Differences Between Thin-Film Evaporators and Scraped-Film Evaporators
Structural Differences
Thin-film evaporator
Primarily utilizing rising-film or falling-film configurations, the core structure consists of vertical shell-and-tube heat exchangers. For example:
Film-type: Liquid flows downward along the heating tube wall as a thin film formed by a distributor, relying on gravity to achieve evaporation.
Rising-film type: Liquid rapidly vaporizes when heated within the heating tube, forming an ascending vapor stream that carries the liquid film upward. The aspect ratio (L/d = 100–250) must be controlled to maintain film stability.
Scraped-surface evaporator
Typically a horizontal cylindrical shell, internally equipped with a rotating scraper assembly. The liquid is uniformly dispersed into a thin film through the mechanical action of the scrapers, flowing along the heated wall surface. Simultaneously, the scrapers continuously renew the liquid film to prevent scaling.
Differences in Working Principles
Thin-film evaporator
Natural film formation: Relying on liquid gravity or vapor flow to form a film (e.g., falling film type), or utilizing vapor generated by boiling to propel the liquid film (e.g., rising film type).
High heat transfer efficiency: Thin liquid film (typically <1mm) and rapid evaporation rate make it suitable for heat-sensitive materials (e.g., pharmaceuticals, fruit juices).
Scraped-surface evaporator
Forced Film Formation: Mechanically forming a uniform liquid film through the action of a scraper, suitable for high-viscosity liquids prone to crystallization or scaling (e.g., resins, syrups).
Strong anti-fouling capability: Scrapers continuously remove residues from heat transfer surfaces, reducing scaling risks.
Application Scenario Comparison
| Characteristics | Thin-film evaporator | Scraped-surface evaporator |
| Applicable Liquid Types | Low to medium viscosity, heat-sensitive liquids (e.g., milk, dilute solutions)47 | High-viscosity, easily crystallizing, and easily scaling liquids (such as asphalt, polymer solutions)25 |
| Vacuum Adaptability | Medium vacuum level (e.g., 10–100 mmHg)7 | High vacuum level (down to 5 mmHg or below)7 |
| Energy Consumption and Maintenance | Relying on natural film formation results in lower energy consumption; however, regular cleaning is required to remove scale buildup. | Mechanical scrapers increase energy consumption but have longer maintenance intervals. |
Typical Application Areas
Thin-film evaporator:
Pharmaceutical Industry: Low-temperature concentration of antibiotics and vitamins.
Food Industry: Evaporation and concentration of fruit juices and dairy products.
Scraped-surface evaporator:
Chemical Industry: Desolventization of high-viscosity resins and asphalt.
Environmental Protection Field: Treatment of wastewater containing solid particles.
Key Differences Summary
Film Formation Mechanism: Thin-film evaporators rely on gravity or steam power for natural film formation, while scraped-surface evaporators employ mechanical scrapers to force film formation.
Anti-scaling capability: Scraped-film evaporators are better suited for treating easily contaminated materials due to the continuous surface cleaning provided by the scraping blade.
Vacuum Adaptability: Scraped-film evaporators perform optimally under high vacuum conditions, making them suitable for the separation of substances with extremely low boiling points.
Note: Some scraped-film evaporators (such as scraped-film molecular distillation units) may share similar principles with thin-film evaporators, but feature specialized designs for highly acidic or high-purity separation applications.
