Design a multiple-effect evaporator: evaporator bodies in series, where the vapour from each one reheats the next. Change the number of effects, feed rate and concentrations to see the water evaporated, live steam required, steam economy and energy saving update in real time, and learn how this layout slashes steam use.
Parameters
Number of effects n
Evaporator bodies in series. More effects raise the steam economy
Feed rate ṁ_feed
kg/h
Flow of dilute solution entering the first effect
Feed concentration x_feed
wt%
Mass fraction of solute (solids) in the feed
Product concentration x_prod
wt%
Target concentration of the liquor leaving the last effect
Latent heat of steam λ
kJ/kg
Latent heat of vaporisation for evaporation and condensation
Steam cost
JPY/t
Cost of boiler live steam, used for the annual saving estimate
Live steam enters the first effect; the vapour produced by each effect heats the next one. The feed liquid is concentrated (darker) as it passes through, and the final effect discharges the product.
Product rate ṁ_prod and water evaporated W from a solids balance. ṁ_feed: feed rate, x_feed: feed concentration, x_prod: product concentration. The solute is conserved between feed and product.
The steam economy is ideally equal to the number of effects n, and the live steam required ṁ_steam is the water evaporated W divided by the economy. Steam economy is the kilograms of water evaporated per kilogram of live steam.
What is the Multiple-Effect Evaporator Simulator?
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"Multiple-effect evaporator" sounds complicated — what does the equipment actually do?
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Put simply, it boils a liquid down to make it more concentrated. You use it to boil sugar-cane juice down into sugar, or to drive water off seawater to get fresh water. The catch is that evaporating water takes a huge amount of heat. Doing the equivalent of keeping a kettle boiling, but at factory scale, costs a fortune in steam. So you connect several evaporator bodies — called "effects" — in series and reuse the steam once, then again, then again.
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Reuse the steam? Hasn't steam already given up all its heat after one pass?
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Good question. When the boiler's live steam boils the liquid in the first effect, that liquid gives off "new vapour". This vapour is still close to 100 °C — full of heat. So if you run the second effect at a lower pressure than the first, the boiling point drops, and that same vapour can boil the second effect's liquid. The vapour from the second effect then drives the third, and so on. Stepping the pressure down stage by stage is the whole trick. Try raising the "number of effects" slider on the left — the steam economy jumps up.
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It works — at 3 effects the steam economy becomes 3. Does that mean "1 kg of live steam evaporates 3 kg of water"?
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Exactly. Steam economy is precisely "how many kilograms of water you evaporate per kilogram of live steam". A single-effect unit needs 1 kg of steam to evaporate 1 kg of water, so its economy is 1. A triple-effect unit ideally reaches 3, so the same job takes only one-third of the steam. Look at the "live steam required vs number of effects" chart below — the more effects, the more steam consumption drops, falling as 1/n.
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So if I just keep adding effects, the steam cost goes to almost zero!
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It is not that simple. First, more bodies mean higher capital cost. And the total temperature difference available is fixed — the gap between the first heating steam and the last effect. You have to split that among the effects, so the more effects you add, the smaller the temperature difference each one gets. A small temperature difference makes heat transfer slow, so you need a much larger heat-transfer area to evaporate the same amount. That is why, in a tug-of-war between steam saving and capital investment, plants usually settle around three to six effects.
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I see. So when it says the economy is "ideally n", the real value is something else?
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Right — in a real plant it comes out a little below n, for two reasons. One is "boiling-point rise": when a solute is concentrated, like sugar liquor or black liquor, the boiling point goes above that of pure water, eating into the useful temperature difference in every effect. The other is heat loss from the bodies and pipes. Together these typically pull the real steam economy 10-20% below the ideal n. This tool calculates with the ideal n, so when you size a real plant, discount it a bit.
Frequently Asked Questions
A multiple-effect evaporator is an energy-saving evaporation system that places several evaporator bodies (effects) in series, so the vapour boiled off in one effect becomes the heating steam for the next. Each effect runs at a lower pressure and lower boiling point than the one before it, so even the previous effect's vapour can boil the next liquid. As a result, one kilogram of live steam from the boiler evaporates roughly n kilograms of water, where n is the number of effects, cutting steam consumption far below that of a single-effect unit.
Steam economy is the number of kilograms of water evaporated per kilogram of live steam. Ideally it equals the number of effects n: a triple-effect unit gives an economy of about 3, a quintuple-effect unit about 5. In practice it is typically 10-20% lower than the ideal because the dissolved solute raises the boiling point in each effect, reducing the useful temperature difference, and because of heat losses. This tool uses the ideal value n, so estimate slightly conservatively for a real plant.
For steam consumption, more effects raise the steam economy and the live steam required falls as 1/n. But more effects mean more evaporator bodies and higher capital cost. Also, the total temperature driving force available (between the first heating steam and the last effect) is fixed, so it must be shared among the effects: with more effects each effect has a smaller temperature difference, transfers heat more slowly and needs more heat-transfer area. In practice, balancing the steam-cost saving against the capital cost, three to six effects are most common.
The solids (solute) are conserved between feed and product, so feed rate x feed concentration = product rate x product concentration. The product rate is therefore feed rate x feed concentration / product concentration, and the water evaporated is feed rate - product rate. For a feed of 5000 kg/h at 10 wt% concentrated to 45 wt%, the product rate is 5000 x 10 / 45 = 1111 kg/h and the water evaporated is 5000 - 1111 = 3889 kg/h. The product must be more concentrated than the feed; the tool automatically corrects inputs that violate this.
Real-World Applications
Sugar refineries: Cane and beet juice is a dilute liquid with about 10-15% solids, concentrated to around 65% before crystallisation. Because the amount of water to evaporate is enormous, the multiple-effect evaporator is the heart of the sugar process. Many plants use four or five effects and combine them with "steam bleeding" — extracting part of a body's vapour for heating elsewhere — to juggle the whole factory's heat balance cleverly.
Black-liquor concentration in paper mills: "Black liquor" from kraft pulping is a waste stream containing the wood's organics and the cooking chemicals. A multiple-effect evaporator concentrates it to over 70% solids so the recovery boiler can burn it, recovering chemicals and producing steam. Black liquor is highly viscous and has a large boiling-point rise, so trains of six or more effects are sometimes built.
Seawater desalination (MED): Multi-effect distillation (MED) is one of the standard ways to make fresh water from seawater. Each effect evaporates seawater and the vapour is condensed to recover fresh water. Because it can run at low temperature and pressure, scale is less likely to form, and MED is widely used in desalination plants in regions such as the Middle East.
Dairy and food concentration: Multiple-effect evaporators also concentrate milk before it is spray-dried into powder, and concentrate fruit juice, coffee extract and seasoning liquors. Because food degrades easily with heat, the later effects are run at low temperature (under vacuum) so the product is concentrated at as low a temperature and for as short a time as possible.
Common Misconceptions and Pitfalls
A common misconception is that "adding effects raises the steam economy without limit". The ideal calculation does give steam economy = number of effects n, but in reality boiling-point rise and heat loss pull it 10-20% below n. More importantly, adding effects does not increase throughput. The total temperature difference available is fixed, so the more effects you add, the thinner the temperature difference per effect becomes, and the more heat-transfer area you must add to evaporate the same amount. The steam saving is traded against more heat-transfer area (capital cost), and the optimum number of effects is decided on economics.
Next, the assumption that "boiling-point rise can be ignored". For simplicity this introductory tool handles only the solids balance and the ideal steam economy, but real designs always account for the boiling-point elevation (BPE) in each effect. Concentrated sugar liquor or black liquor can have a boiling-point rise of over 10 °C, and that much is subtracted from the useful temperature difference (heating-steam temperature minus boiling point) in each effect. Overlooking BPE leads to a unit that should evaporate on paper but is under-capacity in practice.
Finally, the idea that "the way the liquid is fed (the flow pattern) does not matter". This tool does not cover it, but feeding the liquid in the same direction as the steam (forward feed), the opposite direction (backward feed), or partway in (mixed feed) changes the required heat-transfer area, pumping power and the product's heat history. Forward feed keeps the concentrated liquor in the last, coolest effect, reducing viscosity trouble and needing no inter-effect pumps, so it is simple. Backward feed puts the concentrated liquor on the hot side, lowering its viscosity for better heat transfer, but needs a pump for each stage. Choosing the flow pattern to suit the liquid's properties — viscosity, heat sensitivity — is where engineering skill shows.
How to Use
Set the number of evaporator effects (2-5 stages) in series configuration
Adding effects beyond 4 stages shows diminishing returns; pressure drop and heat integration losses exceed marginal steam savings in sugar/dairy processing
Feed concentration affects split: higher inlet solids reduce water load per effect; viscous products (starch >15%) require careful staging to maintain heat transfer coefficients
Assume vacuum operation (0.1-0.3 bar) in later effects; actual capital cost vs. operating savings break-even at 8000+ operating hours annually for 3-effect systems