How Do Air Compressors Work?Last updated on: June 10th, 2020 at: 10:49 am
How Do Air Compressors Work?
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In the modern world of pneumatics, air compressors are vital to the operations of factories and workshops across the globe. But they haven’t always been. Air compressors are a relatively recent invention in the context of machine-age history.
Before air compressors, many tools got their power from complicated systems with belts, wheels and other large components. This machinery was massive, heavy and costly, and typically out of reach for many small operations. Today, air compressors come in many shapes and sizes, and you can find them in large shop floors, auto workshops and even your neighbor’s garage. In this guide, we’ll discuss air compressors and how they work.
What Are Air Compressors Used For?
Air compressors can be used for a wide variety of tasks. They can supply air for filling up objects like tires or inflatable pool toys, or they can supply power for operating tools. Some of the equipment that works well with compressed air power includes:
- Nail guns
- Spray guns
From drills to AC units, many versatile air-powered tools and machines are responsible for the comfort, shelter, automation and efficiency of everyday life. The compressors themselves are more compact and lightweight than other centralized power sources. They are also long-lasting, require less maintenance and are easier to move than other old-fashioned machinery.
Piston Air Compressor Functionality
So, how does an air compressor get air? For those that use pistons, it involves two parts: Raising the pressure and reducing the volume of the air. Most compressors use reciprocating piston technology.
An air compressor typically uses:
- An electric or gas motor
- An inlet and a discharge valve to draw in and release air
- A pump to compress the air
- A storage tank
The compressor draws in air and creates a vacuum to reduce its volume. The vacuum pushes the air out of the chamber and into its storage tank. Once the storage tank reaches its maximum air pressure, the compressor turns off. This process is called the duty cycle. The compressor will turn back on when the pressure drops below a specific number.
What Is Air Displacement?
Air displacement is at the core of each air compressor. To compress air, the internal mechanisms within the compressor move to push air through the chamber. There are two primary types of air displacement used for this purpose:
- Positive displacement: Most air compressors use this method, in which air is pulled into a chamber. There, the machine reduces the volume of the chamber to compress the air. Next, it is moved into a storage tank and saved for later use.
- Dynamic displacement: Also called nonpositive displacement, this method uses an impeller with rotating blades to bring air into the chamber. The energy created from the motion of the blades builds up air pressure. Dynamic displacement can be used with turbocompressors because it works quickly and generates large volumes of air. Turbochargers in cars often use dynamic displacement air compressors.
The Mechanics of an Air Compressor
How air compressors work can vary according to design. Piston-based air compressors can have one of two types of compression cycles:
- Single-stage: The piston compresses the air in one stroke. A stroke is one full rotation of the crankshaft driving the piston.
- Two-stage: The first piston compresses the air before moving it to a smaller cylinder, where another piston compresses it further. This design allows the compressor to generate higher pressures.
For many 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 and the production of compressed air. In most operations, you don’t need to reach that pressure limit, so many compressors set air lines to a regulator. With a regulator, you can input the appropriate pressure level for a given tool.
When the specified pressure is reached, the regulator shuts off the pump at any point in its cycle, which means the piston can be halfway through a stroke, with pressurized air in the chamber, when it stops. This air can put undue pressure on the starting circuit, which needs more power to start the motor. An unloader valve is a simple addition that releases the trapped air to avoid this problem.
A regulator is bookended by two gauges: one to monitor the pressure of the tank and another to monitor the pressure within the air line. Also, the tank has an emergency valve that triggers if the pressure switch malfunctions.
What Is a Reciprocating Piston?
The reciprocating piston involves the following parts:
- Connecting rod
- Valve head
It works similarly to a combustion engine in a car. The crankshaft rod raises the piston in the cylinder and pushes air into the compression chamber, decreasing air volume and increasing the pressure. The piston closes, forcing the compressed air into the storage tank. Then the piston opens again to draw in more air and start the process over.
Compressors that use pistons can be louder than some other designs because of the way the machine’s components move and generate friction. But new technologies and advancing designs are bringing dual and multi-piston models that can make things quieter by splitting up the workload.
The Rotary Screw Air Compressor
In many heavy-duty industrial applications, a piston compressor just doesn’t cut it. For the higher pressures needed for complex pneumatic and high-powered tools, professionals tend to choose rotary screw air compressors.
While the piston air compressor uses pulsation and the alternating nature of the piston mechanic, a rotary screw compressor is continuous. A pair of rotors mesh together to pull air in and compress it as it moves through a spiral. The rotary motion moves the air through a chamber and ejects it. Fast rotational rates can minimize leakage.
Many compressor types require you to take measures to minimize vibrations that can hurt the machine, but most rotary screw compressors offer even, vibration-free performance.
Rotary screw compressors can vary widely, with rates from 10 CFM to those in the 4-5 figure range. Control schemes include:
- Stop/start: This approach either provides power to the motor, or it does not, according to the application.
- Load/unload: The compressor is powered continuously, with a slide valve that reduces the tank’s capacity when a specific compression demand is met. This scheme is common in factory environments, and if it involves a stop timer, it is called a dual-control scheme.
- Modulation: Modulation also uses a sliding valve to adjust pressure by throttling/closing the inlet valve, matching the compressor’s capacity to the demand. These adjustments are less effective on rotary screw compressors than other types. Even when set to a 0 capacity, the compressor would still consume about 70 percent of its full power load. Still, modulation is applicable for operations in which frequently stopping the compressor is not an option.
- Variable displacement: This control scheme adjusts the volume of air that is pulled into the compressor. In rotary screw compressors, this method may be used alongside modulating inlet valves to improve efficiency and pressure control accuracy.
- Variable speed: Variable speed is an efficient way to control a rotary compressor’s capacity, though it may respond differently with different types of air compressors. It varies the speed of the motor, which affects the output. This equipment tends to be more delicate than other designs, so it may not be suited for particularly hot or dusty work environments.
How Lubrication Works in Air Compressors: Oil-Flooded vs. Oil-Free
One of the most important things to know about air compressor maintenance is how lubrication works. When you’re looking at oil pumps, you’re dealing with two categories:
- Oil-lubricated pumps: In this design, oil splashes onto walls and bearings within the cylinder. This method is also called oil-flooded lubrication and tends to be more durable. A piston ring is a piece of metal on the piston that helps to create the seal inside the combustion chamber. This ring can help keep oil out of the compressed air, but sometimes it can still seep into the tank.
- Oil-free pumps: Oil-free pumps receive special lasting lubrication that eliminates the need for oil. In many industries where contamination is not an option, like breweries, food production and pharmaceutical manufacturing, oil-free pumps are an excellent option. They ensure that no oil contaminates the air they use in their process or product.
Oil-flooded pumps are somewhat of a mixed bag. For power tools that need lubrication, the presence of oil in the air stream can be beneficial. For tools that need oil, inline sources can distribute oil in even amounts. Many tools, on the other hand, can stop working correctly when even minute amounts of oil are present in the air stream.
For painting or woodworking, oil can interrupt the entire process. It may keep coatings from drying or finishing evenly. Airborne oil can even corrupt the surface of wood projects.
Thankfully, there are tools to prevent oil from entering the tank, like air-line filters and oil separators, but when oil-free air is critical to operation, oil-free compressors and their permanent lubrication is the best option.
Air Compressor Power Ratings: What Is CFM?
When we talk about power in an air compressor, we usually speak in terms of horsepower, but there are many other ways to determine how much pressure a machine can provide. We use cubic feet per minute (CFM) to discuss the rate and volume by which a machine compresses the air. But the rate at which outside air enters the cylinder is influenced by heat, humidity and wind in the surrounding atmosphere.
To consider these internal and external factors, manufacturers use standard cubic feet per minute (SCFM), which combines CFM with those outside factors of pressure and humidity.
Another rating that you might see is displacement CFM, which looks at the efficiency of the compressor pump. It pulls information from the revolutions per minute (RPM) of the motor and the volume of air that the cylinder can displace. This number is more of a theoretical measurement, while you can also measure CFM in terms of delivered air, or how much is actually discharged. This number is called CFM FAD, which stands for free air delivery, and is useful for measuring delivery to certain tools.
Pumps vs. Compressors: Two Instruments for Harnessing Air
Some degree of confusion exists between the words “pump” and “compressor,” with many believing them to be the same thing. In reality, the distinction between the two is a crucial part of discussing air compressors:
- A pump takes liquids or gasses and moves them between places.
- A compressor takes a gas, squeezes it down to a smaller volume and higher pressure and sends it elsewhere.
The most significant distinction is that a pump can work with liquids, while a compressor cannot. Liquids are much harder to compress. You may find a pump within a compressor, such as in a reciprocating air compressor — the part that performs the compression is a pump. The functions of pumps and compressors can overlap on machines where the pressure rises with each revolution.
Take a tire pump, for example. While it happens to perform both tasks — moving air and reducing its volume — it’s purpose is to move outside air somewhere else, into the air-tight space of the tire. Since its goal is not to reduce the volume, it’s not technically considered a compressor. An alternative example would be using pneumatic tools, which require compressed air. The device that reduces the volume of the air is a compressor.
Air pumps generally fall into one of two categories:
- Reciprocating pumps, which move back and forth. A bicycle pump is a reciprocating pump, where the cylinder pulls outside air in with a back-and-forth motion and moves it into the tire.
- Rotary pumps, also called centrifugal pumps, which spin. A rotary pump uses an impeller, which is basically an enclosed propeller. It has blades that move incoming fluid and send it through an outlet at high speed. This pump uses motorized energy to pull fluids from one place to another, and is not to be confused with a turbine, which captures fluids that are already moving.
Compressed Air in Everyday Life
From pneumatic drills and braking systems to HVAC 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 on a given day, air tools helped someone sand the wood, paint the walls and hammer beams and plasterboards into place. On shop floors across the world, people use compressed air to add coats of paint and blast dust and debris away.
It’s nothing short of remarkable that mankind discovered a way to take ambient air, perhaps the most abundant resource on the planet, and transform it to power motorized equipment for a wide variety of purposes.
Quincy Compressor offers high-quality air compressors in many styles, including rotary screw, reciprocating/piston and oil-free. Use our sales and service locator to find a dealer near you.