Free Quotation SurveyAir conditioning
Air conditioning can refer to any form of cooling, heating, ventilation or disinfection that modifies the condition of the air. Air conditioning is the most common form of thermal comfort which is done via a refrigeration method.
An air conditioner is an appliance, system or mechanism used for extracting heat from a particular area and using a refrigeration cycle cooling the air to a comfortable atmosphere.
The most common uses for air conditioning is within the work environment and on some of the newer vehicle models.
The following addresses air conditioning in a more general over view.
History
Air conditioning is a fairly modern invention, were as the cooling of buildings is an ancient idea dating back to the era of ancient Egyptians.
The ancient Egyptians circulated aqueduct water throughout the walls of houses which cooled them. As this was a luxury due to water usage being expensive only the wealthiest could afford it.
In medieval Persia buildings had their own cisterns and wind towers cooling buildings during the hot season. Cisterns were generally large outdoor pools which collected the rain water; wind towers used windows to catch the wind and internal vanes which directed the air flow down in to the building over the cistern tanks and out through a down wind cooling tower. The cistern of water evaporated which in turn cooled the air within the building.
It was not until 1820 when a British scientist and inventor Michael Faraday discovered that compressing and liquefying ammonia could chill air when the liquefied ammonia was allowed to evaporate.
In 1842 a Dr. John Gorrie used compressor technology to create an ice-making machine, which he then used to cool down his hospital patients. He hoped that his ice-making machine would be used to regulate the temperature in buildings. He also envisaged that his ice-making machine could eventually be used to cool entire cities at anyone time. The prototype that Gorrie used leaked and was unreliable but he was still granted is patent in 1851. His success was short lived due to the death of his main financial backer; he did not get the money required for the machines manufacture. His biographer Vivian M Sherlock said that Gorrie had blamed Frederic Tudor, nick named the Ice King, for his failure, he believed that Tudor had created a smear campaign against his machine.
Dr Gorrie was bankrupt when he died in 1855 and the idea of the air conditioning system died until the idea was resurrected 50 years later.
The early uses for air conditioning were for the industrial sector rather than personal comfort. It was in 1902 when the first electrical air conditioner was invented. The inventor William Havilland Carrier first designed the system to improve manufacturing processes within a printing plant. It was used to control not only the temperature but also the humidity. The printing plant used the air conditioner to maintain a constant temperature and humidity which helped with the paper dimensions and ink alignment. It was not until much later when the systems were used to cool buildings, and eventually automobiles.
In 1906 Stuart W Crammer was trying to add moisture to the air in his textile mill. The term air conditioning was coined by him and using it in a patent claim he filed that same year as an analogue “Water Conditioning”; which was a well known process in the textile industry, which made the working process of textiles easier.
To change the air within the factories he combined the moisture with ventilation which caused the “condition” to be able to control the humidity necessary for textile plants. Willis Carrier then adopted the term and incorporated it into his company name. This process of water in air providing cooling effect is now known better as evaporative cooling.
The first styles of air conditioners and refrigerators used toxic or flammable gasses like ammonia, methyl chloride and propane, which if leaked caused fatal accidents. In 1928 Thomas Midgley Jr. was the first to create Chlorofluorocarbon (CFC) gas, Freon. All though the gas was safer for individuals it was found to be harmful to the earth’s ozone layer. The harmful CFC gases used in refrigeration equipment are due to be phased out by 2020 and replaced by the more friendly refrigerants being developed, including the R-410A gas otherwise known as Puron.
The recent emphasis placed on energy and efficiency for improving the quality of the air indoors means that technology and innovations continue.
Air conditioning applications
Air conditioning applications are broadly divided into two applications: Comfort and Process.
Comfort applications
Comfort applications provide internal environments which are kept at a constant ambient temperature preferred by people. The system does this despite the external or internal weather conditions or internal heat loads.
The most performance for tasks performed by staff working within an office environment will occur at approximately 22.2°C. If the temperature raises by 2° the performance of your staff decreases by about 1%. Where as standing tasks are able to be carried out at a slightly lower temperature. The highest performance tasks taken by smaller people are a slightly higher temperature. The Hawthorne effect debates the fact that worker productivity is affected by thermal comfort.
Deep plan buildings would not be feasible if it were not for comfort air conditioning. Buildings would have to be built narrower. Inner spaces would have to have light wells and receive air via natural ventilation. Air conditioning has also made it possible for buildings to be built taller. The taller the building the less practical natural ventilation is due to the increase in wind speed.
There are many types of buildings comfort air conditioning can be used in, they are catorigized as:
- Lower rise residential buildings - including family homes, duplexes and small apartment buildings.
- High rise residential buildings - such as tall dormitories and apartment blocks.
- Commercial buildings – Office blocks, shopping centres, restaurants and hotels.
- Institutional Buildings - which includes hospitals, government and academic.
- Industrial Spaces - where thermal comfort of workers is desired.
Air conditioning is now being used for a wider variety of applications, in addition to buildings. These applications include vehicles, trains, ships, air craft and even space craft.
Process applications
The applications are to provide and environment which a particular process is carried out, regardless of internal heat loads and external weather conditions. The needs of the process are taken into consideration and not the persons performing the process.
Process applications include the following:
A hospital theatre, air is filtered to higher levels which reduce the risk of infection. The humidity is also controlled which helps stop dehydration within the patient. The temperature range for these applications range from low temperatures for open heart surgery to very warm temperatures for neonatal areas.
Clean rooms for the production of circuits and pharmaceuticals which a very high level of cleanliness is required.
Laboratory animals are kept in rooms which mimic the season that they are more likely to reproduce in causing them to reproduce all year round.
Aircraft air conditioning is normally for the comfort of the crew and passengers. The air conditioning within the aircraft is specialised due to the air pressure difference within and outside the aircraft.
In both comfort and process applications the objective to not only to control temperature, but also humidity, air movement and air quality.
Humidity control
Refrigeration air conditioning equipment is used to reduce the humidity of the air processed by the system. The relatively cold evaporator coil condenses heater vapour from the processed air, much like a cold drink will condensate water on the outside of the glass, sending the water to drain and removing water vapour from the cooled source which in return lowers the relative humidity.
People perspire as away to reduce their temperature so they are comfortable. This works by the sweat produced being evaporated thus lowering temperature. Air conditioners are designed so they create a 40% to 60% relative humidity in the occupied space. In the food retail industry the large refrigerators used act as a highly effective air dehumidifying unit.
Some air conditioning units actually dry the air without cooling air and are classed as dehumidifiers. They work like normal air conditioning units except that they contain a heat exchanger which is placed between the intake and exhaust. To achieve a comfort level in tropical humid areas they contain convection fans, but only consume about 1/3 of the electricity. Dehumidifiers are also preferred by people who do not like the draft created by air coolers.
Vapour - compression refrigeration cycle
In the vapour compression refrigeration cycle heat is transferred from a lower temperature source to a higher temperature heat sink. Heat naturally flows in the opposite direction this is due to the second law of thermodynamics; work is required to move heat from cold to hot. A freezer works in much the same way; it moves heat out the interior into the room in which it stands.
A compressor driven by a motor creates the most common refrigeration cycle. In an automobile the compressor is usually belt driven which is connected to a pulley on the engines crank shaft; which means both use electric motors for air circulation.
Evaporation occurs when the heat is absorbed and condensation occurs when heat is realised, air conditioners are designed to use a compressor to cause pressure changes between two compartments, and also they actively pump a refrigerant around. The refrigerant is pumped into the low pressure compartment, where, despite the low temperature, the low pressure causes the refrigerant to evaporate into vapour, which takes the heat with it. In the second compartment the refrigerant vapour is compressed and forced through another heat exchange coil, condensing into a liquid, rejecting the heat previously absorbed from the cooled area. The heat exchanger in the condenser section is cooled most often by a fan blowing outside air though it, but in some cases it is possible to cool it by other means like water for on some ships.
Refrigerants
“Freon” is a trade name for a family of halokane refrigerants. These refrigerants were more commonly used due to them being safer than the original toxic fluids that were being used.
Unfortunately recent studies have pointed toward the fact that these gases reach our upper atmosphere when they escape. The chemistry for this is poorly understood but general agreement seems to be that the CFC’s break up in the earth’s stratosphere due to UV radiation, which in turn releases their chlorine atoms. They act as catalysts in the breakdown of the ozone, which does severe damage to the ozone layer which shields the earth’s surface from the suns strong UV radiation. The chlorine remains active as a catalyst until and unless it combines with another particle which then makes it stable. The older CFC’s were being replaced with safer HCFC’s. HCFC’s in turn are being phased out by the Montreal Protocol and replaced with HFC’s, such as R-410A, which lack chlorine.
Health implications
Air conditioning systems need to be regularly maintained and cleaned so the growth and spread of micro organisms, such as Legionella Pneumophila, which is the agent responsible for Legionnaires disease, or Thermophilic Actinomycetes.
Conversely air conditioning, including filtration, humidification, cooling, disinfection, etc, can be used to create a safe clean hypoallergenic atmosphere within hospital operating theatres and other environments where the well being of the patient is critical. Air conditioners can also have positive results with people who suffer with allergies or asthma.
Air conditioning can also be used to save the lives of the elderly in a serious heat wave.
Air Conditioning Systems which are not maintained correctly can become extremely noisy and eventually affect your hearing. These levels can reach the same levels as living near to busy roads or airports, well maintained air conditioners do not have this problem and will be much quieter.
Energy use
In a thermodynamically closed system, any energy input into the system that is being maintained at a constant temperature requires the energy removal rate from the air conditioner to increase. This increase has the effect that for each unit of energy input the system requires the air conditioner to remove that energy. In order for the air conditioner to be able to do this it must increase its consumption by the inverse of its efficiency times the input unit of energy. A good example to use is to presume that inside a closed system of 100 watt light bulb is activated, and the air conditioner has an efficiency of 50%. The air conditioners energy consumption will increase by 200 watts to compensate for this, thus making the 100 watt light bulb utilise a total of 300 watts of energy.
Portable air conditioners
Portable air conditioners are commonly available, but there are primarily two types; movable spot cooler and semi-permanent.
The general consensus is that spot coolers are used for cooling people or equipment for a temporary period.
Semi-permanent air conditioners are also split into two groups; one with a duct or hose which goes out of a window and the other is where the heat rejection part is separate from the air conditioning unit. The units with ducting out of the window are less efficient. This is due to the fact that the ejection of air creates a low pressure environment in the room, which will cause air flow into the room or house ultimately from outside again, so this type of air conditioning unit is constantly replacing cooled air with hot air. The types where the heat rejection equipment is outside the conditioned room work quiet well.
