Thermal energy in the vapour evolved from a boiling solution and can be utilized to vaporize more water if there is a temperature drop for heat transfer in the desired direction. In the case of multiple-effect evaporation systems, this temperature drop is created by progressively lowering the boiling point of the solution in a series of evaporators by operating them under lower absolute pressures.
The desired driving force (temperature drop) can also be created by increasing pressure (and, therefore, the condensing temperature) of the vapour evolved by (a) Mechanical recompression or (b) Thermal recompression.
The compressed vapour having a higher condensing temperature is then fed to the steam chest of the evaporator from which it came. So the economy of the evaporator is also increased by recompressing the vapour from the evaporator and condensing it in the steam chest of the same evaporator. In this method, the vapour from the evaporator is compressed to a saturation pressure of steam to upgrade the vapours to the conditioning of the original steam to permit the use of heating media. The cost of supplying the required amount of compression is usually smaller than the value of latent heat in the vapour. By this, we can obtain multiple effect economies in a single effect.
Mechanical Recompression :
In this method, the vapour evolved from the evaporator is compressed to a somewhat higher pressure by a positive displacement (or) centrifugal compressor and fed to a heater as steam. As the saturated temperature of the compressed vapour is higher than the boiling point of the solution, heat flows from vapour to solution and more vapours are generated. The principle of mechanical vapour recompression is depicted in the below figure.
In this method, the vapour is compressed by means of a steam jet ejector. Here the high-pressure steam is used to draw and compress the major part of the vapours from the evaporator while the remaining part of the vapour is separately condensed for compensating motive steam added.
Thermal recompression is better suited than mechanical recompression to vacuum operation. Jets are cheaper and easier to maintain than compressors. Disadvantages of thermal recompression include low mechanical efficiency of jets and lack of flexibility to meet changed operating conditions.
By this vapour recompression, we can obtain multiple effect economy in the single effect. The question that arises is why not the compressor the vapour from the evaporator and expand a small amount of heat to get a kg of steam instead of spending a large amount of heat energy in a boiler to obtain the same kg of steam? The major reasons are
(1) The vapour compression evaporator and small temperature drop make compression economical. The optimum temperature drop is 10 oF whereas in multiple effect evaporator may work on a temperature drop of 100 oF. Thus the single effect must have as much surface in one effect as the sum of all the heating surfaces in multiple effects.
(2) The compression equipment is expensive.
(3) They must be a standby steam capacity for supplying heat for starting.
(4) If the boiling point elevation of the solution is appreciable then the energy needed for compression increases very rapidly.