A steam boiler has system circulation of water to converts it into steam continuously. In the Water Tube Boiler, water starts from water treatment system will be distributed to steam boiler through economizer to steam drum. From steam drum water flow to lower header through downcomer then be distributed again to water wall tubes to generate saturated steam. Saturated steam is the generated steam and mixture water-steam in the same temperature with water. The mixture will be separated in steam drum, steam tends to move upward to be heated again in the superheater then to be sent to turbine generator and water move downward to downcomer to be evaporated again. Based on circulation of water in the steam boiler, circulation can be commonly classified as follow:
1. Natural or thermal circulation
Water from steam drum is distributed to water drum and lower drum through down comer. Water in down comer is unheated water. Evaporation occurs when water flow to water wall then get heat transfer from combustion process in the furnace. Heat transfer generates steam and mixture water-steam in water wall. Steam and mixture steam-water has less density than water, so gravity can cause the water will flow downward to downcomer, and steam/mixture will move upward into steam drum of steam boiler. The difference of density between water and mixture water-steam will influence the rate of circulation. The rate of circulation is affected by:
- The height of steam boiler; if the steam boiler is taller will cause higher difference of pressure between unheated segment (downcomer) and heated segment (water wall tubes), so this condition will increase the rate of circulation.
- The operating pressure of steam boiler; if the steam drum has high pressure will cause the density of steam and mixture water-steam is higher and cause the difference pressure between water and steam/mixture is less and decrease the rate of circulation.
- The rate of heat input: if the rate of heat input is increased will generate a lot of steam and reduce density of mixture steam-water, so increase the difference of density and the rate of circulation.
- The free flow areas of component; the increase of the free flow area will increase the rate of circulation.
2. Forced Circulation
High pressure pump is installed in downcomer pipe of steam boiler to force and control the water flow to water wall tubes. Forced circulation may be used at high pressures when natural circulation forces are small or maybe used at lower pressure if designer want to design tube and other pressure part layout as he likes.