The objective of evaporation is to concentrate a non-volatile solute in a solution by vaporizing the solvent, usually water. This is done by boiling off the solvent.

Evaporators are employed for the concentration of a solution by the evaporation of water. If any other fluid vaporizes besides water, the unit is called as a vaporizer.

An evaporator consists of a heat exchanger for boiling the solution and a means to separate the vapor from the boiling liquid. Different types are categorized by the length and alignment (horizontal or vertical) of the evaporator tubes. The evaporation tubes may be located inside or outside of the main vessel where the vapor is driven off.

Because many materials cannot tolerate high temperatures, evaporators often operate at reduced pressure so that the boiling point will also be reduced.

There are three main measures of evaporator performance:

  1. Capacity (kg vaporized/hr)

  2. Economy (kg vaporized/kg steam input)

  3. Steam consumption (kg/hr)

Note that the measures are related, since \[\text{Steam Consumption} = \frac{\text{Capacity}}{\text{Economy}}\]

Economy calculations are determined using enthalpy balances.

The key factor in determining the economy of an evaporator is the number of effects. The economy of a single effect evaporator is always less than 1.0. Multiple effect evaporators have higher economy but lower capacity than single effect.

Boiling Point Elevation

  • For low concentrations of solute, boiling point of aqueous solution is equal to be the of boiling point of water. With increasing concentration of solute, the boiling point of solution is higher than that of water.

  • The vapor pressure of an aqueous solution is less than that of pure water at the same temperature; so the boiling point of the solution will be higher than that of the water. This is called boiling point elevation or vapor pressure lowering.

  • Since the solution and leaving vapor are in equilibrium, the vapor rises at the solution boiling point. For solution with boiling point elevation, the vapor is at superheated condition with reference to the pressure inside the evaporator.