Such units are currently being designed for capacities from 750,000 to 3,000,000 or more lb of steam per hr at pressures of 1200 to 5000 psi and temperatures of 950° to 1200° F. Because of the quantity of fuel burned, they are designed for efficiencies of 87 to 90 per cent and always include a large air heater. They are fired by pulverized coal or cyclone furnaces, or, where the economics of the situation permit, by gas or oil or a combination of these fuels. Since it is stan­dard practice to install one steam generator per turbine, they are very carefully designed to insure reliable and continuous operation for long periods of time. Depending on boiler insurance requirements and state laws, they may be operated for two to three years without a major shutdown for cleaning and overhaul.

In a single-drum unit having a capacity of 800,000 lb per hr at 1350 psig and 955° F superheat temperatures two large downcomers deliver water from the steam drum to the four headers that supply the furnace-wall tubes in the front, rear, and side walls of the furnace. These furnace tubes deliver their steam-water mixture to the boiler drum. Practically all the steam is generated in the furnace walls. The steam flows from the boiler drum to a heat exchanger that is used for superheat control and then through the counter-flow superheater. It should be noted that the hot end of the superheater is next to the furnace.

There are four rows of boiler tubes between the superheater and the economizer. Final cooling of the gases occurs in a regenerative air heater.

A considerable number of large steam generators of the forced-circulation type have been installed for operation at pressures from 1800 to 2700 psig. Feedwater is fed through a conventional counter-flow economizer to a boiler drum. Also, steam from the boiler drum flows through a conventional superheater. Water from the boiler drum flows by gravity to a circulating pump which discharges into a distributing header. Water from the distributing header flows through long small-diameter boiler tubes located in the walls and roof of the furnace to the drum, where the steam is separated and the water returns to the pump. Orifices at the inlet to each circuit at the distributing header correctly proportionate the water among the many parallel circuits so that each one receives its proper share. The circulating pump raises the water pressure to about 40 psi above the drum pressure, this being sufficient to overcome the resistance of the flow-controlling orifices and 'the long circuits of small-diameter tubing. These tubes may be constructed of thinner walls than would be required by the larger tubes that are used in natural-circulation boilers and may be arranged so that the flow is upward, horizontal, downward, or any combination thereof.

In the conventional forced-circulation boiler, the amount of water circulated is four to five times the amount of steam generated and an effective steam-separating drum is as essential as in the natural-circulation boiler. Recent development in feedwater treatment has resulted in feedwater of high purity. This has made it possible to build steam-generating units in which the large horizontal steam drum has been eliminated.

The conventional steam drum is replaced by a vertical water separator. Water is pumped through small-diameter tubes in the furnace walls and is converted into steam of high quality which is discharged into the water separator. Here the small amount of unevaporated water is separated from the steam and is blown down to a lower pressure, carrying out with it any impurities that have been concentrated in the water as a result of evaporation. The dry steam from the separator then passes through four- sections of superheater tubing, designated as superheater I, II, III and IV, to the turbine. The steam is resuperheated in the reheater to the initial temperature at a pressure of about 30 per cent of the initial pressure. An economizer and air heater are provided to cool the products of combustion to the low temperature nec­essary for high efficiency.

For operation at pressures above the critical pressure, 3206 psia, water does not boil. No boiler drum is required, and the steam generator becomes essentially a continuous circuit of seamless steel tubing with intermediate headers of small diameter. Such a unit is known as a "once-through" steam generator.

The unit is fired by eight 10-ft diameter cyclone furnaces arranged to discharge from opposite sides into a common secondary furnace. The gases flow upward through the secondary furnace and through three parallel vertical passes to three air heaters. The walls of the three parallel vertical passes are constructed of closely spaced steam-generating tubes. The primary and secondary superheater, the first and second reheaters, and the economizer are located in these passes.

The feedwater flows from the economizer section to the cyclones through outside downcomers. From the cyclone, the fluid flows upward through the secondary furnace-wall tubes and the convection section baffle-wall tubes which form the walls of the three parallel gas passes above the furnace. From the baffle walls, the fluid flows through the primary superheater, a heat exchanger or attemperator that is used for superheat control, and then through the secondary superheater to the superheater outlet and turbine. The transition from water to steam occurs in the upper part of the furnace enclosure.

After expansion in the turbine to an intermediate pressure, the steam is reheated in the first-stage reheater to 1050° F. After further expansion in the turbine, it is reheated in the second-stage reheater to 1050° F. It should be noted that the superheaters and reheaters occupy a major part of the total volume of the installation.

Final superheat and reheat temperatures are controlled by a heat exchanger between the primary and secondary superheater, damper above the economizers in the three parallel vertical gas passes, and recirculation of gas from a location beyond the economizer to the secondary furnace above the cyclones.



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