【Haichuan Illustrated Chemical Equipment】Series Posts -- Rising Film Evaporator

Thread Content

Get to know equipment, understand it, and make good use of it. [Haichuan’s Illustrated Guide to Chemical Equipment] series posts: https://bbs.hcbbs.com/forum.php?mod=viewthread&tid=5719196. Everyone is welcome to participate in the discussions. ----------------------------------------------------------- Rising Film Evaporator: A “forceful ascent” driven by steam. If falling film evaporation is a graceful natural descent, then a rising film evaporator is a “forceful ascent” achieved through the power of steam inside it. It does not require a complex top film applicator, but rather utilizes the powerful kinetic energy generated by thermodynamic phase changes. 1. Working principle and the “four-step” flow pattern change: In a falling-film evaporator, the feed liquid enters from the bottom of the heating tube bundle. When the material inside the tube is heated, its fluid state undergoes extremely complex hydrodynamic changes as it moves upward: Single-phase liquid flow region: Initially, the material is in a pure liquid state when it enters the bottom of the tube, and it is heated to its boiling point. Bubbly Flow and Slug Flow: Boiling begins, resulting in the formation of bubbles. As more bubbles form, they aggregate into larger air plugs. Annular Flow – Core film-forming zone: This is the essence of rising film formation! Since the specific volume of gas is much larger than that of liquid, the large amount of secondary steam generated by intense boiling shoots upward at the center of the tube at extremely high speeds (up to 20 m/s or even higher). The strong shear force generated by the high-speed steam flow (vapor shear) overcomes the gravity of the liquid, pressing it tightly against the inner wall of the tube and carrying this layer of liquid film upward at high speed. Mist Flow: At the top of the tube, if evaporation is excessive, the liquid film may be torn into droplets that are carried away by the steam. 2. Performance characteristics and advantages: High turbulence prevents scaling: Due to the extremely fast internal flow velocity and intense boiling, the turbulence level of the fluid on the inner wall of the tube is very high (high Reynolds number Re). This strong scouring effect makes rising-film evaporators less prone to scaling than falling-film evaporators, making them suitable for handling fluids that have a slight tendency to scale or contain small amounts of solid suspended particles. Excellent defoaming capability: The intense steam shear force physically breaks apart the bubbles formed in the liquid, making it highly suitable for handling materials that tend to form bubbles. 3. Limitations of core technology: Film evaporators require a large heat transfer temperature difference (usually ▲T > 15°C) to generate sufficient quantities of secondary steam fast enough to \"blow\" the liquid film upward. If the temperature difference is insufficient and the steam velocity is too low, the liquid film will fall off, causing the equipment to stop operating – a phenomenon commonly referred to as \"inability to climb up\".