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Arrangement of the superheater in the boiler
Locomotive boiler with heating (green), smoke (yellow) and superheater pipes (red)

A superheater (or steam dryer ) is part of a steam boiler system in which water vapor that has been generated in the boiler is further heated above its evaporation temperature . If a heat engine is operated with this superheated steam, this machine works more efficiently (see Carnot efficiency ).


Superheated steam is also called superheated steam or live steam . The steam temperature is above the saturation temperature (recondensation point). In the case of steam with a pressure of 1.013 bar, the steam is superheated when its temperature is above the atmospheric boiling point, i.e. above 100 ° C. At considerably higher pressures, the critical temperature is also considerably higher.

Boiler systems are equipped with superheaters when working machines ( steam turbine or steam engine ) are driven. Overheating when operating steam turbines increases the efficiency of the system and prevents damage to the turbine blades from condensed liquid droplets. For better energy utilization of the generated steam, one or two reheaters are installed in power plants with higher steam pressures. The high pressure turbine is designed in such a way that the steam behind the turbine has an enthalpy that allows a slight condensation of water vapor. The extracted steam is now heated again in the reheater (ZÜ) of the steam boiler and can then be fed to a medium or low pressure turbine.

In systems with multi-stage machines ( composite effect ), intermediate superheating is also used between the individual stages, i.e. the steam is returned to the boiler from the first stage of the machine, where it is heated up again and passed to the next stage of the machine to take advantage of the To have superheated steam available for the entire process if possible.

Superheaters were first implemented around 1890 by Wilhelm Schmidt , following a suggestion by Carl von Linde , and quickly established themselves in stationary systems. Also, steam engines were fitted around the turn of the century with superheaters. The reasons for the later introduction were because lubricants with a flash point of more than 350 ° C and stuffing boxes made of metallic material for sealing the piston rod to the cylinder housing appeared around the time .

Superheaters in power plants

A distinction is made between radiant superheaters , which are also exposed to thermal radiation from the flame, and contact superheaters , which are only heated by the convective heat transfer from the flue gas. Superheaters are often designed as bulkhead superheaters in water tube boilers . It is a pipe arrangement with 180 ° deflections that lie in one plane and are suspended vertically. The pipe spacing is dimensioned in such a way that contamination and bridging caused by soot deposits between the pipes is minimized. If there is a lot of soot, sootblowers must be installed, which blow the dirt off the pipes using air or steam.

With natural circulation boilers , the wet steam is removed from the drum via a droplet separator and fed to the superheater. The superheater usually consists of partial superheater surfaces that are numbered according to the direction of flow of the superheated steam. The steam from the drum is first passed into the superheater part 1, which is arranged as the last in the flue gas path. The other superheaters are arranged in the opposite direction to the flue gas flow - i.e. towards the hotter area. The individual partial superheaters thus form a countercurrent arrangement . The advantage is a more favorable mean temperature difference between the flue gas temperature and the superheated steam temperature in the respective superheater section. This increases the efficiency by making better use of the flue gas enthalpy and the superheater tubes are protected.

The heat transfer to the superheater is strongly influenced by load fluctuations and, in particular, by soot deposits. It is therefore necessary to regulate the overheating temperature. In new systems, this is usually achieved by injecting boiler feed water between the partial superheaters, which is controlled via the outlet temperatures at the respective superheater. Another variant is superheated steam cooling. The superheated steam is passed through a coil which is arranged in the water-filled part of the steam drum. The colder drum water with the saturated steam temperature thus cools the superheated steam; the heat content remains in the system.

In the power plant sector, steam temperatures of 420 to 570 ° C are reached today, depending on the fuel used and the size of the system, while the trend in large power plants is towards steam temperatures of 600 ° C and above. However, the development is only taking place in very small steps, since comparatively little is known about the creep strength of the heat exchanger materials at these temperatures and pressures of up to 250 bar.

When starting up steam boilers with superheaters, it must be ensured that a minimum amount of steam always flows through the superheater to the start-up valve in order to avoid overheating. The safety valves are located behind the superheater. In the case of superheaters that can be shut off, a safety valve must also be installed on the steam drum. The safety valves are set so that the valve behind the superheater opens earlier than that on the drum. This ensures that there is flow through the superheater even when the overpressure protection is triggered.

Superheaters in steam locomotive boilers

In terms of construction, the superheater in steam locomotives built later is designed so that the steam from the steam dome is directed into numerous U-shaped bent tubes that protrude into the smoke tubes of the steam boiler . This smoke tube superheater was preceded by experiments with other forms of superheating, such as the flame tube superheater and the smoke chamber superheater . The repeated contact with the heat of the combustion gases heats the steam to 300 to 400 ° C and the fine water droplets also evaporate. Without a further increase in pressure, the steam contains more energy, and no steam is lost through condensation in the pipelines on the way to the cylinders . In addition, the lower density of the superheated steam reduces coal consumption by around 20% and water consumption by up to 45%.

However, the high temperatures make the use of suitable lubricants (superheated steam oils) necessary for piston engines. Compared to oils for operation with wet steam, these have a higher viscosity and a higher flash point.


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