A multi-effect evaporator (MEE) concentrates a liquid solution by evaporating water (or solvent) using steam — but instead of wasting the evaporated vapour, it uses it as the heating medium for the next effect. This vapour reuse is the fundamental mechanism behind the energy efficiency of MEE systems. A single effect uses approximately 1.1 kg of steam to evaporate 1 kg of water. A triple-effect reduces this to approximately 0.35–0.38 kg of steam per kilogram of evaporation — a 3× improvement in steam economy.
How Multiple Effects Work
In a multi-effect system, each 'effect' is an evaporator body operating at progressively lower pressure (and therefore lower temperature). The steam evaporated in Effect 1 (the highest-pressure effect, heated by fresh live steam) is fed as the heating medium to Effect 2. Effect 2 evaporates at a lower temperature because its pressure is lower — and the vapour from Effect 2 heats Effect 3, and so on.
For this cascade to work, each successive effect must operate at a lower temperature than the previous effect, so the vapour from Effect N can condense and release its latent heat into Effect N+1. The operating pressure of each effect is set to create this temperature cascade.
The final effect operates under vacuum — typically 50–150 mbar absolute — which lowers the boiling point to 40–60°C, maximising the temperature difference available for heat transfer even though low-grade steam or waste heat is the only motive energy.
Steam Economy
Steam economy (SE) is defined as: kg of water evaporated / kg of live steam consumed. Theoretical maximum SE for an N-effect system is N (1 kg steam evaporates N kg water). Practical values are lower due to heat losses, boiling point elevation (BPE), and non-condensable gases.
Beyond 5–6 effects, the incremental steam saving per additional effect decreases while capital cost and complexity continue to rise. Most industrial MEE systems for chemical and food industries are 2–4 effects. Larger systems (5–7 effects) are common in sugar and pulp & paper industries where steam cost is particularly significant.
| Number of Effects | Theoretical SE | Practical SE (typical) | Steam Consumption (kg steam/kg evaporation) |
|---|---|---|---|
| 1 (Single) | 1.0 | 0.85–0.9 | 1.10–1.18 |
| 2 (Double) | 2.0 | 1.7–1.9 | 0.53–0.59 |
| 3 (Triple) | 3.0 | 2.5–2.7 | 0.37–0.40 |
| 4 (Quadruple) | 4.0 | 3.3–3.5 | 0.29–0.30 |
| 5 (Quintuple) | 5.0 | 4.0–4.3 | 0.23–0.25 |
| 6 (Sextuple) | 6.0 | 4.7–5.0 | 0.20–0.21 |
Feed Arrangements: Forward, Backward and Mixed Feed
The feed arrangement determines the direction in which liquid flows through the effects relative to the steam direction. Each has distinct advantages depending on the liquid's properties.
| Arrangement | Liquid Flow | Temperature of Liquid in Each Effect | Best For |
|---|---|---|---|
| Forward Feed | Same direction as steam (Effect 1 → 2 → 3) | Hottest in Effect 1, coolest in last effect | Feeds that degrade at high temperature; viscous feeds that need pump-free flow (gravity/pressure differential); heat-sensitive products (pharmaceuticals, juices, dairy) |
| Backward Feed | Opposite to steam (last effect → 2 → 1) | Coolest in first effect contacted, hottest just before final product | Feeds with high viscosity at low temperature — backward feed ensures lowest viscosity in the hottest, highest-evaporation effect; feeds that scale at high concentration |
| Mixed Feed | Combination — feed enters at an intermediate effect | Varies | When neither pure forward nor backward feed is optimal; complex systems where specific products need specific temperature profiles |
| Parallel Feed | Fresh feed enters each effect simultaneously | Same as boiling point of that effect | When the feed solution composition changes significantly with concentration; crystallisation systems |
Evaporator Body Types
The type of evaporator body used in each effect determines the heat transfer mechanism, suitability for viscous or fouling liquids, and the residence time of the liquid.
- Falling Film Evaporator: Feed liquid flows as a thin film down the inside of vertical tubes, heated by steam on the outside. Very short residence time. Excellent for heat-sensitive materials (pharmaceuticals, fruit juices, dairy). Requires even liquid distribution across all tubes. Not suitable for heavily scaling or viscous liquids.
- Forced Circulation Evaporator: A circulation pump forces liquid through vertical or horizontal tubes at high velocity, heating it above the boiling point. Flashing occurs in the separator vessel. Suitable for viscous, scaling, or crystallising liquids. Higher power consumption due to circulation pump. Robust and easy to clean.
- Natural Circulation (Short/Long Tube Vertical): Liquid circulates by thermosiphon action through vertical tubes. Simpler than forced circulation. Suitable for non-viscous, non-scaling liquids. Not suitable for viscous or scaling services.
- Plate Evaporator: Uses gasketed or welded plates instead of tubes. Compact, efficient. Suitable for clean, non-fouling, non-scaling liquids. Used in dairy and pharmaceutical applications.
Key Data Required for MEE Sizing
- Feed flow rate (kg/hr) and feed concentration (% dissolved solids or solute)
- Product concentration required (% dissolved solids or Brix)
- Feed temperature (is preheating available?)
- Available steam pressure and temperature
- Available cooling water temperature (determines vacuum achievable in the last effect)
- Liquid properties: viscosity at operating temperatures, boiling point elevation (BPE) as a function of concentration, specific heat, density
- Fouling and scaling tendency of the liquid
- Material of construction requirements (food grade? corrosive?)
- Whether vapour recompression (MVR or TVR) is to be considered for additional energy savings
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