Summary
Highlights
Boilers use water in tubes to absorb heat from burning fuel, producing steam to drive turbines and provide heat. Proper water circulation is crucial to continuously supply water for steam production and prevent tube overheating. The water path includes the economizer, boiler drum, downcomers, headers, and boiler tubes. Feedwater is preheated by combustion gases in the economizer before entering the boiler drum.
Natural circulation in boilers relies on the density difference between cooler water in downcomers and the hotter water-steam mixture in the water walls. This creates a self-sustaining flow. Natural circulation is affected by boiler furnace heat and operating pressure. Controlled circulation uses pumps to force water through the system, increasing flow and heat absorption, allowing for more steam production in a given boiler size.
The boiler drum receives heated feedwater and collects steam produced in the water walls. It separates the water-steam mixture into water, which recirculates, and steam. Baffles (drum shrouds) and moisture separators extract water from the steam. Further dryers remove residual water, preventing damage to downstream components before the steam exits through outlet pipes.
Boiling is the conversion of water to steam. Saturation temperature is the boiling point of water at a given pressure, increasing with pressure. When water is present, the temperature in water walls remains at saturation temperature. Superheated steam is steam heated above its saturation temperature after all water has been removed, providing more energy for turbines and increasing efficiency.
Steam from the drum passes through superheaters to become superheated. Superheaters can be radiant (directly exposed to flames) or convection (heated by hot gases), and are classified as primary or secondary. Attemperators (desuperheaters) regulate steam temperature by spraying water, preventing tube damage. Reheaters, similar to superheaters, increase the temperature of steam returning from the high-pressure turbine before it goes to intermediate and low-pressure sections.
After passing through turbine sections, spent steam flows to the condenser. Here, cooling water absorbs heat, condensing the steam back into water. This water is collected and pumped back to the boiler, completing the closed-loop water and steam cycle.
The critical point (705°F and 3206 PSI) is where water and steam densities are identical, eliminating the distinction between them. Subcritical boilers operate below this point, requiring heat to convert water into steam, leading to inherent inefficiency. Supercritical boilers operate above the critical point, converting all heat to raise steam temperature, thus being more efficient. Once-through boilers, often supercritical, do not recirculate water, simplifying their design by removing the need for a drum or separators.
In drum-type boilers, steam pressure is primarily controlled by the firing rate. Increasing the firing rate increases steam production and pressure, and vice versa. In once-through boilers, steam pressure is mainly controlled by the boiler feed pumps. Changes in firing rate in once-through boilers primarily affect water wall temperature, not significantly the steam pressure.