The present invention relates generally to large commercial boilers, and in particular to a new and useful dual pressure boiler which uses a separate low pressure natural circulation steam generating system in the bottom section of the boiler.
A conventional natural circulation boiler system has a single boiler drum, downcomers, supply tubes, furnace wall tubes, riser tubes and steam/water separators inside the drum. Typically, heated feedwater enters the drum via a feedwater distribution system whose task it is to thoroughly mix the feedwater with the saturated water separated from the steam-water mixture supplied to the separators via the riser tubes.
The resulting water mixture (usually subcooled, i.e., below saturation temperature) enters and flows through the downcomers and is distributed via a number of supply tubes to inlet headers of the furnace circuits, e.g. the wall tubes.
Circulation is established through the difference in fluid density between the downcomers and the heated furnace circuits. The fluid velocity in the furnace circuits (tubes) must be sufficient to cool the furnace tubes, typically exposed to combustion gases whose temperature readily reach the ambient flame temperature of the fired fuel.
The steam-water mixture eventually reaches the outlet headers of the furnace circuits, from where this mixture is led to and distributed along a baffle space and from there to the steam/water separators inside the steam drum.
As soon as the heated fluid reaches saturation conditions, steam is beginning to form and the fluid becomes a two-phase mixture. The fluid velocity must be sufficient to maintain nucleate boiling (bubble-type boiling), as this is the regime which generates the highest possible heat conductance, i.e., the best cooling between the fluid and the inside tube wall on the heated side. Insufficient fluid velocity in combination with high heat flux and excessive percentage of steam in the steam-water mixture leads to steam blanketing, equivalent to an insulating-type steam film along the heated, inside tube wall, which causes rapid tube failure. The danger of film boiling increases with increasing boiler pressure. The fluid temperature in the boiling (two-phase) regime is strictly dependent on the local internal pressure and is nearly constant from the point where boiling starts to the point where the saturated water leaves the separators.
The separators separate the saturated water from the saturated steam, usually through centrifugal force generated through either tangential entry of the two-phase fluid into cyclones or through stationary propeller-type devices. The centrifugal action literally “squeezes” the steam out of the steam-water mixture.
The saturated steam leaves the top of the drum through saturated connecting tubes which supply the steam to the superheater where the steam is further heated to the desired final temperature before being sent to a turbine or a process. The saturated water, as stated earlier, leaves the bottom of the separators and mixes with the continuously supplied feedwater.
Low pressure recovery boilers (generally less than 800 psig operating pressure) have been operating for years without significant material or corrosion problems in the bottom furnace. The bottom furnaces of these units have been designed with much lower level of corrosion protection, such as pin studs and refractory, which has been sufficient in combating corrosion for long periods of time. As the operating pressure of a recovery boiler and the furnace tube metal temperature increases, the corrosion rate increases significantly which has resulted in the need for more exotic corrosion protection in the bottom furnace. However, use of exotic materials has significant disadvantages. Such metals are prone to cracking and require extensive inspection and maintenance efforts.
A full disclosure of steam drums specifically and boilers in general can be found in Steam/Its Generation and Use, 40th Ed., Stultz and Kitto, Eds., © 1992 The Babcock & Wilcox Company.