By Melissa C.
Air Compressor Expert
Millions of people use air compressors every day, but if one was in front of you, could you explain how it works?
You've probably seen
where and how they're used over the years, but knowing the mechanics behind how the parts work together are more applicable than you might think.
Already know the basics? Read our Ultimate Air Compressor Guide
to learn which type of air compressor is perfect for your application. This article is for the gearheads, the techies, and the innately
curious.
Early humans might not have understood anatomy or physics, but they understood the potential of using forced air compressed by their lungs. By blowing on their fires, our ancestors were able to grow the flames into larger, more powerful sources of heat and energy.
Thousands of years later, the societies of the iron and bronze ages had evolved to
create useful tools that could be implemented on a larger scale. Metalworkers in China's Han dynasty used hand and foot-powered bellows to stoke the flames in their forges as early as the Third Century BC.
It wasn't until thousands of years had passed that an engineer named John Smeaton devised a way to compress even greater volumes of air in 1762. His invention employed a water wheel to power a moving cylinder-a piston-that compressed air in a large chamber.
Smeaton's design led to further innovations, such as John Wilkinson's development of a machine with an efficient rotating cylinder in 1776 and George Medhurst's invention of a motorized air compressor in 1799.
Since the days of wheels, belts, and metalwork, the most significant innovations in air compressors have taken place in the last 50 years, and as recent as today as the market continues to evolve to keep up with ever-increasing industrial demands.
The concept of compressed air is simple: when atmospheric
air is stored under pressure, it creates potential energy that can be held inside a tank until it's needed. Just like a balloon being released, when the pressurized air is released, the potential energy is converted into usable kinetic (motion) energy.
By harnessing that transfer of energy through pneumatic (air) tools, we are able to work on tasks that were before impossible.
The core of how air compressors work is boiled down to two methods of air
displacement.
In order to compress air, the internal components of a compressor must move or change position to mechanically force the air through the chamber where it is compressed and stored until use.
Positive Displacement is the method that most compressors use. Air is pulled into a
chamber that opens and closes, where the internal parts reduce the volume of the chamber and compress the air. Once the cycle is complete, the air is pushed through the chamber and into a storage tank where it waits to be used.
Piston, rotary-screw, and scroll-type air compressors all use positive displacement to compress and store air.
Dynamic (nonpositive) Displacement
uses rotating blades on an impeller to pull air into the chamber, where the energy from the motion of the blades builds air pressure. Often used with turbocompressors, this method of air compression produces huge volumes of compressed air quickly, and this technology is commonly found in cars that use turbochargers.
Air compressors that utilize this method of displacement are often reserved for commercial/industrial applications when large volume flow-rates and constant pressure is needed.
Compressor drives are most commonly direct-drive or belt-driven systems. In a belt-driven system, as the motor turns, the belt turns with it, thus activating the pump in the system. This is the more economical option and is widely used across all compressor types because the belts can be adjusted to change with air demands.
In a direct-drive system, the motor attaches directly to the crankshaft of the compressor, allowing for smaller designs and fewer maintenance requirements. Although not much adjustment can be made to these drive systems, they are more efficient because less power is lost in the transfer of power to the crankshaft.
Another benefit of direct-drive is the ability to provide compressed air without needing to fill a storage tank first, like using a DC compressor for off-roading.
Depending on the type of air compressor, basic air compression requires an electric-powered motor, a pump with an internal mechanism to compress the air, an inlet/outlet valve to draw in and release air, and-in most cases-an air storage tank.
Air is drawn into the compressor where the internal components reduce the volume of the air by creating a vacuum, which drives the pressure of the air up as it is pushed into a holding tank. Once the maximum pressure is reached inside the tank, the duty cycle is complete, and the compressor shuts off until the pressure falls below a set threshold. Positive displacement air compressors do this in different ways: by using pistons, screws, and scrolls.
Piston-type air compressors operate similarly to the combustion engine in your car. When the crankshaft's rod raises the piston inside the cylinder, it forces air into the compression chamber, decreases air volume (thus increasing air pressure) as the crankshaft drives the piston closed and pushes the compressed air into a storage tank as the piston opens again to draw more air in.
Piston compressors can achieve a full compression cycle (stroke) in 1 or 2 stages:
Piston compressors are notorious for being louder because the internal components rub together and create friction. However, advancing technology is improving the way they operate by introducing dual and multi-piston, single-stage models that use up to 4 pistons inside the pump. By using multiple pistons, it's possible to extend the life
of the unit by dividing up the work of a single-piston and achieve a supremely quiet air compressor.
Instead of using a piston, rotary screw compressors squeeze air between two helically-opposed screws that don't touch, reducing noise and maintenance.
These compressors were created
for heavy-duty applications that require high power during extended periods and are ideal for maximum air intake and production.
Rotary screw compressors are oil-sealed and have fewer moving parts, so they operate more quietly and require less maintenance because of the design.
The third type of positive displacement compressor, scroll compressors are known for their highly-efficient, oil-free, and quiet operation. Similar to rotary compressors, the scroll design uses two interwoven metal pieces that work together but don't touch to create a vacuum.
Two spiral-shaped circular pieces rotate around one another to compress incoming air. One scroll is fixed in place and doesn't move, and the other fits inside the stationary scroll and moves in a tight circular motion without rotating.
Just like your car uses oil, air compressors need lubrication to continue running smoothly over time.
Lubricated air compressors use oil to reduce wear and friction on their moving parts. In the case of piston compressors, the oil is applied to the system in one of two ways:
Non-lubricated compressors are also referred to as oil-less or oil-free because their parts are coated in special chemicals or materials like Teflon to reduce friction instead of conventional oil. The solution here is permanently lubricated components. The drawback of non-lubricated compressors is that they heat up faster, and aren't the best option for heavy-work environments.
Most oil-less air compressors are used in industries where clean air is required for manufacturing, such as in the food, beverage, and electronics industries. For example, scroll compressors are used in the dental industry because of their continuous clean and quiet operation.
When it comes to sizing an air compressor for airflow and air volume, there are two criteria professionals use to determine if an air compressor can properly handle an
application,
and determine the kinds of tools that will be compatible with your air compressor.
"Pressure"
is the amount of force applied to the surface of a given area. For compressed air and gases, this force is measured in
pounds-force per square inch, or PSI. The higher the PSI rating, the greater the amount of force applied to the air within a compressor.
Airflow volume
is a measure of the rate at which air can move into a compression cylinder and the machine can compress it. For air compressors, the volume of air is measured in
cubic feet per minute, or CFM. The higher the CFM value, the greater the volume of air a compressor can generate.
An easy way to understand how pressure and volume work together is to think of a garden hose. By putting your thumb over part of the hose opening, the same amount of water is forced through a smaller space, thus increasing pressure at a constant flow. And, by increasing the amount of water coming through the hose, more work can get done faster.
It used to be that air compressor technology was expensive to produce, and used a lot of energy to power, making them only affordable for the commercial industry. Even today, commercial air compressors still use a lot of energy.
As a general rule, you can expect to spend about $500 per horsepower (HP), per year using an industrial air compressor in continuous use. For example, a 10HP compressor will generally equal $5,000 in operations cost per year.
In efforts to bring down costs and increase efficiency, new technologies are being introduced each year that innovate the possibilities and capability of compressed air. Engineers are finding ways to make compressors more powerful and more energy-efficient, such as
variable speed drives
(VSD)
that
allow the compressor to change motor speeds and voltage as air demands change, saving energy and money.
Even newer is the
variable frequency drive
(VFD) technology that takes this concept a step further. This technology allows the compressor to control the motor speed and torque by alternating from A/C to D/C power and controlling the input frequency and voltage, meaning the most efficient power usage available on the market.
Other innovations, including changes to the rotor speeds of rotary screw compressors, the use of water as a lubricant in place of oil, and the incorporation of remote monitoring systems all promise to deliver even greater improvements to efficiency in the near future.
The short of it is: air compressors aren't going to disappear, but instead continue to allow millions of people to get more work done faster and more effectively.
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By Melissa C.
Air Compressor Expert
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While a number of compressor types exist—scroll, reciprocating, rotary screw, centrifugal, and more—all air compressors do one thing: compress air.
But how do air compressors compress air? Do the different types of compressors compress air differently and if so, how? And why does it matter, anyway?
Let’s answer the last question first.
Different uses of compressed air require the air stream to have different dimensions.[1] The most critical of these dimensions are air pressure, air flow and air quality. While all compressor types follow the same basic air compression process, some steps in that process differ for each type. Those differences can limit the practical values for some of the dimensions of the air stream they create, so the type of compressor you need depends to some degree on the use you’re putting your compressed air to.
Now that we know why the type of compressor can matter, we can go back to the first question and look at the air compression process in general. But even before we talk about the compression process, let’s take a quick look at how air compressors can get their power.
While compressed air is a source of power for air tools, instruments, and more, the compressor itself requires power. The following are the most common power sources for compressors:
Sullair 185 portable diesel air compressor on a job siteThe major steps in the air compression process are intake, compression, integrated storage, integrated cooling, and discharge, although not all compressors need integrated storage or cooling.
To create compressed air, you need, well, air, so the first part of the compression process is air intake. During air intake, air is drawn into the compressor through an air inlet valve.
The air inlet valve is often preceded by a filter, which protects the compressor by reducing the contaminants entering it.
The air then flows into the compression chamber where it is compressed.
We’ll look more closely at how each type of compressor does this when we cover the compressor types, but two fundamental concepts apply to all compressors.
Compressors compress air through either positive displacement or dynamic displacement (also known as non-positive displacement).
Whatever their means of compression, all compressors are either oil lubricated or oil free.
Depending on the compressor type, next the air may flow into an integral receiver tank (sometimes called a storage tank or an air tank) after being compressed.
Many uses of integrated storage are tied to the compressor type and will be covered there. However, two common reasons are shared across multiple types.
Note the difference between limited duty cycle compressors and modulating compressors.
Compressing air creates heat. While the air does not have to be cooled before leaving the compressor, most three-phase electric compressors and some diesel compressors (regardless of type) come with integrated aftercoolers to lower the air temperature before discharge.
Compressors with aftercoolers will also have water separators to remove the excess moisture that drops out of the air stream during cooling.
Finally, the air flows through the discharge valve, either directly to the point-of-use (e.g., a chipping hammer on a portable diesel compressor) or to a series of dryers and filters first (e.g., for instrument air in a manufacturing plant).
All types of air compressors follow the basic process given above, but the details can vary in important ways. Let’s look briefly at how four common types of compression work and how that affects key steps.
Scroll compressors are positive displacement, oil free compressors.
Scroll compressors use two interleaved scrolls to compress air. Depending on the design, one scroll can be fixed and the other rotate or both can rotate together. Because the scrolls never touch, no lubrication is needed.
The smooth, continuous movement of the scrolls also means that scroll compressors run quietly, have minimal vibration, and provide a pulse-free stream of air.
One notable limitation scroll compressors have is maximum air flow. While in theory a scroll compressor can scale up infinitely, the ever increasing diameter of the scrolls needed set a practical limit (at least to creating air efficiently). They have the lowest maximum flow of any of the compressors covered here.
Because they create pulse free, oil free air, scroll compressors do not require integral storage unless they use modulation.
Reciprocating compressors (sometimes called piston compressors) are positive displacement compressors and can be either oil lubricated or oil free.
Reciprocating compressors work similarly to compression chambers in car engines. On the upstroke, the piston creates a vacuum that allows air to flow into the compression chamber. On the downstroke, the air is compressed and forced out of the chamber. To achieve higher pressures and larger volumes efficiently, some designs use two stages of compression, i.e. the air compressed in the first stage is further compressed in the second stage.
Also, like a car engine, piston compressors are loud; in fact, they are typically the loudest of the compressor types covered here.
To maintain efficiency, the pistons must slide smoothly. They either need to be lubricated, which allows a small amount of oil to enter the air stream, or coated with a substance that reduces friction.
Because compression happens during only half the duty cycle, the air stream has a “pulse” when leaving the chamber rather than continuous flow and pressure.
Because their compression process creates a pulse in the air stream, reciprocating compressors always require integral storage, even when they are oil free. By providing the air from the receiver tank, instead of directly from the compression chamber, the air stream can have a continuous flow and pressure.
Rotary screw compressors are positive displacement compressors and can be either oil lubricated or oil free.
Rotary screw compressors use a pair of helical screws, often referred to as rotors, with lobes running the length of both. Air is forced down the length of the compressor along the lobes from a larger space into a smaller one, which compresses the air. As with reciprocal compressors, to achieve higher pressures and larger volumes efficiently, some designs use two stages of compression.
Oil lubricated screw compressors use a fluid to seal the gaps between the rotors, which also allows one rotor to drive the other, but it introduces a small amount of oil into the air stream.
Oil free screw compressors use timings with very tight tolerances between the rotor lobes instead of a fluid, so there is no oil that can enter the air stream.
Because the rotors run continuously, they produce a pulse-free stream of air. While not as quiet as scroll compressors, they are generally quieter than reciprocating ones.
Whether or not a rotary screw compressor needs integral storage depends on if it is oil lubricated or oil flooded.
Centrifugal compressors are dynamic displacement, oil free compressors.
Centrifugal compressors use an impeller rotating at high speed to initially increase the velocity of the air. Then the air is passed through a diffuser to decrease its velocity, which pressurizes the air. No lubrication is needed in the compression chamber, so no oil is added to the air stream.
Because air is created continuously as the impeller spins, the air flow and pressure are continuous, with no pulsing.
Because they create pulse free, oil free air, centrifugal compressors do not require integral storage unless they use modulation.
While knowing how compressors work can help you understand what type of compressors could work for your application, there’s more to consider: energy efficiency, maintenance costs, projected uptime, and more. Your local Sullair distributor has the experience needed to help you choose the best compressor for your needs.