Evaporation thermal recompression

THERMAL RECOMPRESSION. In a thermal recompression system the vapor is compressed by acting on it with high-pressure steam in a jet ejector. This results in more steam than is needed for boiling the solution, so that excess steam must be vented or condensed. The ratio of motive steam to the vapor from the solution depends on the evaporation pressure for many low-temperature operations, with steam at 8 to 10 atm pressure, the ratio of steam required to the mass of water evaporated is about 0.5. 

Since steam jets can handle large volumes of low-density vapor, thermal recompression is better suited than mechanical recompression to vacuum evaporation. Jets are cheaper and easier to maintain than blowers and compressors. The chief disadvantages of thermal recompression are the low efficiency of the jets and lack of flexibility in the system toward changed operating conditions. 

Recompression evaporation is an alternative to the multiple-effect approach that reduces steam consumption. Section 7.1.5.2B treated both mechanical and thermal recompression. The power consumption or the motive steam usage depends on the increase in pressure that is necessary to achieve the desired condensing temperature. 

The discussion of vapor-recompression evaporation in Section 7.1.5.2B distinguished between mechanical and thermal recompression. Vacuum systems (Section 12.6.1) can be divided in the same way between those relying on compressors and those using ejectors. Hybrid systems using both types are also quite common. Since the amount of vapor that it handles fixes the size of a vacuum producer, removal of evaporated water between... 

Example. We wish to operate an evaporator at 165 kPa with condensing steam at205 kPa. Saturated steam is available at 1,135 kl for thermal recompression of the vapor. From the steam tables, the enthalpies in kJkg" at the connection points are H = 2,782, H2 = 2,442, 7/4 = IJOn. Equation (12) also requires H3, which is the enthalpy at an internal point. Its estimation requires the use of the individual efficiencies described in the text. We assume t) = 0.98, ri2 = 0.85, and in = 0.95. 

That the evaporator could be made as efficient as economically justifiable. Thermal efficiency of an evaporator is increased by multiple-effect operation, by recompression of the vapor, by a combination of these, and by a number of other design features. While sea water evaporators had rarely been made with more than three effects, commercial evaporators of six and seven effects are common and ten-effect evaporators have been used. 

B. Vapor-Recompression Evaporation. The existence of the BPR in a solution means that the condensing temperature of the vapor raised in an evsqiorator will be lower than the boiling point of the solution from which it came. In other words, the vapor as it forms is superheated. When the vapor is used in another effect, the superheat provides very little thermal energy, and the vapor temperature quickly drops to the saturation temperature of pure water at the operating pressure. 

In a single-effect vapor recompression (sometimes called vapor compression) evaporator the vapor is compressed so that its condensing or saturation temperature is increased. This compressed vapor is returned back to the heater of steam chest and condenses so that vapor is formed in the evaporator (B5, Wl, Zl). In this manner the latent heat of the vapor is used and not discarded. The two types of vapor recompression units are the mechanical and the thermal type. 

Compression evaporation could be defined as an evaporation process in which part, or all, of the evaporated vapor is compressed by means of a suitable compressor to a higher pressure level and then condensed the compressed vapor provides part of all of the heat required for evaporation. Compression evaporation is frequently called recompression evaporation. All compression methods use the vapors from the evaporator and recycle them to the heating side of the evaporator. Compression can be achieved with mechanical compressors or with thermal compressors. Thermal compression uses a steam jet to compress a fraction of the overhead vapors with high pressure steam. Mechanical compression uses a compressor driven by a mechanical drive (electric motor or steam turbine) to compress all the overhead vapors. 

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