The commonest was of explaining boiling point elevation is to imagine two insulated containers A and B as shown in Fig. 2.6. A contains sea water and B pure water. The sea water temperature is iA, and The pure water temperature fB, the vapour temperatures are equal and the vapour space between the two connected by a compressor. For equal temperatures in both A and B the vapour pressure in B is greater than that of A, i.e. PB > PA . Thus in order to prevent vapour transport from B to A the compressor must equalise the pressure. If the insulation is removed and temperature variation allowed fA must be increased above tB so that = P-&. In other words the lower vapour pressure for the sea water manifests itself as a boiling point elevation which is a function of the dissolved solids concentration.
Boiling point elevation 21
Operating temperature Fig. 2.7. Boiling point elevation for various concentrations of sea water.
Figure 2.7 shows typical boiling point elevations for various concentrations of sea water over the normal working range of sea water evaporators. For brine of 2 x sea water concentration at 88°C (190°F) and boiling point elevation is 1.11°C (2°F). What this means to the evaporator designer is that the water vapour evolved from brine at 88°C (190°F) in the flash chamber will have a temperature of 86.89°C (188°F) which is a serious loss for high performance ratio plants. Thus in the first stage of the evaporator of Fig. 2.4, the flashing brine exit temperature tx is 88°C and the vapour temperature 7\ = 86.89°C just above the brine surface.
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