Falling Film Evaporators: A Complete Guide
By Jon Thompson, extraktLAB
Last updated: October 27, 2026
A falling film evaporator removes solvent from a liquid by letting it run down the inside of heated vertical tubes as a thin film. The film boils at a low temperature, the vapor leaves through a separator at the bottom, and the concentrated liquid is collected.
What Is a Falling Film Evaporator?
A falling film evaporator (FFE) is a type of evaporator that utilizes gravitational force to spread a thin layer of liquid over a heated surface, promoting efficient heat transfer. The goal of this process is to concentrate a liquid by evaporating its solvent through controlled heating.
Overview of Falling Film Evaporation Process
The evaporation process in a falling film evaporator starts with distributing the feed liquid at the top of a vertical tube. This liquid forms a film along the inner surface as it flows downward under gravity’s influence. As it flows downward, the liquid undergoes partial vaporization. As a result, a concentrated liquid emerges from the bottom while vapor exits separately.
How Falling Film Evaporators Work
At the core of the falling film evaporator’s operation is the heat exchanger, typically containing numerous vertical tubes. Heat supplied to the exchanger causes the thin film of fluid to boil as it flows down. Due to how thin the film is, heat transfer rates are remarkably high. Therefore, less heating is required, which translates into energy savings. A distributor at the top of the evaporator ensures the liquid’s even distribution, a critical step that prevents dry spots and maximizes the tube’s surface area effectiveness. Separators at the end of the tubes separate the vapor from the concentrated fluid, allowing both to be collected efficiently.
- The widespread use of FFEs in diverse industries stems from their energy efficiency.
- Manufacturers select FFEs when gentle treatment of heat-sensitive materials is paramount.
- By varying the feed rate and heating temperature, the process can be finely tuned to the properties of the liquid being concentrated.
- Including features like a vapor separator ensures the delivery of a uniform final product.
Diverse in application, falling film evaporators meet the rigorous demands of industries from pharmaceuticals to food processing. Each implementation of this technology streamlines the concentration process through its sophisticated design and automation capabilities. As a result, they have become common in manufacturing setups requiring both precision and energy-conscious operations.
Falling film evaporator working principle, step by step
- The feed liquid is pumped to the top of the evaporator and enters a distributor.
- The distributor spreads the liquid evenly over the inside wall of each vertical tube.
- Gravity pulls the liquid down as a thin film. Heat passes through the tube wall from the hot fluid on the outside.
- The solvent boils out of the film. Vapor and liquid travel down together, and the moving vapor helps drag the film along.
- At the bottom, a separator splits the vapor from the concentrated liquid.
- The vapor goes to a condenser and turns back into liquid. The concentrate is collected, or sent around again for another pass.
Falling Film Evaporator Design
Key Components of a Falling Film Evaporator
Tube Bundle – The falling film evaporator consists of a bundle of parallel tubes welded together at each end and encased in a jacket. This jacket allows heat transfer fluid to circulate across the outside of the tubes. The design, also known as a shell and tube evaporator, commonly features vertical tube bundles, though horizontal configurations are also possible.
Heat Transfer and Fluid Distribution – In the vertical configuration, the solution to be evaporated is distributed inside the tubes while the heat transfer fluid circulates outside. The distribution is engineered to ensure that only the inside of the tubes contacts the solution, facilitating efficient evaporation.
Vaporization and Condensation – As the solution is heated within the tubes, its components evaporate. The resulting vapor is then directed to a condenser, which condenses it back into a liquid form. Design Considerations for Efficiency – Falling film evaporators are designed to handle free-flowing extracts that coat the inside of the tubes under gravity without causing blockages, known as fouling. The system’s capacity increases with the surface area of the evaporator, which can be enhanced by increasing the number of tubes or using smaller diameter tubes.
How to Optimize Falling Film Evaporator Design for Efficiency
Optimizing the design of a falling film evaporator can be approached from several angles. Uniform distribution of the feed across the tube bundle greatly affects performance. A well-designed distributor can minimize wetting issues and promote consistent film thickness. Utilizing advanced materials for the heat transfer surface can improve heat transfer and enhance corrosion resistance. Engineers can adjust tube length and diameter to meet specific requirements, balancing efficiency with spatial restraints. Moreover, including advanced controls stabilizes operations even with fluctuating feed conditions, directly contributing to the falling film evaporator’s energy efficiency and overall performance.
Falling film evaporator design basics
A falling film evaporator is sized in four steps.
- Heat needed. Add two amounts: the heat to warm the feed to its boiling point, and the heat to boil off the solvent. Warming = feed flow × specific heat × temperature rise. Boiling = amount evaporated × latent heat of vaporization. The result is a power, in kilowatts (kW).
- Heat transfer area. Area = heat needed ÷ (overall heat transfer coefficient × temperature difference). The coefficient (U) is in watts per square meter per kelvin (W/m²·K). The temperature difference is between the heating fluid and the boiling liquid. The result is in square meters (m²).
- Number of tubes. Tubes = area ÷ (π × tube inside diameter × tube length). One tube’s surface area is its circumference times its length.
- Wetting check. Divide the feed flow by the total length around the inside of all the tubes. This is the wetting rate, in kilograms per second per meter (kg/s·m). It has to stay above the minimum for your liquid, or dry patches form on the wall.
Tube length and diameter then get adjusted until the area, wetting and building height all work together. Residence time in the tubes is short, usually from seconds to a minute or two.
Our engineers can size a falling film evaporator for your feed. Talk to an expert.
Falling Film Heat Exchanger
Heat exchangers are the nucleus of falling film evaporators, streamlining the thermal energy transfer needed for evaporation. A falling film evaporator manages this thermal exchange, where the working fluid absorbs heat as it flows down the vertical tubes, resulting in the evaporation of the liquid inside.
The Role of Heat Exchangers in Falling Film Evaporators
A heat exchanger in a falling film evaporator allows the distribution and absorption of heat, which causes the evaporation of the volatile components from a liquid mixture. The thin film is optimal for heat transfer and ensures that the thermal energy provided by the heating medium is efficiently utilized.
How a Falling Film Heat Exchanger Enhances Performance
The design of a falling-film heat exchanger ensures several performance enhancements. Such designs promote rapid heat transfer by generating a thin film of liquid over the heat exchange surfaces. This thin film increases the evaporation rate and reduces the product’s residence time within the system, consequently minimizing the thermal degradation of heat-sensitive materials.
- A falling film evaporator typically operates under lower temperature gradients, thus conserving energy.
- Given their larger surface area for heat exchange, such evaporators present improved thermal efficiency.
- Such systems support the continuous processing of feedstock, a definitive boon for large-scale industrial applications.
The low residence time, coupled with the high surface area to volume ratio in falling film heat exchangers, substantially diminishes the risk of fouling. These factors combine to create a more congenial environment for various heat-sensitive substances, securing their integrity during evaporation procedures.
Falling film vs wiped film vs rising film vs rotovap
These four methods all remove solvent with heat and, usually, vacuum. They differ in how the liquid is spread into a thin layer, and that decides which liquids each one handles well.
| Feature | Falling film | Wiped film | Rising film | Rotary evaporator (rotovap) |
|---|---|---|---|---|
| How the liquid is spread | Gravity pulls it down the inside of vertical tubes | Rotating blades spread it over a heated wall | The liquid boils at the bottom of the tubes and the rising vapor pushes it up the wall | A flask turns in a heated bath and coats its own inner wall |
| Moving parts touching the liquid | None | Rotor and blades | None | The turning flask |
| Free-flowing, thin liquids | Good fit | Works | Good fit | Works |
| Thick or sticky liquids | Poor fit | Good fit | Poor fit | Limited |
| Time the liquid spends hot | Short | Very short | Short | Longer (batch) |
| Typical use | Take most of the solvent out quickly, in a continuous flow | Refine thick, heat-sensitive oils under deep vacuum | Concentrate clear, free-flowing liquids | Small batches and lab work |
| extraktLAB model | fracTron-100, fracTron-1000 | clearSTILL-200, clearSTILL-400 | None | None. See falling film vs. rotovap |
Many processors use two steps: a falling film unit removes most of the solvent, then a wiped film unit refines the oil that is left. More on each: wiped film evaporators and short path vs. wiped film distillation.
The Best Falling Film Evaporator
Get a falling film evaporator built for solvent recovery from extraktLAB! With our falling film evaporator, the fracTron, you will gain the edge you need over your competitors. Talk with an expert today!
Frequently asked questions
What is a falling film evaporator?
“Falling film” refers to a thin film created as the ethanol or liquid solvent falls down the inside of the heated vertical metal tubing, which is heated by circulating oil on the outside of the tubes. Optimal heat transfer on the walls of these tubes allows ethanol oil or other liquid solvent mixtures to evaporate at a high throughput rate before condensing on a chilled collector.
How does a falling film evaporator work?
The feed enters at the top and is spread over the inside wall of heated vertical tubes. It runs down as a thin film and the solvent boils out of it. At the bottom, a separator splits the vapor from the concentrated liquid, and a condenser turns the vapor back into liquid.
What is the difference between falling film and wiped film?
In a falling film unit, gravity forms the film and nothing moves in it. In a wiped film unit, rotating blades spread the film, so it can handle thicker liquids under deep vacuum. Many processors use a falling film unit first to remove most of the solvent, then a wiped film unit to refine the oil.
What are the disadvantages of a falling film evaporator?
It does not suit very thick liquids or liquids that foul or crystallize on the tube wall. It needs an even feed so the whole wall stays wet, one pass may not reach the concentration you want, and the units are tall.
What liquids is a falling film evaporator not suited for?
Thick, sticky or solid-bearing liquids, and any liquid that leaves deposits on the tube wall as it loses solvent.
What is a falling film reboiler?
It is the same tube design placed at the base of a distillation column, where it boils the liquid collected at the bottom of the column.
How much can a falling film evaporator process?
It depends on the unit. extraktLAB’s fracTron-100 handles up to 75.7 L/h (20 gal/h) and the fracTron-1000 up to 151 L/h (40 gal/h), as stated on their product pages.
How do I choose between a falling film evaporator and a rotovap?
A rotovap suits small batches and lab work. A falling film evaporator suits a continuous flow of free-flowing liquid at larger volume. The full comparison is in falling film vs. rotovap.
Related equipment and guides
Equipment
- fracTron-100: single-stage batch falling film unit for solvent recovery.
- fracTron-1000: three independent falling film evaporators for continuous ethanol removal.
- clearSTILL-200 and clearSTILL-400: wiped film distillation for refining viscous oils after the solvent is gone.
- DRAINDROYD: vacuum filtration for removing waxes from winterized ethanol solutions before evaporation.
Guides
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