The air around us is a mixture of gases, mainly nitrogen and oxygen, but containing much smaller amounts of water vapor, argon, and carbon dioxide, and very small amounts of other gases. Air also contains suspended dust, spores, and bacteria. Because of the action of wind, the percent composition of air varies only slightly with altitude and location.
The amount of water in the air varies tremendously with location, temperature, and time. In deserts and at low temperatures, the content of water vapor can be less than 0.1% by volume. In warm, humid zones, the air may contain over 6% water vapor.
Air is the commercial source for many of the gases it contains. It is separated into its components by fractional distillation of liquefied air. Before air is liquefied, water vapor and carbon dioxide are removed, because these substances solidify when cooled and would clog the pipes of the air liquefaction plant. The dry, CO2-free air is compressed to about 200 atmospheres. This compression causes the air to become warm, and the heat is removed by passing the compressed air through radiators. The cooled, compressed air is then allowed to expand rapidly. The rapid expansion causes the air to become cold, so cold that some of it condenses. By the alternate compressing and expanding of air, most of it can be liquefied.
The earliest atmosphere
When Earth formed 4.6 billion years ago from a hot mix of gases and solids, it had almost no atmosphere. The surface was molten. As Earth cooled, an atmosphere formed mainly from gases spewed from volcanoes. It included hydrogen sulfide, methane, and ten to 200 times as much carbon dioxide as today’s atmosphere. After about half a billion years, Earth’s surface cooled and solidified enough for water to collect on it.
As oceanic crust is subducted (that is, dragged down beneath continental crust) down into the depths of the Earth by the cycle of plate tectonics, it releases “volatile” elements into the rock above. These volatile elements contain nitrogen – and its fate could be to either end up locked in minerals or be released as gas into the atmosphere. The chemical composition of the overlying rocks decide the fate of the volatiles.
|Figure 1: Mix of gases from the volcano|
Nitrogen deep in the Earth’s crust will tend to form ammonium ions (NH4+) which get incorporated into solid silicate minerals easily. Silicate minerals are among the most abundant kind of minerals in Earth’s crust. This is presumed to occur to much of the nitrogen on Earth and pretty much all of the nitrogen on Venus and Mars. But when those silicate minerals react under certain conditions, such as in the presence of oxygen or oxygen-containing compounds, the ammonium molecules break down to a mixture of water (H2O) and nitrogen (N2). The latter then finds its way to the surface and the atmosphere through volcanic vents. , 
It is tasteless and inodorous which appears colorless, except in very deep layers when a faint blue color is visible, which has been attributed to its Ozone content. Under normal conditions of 760mmHg and 0oC. The weight of liter of air varies, as a rule, between 1.2927 and 1.2933 gr, which is attributes to the fact that the Air has not a perfectly constant chemical composition. So it is useless to determine with great accuracy the density of a gas relatively to air unless the composition of the latter is simultaneously ascertained.
Table 2: Chemical properties of Air components
Compressed air is regular air, the volume of which has been decreased with the help of a compressor. Compressed air, just like regular air, consists mostly of hydrogen, oxygen and water vapor. Heat is generated when the air is compressed, and the pressure of the air is increased.
Electricity is used to produce compressed air. Approximately 10% of all industrial electricity consumption can be attributed to the production of compressed air. In order to use energy efficiently, the heat that is generated when the air is compressed should be used to heat the worksite. Compressed air is often seen as a clean and safe energy source that can be easily used for a number of different industrial purposes. Because of:
- Low maintenance costs
- Can handle high loads over long periods without the risk of overheating
- Easy to store
- Easy to transport
- Can be used in manufacturing processes that demand a high level of cleanliness
A blowgun consists of a nozzle, or tip, installed on a compressed air gun or line. On an airgun, the nozzle serves as a quick and efficient tool for light cleaning, drying and blowing off of parts or work areas.
|Figure 2: Compressed Air as a Blow gun|
Compressed air cylinders work by converting the potential energy of compressed gasses into mechanical energy. Every air cylinder features a cylinder, a piston and at least one inlet. Compressed air, when it is directed into the cylinder through the inlet, forces the piston to move. Valves control the flow of compressed air to the cylinder.
The two basic types of compressed air cylinders are single acting and double acting. The single acting cylinder is able to perform an operating motion in only one direction. A single acting air cylinder has air pressure on one side of a piston flange, supplying force and motion, and a spring supplying the return force after pressure release. Single acting cylinders require approximately half the amount of air used by a double acting cylinder for a single operating cycle.
A double acting pneumatic cylinder has powered motion in two directions, with pressure on both sides. When a cylinder is pushed out in one direction, compressed air moves it back in the other direction. Air lines running into both ends of the cylinder supply the compressed air. The flow of compressed air is controlled with valves for both single and double acting cylinders.
It can be produced by two processes:
• Dynamic compression (conversion of the air velocity into pressure): radial and axial compressors.
• Displacement compression (reduction of the air volume): reciprocating compressors (piston type) and rotary compressors (screw-, vane-, roots- or liquid ring compressors). The compressed air production includes necessary elements of compressed air treatment.
|Figure 3: Production of compressed Air|
The air receiver enables:
• storage of compressed air in order to meet heavy demands in excess of the capacity of the compressor.
• balancing of pulsations from the compressor.
• cooling of the compressed air and collection of residual condensate.
The air dryer reduces the water vapor content of compressed air. Moisture can cause equipment malfunction, product spoilage and corrosion.
Two methods are used: absorption and refrigeration.
Drains eliminate condensate (condensate water mixed with other impurities generated by compressed air and sources of pollution).
And the separator receives condensate from the drains. It separates oil and water avoiding any polluting discharge.
There are some recent research about Air uses:
A hybrid energy storage system using compressed air and hydrogen as the energy carrier
The thermal integration of two sub-systems allows for efficient storage of large amounts of energy based on the use of pressure tanks with limited volumes. A thermodynamic assessment of the integrated hybrid system was carried out. For the assumed operation parameters, an energy storage efficiency value of 38.15% was obtained, which means the technology is competitive with intensively developed pure hydrogen energy storage technologies. The results obtained for the hybrid system were compared to the results obtained for three reference systems, each of which uses hydrogen generators. The first is a typical Power-to-H2-to-Power system, which integrates hydrogen generators with a fuel cell system. The other two additionally use a compressed air energy storage installation. 
Aimed at building a healthy living environment: An analysis of performance of Clean-Air Heat Pump system for ammonia removal
Clean-Air Heat Pump (CAHP) is an innovative system which combines a desiccant wheel with a heat pump which can improve indoor air quality during the process of hydro-thermal control without using additional energy. The removal of ammonia (NH3)-one typical inorganic pollutant by the CAHP was focused, and the effects of regeneration air temperature of the rotor and NH3 concentration on NH3 removal were elaborated analyzed. The results showed that NH3 could not be continuously removed when the adsorbent-silica gel in the CAHP was regenerated by low-temperature thermal energy like 60 °C. A higher regeneration air temperature, possibly more than 90 °C, was required, in which case NH3 removal and desorption efficiencies reached 86.5% and 93.8% when the NH3 concentration was 2.5 ppm. Even if the NH3 concentration in process air was increased to 7.1 ppm, the air cleaning efficiency was still maintained above 67%.
Experimental characterization of the removal efficiency and energy effectiveness of central air cleaner
This study assessed six commercially available in-duct air cleaning devices which are designed to be mounted in the central ventilation system of offices or commercial buildings. The results showed that two devices, namely a radiant catalytic ionizer and a plasma ionizer, had a very low single pass efficiency against all the challenge pollutants. The association of the plasma ionizer and the electrostatic precipitator did not produce a synergetic effect between the two technologies either, contrary to what their manufacturer claims. Finally, three of the six devices tested were effective in terms of pollutant removal, but only two had an acceptable energy effectiveness in view of their use in low or zero energy buildings. Their energy effectiveness ranged from a few thousand m3/kWh for VOCs at the highest airflow rate (3600 m3/h), to more than 60 000 m3/kWh for particles and bio-contaminants at 1200 or 1600 m3/h. These results are at least one order of magnitude higher than the majority of stand-alone air cleaners.
Smart Air Purifier with Air Quality Monitoring System
The project presents the concept, functional physical model of an air purification system for small public spaces or apartments. The purifier is controlled by a microcontroller of the Arduino UNO series. The model is equipped with a set of sensors which are used to determine the air quality. After exceeding the adopted threshold in the software, the system automatically starts the process of air filtering. The air purification system depends on the optical dust sensor readings as it senses the quality of air in the room and turns the air purifier On and Off accordingly. The body of the air purifier is made by wood and filters. The filters used are pre-filter, dust-filter & fine-filter. The purifier absorbs solid pollutants and reduces VOC pollutants. The system has been equipped with an LCD screen informing the user about the air parameters and the quality of air being purified.
|figure(4): Output of Air purifier|
The Smart Air Purifier was specially designed for old age homes, hospitals, offices etc. This can be used to remove dust, fungus and reducing harmful gases from the air. It is handy and works on direct AC power supply by using an adapter connected to Arduino which works on DC. The technology used in this Air Purifier has a bright future because it works on as and when the dust density is more and thus saves energy. We can thus conclude that the three filters(pre-filter, dust filter & fine filter) were effective in purifying the surrounding air.
Air in pipelines
The presence of air in a pipeline and its impact on operations is probably one of the most misunderstood phenomena in our industry today. Many operational problems are blamed on inadequate thrust blocking, improper pipeline bedding. These problems include broken pumps, valves and pipe, as well as faulty instrumentation readings.
Air in a pressurized, operating pipeline comes from three primary sources. First, prior to start, the line is not empty ‐ it is full of air. To entirely fill a pipeline with fluid, it is necessary to eliminate this air. As the line fills, much of this air will be pushed downstream to be released through hydrants, faucets, etc. But a large amount will become trapped at system high points. This phenomenon will occur because air is lighter than water and therefore, will gravitate to the high points.
|figure 5: The presence of Air in pipelines|
Source number two is the water itself. Water contains approximately 2% air by volume. During system operation, the entrained air will be continuously released from the water and once again accumulate at system high points. To illustrate the potential massive amount of air this 2% represents, consider the following: A 1000 ft. length of pipe could contain a pocket of air 20 ft. long if all the air accumulated in one location. Or a one-mile length of pipe could contain a 100 ft. pocket. This would be true regardless of the size of the pipe. The third source of air is that which enters through mechanical equipment. This includes air being forced into the system by pumps as well as air being drawn in through packing, valves. under vacuum conditions. As one can see, a pressurized pipeline is never without air and typically the volume is substantial.
When air is allowed to accumulate in pressurized pipelines, efficiency is sacrificed, and serious damage can occur. A properly de‐aerated pipeline will not solve all surge problems. However, the elimination of air can solve one of their most common causes. Air Valves are a cost effective, reliable method of improving efficiency and solving air related surge problems. 
Zero air generators produce a continuous flow of clean, dry air with an ultra low residual methane content of less than 0.05 ppm from an existing compressed air supply. Zero air generators are easy to install. All that is required is a standard compressed air line and an electrical outlet. Zero Air is air which has had hydrocarbons removed via a process of oxidative catalysis to ensure it only contains less than 0.1 parts per million (PPM) of total hydrocarbons.
Compressed air is an energy source, used throughout industry for various purposes. As an energy source it must be treated with the same respect as electricity, if it is not used correctly it can be fatal.
Compressed air may be stored in cylinders and air receivers or generated by compressors. Air under pressure is delivered via a regulator, airline and air hoses to air tools and equipment. The tool or equipment is driven by the energy contained in the compressed air.
- Ensure the trigger or operating valve works properly. If it is faulty, the equipment shall be removed from service. Repairs shall be undertaken by a competent person before the equipment is placed back in service.
- Before disconnecting tools, depressurize the airline by isolating at the main air supply valve and operating the connected tool until all the air is expelled.
- Always point air guns away from the user and NEVER directly at anyone else.
- Never use compressed air to pressurize a vessel (unless the vessel is specifically designed for that purpose). For example, do not use compressed air to empty oil from gear boxes. They are not designed to withstand high internal pressures.
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