With rising temperatures and water near its boiling conditions, some molecules gain enough kinetic energy to reach the speed that made him any time off from the liquid into the space above surface, before falling back into the liquid. Further heating causes greater excitation and the number of molecules with enough energy to leave the liquid increases. Taking into account the molecular structure of liquid and vapor, it makes sense that the density of steam is smaller than water, because the steam molecules are far apart from one another. Space that suddenly occurs above the water surface becomes filled with dense steam molecules.
If the number of molecules leaving the liquid surface is larger than the re-entry, then the water evaporates freely. At this point the water has reached boiling point or saturation temperature, which is saturated with heat energy. If the pressure is fix, adding more heat does not cause further increase in temperature but causes water to form saturated steam.
The temperature of boiling water with the saturated steam in the same system is same, but the heat energy per unit mass is larger in the steam. Saturated temperature at atmospheric pressure is 100 ° C. However, if the pressure increases, then there will be the addition of more heat that the increase in temperature without phase changes. Therefore, the increase in pressure will effectively increase the enthalpy of saturated water and temperature. The relationship between saturation temperature and pressure are known as saturated steam curve (Figure 1).
|Figure: Saturated Steam Curve|
Water and steam can exist simultaneously on a variety of pressure on this curve; both will be at saturation temperature. Steam saturation curve in the above condition is known as superheated steam / steam through the saturated:
- Temperature above saturation temperature is called the degree of saturated steam.
- Water on the conditions under the curve is called sub-saturated water.
If the steam flows from the boiler at the same speed with which it produces, the addition of heat will further increase the rate of production. If the same steam leaving the boiler is not retained, and the amount of incoming heat is maintained, the energy flow to the boiler will be greater than the energy that flows out. This excess energy will raise the pressure, which in turn will lead to saturation temperature increases, because the temperature is related to pressure saturated steam.
In this case the water in a boiler is combustion, water through the economizer which has flowed through the heating inside the drum boiler (steam shelter) and then burned in a boiler to be heated further up to a wet steam. The temperature inside the boiler is approximately 400o C - 459o C. Combustion of fuel coal and assisted with air to maintain combustion stability in combustion system. Combustion control system connects the control of heat input to the boiler with the ratio of air / fuel entering the combustion chamber. This control system must be able to ensure sufficient amount of air available for combustion of a fuel efficiently without causing smoke and with minimum particulate discharge from the chimney. After this process within the boiler, steam flow and then proceed to the superheater to be his dry steam, steam temperature was about 520o C - 600o C and ready to turn turbines.