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Energy and Power Generation Handbook: Established and Emerging Technologies

K. R. Rao
K. R. Rao
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ASME Press
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The subject of this chapter is the capital equipment in a steam power plant that is used to condense the exhaust steam from the lowest pressure turbine, by using water as the cooling medium. In the applied heat transfer literature, such heat transfer equipment is often simply referred to as the “Surface Condenser.” A surface condenser is necessarily a large piece of equipment because more than 60% of the thermal energy produced by a power plant ends up as low enthalpy (waste) heat. This is because of the inherent thermodynamic limitation of the Rankine Cycle, which must be rejected by the condenser to the environment. The heat transfer area in a power plant's surface condenser easily dwarfs that in any other heat exchanger in the plant.

Classical thermodynamics holds that the lower the temperature of the heat sink, the higher the efficiency of the Carnot cycle. Therefore, attaining the lowest possible condensing temperature in the heat sink of the Rankine Cycle—the surface condenser—is a primary goal in surface condenser design. Since the saturation temperature and pressure of steam vary in a proportional manner at low pressures, the objective of low condensing temperature translates to that of a low condenser operating pressure. Accordingly, surface condensers are operated at as high a level of vacuum (typically 1.0 to 2.5 in. of mercury absolute) as the quantity and temperature of the cooling water would allow. The subatmospheric condition in the condenser and portions of turbine assembly and auxiliaries promotes leakage of air into the system. In boiling water type of nuclear plants the motive steam acquires additional non-condensibles due to radiological disassociation of water into hydrogen and oxygen. Whatever their origin, these so-called non-condensibles tend to collect in the lowest pressure region in the power cycle which is the steam space in the surface condenser. Unless removed continuously and efficiently, they may sharply interfere with the heat transfer process in the condenser. Kern [8] quotes observed data of Othmer which indicates that even 1% volumetric concentration of air in steam reduces the condensing film coefficient by approximately 45%.

The oxygen in the non-condensibles is another source of concern. Oxygen is known to actuate corrosion of condenser internals. Austenitic stainless steel tubing can become highly susceptible to stress corrosion in the presence of even small concentrations of oxygen. Therefore, efficient collection and removal of non-condensibles is of paramount importance in surface condenser design. A reliable design for collection and expulsion of the non-condensibles is particularly important in the surface condensers used in geothermal plants where the steam extracted from the ground has a large fraction of associated gases.

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