Air Conditioning

Air Conditioning

An air conditioner (often abbreviated to AC in the United States, and air-con in Australia and in Great Britain) is an appliance or mechanism designed to extract heat from an area using a refrigeration cycle. To many people, air conditioners are symbolic of the Western world. In construction, a complete system of heating, ventilation and air conditioning is referred to as HVAC.

An earlier form of air conditioning was invented in Persia (Iran) thousands of years ago in the form of wind shafts on the roof, which caught the wind and passed it through water and blew the cooled air into the building

The 19th century British inventor, Michael Faraday discovered that compressing and liquefying a certain gas could chill air when the liquified gas was allowed to evaporate. His idea remained largely theoretical.

One of the first uses of air conditioning for personal comfort was in 1902 when the New York Stock Exchange’s new building was equipped with a central cooling as well as heating system. Alfred Wolff, an engineer from Hoboken, New Jersey who is considered the forerunner in the quest to cool a working environment, helped design the new system, transferring this budding technology from textile mills to commercial buildings.

Later in 1902, the first modern, electrical air conditioning was invented by Willis Haviland Carrier (1876–1950). His invention differed from Wolff’s in that it controlled not only temperature, but also humidity for improved manufacturing process control for a printing plant in Brooklyn, New York. This specifically helped to provide low heat and humidity for consistent paper dimensions and ink alignment. Later, Carrier’s technology was applied to increase productivity in the workplace, and the Carrier Engineering Company, now called Carrier (a division of United Technologies Corporation), was formed in 1915 to meet the new demand. Later still, air conditioning use was expanded to improve comfort in homes and automobiles. Residential sales didn’t take off until the 1950’s.

In 1906, Stuart Cramer first used the term “air conditioning” as he explored ways to add moisture to the air in his southern textile mill. He combined moisture with ventilation to actually “condition” and change the air in the factories, controlling the humidity so necessary in textile plants.

The first air conditioners and refrigerators employed toxic gases like ammonia and methyl chloride, which resulted in fatal accidents when they leaked. Thomas Midgley, Jr. created the first chlorofluorocarbon gas, dubbed Freon in 1928. The refrigerant proved much safer for humans but not for the atmosphere’s ozone layer. “Freon” is a trade name of Dupont for any CFC, HCFC, or HFC refrigerant, the name of each including a number indicating molecular composition (R-11, R-12, R-22, R-134). The blend most used in direct-expansion comfort cooling is an HCFC known as R-22, and is slated to be phased out for use in new equipment by 2010 and completely discontinued by 2020. R-11 and R-12 are no longer manufactured in the US, the only source for purchase being the cleaned and purified gas recovered from other air conditioner systems.

Types of air conditioning

Refrigeration cycle

Refrigeration Cycle

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.

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

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).

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.

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.

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.

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.

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.

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.

Absorptive chillers

Absorptive chillers

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.

Power

Air conditioner equipment power in the U.S. is often described in terms of “tons of refrigeration”. A “ton of refrigeration” is defined as the cooling power of one ton US (2000 pounds or 907 kilograms) of ice melting in a 24-hour period. This is equal to 12,000 BTU per hour, or 3510 watts (http://physics.nist.gov/Pubs/SP811/appenB9.html). Residential “central air” systems are usually from 1 to 5 tons (3 to 20 kW) in capacity.

The use of electric/compressive air conditioning puts a major demand on the nation’s electrical power grid in warm weather, when most units are operating under heavy load. In the aftermath of the 2003 North America blackout locals were asked to keep their air conditioning off. During peak demand, additional power plants must often be brought online, usually natural gas fired plants because of their rapid startup. A 1995 study of various utility studies of residential air conditioning concluded that the average air conditioner wasted 40% of the input energy. This energy is lost, ironically, in the form of heat, which must be pumped out. There is a huge opportunity to reduce the need for new power plants and to conserve energy.

In an automobile the A/C system will use around 5 hp (4 kW) of the engine’s power.

Insulation

Insulation reduces the required power of the air conditioning system. Thick walls, reflective roofing material, curtains and trees next to building also cut down on system and energy requirements.

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