Steam Turbine Condenser Explained
Steam turbine condensers are used within thermal power stations to condense steam into condensate (water). This article provides an overview of steam turbine condensers, their various designs, parts, and working.
Surface Steam Condenser
Good to know – ‘steam turbine condensers’ are also sometimes called ‘surface condensers’, but surface condensers are actually only one particular type of steam turbine condenser.
What is a steam turbine condenser?
Steam turbine condensers are used in thermal power stations to convert steam into condensate (water). A low-pressure steam turbine sits directly on top of its associated condenser, with a flexible joint connecting the steam turbine to the condenser. Low-pressure exhaust steam is discharged from a low-pressure turbine directly into its associated condenser.
Steam Condenser Highlighted in Orange Box
Good to know – the flexible joint is required to cater for the thermal expansion of parts as they heat-up and cool-down; cracking and resultant leaks would occur if the joint was not present.
Why are steam turbine condensers needed?
- Efficiency – cooling of low-pressure steam causes a resultant system pressure reduction. The differential pressure (DP) across the system is a direct indicator of the system’s efficiency, thus a larger DP is indicative of a more efficient system i.e. the larger the pressure difference between the boiler’s discharge steam pressure and condenser pressure, the greater the system efficiency.
- Water Reuse – changing the low-pressure steam back into water allows for the water to be reused; reusing of the water reduces operational costs as it needs less pretreatment. Water is also easily returned to the deaerator (pre-treatment apparatus) in large quantities, but this is not true for low-pressure steam, which would require large piping, and a greater pressure differential in order to flow.
- Deaeration – condensers liberate gases like oxygen and CO2 from the system, thus reducing the likelihood of corrosion within the feedwater and steam systems.
- Collection Point – condensers serve as the primary collection point for other steam extraction systems and condensate drains.
Good to know – water that has been treated but has not yet entered the boiler, is classified as ‘feed water’. Water that is within the boiler, is classified as ‘boiler water’. Steam that has condensed back to water, is classified as ‘condensate’. Condensate becomes feed water after it has been treated.
Good to know - further information about power plant systems and machinery can be found in our Power Engineering Fundamentals Video Course.
Steam Turbine Condenser Parts
The main parts of a steam turbine condenser are indicated below.
Surface Steam Condenser Parts
Low-Pressure Steam Inlet
Low-pressure steam from the low-pressure steam turbine is discharged to the condenser. The condenser is held at vacuum in order to provide a low backpressure for the turbine exhaust, which increases the overall plant efficiency.
Shell
The shell houses the internals of the condenser, including the tube support plates, tube sheets, tubes, hotwell (lower part of condenser) and extraction piping; it is usually manufactured from heavy steel plates that are welded into one piece. Tubes are spaced away from the shell to allow steam to access all parts of the tubes, thus increasing the condenser’s efficiency and reducing the likelihood of overheating.
Tube Bundle
Tubes that pass through the condenser are assembled to form a ‘bundle’; these are mounted to tube sheets. Turbulators are often installed within the tubes to promote turbulent flow, which increases the heat transfer capability of the tubes and thus the overall efficiency of the condenser.
Tube With Turbulator Installed
Tube Sheets
Tube sheets are mounted at opposite ends of the tube bundles; they hold the tubes in position and lend mechanical strength to the tubes. Each tube is rolled and expanded into its associated tube sheet.
Extraction Piping
Extraction steam enters the condenser through this piping. Exhaust steam from the boiler feedwater pumps is discharged to the condenser also.
Water Boxes
Cooling water enters and leaves a condenser via the water boxes; these are installed at opposite ends of the condenser. One end of the condenser forms the inlet, whilst the other forms the outlet (discharge). Depending upon the size of the unit, a condenser may have one or more inlets and discharges, although more than two is unusual. Water boxes should have a protective corrosive resistant lining covering all water-side surfaces.
Good to know – it is essential that water boxes remain fully primed with cooling water when in service, as a failure to do so could lead to localised overheating of the tubes.
Nozzle
The large shape of a waterbox nozzle is designed to keep water velocities low.
Hotwell
The hotwell (hot well) forms the lower part of the condenser. Condensed steam forms condensate, which gathers in the hotwell and is discharged through piping at the base of the hot well.
Condensate Extraction Pump
Condensate is discharged via a condensate extraction pump. As condensate has a much smaller volume than steam (a ratio of 1600:1), the discharge pipe is much smaller than the condenser’s main steam inlet.
Instrumentation
Sensors are fitted to several areas of the condenser for monitoring purposes. Each sensor records operational data relating to flow, pressure, level, and temperature. All data is then fed to a monitoring system in real-time. Alarms and shutdowns are triggered based upon the data received from the installed sensors.
Low-Pressure Feedwater Heater
Large power stations often install low-pressure feedwater heaters in the neck of the condenser; the two main reasons for this are space and cost.
- Space – a feedwater heater requires a lot of space, this space adds cost to the building construction, which is not desired.
- Cost – installing the feedwater heater within the neck of the condenser reduces the length of extraction steam piping needed, which reduces the number of hangers, joints, valves etc. required; thus, a cost and space saving occurs by locating the LP feedwater heater within the condenser.
How do steam turbine condensers work?
Low pressure exhaust steam transfers its heat (thermal energy) to a cooling medium, which causes it to cool (temperature decrease) and condense, whilst the cooling medium’s temperature increases. The primary purpose of a steam condenser is to cool the exhaust steam and cause it to change state to a liquid.
Heat sinks are required to transfer the thermal energy from the exhaust steam to the environment. A typical heat sink may be a lake, river, or ocean. If a large body of water is not available to act as a heat sink, cooling towers are used, and the cooling water recirculated; air-cooled condensers may also be used although this is rare for large power stations due to their high cooling capacity requirements.
In this example, we assume that a natural draft cooling tower is used as the heat sink. The cooling tower is responsible for dissipating the heat from the steam turbine exhaust. Cooling water from the basin of the cooling tower is directed to the condensers where it flows through the tubes. When exhaust steam enters the condenser, it surrounds the water-filled tubes.
Natural Draft Cooling Tower Cross-Section
As the steam moves over the outside of the tubes, it is cooled by the water inside them, leading to its condensation. The cooling water (within the tubes) absorbs the heat from the steam, which causes its temperature to correspondingly increase. The heated cooling water is then sent back to the cooling tower, where it is cooled down via evaporative cooling before the process repeats.
Water and Air-Cooled Condensers
The working principle of a steam turbine condenser involves heat transfer from the low-pressure steam turbine exhaust to a separate cooling medium, typically water or air.
Good to know – see our latent and sensible heat article to understand heat and its various properties.
Water-Cooled Condenser
In the case of a water-cooled condenser, cooling water flows through tubes within the condenser. Exhaust steam flows around these tubes and condenses into water as it is cooled. This type of condenser is highly efficient in transferring heat due to the large contact surface area between the tubes and steam. The condensed steam, now in the form of condensate, is collected at the bottom of the condenser and pumped back to the deaerator then boiler (usually a watertube boiler). The heated cooling water is cooled using a cooling tower, if no lake, river, or ocean is available (there are various heat sink types).
Air-Cooled Condenser
In air-cooled condensers, cooling is achieved by air flowing over finned tubes through which the exhaust steam passes. Air-cooled condensers are used in areas where water resources are limited. However, air-cooled condensers are generally less efficient than water-cooled types due to their lower heat transfer rate (this is because air has a lower density than water and is thus unable to cool as efficiently).
Surface and Air-Cooled Condensers
There are several main types of steam turbine condenser and it is important to discuss direct and indirect cooling to understand their design.
Surface Condensers – these are the most common type of condensers; they are essentially large shell and tube heat exchangers. They indirectly cool the steam i.e. the steam does not come into direct contact with the cooling water.
Air-cooled Condensers – these are used when there isn't a readily available water source. There are two main types of air-cooled condensers:
- Direct Acting – exhaust steam flows through piping with heat exchanger fins welded to them. Electric fans blow air across the pipes and fins, which cools the steam and causes it to condense.
Direct Air-Cooled Condenser
- Indirect Acting – turbine exhaust steam is condensed by a cooling water loop within a conventional surface condenser. The cooling water then rejects the heat to atmosphere via an air-cooled condenser.
Indirect Air-Cooled Condenser
Factors Influencing Condenser Performance
Ambient Environment
The temperature of the cooling water is influenced by the ambient environment. For instance, in colder climates, the cooling water will have a lower temperature. Colder water improves the overall efficiency of the condenser, but it should not be so cold as to thermally shock the condenser or for it to freeze. If the cooling water is too hot, its heat transfer capability and cooling capacity decreases.
Water Quality
The type of water used (freshwater or saltwater) can affect a condenser's performance. Saltwater, being corrosive, requires the condenser to be made of corrosion-resistant materials. All types of water source will require some sort of filtering prior to use.
Good to know – ‘fresh water’ is sometimes called ‘sweet water’. Fresh water is also spelt ‘fresh-water’ or ‘freshwater’, but all spellings mean the same thing.
Fouling
Deposits (scale etc.) accumulating within the tubes, or on the external surfaces of the tubes, can reduce their heat transfer rate, which causes a corresponding reduction in the condenser’s cooling capacity.
Additional Sources
https://en.wikipedia.org/wiki/Surface_condenser
https://engineering.fandom.com/wiki/Condenser_(steam_turbine)
https://www.powerplantandcalculations.com/2020/05/steam-condenservacuum-and-calculations.html
https://learnmech.com/steam-condenser-types-function-diagram-advantages
https://www.nuclear-power.net/nuclear-power-plant/turbine-generator-power-conversion-system/what-is-steam-turbine-description-and-characteristics/condensing-steam-turbine