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'''Surface condenser''' is the commonly used term for a water-cooled [[shell and tube heat exchanger]] installed on the exhaust steam from the [[steam turbine]]s that drive the [[electrical generators]] in [[thermal power | '''Surface condenser''' is the commonly used term for a water-cooled [[shell and tube heat exchanger]] installed on the exhaust steam from the [[steam turbine]]s that drive the [[electrical generators]] in [[thermal power plant]]s.<ref>{{cite book|author=Robert Thurston Kent (Editor in Chief)|title=Kents’ Mechanical Engineers’ Handbook|edition=Eleventh edition (Two volumes)|publisher=John Wiley & Sons (Wiley Engineering Handbook Series)|year=1936|id=}}</ref><ref name=Babcock>{{cite book|author=Babcock & Wilcox Co.|title=Steam: Its Generation and Use|edition=41st edition|year=2005|id=ISBN 0-9634570-0-4}}</ref><ref name=Elliott>{{cite book|author=Thomas C. Elliott, Kao Chen, Robert Swanekamp (coauthors)|title=Standard Handbook of Powerplant Engineering|edition=2nd edition|publisher=McGraw-Hill Professional|year=1997|id=ISBN 0-07-019435-1}}</ref> These surface condensers are heat exchangers which convert steam from its gaseous to its liquid state at a pressure below [[atmospheric pressure]]. | ||
== Purpose == | == Purpose == | ||
Revision as of 11:22, 12 March 2008
Surface condenser is the commonly used term for a water-cooled shell and tube heat exchanger installed on the exhaust steam from the steam turbines that drive the electrical generators in thermal power plants.[1][2][3] These surface condensers are heat exchangers which convert steam from its gaseous to its liquid state at a pressure below atmospheric pressure.
Purpose
In thermal power plants, the primary purpose of a surface condenser is to condense the exhaust steam from a steam turbine at as low a pressure as possible and to obtain pure water (referred to as steam condensate) so that it may be reused in the steam generator or boiler as boiler feed water.
The steam turbine itself is a device to convert the heat in steam to mechanical work. The difference between the enthalpy of the inlet steam to a turbine and the enthalpy of the exhaust steam represents the heat which is converted to mechanical work. Therefore, the larger the enthalpy difference between inlet steam and exhaust steam, the higher is the amount of work delivered by the turbine. Condensing the exhaust steam of a turbine at a pressure below atmospheric pressure, increases that enthalpy difference and therefore increases the work output of turbine.
Most of the heat liberated by condensing the exhaust steam is transferred to and carried away by the cooling medium (water or air) used by the surface condenser.
Where water is in short supply or unavailable, an air-cooled condenser is often used. however, an air-cooled condenser is significantly more expensive and cannot achieve as low a steam turbine exhaust pressure as a surface condenser.
Diagram of water-cooled surface condenser
The adjacent diagram depicts a typical water-cooled surface condenser as used in power stations to condense the exhaust steam from a steam turbine driving an electrical generator.[2][3][4][5] There are many fabrication design variations depending on the manufacturer, the size of the steam turbine, and other site-specific conditions.
Shell
The shell is the condenser's outermost body and contains the heat exchanger tubes. The shell is fabricated from carbon steel plates and is stiffened as needed to provide rigidity for the shell. When required by the selected design, intermediate plates are installed to serve as baffle plates that provide the desired flow path of the condensing steam. The plates also provide support that help prevent sagging of long tube lengths.
At the bottom of the shell, where the condensate collects, an outlet is installed. In some designs, a sump (often referred to as the hotwell) is provided. Condensate is pumped from the outlet or the hotwell for reuse as boiler feedwater.
For most water-cooled surface condensers, the shell is under vacuum during normal operating conditions.
Vacuum system
For water-cooled surface condensers, the shell's internal vacuum is most commonly supplied by and maintained by an external steam jet ejector system. Such an ejector system uses steam as the motive fluid to remove any non-condensible gases that may be present in the surface condenser.
The Venturi effect, which is a particular case of Bernoulli's principle, applies to the operation of steam jet ejectors.
Motor driven mechanical vacuum pumps, such as liquid ring type vacuum pumps, are also used for this service.
Tube sheets
At each end of the shell, a sheet of sufficient thickness usually made of stainless steel is provided, with holes for the tubes to be inserted and rolled. The inlet end of each tube is also bellmouthed for streamlined entry of water. This is to avoid eddies at the inlet of each tube giving rise to erosion, and to reduce flow friction. Some makers also recommend plastic inserts at the entry of tubes to avoid eddies eroding the inlet end. In smaller units some manufacturers use ferrules to seal the tube ends instead of rolling. To take care of length wise expansion of tubes some designs have expansion joint between the shell and the tube sheet allowing the latter to move longitudinally. In smaller units some sag is given to the tubes to take care of tube expansion with both end water boxes fixed rigidly to the shell.
Tubes
Generally the tubes are made of stainless steel, copper alloys such as brass or bronze, cupro nickel, or titanium depending on several selection criteria. The use of copper bearing alloys such as brass or cupro nickel is rare in new plants, due to environmental concerns of toxic copper alloys. Also depending on the steam cycle water treatment for the boiler, it may be desirable to avoid tube materials containing copper. Titanium condenser tubes are usually the best technical choice, however the use of titanium condenser tubes has been virtually eliminated by the sharp increases in the costs for this material. The tube lengths range to about 55 ft (17 m) for modern power plants, depending on the size of the condenser. The size chosen is based on transportability from the manufacturers’ site and ease of erection at the installation site. The outer diameter of condenser tubes typically ranges from 3/4 inch to 1-1/4 inch, based on condenser cooling water friction considerations and overall condenser size.
Cooling water inlet and outlet
The tube sheet at each end with tube ends rolled, for each end of the condenser is closed by a fabricated box cover known as a waterbox, with flanged connection to the tube sheet or condenser shell. The waterbox is usually provided with man holes on hinged covers to allow inspection and cleaning.
These waterboxes on inlet side will also have flanged connections for cooling water inlet butterfly valves, small vent pipe with hand valve for air venting at higher level, and hand operated drain valve at bottom to drain the waterbox for maintenance. Similarly on the outlet waterbox the cooling water connection will have large flanges, butterfly valves, vent connection also at higher level and drain connections at lower level. Similarly thermometer pockets are located at inlet and outlet pipes for local measurements of cooling water temperature.
Other applications of surface condensers
- Large steam turbines in petroleum refineries and petrochemical plants
- Vacuum evaporation
- Vacuum refrigeration
- Ocean Thermal Energy (OTEC)
- Replacing barometric condensers in steam-driven ejector systems
- Geothermal energy recovery
- Desalination systems
References
- ↑ Robert Thurston Kent (Editor in Chief) (1936). Kents’ Mechanical Engineers’ Handbook, Eleventh edition (Two volumes). John Wiley & Sons (Wiley Engineering Handbook Series).
- ↑ 2.0 2.1 Babcock & Wilcox Co. (2005). Steam: Its Generation and Use, 41st edition. ISBN 0-9634570-0-4.
- ↑ 3.0 3.1 Thomas C. Elliott, Kao Chen, Robert Swanekamp (coauthors) (1997). Standard Handbook of Powerplant Engineering, 2nd edition. McGraw-Hill Professional. ISBN 0-07-019435-1.
- ↑ Air Pollution Control Orientation Course from website of the Air Pollution Training Institute
- ↑ Energy savings in steam systems Figure 3a, Layout of surface condenser (scroll to page 11 of 34 pdf pages)