Air conditioning

:Note: in the broadest sense, "air conditioning" can refer to any form of "heating, ventilation, and air-conditioning." This article is specifically about the use of refrigeration for this purpose.

Types of air conditioning

Refrigeration cycle

In the refrigeration cycle, a heat pump pumps heat from a lower temperature heat source into a higher temperature heat sink. Heat would naturally flow in the opposite direction. This is the most common type of air conditioning.

Related Topics:
Refrigeration cycle - Heat pump

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A refrigerator works in much the same way, as it pumps the heat out of the interior into the room in which it stands.

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This cycle takes advantage of the universal gas law PV = nRT, where P is pressure, V is volume, R is the universal gas constant, T is temperature, and n is the number of moles of gas (1 mole = 6.022 * 10^23 molecules. All quantities are in SI units).

Related Topics:
Universal gas law - Universal gas constant - SI

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The most common refrigeration cycle uses an electric motor to drive a compressor. In an automobile the compressor is driven by a pulley on the engine's crankshaft, with both using electric motors for air circulation. Since evaporation absorbs heat, and condensation releases it, air conditioners are designed to use a compressor to cause pressure changes between two compartments, and actively pump a refrigerant around. A refrigerant is pumped into the cooled compartment (the evaporator coil), where the low pressure and load temperature cause the refrigerant to evaporate into a vapour, taking heat with it. In the other compartment (the condenser), the refrigerant vapour is compressed and forced through another heat exchange coil, condensing into a liquid, rejecting the heat previously absorbed from the cooled space.

Related Topics:
Compressor - Pulley - Crankshaft - Evaporation - Heat - Condensation

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Humidity

Refrigeration air conditioning equipment usually reduces the humidity of the air processed by the system. The relatively cold (below the dewpoint) evaporator coil condenses water vapor from the processed air, (much like an ice cold drink will condense water on the outside of a glass), sending the water to a drain and removing water vapor from the cooled space and lowering the relative humidity. Since humans perspire to provide natural cooling by the evaporation of perspiration from the skin, drier air (up to a point) improves the comfort provided. The comfort air conditioner is designed to create a 40% to 60% relative humidity in the occupied space.

Related Topics:
Humidity - Dewpoint - Evaporation

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Refrigerants

"Freon" is a trade name for a family of flourocarbon refrigerants manufactured by DuPont and other companies. These refrigerants were commonly used due to their superior stability and safety properties. Unfortunately, evidence has accumulated that these chlorine bearing refrigerants reach the upper atmosphere when they escape. The chemistry is poorly understood but general consensus seems to be that CFCs break up in the stratosphere due to UV-radiation, releasing their chlorine atoms. These chlorine atoms act as catalysts in the breakdown of ozone, which does severe damage to the ozone layer that shields the Earth's surface from the strong UV radiation. The chlorine will remain active as a catalyst until and unless it binds with another particle forming a stable molecule. CFC refrigerants in common but receding usage include R-11 and R-12. Newer and more environmentally-safe refrigerants include HCFCs (R-22, used in most homes today) and HFCs (R-134a, used in most cars) have replaced most CFC use.

Related Topics:
Freon - DuPont - Stratosphere - UV - Catalyst - Ozone layer

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Evaporation coolers

The aforementioned Persian cooling systems were evaporation coolers. In very dry climates, such affectionately called "swamp coolers" are popular for improving comfort during hot weather. The evaporative cooler is a device that draws outside air through a wet pad. The sensible heat of the incoming air, as measured by a dry bulb thermometer, is reduced. The total heat (sensible heat plus latent heat) of the entering air is unchanged. Some of the sensible heat of the entering air is converted to latent heat by the evaporation of water in the wet cooler pads. If the entering air is dry enough, the results can be quite comfortable. These coolers cost less and are mechanically simple to understand and maintain.

Related Topics:
Swamp coolers - Weather - Sensible heat - Total heat - Latent heat

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An early type of cooler, using ice for a further effect, was patented by John Gorrie of Apalachicola, FL in 1842, who used the device to cool the patients of his malaria hospital.

Related Topics:
John Gorrie - Apalachicola

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A three-stage absorptive cooler exists that first dehumidifies the air with a spray of salt brine. The brine osmotically absorbs water vapor from the air. The second stage sprays water in the air, evaporatively cooling (via absorptive refrigeration) the air. Finally, to control the humidity, the air passes through another brine spray. The brine is reconcentrated by distillation. The system is used in some hospitals because, with filtering, a sufficiently hot regenerative distillation controls airborne organisms.

Related Topics:
Osmotically - Absorptive refrigeration

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Absorptive chillers

Some buildings use gas turbines to generate electricity. The exhausts of these are hot enough to drive an absorptive chiller that produces cold water. The cold water is then run through radiators in air ducts for hydronic cooling. The dual use of the energy, both to generate electricity and cooling, makes this technology attractive when regional utility and fuel prices are right. Producing heat, power, and cooling in one system is known as trigeneration.

Related Topics:
Absorptive chiller - Trigeneration

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