In the food industry, concentrating foodstuffs requires removing water from the product, and evaporation is one of the most common methods. Devices that carry out this process are called evaporators, typically consisting of three main components: a heat exchanger, a separator, and a condenser. To maintain food quality while keeping energy costs low, high-vacuum evaporators are widely used. Depending on the food’s characteristics and the desired concentrate, either single-effect or multiple-effect evaporators are chosen. In this post, we’ll explore the latest advancements in single-effect evaporators, focusing on their applications and recent improvements.
Main Parts of Single-Effect Evaporators
A typical single-effect evaporator consists of three essential components: a heat exchanger, an evaporating section, and a separator. These parts work together to concentrate liquids by boiling off water, and in many systems, they are integrated into a single vertical cylinder. Let’s break down each part and their roles in the evaporation process.
Heat Exchanger
The heat exchanger, also known as the calandria, is crucial for transferring heat from a heating medium (like steam or hot water) to the liquid that needs to be evaporated. The heat exchanger maximizes heat transfer efficiency, ensuring that the evaporation process is effective. Common types of heat exchangers used in evaporation systems include:
- Shell-and-Tube Heat Exchanger: A cylindrical shell with a bundle of tubes where one fluid flows inside the tubes, and the other circulates outside.
- Plate Heat Exchanger: Made of a series of plates that create alternating channels for fluids, enhancing heat transfer through a larger surface area.
- Spiral Heat Exchanger: Features two fluids flowing in spirals, increasing heat transfer efficiency due to the large surface area.
The heat exchanger operates on the principle of conduction—hot fluid transfers heat through the walls of the tubes or plates to the liquid that’s being evaporated. This enables precise control over evaporation rates and ensures efficient use of energy.
Separator
The separator plays a key role in separating the vapor from the concentrated liquid. Positioned at the top of the evaporator, the separator allows the vapor to rise and be directed for condensation, while the denser liquid settles below. This phase separation is essential for optimizing the evaporation process and minimizing losses.
In some systems, a demister (or mist eliminator) may be included to remove any liquid droplets from the vapor stream, ensuring the vapor is as clean and dry as possible.
Condenser
The condenser is where the vapor is cooled and condensed back into liquid form. Its main function is to transfer heat from the vapor to a cooling medium, like water, causing the vapor to lose energy and turn back into a liquid. There are two primary types of condensers:
- Direct Condenser: The vapor directly contacts the cooling medium, which absorbs the heat and facilitates condensation. This type is highly efficient but requires a good water source.
- Indirect Condenser: The vapor doesn’t come into direct contact with the cooling medium. Instead, heat is transferred through walls or tubes, offering advantages like contamination prevention and energy efficiency.
Understanding these components helps in selecting the right evaporator design based on the type of liquid, energy requirements, and process specifications. By optimizing each part, single-effect evaporators provide an efficient and effective solution for concentrating liquids in industries like food processing.
Evaporator: (A) single-effect and (B) multiple-effect
Types of Single-Effect Evaporators
The main types of single-effect evaporators include pan and batch evaporators, short tube evaporators, rising film evaporators, falling film evaporators, agitated film evaporators, forced circulation evaporators, and plate evaporators. Each type is designed to suit different operational requirements, such as handling varying viscosities, heat sensitivity, and fouling tendencies of the liquids being concentrated. These evaporators differ in their design, heat transfer mechanisms, and the nature of the liquid they are best suited for.
Pan and Batch Evaporators
The pan and batch evaporator is the simplest type of evaporator. It consists of a chamber or pan where the food is heated, surrounded by a network of tubes or plates carrying hot steam. As evaporation occurs, vapor is either released directly or directed to a condenser. Control valves regulate the vapor flow. Once the desired concentration is reached, heating stops, and the product is pumped out. This process typically happens at atmospheric pressure, except when dealing with heat-sensitive products, where a vacuum system is used to preserve product quality.
Short Tube Evaporators
Short tube evaporators improve upon pan and batch systems by increasing heat transfer efficiency. This is achieved through a series of vertically installed tubes inside a vertical pipe. These tubes significantly boost heat transfer between the liquid and the heating medium. These evaporators are simple, affordable, and easy to maintain. Common applications include sugarcane juice and salt concentration. The relatively long residence time inside the evaporator makes it ideal for processes requiring gradual concentration.
Rising Film Evaporators
Rising film evaporators are particularly well-suited for handling heat-sensitive or viscous liquids, as they offer efficient evaporation with minimal risk of product degradation. In this system, the liquid to be concentrated enters from the bottom of vertical tubes, where it forms a thin film on the tube walls. The liquid then flows upward, aided by gravity or pump-assisted distribution, while heat is applied to the outer surface of the tubes. This heating causes the liquid film to evaporate.
As the liquid rises, the vapor, being less dense, moves in the opposite direction in a countercurrent flow, enhancing heat transfer efficiency. This flow pattern ensures that the temperature difference between the rising vapor and the descending liquid film accelerates the evaporation process. The evaporator’s design maximizes heat and mass transfer by keeping the liquid in close contact with the tube walls, where the heat is applied.
One of the key advantages of rising film evaporators is their short residence time. The liquid moves quickly through the tubes, reducing the risk of product degradation, which is critical when concentrating heat-sensitive products. Additionally, the evaporator is easy to clean and maintain, making it an efficient choice for continuous operations.
Rising film evaporators are commonly used in industries where high evaporation rates are necessary, such as in the concentration of juices, milk, and other heat-sensitive liquids. Their design allows for high performance in terms of evaporation capacity and product quality.
Falling Film Evaporators
In a falling film evaporator, the liquid flows downward along the inner surfaces of vertical tubes, in contrast to the upward flow in rising film evaporators. The steam, however, still moves upward in the same direction as in the rising film system. This gravity-driven flow allows the liquid to move faster compared to rising film evaporators, which is particularly beneficial when dealing with viscous fluids.
At the bottom of the evaporator, a vapor-liquid separator is positioned to collect the vapor and direct it to a condenser for cooling and condensation. To aid in the separation of vapor from the liquid, a vacuum system is often employed, ensuring efficient recovery of the vapor phase.
The key advantage of the falling film design is the low residence time—typically between 20 to 40 seconds—which helps preserve the quality of heat-sensitive products like milk and fruit juices. However, to achieve proper concentration, it is often necessary to recirculate the liquid for further evaporation, as the short residence time may not be enough for complete concentration.
Designing a falling film evaporator requires careful attention to ensure the liquid maintains a thin film on the tube walls during its downward flow. If the film becomes too thick, it can impair heat transfer efficiency. Operators need to manage viscosity, feed flow rate, and operating temperature to maintain optimal performance. This type of evaporator requires a skilled operator to adjust these parameters and ensure smooth operation.
Falling film evaporators are widely used in industries that process heat-sensitive or viscous liquids, where rapid evaporation is required without compromising the integrity of the product. They are particularly favored for applications in food processing, where precise control over evaporation and minimal product degradation are paramount.
Agitated Film Evaporators
For products with high viscosity and solid characteristics—such as creamy vegetables and fruit pastes—the agitated film evaporator is the ideal solution. These products tend to cause fouling, a process where substances accumulate on the heat transfer surfaces, reducing heat efficiency and ultimately affecting product quality. Unlike rising or falling film evaporators, the agitated film evaporator uses continuous stirring to prevent fouling.
Inside the evaporator, a rotor with multiple blades ensures that the liquid stays evenly distributed as a thin film along the walls, allowing regular cleaning and preventing blockages. The steam and feed movements in this system are similar to the falling film evaporator, but the key difference is the rotor’s stirring action. With a short residence time of less than 2 minutes, the agitated film evaporator efficiently handles high-volume production of viscous products, ensuring consistent heat transfer and preventing fouling.
By employing this design, manufacturers can manage the challenges of working with thick, sticky products while maintaining both quality and process efficiency. This system is ideal for high-volume production in industries like food processing, where continuous stirring is critical to product consistency.
Forced Circulation Evaporators
A forced circulation evaporator uses external mechanical energy, typically from a pump, to boost the circulation speed of the liquid being concentrated. By increasing the velocity of the liquid within the tubes, this design helps prevent fouling, which can impede heat transfer and reduce evaporation capacity. The circulation velocities in these systems typically range from 7 to 20 ft per second (about 2 to 6 meters per second).
These evaporators consist of several key components: a shell-and-tube heat exchanger (which can be oriented horizontally or vertically), a separator located above the heat exchanger, and a circulating pump. To prevent premature boiling, a hydrostatic head is maintained above the top tube plate. Inside the calandria, the product is superheated at a higher pressure than its normal boiling pressure. Upon entering the separator, where pressure is slightly reduced, the liquid flashes into vapor.
To minimize temperature increases across the tube bundle (often limited to 2–3°C), the evaporator maintains a high recirculation ratio—typically between 100 and 150 kg of liquid per kg of water evaporated. The high recirculation rate ensures high liquid velocity through the tubes, preventing deposit formation or crystallization on the heating surface. As a result, forced circulation evaporators achieve high heat transfer coefficients.
While these evaporators are highly effective, they come with higher initial and operating costs. They are more expensive than film evaporators due to the need for large-bore piping and powerful recirculating pumps. Additionally, the longer residence time of the liquid within the heating zone is a limitation. Forced circulation evaporators are ideal for handling viscous liquids, feeds containing solids, and liquids prone to crystallization or fouling.
Plate Evaporators
Plate evaporators resemble plate heat exchangers but are specifically designed to handle the boiling of viscous liquids and efficiently isolate the water vapor produced during heating. These evaporators operate on the same principles as long-tube evaporators, but their compact design offers several advantages, including easier installation, cleaning, and maintenance.
In a plate evaporator, multiple interconnected plates with gaskets between them facilitate both steam and liquid product flow. As the liquid moves through each compartment, it gradually reaches a vapor separator, where the vapor is separated and directed to the condenser or recirculated for the next cycle. The compact structure allows for rapid heating, making plate evaporators ideal for high-throughput operations.
These evaporators are particularly effective for heat-sensitive materials, such as milk, which is why they are widely used in the dairy industry. Their combination of efficiency, space-saving design, and suitability for heat-sensitive product concentration makes them a preferred choice in various food processing applications.
Advantages and Disadvantages of Single-Effect Evaporators
Single-effect evaporators offer a range of benefits, making them an appealing choice for many industries. From simplicity in design to cost-effectiveness and versatility, they are a practical solution for evaporating liquids across diverse applications, including food processing, pharmaceuticals, and chemicals.
Advantages:
- Simplicity: Single-effect evaporators have a simple design and operating principle, making them easy to understand and operate. With just a single heat exchanger, they are less complex and have fewer components, which reduces maintenance needs.
- Cost-Effective: These evaporators are generally more affordable in terms of both capital and operational costs compared to multieffect evaporators. Their simplicity makes them a popular choice for small-scale operations or when steam costs are relatively low.
- Energy Efficiency: While not as energy-efficient as multieffect evaporators, single-effect systems still offer significant energy savings compared to other evaporation methods. They efficiently use steam for heat transfer and concentrate the liquid effectively.
- Versatility: Single-effect evaporators can handle a variety of feed materials, from low- to high-viscosity liquids, as well as heat-sensitive substances. This makes them ideal for concentrating products in industries such as food, chemicals, and pharmaceuticals.
- Ease of Operation and Maintenance: Their straightforward design makes single-effect evaporators easy to operate and maintain. Cleaning and inspection processes are simplified, helping reduce downtime and ensure consistent performance.
- Flexibility: These evaporators can operate at ambient pressure, making them suitable for heat-sensitive materials that need to be evaporated at lower temperatures. They can also be equipped with vacuum systems for reduced-pressure evaporation, offering additional flexibility for specialized products.
- Scalability: Single-effect evaporators can be adapted to different production capacities. They can be scaled up or down with minimal adjustments to the design, providing flexibility as production needs change.
Disadvantages:
Despite these benefits, single-effect evaporators do have limitations. One of the main drawbacks is their lower evaporation rate compared to more advanced multieffect evaporators. Therefore, for processes requiring high evaporation rates and energy savings, multieffect systems may be a better choice. Additionally, while they are versatile and easy to operate, the choice of a single-effect evaporator depends on factors like the desired output, energy efficiency goals, product properties, and budget constraints.
Conclusion
Single-effect evaporators are essential in various industries, particularly in food processing, where they are used to concentrate liquids efficiently. The different types of single-effect evaporators—such as pan and batch, short tube, rising film, falling film, agitated film, forced circulation, and plate evaporators—offer diverse solutions for handling different feed materials, from heat-sensitive liquids to viscous products. These systems provide advantages like simplicity, cost-effectiveness, versatility, and ease of operation, making them ideal for small-scale and medium-scale applications where energy efficiency and process flexibility are key.
However, single-effect evaporators also have limitations, including lower evaporation rates compared to multi-effect evaporators and a reduced capacity for large-scale production. Despite these drawbacks, the appropriate selection and design of the evaporator type can mitigate these challenges, ensuring optimal performance for specific operational needs. Overall, single-effect evaporators remain a vital technology in many industries, offering a reliable and efficient solution for liquid concentration across various production scales.