Evaporator with steam export and separate condensate pre-heater

Evaporator with steam export and separate condensate pre-heater

Evaporator with steam export and separate condensate pre-heater

This system is in fact a 2-stage waste heat boiler in which the deaerator and LP-drum are integrated in a single vessel, often referred to as Internal Drum Deaerator (IDD). A part of the steam that is produced by the LP evaporator (or deaerator – evaporator) is used to heat the incoming condensate and for deaeration. The remainder is used as steam export (e.g. to steam turbine or process).

The system can be operated at constant pressure or sliding pressure. In case of sliding pressure a certain minimum pressure (and pressure drop rate) must be maintained.

The LP drum deaerator has two functions, namely:

  • The deaeration of condensate
  • The supply of LP steam.

These two functions must be more or less physically separated in the vessel. The steam which comes into contact with condensate that has not yet been fully deaerated must not be used as export steam since oxygen will still be present.

The quantity of steam, which is supplied to the deaeration section must be smaller than the quantity of steam required to heat the condensate. Under all operating conditions. As a result, a net steam transport is maintained from the export steam section to the deaeration section. This avoids steam containing oxygen being used as export steam.

When designing the LP drum, it must be determined how many and which of the lines from the condenser bundle (risers) need to be assigned to the deaeration section. To this end, the boiler manufacturer/system designer must provide the necessary information regarding the distribution of steam across the risers for each load situation.

Internal Drum Deaerator (IDD)

An ideal separation between the export steam and deaeration sections is achieved by using the Stork Internal Drum Deaerator (IDD). In this IDD design condensate heating and deaeration take place in a separate unit above the water level in the LP-drum. The required steam is extracted from the steam space in the LP drum.