The water drum is larger than the header, but both are smaller than the steam drum. The water drum equalizes the distribution of water to the generating tubes. Both the water drum and the header collect the deposits of loose scale and other solid matter present in the boiler water. Both the drum and the header have bottom blowdown valves. When these valves are opened, some of the water is forced out of the drum or header and carries any loose particles with it. DO NOT OPEN THE BOTTOM BLOWDOWN VALVES ON A STEAMING BOILER. Opening these valves will interrupt the circulation of the steam cycle.

DOWNCOMER TUBES

At each end of the steam drum are a number of large tubes fig 4-3 that lead to the water drum

Figure 4-3.-Downcomer tubes.

and sidewall header. These tubes are the downcomers through which water flows downward from the steam drum to the water drum and the header. The downcomers range in diameter from 3 to 8 inches.

GENERATING TUBES

Many tubes link the steam drum to the water drum and to the header. The tubes that lead from the water drum to the steam drum are the generating tubes figure 4-4. They are arranged in the furnace so the gases and the heat of combustion can flow around them. The large arrows in figure 4-4 show the direction of flow of the combustion gases.

The generating tubes are made of steel that is strong enough to withstand the high pressures and temperatures within the boiler. In most boilers these tubes are usually 1 to 2 inches in diameter, but some may be 3 inches. These small tubes present a large surface area to absorb furnace heat. A 2-inch tube has twice the surface area of a 1-inch tube but four times the volume. A 3-inch tube has three

Figure 4-4.-Generating tubes and furnace area.

times the surface area of a 1-inch tube but nine times the volume. The smaller the diameter of the tube, the higher is the ratio of absorption surface to the volume of water.

Normally, only one row of tubes leads from the steam drum to the sidewall header. These are the sidewall (water wall) tubes. Their function is to cool and protect the side wall of the furnace.

So far, we have assembled the drums, header, downcomers, and generating tubes. Before going any further with the assembly, let us trace the path of the water through the boiler.

As the water is heated, it becomes less dense, and steam is formed in the tubes. The water in the steam drum is much cooler than the steam and has greater density. As the hotter water and steam rise through the generating tubes, the cooler more dense water drops through the downcomers to the water drum and headers. The arrows in figure 4-5 show the circulation path of the water as it leaves the steam drum and returns to the steam drum as steam. Notice that the caption under figure 4-5 states that it is an accelerated type. This is indicated by the inclination of the tubes. The tubes

Figure 4-5.-Natural circulation (accelerated type).

shown are almost vertical. The greater the incline, the greater the acceleration.

So far, we have learned how the steam is formed in a boiler. Next, let's find out what happens to the steam once it returns to the steam drum from the generating tubes.