Drum (steam boiler)
The drum (so called because of its usually cylindrical shape) is a central container in water pipe - circulation steam boilers . It acts as a reservoir, distributor and separator for the boiler's evaporator .
Layout and function
The drum is the central node of a boiler: it communicates with all three sections of the boiler water-side: the evaporator, the heater (economizer) and the superheater (where present).
The drum acts as a buffer storage for the evaporator and ensures that it is always and evenly supplied with feed water and thus does not overheat. The circuit is fed via downpipes , which run from the drum above, usually unheated, down from the furnace outside. At the bottom , distribution pipes , the lower collectors , are used to distribute to the individual pipes of the evaporator.
In the evaporator heating surfaces , which are in direct contact with the flue gas of the furnace, the heat is transferred and the feed water is brought to the boil . In circulation boilers , however, the water is not completely evaporated, but only partially, ie a two-phase mixture of boiling water and saturated steam is formed in the evaporator circuit . Depending on the number of revolutions of the evaporator, the mass flow ratio of water to steam at the outlet of the evaporator is between 3: 1 and about 20: 1.
The mixture, enriched with steam, exits the evaporator at the top via the overflow or riser pipes and re-enters the drum above. There the liquid phase ( water ) and the gaseous phase ( water vapor ) separate:
The steam bubbles rise to the surface of the water and are withdrawn from the top of the drum. Any water droplets that may be entrained are retained by suitable built-in components ( centrifugal separators ). This ensures that no drops get into the superheater, where they would evaporate and non-volatile components of the boiler water would be deposited as deposits.
The liquid phase remains in the drum and is fed back to the evaporator. The steam produced is topped up by adding feed water from the economizer. In this way the level in the drum is kept almost constant.
Since the non-volatile constituents of the feed water accumulate in the drum as a result of evaporation, a small partial flow has to be drawn off continuously from the drum (or alternatively from the lower collectors) so that salts and other impurities cannot concentrate indefinitely ( desalination or Blowdown ).
The drum as a limiting component
The drum is a pressure vessel . Because of the high pressures and temperatures in modern high pressure vessels, drums in large vessels also have very large wall thicknesses (up to 200 mm). These in turn lead to thermal stresses during rapid heating or cooling , which can go up to the load limit of the material. Thick-walled drums therefore limit the maximum permissible approach and shutdown gradients of the boiler. In order to remain flexible with high capacities, continuous flow boilers have been developed that do not require a drum.
Special designs
Multi-drum boiler
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In some types of boiler, the lower headers have such a large diameter that they are also referred to as the drum. One then speaks of a multi-drum boiler and differentiates between upper and lower drum (s) depending on the position in relation to the combustion chamber . Depending on the shape of the combustion chamber, there may be more than one of the two drum types.
In the case of multi-drum boilers, the two-phase separation function always takes place in the upper drum (s). The lower drums only act as a reservoir and distributor and are used, if necessary, for blowdown / blowdown.
Heated drum
The first and most primitive kettles of all, the so-called roller or elephant kettles , consisted only of one or more connected large drums that were heated directly from the outside by the fire. This design of the shell boiler represents the borderline case between smoke / flame tube and water tube boilers .