Performance of Evaporator

This post provides some information about what is mean by performance of evaporator, how performance of evaporator can be increased.

A rising film evaporator

 

Basically the performance of evaporator depends upon its capacity and economy. Let us discuss these terms one by one.

1) Capacity of evaporator:

Capacity of evaporator is defined as the number of kilograms of water vaporized/evaporated per hour.

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

Therefore  Q = U×A×?T

Where

  • Q = Rate of heat transfer
  • A = area of the heat transfer surface
  • ?T = overall temperature drop

The capacity of an evaporator depends upon the temperature of the feed solution. If the feed solution is at the boiling temperature corresponding to the pressure in vapor space of an evaporator, all the heat supplied will be utilized for evaporation, thus increasing the capacity of evaporator.

Similarly if the cold feed solution is fed in the evaporator, initially some energy will be required to increase the temperature of the feed solution to the boiling point of the solution corresponding to the vapor pressure inside an evaporator. In some cases this amount energy may be very high. Thus the capacity of evaporator will be reduced.

2) Evaporator Economy:

Economy of the evaporator is another important parameter which decides the performance of evaporator. It may be defined as the amount of steam used and is expressed in terms of pounds of vapor produced per pound of steam supplied to the evaporator train.

Following are some of the methods to increase the economy of evaporator;

a)      Use of multiple effect evaporation system ( Vapor recompression )

In multiple effect evaporation system, the vapor produced in the first stage is used as energy source (heat) for the second stage and so on. Thus increasing the economy of evaporator.

multiple effect evaporator

For example, in three effect evaporator if 1Kg. of steam is supplied to the first stage 2.5 Kg. steam is produced.

In vapor recompression method, vapor from the evaporator is compressed to increase its temperature so that it will condensate at temperature higher enough to permit its use as a heating media in the same evaporator.

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What is evaporation and what are the types of evaporation process

Evaporation process and Evaporators terminology.

In this article we are going to discuss what is evaporation process?, its definition, types of evaporation processes.

A rising film evaporator
A rising film evaporator

Evaporation:

Evaporation process may be defined as a process in which aqueous solution is concentrated in a vessel or group of vessel in which concentrated solution is the desired product and indirect heating (usually steam) is the energy source.

Evaporators can be arranged in forward-feed, reverse feed or parallel feed. vapors of the previous stage is the energy source for a next stage .

The efficiency of the evaporators can be increased by arranging number of evaporators in series.

Evaporator’s terminology:

Following are some of the basic terms used in the evaporation process.

1) Single effect evaporation:

In single effect evaporation process the dilute solution which is to be concentrated is contacted only once with the heat.

Following figure shows the single effect evaporation process.

single effect evaporator
single effect evaporator

2) Multiple effect evaporation:

Multiple effect evaporation is the process of evaporation in which the vapors coming out in the one stage is used as an energy source (as a heat) for the next stage.

In short, vapors of first stage are used as energy source for the next stage.

Examples: 1) In paper industries for Kraft liquor concentration six-effect evaporators are used. 2) 20 effect evaporators are used in desalinization plants.

Following figure shows the two stage evaporator.

multiple effect evaporator
multiple effect evaporator

3) Boiling point rise:

It may be defined as the difference in °F of boiling point of constant composition solution and boiling point of pure liquid at the same pressure.

4) Economy:

Economy of evaporator may be defined as the amount of steam used and is expressed in pounds of vapors produced per pound of steam supplied to the evaporator train.

5) Capacity:

Capacity of evaporator is measured in terms of its evaporating capability.

It is the pounds of vapor produced per unit time.

Steam requirement = Capacity ÷ Economy.

6) Co-current operation:

In evaporator if the feed and steam follow parallel path (i.e. in same direction) through the evaporator train.

7) Counter-current operation:

The feed and steam enter the evaporator train at opposite ends.

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