Central air conditioning
air conditioning, commonly referred to as central air (U.S.) or air-con (UK), is an air conditioning system which uses ducts to distribute cooled and/or dehumidified air to
more than one room, or uses pipes to distribute chilled water to heat exchangers in more than one room, and which is not plugged
into a standard electrical outlet.
With a typical split system, the condenser and
compressor are located in an outdoor unit; the evaporator is mounted in the air handler unit. With a package system, all components are located in a single outdoor unit that may
be located on the ground or roof.
Central air conditioning performs like a regular air
conditioner but has several added benefits:
- When the air handling unit turns
on, room air is drawn in from various parts of the building through return-air ducts. This air is pulled through a filter where airborne particles such as dust and lint are removed. Sophisticated filters may remove microscopic pollutants as well. The filtered air is routed to air supply ductwork that carries it back to rooms. Whenever
the air conditioner is running, this cycle repeats continually.
the condenser unit (with its fan and the compressor) is located outside the home, it offers a lower level of indoor noise than a free-standing air conditioning unit.
An Air Conditioner
the 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 (ofte
n referred to as AC or air con.) is an appliance, system, or machine 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.
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.
the refrigeration cycle, a heat pump transfers heat from a lower-temperature heat source into a higher-temperature heat sink. Heat would naturally flow in the opposite direction. This is the most common type of air conditioning.
A refrigerator works in much the same way, as it pumps the heat out of the interior and into the room in which
This cycle takes advantage of the way phase changes work, where latent heat is released at a constant temperature during a liquid/gas phase change, and where varying the pressure of a pure substance also varies its condensation/boiling point.
The most common refrigeration cycle uses an electric motor to drive a compressor. In an automobile, the compressor is driven by a belt over a pulley, the belt being driven by the engine's crankshaft (similar to the driving of the pulleys for the alternator, power steering, etc.). Whether in a car or building, both use electric fan motors for air circulation. Since evaporation occurs when heat is absorbed, and condensation occurs when heat is released, air conditioners use
a compressor to cause pressure changes between two compartments, and actively condense and pump a refrigerant around. A refrigerant is pumped into the cooled compartment (the evaporator coil), where the low pressure causes the refrigerant to evaporate into a vapor, taking heat with
it. In the other compartment (the condenser), the refrigerant vapor is compressed and forced through another heat exchange coil, condensing
into a liquid, rejecting the heat previously absorbed from the cooled space.
unloaders are a method of load control used mainly in commercial air conditioning systems. On a semi-hermetic (or open) compressor, the heads can be fitted with unloaders which remove a portion of the load
from the compressor so that it can run better when full cooling is not needed. Unloaders can be electrical or mechanical.
A Heat Pump
pump is a machine or device that moves heat from one location (the 'source') to another location (the 'sink' or 'heat sink') using mechanical work. Most heat pump technology moves heat from a low temperature heat source to a higher temperature
heat sink. Common examples are food refrigerators and freezers, air conditioners, and reversible-cycle heat pumps for providing thermal comfort.
Heat pumps can be thought of as a heat engine which is operating in reverse. One common type of heat pump works by exploiting the physical properties
of an evaporating and condensing fluid known as a refrigerant. In heating, ventilation, and air conditioning (HVAC) applications, a heat pump normally refers to a vapor-compression refrigeration device that includes a reversing valve and optimized heat exchangers so that the direction of heat flow may be reversed. Most commonly, heat pumps draw heat from the
air or from the ground. Some air-source heat pumps do not work as well when temperatures fall below around −5 °C
When comparing the performance of heat pumps, it is best to avoid the
word "efficiency" which has a very specific thermodynamic definition. The term coefficient of performance (COP) is used to describe the ratio of useful heat movement to work input. Most vapor-compression
heat pumps utilize electrically powered motors for their work input. However, in most vehicle applications, shaft work, via
their internal combustion engines, provide the needed work.
When used for heating a building
on a mild day, a typical air-source heat pump has a COP of 3 to 4, whereas a typical electric resistance heater has a COP of 1.0. That is, one joule of electrical energy will cause a resistance heater to produce one joule of useful heat, while under
ideal conditions, one joule of electrical energy can cause a heat pump to move much more than one joule of heat from a cooler
place to a warmer place.
Note that when there is a wide temperature differential,
e.g., when an air-source heat pump is used to heat a house on a very cold winter day, it takes more work to move the same
amount of heat indoors than on a mild day. Ultimately, due to Carnot efficiency limits, the heat pump's performance will approach 1.0 as the outdoor-to-indoor temperature difference
increases. This typically occurs around −18 °C (0 °F) outdoor temperature for air source heat pumps. Also, as
the heat pump takes heat out of the air, some moisture in the outdoor air may condense and possibly freeze on the outdoor
heat exchanger. The system must periodically melt this ice. In other words, when it is extremely cold outside, it is simpler,
and wears the machine less, to heat using an electric-resistance heater than to strain an air-source heat pump.
Air-source heat pumps
Air source heat pumps are relatively easy (and inexpensive) to install and have therefore historically been the most widely
used heat pump type. However, they suffer limitations due to their use of the outside air as a heat source or sink. The higher
temperature differential during periods of extreme cold or heat leads to declining efficiency, as explained above. In mild
weather, COP may be around 4.0, while at temperatures below around −8 °C (17 °F) an air-source heat
pump can achieve a COP of 2.5 or better, which is considerably more than the COP that may be achieved by conventional heating
systems. The average COP over seasonal variation is typically 2.5-2.8, with exceptional models able to exceed 6.0 (2.8 kW).
There are different types of standard heating systems. Central heating is often used in cold climates to heat private houses and public buildings. Such a system contains
a boiler, furnace, or heat pump to heat water, steam, or air, all in a central location such as a furnace room in a home or a mechanical room in a large building. The use of water as the heat transfer medium is known as hydronics. The system also contains either ductwork, for forced air systems, or piping to distribute a heated
fluid and radiators to transfer this heat to the air. The term radiator in this context is misleading since
most heat transfer from the heat exchanger is by convection, not radiation. The radiators may be mounted on walls or buried in the floor to give under-floor heat.
In boiler fed or radiant heating systems, all but the simplest systems have a pump to circulate the water
and ensure an equal supply of heat to all the radiators. The heated water can also be fed through another (secondary) heat
exchanger inside a storage cylinder to provide hot running water.
Forced air systems send heated air through ductwork. During warm weather the same ductwork can be used for air conditioning. The forced air can also
be filtered or put through air cleaners.
Heating can also be provided from electric,
or resistance heating using a filament that becomes hot when electric current is caused to pass through it. This type
of heat can be found in electric baseboard heaters, portable electric heaters, and as backup or supplemental heating for heat
pump (or reverse heating) system.
The heating elements (radiators or vents) should
be located in the coldest part of the room, typically next to the windows to minimize condensation and offset the convective
air current formed in the room due to the air next to the window becoming negatively buoyant due to the cold glass. Devices
that direct vents away from windows to prevent "wasted" heat defeat this design intent. Cold air drafts can contribute
significantly to subjectively feeling colder than the average room temperature. Therefore, it is important to control the
air leaks from outside in addition to proper design of the heating system.
of central heating is often credited to the ancient Romans, who installed a system of air ducts called a hypocaust in the walls and floors of public baths and private villas.
is the process of "changing" or replacing
air in any space to control temperature or remove moisture, odors, smoke, heat, dust, airborne bacteria, carbon dioxide, and
to replenish oxygen. Ventilation includes both the exchange of air to the outside as well as circulation of air within the
building. It is one of the most important factors for maintaining acceptable indoor air quality in buildings. Methods for ventilating a building may be divided into mechanical/forced
and natural types. Ventilation is used to remove unpleasant smells and excessive moisture, introduce outside air,
and to keep interior building air circulating, to prevent stagnation of the interior air.
Mechanical or forced ventilation
or "forced" ventilation is used to control indoor air quality. Excess humidity, odors, and contaminants can often be controlled via dilution or replacement with outside air. However,
in humid climates much energy is required to remove excess moisture from ventilation air.
and bathrooms typically have mechanical exhaust to control odors and sometimes humidity. Factors in the design of such systems
include the flow rate (which is a function of the fan speed and exhaust vent size) and noise level. If the ducting for the
fans traverse unheated space (e.g., an attic), the ducting should be insulated as well to prevent condensation on the ducting.
Direct drive fans are available for many applications, and can reduce maintenance needs.
fans and table/floor fans circulate air within a room for the purpose of reducing the perceived temperature
because of evaporation of perspiration on the skin of the occupants. Because hot air rises, ceiling fans may be used to keep
a room warmer in the winter by circulating the warm stratified air from the ceiling to the floor. Ceiling fans do not provide
ventilation as defined as the introduction of outside air.
Ducts are used in heating, ventilation, and air conditioning (HVAC) to deliver and remove air. These needed airflows include, for example, supply air, return
air, and exhaust air. Ducts also deliver, most commonly as part of the supply air, As such, air ducts
are one method of ensuring acceptable indoor air quality as well as thermal comfort.
A duct system is often called ductwork.
Planning ('laying out'), sizing, optimizing, detailing, and finding the pressure losses through a duct system is called duct
Polyurethane and Phenolic insulation
panels (preinsulated aluminum ducts)
Traditionally, air ductwork is made
of sheet metal which is installed first and then lagged with insulation as a secondary operation. Ductwork manufactured from
rigid insulation panels does not need any further insulation and is installed in a single fix. Light weight and installation
speed are among the features of preinsulated aluminium ductwork, also custom or special shapes of ducts can be easily fabricated
in the shop or on site.
The ductwork construction starts with the tracing of the
duct outline onto the aluminium preinsulated panel, then the parts are typically cut at 45 degree, bent if required to obtain
the different fittings (i.e. elbows, tapers) and finally assembled with glue. Aluminium tape is applied to all seams where
the external surface of the aluminium foil has been cut. A variety of flanges are available to suit various installation requirements.
All internal joints are sealed with sealant.
Among the various types of rigid polyurethane
foam panels available, a new water formulated panel stands out. In this particular panel, the foaming process is obtained
through the use of water instead of the CFC, HCFC, HFC and HC gasses. The foam panels are then coated with aluminum sheets
on either side, with thicknesses that can vary from 80 micrometres for indoor use to 200 micrometres for external use in order
to guarantee the high mechanical characteristics of the duct
A rigid phenolic insulation
ductwork system is available and complies with the UL 181 standard for class 1 air ductwork.
Fiberglass duct board (preinsulated non metallic ductwork)
the fiberglass panels provide built-in thermal insulation and the interior surface absorbs sound, helping to provide quiet operation of the HVAC system. The duct board is formed by sliding a specially-designed
knife along the board using a straightedge as a guide; the knife automatically trims out a "valley"
with 45° sides; the valley does not quite penetrate the entire depth of the duct board, providing a thin section that
acts as a hinge. The duct board can then be folded along the valleys to produce 90° folds, making the rectangular
duct shape in the fabricator's desired size. The duct is then closed with staples and special aluminum or similar 'metal-backed'
tape. Commonly available duct tape should not be used on air ducts, metal, fiberglass, or otherwise, that are intended for long-term
use; the adhesive on so called 'duct tape' dries and releases with time.
ducts, known as flex, have a variety of configurations, but for HVAC applications, they are typically flexible plastic
over a metal wire coil to make round, flexible duct. In the United States, the insulation is usually glass wool, but other
markets such as Australia, use both polyester fibre and glass wool for thermal insulation. A protective layer surrounds the insulation, and is
usually composed of polyethylene or metalised PET. Flexible duct is very convenient for attaching supply air outlets to the rigid ductwork. However,
the pressure loss through flex is higher than for most other types of ducts. As such, designers and installers attempt to
keep their installed lengths (runs) short, e.g., less than 15 feet or so, and to minimize turns. Kinks in flex must
be avoided. Some flexible duct markets prefer to avoid using flexible duct on the return air portions of HVAC systems, however
flexible duct can tolerate moderate negative pressures - the UL181 test requires a negative pressure of 200 Pa.
Fabric ducting, also known as air socks, duct socks or textile ducts, are designed for even air distribution throughout the entire length.
Usually made of special permeable polyester material, fabric ducts act like a conventional system with much more diffusers.
Fabric ducts are normally used where even air distribution
is essential. Due to the nature or the air distribution, textile ducts are not usually concealed within false ceilings, because of this, they are manufactured in various colours to coordinate with the interior.
Fabric ducts fitted above a ceiling will need to be pvc coated so that no maintenance is required to maintain
Because air passes through the body of a textile duct, it is impossible
for condensation to form on the ducts surface, they can therefore be used where air is to be supplied below the dew point, without needing to be insulated.
zone damper (also known as a Volume Control Damper or VCD) is a specific type of damper
used to control the flow of air in an HVAC heating or cooling system. In order to improve efficiency and occupant comfort, HVAC systems are
commonly divided up into multiple zones. For example, in a house, the main floor may be served by one heating zone while the
upstairs bedrooms are served by another. In this way, the heat can be directed principally to the main floor during the day
and principally to the bedrooms at night, allowing the unoccupied areas to cool down.
dampers as used in home HVAC systems are usually electrically powered. In large commercial installations, vacuum or compressed air may be used instead. In either case, the motor is usually connected to the damper via a mechanical
For electrical zone dampers, there are two principal designs.
In one design, the motor is often a small shaded-pole synchronous motor combined with a rotary switch that can disconnect the motor at either of the two stopping points
("damper open" or "damper closed"). In this way, applying power to the "open damper" terminal
causes the motor to run until the damper is open while applying power at the "close damper" terminal causes the
motor to run until the damper is closed. The motor is commonly powered from the same 24 volt ac power source that is used for the rest of the control system. This allows the zone dampers to be
directly controlled by low-voltage thermostats and wired with low-voltage wiring. Because simultaneous closure of all dampers might harm the furnace
or air handler, this style of damper is often designed to only obstruct a portion of the air duct, for example, 75%.
Another style of electrically powered damper uses a spring-return mechanism and a shaded-pole synchronous motor. In this case, the damper is normally opened
by the force of the spring but can be closed by the force of the motor. Removal of electrical power re-opens the damper. This
style of damper is advantageous because it is "fail safe"; if the control to the damper fails, the damper opens
and allows air to flow. However, in most applications "fail safe" indicates the damper will close upon loss of power
thus preventing the spread of smoke and fire to other areas. These dampers also may allow adjustment of the "closed"
position so that they only obstruct, for example, 75% of the air flow when closed.
vacuum- or pneumatically-operated zone dampers, the thermostat usually switches the pressure or vacuum on or off, causing
a spring-loaded rubber diaphragm to move and actuate the damper. As with the second style of electrical zone dampers,
these dampers automatically return to the default position without the application of any power, and the default position
is usually "open", allowing air to flow. Like the second style of electrical zone damper, these dampers may allow
adjustment of the "closed" position.
Highly sophisticated systems may
use some form of building automation such as BACnet or LonWorks to control the zone dampers. The dampers may also support positions other than fully open or fully
closed and are usually capable of reporting their current position and, often, the temperature and volume of the air flowing
past the smart damper.
Regardless of the style of damper employed, the systems are
often designed so that when no thermostat is calling for air, all dampers in the system are opened. This allows air to continue
to flow while the heat exchanger in a furnace cools down after a heating period completes.
Smoke and Fire dampers are found
in ductwork, where the duct passes through a firewall or firecurtain. Smoke dampers are automated with the use of a mechanical
motor often referred to as an Actuator. A probe connected to the motor is installed in the run of duct, and detects smoke
within the duct system which has been extracted from a room, or which is being supplied from the AHU (Air Handling Unit) or
elsewhere within the run. Once smoke is detected within the duct, the Actuator triggers the motor release and the smoke damper
will automatically close until manually re-opened.
You will also find Fire dampers in the same places as smoke dampers,
depending on the application of the area after the firewall. Unlike smoke dampers, they are not triggered by any electrical
system, which is perfect in the event of an electrical failure where the Smoke dampers would fail to close. A fire damper
is held open by a bar crossing the corrigated screen, which will break and allow the damper to close when air in the duct
is above a certain temperature. This again will then have to be manually re-opened.
Thermostats control the operation
of HVAC systems, turning on the heating or cooling systems to bring the building to the set temperature. Typically the heating
and cooling systems have separate control systems (even though they may share a thermostat) so that the temperature is only
controlled "one-way." That is, in cold weather, a building that is too hot will not be cooled by the thermostat.
Thermostats may also be incorporated into facility energy management systems in which the power utility customer may control the overall energy expenditure. In addition, a growing
number of power utilities have made available a device which, when professionally installed, will control or limit the power
to an HVAC system during peak use times in order to avoid necessitating the use of rolling blackouts. The customer is given a credit of some sort in exchange, so it is often to the advantage of the consumer to buy the most efficient
equipment power in the U.S. is often described in terms of "tons of refrigeration." A "ton of refrigeration" is defined as the cooling power of one short ton (2000 pounds or 907 kilograms) of ice melting in a 24-hour period. This is equal to 12,000 BTU per hour, or 3517 watts. Residential central air systems are usually from 1 to 5 tons (3 to 20 kilowatts (kW)) in capacity.
The use of electric/compressive air conditioning
puts a major demand on the electrical power grid in hot weather, when most units are operating under heavy load. In the aftermath of the 2003 North America blackout locals were asked to keep their air conditioning off. During peak demand, additional power plants must often be brought online, usually expensive peaker plants. A 1995 meta-analysis of various utility studies concluded that the average air conditioner wasted 40% of the input energy.
This energy is lost in the form of heat, which must be pumped out. There is a huge opportunity to reduce the need for new
power plants and to conserve energy.
In an automobile, the A/C system will use around 5
horsepower (4 kW) of the engine's power.
"Freon" is a trade name for a family of haloalkane refrigerants manufactured by DuPont and other companies. These refrigerants were commonly used due to their superior stability and safety
properties. Unfortunately, evidence has accumulated that these chlorine-bearing refrigerants reach the upper atmosphere when they escape. Once the refrigerant reaches the stratosphere, UV radiation from the Sun cleaves the chlorine-carbon bond, yielding a chlorine radical. These chlorine atoms catalyze the breakdown of ozone into diatomic oxygen, depleting the ozone layer that shields the Earth's surface from strong UV radiation. Each chlorine radical remains active
as a catalyst unless it binds with another chlorine radical, forming a stable molecule and breaking the chain reaction. CFC refrigerants is common, but decreasing usage include R-11 and R-12. In light of these environmental concerns, beginning on November 14, 1994, the Environmental Protection
Agency has restricted the sale, possession and use of refrigerant to only licensed technicians, per Rules 608 and 609 of the
EPA rules and regulations;failure to comply may result in criminal and civil sanctions. Newer and more environmentally-safe
refrigerants such as HCFCs (R-22, used in most homes today) and HFCs (R-134a, used in most cars) have replaced most CFC use. HCFCs in turn are being phased out under the Montreal Protocol and replaced by hydrofluorocarbons (HFCs) such as R-410A, which lack chlorine. Carbon Dioxide (R-744) is being rapidly adopted as a refrigerant in Europe
and Japan with Volkswagen being one of the first automotive manufacturers to roll out the new systems. R-744 must use higher
compression to produce an equivalent cooling effect but has the advantage of being about 10% more efficient as compared to
R-134A. R-744 also has a co2 factor of 1
The History of Air Conditioning
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.
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
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. Wind catchers were widely used throughout the medieval Muslim world, where they were used for air conditioning in many cities.
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.
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.
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 CO2. As an alternative to conventional
refrigerants, natural alternatives like CO2 (R-744) have been proposed.
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.
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.
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
The engineering of physical and thermodynamic properties of gas-vapor mixtures
is named Psychrometrics.