The Early Roots of the Air Compressor
Devices for compressing air have been around for thousands of years, and without which modern civilization would have not been able to develop. Compressed air or other gasses have many useful properties, such as enhancing combustion in forges, displacing liquids, and storing and transmitting energy in many useful ways.
Bellows
One of the earliest types of air compressor, the
bellows, is simply a sealed chamber which can be expanded or contracted, with an opening for air to enter through a restricted space. Two familiar variations of bellows are the bladder for an
accordion, and the old-fashioned fireplace blower, made of two hinged wooden plates with canvas or leather side panels to make a crude expandable air chamber. An animal breathes by the same principle. When the bellows are opened, air rushes in to fill the chamber, when they are closed, the air is expelled through a small opening. Large bellows were built thousands of years ago and used to fan fires hot enough to
smelt various ores of
copper,
tin,
lead, and
iron into more usable metals. Bellows were used up until the early 20th century to provide the air needed to drive
pipe organs, and are still used today in some medical applications where it is important not to contaminate the air being compressed.
A refinement of the crude bellows is the addition of a check valve, which allows air to rush into the bellows relatively unrestricted, but prevents the air from exiting the same opening. This makes the bellows easier to operate, and more energy can be spent forcing air out of the bellows than getting it in. A familiar form of check valve, known as the reed valve was originally a reed fastened on one end inside a bellows. If the reed valve was mounted on a flat surface covering a hole inside of the bellows, air trying to exit the bellows past the reed valve would force it tight against the hole, while air trying to rush into the bellows would push it away from the hole, allowing the air to flow. Many light duty air compressors today still use reed valves, though today they are usually made from spring steel rather than reeds.
Another type of check valve still commonly used today is a ball or disc type valve. A ball or disc sits in a cavity, which has a vented seat on one end, and a seat which seals tightly against the ball or disc on the other end. When air flows into the cavity from the vented end, the ball is forced against the seal, stopping the airflow. When air flows the opposite way, the ball or disk is pushed against the vented seat, allowing air to continue flowing. Adding a spring between the ball or disc and the vented seat to push the ball against the sealed end of the cavity now provides the ability to regulate the pressure needed to move the ball away from the seat. This has the very useful property of providing a steady continuous flow of air, and to safely store compressed air by allowing excess pressure to escape.
The Industrial Revolution, Steam Engines, and Air Compressors
Up until the Industrial Revolution, generating compressed air meant using human, water, wind, or animal power to drive huge bellows, pistons, or fans. While it was well-known since antiquity that boiling water creates large quantities of gas, and boiling it in a sealed chamber creates pressurized gas, able to cause even heavy containers to rupture, a safe and practical way to harness this potentially plentiful source of energy remained elusive until towards the end of the 18th century. Steam powered mechanical devices date at least back to 300 BC, Hero of Alexandria has one, but it was only considered a toy. By the 18th century, inventors were looking into more serious uses of steam power, such as pumping water, running mills, and powering boats and ships. Thomas Savery, Thomas Neucomen, and James Watt led the way with increasingly more efficient ways of converting steam to do useful work, though James Watt typically gets most of the credit for making steam power practical. His piston and valve type steam engine was one of the seminal inventions of the industrial revolution. Steam Power deserves a good node on its own, but to make a long story short, Savery, Neucomen, and Watt's work made possible the modern air compressor. Steam engines provided not only plentiful power to run the air compressors, but the engines themselves provided the design template for the modern air compressor, because a steam engine is merely an air compressor in reverse. Running air compressors became a major use of steam engines, in fact. Unlike steam, compressed air can transmit the the energy stored by the work necessary to compress the air long distances, because steam cools and condenses as it travels through a pipe. In relative terms, compressed air is also much safer to handle than high-pressure steam. A ruptured steam line often results in serious or fatal burns, while a ruptured compressed air line of the same pressure only causes mechanical hazards. Compressed air also has the advantage of being breathable, and one major early use of compressed air was in diving apparatus and in caissons used to build underwater foundations for bridges, piers, and tunnels. Another major use of compressed air was on railroad locomotives. Steam engines aboard locomotives also drove air compressors as well as the train's wheels. The air generated by the onboard air compressors was used to actuate the air brakes on trains. Air Brakes are still the preferred method of stopping trains and large trucks up to this day.
While not exactly air compressors in the strictest sense of the word, modern
refrigeration owes its existence to the air compressor. Refrigerators work by compressing warm low pressure gas into a hot high pressure gas. The hot high-pressure gas is then cooled by condensing it into a radiator, which condenses it into a cool high pressure liquid. The cool high-pressure liquid then is released into an
expansion valve which admits it into an
evaporator coil. The high-pressure liquid boils off into a cold low pressure gas, cooling the evaporator coil, and any air or liquid passing over it. At the exit of the evaporator coil, the warmed gas is recycled through the compressor again, repeating the cycle. This effect can be illustrated by filling and emptying the tank of a common air compressor you might see at a tire shop or perhaps in your own garage. As air is compressed, it heats up, due to the squeezing perhaps 10 times the normal volume of air into a space. The compressor itself, and the line of the compressor into the tank will be quite hot, and the tank itself will get pretty warm. Much of this excess heat will radiate into the environment for several minutes after the compressor is shut off. If the now full tank of air is allowed to cool to room temperature, then drained, the escaping air and the air in the tank will cool, often quite a bit. By doing this, you have created a crude
refrigerator. Liquids with low boiling points such as
Propane,
Freon and other
CFCs, and
Ammonia make very useful
refrigerants, though each has its own drawbacks.
Types of air compressors
There are a number of different ways to compress air, here are a few of the main ones
Piston Type: This is the type of compressor you may have in your garage. It traces its roots back to Watt's Steam Engine. It consists of an electric motor or engine driven crankshaft and piston, not unlike that of a gasoline engine. On the downstroke, a check valve allows air to flow into the cylinder on the low pressure side, while a similar check valve prevents air from leaking back from the high pressure side. On the upstroke, the low-pressure side valve closes, and when the cylinder presssure exceeds the pressure on the high side, air flows from the cylinder to the high-pressure side. Poppet valves, similar to those in a 4 cycle gasoline or diesel engine are sometimes used, but reed valves are more common in lighter duty models. A quieter and lower flow variation of a piston pump is a diaphragm pump. A rubber diaphragm is attached to a motor driven cam or a magnetic core moved by an electromagnet running on AC current. The resulting vibrations of the diaphragm move air in a manner similar to that of a piston pump. A familiar example of this is the aquarium pump. Fans and Turbines:
Fans and Turbines have been around for thousands of years, and are in principle as simple as a child's pinwheel, or as complex as a jet engine. Turbines convert fluid pressure into rotational inertia as is the case in a windmill, gas turbine, or hydroelectric generator, or it can convert rotational inertia into fluid pressure as a pump. The turbocharger found on many high performance gasoline or diesel engines does both. Hot expanding gasses on the exhaust side of a turbocharger cause it to spin at a very high speed, the energy captured is used to drive an impeller that is used to compress incoming air to enhance combustion within the engine. Windmills and fans have been around for thousands of years, but even today are more well suited to providing large volumes of compressed air at relatively low pressure than smaller volumes of compressed air at higher pressures.
Rotary Compressors:
Rotary compressors work by spinning a vaned rotor inside of an elongated chamber. They are of similar design to the
Wankel rotary engine, which was used in the
Mazda RX-7, as well as a few earlier
Mazda automobiles. Rotary engines are still used as
APUs on
airliners. As the rotor spins, air is pulled into the intake side as one of the vanes passes the intake port. As the vane continues around, the distance from the rotor to the sides of the chamber increases, effectively causing a vacuum on the intake side, which air rushes into. As the rotor spins, another vane passes the intake port, trapping this air, and readying another gulp. The air, now trapped between two vanes is compressed as the area between the two vanes, the rotor and the wall peaks, then gets smaller. An exhaust port near where this area of trapped air is at its smallest allows the air to escape under pressure. Pumps of this design are often used to provide continuous flow of smallish amounts of compressed air or
vacuum for various industrial uses. Compared to piston pumps, they are fairly quiet and reliable, without much of the noise and vibration associated with piston pumps. Without the need for reed valves, they eliminate the problems associated with them, though the vanes sometimes wear out or jam. They are also a poor choice for pumping gasses contaminated by abrasive or corrosive pollutants, which can foul the vanes or abrade the finely machined parts inside. Rotary compressors have replaced piston type compressors in many refrigeration applications due to their reliability and lack of vibration and noise.
Other Types of Compressors
Other types of compressors are the screw type compressor. The screw type is used to pump large volumes of air at low to moderate pressures. One of the most common types is the
roots blower, which was originally developed as a
supercharger on some types of diesel engines, but has been adapted for use for racing on gasoline engines as well.
Intercoolers
Intercoolers are an additional stage that is added near the output of the compression chamber. Compressing air heats it up not only due to the heating effect of compression, but due to friction within the compressor itself. An intercooler is a heat exchanger which allows the compressed air to cool before being stored or used, increasing the efficiency of the compressor. Intercoolers are often used on heavy-duty compressors, and are also used in automotive applications to enhance combustion and prevent detonation.
Industrial Uses for Air Compressors
There are almost unlimited uses for compressed air in industry besides inflating pneumatic tires, and there is no way I can document them all. Along with the steam engine, the air compressor was one of the most important machines developed during the industrial revolution, and serves critical roles in road, rail, and marine transportation, construction, and in countless industrial processes. That being said, I have alluded to the fact that compressed air has many desirable characteristics due to the ability to quickly store and deliver large amounts of energy for a short period of time. Compressed air makes an excellent energy source for tools and machines that need large but short bursts of energy. Many of these tools, such as jackhammers, hammer drills, and so on are direct descendants of their steam driven counterparts, but are much safer when operated on compressed air. On the factory floor, compressed air runs impact wrenches, riveters, saws, grinders, clamps, presses, and much more. Handheld tools in particular are good applications for compressed air. A 3/8 inch air line operating at 125 PSI can deliver the equivalent of several horsepower in short bursts, and the tools themselves can be much lighter and easier to handle than their electrically driven counterparts. Tools which run on compressed air can be also easily controlled by throttling the amount of air with a simple trigger control.
Compressed air also an excellent way to deliver a controlling force to an object without physically touching it. Paint sprayers work in this way, as jets of air act to break up a jet of paint into a fine mist. Some of the paper handling mechanisms on the equipment I work on depend upon compressed air to do their jobs. Jets of compressed air are good at cleaning debris from hard to reach areas around the shop. Compressed air is also used directly to pressurize other vessels containing liquids, such as the pressure tank for houses which depend on a well.
Using an Air Compressor around the Home
What to Look For in a compressor
If you have a garage or basement workshop, an air compressor can be a very practical addition which will greatly enhance your ability to do auto repairs, spray paint, or do construction around the house. For about $200 USD or so (2005), you should be able to purchase a compressor which will more than adequately serve most jobs around the average homeowner's garage, but keep in mind that certain tools require a lot of air to operate correctly. Most air compressors are rated by the volume of air that can be supplied at a certain line pressure usually measured in
CFM at a certain pressure. Tools and accessories are also rated in this way. A large tank will allow the user to draw quickly on a large reservoir of air to operate tools that use more air than the compressor can instantly provide, but have a low
duty cycle. A compressor in the $200 USD range will probably have a 15 amp motor and run on 120 volts in the USA. A typical compressor in this price range will supply about 4 to 6 cubic feet per minute at about 90 PSI, a larger amount at a lower pressure. It will likely have a maximum pressure of about 125
psi, with a gauge and adjustable regulator to adjust the outlet pressure. If you plan on spraying paint, sandblasting, or grinding with an air powered grinding wheel, keep in mind that these tools use a lot of air and have a high duty cycle. You may want to consider a larger compressor if you will use air hungry tools very much. It is best to use a 20 amp circuit on a dedicated line, mandatory if you leave it on and unattended, but at least make sure you are not sharing the circuit with other appliances not under your direct control if you only use it occasionally. For larger compressors that run on 220 volts in the US, you will have to have available or run a special 220 volt line for this purpose.
For occasional use, a compressor with an oilless cylinder is probably adequate, but will wear out in a few years if used daily for more than a few minutes. They have the advantage that they do not have an oil-filled crankcase, which needs to be checked periodically. You also don’t need to worry about crankcase oil from your compressor contaminating the air being compressed. For a few bucks more, a piston type compressor with a cast iron cylinder and an oil type crankcase will provide longer service if you use it often, such as in a light commercial environment such as a small auto repair garage, farm, or machine shop. For heavier commercial and industrial applications, compressors can be purchased which have ratings up to hundreds of horsepower, and have 3 phase AC motors, or gasoline or diesel engines to drive them.
Safety Considerations:
An air compressor has the ability to store large amounts of energy, which can be released very quickly. Make yourself familiar with the safety warnings not only for your air compressor, but also any accessories you might use with it. In addition to having adequate electric service to the compressor, make sure the compressor is protected against getting wet, is placed where it can't be overturned or fall over, or be easily run into or damaged by other equipment. If you run fixed air lines, building codes may require them to be inspected, but nonetheless, make sure they are protected against damage, are well-secured, and are able to handle the pressure expected with a good safety margin.
An air compressor is not a toy to let small children around when being used, or larger children to use without close supervision. In the hands of a small child, even a blowgun used for cleaning sawdust off a tabletop can be dangerous. Like with any other power tool, teach older children not to be afraid of it, but to respect its power and teach its proper use. An air compressor and many of the accessories which go with it are noisy, and can cause hearing damage if operated too close to the ears, or without hearing protection. Using an air compressor with an air sprayer requires special precautions, the paint mist can form explosive vapors in a confined space, these same vapors can also be harmful to breathe. Proper breathing protection is required when painting or sandblasting with compressed air. Finally, the compressor and air lines should be drained if the compressor is to be unused overnight or longer. Moisture tends to collect in the bottom of the tank, which can cause corrosion and even lead to eventual tank rupture, which can be very dangerous. A petcock located on the bottom of the tank should be opened regularly to drain off any condensation which collects in the tank.
Accessories
Without accessories, an air compressor is just a big, heavy, and noisy lump of metal sitting in the corner of the garage. There are literally dozens of accessories which can be purchased to make use of all of that compressed air. Here are a few of the most popular ones:
Quick ConnectorsQuick Connectors are a must have if you use your compressor to do more than just inflate tires. A fitting with a spring mounted sleeve attaches to the air hose, and fittings with barbed ends attach to the air inlets on other accessories. Additional lengths of air hose can be hooked together in this way as well. Retracting the sleeve on the hose allows accessories to be connected or disconnected easily. The accessory seals and locks into place when the collar pulls back.
Air Chuck: An air chuck is used for inflating tires. The better ones have a gauge built in.
Driers: A drier is a device installed in an air line to trap moisture and other contaminants, such as compressor oil from reaching the accessories. They are particularly critical when the air is used for spray painting, or for cleaning sensitive equipment such as electronics, where contaminants can ruin the work.
Blowgun: A blowgun is used for general purpose cleaning of machinery and work. I also use one at work for clearing paper jams, which occasionally plug up vacuum lines in my equipment at work. One type has a rubber tip, which is useful for clearing clogged lines, another type has a silencer type tip, which helps quiet it when used for general purpose cleaning.
Air powered Impact Wrenches: If you do a lot of automotive repair, this will probably be the first accessory you will want after a blowgun and an air chuck. An air impact wrench makes quick work of stubborn nuts and bolts. Impact wrenches come in various sizes and configurations for specialized needs, as well as the big 1/2 inch drive brutes.
Grinders, Sanders, and Cutoff Tools: Air powered handheld grinders and cutoff tools are useful for working on things like exhaust systems, bodywork, and so on. They are lighter, faster, and more powerful than their electrically operated counterparts. Be aware that many of these tools use lots of air, so make sure that your compressor can handle the load.
Nailers and Staplers: Air powered nailguns and staplers have taken over many of the tasks that carpenters and roofers in particular traditionally did with hammer and nails. With one quick pull of the trigger, a nail or staple is fully set. Fasteners for these pneumatic wonders are sold in clips, which let the user set dozens of fasteners before reloading. Powerful, but compact and relatively lightweight pancake compressors have been developed over the last few years to cater to their needs. Prices for these tools have dropped to the point that they are affordable to homeowners tackling large do it yourself projects.
Sandblasting: A sandblasting attachment is an effective way to remove rust and old finishes from metal, and can also be used to etch glass. Various types of abrasives can be used to sandblast, depending on the material being sandblasted, and the type of deposit to be removed. Common sandblasting materials are of course, sand, glass beads, ground walnut shells, and even baking soda. Sandblasting can create lots of choking dust, so wear a good dust mask, or do your sandblasting inside a cabinet designed for this purpose, if possible. Sandblasters use a lot of air, if you plan to do much sandblasting, make sure your compressor can handle the requirements of your sandblaster.
Painting with Air: Air type sprayers are available in various types, from the standard automotive air sprayer, to small airbrushes, to sprayers suitable for painting houses. For applying large amounts paint or clear finishes for furniture and the like, a compressed air sprayer will give faster, better, and more economical results than trying to use spray cans, and a much smoother finish than can be achieved with a brush. Be sure to use an air dryer in your air line. Moisture in compressed air will condense into droplets, which can leave "fish eyes" and other imperfections in the finish.
Final Thoughts: If you have any important or interesting additions to this node, feel free to /msg me, and I will incorporate them in. For more information on the early history of air compressors, check out this URL:
http://www.history.rochester.edu/appleton/a/ac1.html#lp