The steam temperature in packaged boilers is often controlled from 60% to 100% load by using a two-stage superheater design with interstage attemperation as shown in Fig. 3.18. Steam temperature can also be maintained from 10% to 100%; however, this calls for a much larger superheater surface area. Deminer­alized water should be used for attemperation, because it does not add solids to

The steam. The solids in the feedwater used for attemperation should be in the same range as the final steam purity desired, which could be as low as 30-100 ppb. If solids are deposited inside the superheater, the tubes can become overheated, particularly if operated at high loads and high heat flux conditions. The convective superheaters are generally oversized at 100% load as explained earlier. The quantity of water spray is larger at higher load. In the radiant design, the steam temperature remains nearly flat over the load range because the radiant component of energy increases at lower loads and decreases at higher loads. Thus many radiant superheaters do not use a two-stage design. However, reviewing other concerns such as possible overheating of tubes and higher tube wall temperatures, the choice is left to the user.

When demineralized water is not available, a portion of the saturated steam from the drum is taken and cooled in a heat exchanger, preheating the feedwater as shown in Fig. 3.18. The condensed water is then sprayed into the attemperator between the two stages of the superheater. Often, in order to balance the pressure drops in the two parallel paths, a resistance is introduced into each path or the exchanger is located vertically up, say 30-40 ft above the boiler, to provide additional head for the spray water control valve operation.

Spraying downstream of the superheater for steam temperature control is not recommended, because the steam temperature at the superheater exit increases with load, thus increasing the superheater tube wall temperature, which can lead to tube failures. For example, if 800°F is the final steam temperature desired, the steam temperature at the superheater exit may run as high as 875-925°F, which will diminish the life of the tubes over a period of time. Also, the water droplets may not evaporate completely in the piping and the steam turbine could end up with water droplets and the solids present in the water, leading to deposits on turbine blades.