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Function of an Air conditioner

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Humidity in air

Relative Humidity

A measure of of much water is in the air relative to the maximum amount air can hol at that tmperature

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http://www.ae.iastate.edu/Ast473/Lectures/%285%29Psychrometric_Chart/sld024.htm

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Principle

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Expansion Valve
Compressor

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Arrangement

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TYPES OF AIR CONDITIONERS

Room air conditioners
Central air conditioning systems
Heat pumps
Evaporative coolers

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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.

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Air Conditioning

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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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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.

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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 cooled 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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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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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.

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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 (not RH) as the air flows through the system.

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Total Air Conditioning

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Some refinements to the basic system compensate for the different heat requirements of different rooms in the building. Figure 4 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.

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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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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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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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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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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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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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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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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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Numerical Problem

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