2. Introduction
• Air conditioning has fundamentally changed how
people experience the world. When it's hot
outside, walking into an air-conditioned house is
like walking into another season. Few pieces of
technology have had such a striking effect on
people's daily lives.
• The first modern air conditioning system was
developed in 1902 by a young electrical engineer
named Willis Haviland Carrier.
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3. Principle
• 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.
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5. Parts
• Evaporator - Receives the liquid refrigerant
• Condenser - Facilitates heat transfer
• Expansion valve - regulates refrigerant flow
into the evaporator
• Compressor - A pump that pressurizes
refrigerant
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7. Working…
• The cold side 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.
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8. Working
• 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.
EGEE 102 - Pisupati
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9. TYPES OF AIR CONDITIONERS
•
•
•
•
Room air conditioners
Central air conditioning systems
Heat pumps
Evaporative coolers
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11. Room air conditioner
• Room air conditioners cool rooms rather than the
entire home.
• Less expensive to operate than central units
• Their efficiency is generally lower than that of central
air conditioners.
• Can be plugged into any 15- or 20-amp, 115-volt
household circuit that is not shared with any other
major appliances
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13. Central Air conditioning
• Circulate cool air through a system of supply and
return ducts. Supply ducts and registers (i.e.,
openings in the walls, floors, or ceilings covered by
grills) carry cooled air from the air conditioner to the
home.
• This cool air becomes warmer as it circulates through
the home; then it flows back to the central air
conditioner through return ducts and registers
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14. Types of Central AC
• Split-system
– an outdoor metal cabinet contains the condenser
and compressor, and an indoor cabinet contains
the evaporator
• Packaged
– the evaporator, condenser, and compressor are all
located in one cabinet
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16. Large air conditioning systems
•
Outside air is drawn in, filtered
and heated before it passes
through the main air
conditioning devices. The
colored lines in the lower part
of the diagram show the
changes of temperature and of
water vapor concentration as
the air flows through the
system.
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18. Heat Pumps
• Variable fresh air mixer and dust and pollutant
filtration.
• Supplementary heating with radiators in the
outer rooms and individual mini heater and
• Humidifier in the air stream to each room.
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19. Sizing Air Conditioners
• How large your home is and how many windows it
has
• How much shade is on your home's windows, walls,
and roof
• How much insulation is in your home's ceiling and
walls
• How much air leaks into your home from the outside
and
• How much heat the occupants and appliances in
your home generate
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20. Energy Consumption
• Air conditioners are rated by the number of British
Thermal Units (Btu) of heat they can remove per
hour. Another common rating term for air
conditioning size is the "ton," which is 12,000 Btu per
hour.
• Room air conditioners range from 5,500 Btu per hour
to 14,000 Btu per hour.
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21. Energy Efficiency
• Today's best air conditioners use 30% to 50% less
energy than 1970s
• Even if your air conditioner is only 10 years old, you
may save 20% to 40% of your cooling energy costs by
replacing it with a newer, more efficient model
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22. Energy Efficiency
• Rating is based on how many Btu per hour are
removed for each watt of power it draws
• For room air conditioners, this efficiency rating is the
Energy Efficiency Ratio, or EER
• For central air conditioners, it is the Seasonal Energy
Efficiency Ratio, or SEER
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23. Room Air Conditioners
• Built after January 1, 1990, need have an EER
of 8.0 or greater
– EER of at least 9.0 if you live in a mild climate
– EER over 10 for warmer climates
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24. Central AC
• National minimum standards for central air
conditioners require a SEER of
– 9.7 for single-package and
– 10.0 for split-systems
– Units are available with SEERs reaching nearly 17
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25. Energy Saving Methods
• Locate the air conditioner in a window or wall area
near the center of the room and on the shadiest side
of the house.
• Minimize air leakage by fitting the room air
conditioner snugly into its opening and sealing gaps
with a foam weather stripping material.
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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.
Room air conditioners use the standard compressor cycle and are sized to cool just one room. To cool an entire house, several room units are necessary. Central air conditioning systems also operate on the compressor cycle principle and are designed to cool the entire house. The cooled air is distributed throughout the house using air ducts, which may be the same ducts that are used by the heating system.
Heat pumps, described in the heating section, use the compressor cycle, but it is reversible. In the summer, the heat pump transfers heat from indoors to outdoors. In the winter, the heat pump transfers heat from outdoors to indoors. Heat pumps may be powered by electricity or natural gas.
Evaporative coolers, also called "swamp coolers", do not use the compressor cycle. Instead, they cool air by blowing it over a wet surface. You have experienced this phenomenon when you get out of a swimming pool while a breeze is blowing. As water evaporates, it absorbs heat from the air. Evaporative cooling systems depend on the ability of air to absorb moisture, and so they only work in dry climates such as the Southwest U.S.
That is all there is to the part of the system in the room, which is sketched on the left in figure 2. The bit that is more difficult to understand, or at least unfamiliar to most people, is how the cooling fluid is produced and controlled. That is the part on the right of the diagram.
The cooling fluid used to be a chlorofluorocarbon compound, and often still is, though they all more or less ravage the earth's ozone layer. The essential characteristics of these fluids is that they have quite a low boiling point at atmospheric pressure and that they can stay in the pipes for a long time without decomposing either themselves or the pipes. Finally they need to have some lubricating ability, or the ability to carry a lubricant, because the fluid has to be compressed and pumped round the system. This rare set of necessary properties has proved difficult to combine with friendliness to the earth's atmosphere.
The liquid is let into the cooling unit through a valve marked B on the diagram. It evaporates while it passes through the pipe, taking heat from the air just as water evaporating from a towel laid on your fevered brow cools you when on holiday in the Mediterranean. The temperature in the cooling coil depends partly on the amount of fluid let in by the valve, which is controlled by the thermostat or the humidistat. But now comes a crucial difference from your Mediterranean experience: the minimum temperature at the cold surface can be fixed by controlling the pressure in the cooling coil, with the valve marked A on the diagram. The boiling point of any liquid depends on the pressure. One could use water in the cooling coil, if the pressure is kept low enough. At 1000 Pa pressure, which seems a lot but is just 1% of atmospheric pressure, water boils at 7 degrees. It isn't used in cooling coils of this evaporative type because it has practical disadvantages.
The reason for wanting to limit the minimum temperature is to stop ice clogging the air passage. There are clever systems which notice when ice has formed and hold a melting pause, but that adds to the cost. The pressure controller is therefore set to make the cooling fluid boil at the lowest temperature that is likely to be needed to control the humidity, but always over zero degrees. The temperature needed for cooling is nearly always higher than that needed for dehumidification so it is the RH setting that is decisive.
This brings me to the first point that conservators need to understand: it is expensive to produce air at a dew point below about 4 degrees in this type of equipment. This dewpoint corresponds to 50% RH at 15°C. This sort of air conditioning is entirely suitable for keeping people comfortable but it is not good for specialised stores, for films or for furs, for example, where one needs a temperature below 15 degrees. Such equipment is, however, often used for such places. A better solution is to use an absorption dehumidifier, which will be described in a later article.
Now back to the main story: The vapour that emerges through the pressure controller is gathered up by a compressor. The compression also heats the gas, as will be understood by anyone who has pumped up a cycle tyre. The hot gas is then led away from the room, to be cooled down. This is often done on the roof or in a small enclosure which vibrates to the roar of the fan blowing air over the fins of a condenser. The cooled, now liquid coolant is piped back to the reservoir, ready for its next tour through the room air conditioner.
The entire process described above is inefficient and uses electricity, which is itself produced by inefficient conversion of heat energy. Such systems are therefore confined to small places where the inefficiency is compensated by the generally high reliability and freedom from maintenance.
In a split-system central air conditioner, an outdoor metal cabinet contains the condenser and compressor, and an indoor cabinet contains the evaporator. In many split-system air conditioners, this indoor cabinet also contains a furnace or the indoor part of a heat pump. The air conditioner's evaporator coil is installed in the cabinet or main supply duct of this furnace or heat pump. If your home already has a furnace but no air conditioner, a split-system is the most economical central air conditioner to install.
In a packaged central air conditioner, the evaporator, condenser, and compressor are all located in one cabinet, which usually is placed on a roof or on a concrete slab next to the house's foundation. This type of air conditioner also is used in small commercial buildings. Air supply and return ducts come from indoors through the home's exterior wall or roof to connect with the packaged air conditioner, which is usually located outdoors. Packaged air conditioners often include electric heating coils or a natural gas furnace. This combination of air conditioner and central heater eliminates the need for a separate furnace indoors.
The principle of operation is the same as that of the small system described above except that the cooling fluid is usually water, which has itself been cooled by the refrigeration system described above. The air is circulated through ducts, with a portion of fresh air added. There is therefore a pre-heater, because the outside air may be below zero and will therefore freeze the water in the cooling coil. A humidifier and various filters have also been added in figure
Some refinements to the basic system to compensate for the different heat requirements of different rooms in the building. Figure shows a complete system, with two details that have not been mentioned yet: the outer zone of the building, which loses more heat in winter, has radiators to supplement the heat supply through the air conditioning. The inner zone has, in this example, an archive room that is not much used and so is cooler, and drier, than the rooms with people, computers and coffee machines. To keep the climate uniform throughout the building there is a little local heater and humidifier placed just before the air reaches the room. The main air supply is kept a little too cold and a little too dry. Any one of these local humidifiers can give trouble, with rapid over-humidification of the room. Again, here is a dangerous detail that is provided by the engineer to protect himself against complaints that the equipment does not achieve the standard required.
