How Do Air Compressors Work?
In today’s world of pneumatic operations, it’s hard to imagine a time when air compressors were nonexistent in factories or workshops. The fact is, in the context of machine-age history, air compressors are a relatively recent innovation. Not long ago, the air tools used in workshops typically drew power from complex systems comprised of belts, wheels and other large components. For the most part, such machinery was too massive, heavy and costly for smaller operations, and was therefore confined primarily to larger companies.
Today, however, air compressors are usually found at factories where products are assembled or in most places where cars are serviced, such as gas stations and auto workshops. The list of tools that run on compressed air is long, but some of the most common pneumatic tools include the following:
- Nail guns
- Spray guns
The most significant benefit of the standard workshop air compressor is its compact and relatively lightweight dimensions, which stand in contrast to centralized sources of power that generally utilize large motors. Additionally, air compressors last longer, require less maintenance, are easier to move from worksite to worksite and are far less noisy than old-fashioned machinery.
Piston Air Compressor Functionality
So, How Do Air Compressors Work?
Air compression is essentially a twofold process in which the pressure of air rises while the volume drops. In most cases, compression is accomplished with reciprocating piston technology, which makes up the vast majority of compressors on the market.
Every compressor with a reciprocating piston has the following parts:
- connecting rod
- valve head
Air compressors, for the most part, are powered by either gas or electric motors — it varies by model. At one end of the cylinder are the inlet and discharge valves. Shaped like metal flaps, the two valves appear at opposite sides of the cylinder’s top end. The inlet sucks air in for the piston to compress. The compressed air is then released through the discharge valve.
In certain air compressor models, the pressure is produced with rotating impellers. However, the models that are typically used by mechanics, construction workers and crafts people tend to run on positive displacement, in which air is compressed within compartments that reduce its space. Even though some of the smallest air compressors consist of merely a motor and pump, the vast majority have air tanks. The purpose of the air tank is to store amounts of air within specified ranges of pressure until it’s needed to perform work. In turn, the compressed air is used to power the pneumatic tools connected to the unit supply lines. While all of this is going on, the motor repeatedly starts and stops to keep the pressure at a desired consistency.
What the piston effectively does with its back and forth movements is create a vacuum. As the piston retracts, the space in front gets filled with air, which is sucked through the inlets from the outside. When the piston extends, that same air is compressed and therefore given the strength to push through the discharge valve — simultaneously holding the inlet shut — and into the tank. As more air is sent into the tank, the pressure gains intensity.
How Lubrication Works in Air Compressors: Oil Flooded vs. Oil Free
One of the most important things to know about air compressor maintenance is the way that lubrication works. When it comes to oil, pumps fall into two categories:
- Oil-lubricated pumps: Oil splashes onto the walls and bearings within the cylinder, i.e. an oil bath. This is known as “oil flooded” lubrication. Even though the piston rings are meant to keep the oil away from the compressed air, tiny amounts can sometimes seep into the tank. Generally, this type is more durable.
- Oil-free pumps: The bearings are treated with lasting lubrication. While these require less maintenance than oil-lubricated pumps, they can be noisier and put more strain on the motor.
Oil flooded can be a mixed blessing overall. When it comes to using power tools that need lubrication, the presence of oil in the air stream can be beneficial. For the tools that actually require oil, inline sources can be added that will distribute oil through such tools in even amounts.
However, there are some tools and operations where even the smallest bit of oily mist could have undesired effects. Certain tools, including nailers and sanders, are specifically intended to function without the slightest amount of oil. When the operation involves paint or woodwork, oil at any capacity will undermine the process. For instance, oil within spray paint will make it difficult for coatings to dry or finish evenly. On wood, airborne oil could cause surface corruption.
There are ways to prevent oil from entering the air tank in most circumstances, such as the use of air-line filters and oil separators. Nonetheless, for critical applications, the best way to prevent the air from ever being corrupted with oil is to simply opt for an oil-free compressor with permanent lubrication on the bearings.
One- vs. Two-Cylinder Air Compressors
In order to accommodate the vast range of pneumatic tools on the market, air compressors are manufactured in both one- and two-cylinder varieties. However, compressors used by private craftspeople and contractors often contain two-cylinders that function almost identically to single cylinders, the only real difference being that two strokes occur during each revolution. Some two-cylinder machines that are marketed to the public also work in two stages, where one piston sends compressed air to another cylinder for further compression.
For most single-stage air compressors, the preset pressure limit is 125 psi. When this limit is reached, a pressure switch goes off to stop the motor. In most operations, however, there’s no need to even reach the pressure limit. For that reason, the compressor’s air line is set to a regulator, where you input the appropriate pressure level for a given tool. The regulator is bookended by two gauges: one that comes in front to monitor the pressure of the tank, and another gauge at the end to keep the pressure of the air line in check. Furthermore, the tank is equipped with an emergency valve that triggers in the event of a mishap with the pressure switch. On some models, the switch might connect with an unloader valve, which can help reduce stress to the tank at times when the machine is deactivated.
In contrast to the automotive type pistons of most compressors, some models instead use a connecting rod without a wrist pin. On these models, the piston extends and retracts while leaning side to side as a surrounding seal holds the air inside.
Air Compressor Power Ratings: What Is CFM?
The amount of power being used by an active air compressor can be measured in terms of horsepower. Still, other things need to be taken into consideration in order to determine how much pressure a given machine can offer. The rate and volume by which a machine can offer compression is measured in cubic feet per minute (cfm). However, the rate at which outside air moves into the cylinder is impacted by the heat, humidity and wind of the surrounding environment. Therefore, the atmosphere itself will play a part in the cfm of a given machine.
As a way of taking internal and external factors into consideration, manufacturers use standard cubic feet per minute (scfm) to determine cfm in combination with outside pressure and humidity. Another type of rating is known as displacement cfm, which determines the efficiency of the compressor pump. This rating is drawn from a combination of motor rpm and cylinder displacement.
In order to determine which kind of tools an air compressor will be able to support, it’s crucial to look at a given machine’s cfm and psi ratings.
Pumps vs. Compressors: Two Instruments for Harnessing Air
A certain degree of confusion exists between the words “pump” and “compressor,” which are often perceived as being synonymous with one another. Technically, there are crucial distinctions between the two, the main differences being:
- A pump takes liquids or gases and moves them between places.
- A compressor takes a gas, squeezes the volume of that gas and then sends it elsewhere.
Basically, a pump will work with any fluid, whether liquid or gas, whereas a compressor can only do its function with gas because liquids are a lot harder to compress. In some cases, the portion of a reciprocating air compressor that performs the compression function is called a pump. However, pump and compressor functions overlap on pumps that have pressure gauges that rise with each revolution. One example of this dual function is a tire pump, which basically does both functions — moving air and creating a tighter air compartment — while filling a tire with air. Still, since the purpose of a foot pump is to move outside air into an air-tight space, the tool is not technically considered a compressor. After all, the purpose behind a compressor is to put compressed air to use, such as when pneumatic tools are powered.
Air pumps basically fall into two categories:
- Reciprocating pumps: the kind that move back and forth
- Rotary pumps: the kind that spin around
A perfect example in the reciprocating category is the bicycle pump, which holds a piston within a cylinder through which outside air is sucked in via back-and-forth motions and ultimately packed into a tire.
With rotary pumps, the action is a lot different because it involves an impeller, which is basically an enclosed propeller. The impeller is equipped with blades that spin incoming fluid, which is then sent through an outlet at high speed. Alternately, rotary pumps are often referred to as “centrifugal pumps” due to the action of sending fluid forward via spinning motions. Rotary pumps usually run on motorized energy to pull fluids from one place to another, and are therefore not to be confused with turbines, which capture fluids that are already moving.
The Rotary Screw Air Compressor
For certain heavy-duty industrial operations, piston compressors are considered insufficient. In order to get the pressure intensity needed for complex pneumatic and other high-powered tools, professionals will generally opt for rotary screw air compressors. Unlike the piston air compressor, which relies on pulsation, the rotary screw air compressor produces an ongoing movement to generate power.
In a rotary screw compressor, air is compressed with a meshing pair of rotors. As the screws move in rotation, fluids gets sucked in, compressed and ejected. In order to keep leakage rates at an absolute minimum, fast rotational rates are vital throughout the operation. Due to their even, vibration-free performance, rotary screw air compressors generally don’t need spring suspension. Some of the faster models, however, are mounted with tremor-absorbent rubber.
Rotary screw compressors range in size from 10 cfm to anywhere in the 4-5 figures. Control schemes vary based on the model, but a general rundown of the different schemes is as follows:
- Stop/start: Under this scheme, the machine powers and un-powers the motor according to the needs of the operation.
- Load/unload: With this scheme, the compressor is powered non-stop, but a slide valve activates to reduce the capacity whenever a compression demand has been met. Load/unload is the most common scheme in factory settings. In unload mode, a compressor draws upon only 25% of its overall capacity. On machines where this scheme also includes a stop timer, it’s referred to as a dual-control scheme.
- Modulation: As with the load/unload scheme, modulation relies on a sliding valve to adjust pressure according to the demands of a compression operation. However, adjustments under the modulation scheme are more limited. Even when modulated to zero-load capacity, a machine’s power consumption will still hover around 70%. Nonetheless, modulation is the most applicable scheme in operations where the frequent stopping of a compressor would be unfeasible.
- Variable displacement: Under this scheme, the volume of air swept into the compressor is adjusted according to the needs of the operation. On some models, variable displacement is achieved with plumbed lifting valves along the suction end.
- Variable speed: A self-explanatory scheme that has its pros and cons in terms of cost. While typically less expensive to operate than a load/unload compressor, the power inverter that enables variable speed in select models can also come with higher design costs. The equipment used in variable speed drives is also generally more delicate, and thus not exactly suited for hotter, dustier working environments.
Air Compression in Everyday Life
There are some inventions that actively engage people on a regular, day-to-day basis, and other inventions that make the world go around from behind the scenes. Air compressors — and just as importantly, the inner workings of such machinery that make it possible to transform air into power — fall into the latter category. From pneumatic drills to AC units, a vast range of air-powered tools and machines are responsible for the comfort, shelter, automation and efficiency of everyday life. In nearly every building you walk through or pass by in a given day, air tools have been used to sand the wood, paint the walls and nail and hammer beams and plaster boards into place.
Put simply, it’s nothing short of remarkable that mankind discovered a way to take ambient air — perhaps the most abundant of resources on the planet — and transform it in a way that can power motorized equipment for the purposes of construction, manufacturing, automation and everyday convenience.
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