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Air
Conditioners
How Air Conditioners Work
The
first modern air conditioning system was developed in 1902 by a young
electrical engineer named Willis Haviland Carrier.
It
was designed to solve a humidity problem at the Sackett-Wilhelms Lithographing
and Publishing Company in Brooklyn, N.Y.
Paper
stock at the plant would sometimes absorb moisture from the warm summer air,
making it difficult to apply the layered inking techniques of the time.
Carrier
treated the air inside the building by blowing it across chilled pipes.
The
air cooled as it passed across the cold pipes, and since cool air can't carry
as much moisture as warm air, the process reduced the humidity in the plant and
stabilized the moisture content of the paper.
Reducing
the humidity also had the side benefit of lowering the air temperature -- and a
new technology was born.
Carrier
realized he'd developed something with far-reaching potential, and it wasn't
long before air-conditioning systems started popping up in theaters and
stores, making the long, hot summer months much more comfortable [source: Time].
The
actual process air conditioners use to reduce the ambient air temperature in a
room is based on a very simple scientific principle.
The
rest is achieved with the application of a few clever mechanical techniques.
Actually,
an air conditioner is very similar to another appliance in your home – the refrigerator.
Air
conditioners don't have the exterior housing a refrigerator relies on to
insulate its cold box. Instead, the walls in your home keep cold air in and hot
air out.
Let's
move on to the next page where we'll discover what happens to all that hot air
when you use your air conditioner.
Air-conditioning
Basics
Air
conditioners use refrigeration to chill indoor air, taking advantage of a
remarkable physical law: When a liquid converts to a gas (in a
process called phase conversion), it absorbs heat.
Air
conditioners exploit this feature of phase conversion by forcing special
chemical compounds to evaporate and condense over and over again in a closed
system of coils.
The
compounds involved are refrigerants that have properties
enabling them to change at relatively low temperatures.
Air
conditioners also contain fans that move warm interior air over these cold,
refrigerant-filled coils.
In
fact, central air conditioners have a whole system of ducts designed to funnel
air to and from these serpentine, air-chilling coils.
When
hot air flows over the cold, low-pressure evaporator coils, the
refrigerant inside absorbs heat as it changes from a liquid to a gaseous state.
To
keep cooling efficiently, the air conditioner has to convert the refrigerant
gas back to a liquid again.
To
do that, a compressor puts the gas under high pressure, a process that creates
unwanted heat.
All
the extra heat created by compressing the gas is then evacuated to the outdoors
with the help of a second set of coils called condenser coils, and
a second fan.
As
the gas cools, it changes back to a liquid, and the process starts all over
again.
Think
of it as an endless, elegant cycle: liquid refrigerant, phase conversion to a
gas/ heat absorption, compression and phase transition back to a liquid again.
It's
easy to see that there are two distinct things going on in an air conditioner.
Refrigerant
is chilling the indoor air, and the resulting gas is being continually
compressed and cooled for conversion back to a liquid again.
On
the next page, we'll look at how the different parts of an air conditioner work
to make all that possible.
COOL THE
GREEN WAY
The chemical composition of modern refrigerant
compounds has changed over the last few decades as a result of environmental
concerns and international treaty agreements like the Montreal Protocol.
Older
refrigerant formulas containing chlorine atoms that had the potential to damage
the ozone layer have slowly been phased out in favor of more environmentally
friendly coolants [source: EPA].
The Parts of an
Air Conditioner
Let's get some housekeeping topics out of the way
before we tackle the unique components that make up a standard air conditioner.
The
biggest job an air conditioner has to do is to cool the indoor air. That's not
all it does, though.
Air
conditioners monitor and regulate the air temperature via a thermostat.
They
also have an onboard filter that removes airborne particulates from the
circulating air. Air conditioners function as dehumidifiers.
Because
temperature is a key component of relative humidity, reducing the temperature
of a volume of humid air causes it to release a portion of its moisture.
That's
why there are drains and moisture-collecting pans near or attached to air
conditioners, and why air conditioners discharge water when they operate on
humid days.
Still,
the major parts of an air conditioner manage refrigerant and move air in two
directions: indoors and outside:
· Evaporator - Receives
the liquid refrigerant
· Condenser - Facilitates
heat transfer
· Expansion valve - regulates refrigerant flow into the evaporator
· Compressor - A
pump that pressurizes refrigerant
The
cold side of an air conditioner contains the evaporator and a fan that blows
air over the chilled coils and into the room.
The
hot side contains the compressor, condenser and another fan to vent hot air
coming off the compressed refrigerant to the outdoors.
In
between the two sets of coils, there's an expansion valve. It
regulates the amount of compressed liquid refrigerant moving into the
evaporator.
Once
in the evaporator, the refrigerant experiences a pressure drop, expands and
changes back into a gas.
The compressor is
actually a large electric pump that pressurizes the refrigerant gas as part of
the process of turning it back into a liquid.
There
are some additional sensors, timers and valves, but the evaporator, compressor,
condenser and expansion valve are the main components of an air conditioner.
Although
this is a conventional setup for an air conditioner, there are a couple of
variations you should know about.
Window
air conditioners have all these components mounted into a relatively small
metal box that installs into a window opening.
The
hot air vents from the back of the unit, while the condenser coils and a fan
cool and re-circulate indoor air.
Bigger
air conditioners work a little differently: Central air conditioners share a
control thermostat with a home's heating system, and the compressor and condenser,
the hot side of the unit, isn't even in the house.
It's
in a separate all-weather housing outdoors. In very large buildings, like
hotels and hospitals, the exterior condensing unit is often mounted somewhere
on the roof.
Window and
Split-system AC Units
A window air conditioner unit implements a complete
air conditioner in a small space.
The
units are made small enough to fit into a standard window frame. You close the
window down on the unit, plug it in and turn it on to get cool air.
If
you take the cover off of an unplugged window unit, you'll find that it
contains:
· A compressor
· An expansion valve
· A hot coil (on the outside)
· A chilled coil (on the inside)
· Two fans
· A control unit
The
fans blow air over the coils to improve their ability to dissipate heat (to the
outside air) and cold (to the room being cooled).
When
you get into larger air-conditioning applications, its time to start looking at
split-system units.
A
split-system air conditioner splits the hot side from the cold side of the
system, as in the diagram below.
The
cold side, consisting of the expansion valve and the cold coil, is generally
placed into a furnace or some other air handler.
The
air handler blows air through the coil and routes the air throughout the
building using a series of ducts. The hot side, known as the condensing unit,
lives outside the building.
The
unit consists of a long, spiral coil shaped like a cylinder. Inside the coil is
a fan, to blow air through the coil, along with a weather-resistant compressor
and some control logic.
This
approach has evolved over the years because it's low-cost, and also because it
normally results in reduced noise inside the house (at the expense of increased
noise outside the house).
Other
than the fact that the hot and cold sides are split apart and the capacity is
higher (making the coils and compressor larger), there's no difference between
a split-system and a window air conditioner.
In
warehouses, large business offices, malls, big department stores and other
sizeable buildings, the condensing unit normally lives on the roof and can be
quite massive.
Alternatively,
there may be many smaller units on the roof, each attached inside to a small
air handler that cools a specific zone in the building.
In
larger buildings and particularly in multi-story buildings, the split-system
approach begins to run into problems.
Either
running the pipe between the condenser and the air handler exceeds distance
limitations (runs that are too long start to cause lubrication difficulties in
the compressor), or the amount of duct work and the length of ducts becomes
unmanageable.
At
this point, it's time to think about a chilled-water system.
Chilled-water
and Cooling-tower AC Units
Although standard air conditioners are very popular,
they can use a lot of energy and generate quite a bit of heat.
For
large installations like office buildings, air handling and conditioning is
sometimes managed a little differently.
Some
systems use water as part of the cooling process. The two most well-known
are chilled water systems and cooling tower air conditioners.
· Chilled water systems - In a chilled-water system, the entire air
conditioner is installed on the roof or behind the building. It cools water to
between 40 and 45 degrees Fahrenheit (4.4 and 7.2 degrees Celsius).
The chilled water is then piped throughout the
building and connected to air handlers. This can be a versatile system where
the water pipes work like the evaporator coils in a standard air conditioner.
If it's well-insulated, there's no practical distance
limitation to the length of a chilled-water pipe.
· Cooling tower technology - In all of the air conditioning systems we've
described so far, air is used to dissipate heat from the compressor coils.
In some large systems, a cooling tower is used
instead. The tower creates a stream of cold water that runs through a heat
exchanger, cooling the hot condenser coils.
The tower blows air through a stream of water causing
some of it to evaporate, and the evaporation cools the water stream.
One of the disadvantages of this type of system is
that water has to be added regularly to make up for liquid lost through
evaporation.
The actual amount of cooling that an air conditioning
system gets from a cooling tower depends on the relative humidity of the air
and the barometric pressure.
Because
of rising electrical costs and environmental concerns, some other air
cooling methods are being explored, too. One is off-peak or ice-cooling
technology.
An off-peak cooling
system uses ice frozen during the evening hours to chill interior air during
the hottest part of the day.
Although
the system does use energy, the largest energy drain is when community demand
for power is at its lowest.
Energy
is less expensive during off-peak hours, and the lowered consumption during
peak times eases the demand on the power grid.
Another
option is geo-thermal heating. It varies, but at around 6 feet (1.8 meters)
underground, the earth's temperature ranges from 45 to 75 degrees Fahrenheit
(7.2 to 23.8 degrees Celsius).
The
basic idea behind geo-thermal cooling is to use this constant
temperature as a heat or cold source instead of using electricity to generate
heat or cold.
The
most common type of geo-thermal unit for the home is a closed-loop system.
Polyethylene pipes filled with a liquid mixture are buried underground.
During
the winter, the fluid collects heat from the earth and carries it through the
system and into the building.
During
the summer, the system reverses itself to cool the building by pulling heat
through the pipes to deposit it underground [source: Geo Heating].
For
real energy efficiency, solar powered air conditioners are also making their
debut.
There
may still be some kinks to work out, but around 5 percent of all electricity
consumed in the U.S. is used to power air conditioning of one type or another,
so there's a big market for energy-friendly air conditioning options [source: ACEEE].
BTU
and EER
Most air conditioners have their capacity rated in
British thermal units (Btu).
A
Btu is the amount of heat necessary to raise the temperature of 1 pound (0.45
kilograms) of water one degree Fahrenheit (0.56 degrees Celsius).
One
Btu equals 1,055 joules. In heating and cooling terms, one ton equals 12,000
Btu.
A
typical window air conditioner might be rated at 10,000 Btu.
For
comparison, a typical 2,000-square-foot (185.8 square meters) house might have
a 5-ton (60,000-Btu) air conditioning system, implying that you might need
perhaps 30 Btu per square foot.
These
are rough estimates. To size an air conditioner accurately for your specific
application, you should contact an HVAC contractor.
The
energy efficiency rating (EER) of an air conditioner is its Btu rating over
its wattage.
As
an example, if a 10,000-Btu air conditioner consumes 1,200 watts, its EER is
8.3 (10,000 Btu/1,200 watts).
Obviously,
you would like the EER to be as high as possible, but normally a higher EER is
accompanied by a higher price.
Let's
say you have a choice between two 10,000-Btu units. One has an EER of 8.3 and
consumes 1,200 watts, and the other has an EER of 10 and consumes 1,000 watts.
Let's
also say that the price difference is $100. To determine the payback period on
the more expensive unit, you need to know approximately how many hours per year
you will be operating the air conditioner and how much a kilowatt-hour (kWh)
costs in your area.
Assuming
you plan to use the air conditioner six hours a day for four months of the
year, at a cost of $0.10/kWh.
The
difference in energy consumption between the two units is 200 watts.
This
means that every five hours the less expensive unit will consume one additional
kWh (or $0.10) more than the more expensive unit.
Let's
do the math: With roughly 30 days in a month, you're operating the air
conditioner:
4 months x 30 days per month x 6 hours per day = 720
hours
[(720 hours x 200 watts) / (1000 watts/kilowatt)] x
$0.10/kilowatt hours = $14.40
The
more expensive air conditioning unit costs $100 more to purchase but less money
to operate. In our example, it'll take seven years for the higher priced unit
to break even.
THE HEAT
BEHIND HUMIDITY
Humans use perspiration to stay cool, and a relative
humidity of around 45 percent is just about perfect to sweat.
Very
humid conditions are so uncomfortable because the air becomes saturated with
moisture, and all that nice, cooling sweat can't evaporate. It has no place to
go. Just think of that shiny glow as your body's personal AC -- when it isn't
too humid out [source: HSW].
Energy
Efficient Cooling Systems
Because of the rising costs of electricity and a
growing trend to "go green," more people are turning to alternative
cooling methods to spare their pocketbooks and the environment.
Big
businesses are even jumping on board in an effort to improve their public image
and lower their overhead.
Ice
cooling systems are one way that businesses are combating high electricity
costs during the summer.
Ice
cooling is as simple as it sounds. Large tanks of water freeze into ice at
night, when energy demands are lower.
The
next day, a system much like a conventional air conditioner pumps the cool air
from the ice into the building.
Ice
cooling saves money, cuts pollution, eases the strain on the power grid and can
be used alongside traditional systems.
The
downside of ice cooling is that the systems are expensive to install and
require a lot of space. Even with the high startup costs, more than 3,000
systems are in use worldwide [source: CNN].
An
ice cooling system is a great way to save money and conserve energy, but its
price tag and space requirements limit it to large buildings.
One
way that homeowners can save on energy costs is by installing geo-thermal
heating and cooling systems, also known as ground source heat pumps (GSHP).
The
Environmental Protection Agency recently named geo-thermal units "the most
energy-efficient and environmentally sensitive of all space conditioning
systems" [source: EPA].
Although
it varies, at six feet underground the Earth's temperatures range from 45 to 75
degrees Fahrenheit.
The
basic principle behind geo-thermal cooling is to use this constant temperature
as a heat source instead of generating heat with electricity.
The
most common type of geo-thermal unit for homes is the closed-loop system.
Polyethylene
pipes are buried under the ground, either vertically like a well or
horizontally in three- to six-foot trenches. They can also be buried under
ponds. Water or an anti-freeze/water mixture is pumped through the pipes.
During
the winter, the fluid collects heat from the earth and carries it through the
system and into the building.
During
the summer, the system reverses itself to cool the building by pulling heat
from the building, carrying it through the system and placing it in the ground [source: Geo
Heating].
Homeowners
can save 30 to 50 percent on their cooling bills by replacing their traditional
HVAC systems with ground source heat pumps.
The
initial costs can be up to 30 percent more, but that money can be recouped in
three to five years, and most states offer financial purchase incentives.
Another benefit is that the system lasts longer than traditional units because
it's protected from the elements and immune to theft [source: Geo
Exchange].
PASSIVE
COOLING
Some people go to the extreme and get rid of their AC
units entirely.
Passive
cooling is the greenest of trends and a great way to save money.
Passive
cooling revolves around the concept of removing warm air from your home using
the interaction between the house and its surroundings.
There
are several ways to block and remove heat, including shading through
landscaping, using a dark exterior paint, installing a radiant barrier in the
roof rafters and good old- fashioned insulation.
Another
way is through thermal siphoning, the process of removing heat through
controlled airflow.
Opening
the lower windows on the breezy side of your house and the upper windows on the
opposite side creates a vacuum that draws out the hot air. Ceiling fans and
roof vents are other ways to direct heat out at low cost [source: Earth Easy].
Marshall Brain, Founder
Marshall Brain is the founder of HowStuffWorks. He holds a bachelor's degree in electrical engineering from Rensselaer Polytechnic Institute and a master's degree in computer science from North Carolina State University. Before founding HowStuffWorks, Marshall taught in the computer science department at NCSU and ran a software training and consulting company. Learn more at his site.
Marshall Brain is the founder of HowStuffWorks. He holds a bachelor's degree in electrical engineering from Rensselaer Polytechnic Institute and a master's degree in computer science from North Carolina State University. Before founding HowStuffWorks, Marshall taught in the computer science department at NCSU and ran a software training and consulting company. Learn more at his site.
Charles W.(Chuck) Bryant co-hosts the 'Stuff You Should Know' podcast along with his trusty sidekick, Josh Clark. He was
born in Atlanta in the early 1970s under the sign of Pisces. Twenty-four years
later, he earned an English degree at the University of Georgia. He spent the
next decade traveling, pursuing creative endeavors and getting street smart. He
and his wife-to-be moved back to Atlanta in 2004, with four pets in tow. He
hooked up with HowStuffWorks.comshortly after co-host Josh was hired,
and the pair bonded immediately over their love of Hunter S. Thompson, the
fight-or-flight response and dive bars. In his off-time, Chuck enjoys hanging
out with his wife, cooking and playing in his old-man band. He loves his neti
pot and hates cold bathroom floors. You can find Chuck on Twitter at @SYSKPodcast and on
Facebook at the official Stuff You Should Know page.
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