Worked problem

Single-Effect Evaporator: Water Removed and Product Flow

Concentrate a 5,000 kg/h feed from 10% to 35% solids and find the product flow, water evaporated, heat duty, steam use and steam economy.

Chemical & Process · Last reviewed

Given

A single-effect evaporator concentrates 5,000 kg/h of feed from 10% to 35% nonvolatile solids by mass. The feed enters at 30 °C and the liquid boils at 100 °C (neglect boiling-point rise). Supplied thermal data for the exercise: feed heat capacity 3.8 kJ/kg·K, latent heat of water at 100 °C 2,257 kJ/kg, and latent heat of the heating steam 2,174 kJ/kg.

Find

The product flow, the water evaporated, the heat duty, the steam flow and the steam economy.

Solution

  1. Solids in the feed: 5,000 × 0.10 = 500 kg/h. The solids all stay in the product.
  2. Product flow = 500 ÷ 0.35 = 1,428.6 kg/h.
  3. Water evaporated = 5,000 − 1,428.6 = 3,571.4 kg/h.
  4. Heat to warm the feed to boiling: 5,000 × 3.8 × (100 − 30) = 1.33 × 10⁶ kJ/h.
  5. Heat to evaporate the water: 3,571.4 × 2,257 = 8.06 × 10⁶ kJ/h.
  6. Total duty = 9.39 × 10⁶ kJ/h = 2,609 kW.
  7. Steam flow = 9.39 × 10⁶ ÷ 2,174 = 4,320 kg/h. Steam economy = 3,571 ÷ 4,320 = 0.83 kg of water evaporated per kg of steam.

Answer

product 1,429 kg/h | water removed 3,571 kg/h | duty 2,609 kW | steam 4,320 kg/h

What it shows

  • The product and evaporation rates follow from mass balances alone: one on solids, one on total mass.
  • The energy duty is dominated by the latent heat. Preheating the feed is only 14% of it.
  • A single effect uses roughly one kilogram of steam for each kilogram of water removed. Multiple-effect evaporators reuse the vapor to raise the economy.

Common mistakes

  • Balancing on solution instead of solids. The solids are the only component that does not leave with the vapor.
  • Reading 35% as 35 kg of solids per 100 kg of feed.
  • Leaving out the sensible heat of the feed, which matters when the feed is cold.
  • Ignoring boiling-point rise for strong solutions, and the changing heat capacity of the product. This problem neglects both.

Related tools and labs

Calculators, labs and simulators are part of the All-Access Pass.

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Questions

Why is steam economy less than 1 here?

Heat is also needed to warm the feed, and real evaporators have heat losses. An economy near 0.8 to 1.0 is typical for a single effect.

How does a multiple-effect evaporator help?

The vapor from one effect heats the next, which runs at a lower pressure. The economy rises roughly in proportion to the number of effects.

Does this size the evaporator?

No. Sizing needs the heat-transfer area, which follows from the duty, the overall heat-transfer coefficient and the temperature difference between the steam and the boiling liquid.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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