Vapor-compression evaporation

Vapor-Compression Evaporation and Waste Heat Evaporation. Both of these processes remove water from contaminants rather than contaminants from water. They are better suited for industrial installations where excess energy is available. The water thus produced is of high quaUty and can be used directly. An important advantage is the concentration of waste-residue volume with attendant economies of handling and transportation... 

Mechanical efficiency may range from 25-75% of the theoretical evaporation rate. Efficiencies may be raised with the application of multieffect or vapor compression evaporators. The more complicated efficient systems can seldom be warranted due to the short service offered. 

Vapor compression evaporator, see Evaporator, vapor compression ... 

It is evident that for any At less than about 12° F. in the vapor-compression evaporator, the energy cost will be less than that of the multiple-effect evaporator even if the latter had an infinite number of effects. With a 15° F. At in vapor compression it would require a 20-effect evaporator for equivalent energy cost. 

In the 1950s Hickman developed a centrifugal vapor compression evaporator for seawater desalination (53). This device consisted of multiple spinning discs. Seawater sprayed on one side of the disc evaporated, while the centrifugal force removed the residue from the plate surface. The vapor was compressed and returned to the opposite side of the plate, where condensation provided the heat for evaporation and the desired freshwater for recovery. Overall heat transfer coefficients of 18 kW/m2-K are about three times higher than those achieved in steam turbine condensers. 

The ability of the spinning disc to operate with very small driving temperature and concentration differences can improve the thermodynamic efficiency of the overall process system. This is clearly the case with the Hickman vapor compression evaporator, and it is also exemplified in the applications described next. In general, the power needed to rotate the spinning disc assembly is a small fraction of that saved by virtue of the establishment of an intensified fluid dynamic environment. 

The basic t5q)es of evaporators are pot evaporators and circulation—either natural or forced—evaporators. Figure 11.20 shows a natural-circulation evaporator. To improve the economy of the process, vapor compression may be employed. Vapor-compression evaporators make the latent heat of condensation available at a higher temperature to use the energy potentials of vapors by compressing it and combining it with fresh steam input. 

Temperature difference in vapor-compression evaporator is 5° F. (boiling-type evaporator). 

Dodge, B. F., Eshaya, A. M., Development of a Pilot Plant for a Forced-Circulation and Drop wise Condensation Vapor-Compression Evaporator and an Experimental Program, Rept. to OSW under Contract 14-01-001-172 (September 1959). 

When raw sea water was fed to the vapor compression evaporators the 6-p.s.i. cutoff pressure differential was exceeded in 150 hours. Mg(OH) > and CaS04.-v2h2o were found by x-ray analysis in the tube scale samples. The evaporation path for raw sea water as shown in Figure 1 indicated that CaS0.1/2H20 precipitation was expected and did occur. 

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. 

Venting is especially important for a vapor compression evaporator because reduced evaporator capacity can result in compressor surge. 

Figure 19-10 Performance curve of vapor compression evaporator with inlet guide vane control.

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