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 The Eckrohrkessel design is a distinctive type of single drum, high-circulation, watertube boiler developed by Eckrohrkessel GMBH of Germany.
The Eckrorhkessel, or Corner Tube boiler developed out of a crisis in the supply of transport fuels in Germany during the 2nd World War creating a need to convert trucks to steam power, fuelled by coal. While it was considered desirable to avoid the complications of forced circulation boilers, existing natural circulation boilers would clearly not be successful for such a small boiler. Working from first principles, a 300 Kg/hr, 40 bar,
400 degrees Celcius boiler was developed which gave the maximum circulation possible in such a small natural circulation boiler. This high circulation was achieved by the use of relatively large unheated downcomers at each corner of the boiler & hence the name "Eckrohrkessel" which translates as "Corner Tube Boiler".
The Eckrohrkessel design is then a modern, natural circulation watertube boiler, with the main characteristics a pre-separation of the steam outside the drum & large unheated downcomers.
Due to the large downcomers & careful design of water distribution & steam separation piping, an intense water circulation is obtained immediately upon starting. Since all the water is brought quickly into circulation there are no differential temperatures within the boiler that restrict the startup rate. A 2-drum boiler with its expanded tubes & relatively stagnant water in the mud drum must be started up much more carefully in order to limit the strains in these components.
The boiler is very quickly able to respond to major load fluctuations while still maintaining pressure because of its positive circulation system. Downcomers are connected, as required from the top headers to directly supply water to the riser tubes. Hence, typically only 50% of the boiler water circulates via the drum.
The very positive circulation achieved with the Eckrohrkessel design allows a unique construction of convective heating surfaces. Relatively small diameter tubes are connected directly to sidewall tubes having a partial baffle between the horizontal water flow & steam flow connections. This construction has been very well proven over many years & permits a great deal of flexibility in configuring the boiler.
Due to the relatively small convection tubes, which no longer have to act as downcomers as in a conventional boiler, high heat, transfer coefficients can be obtained with relatively low gas velocities. The tube layout may be staggered or in-line & the gas can flow either horizontally or vertically over the tubes. A vertical flow configuration is particularly advantageous for a coal-fired boiler since this eliminates areas for ash to build up & allows the manufacturer to build hoppers below the banks to collect the ash.
 The furnace wall construction, as with other boilers, may be of membrane wall or skin construction. Eckrohrkessel have developed a very successful skin casing construction that is significantly cheaper than membrane wall construction while suffering none of the disadvantages of traditional skin-casing design. The essential features of a good design are that the skin casing should be maintained at the same temperature as the tubes. This is achieved by ensuring that the gaps between the tubes are extremely small & that the relatively thin 2mm casing is in intimate contact with the tubes at all times. This is done by welding the casing in narrow strips to the boiler tubes so that furnace pressure or differential cannot cause the sheets to "belly-out".
To achieve the narrow pitching required, the tubes are cold-swaged for a short distance at their ends to allow access for welding of the tubes to the header. Actual measurements on the skin casing have shown that the casing temperatures stay within
5-15 degrees Celcius of the steam temperature. This construction has been used on hundreds of boilers, fired on all fuels, without any problems due to leaks, corrosion or deposits behind the tubes.
Significantly, for coal-fired boilers the configuration of the furnace is dictated by combustion requirements & is independent of the required space for adjoining convection banks. The Eckrohrkessel system allows a tremendous amount of flexibility to tailor a boiler to meet specific requirements.
The Eckrohrkessel design offers many significant performance advantages to users of coal-fired boilers while also ensuring a cost competitive boiler especially at the higher steam pressures increasingly required today. While the Eckrohrkessel design started off as a very small boiler it has now been manufactured in sizes up to 250 tonnes/hr of steam & 175 MW as a hot-water boiler.
Contributed by Rolls Royce (NZ) Ltd. |
