Cooling towers are relatively simple yet ingenious pieces of technology which tend to be taken for granted by many people.
Typically placed on the roofs of commercial buildings, they form part of the system to provide air conditioning or refrigeration on a large scale.
As the name suggests, cooling towers in HVAC applications remove heat from buildings by shedding the waste heat to atmosphere.
Cooling tower origins
Cooling towers are by no means a new idea. They were originally developed in the 19th century as a result of the popularity of condensers 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).
However, a problem with cooling towers for air conditioning applications is that they produce warm water vapour and there is a risk of potentially fatal pathogens such as legionella being distributed in the air.
Managing the legionella risk
That’s why cooling towers must be managed effectively. The Health and Safety Executive 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.
Cooling tower types
There are several types of cooling towers designed for different applications. They include open and closed-circuit cooling towers, evaporative condensers and adiabatic systems.
Open-circuit cooling towers
Evaporative cooling towers exploit a simple and natural physical principle: forced evaporation of a minimal water quantity lowers the temperature of the main water mass. Therefore, cooling towers represent 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 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.
Towers of strength
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. Simply contact us here – firstname.lastname@example.org