Cooling towers explained

Cooling towers are relatively simple yet ingenious pieces of technology which tend to be taken for granted by many people. 

The distinctive architecture of hyperboloid cooling towers at power stations is an iconic sight from trains and motorways across the UK, but cooling towers vary in size and design depending on the application. 

Essential to many commercial buildings and industrial processes, cooling towers require careful maintenance and management to ensure operations are energy efficient, cost effective and safe. 

What is a cooling tower?

As the name suggests, cooling towers remove heat from commercial buildings and industrial processes by shedding the waste heat to atmosphere through evaporation. 

Originally developed in the 19th century for use with steam engines, condensers use relatively cool water to condense the steam emerging from the cylinders or turbines, thus reducing back pressure (the force opposing the desired flow of fluid through pipes).

What industries use cooling towers?

Typically placed on the roofs of commercial buildings, cooling towers contribute to large scale air conditioning and refrigeration in hospitals, airports and hotels. They are an essential component of many industrial processes at power plants, refineries and manufacturing facilities. 

What are the main types of cooling tower?

The four types commonly used for industrial and commercial HVAC are:

  • Open-circuit cooling towers
  • Closed-loop cooling towers
  • Evaporative condensers
  • Adiabatic systems.

OPEN-CIRCUIT COOLING TOWERS

Open-circuit cooling towers exploit a simple and natural physical principle: forced evaporation of a minimal water quantity lowers the temperature of the main water mass, representing the most widely employed cooling system in civil and industrial applications still today.

Wet bulb temperature in the installation area is the minimum outlet temperature which can be obtained theoretically from a cooling tower: this value is always lower than the dry bulb temperature.

A well-sized cooling tower can cool circulating water down to a temperature 2-3 deg C above the wet bulb temperature.

CLOSED-CIRCUIT COOLING TOWERS

Closed-circuit cooling towers exploit the same physical principles as their open circuit counterparts in order to dissipate the heat. This involves the forced evaporation of a small quantity of water to lower the temperature of the main water mass.

Process fluid to be cooled enters the upper header of a coil which is constantly drenched by water recirculating in the small, pre-assembled evaporative circuit of the unit.

Evaporation of a fraction of recirculating water removes heat from the coil exchanging surface. Cooling performance of a closed-circuit cooling tower depends on the ambient wet bulb temperature.

EVAPORATIVE CONDENSERS

Evaporative condensers exploit the same physical principle as cooling towers in order to dissipate heat.

The refrigerant to be condensed (hydro halocarbon or ammonia) enters the upper header of a coil. As in closed loop cooling towers, the coil is constantly saturated by water recirculating in the small, pre-assembled evaporative circuit of the unit.

Evaporation of a fraction of recirculating water removes heat from coil exchanging surface. Again, performances depend on ambient wet bulb temperature as for water cooling towers.

ADIABATIC COOLERS AND CONDENSERS

Adiabatic cooling is a thermodynamic phenomenon by which the air temperature is decreased through humidification. 

Adiabatic coolers can be used for water/glycol mixtures cooling or refrigerant condensation.

The temperature that can be reached is called ‘adiabatic saturation temperature’: it can be much lower than the ‘dry’ air temperature, depending on humidification system efficiency.

What problems are common in cooling towers?

Energy consumption: Cooling towers consume a huge amount of energy and are prone to corrosion and scale, which exacerbates problems with efficiency and degradation. 

Legionella: Legionella is also a key concern for facilities managers and site supervisors. Cooling towers for air conditioning applications produce warm water vapour and there is an increased risk of potentially fatal pathogens being distributed in the air.

Planned management and preventative maintenance is critical to ensure operational efficiency, reduced maintenance costs and health and safety compliance. 

How do you manage the risk of legionella in cooling towers?

The Health and Safety Executive (HSE) recommends a scheme that includes inspection and maintenance; cleaning and disinfection; dosing the tower with a biocide (a chemical which prevents legionella bacteria from growing); and monitoring to ensure that this biocide dosing is effective.

Employers and those responsible for premises are obliged, as appropriate, to:

  • Ensure that the release of water spray is properly controlled.
  • Avoid water temperatures and conditions that favour the growth of legionella and other micro-organisms.
  • Ensure water cannot stagnate anywhere in the system by keeping pipe lengths as short as possible or by removing redundant pipework.
  • Avoid materials that encourage the growth of legionella.
  • Keep the system and the water in it clean.
  • Treat water to either kill legionella and other micro-organisms or limit their ability to grow.

TOWERS OF STRENGTH – Leading-edge cooling technology from Klima Therm

Klima-Therm distributes several of Mita’s leading edge cooling technologies including open and closed-circuit cooling towers, evaporative condensers and dry coolers (including adiabatic versions) – see MITA Cooling Technologies. We can offer advice and support on these and other products.

Contact us to discuss your cooling requirements.