How is Steam Generated at a Given Pressure?

Consider one kg of water at 0°C. Also let us decide upon the pressure under which one kg of water is to be heated. Consider a cylinder with a frictionless piston loaded such that the pressure P under consideration is acting on water.

Heat is added to the water so that its temperature increases from 0°C to t_sat (saturation temperature) at which the water boils at pressure P. Volume of water increases to V_f, This is shown in Fig. 10.4(a).

Heat supplied or added to 1 kg water to raise its temperature from 0°C to saturation temperature t_sat is called sensible heat because it can be detected by the sense of touch and produces a rise in temperature to be seen on a thermometer. Generally this sensible heat is denoted by h_f.

Note:

V_f means volume of fluid which is taken as liquid.

Heating is continued after the water reaches saturation temperature. Evaporation of water takes place and steam is generated at pressure P. This heating is continued till all the water is evaporated and the temperature of the steam is saturation temperature. V_g or V_sat denotes the volume of steam at this condition. The external work done during evaporation is given by W = P (V_sat – V_f) or p (V_g – V_f) with proper units.

Because the heat during this stage of evaporation cannot be recorded by a rise in temperature on the thermom­eter, it is called Latent heat (hidden heat).

The amount of heat required to evaporate 1 kg of water at saturation temperature to steam or vapour at saturation temperature is called Latent heat or enthalpy of evaporation and is generally denoted by h_fg. This is shown in Fig. 10.4(b).

Once all the water is evaporated to saturated steam, if heat is added further, the steam behaves approximately as an ideal gas and the heating of saturated steam increases its temperature. The steam whose temperature is greater than saturation temperature at the given pressure, is called a superheated steam; and let its temperature be denoted by t_sup. Its volume will also increase to V_sup.

The amount of heat required to raise the temperature of l kg steam from t_sat to t_sup is called superheat and the temperature difference called degrees of superheat.