What are natural draft cooling towers?
Natural draft cooling towers (natural convection cooling towers) use the principle of convective flow to provide air circulation. Natural draft towers are usually very tall in order to induce adequate air flow, they are also expensive to construct and are only used for applications where a large constant cooling requirement over many years is required; a thermal power plant is one such application.
Natural Draft Cooling Towers
How a Natural Draft Cooling Tower Works
The below video is an extract from our Introduction to Cooling Towers Online Video Course.
Cool cooling water is pumped from the cooling tower basin to the power plant. The cool cooling water is heated by the process and its temperature increases. The warm cooling water is now pumped back to the cooling tower to be cooled.
Natural Draft Cooling Tower Water System
The incoming warm water is distributed through spray nozzles inside the tower. The spray nozzles spray the warm water evenly over the entire fill. Water passes downwards through the fill whilst air passes upwards.
Natural Draft Cooling Tower Components
As the water travels downwards through the fill, some of it evaporates which causes the remaining water to be cooled (evaporative cooling).
Natural Draft Cooling Tower Cross Section
As air travels through the fill, its temperature increases and it rises to the top of the cooling tower due to the stack effect (hot air is less dense than cool air and thus rises above it). The air exiting the top of the tower draws in more air at the base of the tower, creating a natural air flow from the base to the top of the tower; this is the stack effect and it is continuous providing cooling water is constantly circulated.
Air and Water Flow
Counter and Cross Flow
Natural draft cooling towers may be of the counter flow or cross flow design. Cross flow natural draft cooling towers have a much wider base than counter flow natural draft cooling towers.
Counter Flow Natural Draft Cooling Tower
Cross Flow Natural Draft Cooling Tower
Advantages and Disadvantages
Advantages
- Low maintenance costs.
- Low operational costs.
- Low system losses (typically less than 1% of total flow).
- Low noise level as there are no fans used.
- Large cooling capacity.
Disadvantages
- Large initial capital investment.
- Difficult sometimes to get planning permission due to the large structure and aesthetic affect upon the local area.
Winterization
Natural draft cooling towers may operate in sub-zero environments which may lead to the reservoir water freezing. There are several methods employed to prevent this occurring.
- Drain the tower of cooling water. A simple and effective way to remove the possibility of damage due to the water freezing, but it renders the power station inoperable.
- Circulate the cooling water to maintain a temperature above sub-zero in the cooling tower basin. This method is used by most plants as it is economical and relatively easy to implement.
- Dose the cooling tower water with anti-freeze. This would prevent the water freezing, but it is not a financially viable solution due to the large volume of water in the system.
- Heat the cooling tower basin. This option is not usually economically viable.
Why do natural draft cooling towers have such a weird shape?
Natural draft cooling towers have a very unique shape for several reasons. The first reason is that the shape reduces the amount of construction material required when building such a large tower. The second reason is that the paraboloid shape of the tower accelerates the air flow through the tower, which increases the tower’s cooling capacity. Natural draft cooling towers are sometimes referred to as hyperbolic towers although the correct term is hyperboloid.
What is the stack effect?
When air is heated, it becomes less dense. Because warm air is less dense than cold air, it will rise above cold air due to the density difference. This process of warm and cold air separating based upon densities is known as the stack effect or chimney effect.
For example, a hot air balloon flies in the air due to the hot air that is contained within the balloon. The hot air is less dense than the surrounding ambient air, which is why the hot air rises, pulling the balloon up with it.
Hot Air Balloon
3D Model Components
This 3D model shows all major components associated with a typical induced draft hyperboloid cooling tower, these include:
- Fill (Heat Exchanger)
- Spray Nozzles
- Piping
- Basin/Reservoir
- Drift Eliminator
- Fan
- Tower Structure (Hyperboloid Tower)
This is a 3D model of a Natural Draft Cooling Tower.
3D Model Annotations
Natural Draft Cooling Tower Components
Natural draft cooling towers (natural convection cooling towers) use the principle of convective flow to provide air circulation. They are tall in order to induce adequate air flow. They are also expensive to construct, and are only used for applications where a large constant cooling requirement over many years is required; a thermal power plant is one such application.
Collecting Basin (reservoir)
The collecting basin is a receptacle beneath the cooling tower for collecting the water cooled by the cooling tower; it is usually constructed from concrete. 'Cold' cooling water is pumped from the cooling tower basin to the process e.g. power plant.
Distribution System
The portion of a cooling tower that distributes water over the fill area usually consists of flanged inlets, flow control valves, spray branches, metering orifices, spray nozzles and other related components. The purpose of the distribution system is to ensure water is distributed evenly to all spray nozzles.
Sump
The sump (pump suction bay) is a depressed portion of the collecting basin from which ‘cold’ water is drawn to the centrifugal pump(s), then discharged to the process e.g. power plant. The sump usually contains strainers, anti-vortex devices and a drain or clean-out connection.
Fill
The fill is essentially a heat exchanger that maximises the contact surface area between the cooling water and air. Air passes upwards across the fill whilst water falls downwards due to gravity. As air passes over the water, some of the water evaporates, and the remaining water is cooled (evaporative cooling). Cooling towers use two main fill designs, the ‘film fill’ and ‘splash fill’ designs. Film fill is more efficient, but more expensive, and more prone to fouling.
Drift Eliminators
‘Drift’ is the name given to water molecules that are lost from the cooling water system due to evaporation. A large plume of white moisture can often be seen rising from natural draft cooling towers, this is ‘drift’, and it represents a financial loss (lost water must be replaced). Drift eliminators reduce water losses and consequently reduce operational running costs.
Drift eliminators consist of parallel blades arranged on the air discharge side of the tower to remove entrained water droplets from the exiting air stream. The shape of the drift eliminator requires the entrained water droplets to change direction several times (torturous flow path) prior to being discharged from the tower. Air has little problem changing direction and passing through the drift eliminator, but water droplets impinge upon the drift eliminator, condense, then drip back down onto the fill, and return to the cooling tower basin.
Hyperboloid Tower
There are two main reasons why natural draft cooling towers having such a unique shape. The first reason is that the shape reduces the amount of construction material required when building such a large tower. The second reason is that the hyperboloid shape of the tower accelerates the air flow through the tower, which increases the tower’s cooling capacity. ‘Cold’ air enters the base of the tower, is drawn upwards across the fill, is heated, then exits through the top of the tower. It is the difference in air temperature -and consequently air density- which causes convective air flow through the tower (hotter air is less dense and rises upwards through the tower, which leads to the ambient colder air being drawn in through the base of the tower). The process of hot air (less dense) rising above colder air (more dense), is refereed to as the ‘stack effect’ or ‘chimney effect’.
Spray Nozzles
The purpose of the spray nozzles is to spread water evenly across the fill and thus maximise the contact surface area between the water and air. A large contact surface area is desired because it yields a much greater heat transfer capacity. All heat exchangers rely upon a large contact surface area between the flowing mediums, as this ensures high thermal contact between them.
Cooling Water Outlet
Cooling water is drawn from the reservoir and sent to the steam turbine condenser(s) through this connection.
Cooling Water Inlet
Cooling water returned from the condenser(s) enters through this connection.
Heated Air
As air passes through the fill, its temperature increases, and it rises up through the tower.
Air Inlet
Ambient air is drawn into the tower through the tower base.
Additional Resources
https://en.wikipedia.org/wiki/Cooling_tower
https://www.enexio.com/cooling-solutions/wet-cooling-towers/natural-draft-cooling-towers
https://www.sciencedirect.com/topics/engineering/natural-draft-cooling-tower