If a single pressure steam cycle was used for magnox reactors, poor overall efficiencies and net power out­puts would result. Figure 2.16 shows a typical heat diagram for the single cycle w’here it is apparent that the attainable steam pressure is dictated by the minimum temperature difference between the gas and saturated water at the ‘pinch point’. Attempts to im­prove the efficiency by introducing feed heating are offset by the necessary reduction in cycle pressure, as shown by the dotted line. In order to maintain the same cycle pressure and obtain the benefits of feed heating it would be necessary to increase the reactor gas inlet temperature, i. e., raise the gas line. However, blower power is very sensitive to this tem­perature and the effect would be to reduce the net electrical output from the plant.

Dual pressure cycle

The disadvantages of the single pressure cycle can be partially overcome by having a dual pressure cycle. A proportion of the steam is raised in a high pres­sure section which is in series on the gas side (as shown in Fig 2.17) with a low pressure section, which effectively cools the gas leaving the boiler to tem­peratures that enable blower power to be minimised. Increased complexity and capital cost limit a practical arrangement to two pressures.

The temperature heat diagram (Fig 2.18), shows how a higher operating pressure is possible for part of the cycle. As cycle efficiency is approximately proportional to the area under the water/steam line, and a higher feed temperature is possible, the overall thermal efficiency is much increased without a loss in net electrical output. For example, with a dual pres­sure system at 14 bar and 56 bar instead of a single pressure system at 14 bar, and w-ith a boiler gas inlet temperature of 385°C and a gas outlet temperature of 172°C, the net electrical output is increased from

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HEAT CONTENT

Fig. 2.16 Typical single cycle heat diagram

Fig. 2.17 Dual pressure steam cycle

62 MW to 69 MW whilst the thermal efficiency in­creases from 25% to 28%. The proportion of HP (o LP steam flow is typically 60% HP and 40% LP with pressure ratios of around 4:1.