Even though cooling towers are not as common today as they were in years gone by, you will almost certainly come across at least one cooling tower in your daily travels. They play an integral part in many different industries and while on the surface they seem fairly straightforward, these are actually extremely well structured, well designed and highly efficient heat rejection systems.
History of cooling towers
Before we look at how a cooling tower works, it is worth reminding ourselves that the first cooling towers were built back in the 19th century. This was a time when steam engines were pushing the boundaries of travel and speed but required a large amount of cooling water to maximise fuel efficiency and increase power. As there was obviously no natural continuous supply of fresh cooling water, we saw the emergence of the first “cooling towers” which allowed boiling hot water to be cooled, recycled and used time and time again on the steam engines.
In 1918 Dutch engineers Frederick Van Iterson and Gerard Kuypers patented the first hyperboloid cooling tower which became the standard for many years to come. This was a time when steam was used to turn turbines to create power then the boiling hot water was cooled, recycled and the process began again. There are many uses for cooling towers today but the principal of the historic and the modern day cooling tower has not changed.
How does a natural draft cooling tower work?
There are many processes where cool water is used to extract heat from mechanical operations such as power production. One such type of cooling tower is the natural draft cooling tower which as the name suggests makes use of natural airflow. These large towers can be in excess of 200 m high and while a number of elements will impact efficiency, the general process is the same.
Hot water is pumped into the cooling towers across an array of water distribution devices such as water jets. These jets force the water onto what is known as the exchange surface which spreads the water across a larger area thus speeding up the cooling process. As the hot water is forced through the jets, steam will rise up the tower, some of which will be released into the atmosphere, while the larger droplets are caught by what are known as drift eliminators. As the smaller droplets of steam travel through the drift eliminators up through the top of the cooling tower this draws in natural air from the bottom to fill the vacuum. The cold air drawn in through the bottom hits the exchange surface, cooling down the hot water which then drops into the basin at the bottom of the cooling tower.
This water is then funnelled back into the industrial system/industrial process from where it came and the whole cycle begins again. It is estimated that around 2% of the water which flows through a cooling tower will escape as steam. This lost water is replaced by an additional water supply which could be from a local river or lake, ensuring that the same volume of water flows through the industrial process time and time again. The original water supply and that used to maintain same volume flow are not pure and deposits are left on the bottom of the cooling tower. These deposits are heavy and tend to sink to the bottom of the water basin with some cooling towers having a waste/sewage outlet which is opened periodically to remove some of the pollutants.
How does a mechanical/induced draft cooling tower work?
The general process associated with a mechanical/induced draft cooling tower is similar to that of a natural draft cooling tower system except, as the name suggest, there is an artificial airflow. Large fans are placed towards the top of the cooling tower just below the pipes which bring the boiling hot water into the tower. This not only cools the water as it flows through the pipes, prior to hitting the water distribution system, but also helps to push steam up through the top of the tower. This process quickens the rate of air dragged in at the bottom of the tower, which hits the exchange surface, which cools the boiling hot water which falls into the basin to be funnelled away and recycled.
Common uses of cooling towers
Cooling towers are still used to cool water associated with oil refineries, petrochemical and chemical plants and power stations including nuclear power stations. Many people associate cooling towers with large power stations and while traditionally they tended to be coal power stations, they are still used in more modern thermal power stations today. It will surprise many to learn that they will come across much smaller cooling towers every day, such as those used in large tower blocks to assist in the process of discharging heat from air-conditioning systems. The amazing cooling tower played a massive part in the industrial revolution and still have a role to play today.
