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Part 3 - Air Compressors

The first air compressor was the human lung as people discovered that they could blow air onto a glowing ember to start a fire. (This works because exhaled air still contains some oxygen and is the same reason that rescue breathing used in cardio-pulmonary resuscitation (CPR) can revive a person no longer breathing or even someone without a heart beat).

 When people started using fire to extract metal from ores about 3000 BCE, they found ways to deflect strong winds to stoke the fires. The first air compressor used to pump air into a fire more reliably was the bellows, invented about 1500 BCE. This was a bag of leather and wood with a flap valve that forced air trapped in the bag to exit through a nozzle. Initially this was pumped by hand and two or more could be used to provide a steady, concentrated blast of air to create a fire hot enough to smelt metal.

Later, two bellows were attached together so that the metal worker could step on a pedal to compress one and, at the same time, lift a weight that operated the other bellows so that the two bellows operated alternately.

In 1762, John Smeaton developed a waterwheel driven, blowing cylinder and, in 1776, John Wilkinson, invented an air compressor capable of supplying air up to 14.5 psi (pounds per square inch). These slowly replaced the bellows for metal working.

In 1861, the Mont Cenis Tunnel in Switzerland was cut using an 87 psi (pounds per square inch, 600 kPa) compressed air supply for pneumatic drills which was much cheaper than the traditional pick and shovel techniques and reduced construction time by 16 years (!). The compressors were driven by waterwheels. Compressed air drills were not only much safer than gunpowder for mining and tunnelling but were considerably more efficient.

Austrian engineer Viktor Popp installed a 1,500 kW compressor plant in Paris, in 1888, for power transmission. A municipal system of pipes supplied compressed air to drive electrical generators for lighting and other machines. (Compressed air did not lose pressure when piped for long distances whereas steam lost pressure by condensation). Three years later the compressor plant had grown to 18,000 kW. The air compressors were generally driven by steam engines.

George Westinghouse considerably improve rail safety in the USA in 1869 when he invented air brakes for trains. The brakes were automatically applied whenever the air pressure dropped, as would happen if the carriages were accidentally decoupled.  

In 1871 USA, Simon Ingersoll developed a pneumatic rock drill (jack hammer). In 1927 Ingersoll Rand supplied the drills and compressors to make the gigantic statues on Mt. Rushmore.

A diving bell was described by Aristotle in the 4th century BCE as a bell shaped metal container that could be lowered into the water with a man inside. As the air remained trapped in the container, the man was able to work until the oxygen was exhausted.

In 1535, Guglielmo de Lorena invented the modern diving bell and between 1663 and 1665 Albrecht von Treileben's divers, working from a diving bell, recovered most of the cannons from the warship Vasa, which sank in Stockholm harbor in 1628. 

In 1689, Denis Papin suggested that the pressure and fresh air inside a diving bell could be maintained by a force pump or bellows. In 1789 Britain, engineer John Smeaton utilized this idea.

The Brooklyn Bridge, in New York was completed in 1883 because workers were able to excavate the river bottom down to the bedrock for the foundations by working in caissons. (Caissons are generally circular dams with closed tops that use compressed air to force water out of the caisson and permit workers to build foundation below water level).

Workers breathing air under pressure, sometimes died from 'caisson' disease or decompression sickness. The cause was later discovered to be bubbles of nitrogen in the blood which could be prevented by slowly decompressing workers. Decompression chamber are still used by scuba divers who resurface too quickly from depths over 30 feet.

Commonly used gas compressors include;- reciprocating, screw, rotary vane, centrifugal and axial flow.

The simplest piston-type air compressor is the familiar bicycle pump. Traditionally this was single acting (forcing air out when pushed down and sucking air in on the return stroke) but these may be double acting (using both sides of the piston to suck and blow alternately on the down and the return strokes). A tubular rubber check valve on the bicycle tire prevents air from escaping.

Power driven piston (reciprocating) compressors are similar to internal combustion engines (both use reciprocating pistons in cylinders) except that the compressed air is not mixed with fuel and ignited to provide power but is instead delivered to a storage tank. The tank is equipped with high pressure and low pressure limit switches to stop the compressor motor exceeding the recommended tank pressure and restart it when the pressure drops.

Small air compressors (up to 5 horsepower (hp)) generally have a single cylinder while larger compressors may have multiple cylinders and may also be two stage which means that one compressor provides air at a low pressure to a second compressor that increases the pressure. This type (typically up to 30 hp) often has a radiator to cool the air heated by the first compressor before it is delivered to the second compressor. This added complexity provides higher efficiency as less power is required to pressurize the same amount of air.

Rotary screw compressors: use two matching helical screws to force air into a smaller volume as they rotate.

Vane compressors: use a slotted rotor with sliding blades to compress air as it rotates in an eccentric chamber.

Centrifugal compressors are essentially rotating fans that drive air out into a smaller space, typically used to move large volumes of air at lower pressures.

Axial compressors are rotating fans ranging from the domestic fan (use to create air movement for comfort) to the axial flow turbines used to compress air in the gas turbine (jet) engines used by aircraft.

Air compressors are used extensively in industries of all kinds. Probably the most familiar being the mobile compressors used for pneumatic chisels (jack hammers) on demolition jobs or on road and sewer maintenance. But air compressors are used in every air conditioning unit and every refrigerator, by every cement plant, every petroleum refinery, every wastewater treatment facility, in every power plant and in most industrial operations.

They are also used to fill scuba diving tanks, operate pneumatic brakes on cars, trucks and locomotives, fill pneumatic tires, clean and drill teeth in dental offices, spray paint, blow glass bottles and thermoformed plastics, aerate water supplies and pressurize aircraft flying above 10,000 ft.

Pressure testing for all pressure vessels, like boiler and compressed gas tanks, is mandatory before use and periodically to check for deterioration while in service. Typically, the vessel is filled with water that is pumped to twice the maximum operating pressure. (If the vessel fails, water does not expand explosively unlike pressurized gas).

Vacuum pumps work in the same way as compressors except that air is sucked out of a tank instead of being forced into a pressure tank. A major industrial use of vacuums is in the plastic thermo-forming business when a sheet of heated plastic is sucked into a mold to make plastic components or for packaging.