Describe the process of 'dry-out' in a boiler and its consequences.

Dry-out in a boiler refers to the condition where the water film that normally protects the boiler tubes from overheating is lost, leading to direct contact between the hot combustion gases and the tube metal. This occurs when the heat input exceeds the rate at which water can be supplied to the tube surface to remove the heat through boiling. The water film acts as a coolant, preventing the tube metal from reaching excessively high temperatures. When dry-out occurs, the tube metal temperature rises rapidly, significantly reducing its strength and increasing the risk of tube failure. This is especially critical in areas of high heat flux, such as the furnace walls. The consequences of dry-out are severe and can include: Tube overheating and bulging: The increased metal temperature causes the tubes to weaken and deform under pressure. Tube rupture: If the metal temperature becomes high enough, the tubes can rupture, releasing high-pressure steam or water into the boiler room. This can cause significant damage and pose a serious safety hazard. Reduced boiler efficiency: Dry-out can lead to scale formation on the tube surfaces, which reduces heat transfer and lowers boiler efficiency. Increased maintenance costs: Repairing or replacing damaged boiler tubes is expensive and time-consuming. Dry-out can be caused by several factors, including: Low water level: Insufficient water in the boiler reduces the area available for heat transfer and increases the risk of dry-out. Overfiring: Excessive heat input can overwhelm the boiler's cooling capacity. Scale buildup: Scale on the tube surfaces reduces heat transfer and can lead to localized hot spots and dry-out. Flow restrictions: Blockages in the water circulation system can reduce the flow of water to critical areas. For example, if a boiler's water level drops too low due to a malfunctioning feedwater pump, the upper tubes in the furnace walls may experience dry-out, leading to overheating and potential tube rupture.