Thermal energy in the vapor evolved from a boiling solution can be utilized to vaporize more water if at all there is a temperature drop for heat transfer in the desired direction. In case of multiple effect evaporation systems, this temperature drop is created by progressively lowering the boiling point of the solution in 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 vapor evolved by (a) Mechanical recompression or (b) Thermal recompression.
The compressed vapor having higher condensing temperature is then fed to the steam chest of the evaporator from which it came. So economy of the evaporator is also increased by recompressing the vapor from evaporator and condensing it in the steam chest of the same evaporator. In this method, the vapour from the evaporator are compressed to a saturation pressure of steam to upgrade the vapors to the conditioning of original steam to permit the use as heating media. The cost of supplying the required amount of compression usually smaller then the value of latent heat in vapor. By this we can obtain multiple effect economy in single effect.
Mechanical Recompression :
In this method the vapor evolved from the evaporator is compressed to some what higher pressure by positive displacement (or) centrifugal compressor and fed to a heater as a steam. As the saturated temperature of compressed vapor is higher than boiling point of solution, heat flows from vapor to solution and more vapors are generated. The principle of mechanical vapor recompression is depicted in below figure.
In this method vapor is compressed by means of steam jet ejector. Here the high pressure steam is used to draw and compress the major part of vapors from the evaporator while remaining part of vapor is separately condensate 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 vapor recompression, we can obtain multiple effect economy in single effect. The question that arises have is why not the compressor the vapor from evaporator and expand small amount of heat to get a kg of steam instead of spending large amount of heat energy in a boiler to obtain the same kg of steam? The major reasons are
(1) The vapor compression evaporator and small temperature drop to make compression economical. Optimum temperature drop is 10 oF where as in multiple effect evaporator may work on temperature drop of 100 oF. Thus the single effect must have a much surface in one effect as the sum of all the heating surfaces in a multiple effect.
(2) The compression equipment is expensive.
(3) They must be stand by 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.