Industrial evaporator heat-transfer surfaces are either tubular or flat plates, packaged into a variety of evaporator types, as outlined here.
Batch evaporators are vessels with a heating jacket or internal coil, an overhead condenser, a condensate receiver, and usually, a vacuum source. After feed is charged into the vessel, heat is applied and evaporated vapor is condensed overhead, while the contents of the vessel decrease in volume and increase in concentration of non-volatile materials.
Short-tube vertical evaporators
A shell-and-tube heat exchanger is situated inside the evaporator vessel, near the bottom. The heat exchanger, called the calandria, contains an open area at the center, known as the downtake. Process fluid circulates upward through the calandria tubes, against condensing steam on the shell side. The vapor formed travels to the top of the evaporator, where entrained liquid droplets coalesce on a mesh pad and fall back into the boiling liquid. Meanwhile, the liquid emerging from the calandria tubes travels downward through the downtake, then back up through the tubes for a subsequent pass. Circulation occurs by natural convection.
Long-tube vertical evaporators
These evaporators operate with a thin film of liquid on the heat-transfer surface. As evaporation takes place, vapor fills the core of the flow channel, which thins and accelerates the film.
With this type, process fluid circulates from the vapor-liquid separator (flash chamber) through the heat exchanger and back. The orifice plate applies enough backpressure to prevent boiling in the heat exchanger. Only sensible heat (no latent heat) is transferred in the heat exchanger, and the process liquid exits the heat exchanger at a temperature above the boiling point at the prevailing pressure in the flash chamber. The flashed vapor is directed to an overhead condenser, usually via a mesh pad to recover entrained liquid droplets. Meanwhile, the concentrated liquid makes another pass through the heat exchanger, with some new feed added, and some concentrate removed as product.
This type has a series of corrugated metal plates, separated by gaskets around the periphery of each plate. The plates are pressed together to form a series of flow channels. Steam and process fluid are directed to alternate channels, and heat is transferred across each plate from the steam side to the process side. Slotted openings and seals direct the various streams. Baffles create a tortuous flow path for the fluid, increasing local velocity, and in turn, heat-transfer efficiency.
Agitated thin-film evaporators
These evaporators form films mechanically, using a rotating blade near (or contacting) the heat-transfer surface. The device has a mechanical rotor concentric to a cylindrical, jacketed body. Feed entering the top is distributed evenly by the rotor, then the fluid spirals down the heated wall. Bow waves generated by the rotor blades cause high turbulence, so heat-transfer coefficients are high. Concentrate exits the bottom. Vapor travels from the heated surface to an internal condenser, concentric to the evaporator body, and the resulting condensate (distillate) proceeds downward to exit.
Editor’s note: This content is adapted from: Gabelman, A., Evaporators: Design Concepts and Equipment Selection, Chem. Eng., January 2020, pp. 27–38.
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