The density of saturated water, which is 958 kg/m3 (59.8 lb/m3) at 100°C, decreases with increasing pressure, as the saturation temperature increases, making the water expand. The density of steam, on the contrary, increases with increasing pressure as it is com­pressed. This density differential at any pressure is the driving force behind promoting circulation in an evaporator circuit. At a pressure of 70 bar, water is —20 times heavier than steam. At a pressure of 200 bar, the density differential narrows to —3, which is still ade­quate to have natural circulation in boilers. At the critical point the two densities become equal—315.5 kg/m3 (19.7 lb/ft3), as shown in Figure 2.2.

2.2.1 Heat Distribution at Various Pressure Levels

With increasing steam pressure, the latent heat addition reduces. Consequently, the work­loads of the superheater (SH) and reheater (RH) keep increasing while that of evaporator/ boiler keeps reducing. Table 2.1 illustrates this point. Figure 2.3 illustrates this for pressures >100 bar. The heating surfaces (HSs) also maintain a similar trend as the pressures increase.