When the temperature outside begins to climb, many people
seek the cool comfort of indoor air conditioning. Like water towers
and power
lines, air conditioners are one of those things that we
see every day but seldom pay much attention to.
Air conditioners come in various sizes, cooling capacities
and prices. One type that we see all the time is the window
air conditioner.
Window air conditioners are an easy and
economical way to cool a small
area.
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Most people who live in suburban areas usually have one of
these in their backyard:
If you live in an apartment complex, this is probably a
familiar sight:
Most businesses and office buildings have condensing units
on their roofs, and as you fly into any airport
you notice that warehouses and malls may have 10 or 20
condensing units hidden on their roofs:
And then if you go around back at many hospitals,
universities and office complexes, you find large cooling
towers that are connected to the air conditioning system:
Wouldn't it be nice to know what all of this equipment is
doing? In this edition of HowStuffWorks,
we will examine air conditioners -- from small to huge -- so
you know more about what you're seeing!
The Basic Idea
An air conditioner is
basically a refrigerator
without the insulated box. It uses the evaporation of a
refrigerant, like Freon, to provide cooling. The
mechanics of the Freon evaporation cycle are the same
in a refrigerator as in an air conditioner. According to the
Merriam-Webster
Dictionary Online, the term Freon is generically "used for
any of various nonflammable fluorocarbons used as refrigerants
and as propellants for aerosols."
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Diagram of a typical air
conditioner |
This is how the evaporation cycle in an air conditioner
works (See How
Refrigerators Work for complete details on this cycle):
- The compressor compresses cool Freon gas, causing
it to become hot, high-pressure Freon gas (red in the
diagram above).
- This hot gas runs through a set of coils so it can
dissipate its heat, and it condenses into a liquid.
- The Freon liquid runs through an expansion valve, and in
the process it evaporates to become cold, low-pressure
Freon gas (light blue in the diagram above).
- This cold gas runs through a set of coils that allow the
gas to absorb heat and cool down the air inside the
building.
Mixed in with the Freon is a small amount of a lightweight
oil.
This oil lubricates the compressor.
Window 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 the unit in and turn it on to get cool air.
If you take the cover off of an unplugged window unit, you
will 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).
BTU and EER
Most air conditioners have their
capacity rated in British thermal units (BTU).
Generally speaking, a BTU is the amount of heat required to
raise the temperature of one pound (0.45 kg) of water 1 degree
Fahrenheit (0.56 degrees Celsius). Specifically, 1 BTU equals
1,055 joules. In heating and cooling terms, 1 "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
m2) house might have a 5-ton
(60,000-BTU) air conditioning system, implying that you might
need perhaps 30 BTU per square foot. (Keep in mind that these
are rough estimates. To size an air conditioner for your
specific needs, contact an HVAC contractor.)
The energy efficiency rating (EER) of an air
conditioner is its BTU rating over its wattage. For 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.
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Is the higher EER is worth it?
Let's say that 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 understand what
the payback period is on the more expensive unit, you
need to know:
- Approximately how many hours per year you will be
operating the unit
- How much a kilowatt-hour (kWh) costs in your area
Let's say that you plan to use the air
conditioner in the summer (four months a year)
and it will be operating about six hours a day.
Let's also imagine that the cost in your area is
$0.10/kWh. The difference in energy consumption
between the two units is 200 watts, which means that
every five hours the less expensive unit will consume 1
additional kWh (and therefore $0.10 more) than the more
expensive unit.
Assuming that there are 30 days in a month, you find
that during the summer you are operating the air
conditioner:
4 mo. x 30 days/mo. x 6 hr/day = 720 hours
(720 hrs x 200 watts/hr) / (1000 watts/kW x
$0.10/kWh) = $14.40
Since the more expensive unit costs $100 more, that
means that it will take about seven years for the more
expensive unit to break even.
See this
page for a great explanation of seasonal energy
efficiency rating (SEER).
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Split-system Units
A split-system air
conditioner splits the hot side from the cold side of the
system, like this:
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. In most
home installations, the unit looks something like this:
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 is low-cost, and also
because it normally results in reduced noise inside the house
(at the expense of increased noise outside the house). Besides
the fact that the hot and cold sides are split apart and the
capacity is higher (making the coils and compressor larger),
there is no difference between a split-system and a window air
conditioner.
In warehouses, businesses, malls, large department stores,
etc., 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.
Let's take a look now at a chilled-water air conditioner.
Chilled-water System
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 is time to think about a chilled-water system.
In a chilled-water system, the entire air conditioner lives
on the roof or behind the building. It cools water to between
40 and 45 F (4.4 and 7.2 C). This chilled water is then piped
throughout the building and connected to air handlers as
needed. There is no practical limit to the length of a
chilled-water pipe if it is well-insulated.
You can see in this diagram that the air conditioner (on
the left) is completely standard. The heat exchanger lets the
cold Freon chill the water that runs throughout the building.
Cooling Towers
In all of the systems
described above, air is used to dissipate the heat from the
outside coil. In large systems, the efficiency can be improved
significantly by using a cooling tower. The cooling
tower creates a stream of lower-temperature water. This water
runs through a heat exchanger and cools the hot coils of the
air conditioner unit. It costs more to buy the system
initially, but the energy savings can be significant over time
(especially in areas with low humidity), so the system pays
for itself fairly quickly.
Cooling
towers come in all shapes and sizes. They all work on the same
principle:
- A cooling tower blows air through a stream of water so
that some of the water evaporates.
- Generally, the water trickles through a thick sheet of
open plastic mesh.
- Air blows through the mesh at right angles to the water
flow.
- The evaporation cools the stream of water.
- Because some of the water is lost to evaporation, the
cooling tower constantly adds water to the system to make up
the difference.
The amount of cooling that you get from a cooling tower
depends on the relative
humidity of the air and the barometric
pressure.
For example, assuming a 95 F (35 C) day, barometric
pressure of 29.92 inches (sea-level normal pressure) and
80-percent humidity, the temperature of the water in the
cooling tower will drop about 6 degrees to 89 F (3.36 degrees
to 31.7 C).
If the humidity is 50 percent, then the water temperature
will drop perhaps 15 degrees to 80 F (8.4 degrees to 26.7 C).
If the humidity is 20 percent, then the water temperature
will drop about 28 degrees to 67 F (15.7 degrees to 19.4 C).
Even small temperature drops can have a significant effect on
energy consumption.
These links can help you to understand how the relative
humidity and atmospheric pressure control the temperature drop
in a cooling tower on any given day:
Whenever you walk behind a building and find a unit that
has large quantities of water running through a plastic mesh,
you will know you have found a cooling tower!
In many office complexes and college campuses, cooling
towers and air conditioning equipment are centralized, and
chilled water is routed to all of the buildings through miles
of underground pipes. This
article shows the centralized approach on a typical
college campus.
For more information, check out the links on the next page.
Lots More Information!
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