Condenser and cooling system

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Condenser and cooling system

The condensers and cooling systems involved in condensing the exhaust steam from a steam turbine and transferring the waste heat away from the power station.
The environmental effects of these systems will also be briefly discussed.

Condensers

The function of the condenser is to condense exhaust steam from the steam turbine by rejecting the heat of vaporisation to the cooling water passing through the condenser. The temperature of the condensate determines the pressure in the steam/condensate side of the condenser. This pressure is called the turbine backpressure and is usually a vacuum. Decreasing the condensate temperature will result in a lowering of the turbine backpressure. Note: Within limits, decreasing the turbine backpressure will increase the thermal efficiency of the turbine.
The condenser also has the following secondary functions:

The condensate is collected in the condenser hot well, from which the condensate pumps take their suction;

Provide short-term storage of condensate;

Provide a low-pressure collection point for condensate drains from other systems in the plant; and

Provide for de-aeration of the collected condensate.

A typical power plant condenser has the following functional arrangement.

Large power plant condensers are usually 'shell and tube' heat exchangers. These types of condensers are also classified:

As single pressure or multi-pressure, depending on whether the cooling water flow path creates one or more turbine backpressures;

By the number of shells (which is dependent on the number of low-pressure turbine casings); and

As either single pass or two-pass, depending on the number of parallel water flow paths through each shell.

Other types of condensers are:

Plate types consisting of a series of parallel plates that provide paths for the steam and the cooling water. Plate condensers are used mainly for smaller power plants; and

Direct contact types where the cooling water is sprayed directly into the steam. This type of condenser is used in applications where the cooling water is the same quality as the steam condensate. Systems that have dry cooling (described in a following section) sometime use direct contact condensers.

The parts of shell and tube condensers and plate condensers involved in the transfer of heat from the steam and condensate to the cooling water should have the following properties:

Be resistant to corrosion from both the steam/condensate and the cooling water;

Have a minimal resistance to the flow of heat from the steam/condensate through the material into the cooling water; and

Provide mechanisms to remove organic and inorganic deposits on the heat transfer surfaces in contact with the cooling water.

Types of Cooling Systems

Some power stations have an open cycle (once through) cooling water system where water is taken from a body of water, such as a river, lake or ocean, pumped through the plant condenser and discharged back to the source. Inland plants away from large water bodies prefer to use closed cycle wet cooling system with wet cooling towers. Plants in remote dry areas without economic water supplies use closed cycle dry cooling systems that do not require water for cooling. Hybrid cooling systems are used in particular circumstances.

The type of cooling system used is therefore heavily influenced by the location of the plant and on the availability of water suitable for cooling purposes. The selection process is also influenced by the cooling system's environmental impacts (refer to a following section for a brief discussion on this topic).

Open Cycle Cooling Systems

Open cycle (once through) cooling systems may be used for plants sited beside large water bodies such as the sea, lakes or large rivers that have the ability to dissipate the waste heat from the steam cycle. In the open system, water pumped from intakes on one side of the power plant passes through the condensers and is discharged at a point remote from the intake (to prevent recycling of the warm water discharge).

Open systems typically have high flow rates and relatively low temperature rises to limit the rise in temperature in the receiving waters. A typical 350 MW unit would have a flow of some 15000 to 20000 L/s.

Lake cooling systems are a variant on a true open system as the temperature of the lake is increased from the circulation of the warm water. Environmental requirements have become more stringent on the allowable rise in temperature of the receiving waters, so that closed systems are now more commonly used in Australia.

Open Cycle with Helper Cooling Tower

In this system, cooling towers are installed on the discharge from open systems in order to remove part of the waste heat, so that the load on the receiving waters is contained within pre set limits. Systems with helper cooling towers are common in Germany and France where cooling supplies are drawn from the large rivers. The helper towers are used in the warmer summer periods to limit the temperature of the discharged cooling water, usually to less than 30º C.

Closed Cycle Wet Cooling Systems

In closed cycle wet cooling systems, the waste energy that is rejected by the turbine is transferred to the cooling water system via the condenser. The waste heat in the cooling water is then discharged to the atmosphere by the cooling tower.
In the cooling tower, heat is removed from the falling water and transferred to the rising air by the evaporative cooling process. The falling water is broken up into droplets or films by the extended surfaces of the tower 'fill'. This 'fill' in the later Queensland towers is manufactured from plastic.

Some of the warm water, typically 1 to 1.5% of the cooling water flow, is transferred to the rising air, and this is visible in the plume of water vapour above towers in times of high humidity. The evaporation rates of the Queensland 350 MW cooling systems are typically 1.8 litres of water per kWh of power generated.

The major components of a closed cycle wet cooling water system are:
Cooling towers - two types are commonly used, concrete natural draught towers and mechanical draught towers; and
Pumps and pipes.

Natural Draught Towers

Concrete natural draught towers have a large concrete shell. The heat exchange 'fill' is in a layer above the cold air inlet at the base of the shell as shown in the tower sectional view. The warm air rises up through the shell by the 'chimney effect', creating the natural draught to provide airflow and operate the tower. These towers therefore do not require fans and have low operating costs.

The cooling towers have two basic configurations for the directions of the flow of air in relation to the falling water through the tower fill:

The counter-flow tower where the air travels vertically up through the fill (a diagram of this type of tower is shown below); and
The cross-flow tower where the air travels horizontally through the fill.

Natural draught towers are only economic in large sizes, which justifies the cost of the large concrete shell. Natural draught towers are the most common towers for large generating units in Europe, South Africa and Eastern USA. They are not used in the drier areas of Western USA, as their performance is better suited to cooler and more humid areas. This performance limitation also limits their use in Australia.

Mechanical Draught Cooling Towers

In mechanical draught cooling towers, large axial flow fans provide the airflow. While fans have the disadvantage of requiring auxiliary power, typically 1.5 to 2.0 MW for a 420 MW unit, fans have the advantage of being able to provide lower water temperatures than natural draught towers, particularly on hot dry days.

Mechanical draught towers are used exclusively in central and western USA as their climate can vary from freezing to hot with low humidity, and the mechanical towers can provide a more controlled performance over this wide range of conditions.

The most common materials used in large mechanical draught cooling towers are timber for the framing and plastic for the cladding and internals.

Pumps and Pipes in a Cooling Water System

Circulating water pumps supply cooling water at the required flow rate and pressure to the power plant condenser and the plant auxiliary cooling water heat exchangers. These pumps are required to operate economically and reliably over the life of the plant.
The three types of pumps commonly used for circulating water service are 'vertical wet pit', 'horizontal dry pit' and 'vertical dry pit'. For once through systems, vertical wet pit pumps are in common usage. For re-circulating cooling systems, vertical wet pit and horizontal dry pit are used about equally, with occasional use of vertical dry pit pumps.

Circulating water piping carries the cooling water from the circulating water pumps to the condenser and returns the water to the cooling tower or discharge structure. The large flow rates associated with circulating water systems typically require the use of large diameter piping in the range 900 mm to 2400 mm diameter.

The design of the pipework must consider the environment internal to the pipe as well as the external environment. Pipe materials used include steel, fibre reinforced plastic and reinforced concrete. The large water requirement generally makes it uneconomical to use high quality water sources. The source of water for the plant generally depends on the plant's location. Coastal sites generally use seawater or brackish water as the circulating water source, either by pumping directly from the sea or extracting the water from the local bores.

Water from many sources can contain high concentrations of corrosive contaminants. Any pipe materials considered must include measures to protect the pipe for the service life of the plant. For example, carbon steel pipes in seawater service require either an internal coating, or a cathodic protection system, or both. Concrete pipes may require a dense concrete mix to withstand chloride attack. These protective measures significantly increase the capital cost of an installation such that it can be as economical to install fibre reinforced plastic pipe to obtain the same service life. As existing water sources become strained and new water sources more scarce and expensive to develop, the quality of circulating water in future power plants is expected to decline further. This will increase the trend towards corrosion resistant piping materials.

Closed Cycle Dry Cooling Systems

Dry cooling systems are used where there is insufficient water, or where the water is too expensive to be used in an evaporative system. Dry cooling systems are the least used systems as they have a much higher capital cost, higher operating temperatures, and lower efficiency than wet cooling systems.

In the dry cooling system, heat transfer is by air to finned tubes. The minimum temperature that can be theoretically provided is that of the dry air, which can be regularly over 30º C and up to 40º C on typical summer afternoons in Queensland. Compare this to wet cooling towers, which cool towards the wet bulb temperature, which is typically 20º C on summer afternoons. The steam condensing pressures and temperatures of a dry cooled unit are significantly higher than a wet cooled unit, due to the low transfer rates of dry cooling and operation at the dry bulb temperature.

There are two basic types of dry cooling systems:

1. The direct dry cooling system; and
2. The indirect dry cooling system.
Variations on the full dry and full wet systems are hybrid systems, which may be wet with some dry or dry with part wet.

Direct Dry Cooling System

In the direct dry system, the turbine exhaust steam is piped directly to the air-cooled, finned tube, condenser. The finned tubes are usually arranged in the form of an 'A' frame or delta over a forced draught fan to reduce the land area. The steam trunk main has a large diameter and is as short as possible to reduce pressure losses, so that the cooling banks are usually as close as possible to the turbine.

The direct system is the most commonly used as it has the lowest capital cost, but significantly higher operating costs. The power required to operate the fans of this system is several times that required for wet towers, being typically 4 to 5 MW for a 420 MW unit.

Indirect Dry Cooling System

Indirect dry cooling systems have a condenser and turbine exhaust system as for wet systems, with the circulating water being passed through finned tubes in a natural draught cooling tower. The water pipework allows the towers to be sited away from the station.

A variation on this type of indirect system is the system that uses a direct contact condenser in place of the traditional tube type condenser. In the spray condenser, the water from the cooling cycle mixes with the boiler water. The maintenance of the water quality to suit all circuits is critical to the successful operation of the system.

Hybrid Systems

There are two common hybrid systems, which have been developed to overcome some of the disadvantages of the full wet and full dry systems.

Wet with Part Dry

One of the problems with wet towers is that in cold and humid climates the towers plume, can create fog. In the part dry or plume abatement tower, a dry section above the wet zone provides some dry cooling to the exhaust plume to remove the condensing water vapour. These towers are common in Germany and England where environmental problems with mechanical towers have arisen.

Dry with Part Wet

Problems with full dry towers are centred on loss of performance in hot weather. With the part wet towers, there is provision for water sprays to evaporatively cool the finned tubes for short periods of extreme temperature.

Environmental Effects of Cooling Systems

All the heat transferred from the exhaust steam to the cooling system eventually finds its way into the earth's atmosphere.

In the once-through cooling water system, heat is removed from the steam turbine and transferred to the source body of water. The heat is then gradually transferred to the atmosphere by evaporation, convection and radiation. However, this waste heat transfer process may negatively affect the body of water buy increasing the temperature of the water.

In a re-circulating cooling system, the cooling water carries waste heat removed from the steam turbine exhaust to the cooling tower, which rejects the heat directly to the atmosphere. Because of this direct path to the atmosphere, surrounding water bodies typically do not suffer adverse thermal effects. Some water is discharged from the cooling water system to maintain the concentration of chemicals in the cooling water below licensed limits. This water is often discharged to surrounding watercourses.

In dry cooling systems, the waste heat is transferred directly to the atmosphere.