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.
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 12952, or TCVN 7704. 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 12952, TCVN 7704, 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.
4. Boiler or Steam Generator – Which One Best Suits Your Process Needs?
When designing and investing in an industrial thermal system, choosing between a boiler and a steam generator (evaporator) is a strategic decision. Both types of equipment serve the common purpose of producing steam for industrial processes. However, differences in operational mechanisms, technical conditions, and application suitability mean that they are not interchangeable in every situation.
4.1. Understanding the Essence: Steam is Not Always the Same
A boiler generates steam by directly heating water under high pressure. It is commonly used in factories that require large, stable amounts of high-pressure steam, such as in wood processing, textile dyeing, or for driving turbines to generate electricity. Thanks to its ability to rapidly and powerfully generate steam, the boiler acts as the “heart” of many large-scale industrial systems.
In contrast, a steam generator does not produce steam through direct combustion but instead uses indirect heating—usually via a heat transfer fluid such as thermal oil. This equipment is more suitable for processes that require clean steam, light steam, and precise temperature control, such as in the food, pharmaceutical, cosmetic, or sensitive chemical industries. The steam from a steam generator is usually at low pressure, close to atmospheric level, but offers high quality and stability.
4.2. Steam Generator in Thermal Oil Systems: A Safe and Flexible Solution
In many modern factories, steam generators are integrated as part of a thermal oil system. Instead of feeding water into a boiler to generate pressure directly, this system uses a thermal oil heater to heat the oil, which then transfers heat through a heat exchanger surface to a separate water tank—the steam generator. The water is heated indirectly and evaporates, without coming into direct contact with flames or combustion gases.
This solution offers several safety advantages, especially in environments that require cleanliness or involve flammable substances. Additionally, it allows for more flexible temperature control, reduces system pressure, and minimizes corrosion caused by impure water.
4.3. When Should You Choose a Boiler?
If your process requires a large amount of high-pressure steam for drying, rapid heating, or transmitting energy over long pipelines, a boiler is the optimal choice. It is a powerful solution for heavy industries such as textile dyeing, wood processing, large-scale food production, or in-house electricity generation. Boilers are also easier to maintain in environments that do not have strict steam cleanliness requirements.
However, boilers operate under high pressure, which means they are subject to stricter requirements for operation, monitoring, certification, and periodic safety inspections.
4.4. When Should You Choose a Steam Generator?
In applications that demand precise temperature control, clean steam without impurities, or evaporation of heat-sensitive solvents, a steam generator is the ideal choice. These devices are commonly used in distillation, concentration, indirect sterilization, or solvent evaporation in chemical processes.
With an integrated thermal oil heating system, a steam generator can operate with consistent efficiency, minimize the risk of fire or explosion, and ensure sanitary safety for the end product.