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What Is an Evaporator? | Process Engineering Glossary
What Is an Evaporator?
In piping engineering and process engineering, an evaporator is a heat transfer device that concentrates a solution by boiling off the solvent, typically water, under controlled conditions of temperature and pressure. The vapour produced is condensed and either recovered or discarded, while the concentrated product remains. Evaporators appear throughout food, chemical, pharmaceutical, pulp and paper, and desalination industries wherever concentration by evaporation is more practical or economical than other separation methods.
Applications of Evaporators
Sugar Refining
Multiple effect falling film or Robert evaporators concentrate clarified sugar juice from approximately 14 degrees Brix to 65 degrees Brix before crystallisation. Steam economy is critical because the sugar refinery generates its own steam from bagasse combustion. Maximising the number of effects reduces the steam consumption per tonne of sugar produced and improves the energy self-sufficiency of the plant.
Black Liquor Concentration
Kraft pulp mills concentrate black liquor from approximately 15 percent dry solids content at the digester outlet to 65 to 80 percent dry solids before combustion in the recovery boiler. Multiple effect evaporators with forced circulation handle the scaling tendency and high viscosity of black liquor at high concentrations. The recovered heat from the recovery boiler supplies the steam for the evaporation system, completing the energy recovery cycle of the kraft process.
Seawater Desalination
Multi-effect distillation evaporators produce fresh water from seawater by evaporation in a series of effects at decreasing temperature and pressure. Each effect uses the latent heat from condensing vapour generated in the previous effect. Boiling point elevation due to seawater salinity reduces the available temperature driving force in each effect and limits the practical number of effects in a multi-effect distillation unit.
Pharmaceutical Concentration
Pharmaceutical manufacturers use falling film evaporators under vacuum to concentrate heat-sensitive active ingredient solutions. The low operating temperature achieved under vacuum preserves product quality. Short residence time prevents thermal degradation. The evaporator operates under strictly controlled conditions and full clean-in-place capability to meet pharmaceutical manufacturing hygiene standards.
Benefits of Evaporators
High Throughput Concentration
Evaporation removes large volumes of solvent in a single continuous operation. Multiple effect designs achieve this at a fraction of the steam cost of single-effect operation. No other concentration method approaches the throughput and energy efficiency of a well-designed multiple effect evaporator for high-volume aqueous solutions.
Flexible Product Concentration
The evaporator product concentration adjusts readily by changing the product draw-off rate or the steam supply rate without process shutdown. This flexibility allows the plant to respond to feed composition variations and market demand changes without equipment modification.
Solvent Recovery
The condensed vapour from the evaporator is often recovered as clean process water or solvent for reuse in the plant. In pharmaceutical and food applications, high-purity condensate provides a valuable water source that reduces the demand on water treatment systems.
Limitations to Consider
Fouling and Scaling
Many process solutions deposit scale on heat transfer surfaces as concentration increases. Scale accumulation reduces the heat transfer coefficient and eventually blocks the heating tubes entirely. Forced circulation designs resist scaling better than natural circulation or falling film designs because the high liquid velocity reduces the tendency for deposits to form. However, all evaporator types require periodic cleaning to restore design heat transfer performance.
Boiling Point Elevation Losses
As discussed above, boiling point elevation reduces the effective temperature driving force in each evaporation stage. For solutions with high dissolved solids concentration, such as caustic soda and some salt solutions, the elevation losses are large enough to limit the achievable product concentration from a multiple effect system without very high steam temperatures.
Energy Intensity
Even with multiple effect operation, evaporation remains an energy-intensive operation compared to membrane concentration or crystallisation. In applications where large volumes of solvent must be removed from dilute solutions, the steam consumption of the evaporation plant represents a major fraction of the total plant operating cost.
Evaporator FAQ
What is an evaporator in process engineering? An evaporator is a heat transfer device that concentrates a solution by boiling off the solvent under controlled temperature and pressure conditions. Steam or hot utility fluid supplies heat through a tube or plate surface to the boiling liquid. The generated vapour leaves through the vapour outlet to a condenser. Multiple effect designs reuse the vapour from each stage as the heating medium for the next, improving steam economy significantly over single-effect operation.
What is the difference between a falling film and a forced circulation evaporator? A falling film evaporator passes feed liquid as a thin film down the inside of vertical heated tubes, achieving high heat transfer coefficients and short residence times. It suits low-viscosity, heat-sensitive products. A forced circulation evaporator pumps liquid at high velocity through a shell and tube heater, then flashes it in a separate vessel. It suits viscous, crystallising, or fouling services where falling film operation would produce unacceptable scale deposits or tube dry-out.
How does boiling point elevation affect evaporator design? Boiling point elevation raises the boiling temperature of the process solution above that of pure water at the same pressure. This elevation reduces the temperature difference between the heating steam and the boiling liquid, which is the driving force for heat transfer. Engineers subtract the boiling point elevation from the apparent steam-to-liquid temperature difference when sizing the heat transfer area. In multiple effect systems, the elevation losses in each effect sum to reduce the total available temperature budget for the entire system.
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