15. MAIN COOLING WATER SYSTEMS
15.2 Components of the System
The Cooling Water Source
For economic reasons Power Stations are normally located as near as practicable to the resources they rely upon. This usually means that the provision of an adequate supply of cooling water has already been negotiated at the design stage and the Power Station will be located adjacent to a sea side or fresh water lake or have access to a pumping quota from a nearby river.
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Cooling Water Pump Condensate Pump
Condenser Steam to LP Cylinders
Water Source – River Sea or Lake Cooling Tower Basin Warm water to Cooling Tower
Cooled Water returned to Water Source
Inland Power Stations are more likely to rely upon river water makeup to a Closed Cooling Water System than to have exclusive use of an inland Lake as a cooling medium. It is the Cooling Towers associated with a Closed Cooling Water System that will now be examined in more detail.
Cooling Towers
Cooling Towers are Air/Water Heat Exchangers in which the water to be cooled is brought into intimate contact with a stream of ambient air resulting in a transfer of heat from the water to the atmosphere. Heat transfer occurs through:
Sensible heat exchange, seen as an increase in the air temperature
Latent heat exchange, in which a portion of the water is evaporated and lost from the cooling water circuit, taking with it the extra heat load required to create the water/steam phase change. (This accounts for the major part of the heat loss from the returning cooling water).
A small portion of water is also lost from the system due to drift or entrainment in the air stream. This water has to be replaced from a make up source, which is usually colder than the return water temperature, resulting in a reduction of the overall cooling water temperature (although not caused by heat transfer as such)
The type and size of cooling tower used will depend upon:
The amount of heat rejected by the turbine and auxiliary plant at maximum load.
The average and extreme conditions of ambient temperature and humidity experienced at the Cooling Tower site.
The design supply and return cooling water temperatures for the Cooling Water System (which are related to the mass flow of cooling water and the condenser design)
Cooling Towers may be of a Natural or Fan Assisted Flow design.
A Natural Draft Cooling Tower relies on what is termed as a
“stack (or chimney) effect” to create a rising air flow through
the tower. This “stack effect” is produced by the warm, less dense air being driven from the top of the tower as it is displaced by the cool, more dense air entering the base.
Fan Assisted Cooling Towers incorporate a mechanical fan to promote a flow of air through the Tower.
Natural Draft Cooling Towers
The driving pressure, which maintains the air flow through a Natural Draft Cooling Tower, is dependent on the difference in densities between the inside and outside air and the height of the tower. As the difference in densities is often quite small, the height of the tower becomes the most important design criteria.
This increased demand for height brings with it problems in construction due to a need for superior strength and resistance to the high wind loading that can be directed against such a large surface area.
The hyperbolic shape (shown in Figure 85) offers the most suitable profile for strength and wind resistance.
The performance of Natural Draft Cooling Towers is poor in hot dry inland areas where low relative humidity conditions are common and the air density outside of the cooling tower may not be high enough to displace the moisture laden air inside the tower. Natural Draft Cooling Towers are, however, well suited to locations with consistently high relative humidity, a cool, humid climate and a high winter power demand.
High initial costs tend to relegate the Natural Draft Cooling Tower to higher output Power Stations where long term gains made from the non use of mechanical fans offset the initial cost.
Figure 85: Cutaway View of Natural Draft Cooling Tower
Cold Air In Warm Air
Out
Cool Water Collected in Cooling Tower Basin Drift Eliminators
Fill
Hot Water Distribution
System
Hot Water In Warm Air Out
Cool Water Out
Fan Assisted Cooling Towers
Where initial cost, climatic conditions and available space become a concern an alternative to Natural Draft type Cooling Towers must be found.
By reducing the total height and size of a cooling tower, the natural “Stack effect,” which induces air flow is also reduced and it becomes necessary to use a fan to create the required air flow.
Fan assisted cooling towers provide an alternative to the natural draft type, having a lower initial cost, but incurring an ongoing cost associated with fan useage.
Fan Assisted Cooling Towers may be of a Forced or induced Draft type.
Forced Draft Cooling Towers
The fan (or fans) in a Forced Draft Cooling Tower is in the air stream entering the tower. This design allows:
greater ease of access to the fans for inspection and maintenance
reduced fan power demand due to the drier less dense air being passed by the fan
But incurs the following disadvantages:
heat generated by the fan is added to the Turbine Heat Load within the Cooling Tower
a portion of the Hot Air and Moisture from the Cooling Tower discharge can be re-entrained into the Fan intake and recirculated
difficulty is encountered in maintaining even air distribution through out the tower
as the tower is pressurised leakage can occur from the casing
during cold weather operation in winter, frost can accumulate around the fan intake
Owing to the above disadvantages, the majority of Fan assisted Cooling Towers are of the Induced Draft Type.
Induced Draft Cooling Towers.
The fan in an Induced Draft Cooling Tower is placed at the top of the Cooling Tower above the Hot Water Distribution System. The fan draws air from the surrounding area through the open sided base of the tower and induces it to flow through the water distribution system before discharging to atmosphere above the tower.
Cooling Towers can be either crossflow or counterflow.
A Counterflow Cooling Tower (shown in Figure 86 draws air into the tower and directs it to flow vertically upward through the falling water curtain and fill.
A Crossflow Cooling Tower (shown inFigure 87) draws air into the tower horizontally while the water curtain is falling vertically.
Figure 86: Counterflow Induced Draft Cooling Tower
Hot Water In
Hot Water Distributors
Fill Material Warm Air Out
Cool Air In Cool Air In
Drift Eliminators
Cool Water Collected in Cooling Tower Basin Induced
Draft Fan External Fan
Drive Unit Fan Cowl
Cool Water Out
Figure 87: Cross Flow Induced Draft Cooling Tower Air Entry
louvres Hot Water
In
Hot Water Distributor
Fill Material
Warm Air Out
Cool Air In Cool Air
In
Induced Draft Fan
External Fan Drive
Unit Fan Cowl
Cool Water
Out Cool Water Collected in Cooling Tower Basin
Hot Water Distribution Systems
Hot Water, returning from the condenser, is pumped to the Cooling Tower under pressure and evenly distributed throughout the cooling tower cells. This ensures maximum contact and maximum heat transfer between the air and water. The distribution system does this by breaking the flow into fine droplets (Spray Distribution) and/or reducing the velocity of the water flow into the tower (Gravity Distribution).
Spray Distribution uses a grid of spray distributor nozzles fed through branched piping taken from the main inlet manifold.
The spray system allows maximum wetting of the Cooling Tower and enhanced water/air stream contact. Spray Distribution is used mainly on Counterflow Cooling Towers (see Error! Reference source not found.).
A Gravity Distribution system first reduces the return water velocity by discharging from the return pipework into a basin above the cooling tower fill. The hot water, with a reduced head, then flows through a grid of orifices. Diffuser heads can be inserted into the orifices to give the required spray pattern on to the fill material below. Gravity Distribution is used mainly on Cross Flow Cooling Towers (see Error! Reference source not found.).
Cooling Tower Fill
To increase the heat transfer capacity of a Cooling Tower the air and water must be mixed as intimately as possible.
This is done by:
increasing the time the water takes to fall from the inlet to the holding basin and
increasing the surface area that is presented to the air stream.
The use of fill or “wet deck” within a cooling tower achieves both of the above. The fill is placed between the hot water distribution system and the holding basin.
Splash Fill is made up of a series of rectangular bars ( or planks depending on the material used) with a small vertical
dimension and a larger horizontal dimension, arranged in tiers within the cooling tower. The small vertical dimension gives little impedance to the air flow while the broader horizontal dimension impedes the water flow, causing the stream to be repeatedly broken up and thinly distributed across the broad face of the bars. This increases both the surface area in contact with the air stream and the time the water is in contact with the air stream before it finally reaches the basin below. Figure 88 shows a simplified flow diagram of the air and water through a section of splash type fill.
Film Type Fill is made up of many hard plastic sheets (which are formed in a range of rippled patterns dependent on the supplier) placed together to form hundreds of separate flow paths. The water tends to flow as a thin film down the sides of the fill while the air flows up through the centre.
The rippled patterns:
present a greater water surface area to the air flow
increase the time that the water is in contact with the air stream and
create turbulence in the air stream to ensure more intimate contact between the air and water
By arranging the sheets so that the paths are not vertical but zig-zagged the contact time and surface area are further extended.
Figure 89 is a simplified diagram of film type fill showing the air and water paths.
Figure 88: Splash Fill – Most Suitable for Cross Flow Cooling Towers
Figure 89: Film Type Fill – Equally Suitable for Cross or Counter Flow Water Flow Consistently Broken and
Slowed Down by Splash Bars
Air Flow Horizontal and
Water Curtain Vertical Splash Bars
Air passes over a fine film of water flowing down the surface of the fill medium
Cross or Counter flow are equally appropriate for film
type fillmedia
Cooling Tower Fans
Cooling Tower Fans may be of either the centrifugal or axial flow type. Centrifugal fans operate against increased discharge heads and so are more likely to be used for forced draft Cooling Tower applications. Axial flow fans are most prominent in Induced Draft Cooling Towers where they are capable of moving large volumes of air for a relatively low power demand.
Air Flow and Water Temperature Control
Air Flow through the Cooling Tower can be regulated by a number of mechanisms:
Fan speed adjustment
Fan Blade Pitch adjustment (axial Flow Fans)
Shutting down and placing fans in service as air flow demand dictates
As the Heat Load transferred to the Main Cooling Water System may vary dependent on the total steam flow being passed to the Turbine Condenser and the load being contributed from the Auxiliary Heat Exchangers, Cooling Towers for larger installation tend to be of a multi-cellular construction. Each Cell is fitted with its own fan, hot water distribution system and “wet deck” or Fill.
This allows the Cooling Tower power demand to be „turned down‟ during times of low heat transfer demand. Fans can be selectively taken out of service or fan blade pitch changed to reduce the total air flow through the tower to prevent overcooling of the water. Where multiple Main Cooling Water Pumps are provided ( each with less than 100% flow capacity) cooling water flow can be altered by varying the number of
initially filled from an external source (Sea, lake or river) and the operating level is maintained from the same source. The Basin‟s size should be calculated to allow the system to operate without makeup for sufficient time to carry out regular in-service maintenance.
The Cooling Tower Basin is normally fitted with the following:
Valved Cooling Water Makeup Supply Line Valved Drain Line
Overflow Line
Main Cooling Water Pump Forebay (Usually of a greater depth than the main basin area to prevent pump vortexing and cavitation)
Debris Screens at the pump forebay entry Chemical Dosing Facilities
Facilities to monitor Water Quality and blowdown
Where on site water resources are limited Cooling Tower Basins have been used as an emergency source of water for Fire Fighting. Alternate valved pipework is installed to supply the Fire Fighting Pumps‟ suction.
Cooling Tower Makeup
Water is lost from the Main Cooling Water Circuit due to:
Evaporation Losses in the Cooling Tower (approximately 1 to 1.5% total Cooling Water flow rate)
Drift Losses from the Cooling Tower ( approximately 0.02 to 0.03% total flow rate)
Blowdown from the Cooling Tower Basin to control the concentration of dissolved solids (approximately 0.2 to 1.5
% total Cooling Water flow rate dependent on allowable
Blowdown and Chemical Dosing
With an evaporation rate of 1 to 1.5% the water within the Cooling Tower Basin would have a concentration of solids of 2 to 2.5 times that of the makeup water with every 100 cycles of the basin‟s volume through the system. Dependent on whether the primary source is sea water, lake or river water the initial concentration of solids will vary. Chemical analysis of the water will determine the allowable concentration levels and the degree of blowdown required to maintain acceptable concentrations within the system. If the total concentration of solids reach saturation point scaling will occur within the cooling water circuit and the heat exchange capacity of the system will deteriorate. It is therefore necessary to continually remove a percentage of the cooling water from the circuit and to replace it with makeup water with a lower solids concentration.
Air moving through the Cooling Tower carries with it dust and debris which is washed from the air by the cooling water.
This silt enters the system and, if the water is not treated to prevent it, precipitates out, forming a film over the heat exchange surfaces.
Biological contaminants in the form of marine and fresh water molluscs and crustaceans, water resident plants, algae and bacteria can cause fouling and corrosion within the systems pipework and the heat exchange surfaces.
Crossflow and Counterflow Cooling Towers without air entry louvres tend to grow more algae due to the increased amounts of sunlight entering the tower.
Breakdown and decomposition of biological material can generate Hydrogen Sulphide and Carbon Monoxide, which readily combine with water to form corrosive solutions.
To counter the above scaling and corrosion effects, antiscalant and anticorrosion chemical dosing is normally carried out (if required) on a regular basis with dedicated dosing pumps delivering a metered dose from chemical storage tanks.
Biological control tends to be irregular in the form of “shock”
dosing to prevent molluscs etc from developing a learned response and subsequently withdrawing themselves from the dosing stream prior to the dose being delivered.
Cooling Tower Wetdown System
Where the main structural components of the cooling tower are made from wood a Wetdown System is normally installed.
Such a system uses low pressure sprays to douse the cooling tower internals and prevent dryout and distortion of the wooden structure during periods when the cooling water circuit is out of service. The risk of fire within the cooling tower is also reduced by keeping the wooden structure damp.
Circulating Water Pumps
Cooling Water Pumps may be of the Centrifugal, Axial Flow or Mixed Flow types dependent on the total System Discharge Head and mass flow required. Axial Flow pumps are well suited to Open Cooling Systems while Centrifugal Pumps perform well in Closed Systems.
Debris Screens
Depending on the water source a variety of debris screens are used to prevent fouling of the pumps and heat exchangers by large particulate matter.
Where salt or fresh water molluscs and crustaceans are plentiful care must be taken to prevent a build up of shells and grit within the system. In such cases the intake from the water source needs to be screened and where a cooling tower forms a component of the system a further debris screen needs to be added at the Cooling Tower Basin Outlet.
Screens can take the form of:
Fixed Screens with a means of raking the debris from the screen and discarding it to waste
Rotating Screens with a self cleaning water spray which flushes water borne fauna and debris to waste
Removable Series Screens, which allow any one screen to be removed and cleaned while subsequent screens remain active in the flow path.
The condition of the screens may be monitored by the installation of a differential pressure switch across the screen with alarm contacts included to initiate an automatic self
separate closed system completely divorced from the Main Cooling Water System
closed system which includes a heat exchanger cooled by a branch line from the Main Cooling Water System thereby transferring its heat to the same heat sink as the Main System
Figure 90: Auxiliary Cooling Water System Utilising Main/Auxiliary Cooling Water Heat Exchanger
In a system such as that shown in Figure 90 the recirculating Cooling Medium is usually of a high quality (eg.
Demineralised Water). Provision is made for the addition of makeup and for the expansion of the system through a raised head tank which also serves to maintain a positive suction head on the circulating pumps. Chemical dosing and/or other methods of water quality maintenance and control may also be used dependent on the circulating fluids in the heat exchangers to be cooled.
System pressures within Auxiliary Cooling Water System Heat Exchangers normally maintain a positive pressure differential between the fluid being cooled and the fluid coolant to prevent contamination of the primary fluid should a leak occur within the heat exchanger. An example can be seen in a Lubricating Oil Cooler. The system pressure of the Lubricating Oil would be higher than the Auxiliary Cooling
Water Pressure to ensure any leakage would result in oil migrating into the cooling water circuit rather than vice versa. Th higher pressure system is also placed into service before the cooling circuit and removed from service after the cooling circuit
Typical Heat Exchange Circuits served by the Auxiliary Cooling Water System can include but are not limited to :
Turbine Lubricating Oil Coolers
Steam and Hot Water Sample Coolers
Steam and Hot Water Sample Coolers