Resumo de Steam Turbine Exhaust Condensers

Steam Turbine Exhaust Condensers

For historical reasons, steam turbine exhaust condensers are often referred to as surface condensers. In principle, they are no different from the shell-side condensers are described in section, in particular, the X-type. The practice, there are certain severe demands, arise from the large heat duties that they must perform, and from the necessity to maintain a low-condensing temperature to achieve the highest possible power station efficiency.

The aim is to operate with the condensing temperature only a few degrees above the cooling water temperature only a few degrees above the cooling water temperatures. Typically, the cooling water is about 20 c, with condensation taking place at around 30 c. Saturation pressure of water at this temperature at this temperature is 0.042 bar absolute, which is typical operating pressure for these condensers. Clearly, there is a littlepressure available for the pressure drop through the unit.

There is also small temperature difference to spare to overcome the effect of noncondensable gases. Hence, the design of surface condensers is governed by the need for good venting and low-pressure drop. These very large condensers often have box-shaped shells, the smaller ones, with surface areas less than about 5000m, may have cylindrical shells.

As with any other shell-and-tube unit, tubes in surface condensers must be supported at regular intervals along their lengths with tube supporting plates. Such support plates also have the advantaging of deliberately preventing any axial flow of vapor, thus making it easier for designers to ensure that vapor flow paths through the bundles are relatively straightforward, giving rise to no recirculation pockets where noncondensables can accumulate.

It is desirable to subcool any condensate leaving a condenser to prevent flashingin the piping circuit and equipment downstream of the condenser. One solution is to design a separate unit as a subcooling exchanger. One can also overdesign the condenser, a subcooling section can be added as a second pass. This second section often has a closer baffling to increase the heat transfer coefficient in this region. For eg. The unit may be designed will all the required condensation in the first pass. The vent line would then be placed in the header the end of the first pass. The liquid level would be maintained in this header and the second pass would be full of liquid. With this particular method of achieved subcooling, however, a much smaller, a much smaller number of tubes would normally be required in the second pass to give high velocities to ensure good heat transfer. There is such a variety of different surface condenser designs that it is impossible to illustrate them all here, but many examples of modern condensers are described by Sebald.

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