Superheater

A superheater (or steam dryer) is part of a steam boiler system, is further heated in the water vapor generated in the boiler on its vaporization temperature for addition. One operates with this superheated steam, a heat engine, this machine works more effectively (see Carnot efficiency ).

General

This vapor is called superheated steam or hot vapor or steam. The vapor temperature is higher than the saturation temperature ( condensation point Re ). In the case of water vapor at a pressure of 1.013 bar to the vapor that is about 100 ° is then superheated, when its temperature is above the atmospheric boiling point, C.

Boilers are equipped with superheaters when working machinery (steam turbine or steam engine ) are driven. The overheating in the operation of steam turbines is used to increase plant efficiency and avoids damage to the turbine blades by condensed liquid droplets. To improve energy efficiency of the steam produced one to two reheater be installed in power plants with higher vapor pressures. The high-pressure turbine is designed so that the steam downstream of the turbine has an enthalpy which allows a slight condensation of water vapor. The extracted steam is heated now in the intermediate superheater ( reheat ) of the boiler again and can then be passed to a low pressure turbine.

For systems with multi-stage machines ( composite action ) reheating is also between the individual levels applied, that is, the vapor is passed from the first stage of the work machine back to the boiler, heated there again and passed to the next stage of the work machine to take advantage of the possible to have hot steam for the entire process available.

Superheater were first performed in 1890 by Wilhelm Schmidt, a suggestion of Carl von Linde following and established rapidly in stationary installations. Also steam locomotives were equipped with superheaters short time later.

Superheater in power plants

A distinction is made between the radiation superheaters that are exposed to the thermal radiation of the flame and touching superheaters which are heated only by the convective heat transfer from the flue gas. Superheater are often carried out in water -tube boilers as Scots superheater. It is to tubing with 180 ° bends, which are in one plane and are vertically suspended. The tube spacing is such that the contamination and bridging is minimized by carbon deposits between the tubes. In strong Rußanfall soot blowers are required, blow off the dirt from the pipes by means of air or steam.

With natural circulation boilers the wet steam is taken out through a droplet out of the drum and supplied to the superheater. The superheater generally consists of part of the superheater surfaces, which are numbered according to the direction of flow of the superheated steam. The steam from the drum is first fed into the superheater portion 1 which is disposed last in the flue gas. The other superheater are in the opposite direction to the flue gas stream - arranged - so the hotter portion. The individual sub- superheater thus form a counterflow arrangement. The advantage is a better average temperature difference between the flue gas temperature and the steam temperature in the respective part of the superheater. Thus the efficiency is increased by better utilization of the Rauchgasenthalpie and achieves a protection of the superheater tubes.

Because of load fluctuations and in particular by soot, the heat transfer to the superheater is strongly influenced. Therefore, it is necessary to control the superheat temperature. For new installations, this is usually achieved by injection of boiler feedwater between the part of superheaters, which is controlled by the outlet temperature of each superheater. Another variant is the steam cooling. The superheated steam is in this case passed through a pipe coil which is arranged in the water-filled portion of the steam drum. The colder water drum with the saturated steam temperature thus cools the superheated steam; the heat content remains in the system.

In the power plant area of ​​the plant steam temperatures 420-570 ° C can be achieved today, depending on the fuel used and size, development tends at large power plants to steam temperatures of 600 ° C and above. However, the development takes place only in very small increments, there's cash on the creep strength of the heat exchanger materials at these temperatures and pressures of up to 250 still comparatively little known.

During start-up of steam boilers with superheaters must be taken to ensure that a minimum amount of vapor always flows through the superheater to the diverter valve to prevent overheating. The safety valves are located behind the superheater. With lockable superheaters also a safety valve on the steam drum must be installed. The safety valves are adjusted so that the valve opens earlier than to the drum behind the superheater. This ensures that the response of the overpressure protection of the superheater is traversed.

Superheater in steam locomotive boilers

Structurally, the superheater direction at later built steam locomotives designed so that the steam is led by the steam dome in numerous U- shaped bent tubes that protrude into the flue pipes of the boiler. This smoke tube superheaters went ahead experiments with other forms of overheating, so the flame tube superheater and the smoke chamber superheater. However, by the repeated contact with the heat of the combustion gas, the steam is heated to 300 to 400 ° C, and the fine droplets of water evaporate. Without further pressure increase so that the steam contains more energy, also is on the way to the cylinders no steam condensation in the pipes lost. In addition, the lower density of the superheated steam reduces coal consumption by about 20 % and water consumption by up to 45%.

The high temperatures make but with reciprocating machines using suitable lubricant (hot steam oils ) are required. These have a higher viscosity and a higher flash point compared to oils for use with wet steam.