Cooling systems play an increasingly important role in the management of internal workplace environments. In this guide the experts at Water Treatment Services compare two of the most common types of cooling system, adiabatic cooling systems and open evaporative systems. The guide looks at how each one operates and the important differences between the two including the risks associated with legionella, how energy efficient they are, how much water they use, water treatment considerations and annual running costs.
Adiabatic cooling and open evaporative cooling systems
In terms of the most energy efficient way of cooling air, open or evaporative cooling towers that use a water spray are most definitely the best option.
Evaporative cooling towers take a small amount of the water circulating through them and allow it to evaporate, an action which creates a very efficient cooling effect.
One of the main risks in any evaporative cooling tower are the risks associated with the growth and proliferation of Legionella bacteria, the organism that can cause Legionnaires’ disease.
This risk means that cooling towers should be properly maintained, inspected and treated to guard against the growth of legionella and other bacteria.
In the UK owners and operators of cooling towers have legal obligations including those under COSHH and the Health and Safety at Work etc. Act to manage their cooling towers safely and so keep people safe from harm.
This legislation is supported by the Health and Safety Executives Approved Code of Practice (ACOP) L8, and the Health and Safety guidance HSG274 Part 1, which deals with the control of legionella in cooling systems.
How do adiabatic cooling systems work?
An adiabatic cooling system is different to an open or evaporative cooler in that for most of the year, it operates as a dry cooling system.
Water is only used to cool the air coming into the system when it’s particularly warm outside, or when the demands on the cooling system are heavier.
Most adiabatic coolers have a two-stage process when water is being pulled into the system to help with the cooling process.
As the air is pulled into the cooler, water is used to reduce its temperature.
This is usually done either by pouring small quantities of cooled water over a medium the air passes through, or by spraying water directly into the air stream as it enters.
Then, the already cooler air is passed over a heat exchanger (usually a finned coil) which aids the cooling process further.
The combination of these two processes ensures that all remaining water evaporates away.
The other main advantage of an adiabatic cooling system is that it is cheaper to run when compared with a standard evaporative cooler tower.
Energy bills may be more, but when all other costs are taken into account, the overall running costs are usually less than 50% than those of an evaporative cooling system.
Understanding the differences between adiabatic and open evaporative cooling
There’s lots of confusion about the differences between adiabatic cooling and open evaporative cooling systems.
Here, we’ve pulled together some of the most misunderstood issues, and we’ll help you unpick the fact from the fiction.
Do adiabatic systems create a legionella risk?
Any risk to health from Legionella bacteria is significantly lower when running an adiabatic cooling system, compared to a conventional open evaporative cooling tower.
However, it’s important to remember that adiabatic coolers are not risk-free.
They still require a robust maintenance process to keep the risk of bacterial growth at safe levels.
This is exactly the same principle as with the regular maintenance required on evaporative cooling towers.
Do adiabatic systems need to be registered with the local authority?
Companies which operate wet cooling towers and open evaporative condensers are well aware of their obligations to register the presence of their towers under a law called the Notification of Cooling Towers and Evaporative Condensers Regulations, or NCTEC.
Adiabatic cooling systems do not need to be registered with the local authority in the same way.
However, that doesn’t mean that companies operating this type of cooling system can ignore the requirements completely.
Each operator should take steps to assess any potential dangers, usually starting with a detailed legionella risk assessment.
Do adiabatic cooling systems need a legionella risk assessment?
An adiabatic system works by using water in a way which creates an aerosol spray – fine droplets of water suspended in the air.
Under certain circumstances these tiny airborne droplets of water can become contaminated with Legionella bacteria.
These two factors combined can create significant risk as the contaminated water droplets disperse in to the atmosphere where people can breathe them in.
A legionella risk assessment is therefore essential for any businesses operating this type of wet cooling tower.
Comparing the energy efficiency of each system
As already highlighted, evaporative cooling systems are a very energy efficient way of cooling air.
Adiabatic coolers vary in energy use but most use between 50% and 100% more energy than evaporative coolers.
They also typically take up as much as twice the space of a similar capacity evaporative cooling tower.
Cooling water treatment considerations
As adiabatic cooling systems typically create a lower risk for the growth of Legionella bacteria than evaporative cooling towers, a lower chemical dosage (biocide) is usually needed to keep things under control.
The use of ultraviolet light treatment is popular for adiabatic coolers where it is used to treat the water in the system where it helps to protect against the growth of Legionella bacteria.
Adiabatic cooling systems also require some form of scale inhibition to control the build-up of scale on heat transfer surfaces including chiller coils.
There should be a regular water treatment programme of any closed system in order to prevent scale, rust or bacteria causing problems within the system.
Depending on the exact composition of the cooling system, additional filtration or pre-treatment might also be needed to protect delicate areas of the system.
Frost protection for cooling systems
As adiabatic coolers are usually only used when the weather gets hot, there’s a risk of the system freezing in the colder months.
Guard against this risk by draining down the system, adding anti-freeze or providing an external heat source to the cooling system to stop the temperature dropping below freezing point.
The disadvantage of using a heat source is that if there were to be a power failure, the heating may fail or might not be sufficient to stop potential damage to the coil, pipes and other vulnerable components.
Depending on the configuration of the adiabatic system, water can be drained into a tank to leave it empty.
The disadvantage of this is that it can increase rust and corrosion and if the system does not drain fully, there is still the issue of freezing.
How much water does an adiabatic system use?
For most of the year, an adiabatic cooling system uses no water at all and operates as a dry cooling system.
Water is only added into the system when the temperature rises and the cooling demand on the system increases.
As such, the water use from a adiabatic cooler is far lower than an open evaporative cooling tower.
Depending on the type of cooling system this could be up to 90% less.
What are the running costs of adiabatic coolers?
The comparison of costs between adiabatic coolers and evaporative cooling towers will depend on the configuration of the two systems.
However, in general terms an adiabatic cooling system is cheaper to run overall, but uses more energy.
Add in the lower level of water treatment, water use and costs of maintaining it, and adiabatic coolers may still work out cheaper.
What is the lifespan of evaporative cooling towers and adiabatic coolers?
Although it’s hard to give exact figures, the average lifespan of a well maintained evaporative cooling tower could be anything up to 30 years.
Adiabatic systems can be used for up to 20 years when well maintained.
These figures will be greatly affected by the way the systems are maintained, local water conditions and effectiveness of the water treatment, and the types of system on which they are operating.
Specialist cooling solutions for business
Water Treatment Services offer a comprehensive range of innovative engineering solutions for the management of industrial and commercial cooling systems for business.
Our experts can help you optimise costs and operational efficiencies, improve plant safety and regulatory compliance, reduce downtime, achieve water and energy savings, and increase plant reliability.
With offices in London serving the South and South East England, Manchester (North West), Birmingham (Midlands), Bristol (South East England and Wales), Leeds (North and North East) and Glasgow (Scotland), supported by regional teams of specially trained technicians and consultants we can offer professional, cost effective cooling system engineering services throughout the UK and Internationally.
Contact us today to learn how our cooling tower management solutions can help you.
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