Draft (furnace)

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Shell boiler with two horizontal flues (flue gas path see arrows)

As a train is at a firing (for example in a furnace or boiler ), a portion of the flue gas indicated -Weges, the flow through the flue gas without substantial change of direction. A new train begins at each deflection (kink) .

backgrounds

The flue gas path is divided into puffs and “folded” so that a long smoke gas path can be accommodated even in a narrow space. The length of the flue gas path determines the residence time of the flue gas in the hot zone of the furnace. A certain dwell time is required so that the flue gas can give off its heat to the walls of the combustion chamber and the heating surfaces and good burnout is achieved. With typical dwell times of 5 to 10 seconds and flow speeds of 5 to 20 m / s, the result is a flue gas path that is normally significantly larger than the room in which the furnace is installed ( boiler house or similar). A compact design is particularly important for mobile use on steam locomotives or steam ships .

Changes in direction are basically undesirable for various reasons: The centrifugal force causes a separation effect at the deflection points. The ash that falls there must be removed. For this purpose, tiled stoves are provided with round cleaning openings. Sparks that are still burning hit the wall of the deflection point and go out, which worsens the burnout. For the gas phase, too, deflections lead to imbalances, eddies and increased pressure loss . These disadvantages are accepted in favor of the compact design. It is positive that the eddies contribute to a thorough mixing and dissolution of temperature and concentration strands . The diversions can also be used to support ash removal.

Arrangement of trains

A train can be arranged horizontally, vertically rising, vertically falling or diagonally. Because of the direction of gravity , the type of arrangement has a decisive influence on the type of heating surface cleaning and ash removal . With a horizontal draft there is a risk of ash and unburned fuel being deposited. Above a certain length, the ashes must be collected and drawn off in funnels attached below. Such a horizontal train with ash hoppers is also called a dachshund train because of its shape reminiscent of a dachshund .

In shell boilers (including steam locomotive boilers ), the diversion often takes place in turning chambers at the ends of the boiler. The puffs are horizontal, the first puff is the flame tube , as further puffs follow the smoke tubes up to the exit into the chimney. Due to their high efficiency, 3-pass boilers are mostly used. Some boilers also have a separate fourth pass to use waste heat from processes or from the exhaust gas from combined heat and power plants. When passing through, the hot gases give off available waste heat to the water in the steam generator before they are discharged through the chimney.

So-called tower boilers contain only a single vertically rising draft that includes the combustion chamber. In large tower boilers, this train can reach heights of 150 m and more.

The last part of the flue gas path, the chimney, was and is also called induced draft because of the sucking effect of the chimney effect (natural draft) . However, when specifying the speed of the firing, i.e. the number of puffs, it is not counted as a puff, as it does not belong to the hot zone.

In the case of ovens, a horizontal train that was designed as a brick channel is also known as a fox .

literature

  • Hans A. Behrens: Handbook of industrial furnace construction . Stahleisen, 2001, ISBN 978-3-514-00642-3 .
  • Karl Strauss: Power plant technology . 2nd Edition. Springer, Berlin / Heidelberg / New York 1994, ISBN 3-540-57343-7 .
  • Fritz Mayr (Ed.): Boiler operating technology . 7th edition. Dr. Ingo Resch, Graefelfing 1997, ISBN 3-930039-13-3 .
  • Helmut Effenberger: Steam generation . Springer, 1999, ISBN 978-3-540-64175-9 .

Individual evidence

  1. ↑ 1 × 1 tiled stove. Dictionary. Archived from the original on December 19, 2010 ; Retrieved February 2, 2011 .
  2. ↑ a b Mayr, Section 2: Boiler types ... (p. 59 ff.).
  3. ↑ a b c Strauss, Section 4: Use of fossil fuels ... (p. 91 ff.).
  4. ↑ Effenberger, p. 161.
  5. ^ Fritz Brandt: Fuels and combustion calculation . Vulkan, 1999.