Air conditioning is the cooling and
dehumidification of indoor air for
thermal comfort. In a broader sense, the
term can refer to any form of
cooling, heating,
ventilation, or
disinfection that
modifies the condition of air. An
air
conditioner (often referred to as AC or air con.) is an
appliance,
system, or
mechanism
designed to stabilise the air temperature and humidity within an
area (used for cooling as well as heating depending on the air
properties at a given time), typically using a
refrigeration cycle but sometimes using
evaporation, commonly for comfort
cooling in buildings and motor vehicles.
The concept of air conditioning is known to have been applied in
Ancient Rome, where
aqueduct water was circulated through the walls of
certain houses to cool them.
Similar techniques in medieval Persia
involved the
use of cisterns and wind towers to cool buildings during the hot
season. Modern air conditioning emerged from advances in
chemistry during the 19th century, and the
first large-scale electrical air conditioning was invented and used
in 1902 by
Willis Haviland
Carrier.
History
While moving heat via machinery to provide air conditioning is a
relatively modern invention, the cooling of buildings is not.
Wealthy
ancient Romans circulated
aqueduct water through walls to cool their
luxurious houses.
The 2nd
century Chinese
inventor
Ding Huan (fl. 180) of the
Han
Dynasty invented a
rotary fan
for air conditioning, with seven wheels 3 m (10 ft) in diameter and
manually powered. In 747,
Emperor Xuanzong (r. 712–762) of
the
Tang Dynasty (618–907) had the Cool
Hall (
Liang Tian) built in the imperial palace, which the
Tang Yulin describes as having
water-powered fan wheels for air conditioning as
well as rising jet streams of water from fountains. During the
subsequent
Song Dynasty (960–1279),
written sources mentioned the air conditioning rotary fan as even
more widely used.
Medieval
Persia had buildings that
used
cisterns and
wind
towers to cool buildings during the hot season: cisterns (large
open pools in central courtyards, not underground tanks) collected
rain water; wind towers had windows that could catch wind and
internal vanes to direct the airflow down into the building,
usually over the cistern and out through a downwind cooling tower.
Cistern water evaporated, cooling the air in the building.
Ventilators were invented in
medieval Egypt and were widely
used in many houses throughout Cairo
during the
Middle Ages. These ventilators
were later described in detail by
Abd
al-Latif al-Baghdadi in 1200, who reported that almost every
house in Cairo has a ventilator, and that they cost anywhere from 1
to 500
dinars depending on their sizes and
shapes. Most ventilators in the city were oriented towards the
Qibla, as was the city in general.
In the 1600s
Cornelius Drebbel
demonstrated "turning Summer into Winter" for
James I of England by adding salt to
water.
In 1758,
Benjamin Franklin and
John Hadley, professor of chemistry at Cambridge University,
conducted an experiment to explore the principle of evaporation as
a means to rapidly cool an object. Franklin and Hadley confirmed
that evaporation of highly volatile liquids such as alcohol and
ether could be used to drive down the temperature of an object past
the freezing point of water. They conducted their experiment with
the bulb of a mercury thermometer as their object and with a
bellows used to "quicken" the evaporation; they lowered the
temperature of the thermometer bulb down to 7°F while the ambient
temperature was 65°F. Franklin noted that soon after they passed
the freezing point of water (32°F) a thin film of ice formed on the
surface of the thermometer's bulb and that the ice mass was about a
quarter inch thick when they stopped the experiment upon reaching
7°F. Franklin concluded, "From this experiment, one may see the
possibility of freezing a man to death on a warm summer's
day".
In 1820,
British
scientist and inventor
Michael Faraday discovered that
compressing and liquefying ammonia could
chill air when the liquefied ammonia was allowed to
evaporate. In 1842, Florida
physician
John Gorrie used compressor technology
to create ice, which he used to cool air for his patients in his
hospital in Apalachicola,
Florida
. He hoped eventually to use his ice-making
machine to regulate the temperature of buildings. He even
envisioned centralized air conditioning that could cool entire
cities. Though his prototype leaked and performed irregularly,
Gorrie was granted a patent in 1851 for his ice-making machine. His
hopes for its success vanished soon afterwards when his chief
financial backer died; Gorrie did not get the money he needed to
develop the machine. According to his biographer,
Vivian M. Sherlock, he blamed the "Ice King",
Frederic Tudor, for his failure,
suspecting that Tudor had launched a
smear campaign against his invention. Dr.
Gorrie died impoverished in 1855 and the idea of air conditioning
faded away for 50 years.
In 1902, the first modern electrical air conditioning unit was
invented by
Willis Haviland Carrier in Buffalo,
NY. After graduating from Cornell University, Carrier, a native of
Angola, NY, found a job at the Buffalo Forge Company. While at
Buffalo Forge, Carrier began experimentation with air conditioning
as a way to solve an application problem for the Sackett-Wilhelms
Lithographing and Publishing Company in Brooklyn, New York, and the
first "air conditioner," designed and built in Buffalo by Carrier,
began working 17 July 1902.
Designed to improve manufacturing process control in a printing
plant, Carrier's invention controlled not only
temperature but also
humidity. Carrier used his knowledge of the heating
of objects with steam and reversed the process. Instead of sending
air through hot coils, he sent it through cold coils (ones filled
with cold water). The air blowing over the cold coils cooled the
air, and one could thereby control the amount of moisture the
colder air could hold. In turn, the humidity in the room could be
controlled. The low heat and humidity were to help maintain
consistent paper dimensions and ink alignment. Later, Carrier's
technology was applied to increase productivity in the workplace,
and
The Carrier Air Conditioning
Company of America was formed to meet rising demand. Over time,
air conditioning came to be used to improve comfort in homes and
automobiles as well. Residential sales
expanded dramatically in the 1950s.
In 1906,
Stuart W. Cramer of Charlotte, North
Carolina
, USA
, was exploring ways to add moisture to the air in
his textile mill. Cramer coined the term "air conditioning",
using it in a patent claim he filed that year as an analogue to
"water conditioning", then a well-known process for making textiles
easier to process. He combined moisture with ventilation to
"condition" and change the air in the factories, controlling the
humidity so necessary in textile plants. Willis Carrier adopted the
term and incorporated it into the name of his company. This
evaporation of water in air, to provide a cooling effect, is now
known as
evaporative
cooling.
The first air conditioners and
refrigerators employed toxic or flammable gases
like
ammonia,
methyl chloride, and
propane which could result in fatal accidents when
they leaked.
Thomas Midgley, Jr.
created the first chlorofluorocarbon gas,
Freon, in 1928.
Freon is a
trademark name of
DuPont for any
Chlorofluorocarbon (CFC), Hydrogenated
CFC (
HCFC), or
Hydrofluorocarbon (HFC) refrigerant, the
name of each including a number indicating molecular composition
(R-11, R-12, R-22, R-134A). The blend most used in direct-expansion
home and building comfort cooling is an HCFC known as
R-22. It is to be phased out for use
in new equipment by 2010 and completely discontinued by 2020. R-12
was the most common blend used in automobiles in the US until 1994
when most changed to R-134A. R-11 and R-12 are no longer
manufactured in the US for this type of application, the only
source for air conditioning purchase being the cleaned and purified
gas recovered from other air conditioner systems. Several non-ozone
depleting refrigerants have been developed as alternatives,
including
R-410A, invented by Honeywell
(formerly AlliedSignal) in Buffalo, NY, and sold under the Genetron
(R) AZ-20 name. It was first commercially used by Carrier under the
brand name
Puron.
Innovation in air conditioning technologies continues, with much
recent emphasis placed on energy efficiency, and on improving
indoor air quality. Reducing
climate change impact is an important area of innovation, because
in addition to greenhouse gas emissions associated with energy use,
CFCs, HCFCs and HFCs are, themselves, potent
greenhouse gases when leaked to the
atmosphere. For example, R-22 (also known as HCFC-22) has a
global warming potential
about 1,800 times higher than CO
2. As an alternative to
conventional refrigerants, natural alternatives like CO
2
(
R-744) have been proposed.
Air conditioning applications
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An air conditioner.
Air conditioning engineers broadly divide air conditioning
applications into
comfort and
process.
Comfort applications aim to provide a
building indoor
environment that remains relatively constant in a range
preferred by humans despite changes
in external weather conditions or in internal heat loads.
Air conditioning makes
deep plan buildings
feasible, for otherwise they'd have to be built narrower or with
light wells so that inner spaces receive sufficient outdoor air via
natural ventilation. Air
conditioning also allows buildings to be taller since
wind speed increases significantly with
altitude making natural ventilation impractical for very tall
buildings. Comfort applications for various building types are
quite different and may be categorized as
- Low-Rise Residential buildings, including single
family houses, duplexes, and small apartment buildings
- High-Rise Residential buildings, such as tall
dormitories and apartment blocks
- Commercial buildings, which are built for commerce,
including offices, malls, shopping centers, restaurants, etc.
- Institutional buildings, which includes hospitals,
governmental, academic, and so on.
- Industrial spaces where thermal comfort of workers is
desired.
In addition to buildings, air conditioning can be used for many
types of transportation - motor-cars and other land vehicles,
trains, ships, aircraft, and spacecraft.
Process applications aim to provide a
suitable environment for a process being carried out, regardless of
internal heat and humidity loads and external weather conditions.
Although often in the comfort range, it is the needs of the process
that determine conditions, not human preference. Process
applications include these:
- Hospital operating theatres,
in which air is filtered to high levels to reduce infection risk and the humidity controlled to
limit patient dehydration. Although temperatures are often in the
comfort range, some specialist procedures such as open heart surgery require low
temperatures (about 18 °C, 64 °F) and others such as neonatal relatively high temperatures (about 28 °C,
82 °F).
- Facilities for breeding laboratory
animals. Since many animals normally only reproduce in spring,
holding them in rooms at which conditions mirror spring all year
can cause them to reproduce year-round.
- Aircraft air conditioning. Although nominally aimed at
providing comfort for passengers and cooling of equipment, aircraft
air conditioning presents a special challenge because of the
changing density altitude associated with changes in altitude,
humidity and temperature of the outside air .
In both comfort and process applications the objective may be to
not only control temperature, but also
humidity, air quality and air movement from space
to space.
Humidity control
Refrigeration air conditioning equipment usually reduces the
humidity of the air processed by the
system. The relatively cold (below the
dewpoint) evaporator coil condenses water vapor
from the processed air, (much like an ice-cold drink will condense
water on the outside of a glass), sending the water to a drain and
removing water vapor from the cooled space and lowering the
relative humidity. Since humans perspire to provide natural cooling
by the
evaporation of perspiration from
the skin, drier air (up to a point) improves the comfort provided.
The comfort air conditioner is designed to create a 40% to 60%
relative humidity in the occupied space. In food retailing
establishments large open chiller cabinets act as highly effective
air dehumidifying units.
A specific type of air conditioner that is used only for
dehumidifying is called a
dehumidifier.
A dehumidifier is different from a regular air conditioner in that
both the evaporator and condensor coils are placed in the same air
path, and the entire unit is placed in the environment that is
intended to be conditioned (in this case dehumidified), rather than
requiring the condensor coil to be outdoors. Having the condensor
coil in the same air path as the evaporator coil produces warm,
dehumidified air. The evaporator (cold) coil is placed first in the
air path, dehumidifying the air exactly as a regular air
conditioner does. The air next passes over the condensor coil
re-warming the now dehumidified air. Note that the terms "condensor
coil" and "evaporator coil" do not refer to the behavior of water
in the air as it passes over each coil; instead they refer to the
phases of the
refrigeration
cycle. Having the condensor coil in the main air path rather
than in a separate, outdoor air path (as in a regular air
conditioner) results in two consequences—the output air is warm
rather than cold, and the unit is able to be placed anywhere in the
environment to be conditioned, without a need to have the condensor
outdoors.
Unlike a regular air conditioner, a dehumidifier will actually heat
a room just as an
electric heater
that draws the same amount of power (
watts) as
the dehumidifier. A regular air conditioner transfers energy out of
the room by means of the condensor coil, which is outside the room
(outdoors). This is a
thermodynamic
system where the room serves as the system and energy is
transferred out of the system. Conversely with a dehumidifier, no
energy is transferred out of the thermodynamic system (room)
because the air conditioning unit (dehumidifier) is entirely inside
the room. Therefore all of the power consumed by the dehumidifier
is energy that is input into the thermodynamic system (the room),
and remains in the room (as heat). In addition, if the condensed
water has been removed from the room, the amount of heat needed to
boil that water has been added to the room. This is the inverse of
adding water to the room with an
evaporative cooler.
Dehumidifiers are commonly used in cold, damp climates to prevent
mold growth indoors, especially in basements.
They are also sometimes used in hot, humid climates for comfort
because they reduce the humidity which causes discomfort (just as a
regular air conditioner, but without cooling the room).
The engineering of physical and thermodynamic properties of
gas-vapor mixtures is named
Psychrometrics.
Health implications
A poorly maintained air-conditioning system can occasionally
promote the growth and spread of microorganisms, such as
Legionella
pneumophila, the infectious agent responsible for
Legionnaires' disease, or thermophilic
actinomycetes, but as long as the air
conditioner is kept clean these health hazards can be avoided.
Conversely, air conditioning, including filtration, humidification,
cooling, disinfection, etc., can be used to provide a clean, safe,
hypoallergenic atmosphere in hospital
operating rooms and other environments where an appropriate
atmosphere is critical to patient safety and well-being. Air
conditioning can have a positive effect on sufferers of
allergies and
asthma.
In serious
heat waves, air conditioning
can save the lives of the elderly. Some local authorities even set
up public
cooling centers for the
benefit of those without air conditioning at home.
Energy use
It should be noted that in a thermodynamically
closed system, any energy input into the
system that is being maintained at a set temperature (which is a
standard mode of operation for modern air conditioners) requires
that the energy removal rate from the air conditioner increases.
This increase has the effect that for each unit of energy input
into the system (say to power a light bulb in the closed system)
this requires the air conditioner to remove that energy. In order
to do that the air conditioner must increase its consumption by the
inverse of its efficiency times the input of energy. As an example,
presume that inside the closed system a 100 watt light bulb is
activated, and the air conditioner has an efficiency of 200%. The
air conditioner's energy consumption will increase by 50 watts to
compensate for this, thus making the 100 W light bulb use a
total of 150 W of energy.
It is typical for air conditioners to operate at "efficiencies" of
significantly greater than 100%. .However it may be noted that the
input (electrical) energy is of higher thermodynamic quality than
the output which is basically thermal energy (heat dissipated), See
Coefficient of
performance.
Automobile air conditioners
Air conditioner systems are designed to allow the driver and or
passengers to feel more comfortable during uncomfortably warm humid
or hot trips in a vehicle. Cars in hot climates often are fitted
with air conditioning. There has been much debate and discussion on
what the usage of an air conditioner does to the fuel efficiency of
a vehicle. Factors such as wind resistance aerodynamics and engine
power and weight have to be factored into finding the true variance
between using the air conditioning system and not using it when
figuring out difference in actual gas mileage. Other factors on the
impact on the engine and an overall engine heat increase can have
an impact on the cooling system of the vehicle.
The
Packard Motor Car Company was the first
automobile manufacturer to build air conditioners into its cars,
beginning in 1939. These air conditioners were originally optional,
and could be installed for an extra $274 (about $4,050 in ). The
system took up the entire trunk space, was not very efficient, and
had no thermostat or independent shut-off mechanism. The option was
discontinued after 1941.
In 1954 the
Nash Ambassador was the
first American automobile to boast front-end, fully-integrated
heating, ventilating, and air-conditioning system. The
Nash-Kelvinator corporation used its
experience in refrigeration to introduce the automobile industry's
first compact and affordable, single-unit heating and air
conditioning system optional for its 1954
Nash models. This was the first system for the
mass market with controls on the dash and an electric clutch.
Marketed under the name of "
All-Weather
Eye", the Nash system was "a good and remarkably inexpensive"
system. Entirely incorporated within the engine bay, the combined
heating and cooling system had cold air for passengers enter
through dash-mounted vents. Nash's exclusive "remarkable advance"
was not only the "sophisticated" unified system, but also its $345
price that beat all other systems.
Most competing systems used a separate heating system and an
engine-mounted compressor with an evaporator in the car's trunk to
deliver cold air through the rear parcel shelf and overhead vents.
General Motors made a front mounted air conditioning system
optional in 1954 on
Chevrolets and
Pontiacs with a
V8
engine that added separate controls and air distribution. The
alternative layout pioneered by Nash "became established practice
and continues to form the basis of the modern and more
sophisticated automatic climate control systems."
The innovation was adopted quickly, and by 1960 about 20% of all
cars in the United States had air-conditioning with the percentage
increasing to 80% in the desert areas of the Southwest.
American Motors made air conditioning
standard equipment on all
AMC
Ambassadors starting with the 1968 model year, a first in the
mass market with a base price starting at $2,671. By 1969, over
half (54%) of the domestic automobiles were equipped with air
conditioning; with the system needed not only for passenger
comfort, but also to increase the car's resale value.
Portable air conditioners
A portable air conditioner is one on wheels that can be easily
transported inside a home or office. They are currently available
with capacities of about 6,000 to 60,000
BTU/h
(1,800 to 18,000 watts output) and with and without electric
resistance heaters. Portable true air conditioners come in two
forms, split and hose. Evaporative coolers, sometimes called
conditioners, are also portable.
Air-cooled portable air conditioners are compressor-based
refrigerant system that use air to exchange heat, in the same way
as a car or typical household air conditioner. With this type of
system the air is dehumidified as it is cooled. They collect water
condensed from the cooled air, and produce hot air which must be
vented outside of the cooled area (they transfer heat from the air
in the cooled area to air which must be vented).
A split system has an indoor unit on wheels connected to an outdoor
unit via flexible pipes, similar to a permanently fixed installed
unit.
Hose systems, which can be
Air-to-Air and
Monoblock, are vented to the outside via air
ducts. The "monoblock" version collects
the water in a bucket or tray and stops when full. The
Air-to-Air version re-evaporates the water and
discharges it through the ducted hose, and can run
continuously.
A single-duct unit draws air out of the room to cool its condenser,
and then vents it outside. This air is replaced by hot air from
outside or other rooms, thus reducing efficiency. Modern units run
on approximately 1 to 3 ratio i.e., to produce 3 kW of cooling
this will use 1 kW of electricity. A dual-duct unit draws air
from outside to cool its condenser instead of from inside the room,
and thus is more efficient than most single-duct units.
Evaporative air coolers,
sometimes called swamp air conditioners, do not have a compressor
or condenser. Liquid water is evaporated on the cooling fins,
releasing the vapour into the cooled area. Evaporating water
absorbs a significant amount of heat, the
latent heat of vaporisation,
cooling the air—humans and other animals use the same mechanism to
cool themselves by
sweating.
Disadvantages are that unless ambient
humidity is low (dry
climate) cooling is limited and the cooled air is
very humid and can feel clammy. They have the advantage of needing
no hoses to vent heat outside the cooled area, making them truly
portable; and they are very cheap to install and use less energy
than refrigerative air conditioners.
Heat pumps
Heat pump is a term for a type of air conditioner in which
the
refrigeration cycle is able
to be reversed, producing heat instead of cold in the indoor
environment. They are also commonly referred to, and marketed as, a
reverse cycle air conditioner. Using an air conditioner in this way
to produce heat is significantly more efficient than
electric resistance heating.
Some home-owners elect to have a heat pump system installed, which
is actually simply a central air conditioner with heat pump
functionality (the refrigeration cycle is reversed in the winter).
When the heat pump is enabled, the indoor evaporator coil switches
roles and becomes the condensor coil, producing heat. The outdoor
condensor unit also switches roles to serve as the evaporator, and
produces cold air (colder than the ambient outdoor air).
Heat pumps are more popular in milder winter climates where the
temperature is frequently in the range of 40-55°F (4-13°C), because
heat pumps become inefficient in more extreme cold. This is due to
the problem of the outdoor unit's coil forming ice, which blocks
air flow over the coil. To compensate for this, the heat pump
system must temporarily switch back into the regular air
conditioning mode to switch the outdoor evaporator coil
back to being the condensor coil so that it can heat up
and de-ice. A heat pump system therefore will have a form of
electric resistance heating in the indoor air path that is
activated only in this mode in order to compensate for the
temporary air conditioning, which would otherwise generate
undesirable cold air in the winter. The icing problem becomes much
more prevalent with lower outdoor temperatures, so heat pumps are
commonly installed in tandem with a more conventional form of
heating, such as a
natural gas or
oil furnace,
which is used instead of the heat pump during harsher winter
temperatures. In this case, the heat pump is used efficiently
during the milder temperatures, and the system is switched to the
conventional heat source when the outdoor temperature is
lower.
Some more expensive window air conditioning units have the heat
pump function. However, a window unit that has a "heat" selection
is not necessarily a heat pump because some units use electric
resistance heat when heating is desired. A unit that has true heat
pump functionality will be indicated in its literature by the term
"heat pump".
Professional bodies
American Society of Heating, Refrigerating, and
Air-Conditioning Engineers
ASHRAE (American Society of Heating, Refrigerating, and
Air-Conditioning Engineers) is an organization devoted to the
advancement of indoor-environment-control technology in the
heating, ventilation, and air conditioning (HVAC) industry. ASHRAE
was founded in 1894 to serve as a source of technical standards and
guidelines. Since that time, it has grown into an international
society that offers educational information, courses, seminars,
career guidance, and publications. The organization also promotes a
code of ethics for HVAC professionals and provides for liaison with
the general public. Its headquarters are in Atlanta, GA.
Australian Institute of Refrigeration Air Conditioning and
Heating
The Australian Institute of Refrigeration Air Conditioning and
Heating (AIRAH) was founded in 1920 and currently has around 10,000
members. AIRAH is the official Australian secretariat of the
International Institute of Refrigeration (IIR) and collaborates
closely with the American Society of Heating, Refrigeration and Air
Conditioning Engineers (ASHRAE).
Air Conditioning and Mechanical Contractors Association of
Australia
The Air Conditioning and Mechanical Contractors Association of
Australia (AMCA) is a nation wide industry association dedicated to
represent and service the air conditioning and mechanical services
industry in Australia. Members of AMCA design, install and provide
ongoing service of air conditioning and mechanical ventilation
systems.
Air Conditioning Contractors of America (ACCA)
The ACCA is a large organization of American HVACR professionals.
They have over four thousand members and have individual charters
in each state.
See also
References
- ASHRAE Terminology of HVAC&R, ASHRAE, Inc., Atlanta,
1991,
- Needham, Joseph (1986). Science and Civilization: Volume 4,
Physics and Physical Technology, Part 2, Mechanical Engineering.
Taipei: Caves Books Ltd. Pages 99, 151, 233.
- Needham, Joseph (1986). Science and Civilization: Volume 4,
Physics and Physical Technology, Part 2, Mechanical Engineering.
Taipei: Caves Books Ltd. Pages 134 & 151.
- Needham, Joseph (1986). Science and Civilization: Volume 4,
Physics and Physical Technology, Part 2, Mechanical Engineering.
Taipei: Caves Books Ltd. Page 151.
- Ancient air conditioning, The World, January 11, 2007 [1]
- David A. King (1984). "Architecture and Astronomy: The
Ventilators of Medieval Cairo and Their Secrets", Journal of
the American Oriental Society 104 (1), p.
97-133.
- Cooling by Evaporation (Letter to John Lining).
Benjamin Franklin, London, June 17, 1758
- History of Air Conditioning Source: Jones Jr.,
Malcolm. "Air Conditioning". Newsweek. Winter 1997 v130 n24-A
p42(2). Retrieved 1 January 2007.
- The History of Air Conditioning Lou Kren, Properties
Magazine Inc. Retrieved 1 January 2007.
- IPCC Fourth Assessment Report, Table 2.14
- The current status in Air Conditioning - papers
& presentations
- Sick building syndrome
- Home Control of Asthma & Allergies
- Michigan Fast Facts and Trivia, retrieved on
2009-08-29.
- "Air Conditioning and Refrigeration Timeline", National
Academy of Engineering, retrieved 2009-08-29.
- "Air Conditioning and Refrigeration History - part 4",
National Academy of Engineering, retrieved 2009-08-29.
- Al Binder and the Ward's staff, "Rearview Mirror",
Ward's AutoWorld, 2001-02-01, retrieved on
2009-08-26.
- "1953-1955 Nash and Hudson Ramblers" by the editors
of Consumer Guide, 2007-11-29, retrieved on
2009-08-29.
- ACCA home
page