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Evaporative Coolers


Schematic diagram of an ancient Iranian windcatcher and qanat, used for evaporative cooling of buildings (click image to enlarge)

Civilizations throughout the ages have found ingenious ways to combat
the heat in their region. An earlier form of air cooling, the windcatcher (Bâd gir), was invented in Persia (Iran) thousands of years ago in the form of wind shafts on the roof, which caught the wind, passed it over subterranean water in a qanat and discharged the cooled air into the building. Nowadays Iranians have changed the windcatcher into an evaporative cooler (Coolere Âbi) and use it widely.[1] There are 9 million evaporative coolers in central Iran, and in just the first two months of year 1385 in the Persian/Iranian calendar (April–May 2006) 130,000 evaporative coolers were sold in Iran.[2][not in citation given]

The evaporative cooler was the subject of numerous US patents in the 20th century; many of these, starting in 1906,[3] suggested or assumed the use of excelsior (wood wool)
pads as the elements to bring a large volume of water in contact with
moving air to allow evaporation to occur. A typical design, as shown in a
1945 patent, includes a water reservoir (usually with level controlled
by a float valve), a pump to circulate water over the excelsior pads and a squirrel-cage fan to draw air through the pads and into the house.[4] This design and this material remain dominant in evaporative coolers in the American Southwest, where they are also used to increase humidity.[5] In the United States, the use of the term swamp cooler may be due to the odor of algae produced by early units.[6]

Evaporative cooling was in vogue for aircraft engines in the 1930s, for example with the Beardmore Tornado airship engine. Here the system was used to reduce, or eliminate completely, the radiator which would otherwise create considerable drag.
In these systems the water in the engine was kept under pressure with
pumps, allowing it to heat to temperatures above 100°C, as the actual boiling point
is a function of the pressure. The superheated water was then sprayed
through a nozzle into an open tube, where it flashed into steam,
releasing its heat. The tubes could be placed under the skin of the
aircraft, resulting in a zero-drag cooling system.[citation needed]

However these systems also had serious disadvantages. Since the
amount of tubing needed to cool the water was large, the cooling system
covered a significant portion of the plane even though it was hidden.
This added complexity and reliability issues. In addition this large
size meant it was very easy for it to be hit by enemy fire, and
practically impossible to armor. British and U.S. developers used ethylene glycol
instead, cooling the liquid in radiators. The Germans instead used
streamlining and positioning of traditional radiators. Even the method's
most ardent supporters, Heinkel's Günter brothers, eventually gave up on it in 1940.[citation needed]

Externally-mounted evaporative[7]
cooling devices to cool interior air were used in some automobiles,
often as aftermarket accessories, until modern vapor-compression air
conditioning became widely available.

Physical principles

Evaporative cooling is a physical phenomenon in which evaporation of a liquid, typically into surrounding air, cools an object or a liquid in contact with it. Latent heat,
the amount of heat that is needed to evaporate the liquid, is drawn
from the air. When considering water evaporating into air, the wet-bulb temperature which takes both temperature and humidity into account, as compared to the actual air temperature (dry-bulb temperature),
is a measure of the potential for evaporative cooling. The greater the
difference between the two temperatures, the greater the evaporative
cooling effect. When the temperatures are the same, no net evaporation
of water in air occurs, thus there is no cooling effect. The wet-bulb
temperature is essentially the lowest temperature which can be attained
by evaporative cooling at a given temperature and humidity.

A simple example of natural evaporative cooling is perspiration,
or sweat, secreted by the body, evaporation of which cools the body.
The amount of heat transfer depends on the evaporation rate, however for
each kilogram of water vaporized 2,257 kJ of energy (about 890 BTU per
pound of pure water, at 95°F) are transferred. The evaporation rate
depends on the temperature and humidity of the air, which is why sweat
accumulates more on hot, humid days, as it does not evaporate fast

Vapor-compression refrigeration
uses evaporative cooling, but the evaporated vapor is within a sealed
system, and is then compressed ready to evaporate again, using energy to
do so. A simple evaporative cooler's water is evaporated into the
environment, and not recovered. In an interior space cooling unit, the
evaporated water is introduced into the space along with the now-cooled
air; in an evaporative tower the evaporated water is carried off in the
airflow exhaust.

Other types of phase-change cooling

A closely related process, sublimation cooling differs from evaporative cooling in that a phase transition from solid to vapor, rather than liquid to vapor occurs.

Sublimation cooling has been observed to operate on a planetary scale on the planetoid Pluto, where it has been called an anti-greenhouse effect.[citation needed]

Another application of a phase change to cooling is the
"self-refrigerating" beverage can. A separate compartment inside the can
contains a desiccant
and a liquid. Just before drinking, a tab is pulled so that the
desiccant comes into contact with the liquid and dissolves. As it does
so it absorbs an amount of heat energy called the latent heat of fusion. Evaporative cooling works with the phase change of liquid into vapor and the latent heat of vaporization, but the self-cooling can uses a change from solid to liquid, and the latent heat of fusion to achieve the same result.


Before the advent of refrigeration, evaporative cooling was used for millennia. A porous earthenware vessel would cool water by evaporation through its walls; frescoes from about 2500 BC show slaves fanning jars of water to cool rooms[8].
A vessel could also be placed in a bowl of water, covered with a wet
cloth dipping into the water, to keep milk or butter as fresh as

California ranch house with evaporative cooler box on roof ridgeline (click image to enlarge)

Evaporative cooling is a common form of cooling buildings for thermal comfort
since it is relatively cheap and requires less energy than other forms
of cooling. However, evaporative cooling requires an abundant water
source as an evaporate, and is only efficient when the relative humidity
is low, restricting its effective use to dry climates. Evaporative
cooling also raises the internal humidity level significantly, which can
cause problems such as lumpy table salt; swelling and warping of wood
paneling, doors and trim; pianos going out of tune or suffering internal
rusting, etc.

Evaporative cooling is especially well suited for climates where the air is hot and humidity
is low. In the United States, the western/mountain states are good
locations, with evaporative coolers prevalent in cities like Denver, Salt Lake City, Albuquerque, El Paso, Tucson, and Fresno
where sufficient water is available. Evaporative air conditioning is
also popular and well-suited to the southern (temperate) part of Australia.
In dry, arid climates, the installation and operating cost of an
evaporative cooler can be much lower than that of refrigerative air
conditioning, often by 80% or so. However, evaporative cooling and
vapor-compression air conditioning are sometimes used in combination to
yield optimal cooling results. Some evaporative coolers may also serve
as humidifiers in the heating season.

In locations with moderate humidity there are many cost-effective
uses for evaporative cooling, in addition to their widespread use in dry
climates. For example, industrial plants, commercial kitchens, laundries, dry cleaners, greenhouses,
spot cooling (loading docks, warehouses, factories, construction sites,
athletic events, workshops, garages, and kennels) and confinement
farming (poultry ranches, hog, and dairy) often employ evaporative
cooling. In highly humid climates, evaporative cooling may have little thermal comfort benefit beyond the increased ventilation and air movement it provides.

Other examples

On Earth, trees transpire large amounts of water through pores in their leaves called stomata, and through this process of evaporative cooling, forests interact with climate at local and global scales.[10]

Evaporative cooling is commonly used in cryogenic
applications. The vapor above a reservoir of cryogenic liquid is pumped
away, and the liquid continuously evaporates as long as the liquid's vapor pressure is significant. Evaporative cooling of ordinary helium forms a 1-K pot, which can cool to at least 1.2 K. Evaporative cooling of helium-3 can provide temperatures below 300 mK. These techniques can be used to make cryocoolers, or as components of lower-temperature cryostats such as dilution refrigerators.
As the temperature decreases, the vapor pressure of the liquid also
falls, and cooling becomes less effective. This sets a lower limit to
the temperature attainable with a given liquid.

Evaporative cooling is also the last cooling step in order to reach the ultra-low temperatures required for Bose–Einstein condensation
(BEC). Here, so-called forced evaporative cooling is used to
selectively remove high-energetic ("hot") atoms from an atom cloud until
the remaining cloud is cooled below the BEC transition temperature. For
a cloud of 1 million alkali atoms, this temperature is about 1μK.

Although robotic spacecraft use thermal radiation
almost exclusively, many manned spacecraft have short missions that
permit open-cycle evaporative cooling. Examples include the Space Shuttle, the Apollo Command/Service Module (CSM), Lunar Module and Portable Life Support System. The Apollo CSM and the Space Shuttle also had radiators, and the Shuttle could evaporate ammonia as well as water. The Apollo spacecraft used sublimators, compact and largely passive devices that dump waste heat in water vapor (steam) that is vented to space.[citation needed]
When liquid water is exposed to vacuum it boils vigorously, carrying
away enough heat to freeze the remainder to ice that covers the
sublimator and automatically regulates the feedwater flow depending on
the heat load. The water expended is often available in surplus from the
fuel cells used by many manned spacecraft to produce electricity.

Evaporative cooler designs

Evaporative cooler illustration

Most designs take advantage of the fact that water has one of the highest known enthalpy of vaporization (latent heat of vaporization) values of any common substance.

Direct evaporative cooling (open circuit) is used to lower the
temperature of air by using latent heat of evaporation, changing liquid
water to water vapor. In this process, the energy in the air does not
change. Warm dry air is changed to cool moist air. The heat of the
outside air is used to evaporate water.

Indirect evaporative cooling (closed circuit) is similar to direct evaporative cooling, but uses some type of heat exchanger. The cooled moist air never comes in direct contact with the conditioned environment.

Two-stage evaporative cooling, or indirect-direct.
Traditional evaporative coolers use only a fraction of the energy of
vapor-compression or absorption air conditioning systems. Unfortunately,
except in very dry climates they can increase humidity to a level that
makes occupants uncomfortable. Two-stage evaporative coolers do not
produce humidity levels as high as that produced by traditional
single-stage evaporative coolers.

In the first stage of a two-stage cooler, warm air is pre-cooled
indirectly without adding humidity (by passing inside a heat exchanger
that is cooled by evaporation on the outside). In the direct stage, the
pre-cooled air passes through a water-soaked pad and picks up humidity
as it cools. Since the air supply is pre-cooled in the first stage, less
humidity is transferred in the direct stage, to reach the desired
cooling temperatures. The result, according to manufacturers, is cooler
air with a relative humidity between 50-70%, depending on the climate,
compared to a traditional system that produces about 70–80% relative
humidity in the conditioned air.[citation needed]

Traditionally, evaporative cooler pads consist of excelsior (wood wool) (aspen wood fiber) inside a containment net, but more modern materials, such as some plastics and melamine
paper, are entering use as cooler-pad media. Wood absorbs some of the
water, which allows the wood fibers to cool passing air to a lower
temperature than some synthetic materials.[dubious ]

Typical installations

Typically, residential and industrial evaporative coolers use direct
evaporation, and can be described as an enclosed metal or plastic box
with vented sides. Air is moved by a centrifugal fan or blower, (usually driven by an electric motor with pulleys known as "sheaves" in HVAC
terminology, or a direct-driven axial fan), and a water pump is used to
wet the evaporative cooling pads. The cooling units can be mounted on
the roof (down draft, or downflow), or exterior walls or windows (side
draft, or horizontal flow) of buildings. To cool, the fan draws ambient
air through vents on the unit's sides and through the damp pads. Heat in
the air evaporates water from the pads which are constantly re-dampened
to continue the cooling process. Then cooled, moist air is delivered
into the building via a vent in the roof or wall.

Because the cooling air originates outside the building, one or more
large vents must exist to allow air to move from inside to outside. Air
should only be allowed to pass once through the system, or the cooling
effect will decrease. This is due to the air reaching the saturation
point. Often 15 or so air changes per hour (ACHs) occur in spaces
served by evaporative coolers, a relatively high rate of air exchange.

Evaporative (wet) cooling towers

Large hyperboloid cooling towers for a power plant in Kharkov (Ukraine)

Cooling towers are structures for cooling water or other heat
transfer media to near-ambient wet-bulb temperature. Wet cooling towers
operate on the evaporative cooling principle, but are optimized to cool
the water rather than the air. Cooling towers can often be found on
large buildings or on industrial sites. They transfer heat to the
environment from chillers, industrial processes, or the Rankine power cycle, for example.

Misting systems

Mist spraying system with water pump beneath

Misting systems work by forcing water via a high pressure pump and
tubing through a brass and stainless steel mist nozzle that has an
orifice of about 5 micrometres,
thereby producing a micro-fine mist. The water droplets that create the
mist are so small that they instantly flash evaporate. Flash evaporation can reduce the surrounding air temperature by as much as 35 F° (20 C°) in just seconds.[11]
For patio systems, it is ideal to mount the mist line approximately 8
to 10 feet (2.4 to 3.0 m) above the ground for optimum cooling. Misting
is used for applications such as flowerbeds, pets, livestock, kennels,
insect control, odor control, zoos, veterinary clinics, cooling of
produce, and greenhouses.

Misting fans

A misting fan is similar to a humidifier.
A fan blows a fine mist of water into the air. If the air is not too
humid, the water evaporates, absorbing heat from the air, allowing the
misting fan to also work as an air cooler. A misting fan may be used
outdoors, especially in a dry climate.