Tag: cooling tower

The inside of a cooling tower looks like no place on earth

The silhouette of cooling towers on the horizon is one of the most recognisable symbols of electricity generation around the world. But inside these massive structures is an environment unlike any other.

When cooling towers are in operation, torrents of warm water cascade down to a huge pond at its base, the air cooling it as it falls. Plumes of water vapour rise through the structure and into the air.

But when shut down – for maintenance, for example – the inside of a cooling tower is a very different place. The vast emptiness of the space can be eerily silent. Even the smallest noise echoes around its concrete shell.

Standing at over 114 metres high, each of Drax’s 12 cooling towers are 86 metres in diameter at their base, 53 metres at their summit, and could comfortably fit the Statue of Liberty inside. Everything about them is huge, but they are not the unsophisticated masses of concrete they appear from afar.

“Look at a cooling tower and you might think it’s a substantial, thick structure. It’s not,” explains Nick Smith, a civil engineer at Drax. “It’s basically like an egg shell. It is the shape that gives it its strength.” For the majority of their height, a typical cooling tower is between just 178 and 180 mm – or 7 inches – thick.

It’s a testament to the original design and construction that they require such limited maintenance more than half a century after plans were first drawn up. Especially considering they are in daily use.

What does a cooling tower do?

Water is an essential part of thermal electricity generation. It is turned into high-pressure steam in the extreme temperatures of a boiler before being used to spin turbines and generate electricity. Water within the boiler is ‘de-mineralised’ and purified to prevent damage to the turbine blades and infrastructure.

Once it leaves the turbine, the steam is cooled to pure water again in the condenser so it can be reused in the boiler. To do this the steam is passed over pipes containing cold water from the cooling towers, which cools and condenses the steam while also heating up the cold water to roughly 40 degrees Celsius, the temperature it is at when it enters the cooling tower.

Inside the towers the warm water is poured over what’s known as the cooling tower pack, a series of stacks of corrugated plastic that sit roughly 30 metres up the tower. The heat and the tower’s height create a natural draught. This pulls air in from the cavities at the base of the tower – called the throat – which cools the water to around 20°C as it cascades down the stack into a pond below. It is then returned to the condenser where the cooling cycle starts all over again.

Only around 2% of the water escapes through the top of the cooling towers as water vapour – which is what can be seen exiting the top of the towers – with a further 1% returned to the River Ouse to control water levels. These small losses are replenished with water taken from the Ouse. It highlights the genius of the towers’ design that their shape alone can cool water so efficiently on an industrial scale with minimal environmental impact. 

A lasting design

A cooling tower’s iconic shape is known as a hyperboloid, referring to its inward curve. This makes them very stable, but to make them strong enough to last as long as they have, Drax’s cooling towers have the added assistance of reinforced concrete.

“Concrete is very strong in compression, but it has hardly any tensile strength,” says Smith. “Therefore our cooling towers have both vertical and horizontal hoop reinforcement to take any tensile forces generated. It is the concrete and steel working together that gives the reinforced concrete its strength.”

The level of design and engineering of Drax’s cooling towers are all the more impressive considering their age. “The construction of our first tower was completed in 1970 and designed in the mid 1960s,” says Smith, pointing out, “they were designed at a time where there wasn’t huge computing processing power, so they would likely have been designed by hand.”

“They were constructed to a very high degree of accuracy even when a lot of the equipment used would have been manual,” he adds.

Designing and building a cooling tower today, he adds, would require significant computing power and sophisticated setting-out equipment to ensure the accuracy of the construction. However, the underlying principles of the towers’ shape and how well they have continued to perform since their construction would give little reason to deviate from the current design.

In fact, that consistent performance means that even as the nature of generating electricity develops to include new fuels and technologies, cooling towers remain an integral part of the process. Drax’s Repower project– which could see the conversion of the plant’s remaining coal units to gas and the installation of a giant battery facility – is a significant step forward in the evolution of power generation, yet the design and purpose of the cooling towers would remain the same.

The structures that will shape the landscape of the future of electricity generation may include wind turbines, biomass domes and solar panels. But the enduring functionality of natural draught concrete cooling towers means they will still play a role in producing the country’s electricity – even as generation diversifies.

Watch Inside a cooling tower

How electricity is made

Every morning we take electricity as a given. We switch on lights, charge phones and boil kettles without thinking about where this power comes from.

The electronic devices and appliances that make up our daily routines are not particularly energy intensive. Boiling a kettle only uses 93 watts, toasting for three minutes only requires 60 watts, while cooking in a microwave for five minutes takes 100 watts.

However, when people are waking up and making breakfast in almost 30 million households around the UK, those small amounts soon create a significant demand for electricity. On a typical winter’s morning, this combined demand spikes to more than 45 gigawatts (GW).

So this is what it takes to power your breakfast – from the everyday toaster in your kitchen backwards through thousands of miles of cables to the hundreds of thousands of tonnes of machinery in wind farms, hydro-electric dams and at power stations such as Drax where electricity generation begins.

The grid 

The journey starts in the home where all our electricity usage is tracked by meters. These are becoming increasingly ‘smart’, displaying near real-time information on energy consumption in financial terms and allowing more accurate billing. There are already 7.7 million smart meters installed around the UK, but that number is set to triple this year, paving the way for a smarter grid overall.

What brings electricity into homes is perhaps the most visible part of the energy system on the UK’s landscape. The transmission system is made up of almost 4,500 miles of overhead electricity lines, nearly 90,000 pylons and 342 substations, all bringing electricity from power stations into our homes.

Making sure all this happens safely and as efficiently as possible falls to the UK’s nine regional electricity networks and National Grid. Regional networks ensure all the equipment is in place and properly maintained to bring electricity safely across the country, while National Grid is tasked with making sure demand for electricity is met and that the entire grid remains balanced.

The station cools down

One of the most distinctive symbols of power generation, cooling towers carry out an important task on a massive scale.

Water plays a crucial role in electricity generation, but before it can be safely returned to the environment it must be cooled. Water enters cooling towers at around 40 degrees Celsius, and is cooled by air naturally pulled into the structure by its unique shape.

This means those plumes exiting from the top of the towers are, rather than any form of pollution, only water vapour. And this accounts for just 2% of the water pumped into the towers.

Drax counts 12 cooling towers, each 114 metres tall – enough to hold the Statue of Liberty with room to spare. Once the water is cooled it is safe to re-enter the nearby River Ouse.

The station’s bird’s-eye view

The control room is the nerve centre of Drax Power Station. From here technicians have a view into every stage of the power generation process.  The entire system controls roughly 100,000 signals from across the power station’s six generating units, water cooling, air compressors and more.

While once this area was made up of analogue dials and controls, it has recently been updated and modernised to include digital interfaces, display screens and workstations specially designed by Drax to enable operators to monitor and adjust activity around the plant.

The heart of power generation 

At the epicentre of electricity generation is Drax’s six turbines. These heavy-duty pieces of equipment do the major work involved in generating electricity.

High-pressure steam drive the blades which rotates the turbine at 3,000 revolutions per minute (rpm). This in turn spins the generator where energy is converted into the electricity that will eventually make it into our homes.

These are rugged pieces of kit operating in extreme conditions of 165 bar of pressure and temperatures of 565 degrees Celsius. Each of the six turbine shaft lines weighs 300 tonnes and is capable of exporting over 600 megawatts (MW) into the grid.

One of the most important parts of the entire process, turbines are carefully maintained to ensure maximum efficiency. Even a slight percentage increase in performance can translate into millions of pounds in savings.

Turning fuel to fire

To create the steam needed to spin turbines at 3,000 rpm, Drax needs to heat up vast amounts of water quickly and this takes a lot of heat.

The power station’s furnaces swirl with clouds of the burning fuel to heat the boiler. Biomass is injected into the furnace in the form of a finely ground powder. This gives the solid fuel the properties of a gas, enabling it to ignite quickly. Additional air is pumped into the boiler to drive further combustion and optimise the fuel’s performance.


How do you turn hundreds of tonnes of biomass pellets into a powder every day? That’s the task the pulveriser take on. In each of the power plant’s 60 mills, 10 steel and nickel balls, each weighing 1.2 tonnes, operate in extreme conditions to crush, crunch and pulverise fuel.

These metal balls rotate 37 times a minute at roughly 3 mph, exerting 80 tonnes of pressure, crushing all fuel in their path. Air is then blasted in at 190 degrees Celsius to dry the crushed fuel and blow it into the boiler at a rate of 40 tonnes per hour.

The journey begins: biomass arrives

Biomass arrives at Drax by the train-load. Roughly 14 arrive every day at the power station, delivering up to 20,000 tonnes ready to be used as fuel.

These trains arrive from ports in Liverpool, Tyne, Immingham and Hull and are specially designed to maximise the efficiency of the entire delivery process, allowing a full train to unload in 40 minutes without stopping.

The biomass is then taken to be stored inside Drax’s four huge storage domes. Each capable of fitting the Albert Hall inside, the domes can hold 300,000 tonnes of compressed wood pellets between them.

Here the biomass waits until it’s needed, at which point it makes its way along a conveyor belt to the pulveriser and the process of generating the electricity that powers your breakfast begins.

Everything you ever wanted to know about cooling towers

Close up image of Drax cooling tower

Cooling towers aren’t beautiful buildings in the traditional sense, but it’s undeniable they are icons of 20th century architecture. They’re a ubiquitous part of our landscape – each one a reminder of our industrial heritage.

Yet despite the familiarity we have with them, knowledge about what a cooling tower actually does remains limited. A common misconception is that they release pollution. In fact, what they actually release is water vapour – similar to, but nowhere near as hot, as the steam coming out of your kettle every morning. And this probably isn’t the only thing you never knew about cooling towers. 

What does a cooling tower do?

As the name suggests, a cooling tower’s primary function is to lower temperatures – specifically of water, or ‘cooling water’ as it’s known at Drax.

Power stations utilise a substantial amount of water in the generation of electricity. At a thermal power plant, such as Drax, fuel is used to heat demineralised water to turn it to high pressure steam. This steam is used to spin turbines and generate electricity before being cooled by the cooling water, which flows through two condensers on either side of each of the steam turbines, and then returning to the boiler. It is this process that the cooling towers support – and it plays a pivotal role in the efficiency of electricity generation at Drax’s North Yorkshire site.

To optimise water utilisation, some power stations cycle it. To do this, they have cooling towers, of which at Drax there are 12. These large towers recover the warmed water, which then continues to be circulated where chemistry is permitting.

The warmed water (about 40°C) is pumped into the tower and sprayed out of a set of sprinklers onto a large volume of plastic packing, where it is cooled by the air naturally drawn through the tower. The plastic packing provides a large surface area to help cool the water, which then falls in to the large flat area at the bottom of the massive structure called the cooling tower pond.

As the water cools down, some of it (approximately 2%) escapes the top of the tower as water vapour. This water vapour, which is commonly mistakenly referred to as steam, may be the most visible part of the process but it’s only a by-product of the cooling process.

The majority of the water utilised by Drax Power Station is returned back to the environment, either as vapour from the top of the towers or safely discharged back to the River Ouse. Each year, about half of the water removed from the river is returned there. In effect, it is a huge amount of water recycling and in environmental terms, it is not a consumptive process.

Close-up of side of Drax cooling towers

How do you build a cooling tower?

The history of cooling towers as we know them today dates back to the beginning of the 20th century, when two Dutch engineers were the first to build a tower using a ‘hyperboloid’ shape. Very wide on the bottom, curved in the centre and flared at the top, the structure meant fewer materials were required to construct each tower, it was naturally more robust, and it helped draw in air and aid its flow upwards. It quickly became the de facto design for towers across the world.

The Dutch engineers’ tower measured 34 metres, which at the time was a substantial achievement, but as engineering and construction abilities progressed, so too did the size of cooling towers.

Today, each of 12 towers measures 115 metres tall – big enough to fit the dome of St Paul’s Cathedral or the whole of the Statue of Liberty, with room to spare. If scaled down to the size of an egg, the concrete of each cooling tower would be the same thinness as egg shell.

The structures at Drax are dwarfed by the cooling towers at the Kalisindh power plant in Rajasthan, India, the tallest in the world. Each stands an impressive 202 metres tall – twice the height of the tower housing Big Ben and just a touch taller than the UK’s joint fifth tallest skyscraper, the HSBC Tower at 8 Canada Square in London’s Canary Wharf.

The industrial icon of the future

Today’s energy mix is not what is used to be. The increased use of renewables means we’re no longer as reliant on fossil fuels, and this has an effect on cooling towers. Already a large proportion of the UK’s most prominent towers have been demolished, going the same way as the coal they were once in service to. But this doesn’t mean cooling towers will disappear completely.

Power stations such as Drax, which has upgraded four of its boilers to super-heat water with sustainably-sourced compressed wood pellets instead of coal, the dwindling coal fleet, and some gas facilities still rely on cooling towers. As they continue to be part of our energy mix, the cooling tower will remain an icon of electricity generation for the time being. But it’ll be a mantle it shares with biomass domes, gigantic offshore wind turbines and field-upon-field of solar panels – the icons of today’s diverse energy mix.

View our water cooling towers close up. Drax Power Station is open for individual and group visits. See the Visit Us section for further information.

If you’re afraid of heights, don’t do this job

Reparing the colling tower at Drax Power Station

Be they for nuclear, coal, or biomass power, cooling towers and their colossal, tapering silhouettes are the most iconic element of the architecture of energy. Drax has 12 of them.

But a structure of that size poses a considerable maintenance challenge. For the first time since Drax’s six towers were constructed between 1967 and 1974, one of them was in need of repair.

Ladder up a Drax cooling tower

What could possibly go wrong?

No matter how well you build something, things can go wrong after more than five decades of continuous operation. Each tower is made from concrete that varies in thickness from seven inches in the middle to around 15 inches at the top and bottom. Over time, even a structure this solid can begin to weaken.

Cooling towers are reinforced with steel bars embedded within their concrete which can rust and expand, causing the concrete around it to crack – a process called spalling. Water vapour, which passes through the towers on an almost constant basis, can also migrate into poorly compacted concrete inside the tower and cause further cracks.

Before the Drax team could set about repairing the towers, they needed to know where these cracks were. Inspecting a structure that tall needed an innovative solution. It needed drones.

Surveying the damage

Drones were used to make a comprehensive, photographic record of the towers that could be inspected for signs of damage. The drones also helped produce a 3D model of the structure to visualise the tower’s defects. It was the first-of-a-kind for the company.

The drone survey found that on tower 3B there were a number of cracked concrete patches on the towers that needed repairing and maintenance was scheduled to coincide with Drax’s 2016 outages – periods during the summer months when electricity demand is lower and parts of the power station undergo routine repair work.

The next challenge was how to carry out these repairs on a structure taller than the Statue of Liberty.

A 3D model of a Drax cooling tower

To inspect the cooling tower, Drax created a 3D model with the help of CyberHawk.

Engineering at an altitude

Drax tower 3B is nearly 115 metres tall, enough to fit in the Statue of Liberty or St Paul’s Cathedral with room to spare.

How do you go about repairing a structure like this? The answer: Steeplejacks. Steeplejacks were called so because, originally, they were the people used for scaling the side of church steeples to make repairs. But a cooling tower presents a distinctly different structure that can’t necessarily be climbed up from the bottom. To scale it and make the repairs, a different approach was needed.

Drax reached out to specialist steeplejack contractors Zenith Structural Access, who build devices that allow the scaling of industrial-scale structures.

Zenith’s solution was to fix a metal frame to the top of the tower, which then lowers a walkway suspended by strong metal cables down its side. From a perch suspended from the top of tower 3B, workmen were deployed to make the repairs – more than 100 metres above ground.

Suspended in their cradles, the teams sealed the surface of each crack and then injected resin to fix the cracks in the concrete shell. Where the concrete had spalled, new specialist repair mortars were applied.

Repairs on Drax Tower 3B

Regular repairs

With the identified defects on the tower fully repaired, attention can now move on to others on site. Routine inspections using drones and binoculars have been planned to take place every three years. These will monitor the condition of all the towers and allow for future maintenance to be planned in advance.

Two more towers are already scheduled for repair in 2017’s outages. Once again, it’ll be case for engineering work at elevation.