Air conditioning

An air conditioner (often abbreviated to A/C in the United States and Canada, and air-con in Britain and Japan) is an appliance, system or mechanism designed to extract heat from an area using a refrigeration cycle. In construction, a complete system of heating, ventilation and air conditioning is referred to as HVAC.

Air conditioning engineers broadly divide air conditioning applications into comfort and process.

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Comfort applications aim to provide an indoor environment that remains relatively constant in a range preferred by humans despite changes in external weather conditions or in internal heat loads. Some have claimed that comfort air conditioning increases worker productivity but this claim is disputed, one counter argument being that apparent increases in productivity can be explained as resulting from workers perceiving that their employer shows an interest in their welfare. (See Hawthorne effect). What is certain is that comfort air conditioning makes deep plan buildings feasible. Without air conditioning, buildings must be built narrower or with light wells so that inner spaces receive sufficient fresh air. Air conditioning also allows building to be taller since wind speed increases significantly with altitude making natural ventilation impractical for very tall buildings.

Process applications aim to provide an indoor environment suit a process being carried out that remains relatively constant despite changes in external weather conditions or in internal heat loads. Although often in the comfort range it is the process that determines conditions not human preference. Process applications include:

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Hospital operating theatres in which air is filtered to high levels to reduce infection risk and the humidity controlled to limit patient dehydration. Although temperatures are often in the comfort range, some specialist procedures such as open heart surgery require low temperatures (about 18°C, 64°F) and others such as neonatal relatively high temperatures (about 28°C, 82°F).
Clean rooms for the production of integrated circuits, pharmaceuticals and the like in which very high levels of air cleanliness and control of temperature and humidity are required for the success of the process.
Facilities for breeding laboratory animals. Since many animals normally only reproduce in spring, holding them in rooms at which conditions mirror spring all year can cause them to reproduce year round.
Aircraft air conditioning. Although nominally aimed at providing comfort for passengers and cooling of equipment, aircraft air conditioning presents a special process because of the low air pressure outside the aircraft.
In both comfort and process applications not only is the objective to control temperature (although in some comfort applications this is all that is controlled) but other factors including humidity, air movement and air quality.

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The efficiency of air conditioners are usually rated by the SEER (Seasonal Energy Efficiency Ratio) rating system. The SEER rating is calculated by dividing the total number of BTUs of heat removed from the air by the total amount of energy required by the air conditioner in watt-hours. The higher the ratio, the more energy efficient the air conditioner.

Looking at it on a power basis, the SEER ratio relates the cooling power of the air conditioner (in BTU per hour) to the electrical power consumption (in watts). For example if an air conditioner has a cooling power of 5000 BTU/hour, and an SEER rating of 10, then on average it will consume 500 watts of electric power (5000 divided by 10).

Today, it is rare to see systems rated below SEER 9 in the United States, as people there are increasingly using higher efficiency units. In addition some countries set minimum values for energy efficiency, although not always in terms of SEER. (For example, the United States [4] requires new systems to have a minimum SEER rating of 13.) Substantial energy savings can be obtained from more efficient systems. For example by upgrading from SEER 9 to SEER 13, the power consumption is reduced by over 30% (equal to 1 - 9/13). It is claimed that this can result in an energy cost saving of $US 300 per year, depending on energy tariffs, usage and climate. A common misconception held by people is that the SEER rating system also applies to the efficiency of heating systems. This however is not true, as SEER ratings only apply to air conditioning.

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Air conditioners (for cooling) and heat pumps (for heating) both work similarly in that heat is transferred or "pumped" from a cooler "heat-source" to a warmer "heat-sink". (Note: This process requires energy input according to the second law of thermodynamics)

Air conditioners and heat pumps usually operate most effectively at temperatures around 50-55 degrees Fahrenheit. Typically when the heat source temperature falls below 40 deg F, the system begins to reach a point called the "balance point", where the system is not able to "pull" any more heat out of the heat-source. Similarly, when the heat-sink temperature rises, the system will also operate less effectively, and the system will not be able to "push" out any more heat typically when the heat-sink temperature reaches about 120 deg F.

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