Other features that will be found in a quality compressor include:

An ASME certified safety relief valve which will allow air to escape automatically if pressure in the tank should ever exceed the maximum. This valve will have a pull ring attached to it to allow you to check the valve to make certain the valve is not clogged or corroded.

An oil level sight glass, a tank pressure gauge and, of course, a pressure regulator and gauge are important, as each tool and job has a specific pressure requirement. The tank maintains air at maximum pressure from 100 to 125 pounds of pressure per square inch. PSI is the force of the pressurized air delivered to the tool. Projects and tools have both pressure and volume requirements. Volume is measured in cubic feet per minute (CFM) or standard cubic feet per minute (SCFM). When selecting and using a compressor, it is the relationship of CFM (volume of air) and PSI (force or pressure of air) that is important.
A manual thermal overload button is important in case of overloads or if the motor overheats. If the motor overheats, it automatically shuts off. This protective control button must be pushed for the compressor to run once the motor has cooled down, preventing a sudden and unexpected restart. The best air intake filtration system will be enclosed and mounted on the side of the compressor. This does more to protect the filtering foam, inside the housing, keeping the compressor cleaner, longer.

A belt guard is indispensable protection and the compressor should never be turned on without the guard in place. A hose rack is desirable for convenience and for protecting the air hose when not in use.

One other feature, most desirable to the do it yourselfer, is a toll free number to call should a question or problem arise.

There are five basic steps in the operation of an air compressor.

1. Check the oil level to make sure the compressor is properly lubricated.
2. Plug the unit into the correct grounded, 3-pronged outlet. Turn the pressure switch on and close the tank drain valve.
3. Adjust the pressure for the tool you will be using and the job you will be doing. Never exceed recommended pressure for the tool or the job.
4. When finished, shut off the motor, unplug the unit, and turn off the regulator valve. Then bleed the air out of the hose, remove the tool and open the regulator to bleed the air in the tank. If you have a quick connect, you must either remove the hose to bleed off the air from the tank or bleed the air through the drain cocks.
5. After storing the hose, open the drain cock to release any accumulated moisture. Leave it open until the next time the compressor is used.

Attachments

Now let’s talk about the available tool attachments. If kept properly cleaned and lubricated, air tools are virtually indestructible. With few actual moving parts, maintenance is minimal. They run cool, since their power source is the compressor.
Perhaps two of the most obvious and useful tools are an inflation kit and quick connect couplers. The quick connect couplers make it fast and simple to change tools. The inflation kit attachments allow you to inflate everything from beach balls to automobile tires.

Blo-gun. This attachment is great for blasting away dirt, grease, and dust from hard-to-reach areas. Never point the gun at the eyes or other parts of the body.

Nail gun. Always be sure the gun is flat against the surface being nailed and know what is on the other side, so you won’t cause damage or injury with the high pressure of the gun.

Air stapler. Again, be sure the stapler is flat against the surface being stapled. Larger staplers are available for attaching roofing shingles and so forth.

Air sander. The dual-action air sander should always be touching the surface when it is turned on. This type sander is frequently used in automotive work but many other uses around the house, such as rust removal or paint preparation, make it a handy tool to have.

Spray gun. This speeds up paint application and gives a smooth finish. There are a variety of spray gun designs on the market for various types of painting. Many times you can reduce the time required to do a job by 50% or more.

Sandblaster. This works well for removing rust and old paint and for preparing surfaces for painting. This same equipment can be adapted for use with soap and water for pressure cleaning such as degreasing auto engines and lawn and garden equipment.

Caulking gun. This tool takes the toil out of caulking, by giving a fast, uniform bead. Uniform and consistent pressure makes for a stronger bead. This tool can be used for any tube material such as adhesive or grease.

Air ratchet wrench. This is great for tightening bolts, whether building a deck, working on an automobile engine or installing a muffler.

Air hammer/chisel. The masters jobs from masonry to tailpipe removal. It must be up against the surface when started.

Air drill. An air drill makes drilling into any surface an effortless task.

Impact wrench. This is used in automotive and assembly work.

Most air tools are available at hardware stores and home centers. Specialty air tools can be rented. Instructions for each tool attachment are included with the purchase or rental. Read these instructions carefully before attempting to use the tool.

source: http://www.doityourself.com

Instructions

1. Check to make sure that the cord is plugged into the outlet firmly and that the power switch is on. If power is not restored, locate the “Reset” button located on the side of the compressor motor, typically near where the power cord is attached. Look for a round black or red button. Depress the button to reset the circuit breaker. If power is not restored, continue to the next step
2. Trace the cord to ensure that all connections are snug. Plug any cords in that are loose.
3. Locate the circuit breaker box and check for circuit breakers that have tripped. Look for a small red square near the breaker to indicate a breaker that has tripped. Flip tripped breakers to the full “Off” position then back to “On.” If power is not restored, continue to the next step.
4. Test the outlet by plugging in a small radio or lamp. If the outlet works, remove any extension cords and plug the compressor cord directly into the outlet. Test the compressor again. If power is not restored, take the compressor for professional servicing.

Building Pressure

1. Unplug hoses and tools. Turn the compressor on and allow it to charge. Spray soapy water around the fittings of the compressor and check for bubbling to indicate air leaks. Release the pressure from the tank by pulling the air release valve. Look for a small plastic/metal valve near the air outlet with a pull ring attached.
2. Remove fittings that have actively bubbling soapy water surrounding them with a wrench. Turn the fittings counterclockwise to remove them. Wrap teflon plumber’s tape around the threads of the fittings.
3. Thread the fittings back into the compressor and tighten with the wrench. Test again, charging the compressor and spraying with soapy water. Replace fittings that still leak.
4. Charge the compressor. Locate the bleed valve on the bottom of the air tank. Grip the valve with locking pliers and turn it counterclockwise to open it. Allow the air to blow the condensation from the tank until the air escaping is dry. Tighten the valve by turning clockwise.

Adjusting Air Flow

1. Remove the plastic shroud from the top of the compressor by removing the screws with a screwdriver. Lift the cover off to expose the pressure limiter screws; there will be two screws. The lower control screw, closest to you, controls when the compressor comes on. The upper screw controls when the compressor goes off.
2. Start the compressor and make a note of the pressure setting when the compressor kicks off. Adjust pressure up or down with the upper control screw. Tighten the screw clockwise to raise the pressure, or counterclockwise to lower it.
3. Pull the pressure release valve and make a note of the pressure when the compressor comes on. Adjust the lower control screw to raise or lower the setting. Replace the cover and replace and tighten the screws with a screwdriver.

source: http://www.ehow.com/how_7872768_repair-air-compressors.html

Axial compressors consist of rotating and stationary components. A shaft drives a central drum, retained by bearings, which has a number of annular airfoil rows attached. These rotate between a similar number of stationary airfoil rows attached to a stationary tubular casing. The rows alternate between the rotating airfoils (rotors) and stationary airfoils (stators), with the rotors imparting energy into the fluid, and the stators converting the increased rotational kinetic energy into static pressure through diffusion. A pair of rotating and stationary airfoils is called a stage. The cross-sectional area between rotor drum and casing is reduced in the flow direction to maintain axial velocity as the fluid is compressed.

Axial compressors are widely used in gas turbines, such as jet engines, high speed ship engines, and small scale power stations. They are also used in industrial applications such as large volume air separation plants, blast furnace air, fluid catalytic cracking air, and propane dehydrogenation. Axial compressors, known as superchargers, have also been used to boost the power of automotive reciprocating engines by compressing the intake air, though these are very rare.

The increase in pressure produced by a single stage is limited by the relative velocity between the rotor and the fluid, and the turning and diffusion capabilities of the airfoils. A typical stage in a commercial compressor will produce a pressure increase of between 15% and 60% (pressure ratios of 1.15-1.6) at design conditions with a polytropic efficiency in the region of 90-95%. To achieve different pressure ratios, axial compressors are designed with different numbers of stages and rotational speeds. As a general rule-of-thumb we can assume that each stage in a given compressor has the same temperature rise (deltaT). Therefore, as the entry temperature (Tstage entry) to each stage must increase progressively through the compressor, the ratio deltaT/Tstage entry must decrease, thus implying a progressive reduction in stage pressure ratio through the unit. Hence the rear stage develops a significantly lower pressure ratio than the first stage.

Higher stage pressure ratios are also possible if the relative velocity between fluid and rotors is supersonic, however this is achieved at the expense of efficiency and operability. Such compressors, with stage pressure ratios of over 2, are only used where minimising the compressor size, weight or complexity is critical, such as in military jets.

A compressor in which the fluid enters and leaves in the axial direction is called axial flow compressor. So, the centrifugal component in the energy equation does not come into play. Here the compression is fully based on diffusing action of the passages. The main parts include a stationary (stator) part and a moving (rotor) part. The diffusing action in stator converts absolute kinetic head of the fluid into rise in pressure. The relative kinetic head in the energy equation is a term that exists only because of the rotation of the rotor. The rotor reduces the relative kinetic head of the fluid and adds it to the absolute kinetic head of the fluid i.e., the impact of the rotor on the fluid particles increases its velocity (absolute) and thereby reduces the relative velocity of the between the fluid and the rotor. In short, the rotor increases the absolute velocity of the fluid and the stator converts this into pressure rise. Designing the rotor passage with a diffusing capability can produce a pressure rise in addition to its normal functioning. This produces greater pressure rise per stage which constitutes a stator and a rotor together. This is the reaction principle in turbomachines. If 50% of the pressure rise in a stage is obtained at the rotor section, it is said to have a 50% reaction

The first instrument to be named the “airbrush” was developed by Abner Peeler “for the painting of watercolors and other artistic purposes” and used a hand-operated compressor. It was rather crude, being based on a number of spare parts in a jeweller’s workshop such as old screwdrivers and welding torches. It took 4 years of further development before a truly practical device was developed. This was marketed by Liberty Walkup, who taught airbrush technique to American Impressionist master Wilson Irvine at the Air Brush School in Rockford, Illinois. The first certain ‘atomising’ type airbrush was invented by Charles Burdick in 1893 and presented by Thayer and Chandler art materials company at the World Columbian Exposition in Chicago. Burdick founded the Fountain Brush Company in the US, and launched the first series of airbrushes onto the market. This device was essentially the same as a modern airbrush, resembling a pen and working in a different manner than Peeler’s device. Aerograph, Burdick’s original company, still makes and sells airbrushes in England. Thayer and Chandler were acquired by Badger Air-Brush Co. in 2000. Badger Air-Brush continues the Thayer and Chandler tradition of manufacturing quality airbrush guns, tools and compressors out of Franklin Park, Illinois.

For more a detailed academic study, the University of Wales Library holds a detailed PhD on Airbrush History. The Franklin Institute in Philadelphia, The Public Library in Rockford Illinois and the Conservation Department of New York University retain copies. This was authored by Dr. Andy Penaluna, now Professor of Creative Entrepreneurship at Swansea Metropolitan University. Professor Penaluna has also advised the International Museum of Photography at George Eastman House, Rochester, New York.

An airbrush works by passing a stream of fast moving (compressed) air through a venturi, which creates a local reduction in air pressure (suction) that allows paint to be pulled from an interconnected reservoir at normal atmospheric pressure. The high velocity of the air atomizes the paint into very tiny droplets as it blows past a very fine paint-metering component. The paint is carried onto paper or other surface. The operator controls the amount of paint using a variable trigger which opens more or less a very fine tapered needle that is the control element of the paint-metering component. An extremely fine degree of atomization is what allows an artist to create such smooth blending effects using the airbrush.

The technique allows for the blending of two or more colors in a seamless way, with one color slowly becoming another color. Freehand airbrushed images, without the aid of stencils or friskets, have a floating quality, with softly defined edges between colors, and between foreground and background colors. A well skilled airbrush artist can produce paintings of photographic realism or can simulate almost any painting medium. Painting at this skill level involves supplementary tools, such as masks and friskets, and very careful planning.

Some airbrushes use pressures as low as 20 psi (1.38 bar) while others use pressures in the region of 30-35 psi (2-2.4 bar). Larger “spray guns” as used for automobile spray-painting need 100 psi (6.8 bar) or more to adequately atomize a thicker paint using less solvent. They are capable of delivering a heavier coating more rapidly over a wide area. Even with small artist airbrushes using acrylic paint, artists must be careful not to breathe in the atomized paint, which floats in the air for minutes and can go deep into the lungs. With commercial spray guns for automobiles, it is vital that the painter have a clean air source to breathe, because automotive paint is far more harmful to the lungs than acrylic. Certain spray guns, called High-Volume Low-Pressure (HVLP) spray guns, are designed to deliver the same high volumes of paint without requiring such high pressures.

Compression has many implications in materials science, physics and structural engineering, for compression yields noticeable amounts of stress and tension. By inducing compression, mechanical properties such as compressive strength or modulus of elasticity, can be measured. Scientists and engineers may utilize compression machines to measure the resistance of materials and structures to compression. Compression machines range from very small table top systems to ones with over 53 MN capacity

Internal combustion engines

In internal combustion engines it is a necessary condition of economy to compress the explosive mixture before it is ignited: in the Otto cycle, for instance, the second stroke of the piston effects the compression of the charge which has been drawn into the cylinder by the first forward stroke.

Steam engines

The term is applied to the arrangement by which the exhaust valve of a steam engine is made to close, shutting a portion of the exhaust steam in the cylinder, before the stroke of the piston is quite complete. This steam being compressed as the stroke is completed, a cushion is formed against which the piston does work while its velocity is being rapidly reduced, and thus the stresses in the mechanism due to the inertia of the reciprocating parts are lessened. This compression, moreover, obviates the shock which would otherwise be caused by the admission of the fresh steam for the return stroke.

Operation

Rotary screw compressors use two meshing helical screws, known as rotors, to compress the gas. In a dry running rotary screw compressor, timing gears ensure that the male and female rotors maintain precise alignment. In an oil-flooded rotary screw compressor, lubricating oil bridges the space between the rotors, both providing a hydraulic seal and transferring mechanical energy between the driving and driven rotor. Gas enters at the suction side and moves through the threads as the screws rotate. The meshing rotors force the gas through the compressor, and the gas exits at the end of the screws.

The effectiveness of this mechanism is dependent on precisely fitting clearances between the helical rotors, and between the rotors and the chamber for sealing of the compression cavities.

Size

Rotary screw compressors tend to be compact and smooth running with limited vibration and thus do not require spring suspension. Many rotary screw compressors are, however, mounted using elastomer vibration isolating mounts to absorb high-frequency vibrations, especially in rotary screw compressors that operate at high rotational speeds. Rotary screw compressors are produced in sizes that range from 10 cubic feet per minute to several thousand CFM. Rotary screw compressors are typically used in applications requiring more airflow than is produced by small reciprocating compressors but less than is produced by centrifugal compressors.

Applications

Typically, they are used to supply compressed air for general industrial applications. Trailer mounted diesel powered units are often seen at construction sites, and are used to power air operated construction machinery.

In an oil-free compressor, the air is compressed entirely through the action of the screws, without the assistance of an oil seal. They usually have lower maximum discharge pressure capability as a result. However, multi-stage oil-free compressors, where the air is compressed by several sets of screws, can achieve pressures of over 150 psig, and output volume of over 2000 cubic feet (56.634 cubic meters) per minute (measured at 60 °C and atmospheric pressure).

Oil-free compressors are used in applications where entrained oil carry-over is not acceptable, such as medical research and semiconductor manufacturing. However, this does not preclude the need for filtration as hydrocarbons and other contaminants ingested from the ambient air must also be removed prior to the point-of-use. Subsequently, air treatment identical to that used for an Oil-flooded screw compressor is frequently still required to ensure a given quality of compressed air.

In an oil-flooded rotary screw compressor, oil is injected into the compression cavities to aid sealing and provide cooling sink for the gas charge. The oil is separated from the discharge stream, then cooled, filtered and recycled. The oil captures non-polar particulates from the incoming air, effectively reducing the particle loading of compressed air particulate filtration. It is usual for some entrained compressor oil to carry over into the compressed gas stream downstream of the compressor. In many applications, this is rectified by coalescer/filter vessels.^[3] In other applications, this is rectified by the use of receiver tanks that reduce the local velocity of compressed air, allowing oil to condense and drop out of the air stream to be removed from the compressed air system via condensate management equipment.

Standard oil-flooded compressors are capable of achieving output pressures over 200 psig, and output volumes of over 1500 cubic feet per minute (measured as Free Air Delivery at 60 °C and atmospheric pressure).

Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas. Liquids are relatively incompressible, while some can be compressed, the main action of a pump is to pressurize and transport liquids.

Hermetically sealed, open, or semi-hermetic

Compressors are often described as being either open, hermetic, or semi-hermetic, to describe how the compressor and motor drive is situated in relation to the gas or vapour being compressed. The industry name for a hermetic is hermetically sealed compressor, while a semi- is commonly called a semi-hermetic compressor.

In hermetic and most semi-hermetic compressors, the compressor and motor driving the compressor are integrated, and operate within the pressurized gas envelope of the system. The motor is designed to operate and be cooled by the gas or vapour being compressed.

The difference between the hermetic and semi-hermetic, is that the hermetic uses a one-piece welded steel casing that cannot be opened for repair; if the hermetic fails it is simply replaced with an entire new unit. A semi-hermetic uses a large cast metal shell with gasketed covers that can be opened to replace motor and pump components.

The primary advantage of a hermetic and semi-hermetic is that there is no route for the gas to leak out of the system. Open compressors rely on either natural leather or synthetic rubber seals to retain the internal pressure, and these seals require a lubricant such as oil to retain their sealing properties.

An open pressurized system such as an automobile air conditioner can leak its operating gases, if it is not operated frequently enough. Open systems rely on lubricant in the system to splash on pump components and seals. If it is not operated frequently enough, the lubricant on the seals slowly evaporates, and then the seals begin to leak until the system is no longer functional and must be recharged. By comparison, a hermetic system can sit unused for years, and can usually be started up again at any time without requiring maintenance or experiencing any loss of system pressure.

The disadvantage of hermetic compressors is that the motor drive cannot be repaired or maintained, and the entire compressor must be removed if a motor fails. A further disadvantage is that burnt out windings can contaminate whole systems requiring the system to be entirely pumped down and the gas replaced. Typically hermetic compressors are used in low-cost factory-assembled consumer goods where the cost of repair is high compared to the value of the device, and it would be more economical to just purchase a new device.

An advantage of open compressors is that they can be driven by non-electric power sources, such as an internal combustion engine or turbine. However, open compressors that drive refrigeration systems are generally not totally maintenance free throughout the life of the system, since some gas leakage will occur over time.

Positive-displacement air compressors work by forcing air into a chamber whose volume is reduced to effect the compression. Piston-type air compressors use this principle by pumping air into an air chamber through the use of the constant motion of pistons. They use unidirectional valves to guide air into a chamber, where the air is compressed. Rotary screw compressors also use positive-displacement compression by matching two helical screws that, when turned, guide air into a chamber, the volume of which is reduced as the screws turn. Vane compressors use a slotted rotor with varied blade placement to guide air into a chamber and compress the volume.

Negative-displacement air compressors include centrifugal compressors. These devices use centrifugal force generated by a spinning impeller to accelerate and then decelerate captured air, which pressurizes it.

The air compressors seen by the public are used in 5 main applications:

• To supply a high-pressure clean air to fill gas cylinders
• To supply a moderate-pressure clean air to supply air to a submerged surface supplied diver
• To supply a large amount of moderate-pressure air to power pneumatic tools
• For filling tires
• To produce large volumes of moderate-pressure air for macroscopic industrial processes (such as oxidation for petroleum coking or cement plant bag house purge systems).

Most air compressors are either reciprocating piston type or rotary vane or rotary screw. Centrifugal compressors are common in very large applications. There are two main types of air compressor’s pumps: Oil lubed and oil-less. The oil-less system has more technical development, but they are more expensive, louder and last for less time than the oiled lube pumps. However, the air delivered has better quality.