How is a water tube boiler

A Water tube boiler is a steam boiler type. It is characterized by the fact that the water is guided in the pipes. The water tube boiler can be used as a steam or hot water generator.


In the area of ​​gas and oil firing with steam capacities of up to 25 t / h and pressures of up to 32 bar, only shell boilers are used in new systems, since the boiler can be made ready for operation and more efficiently by the manufacturer. The shell boiler is not suitable for the automated combustion of solid fuels or a combustion chamber must be placed upstream.

Large capacity boilers can no longer be used for higher pressures. The pipes in the water-tube boiler have a much smaller diameter than the jacket of large-volume water boilers, so that here much smaller pipe wall thicknesses are sufficient to absorb the internal pressure. The water tube boiler is suitable for the combustion of all fuels if the combustion chamber is designed accordingly. Sootblowers can be inserted into the flue gas path in order to clean the heating surfaces when there is a lot of dust.



Stephen Wilcox patented the first sectional kettle in 1874. The steam output at that time was 2.4 tons of steam per hour. An essential prerequisite for the further increase in performance was the development of the seamless tube by the Mannesmann brothers in 1886.

The partial chamber sectional boiler around 1900 consists of a brick furnace in which the evaporator pipes are arranged in an ascending manner. The flue gases are led through the brick lining at an angle to the axis of the evaporator tubes. The evaporator tubes open into the drum, which is placed on the masonry. The downpipes are arranged in the rear area of ​​the combustion chamber that is sealed off from the flue gases.

In 1918, coal dust was used for the first time in continuous operation to fire a water-tube boiler. In the course of further technical developments, operating pressures of 100 bar were reached in 1927/28. A further increase in the operating pressures in natural circulation boilers is limited by the increasing wall thickness of the drums. As a result, once-through boilers were developed that do not require a drum.

The technical limits of steam generation are at pressures of 300 bar in the supercritical range of water and at overheating temperatures of 600 ° C. In the course of development, the steam output of a power plant unit has risen to currently 2,000 t / h.


A distinction is made according to the type of construction

  • Natural circulation boilers that are only fed with water from the condenser by the feed pump and in which the water circulates naturally between the upper and lower collecting tanks,
  • Forced circulation boilers in which the water circulation is ensured by a circulation pump,
  • Forced flow boilers in which the water is pumped through the pipe system in one direction.

The bulk of the evaporator is largely self-supporting (the pipe walls consist of pipes welded together). The rest of the structure of a water tube boiler consists essentially of steel profiles with concrete foundations. Alternatively, reinforced concrete can also be used for the structure. The outer walls of the structure are thermally insulated and clad with metal sheets for weather and noise protection.

Typical pipe dimensions are:

  • Preheater: 38x3.5 mm
  • Evaporator: 60x5 mm
  • Superheater: 32x5 mm


Quality requirements

Water-tube boilers are pressure equipment within the meaning of the Pressure Equipment Directive 97/23 / EC and may only be placed on the market if the manufacturer has demonstrated through a conformity assessment procedure with the participation of a notified body that the basic safety requirements of the directive have been complied with. The manufacturer affixes the CE mark and issues an EC declaration of conformity. Harmonized product standards for water tube boilers are:

  • EN 12952-1 to 17: water tube boilers

When using this standard, the manufacturer can assume that he meets the basic safety requirements of the directive (presumption of conformity).

Category: Process engineering