3.3.2 Effect of reheat conditions
Now consider the selection of reheat conditions for a single reheat cycle. Having established the steam conditions at entry to the HP turbine, there are several design compromises involved in the choice of reheat pressure and temperature. First, consider the boundary conditions.
The HP cylinder exhaust steam must be sufficiently superheated to avoid any wetness in the cold reheat pipework, which would cause severe erosion. As in the previous section, the exhaust steam from the LP cylinder must be wet, but not greater than 12% wetness. These boundaries are illustrated, together with the effects of changing pressure, on the Mollier diagram (Fig 1.39). Now consider the effect of changing reheat temperature for a constant pressure drop in the HP cylinder (Fig 1.40). The effect of reducing the pressure or of raising the temperature of the reheat steam is to reduce LP exhaust wetness.
Although reducing LP exhaust wetness is desirable, too high a temperature could lead to some difficulties with materials. The common practice has been to reheat to the original superheat steam condition, i.e., 565°C.
Another important factor in the design of the re-heater is the effect of pressure losses in the pipework. The pressure loss results from a throttling effect which reduces the available energy for work (to be discussed later in this section). The pressure at which steam is reheated is about 25% of the stop valve condition, representing approximately a fourfold increase in volume. This necessitates the use of increased pipe sizes which are expensive and inflexible. The plant designer has to weigh the merit of using larger pipes, or pipe runs in parallel, versus the improvement in cycle efficiency. The usual pressure drop is 7.5 to 10% of the HP cylinder exhaust pressure. Finally, constraints will exist in the boiler on the range of conditions attainable.