Comparison between Cooling Tower and Adiabatic Cooler

When it comes to industrial cooling, or even in the refrigeration and air conditioning arena, the message is often thrown around that an adiabatic cooler can replace an evaporative tower in function and performance, justifying this possibility with a number of “one-way” advantages toward the adiabatic system.

In the opinion of MITA Cooling Technologies, which manufactures both systems, some clarity should be brought to this claim.

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by Giorgio Lorenzetti, Technical Advisor

1. Adiabatic and evaporative cooler: some definitions

In order to answer the questions below objectively, it is good to give a nod regarding the operating principle of the two cooling systems under consideration. Indeed, they are totally different from each other and can be summarized as below:

• Adiabatic Cooling System: it exploits sensible heat, that is, the principle whereby two bodies at different temperatures brought into contact (in this case, air and the fins of the finned coil), exchange heat by producing a decrease in the initial temperature difference between the two bodies themselves. The final magnitude of the temperature difference depends on how much finned coil surface area is in contact with air, as well as the mass and temperature of the air. As mentioned above, this depends on the efficiency of the humidification system.
• Evaporative Cooling System: it takes advantage of latent heat, or the principle whereby the phase change of a fluid (in this case, water to be cooled that partly evaporates) removes energy in the form of heat from the remaining fluid. Specifically, the “latent heat of evaporation” is the maximum energy corresponding to the fluid (water) change from liquid to vapor state.

From the above, a fundamental aspect that differentiates the two systems, being indeed the real “discriminating factor” among the two choices: water use.

In the adiabatic system, in fact, water is used only to cool the air that invests the finned coil. Therefore, its use is in no way related to the amount of heat to be dissipated, but only to the efficiency of the humidification system in relation to the mass of air flowing through it. Moreover, in the adiabatic system this need occurs only on the hottest days of the summer period, thus limiting the use of water to the bare minimum.

Then, on this aspect, it is important to pay attention to how the water used for air humidification is managed: from systems that use osmotized and nebulized water through very high-pressure nozzles and pumps, to systems that use disposable water distributed on impregnated cellulose humidifier packs, to still others that intermittently wet flocked PVC packs and recover the water by providing timed purges and/or replacements without requiring any costly treatment. Regarding this point, plant implications and resulting costs must be carefully evaluated.

On the contrary, in evaporative systems water use is almost constant throughout the year and is closely related to the amount of heat to be dissipated, to the extent of about 1 liter for every 600 Kcal or so.

Water use originates several issues that may influence the choice, as mentioned above. Some elements should also be taken into account: cost aside, which may vary from area to area, costs related to the treatment of make-up water, the formation of annoying (as much as harmless) plume of steam in the cold season or the possible formation of ice,  and, above all the possible implications at the sanitary-sanitary level that the presence of water entails in plumbing and industrial systems and that should be managed respecting the guidelines dictated by Eurovent in this regard.

In conclusion, it becomes essential to examine the objective needs of the customer as a whole (as it is normal for each system to have its favorable and unfavorable aspects). The goal is to be able to propose the technical solution that best suits the customer’s needs.

Below, we propose a series of recurring questions that we have tried to answer as objectively as possible and exclusively from a technical point of view.

2. Can an adiabatic cooler always replace an evaporative tower?

This depends on several factors that need to be carefully evaluated, chief among them two.

• Thermal capacity to dissipate. In principle, for single unit powers up to 1 MW and with some “distinctions” related to the performance offered, the two systems can be proposed as alternatives to each other. Higher power would involve such large machines, or more machines, such as to make the adiabatic system uneconomic.
• Temperature required for the fluid output. In adiabatic coolers, where forced cooling of the air is carried out by means of water input, the efficiency of the humidification system greatly conditions the lower limit attainable by the fluid. In any case, this limit can hardly match the one reached by an evaporative cooling tower unless considerable exchange surfaces are employed.

Having said the above, it is therefore not automatic that one can always and indiscriminately propose an adiabatic system in place of an evaporative system: there are limits in thermal potential and in performance that must be taken into due account.

3. Does an adiabatic cooler have the same efficiency as an evaporative tower?

The efficiency of an adiabatic system is closely related to that of the air humidification system one chooses to adopt, as well as to its mode of use. Now, let us take into account that this depends on the rate at which the air passes through the humidification system and considering that sensible-to-air exchange is less efficient than latent-to-wet exchange: it is clear that the claim that an adiabatic cooler can replace an evaporative tower at equal efficiency conditions is not defensible in absolute terms.

In order to increase the performance of the humidification system, it is necessary to reduce the air velocity. While, if the latter is increased, the humidification efficiency is reduced and, in addition, the pressure drop on the air side is exponentially increased: the classic “short blanket.”…

In contrast, in evaporative systems it is the volume of air moved through the exchange pack that determines the efficiency of the system (the higher the velocity, the better the system performs). Less relevant, however, is its temperature, which, indeed, allows smaller approaches the higher the wet bulb temperature.

4. Un raffreddatore adiabatico può fornire le stesse prestazioni di una torre evaporativa, in termini di temperature raggiungibili?

Based on the above, it becomes immediately clear that systems of different functional nature and design content each have their own optimal operating conditions that are not overlapping.

Therefore, to ask an air system, although adiabatic, to operate under the same conditions as an evaporative system is a not inconsiderable conceptual stretch.

5. One must be wary of misleading messages equating adiabatic coolers with evaporative cooling towers

MITA Cooling Technologies knows this, which is why it is able to offer the customer both solutions depending on his or her actual need.

Having said all the above, adiabatic systems should be understood as “complementary” to evaporative systems and not alternatives to them in the strict sense. MITA produces both, so it is in the best position to be able to suggest to the customer the best proposal based on the current problem, sharing with the same the objectives and limits that are to be achieved.