IIF

IIR
INSTITUT INTERNATIONAL DU FROID
INTERNATIONAL INSTITUTE OF REFRIGERATION

EVAPORATIVE COOLING

© shutterstock
UlrichUlrich
may 2023

52nd Informatory Note

on Refrigeration
Technologies
“On average, the energy
consumption of an
evaporative cooling
system can be four times
lower than that of a
conventional device with
the same cooling power.”

Doi: 10.18462/iif.NItec52.05.2023

IIF
IIR
iif-iir@iifiir.org

177, boulevard Malesherbes - 75017 Paris - France

Tel. +33 (0)1 42 27 32 00 / Fax +33 (0)1 47 63 17 98

Copyright © 2023 IIR/IIF All rights reserved/Tous droits réservés

Introduction Operating principles

The principle of evaporative cooling is based on Evaporative cooling is based on two important
the fact that the evaporation of a liquid absorbs phenomena:
significantly more heat than is required to raise its
temperature by a few degrees. A simple example of • at standard temperature and pressure,
this is the cold sensation experienced when getting roughly 60 times more heat is required to
out of an open-air swimming pool, in the wind, even evaporate a certain amount of water than to raise
in warm weather. Water evaporates from the skin. its temperature by 10°C;
The heat required for the evaporation process is • air that is unsaturated with moisture can
thus removed and the skin cools down. absorb a certain additional amount of water
This process is used (in combination with others) vapour, in which case the heat contained in the
in conventional refrigeration and air-conditioning air is absorbed by the vaporisation of the water.
systems. However, the term “evaporative cooling” This liquid-to-vapour phase change causes the
refers to the cooling obtained solely by the simultaneous cooling of the air and the water that
evaporation of water into air. remains in a liquid state.
Then, injecting fine droplets of water into a flow of
The main evaporative cooling processes are: direct
unsaturated air (Figure 1) causes the temperature
evaporative cooling, indirect evaporative cooling
to decrease with a concurrent increase in humidity.
and a combination of both. The current note
The cooling effect depends on the air temperature,
describes the applications and conditions under
the relative humidity and the saturation efficiency of
which these processes may offer a technical and
the humidifier. The saturation efficiency in adiabatic
economic advantage.
humidification is the ratio of the increase in specific
A description and further technical explanations humidity x in the process (x2-x1) to the maximum
can be found in a Technical Note on Evaporative increase obtainable (xwb-x1) by reaching saturation
Cooling[1] on the IIR website: www.iifiir.org with a relative humidity of 100%, as illustrated in
figure 2.
This Informatory Note was prepared by Renato Lazzarin
(President of IIR Section E “Air conditioning, heat pumps An efficiency of 80-90% can be reached. Figure 3
and energy recovery”. It was reviewed by Yunho Hwang, allows to evaluate the specific cooling, kJ for
President of Commission B1 “Thermodynamics & transfer every kg of air, and the temperature drop that
processes” and Xianting Li, President of Commission E1 can be obtained by humidifying air (whose initial
“Air conditioning” under the supervision of Jean-Luc temperature is on abscissa) for various relative
Dupont (Head of the Scientific and Technical Information
humidity rates (%) when the saturation efficiency
Department). It was proofread by Nathalie de Grissac
and designed by Aurélie Durand (IIR head office).
is 100%.
In the following, the processes mainly concern
evaporative cooling of air in all-air conditioning
plants. Of course, the evaporatively cooled air
can generate cool water in a suitable gas-liquid
heat exchanger. According to the heat exchanger
efficiency, the final water temperature may approach
the air temperature.
The highest temperature difference obtainable in
a simple adiabatic saturation process is (t1-twb),
where twb is the saturation temperature.*

*
The saturation temperature can be evaluated in a psychrometric
chart when the adiabatic line from the point that represents the
air condition (identified by dry bulb temperature (abscissa) and
specific humidity (ordinate) meets the saturation curve (100%))

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 52nd IIR Informatory Note

Figure 1:
Scheme of an adiabatic humidifier

Figure 2:
Humidification by adiabatic saturation through the introduction of liquid water

Figure 3:
Specific cooling energy (q) and temperature reduction (final temperature t2 – initial
temperature t1) achievable by adiabatic humidification of the air, with saturation
efficiency η = 100%, as a function of initial temperature (t1) and relative humidity (Φ1)

5
An alternative process can be implemented that The result is achieved by cooling a primary air
allows a higher temperature reduction than the stream by indirect contact with a working air
conventional evaporative cooling described above. stream cooled by evaporation. A recently proposed
This is the so-called Maisotsenko cycle [2]. While scheme is represented in figure 5 [3]
the lowest temperature limit in the conventional
adiabatic humidification process is the saturation
temperature, the limit is extended in this cycle to
the dew point temperature, i.e. the temperature
reached when the air is cooled to a constant
specific humidity until saturation (figure 4).

Figure 4:
Wet bulb and dew point temperatures for air at a dry bulb temperature of 24°C and a
specific humidity of 10gkga-1, respectively 17.5°C and 14°C

A working air stream flows along a wet channel

Figure 5: below the primary air stream channel. The
Perforated counterflow configuration of working stream enters the wet channel through a
an indirect evaporation system based on perforated wall and is cooled as it flows through
the dry channel below. In this way, the working fluid
the Maisotsenko cycle
approaches the wet bulb temperature starting from
continuously decreasing dry bulb temperatures
(figure 6). At the end of the heat exchanger, the
remaining working stream approaches the dew
point temperature so that, in the counterflow heat
exchange, the process fluid can tend towards
this temperature. The potentially achievable
temperature drop must be considered together
with the complexity of the system, which has only
recently been commercialised.

52nd IIR Informatory Note

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 52nd IIR Informatory Note

Direct evaporative cooling is the simplest

Figure 6: process: outdoor air passes through an enclosed
Sequential temperature decrement in the space where it enters into contact with water, either
wet channel, approaching the dew point in the form of fine droplets (Figure 7) or saturating
a porous medium. The water evaporates into the
temperature [3]
air, thereby making it cooler and moister. Because
of the practical limitations of these systems, the
relative humidity does not reach 100%, but a few
percentage points less. Manufacturers call this
process “adiabatic cooling” because the only heat
exchange involved occurs between the air and the
water it is in contact with.
The cooling effect is appreciable when humidity is
quite low, and in some cases it can satisfy the entire
sensible load in an air conditioning plant, while
providing - in very dry climates - the necessary
moisture content . Even if direct evaporative cooling
can be considered an effective and cheap cooling
technology, some drawbacks must be considered:

• It can only be used in a warm, dry climate

where latent loads are low.
The amount of water vapour contained in the
saturated air increases faster than the temperature. • The moisture content of the supplied air
Therefore, evaporative cooling is particularly may become too high, requiring subsequent
interesting in regions with hot and dry climates, dehumidification.
provided that water supply is not an issue. • A water supply is required, with potentially
Conversely, the potential for evaporative cooling high water consumption.
decreases and tends to nil when the air is close
• The humidifier requires proper maintenance
to humidity saturation levels. In humid climates,
to avoid the risk of bacterial growth.
however, evaporative cooling may be used at
the condenser level in conventional refrigeration Several measures enable to overcome these
systems or heat exchangers for industrial drawbacks. The use of pressurised, once-through
processes. water spray humidifiers minimises bacterial growth
and water consumption [4].

Figure 7:
Scheme of an adiabatic humidifier

7
Indirect evaporative cooling: in this case, the Figure 8 represents the process. The ambient
air used to cool the room passes through a heat air (A), before being expelled, is humidified: its
exchanger placed in an evaporatively cooled temperature decreases (C), and it can therefore be
enclosed space. As the quantity of water vapour in used for heat exchange with the fresh air (E), which
the air remains unchanged, the relative air humidity is cooled without changing its moisture content
increases to a lesser extent than in the case of (X). After heat exchange, the air is exhausted at a
direct cooling, with an almost equal temperature higher temperature (O).
drop. The presence of the heat exchanger causes
the temperature to decrease slightly less than in Indirect and direct evaporative cooling:
direct cooling systems. Renewal air may always If climatic conditions allow, both indirect and direct
be cooled indirectly in principle, i.e. using a heat evaporative cooling techniques can be used.
exchanger and a stream of suitably humidified The outside air can be first cooled indirectly by
outside air (in theory, as long as it is not saturated). the suitably humidified discharged air, and then
In many cases, this operation is even more effective subsequently humidified. If this is not sufficient on
and predictable if the stream of cooling air is the its own to reach a suitable delivery temperature
exhaust air. and/or humidity, cooling and/or dehumidification
coils, supplied by refrigeration units, can be used
(Figure 9) [5, 6].

Figure 8:
Representation on the psychrometric diagram of indirect evaporative cooling. A= ambient
exhausted air; C= after humidification; O=exhausted after the heat exchanger; E= fresh
outside air; X: after heat exchange

52nd IIR Informatory Note

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 52nd IIR Informatory Note

Figure 9:
Scheme of indirect and direct evaporative cooling. Fresh outside air is firstly cooled
indirectly. The total air flow is finally cooled by direct evaporation

Energy savings and

water consumption

On average, the energy consumption can be 4 Water quality should be monitored and water
times lower [7] than that of a conventional device treated if necessary, by disinfection, filtration or
with the same cooling power, but can be up to demineralisation:
10 times lower in a hot and dry climate [8, 9]. The
overall operational cost is approximately 20 times for sanitary reasons: particularly in water
lower than that of a standard vapour compression recirculation systems, the risk of legionellosis must
system [1]. be taken into account. On the other hand, in the
case of direct air cooling for houses or workplaces,
Water consumption can be a concern, particularly in there is a risk of aerosol migration and the water
arid regions, where evaporative cooling is the most should not contain pathogens;
beneficial. The evaporation of 1 cubic meter water
can produce a cooling effect of about 2.5 x106 kJ for technical reasons: the more minerals and
(700 kWh). A conventional air-conditioning system particles in suspension in the water, the more
with a coefficient of performance (COP) of 3 would frequent “rinsing” operations are needed, as
require as much as 230 kWh of electricity for the overloaded water may be unusable. This will
same cooling effect. In practice, a fraction (around increase water consumption at an equal cooling
20%) of the water is utilised, not to cool the air, but capacity.
to avoid the deposit of minerals that may result from
excessive concentration due to evaporation.

9
Applications of COOLING OF CONDENSERS IN TRADITIONAL
COOLING SYSTEMS
evaporative cooling In conventional refrigeration and air-conditioning
systems, heat is discharged into the environment
at condenser level. Cooling the condenser
using evaporatively-cooled water can improve
The choice of the cooling system (direct, indirect
the performance of the device. Incidentally,
or two-stage, or even multi-stage, with or without
this principle is widely used in condensers and
recirculation of air or water) is a technical and
adiabatic condensers, even in humid climates.
economic choice that depends on the temperature
and humidity of the ambient air, on the possible
temperature and humidity requirements for the
refreshed premises and on water quality. The air COOLING OF LIQUIDS
cooled by direct cooling can only be pulsed into
the premises if the quality of the water allows for it. The process is similar to indirect cooling described
above, except that here water or another liquid
passes through the exchanger (which is itself
AIR CONDITIONING cooled by direct evaporative cooling, as mentioned
before). The cooled liquid can then be used for air
conditioning or to evacuate the heat generated
The main application of evaporative cooling is the
by an industrial process for example. In the latter
air-conditioning of premises in hot and arid regions,
case, evaporative cooling can have an economic
with the above described air temperature reduction
and technical advantage, even in a relatively humid
and humidity increase. However this increase in
climate.
humidity must be careful evaluated. In hot weather,
the human body’s thermal regulation depends
precisely on evaporative cooling, as perspiration
is the evaporation of water through pores and this STORAGE OF PERISHABLE FOODSTUFFS
natural regulation process is hindered when the air
is too humid. It is generally considered that human In hot countries, evaporative cooling cannot achieve
thermal comfort conditions are met when the the conservation recommended temperatures for
temperature is between 20 and 27°C and relative products of animal origin or for most of the products
humidity between 30 and 65-70%. However, of vegetal origin. However, in some cases, it can
the definition of what temperature and humidity significantly slowdown the deterioration process
conditions are considered “comfortable” for the of tropical fruit and vegetables, resulting in
human body depends, at least to some extent, on appreciable gains in terms of shelf life and time to
individual perception. The role of clothing should market.
also be taken into consideration as regards the The increase in relative humidity reduces the
sensation of freshness and so should relative air wilting and weight loss through evapotranspiration
humidity and velocity (air renewal rates, fans), of fruit and vegetables, but an excess of relative
outdoor temperature and local climate. This is the humidity encourages the proliferation of unwanted
reason why indirect evaporative cooling, which organisms, including fungi (botrytis, penicillium...),
does not increase air humidity, can find frequent resulting in the deterioration of the products, or even
application in less arid climates. in the production of bio-toxins. For the preservation
Air conditioning is not only relevant for humans, but of fruit and vegetables, the recommended humidity
also for livestock: extreme heat has an impact on is generally 85-95%. The risk of corrosion of metal
their health, breeding and growth. In addition, poultry parts is also to be taken into consideration.
lacks skin thermal regulation (evapotranspiration),
so excessive temperature can prove fatal.

52nd IIR Informatory Note

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 52nd IIR Informatory Note

PRE-COOLING
Conclusions
When evaporative cooling cannot achieve the
desired temperatures, it can, in some cases, be
used for pre-cooling operations to reduce the
energy consumption of conventional devices used Because of its low cost and effectiveness under a
to achieve the required temperatures, as well as hot and dry climate, evaporative cooling should be
the sizing of these devices, thus allowing for lower used more widely:
operation and investment costs.
• for air conditioning in homes and workplaces,
Higher temperatures are usually observed in the which is currently the most widespread use,
dry season in continental climate areas, far from
• for air conditioning in rearing houses under
major sources of moisture. This causes strong daily
hot and dry climates,[for every kind of livestock]
thermal amplitude, with very low relative humidity
at the hottest period of the day. A similar situation • for short-term storage of products that can
occurs in temperate climates likely to undergo be subjected to relatively warm temperatures and
episodes of hot, dry winds from the desert, as would deteriorate rapidly in the event of more
on the southern shore of the Mediterranean. In severe heat.
these regions, evaporative cooling is therefore The increasing cost of energy can also enhance
more beneficial during the warmer periods. This is the comparative advantage of evaporative cooling
important, because the performance of conventional for the pre-cooling and cooling of conventional
equipment decreases as the outside temperature refrigeration system condensers and some heat
increases. This also applies to all systems using exchangers under moderately humid hot climates.
evaporative cooling in combination with another
cooling process. The potential advantage of On average, the energy consumption can be 4
evaporative cooling is therefore twofold: on the one times lower than a conventional device with the
hand, it allows reducing the size of the conventional same cooling power, but it can be 10 times lower in
systems operating under usual conditions, and on a hot and dry climate. The evaporation of 1 cubic
the other hand, it allows reducing the oversizing meter water can reach a cooling effect of about
of the devices that would be required to cope with 2.5 x106 kJ (700 kWh). A conventional air
higher temperatures. -conditioning system with a coefficient of
performance (COP) of 3 would require up to
230 kWh electricity for the same cooling effect.
DATA CENTRE COOLING Therefore, provided that water supply is not an issue,
evaporative cooling is strongly recommended.
An increasingly widespread application of
evaporative cooling is data centre cooling.
Data centres use a lot of energy, resulting in a
corresponding heat development that must be
dissipated to maintain acceptable inside conditions
for both operators and IT (Information Technologies)
equipment. Direct evaporative cooling is a very
efficient cooling method, allowing hot air to be
exhausted to the outside while cooling fresh air by
direct water evaporation. When outdoor humidity
conditions prevent direct evaporation, indirect
evaporative cooling is a possible solution.

11
References

[1] Lazzarin, R. , 2015. Evaporative Cooling, 27th Informatory Note on [6] Bruno, F. , 2010. On-site testing of a novel indirect evaporative
refrigeration technologies. https://iifiir.org/en/fridoc/evaporative-cooling- cooler. 9th IIR-Gustav Lorentzen Conference on Natural Working Fluids
27-lt-sup-gt-th-lt-sup-gt-informatory-note-on-refrigeration-138322 (GL2010). Proceedings. Sydney, Australia, April 12-14, 2010.
[2] Dizaji H.S., Hu E.J., Chen L., A comprehensive review of the [7] Energy.gov, 2012. Energy savers: evaporative-coolers. USA
Maisotsenko-cycle based air conditioning systems, Energy, 156, 725- Department of Energy. http://energy.gov/energysaver/articles/
749, 2018. https://doi.org/10.1016/j.energy.2018.05.086 evaporative-coolers
[3] Mahmood, M.H., Sultan M., Miyazaki T., Koyama S., Maisotsenko, [8] Herman, T., 2009. Two-stage evaporative cooling made easy. RACA
V.S., Overview of the Maisotsenko cycle – A way towards dew point Journal, Volume 28, 7.
evaporative cooling, Renew. Sust. Energ. Rev. 66, 537-555, 2016. http://
[9] Jaber, S., et al., 2011. Evaporative cooling as an efficient system
doi.org/10.1016/j.rser.2016.08.022
in Mediterranean region. Applied Thermal Engineering, Volume
[4] Lazzarin R., Nalini L., Air humidification: technical, health and energy 31, Number 14-15, November 2011. http://dx.doi.org/10.1016/j.
aspects, Carel, Brugine, Chapt. 7: Adiabatic humidifiers, pp. 1-600, applthermaleng.2011.04.026
2004.
[5] Jain, S., 2008. Emulating nature: evaporative cooling systems,
ASHRAE Transactions, volume 114, Part 2. ASHRAE, Atlanta, United
States. http://www.thefreelibrary.com/Emulating nature: evaporative
cooling systems.-a0201378222

52nd IIR Informatory Note

12
IIR recommendations

Evaporative cooling technologies may have a very positive

environmental impact by reducing the demand for electricity and
other fuels to operate cooling in various applications in regions
where water availability is not an issue.

The IIR therefore emphasises the need to:

• Develop strong worldwide campaigns on the economic and

environmental benefits of evaporative cooling technologies to
raise awareness among potential users, engineers, architects,
policy makers and industry representatives.

• Organise specific courses for designers and installers for a

proper evaluation of the possible application of direct, indirect or
both technologies to fully or partially meet the cooling demand,
considering all relevant local conditions.

• Set up incentive schemes and guidelines to promote the

use of evaporative cooling technologies in air conditioning
plants, livestock farming, storage of perishable foodstuff and in
precooling processes, where evaporative cooling almost always
makes a useful contribution to energy savings.

IIF
IIR iif-iir@iifiir.org

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Tel. +33 (0)1 42 27 32 00 / Fax +33 (0)1 47 63 17 98

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