Falling Film Evaporators
Falling Film Evaporators
Falling film evaporators rank among the most efficient and widely applied evaporators in industry. Feed liquid is evenly distributed at the top of a vertical tube bundle and flows downward as a thin film along the inner walls of the tubes. Steam or hot water circulates on the shell side, rapidly heating the film and causing solvent (usually water) to evaporate. The vapor-liquid mixture exits the bottom into a separator where vapor is disengaged and the concentrate is collected. These evaporators offer outstanding heat transfer, short residence time, low pressure drop, and minimal product degradation — making them the preferred choice for heat-sensitive materials in food, dairy, pharmaceuticals, chemicals, and wastewater concentration.
Core Design & Components
Falling film evaporators feature a vertical shell-and-tube heat exchanger with long tubes (typically 4–10 m long, 25–50 mm diameter). Essential components include:
- Top liquid distributor (orifice plate, spray nozzles, or calandria) for uniform film formation
- Vertical tube bundle (heating surface)
- Shell-side heating jacket or shell with steam/hot water circulation
- Bottom vapor-liquid separator (cyclone, baffle, or centrifugal type)
- Condenser and vacuum system (to reduce boiling point)
- Concentrate discharge pump and recirculation loop (for high concentration ratios)
Construction materials are usually 316L stainless steel, with sanitary finishes for food/pharma applications.
Evaporation Process Step-by-Step
The continuous process occurs as follows:
- Feed Distribution
Pre-heated feed enters the top distributor and is spread uniformly across all tubes, forming a thin annular film on the inner walls. - Downward Film Flow & Boiling
Gravity pulls the film downward; heat from the shell side causes rapid evaporation, generating vapor bubbles within the film. - Two-Phase Flow
Vapor increases flow velocity inside the tubes, enhancing turbulence and heat transfer as the mixture travels downward. - Separation
At the tube bottom, the vapor-liquid mixture enters a separator where vapor is removed and routed to the condenser; concentrate collects below. - Concentrate Handling
Concentrated liquid is discharged or partially recirculated to maintain stable film flow, especially at high concentration ratios or with viscous products. - Vapor Condensation & Vacuum
Vapor is condensed (often under vacuum) to recover solvent or heat; vacuum lowers boiling point and minimizes thermal stress on the product.
Comparisons
| Feature | Falling Film Evaporator | Rising Film Evaporator |
|---|---|---|
| Liquid Flow Direction | Downward (gravity-driven thin film) | Upward (vapor lift inside tubes) |
| Residence Time | Very short (seconds to minutes) | Moderate (minutes) |
| Heat Transfer Coefficient | Very high (thin, turbulent film) | Good, but lower at low flow rates |
| Fouling Tendency | Low (self-cleaning film flow) | Moderate to high (longer tube residence) |
| Best For | Heat-sensitive, low-to-medium viscosity products | Clean, low-viscosity liquids |
| Energy Efficiency | High | Moderate |
| Feature | Falling Film Evaporator | Forced Circulation Evaporator |
|---|---|---|
| Circulation Method | Gravity-driven film flow | Pump-forced high-velocity recirculation |
| Fouling Resistance | Good (thin film, short contact) | Excellent (high velocity prevents scaling) |
| Viscosity Handling | Low to moderate (up to ~5,000 cP) | High (up to 50,000 cP or more) |
| Energy Consumption | Low (no recirculation pump) | Higher (pump energy required) |
| Best Suited For | Heat-sensitive, clean to moderately fouling liquids | High-viscosity, scaling, crystallizing solutions |
| Residence Time | Very short | Longer (due to recirculation) |
Typical Operating Parameters
| Parameter | Typical Range | Notes |
|---|---|---|
| Evaporation Rate | 1–100 tons water/h | Scales with tube bundle size and number of effects |
| Operating Temperature | 40–120 °C | Lower under vacuum for heat-sensitive products |
| Operating Pressure | 0.05–1 bar (vacuum to atmospheric) | Vacuum operation is standard |
| Residence Time | Seconds to a few minutes | Extremely short to minimize product degradation |
| Heat Transfer Coefficient | 2,000–6,000 W/m²·K | Very high due to thin, turbulent film |
| Viscosity Limit | Up to 5,000–10,000 cP (concentrate) | Recirculation loop used for higher viscosities |
Common Applications & Advantages
| Industry / Application | Typical Process | Primary Goal |
|---|---|---|
| Food & Beverage | Juice concentration, coffee extract, sugar syrup | Preserve flavor, nutrients, and color |
| Dairy | Milk evaporation, whey concentration | High-quality concentrate for powder production |
| Pharmaceuticals | API solutions, herbal extracts, fermentation broths | Minimal thermal degradation |
| Chemicals | Brine concentration, solvent recovery | Efficient solvent removal |
| Wastewater / Zero Liquid Discharge (ZLD) | Brine minimization, effluent concentration | Water recovery, volume reduction |
Key Advantages of Falling Film Evaporators
- Exceptionally high heat transfer rates and evaporation capacity
- Very short residence time — excellent for heat-sensitive products
- Low fouling tendency due to self-cleaning film flow
- High energy efficiency in multi-effect or MVR configurations
- Compact design relative to evaporation rate
- Easy cleaning and CIP compatibility
Also check out, "Wipe Film Evaporators"