhance heat and mass transfer capabilities, leading to more energy-efficient vapor generation. Evaporators are essential to many industries because of their ability to separate phases through a controlled phase change process.

Some air conditioners and refrigerators use compressed liquids with a low boiling point that vaporizes within the system to cool it, whilst emitting the thermal energy into its surroundings.[1][2]

Evaporators are often used to concentrate a solution. An example is the climbing/falling film plate evaporator, which is used to make condensed milk.

Similarly, reduction (cooking) is a process of evaporating liquids from a solution to produce a "reduced" food product, such as wine reduction.

Evaporation is the main process behind distillation, which is used to concentrate alcohol, isolate liquid chemical products, or recover solvents in chemical reactions. The fragrance and essential oil industry uses distillation to purify compounds. Each application uses specialized devices.

In the case of desalination of seawater or in Zero Liquid Discharge plants, the reverse purpose applies; evaporation removes the desirable drinking water from the undesired solute/product, salt.[3]

Chemical engineering uses evaporation in many processes. For example, the multiple-effect evaporator is used in Kraft pulping,[4] the process of producing wood pulp from wood.

Large ships usually carry evaporating plants to produce fresh water, reducing their reliance on shore-based supplies. Steamships must produce high-quality distillate to maintain boiler-water levels. Diesel engine ships often utilize waste heat as an energy source for producing fresh water. In this system, the engine-cooling water is passed through a heat exchange, where it is cooled by concentrated seawater. Because the cooling water, which is chemically treated fresh water, is at a temperature of 70–80 °C (158–176 °F), it would not be possible to flash off any water vapor unless the pressure in the heat exchanger vessel is dropped.

A brine-air ejector venturi pump is then used to create a vacuum inside the vessel, achieving partial evaporation. The vapor then passes through a demister before reaching the condenser section. Seawater is pumped through the condenser section to cool the vapor sufficiently for condensation. The distillate gathers in a tray, from where it is pumped to the storage tanks. A salinometer monitors salt content and diverts the flow of distillate from the storage tanks if the salt content exceeds the alarm limit. Sterilization is carried out after the evaporator.

Evaporators are usually of the shell-and-tube type (known as an Atlas Plant) or of the plate type (such as the type designed by Alfa Laval). Temperature, production and vacuum are controlled by regulating the system valves. Seawater temperature can interfere with production, as can fluctuations in engine load. For this reason, the evaporator is adjusted as seawater temperature changes and shuts down altogether when the ship is maneuvering. An alternative in some vessels, such as naval ships and passenger ships, is the use of the reverse osmosis principle for fresh-water production instead of using evaporators.

Evaporation, or vaporization, is an endothermic phase transition process that is thoroughly understood in the field of thermodynamics. It is intimately related to the vapor pressure of the liquid and surrounding pressure, in addition to the enthalpy of vaporization.

Evaporators work using the same principle design. A heat source is in contact with the liquid causing it to evaporate. The vapor is removed entirely (like in cooking), or it is stored for reuse (like in a refrigerator) or a product for isolation (essential oil).

Rotary evaporators use a vacuum pump to create a low pressure over a solvent while simultaneously rotating the liquid flask to increase surface area and decrease bubble size. Typically, the vapor is passed over a cold finger or coil so that the vaporized material does not damage the pump. The rotary evaporator is best used for removing solvent from solutions containing the desired product that will not vaporize at the operating pressure to separate the volatile components of a mixture from non-volatile materials.

Natural circulation evaporators are based on the natural circulation of the product caused by the density differences that arise from heating (convection). A chamber containing a solution is heated, and the vaporized liquid is collected in a receiving flask.

This type of evaporator is generally made of 4–8 m (13–26 ft) tubes enclosed by steam jackets. The uniform distribution of the solution is important when using this type of evaporator. The solution enters the evaporator and gains velocity as it flows downward. This gain in velocity is attributed to the vapor being evolved against the heating medium, which also flows downward. This evaporator is usually applied to highly viscous solutions, so it is frequently used in the chemical, sugar, food, and fermentation industries.

This type of evaporator is useful in concentrating solutions.[5] The operation is very similar to that of a calandria where the liquid is boiled inside vertical tubes by applying heat to the outside of the tubes. The produced solvent vapor presses the liquid against the walls of the tubes forming a thin film that moves upwards with the vapor. The vapor may be released from the system while the liquid may be recirculated through the evaporator to further concentrate the solute. In many cases, the tubes of a rising film evaporator are usually between 3–10 metres (9.8–32.8 ft) in height with a diameter of between 25–50 millimetres (0.98–1.97 in). Sizing this type of evaporator requires a precise evaluation of the actual level of the liquid inside the tubes and the flow rates of the vapor and film.

Climbing and falling-film plate evaporators have a relatively large surface area. The plates are usually corrugated and are supported by the frame. During evaporation, steam flows through the channels formed by the free spaces between the plates. The steam alternately climbs and falls parallel to the concentrated liquid. The steam follows a co-current, counter-current path with the liquid. The concentrate and the vapor are fed into the separation stage, where the vapor is sent to a condenser. This type of plate evaporator is frequently applied in the dairy and fermentation industries since they have spatial flexibility. A negative point of this type of evaporator is its limited ability to treat viscous or solid-containing products. There are other types of plate evaporators that work with only climbing film.

Unlike single-stage evaporators, these evaporators can be composed of up to seven evaporator stages (effects). The energy consumption for single-effect evaporators is very high and is most of the cost for an evaporation system. Putting together evaporators saves heat and thus requires less energy. Adding one evaporator to the original decreases energy consumption by 50%. Adding another effect reduces it to 33% and so on. A heat-saving-percent equation can estimate how much one will save by adding a certain number of effects.

The number of effects in a multiple-effect evaporator is usually restricted to seven because, after that, the equipment cost approaches the cost savings of the energy-requirement drop.

Two types of feeding can be used when dealing with multiple-effect evaporators:

• Forward feeding: This occurs when the product enters the system through the first effect at the highest temperature. The product is then partially concentrated as some water is transformed into vapor and carried away. It is then fed into the second effect, which is slightly lower in temperature. The second effect uses the heated vapor created in the first stage as its heat source (hence the saving in energy expenditure). The combination of lower temperatures and higher viscosity in subsequent effects provides good conditions for treating heat-sensitive products, such as enzymes and proteins. In this system, an increase in the heating surface area of subsequent effects is required.
• Backwards feeding: In this process, the dilute products are fed into the last effect with the lowest temperature and transferred from effect to effect, with the temperature increasing. The final concentrate is collected in the hottest effect, which provides an advantage in that the product is highly viscous in the last stages, so the heat transfer is better.

In recent years, multiple-effect vacuum evaporator (with heat pump) systems have come into use. These are well known to be energetically and technically more effective than systems with mechanical vapor recompression (MVR). Due to the lower boiling temperature, they can handle highly corrosive liquids or liquids which are prone to forming incrustations.[6]

Agitated thin-film evaporation has been very successful with difficult-to-handle products. Simply stated, the method quickly separates the volatile from the less volatile components using indirect heat transfer and mechanical agitation of the flowing product film under controlled conditions. The separation is normally made under vacuum conditions to maximize ∆T while maintaining the most favorable product temperature so that the product only sees equilibrium conditions inside the evaporator and can maximize volatile stripping and recovery.[7]

Technical problems can arise during evaporation, especially when the process is used in the food industry. Some evaporators are sensitive to differences in viscosity and consistency of the dilute solution. These evaporators could work inefficiently because of a loss of circulation. The pump of an evaporator may need to be changed if the evaporator needs to be used to concentrate a highly viscous solution.

Fouling also occurs when hard deposits form on the surfaces of the heating mediums in the evaporators. In foods, proteins and polysaccharides can create such deposits that reduce the efficiency of heat transfer. Foaming can also create a problem since dealing with excess foam can be costly in time and efficiency. Antifoam agents are used, but only a few can be used when food is being processed.

Corrosion can also occur when acidic solutions such as citrus juices are concentrated. The surface damage caused can shorten the long life of evaporators. The quality and flavor of food can also suffer during evaporation. Overall, when choosing an evaporator, the qualities of the product solution must be taken into careful consideration.

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• Flash evaporation
• Vacuum evaporation
• Centrifugal evaporator
• Rotary evaporator[8]
• Vapor-compression evaporator
• Evaporative cooler
• Pumpable ice technology
• Circulation evaporator