The Clayton Report

SECTION TWO Controlled Forced Circulation Cont.

NATURAL CIRCULATION

There is an inherent lag characteristic with this system which limits operational flexibility such as quick start-up, rapid load change, and quick shut-down. For example, the circulation rate is different at minimum and maximum firing rates. It requires a few moments after a firing rate change to establish the equilibrium which involves circulation, tube wall temperature and other factors. Steam and water storage capacity is large. Water surface area is also large to permit steam release with limited agitation to control moisture carry-over. Moisture carryover is maximum at full load (2 to 3 percent) and higher during sudden steam demand. In some instances, an external steam/moisture separator is required to maintain desirable steam quality. Large water storage, at saturation temperature, presents an explosion hazard. For this reason, shell and tube maintenance is critical and inspection of their condition must be regularly scheduled. The necessary strong shell construction and required large water volume results in great weight and physical size. The uneven tube temperatures which occur in rapid startups and sudden load changes cause high stresses in the fire-tubes, resulting in warped tube sheets, rear door leakage, shortened life and increased down-time for the unit. Depending on design, the fire box, furnace or front and back sections (whichever is applicable) are lined with refractory cement which results in added weight and lost heat during start-up.

Fire-tube or Scotch-Marine type boilers depend entirely on natural thermal circulation of water within the boiler shell. Water is convected upward between the tubes, usually faster in the rear of the boiler than in the front. Colder water flows downward along the shell, then upward around the furnace tubes to complete the pattern.

NATURAL CIRCULATION

STEAM DISCHARGE

STEAM RELEASE AREA

SHELL

WATER SURFACE

FIRE TUBE

FIGURE 2B

FEEDWATER

SLUDGE

BLOWDOWN

The circulation is caused by the difference in density between the water and steam/water mixture. As the steam bubbles form, they rise to the water surface and release in the steam disengaging area.

FORCED CIRCULATION

CIRCULATION WATER PUMP

GRADUATED SIZE COIL TUBE

FURNACE HEAT

FIGURE 2C