Thermodynamics Term

Steam boiler has main task to convert water to high quality steam. When water is heated at atmospheric pressure, water will be converted to steam at temperature 212 F. When water is heated at higher pressure than atmospheric pressure, the boiling temperature will increase.

Enthalpy is the heat energy which is required to heat water from temperature 0 C (3 F) to the boiling temperature. Enthalpy is expressed as Btu/lb (British thermal unit per pound). When temperature from 32 F is increased gradually until 212 F, water will go to the boiling point temperature. If heat temperature is added more than 212 F, the water will converted to steam and the temperature can called as steam temperature.

Saturation temperature is reached when temperature of water is same with temperature of steam. Saturation temperature shows that at the boiling process, temperature is constant. Enthalpy of evaporation which means the heat required to convert water liquid phase to steam phase, will occur when heat temperature is increased from the boiling temperature.

The total enthalpy of saturated steam is the sum of enthalpy of saturated liquid and enthalpy of evaporation. Another means is total heat which is required to convert water and produce steam totally (see figure 1).

From figure 1, can be known that phase 1-2 is water phase, 2-3 is the boiling phase where there are mixture between water and steam at constant temperature. Phase 3-4 is superheated phase where water is converted 100% to steam.

In steam boiler, phases 1-2 occurs in economizer, water drum, downcomer and lower drum, while phase 2-3 occurs on top 1/3 furnace tubes and steam drum. Finally phase 3-4 occurs in superheater and reheater.