Multiple Effect Evaporator Calculator
Calculates the steam consumption per effect and total water evaporated in a multiple-effect evaporator system. Use it when designing industrial concentration processes for food, chemical, or pharmaceutical liquids.
About this calculator
In a multiple-effect evaporator, steam from one effect heats the next, reusing latent heat and improving steam economy. The total water that must be evaporated is found from a mass balance: water_evaporated = F × (C_f − C_i) / C_f, where F is the feed rate (kg/h), C_i is the initial solute concentration (% w/w), and C_f is the final concentration (% w/w). Assuming equal evaporation across each effect, the steam consumption per effect is approximately: steam_per_effect = water_evaporated / N, where N is the number of effects. Ideal steam economy equals N kg water evaporated per kg of steam consumed, so a triple-effect system theoretically requires only one-third the steam of a single-effect unit. Real systems deviate due to boiling point rise and heat losses.
How to use
Suppose a juice concentration plant processes F = 10,000 kg/h of feed at C_i = 5% solids and targets C_f = 40% solids using N = 3 effects. First calculate total water to evaporate: water_evaporated = 10,000 × (40 − 5) / 40 = 10,000 × 0.875 = 8,750 kg/h. Then steam consumption per effect = 8,750 / 3 ≈ 2,917 kg/h. The theoretical steam economy is 3 kg water / kg steam. If live steam enters at 200 kPa (≈120 °C), each subsequent effect operates at successively lower pressures, enabling vapor from one effect to drive the next. Use these results to size heat exchangers with the given overall heat transfer coefficient.
Frequently asked questions
What is steam economy in a multiple effect evaporator and how is it calculated?
Steam economy is the kilograms of water evaporated per kilogram of live steam supplied to the first effect. For an ideal N-effect system, economy ≈ N, meaning a triple-effect evaporator evaporates roughly 3 kg of water per kg of steam. In practice, boiling point elevation, heat losses, and unequal distribution reduce economy to 0.8–0.9 × N. Economy is calculated as total water evaporated divided by steam consumed in the first effect and is the primary economic metric for evaporator design.
How does the number of effects influence capital cost versus operating cost in evaporation?
Adding more effects increases steam economy and reduces steam (energy) costs, but each additional effect requires its own vessel, heat exchanger, and vacuum system, raising capital expenditure. The optimal number of effects is found by balancing annualized capital cost against annual steam savings. For low-energy-cost locations or small plants, two or three effects are typical; energy-intensive industries like sugar or paper may justify five or six effects. Engineers plot total annual cost versus N to identify the economic optimum.
Why does boiling point rise reduce the performance of a multiple effect evaporator?
Boiling point rise (BPR) occurs because dissolved solutes elevate the boiling temperature of a solution above that of pure water at the same pressure. This means the vapor produced in one effect is at a lower temperature than expected, reducing the temperature driving force available to heat the next effect. As concentration increases across effects, BPR grows, further shrinking the effective temperature difference. Designers must account for BPR when calculating heat transfer areas and may need to increase the number of effects or use a larger steam pressure to compensate.