A surface condenser is a type of heat exchanger often used to condense steam under vacuum pressure as part of a thermal power plant. The tube and shell condenser converts steam from its gas phase to liquid as part of a thermodynamic cycle that includes a steam generator, pump, and turbine. The steam is condensed on the outer surface of tubes through which pass cooling water. The water can come from a closed-loop system or a system open to an exterior source. The configuration of a surface condenser is designated by the number of passes, whether the system is divided, and the shape of the shell.
The thermodynamic cycle using a surface condenser incorporates a generator where water is heated to create high pressure steam. The steam pressure drives a turbine to produce power. The system passes the steam exhaust into the surface condenser at a lower pressure. The condenser converts the steam back to liquid. A pump moves the condensed water back to the steam generator so the cycle can repeat.
The physical form of the condenser is a shell with a bundle of tubes passing through it. Cooling water runs through the tubes. The heat exchange takes place when the cooling water absorbs heat from the steam through the surface of the tubes. The loss of heat causes the steam to change into liquid water, known as condensate, and fall to the bottom of the condenser. The cooling water may be recycled in a closed-loop or may be drawn from an outside source such as a lake or river and discharged after passing through the condenser.
Vacuum pressure is important to the efficiency of the system. The shell of the surface condenser is maintained under vacuum by keeping the temperature at a point low enough that the vapor pressure of water is less than ambient pressure. This lowered level helps to increase the pressure drop across the turbine, thus improving the output. The vacuum pressure tends to draw non-condensable gases, such as air, into the condenser. These gases must be removed to prevent a reduction in efficiency caused by the non-condensable gases surrounding the tubes and interfering with the heat exchange. Removal is also necessary to prevent corrosion from the presence of oxygen in the system.
The design of the unit may provide for the cooling water to pass through the tubes once or it may have multiple tube bundles so that the water flows from end to end of the shell two or more times, or passes. A divided surface condenser will have separate sections and tube bundles such that part of the unit can be shut down for maintenance while another part is still functioning. An undivided unit with a single shell and tube bundle must be shut down completely for any servicing or problems. Surface condenser shells may have rectangular or cylindrical shapes depending on the location of the equipment and the capacity of the system.