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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)
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. )
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.
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:
1.Approximately how many hours per year
you will be operating the unit2.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:
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.
Split-system Units
A split-system air conditioner splits the hot
side from the cold side of the system 。
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 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 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