Mixing preheater

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Smoke chamber with mixing box (mixing preheater system type IfS / DR) on a new boiler 39E
(Meiningen, 2003)
dismantled mixing box, chimney side, top left: connections for cold water inlet, above generator exhaust steam, ventilation lines; bottom right: overflow and drain cyclone separator

The mixer preheater is part of the boiler equipment of steam locomotives , which serves both to preheat the water to be replenished and to treat the boiler feed water externally ( degassing and partial softening ) .

Newly built and Reko locomotives were equipped with a mixer preheater to improve efficiency and save feed water . With the IfS / DR type system, which was mainly installed at the Deutsche Reichsbahn, it was possible, for example, to save 15% of the tender water carried and thus considerably increase the operating radius of the steam locomotives.

In addition to the mixing box already mentioned, the components of the mixing preheater system are the feed pump (mostly designed as a compound mixing pump in the DR), exhaust steam collecting tank, sludge separator, oil separator and an overflow mixing tank. The function of the system is monitored from the driver's cab using a remote thermometer and a travel indicator .

Mode of operation of the IfS / DR type

Mixing box open, v. r. n. l .: cyclone separator, injection pipe, partition and overflow walls, degassing chamber and hot water chamber

Cold water flows from the storage tank or from the tender into the overflow mixing tank, where it is mixed with the overflow water from the mixing preheater. The temperature rises to 30 to 40 degrees Celsius. The cold water part of the compound mixing pump sucks in the preheated water and presses it through the cold water pressure line into the mixing box. The incoming cold water is finely atomized in the mixing chamber through the injection pipe . A portion of the steam from the steam engine is constantly diverted at the blowpipe head . This is also applied, together with the exhaust steam of the air and feed pump, as by the cyclone executed oil separator into the mixing chamber. An exhaust-steam collecting tank originally installed in the exhaust-steam lines has not proven itself and was removed again (except for locomotives of the 65.10 series ).

The cold water injected into the mixing chamber mixes with the machine exhaust steam, which gives off its heat to the water and cools it down. A large part of the amount of evaporation is deposited by condensation . In total, around 15% of the total amount of water fed in is recovered. The feed water is heated to 90-100 degrees Celsius and passes through the degassing chamber of the mixer preheater, through a sludge pot and a connecting pipe into the hot water chamber. On the way to the hot water chamber, scale builders are precipitated and the feed water is degassed.

A complicated system of dividing and overflow walls and the division of the mixing preheater box into three chambers prevent hardness builders and corrosive substances from entering the boiler with the hot water or the mixing box from running empty when the regulator is closed. The warm water flows from the warm water chamber to the warm water section of the compound mixing pump, which - overcoming the boiler pressure - feeds it into the boiler.

Dirt separator and compound mixing pump on a Reko locomotive
Compound mixing pump VMP 15-20 in detail

Compound mixing pump

The V erbund M isch P ump VMP 15-20 (type BBW ) is a flywheelless, oscillating two-cylinder compound piston steam pump , the cylinders of which work (unusually next to each other) in one housing. The high pressure part (on the picture on the left) works with full pressure on the hot water part of the pump, while the low pressure cylinder (on the picture on the right) drives the cold water piston as an expansion steam engine. The picture also shows the hot and cold water parts, each with the outgoing water pipes; the shock absorber and the air chamber (right and left on the pump part) as well as the grease gun for supplying the pump with oil (above) can be seen. The pipe leading down to the right is the cold water suction pipe, above it the cold water pressure pipe to the mixer preheater. Similarly, on the bottom left, the hot water suction line, arriving from the sludge separator, and above it the hot water pressure line leading to the boiler feed valve.

The pump delivers 15 m³ / h (250 l / min) of feed water at a steam pressure of 12 bar with 61 double strokes against 14 bar boiler pressure. It is able to deliver up to a maximum boiler counter pressure of 20 bar. The unit has a mass of approx. 700 kg. The VMP 15-20 is designed in such a way that the cold water section always conveys a larger amount than the hot water section. This ensures that the mixer preheater is not sucked dry. The excess water from the preheater, together with the blown and condensed exhaust steam, is returned to the overflow mixing tank and thus to the cold water suction side.