Evaporative cooling or adiabatic cooling?

When we deal with industrial cooling, or even refrigeration for air conditioning or HVAC, sometimes it is send the message that an adiabatic cooler can replace an evaporative cooling tower in function and performance, because of a range of “one-way” advantages toward adiabatic system.

We are going to clarify that statement:

1. Adiabatic and evaporative cooling: concepts

In order to answer objectively to those questions, it is worthwhile to mention the operation principle of both cooling systems, that are very different:

ADIABATIC COOLER: takes sensible heat adventage , that means, principle that two different bodies at different temperatures placed in contact (in this case, air and the cooling equipment finned coil), exchange heat producing a reduction of the initial temperature difference between them. The final entity of the temperature difference depends on the quantity of flutter serpentine surface that is in contact with air, as well as the mass and temperature of that final one, that as it has been mentioned previously, it depends on the efficiency of the humidification system.

EVAPORATIVE TOWER: takes advantage of the latent heat, that is to say, the principle that ensures that the phase exchange of a fluid (in this case water to cool, that partly it is evaporate) removes the energy in heat way of the remaining fluid. Specifically, <> is the maximum energy corresponding to the fluid (water) passage from liquid condition to steam stage.

All these detaches a key aspect that differentiates between the two systems and that it is, in fact, the “real” discriminant when it has to be decided between an evaporative cooling tower or an adiabatic cooler. That aspect is the water consumption.

In adiabatic system, in fact, water is used only to cool the air that fluttered coil invests, so the consumption it´s not related by no means with the heat amount to dissipate, only with the efficiency of the humidification system in connection with the air mass that passes through it.

Also, in the adiabatic system this need takes place only and exclusively in the hottest days of the summer period, limiting the water use only when it is strictly necessary.

As well, it is important to pay attention to the management of the water that is used to humidify the air.

In fact, there are systems that use atomized water through nozzles in high pressure, systems that use disposable water distributed in cellulose-imbued packages and systems that humidify intermittently PVC packages and recover the water providing drains and/or timed spares without the necessity of costly treatments

In the management economy of this point, the implications of plant engineering and the resulting costs have to be assess carefully.

By contrast, in the evaporative system, the water consumption is almost constant through the year and it is closely related with the heat amount that is dissipate, approximately 1 litre per 600 Kcal.

Water consumption raises diverse problems that, as we have said, can influence in the election. In addition to the cost, which can vary between zones, they have to consider the cost related to the replacement of water treatment, the formation of annoying (but harmless) vapour plumes in the cold season or the possible formation of ice. However, there are especially important the possible health and hygienic impacts that the presence of water has in the plumbing and industrial installations and that they have to be manage accordingly with the Eurovent promulgated guidelines.

In conclusion, each system has aspects in favour and against, so it´s essential to examine the objective necessities of the customer as a whole, to propose the technical solution that fits better with his needs.

Then, we propose some recurring questions that we tried to answer in the most objective way and exclusively from a technical point of view.

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

It depends on some factors that have to be assess carefully, mainly two of them:

THERMAL POTENTIAL TO DIFFUSE. In principle, for unit powers up to 1 MW and with some <<distinctions>> related with the offered performance, both systems can be propose like an alternative for the other one. Superior outputs involve higher size machines, or more tools, making adiabatic system no economic.

REQUIRED TEMPERATURE FOR OUTLET FLUID. On the basis of respective functioning principles that have been mentioned previously, in adiabatic coolers, where air forced cooling is carry out with water injection, the effectiveness of the humidification system condition largely the lower reached limit by the fluid, that however, hardly could equal an evaporative cooling tower, unless they use significant exchange surfaces.

That said, it isn´t automatic, therefore, that it can be always and in an indiscriminate manner propose adiabatic system, instead an evaporative system: there are limits in capacity terms and thermal performance that have to take into account.

3. Has an adiabatic cooler the same efficiency as an evaporative cooling tower?

The efficiency of an adiabatic system is closely link to the efficiency of the air humidification system. Considering that, this depends on the speed in which air pass through humidification system and regarding that the exchange of sensitive air to air is less efficient than the latent air exchange. It´s clear that the statement that an adiabatic cooler can replace an evaporative cooling tower in the same efficiency conditions can´t be defensible.

For increasing humidification system efficiency it´s necessary to reduce the air speed while the air speed increases, reducing humidification efficiency and also increasing exponentially pressure losses in air part.

By contrast in evaporative cooling towers, it´s the volume of moving air through filler what determines the efficiency of the system (more speed, more efficiency of the system ) and temperature, by contrast, allows less approach while wet bulb temperature increases.

4. Can an adiabatic cooler guarantee the same performance as an evaporative tower, in terms of achievable temperatures?

Based on previous information, it is clear that different functional and design character systems have their own optimum operation conditions that cannot be superimpose.

Therefore, asking for an air system, although it is adiabatic, operating in the same conditions as an evaporative one is a no indifferent conceptual restriction.