Mass Purging of Evaporation Systems with Uniform and Non-Uniform Heating Along the Tubes.

When starting up steam generators under pressure (in particular during quick starting of generators with a fixed evaporation point), the purged masses of water must be discharged from the drum as quickly as possible for the safety of the superheater. In order to dimension the drain conduits appropriately, the average mass flow expected must be estimated. Evaporator systems with both uniform and non-uniform heating along the tubes have been investigated. The analytical solutions show that with a radiant heating surface and at constant firing rate the mass purging depends on the pressure and in the presence of convection, also on the type of flow. To control or, as necessary, limit the water purging, a few design measures are discussed in both cases with examples. The accuracy of the assumptions made was confirmed by numerous measurements on a quick-starting natural-circulation boiler.