A surface condenser or steam condenser is a water-cooled shell-and-tube heat exchanger, primarily used for condensing steam exhausted from the steam turbines of thermal power plants, allowing the steam to transition from a gaseous to a liquid state at a pressure below atmospheric level.
In surface condensers, steam and cooling water do not mix. Condensate from the boiler can be reused: non-pure water can be used for cooling in surface condensers, whereas in jet condensers, the cooling water must be pure. Despite the higher initial investment cost and space requirements of surface condensers, they remain the preferred equipment in power plants due to their low operating costs and high thermal efficiency.
Surface Condenser Types: Depending on the location of the condensate extraction pump and the condensate flow rate as well as the pipeline arrangement, surface condensers can be categorized into different types.
Streamlined Surface Condenser
In a once-through condenser, the exhaust steam is propelled downward, flowing through cooling water pipes with a dual-channel setup (the lower part of the steam enters downward, while the upper part exits in the opposite direction).
Centrifugal Surface Condenser
In this centrifugal-type surface condenser, steam enters the shell side circuit, and the condensate flows centrally towards the tube bundle.
Reversed-Flow Steam Condenser
In the counterflow condenser, steam enters at the bottom of the shell side, with the exhaust pump connected to the top. The steam first flows upwards, then returns to the bottom of the condenser, where the condensate water extraction pump is connected.
The power plant converts thermal energy into electrical energy through steam-driven turbines: heating water to produce steam that drives the turbines connected to the generator. After the turbines operate, the steam condenses in the condenser and returns to the boiler for recirculation, where it is reheated and evaporated again. This is the basic principle of the Rankine cycle.
Steam from the low-pressure turbine enters the shell side. After passing through the condenser tube bundles, the steam is cooled and converted into water (condensate). The steam jet ejector (or exhaust device driven by a rotating electric motor) continuously removes air and gases from the steam while maintaining a vacuum.
The lower the condenser temperature, the lower the pressure in the condensate vapor, which ensures work efficiency. As the condenser temperature is nearly always kept below 100°C, the steam pressure of the water is significantly lower than atmospheric pressure, hence the condenser usually operates under vacuum. Air (non-condensable) leakage into the closed loop will decrease the vacuum, thereby reducing efficiency, so it must be prevented.
Generally, steam condensation occurs at a cooling water temperature of approximately 25°C (77°F), resulting in an absolute pressure of around 2-7 kPa (0.59-2.07 inHg) in the condenser, which is about -95 kPa (-28 inHg) below atmospheric pressure.
When steam condenses, significantly reducing the vacuum volume helps expel it into the turbine for improved efficiency.
Condensers typically use recirculated cooling water from cooling towers or direct-cycled surface water (from rivers, lakes, or oceans) to discharge excess heat into the atmosphere. In many areas, the use of cooling towers, even for one-time installations, is mandatory to prevent heating of surface water and related biological impacts.
Natural ventilation, forced ventilation, or chiller towers through evaporation can reduce water temperature by approximately 11 to 17°C (20 to 30°F) and emit non-recyclable waste heat into the atmosphere. A 500 MW unit circulates at a rate of about 50,000 m³/hr (500 ft³/s or 225,000 US gal/min) at full load.
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