THE STEAM-GENERATING UNITS
For operation at pressures below the critical pressure, a steam-generating unit consists of a boiler, superheater, air heater, and (or) economizer. The furnace walls are either partially or fully covered with boiler tubes. In general, most of the steam is generated in the furnace-wall tubes since they can absorb radiant energy from the high-temperature flame.
A typical stoker-fired steam-generating unit in the smaller size range, has a capacity of 72,500 lb of steam per hr. The gases as they leave the completely water-cooled furnace pass across the superheater surface, then the convection tubes of the boiler, then upward through a small economizer, downward through a tubular air heater, dust collector, and fan, to the chimney. The boiler is of the two-drum type without gas baffles; that is, it is a single-pass boiler. The internal baffles in the steam drum are so arranged that the last four rows of boiler tubes in which the heat-transfer rate is quite low are downcomers. Since a major item in the cost of a boiler is the drums, as many boiler tubes as possible are placed between the drums. A large amount of surface is required to cool the gases from the temperature at which they leave the superheater to the final temperature.
Depending upon the steam pressure, the feedwater is heated in regenerative feed-water heaters to 275° F to over 600° F, depending on pressure, before being admitted to the economizer. Essentially, the economizer raises the feed-water temperature almost to the saturation temperature, the boiler supplies the latent heat, and the superheater supplies the superheat. It will be noted that, as the pressure increases, a decreasing portion of the total energy absorption occurs in the boiler and that, for pressures above the critical there is no boiler. Supercritical-pressure steam generators essentially are economizers connected to superheaters. There is no steam drum since there is no boiling and no steam to.be separated from water at a constant temperature.
At the higher pressures at which natural circulation boilers may be used, the boiler becomes a smaller part of the installation and the superheater and reheater become a larger portion of the total heat-transfer surface.
Modern high-capacity steam-generating units have been developed to the point that they can be depended upon to carry heavy loads continuously for months at a time. Their reliability is approximately equal to that of modern steam turbines. Consequently, most new central-station power plants are built on the unit system: that is, with each turbine generator supplied with steam from its own steam-generating unit. Thus, turbine-generator units in capacities up to 500,000 kw are being supplied with steam from a single steam-generating unit. One of the major reasons for this arrangement is the decreased cost per unit of capacity which results from increased size.
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