Текст 6. Boiling of water

Water and steam are typically used as heat carriers in heating systems. Steam, the gas phase, water, results from adding sufficient heat to water to cause it to evaporate. This boiler process consists of three main steps: the first step is the adding of heat to the water that raises temperature up to the boiling point of water, also called preheating. The second step is continuing addition of heat to change the phase from water to steam, the actual evaporation, the third step is the heating of

steam beyond the boiling temperature of water, known as superheating. The first step and the third steps are the part where heat addition causes a temperature rise but no phase change. When all the water has been evaporated, the steam is called dry saturated steam. If steam is heated beyond its saturation point, the temperature begins to rise again and the steam becomes superheated steam. Superheated steam is defined by its zero moisture content: it contains no water at all, only 100 % steam.

Evaporation

During the evaporation the enthalpy rises drastically. If water is evaporated at atmospheric pressure from saturated liquid to saturated vapor, the enthalpy rise needed is 2260 kJ/kg, from 430 kJ/kg (saturated water) to 2690 kJ/kg (saturated steam). When the water has reached the dry saturated steam condition, the steam contains a large amount of latent heat, corresponding to the heat that was led to the process under constant pressure and temperature. So despite pressure and tempera- ture is the same for the liquid and the vapour, the amount of heat is much higher in vapour compared to the liquid.

Superheating

If the steam is heated beyond the dry saturated steam condition, the tempera- ture begins to rise again and the properties of the steam start to resemble those of a perfect gas. Steam with higher temperature than that of saturated steam is called superheated steam. It contains no moisture and cannot condense until its tempera- ture has been lowered to that of saturated steam at the same pressure. Superheating the steam is particularly useful for eliminating condensation in steam lines, de- creasing the moisture in the turbine exhaust and increasing the efficiency (i.e. Car- not efficiency) of the power plant.

Effect of pressure on evaporation temperature

It is well known that water boils and evaporates at 100˚ C under atmospheric pressure. By higher pressure, water evaporates at higher temperature – e.g. a pres- sure of 10 bar equals an evaporation temperature of 184º C. The pressure and the

corresponding temperature when a phase change occurs are called the saturation temperature and saturation pressure. During the evaporation process pressure and temperature are constant, but if the vaporization occurs in a closed vessel, the ex- pansion that occurs due to the phase change of water into steam causes the pressure to rise and thus the boiling temperature rises.

When 22.12 Mpa is exceeded (the corresponding temperature is 374º C), the line stops. The reason is that the border between gas phase and liquid phase is blurred out at that pressure. The point, where the different phases cease to exist, is called the critical point of water.