Theory & Operation of Cooling Technologies

How open and closed circuit cooling towers, evaporative condensers and adiabatic systems work: a quick guide to the knowledge of cooling technologies.


Open Circuit Cooling Towers – Theory & Operation

Open Circuit Cooling Towers_How Do They Work

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 dry bulb temperature (naturally unless air is already saturated).

Practically, an adequately sized cooling tower manages to cool circulating water down to a temperature 2-3 °C above the wet bulb temperature. This is due to the effects of performance factors related to air saturation.

On this basis many designers and equipment manufacturers plan to use cooling tower water while sizing cooling circuits and heat exchangers. This way they ensure optimum plant efficiency from the first design stage.

Open Circuit Cooling Towers


Closed Circuit Cooling Towers – Theory & Operation

Closed Circuit Cooling Towers_How Do They Work

Closed circuit cooling towers exploit the same physical principle as their open circuit counterpart in order to dissipate the heat: the forced evaporation of a minimal quantity of water lowers temperature of the main water mass.

Process fluid to be cooled enters the upper header of a coil. This last is constantly wetted 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. Cooling performance of a closed circuit cooling tower depends on the ambient wet bulb temperature.

Closed Circuit & Hybrid Coolers


Evaporative Condensers – Theory & Operation

Evaporative Condensers_How Do They Work

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

The refrigerant (hydrohalocarbon or ammonia) to be condensed enters the upper header of a coil. As for closed loop cooling towers, coil is constantly wetted 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. Here again, performances depend on ambient wet bulb temperature as for water cooling towers.

Evaporative Condensers


Adiabatic Coolers & Condensers

MITA adiabatic coolers exploit the adiabatic cooling principle to improve finned coil’s efficiency. They can be used for water/glycol mixtures cooling or refrigerant condensation.

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

Temperature that can be reached is called “adiabatic saturation temperature”: it can be much lower than “dry” air temperature
depending on humidification system efficiency.

MITA Cooling Technologies developed an air cooling system that allows to fully exploit fan characteristics in terms of flow rate. High humidification efficiency is ensured.

Adiabatic Coolers & CondensersAdiabatic Gas CoolersAdiabatic Subcoolers

Gloxary of Cooling Technologies

Technical solutions, components and accessories, physical notions: the terminology of the evaporative and adiabatic theory from A to Z.


A – C : Adiabatic Cooler – Cooling Tower


Adiabatic cooler. A device that uses adiabatic air cooling to increase sensible heat exchange efficiency in finned coil heat exchangers.

Inner coils of a Closed Circuit Cooling Tower
A closed circuit cooling tower from the inside

Adiabatic cooling. Decrease of air temperature by means of forced increase in its relative humidity, but without heat exchange (isenthalpic).

Adiabatic humidifier: a device which allows to increase the relative air humidity to lower the temperature.

Air cooling system: System that uses air as a fluid to remove heat, usually by means of a coil of finned tubes.

Air liquid cooler. Heat exchanger usually consisting of a coil with copper tubes and aluminium fins, where the latter’s purpose is to increase the surface of contact with the air.

Ambient air. It is the air introduced into the cooler taken from the environment and used for the thermodynamic operation of the appliance.

Approach.  It is the difference between the chilled water temperature and the ambient air wet bulb temperature.

Basin or collection basin.  Basin to be used by the tower to collect cooled water.

Closed circuit. See “closed circuit cooling towers”.


Components of the cooling tower. 

  • Casing.
  • Fill pack or filling material or heat exchange surface, formed to produce a contact between water and air.
  • One or more fans for generating air stream with the required characteristics (flow and pressure).
  • Cold water collection basin.
  • Drop eliminator to avoid excessive water losses due to dragging in the air flow.

Concentration. Quantity of solid or liquid elements unrelated to the treated water contained therein.

Cooling. Process by which the temperature of a gas, a liquid or a solid object, is changed by lowering it. In the case of the liquid, cooling may take place by mainly latent exchange using an evaporative system, or by sensible exchange by contact between two elements at different temperatures: for instance, in air cooling.

Cooling Technologies: different methods, based on different functional principles, for removing heat derived by the production processes of various kinds. The choice for best technology must be made based on the user’s real needs and the environmental conditions of installation place.

PME-E Open Circuit Cooling Tower
Open circuit cooling tower

Cooling Tower. “It is a heat rejection device that rejects waste heat to the atmosphere through the cooling of a water stream to a lower temperature. Cooling towers may either use the evaporation of water to remove process heat and cool the working fluid to near the wet-bulb air temperature or, in the case of closed circuit dry cooling towers, rely solely on air to cool the working fluid to near the dry-bulb air temperature” (Source: Wikipedia).

  • Centrifugal cooling tower: device that uses a centrifugal fan to move the air which is necessary to perform forced evaporation. Generally preferred when there are noise problems, need for air ducting or when sound-absorbing silencers are needed.
  • Closed circuit cooling towers. Tower in which the process fluid which is cooled (often not water, but a cooling fluid) does not come into contact with the cooling water. This is distributed on the outer surface of a tube-bundle or of linked plates and circulates between the distributor and the basin.
  • Counterflow cooling towers. “In a counterflow design, the air flow is directly opposite to the water flow. Air flow first enters an open area beneath the fill media, and is then drawn up vertically. The water is sprayed through pressurized nozzles near the top of the tower, and then flows downward through the fill, opposite to the air flow” (Source: Wikipedia).
  • Factory-assembled cooling tower: small and medium-sized, totally factory assembled in 2 or 3 main sections with dimensions that are compatible for road or sea transport. No assembly time or a few days for modular systems.
  • Field-erected cooling tower. Generally large, in order to reduce transport costs. It is transported completely disassembled and assembled directly at the installation site. Assembly times are roughly variable between 20 and 60 days.
  • Mechanical draft cooling towerEvaporative cooler in which the water is cooled by partial evaporation through contact with a mechanically driven air stream.
  • Open circuit cooling towers. Evaporative system that cools water through direct exchange with air.
  • Opposing current cooling towers (against the current). Towers in which the air flow goes in opposing directions..


D- F : Distribution Manifold – Flow


Distribution manifold. System of channels that distribute hot water to the filler material.

Distribution system. To obtain the maximum yield from the cooling tower, the water entering the tower should be divided into droplets and distributed evenly over the fill.

Dragging. Droplets dragged out of the tower by the air stream.

Drift Eliminator. See “Drop eliminator”.

Different Fill Packs

Different types of fill packs

Drop eliminator or Drift eliminator. System of fins placed inside the tower used for quantities of water droplets which are then dragged out to reduce the outgoing air.

Evaporative cooling. Heat dissipation from a fluid, generally water, by evaporation of a small part of the same fluid (latent exchange).

Evaporative coolers. A device that performs the evaporative cooling in a forced and efficient way, optimising the latent exchange of the fluid to be cooled with the air.

Filling material. See “Fill pack”.

Fill Pack or Filling material or Heat exchange surface. Set of sheets or grilles of plastic material that separate the water in the tower and activate the contact between the fluids and therefore the cooling process. The fill pack’s purpose is to divide the air and water flows in the most uniform and targeted way possible. This would allow the most extensive contact between the fluids. The other purpose of a filling material is to slow down the fall of the water to allow a time when water can remain in the tower, thus creating a thorough cooling. There are two main types of fill packs, defined by the way in which they produce a large contact surface between water and air: film and spray.

  • Film fill pack. This type of fill pack distributes the water over a large surface dividing it into thin films or dispersed droplets. The film fill can be constituted by mobile or grating packs.
  • Spray fill pack. This type of filling material is formed with grates or bars arranged in horizontal planes over which the water falls, breaking into numerous droplets. These, by falling from one level to another, divide further. The grates or the levels below, are placed so as to continuously break the fall of water droplets.

Flow. See the next entries.

  • Air flowIt is the total amount of air, including the dissolved water vapour, which flows into the tower.
  • Inlet water flowQuantity of hot water or other liquid that enters the cooling circuit.


H – P : Heat exchange surface – Power


Heat exchange surface. See “Fill pack”.

Hot water temperature. Water temperature to be cooled in the distribution system.

Hybrid cooler. A device which, in a single solution, has an evaporative cooling and an air function. The former is used during summer, the latter during spring, autumn and winter periods.

Louvers.  Fins or plates mounted on the wall or on the mouths of the tower to direct the air.

Make up. Water added to the circuit to compensate for liquid leaks from the circuit by evaporation, spray entrainment, water drains and various losses.

Louvers - Adiabatic Pack - Coils
Louvers, adiabatic pack and coils inside an adiabatic system

Nozzle. A device through which the flow of liquid is sprayed onto the filling material.

Open circuit. See “open circuit cooling towers”.

Plume or Bleed-off. Cloud or column produced by the steam contained in the air exiting from the tower. It condenses on contact with the colder ambient air.

Power. See the next entries.

  • Thermal power. See “Thermal power”.
  • Power absorbed by the fan motorPower measured at the control axis of the fan, excluding the only motor losses.


R – Z : Recirculation – Wet Bulb Temperature


Recirculation.  Part of the outlet air that is diverted back into the tower from the air inlets.

Scale deposit . Deposits normally due to the mineral salts, or other solids present in the water or, in cases where an adequate water treatment has been provided.

Thermal power. Heat quantity subtracted from the liquid flowing into the tower and disposed of in the air.

Water cooling system. System or circuit that uses water as a fluid to remove heat, usually from another fluid or directly from a production process. The heat exchange can take place indirectly (in a closed circuit through a tube-bundle or plate exchanger), or directly through an evaporative open circuit tower.

Water drain. Water discharged from the circuit to control the concentration of salts or other impurities in the circulating liquid.

Water temperature. Average temperature of the liquid at the outlet of the basin, including the effect of replenishing the cooling air entering the basin.

Water treatment. Without proper management and chemical treatment, the circulating water in a cooling tower would soon adversely affect the cooling system and decrease the exchange yield of both the tower and the entire circuit.

Wet bulb temperature (ambient air to be used for the project). Average air wet bulb temperature at the inlet to the tower, including any air recirculation effect. It is an essential concept for operation.


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