Performance of Tubular Evaporators | Evaporator Performance

The performance of a steam-heated tubular Evaporator is evaluated in terms of (i) Capacity and (ii) Economy.

Capacity of Evaporators

The capacity of an evaporator is defined as the number of kilograms of water vaporized/evaporated per hour.

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The rate of heat transfer Q, through the heating surface of an evaporator, is the product of the area of heat transfer surface A, the overall heat transfer coefficient U, and the overall temperature drop ∆T.

Q = U•A∆T

∆T is the temperature difference between the heating medium and the boiling liquor (saturation temperature of steam minus the boiling point of solution).

If the feed solution is at the boiling temperature corresponding to the pressure in the vapor space of an evaporator, then all the heat transferred through the heating surface is available for evaporation and the capacity is proportional to the heat transfer rate. If the cold feed solution is fed to the evaporator, the heat is required to increase its temperature to the boiling point and it may be quite large thus, the capacity for a given rate of heat transfer will be reduced accordingly as the heat used to increase the temperature to the boiling point is not available for evaporation. When the feed solution to the evaporator is a temperature higher than the boiling point corresponding to the pressure in the vapor space, a portion of the feed evaporates adiabatically and the capacity is greater than that corresponding to the heat transfer rate. The process is called flash evaporation.

Source this image: Indiamart

Evaporator Economy

The economy of an evaporator is defined as the number of kilograms of water evaporated per kilogram of steam fed to the evaporator. It is also called the steam economy.

In a single-effect evaporator, the amount of water evaporated per kg of steam feed is always less than one, and hence economy is less than one. The fact that latent heat of evaporation of water decreases as the pressure increases tends to make the ratio of water vapor produced per kg of steam condensed less than unity.

Note: An increase in the economy of an evaporator is achieved by reusing the vapor produced.

The methods of increasing the economy 

(i) Use of multiple effect evaporation system

(ii) Vapour recompression.

In a multiple-effect evaporation system, the vapor produced in the first effect is fed to the steam chest of the second effect as a heating medium in which boiling takes at low pressure and temperature, and so on. Thus in a triple-effect evaporator, 1 kg of steam fed to the first-effect evaporator is approximately 2.5 kg of steam.

Another method to increase the economy of an evaporator is to use the principle of thermo compression. Here, the vapor from the evaporator is compressed to increase its temperature so that it will condense at a temperature higher enough to permit its use as a heating media in the same evaporator. 

Take these Notes is, Orginal Sources: Unit Operations-II, KA Gavhane