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How does an air compressor work? A simple (and common) question but the answer is a little more complicated than you think, which is why we took the time to write this article explaining in detail a variety of topics to give you a full picture on how an air compressor truly works.
How does an air compressor work?
For anyone who has a workshop, whether you’re a professional or home user, an air compressor is an invaluable piece of equipment. Pneumatic tools are fantastic. Because they use the power of compressed air, instead of an electric motor, pneumatic tools are more compact and lighter than their electric counterparts. In addition to this, pneumatic tools are much more durable. There’s very little that can go wrong with pneumatic tools. There’s no brushes or capacitors to replace. Furthermore, any electric motor has a limited lifespan. A pneumatic tool uses very basic technology and, if correctly maintained, will outlast most other power tools.
Many professionals and DIY enthusiasts use an air compressor for spray painting or sealing wood, concrete or other materials. For these applications, there is no alternative. An air compressor is an essential part of high-pressure spray systems. Portable compressors are very popular with contractors as they provide compressed air for a variety of tools on any job site.
All pneumatic tools use the latent energy of compressed air as their source of power. Essentially, this is a transfer of energy provided by an electric motor or gas engine. By compressing the air it has energy, resulting from the pressure difference in the compressor’s tank and the surrounding atmosphere.
One of the biggest advantages of pneumatic tools is the high torque that they provide. They also don’t generate the heat that comes from an electric motor. This means that when a pneumatic tool is working under extreme conditions, the extra load placed on it, won’t cause the tool to overheat.
Because an air compressor is an integral part of pneumatic technology, it’s a good idea to understand them. So, how does an air compressor actually work?
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The Technology behind Air Compressors
While some air compressors use an impeller pump to compress the air, most use reciprocating piston technology. A compressor that uses a reciprocating piston works in very much the same way as an internal combustion engine. Like in a gas engine, there are a few basic components that make up a piston air compressor: a crankshaft; connecting rod; cylinder; piston and valve head.
Most air compressors will use an electric motor (though some may use a gas engine) to drive the crankshaft. As the shaft turns, the connecting rod pushes the piston up and down inside the cylinder. The valve head has two valves – one inlet and one outlet. There are two effects that are caused by the piston’s motion.
On the downward stroke, a vacuum is created in the head. During the downward stroke, an inlet valve opens and allows air to fill this vacuum. On the upward stroke, the air is compressed by the force of the piston pushing against it. During this compression stroke, an outlet valve opens to allow the air to be forced into a compression tank. With each stroke, more air is forced into the tank, causing the pressure in the tank to build.
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Lubrication for Air Compressors
Like the engine in your car, the cylinder in the compressor needs lubrication. Traditionally, air compressors have always used oil to lubricate the piston. Oil-free lubrication is quite common these days. An oil lubricated compressor has an oil sump or oil bath and the oil is pulled up by the motion of the piston. This oil is splashed onto the walls of the cylinder to lubricate it. Despite the use of piston rings that prevent oil from entering the valve head, microscopic particles of oil are pushed through and mix with the air that’s being compressed.
This oil can be beneficial in some cases, though there are instances where oil in the air has a negative impact. Pneumatic tools that require lubrication during their operation will benefit from the oil contained in the compressed air. Some tools, most importantly, spray guns where paint is used, will be adversely impacted by oil in the air.
Oil will inhibit the ability of paint to adhere to a surface. There are filtration systems to remove oil from the compressed air. However, compressors that don’t use oil lubrication will eliminate the problem of air becoming contaminated by oil. These compressors are treated with a lubricant with a long life (often silicon-based). Oil-lubricated compressors are more durable because the oil can be replaced. This requires a maintenance program but will ensure that the piston remains properly lubricated for the life of the machine.
Compressors that don’t use oil, are sealed and the lubricant cannot be changed or topped up, this means that the life of the compressor is limited to the effective lifespan of the lubricant used. Despite oil-free lubrication being less durable, many prefer this system because there’s no risk of oil contamination in the air and there’s no need to check the oil level or change the oil – this means less time spent on maintenance.
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We’ve already looked at the presence of oil in the compressed air and this can have a positive or negative effect, depending on the intended application. There are some contaminants in the air that must always be avoided.
Dust and other solid particles in the surrounding atmosphere can harm the components used in pneumatic tools. When it comes paint and other spray applications, particles in the air will result in an inferior result. To prevent dust from entering the compressor tank, an air filter is fitted to the compressor’s air intake. The type of filter that’s used can vary, with some manufacturers opting for simple air filters that are cheaper and others using industrial grade air filters. It’s always possible to retrofit a more effective air filtration system if needed. Professional spray booths usually use specialized, multi-stage air filtration systems to ensure the highest quality results.
Water is another factor that is harmful to both spray paint quality and the durability of most pneumatic tools. An air compressor creates heat, both from the motor and the friction of the moving piston. This heat causes increased evaporation of the water molecules in the surrounding air. When the air cools inside the compressor tank, the water condenses and settles at the bottom of the tank. As the water increases inside the tank, it will mix with the air leaving the tank.
Some air compressors using cooling fins to cool the air before it enters the tank. This will reduce the amount of condensation in the tank but not eliminate it completely. All compressor tanks are fitted with a drain valve at the bottom of the tank to extract the water that collects inside it. A water trap can also be fitted to the air outlet. This uses a venturi effect to pull the water suspended in the compressed air into a reservoir at the bottom of the trap, preventing the water from entering the pipe that leads to the tool that you’re using.
Water buildup in the tank is unavoidable, it can be managed, but the tank will need to be drained periodically. How often the tank needs draining will depend on the air intake cooling system and for how long the compressor runs. A larger compressed air tank will require less frequent draining, simply because the larger volume of the tank will mean that there’s lower ratio of water to air in the tank.
Compressor Volume and Pressure
Pneumatic tools work in different ways. Some pneumatic equipment will require High Volume Low Pressure (HVLP), others use Low Volume Low Pressure (LVLP) – these are usually spray guns. Orbital pneumatic tools will vary in pressure and the volume requirements depending on the power or size of the tool. Pneumatic tools have adjustable valves that allow you to control the pressure and volume of the air that you’re using for that machine. Tools typically operate at either 60 PSI or 90PSI.
The volume that’s produced by the air compressor is measured in Cubic Feet per Minute (CFM). This tells you how many cubic feet of air is forced into the tank per minute. A large piston with a high horse power motor will produce the greatest CFM rate. When using pneumatic tools, the size of the tank will have a big effect on how you work. Even if a compressor produces a low air volume, this can be compensated for by using a larger tank. A high volume of compressed air in the tank means that it will take longer for the pressure to drop inside the tank. It’s easy to add additional tanks to the outlet manifold to increase the volume of compressed air that you store.
All compressors are fitted with a pressure control switch that controls the starting and stopping of the compression pump. This is a spring loaded switch. A valve on the tank directs air into the switch. As pressure builds up in the tank, the spring will compress. The spring is connected to a lever that opens or closes the switch and is calibrated for a top pressure setting and a bottom pressure setting. When the spring is compressed to the maximum pressure rated for the tank (usually 120PSI-125PSI), the switch will open and cut the power to the motor. As air is released from the tank, the spring decompresses until it reaches minimum pressure (usually 90PSI – 95PSI). When the tank pressure reaches this point, the switch will close and power will be supplied to the motor.
Some air compressors will be fitted with a regulator at the outlet which allows you to set the pressure exiting the tank. These compressors will have two gauges – one to indicate the tank pressure and one to indicate the pressure on the output line. Compressors are usually fitted with a safety cut-off in case of a failure in the pressure control switch. If the pressure in the tank exceeds its maximum safety point, the power will be switched off. This is usually accompanied by a pressure relief valve that will open and release air from the tank until it reaches a safe level.
Compressors can be very small handheld machines for light duty work. A small, handheld compressor will only be capable of supplying enough compressed air to run a light-duty nail gun or staple gun. Industrial air compressors can reach a size that will run an almost infinite amount of pneumatic tools. There are many sizes in between with portable options available.
Portable air compressors are very popular with contractors and home users who don’t have much space. Some portable air compressors are classified as silent air compressors. Though not completely silent, these compressors are quiet enough to use in an enclosed space. The noise levels on silent air compressors are usually low enough as to not interfere with a conversation in a normal speaking voice.
Choosing the right size air compressor means factoring in the need for portability and the amount or type of pneumatic tools that are going to be used. For spray guns, it’s important to have a constant pressure, as a drop in pressure will negatively affect the quality of the paint finish. A larger volume tank is always preferable when spray painting as this results in a more gradual pressure drop as the air is released from the tank. A low CFM compressor can often result in delays in working time because you may have to wait for the tank to build up pressure when a large volume of air is being used.
We hope you now have a better understanding of air compressors and how they function, and the next someone at a party asks : “How does an air compressor work?” You will be able to answer them confidently.
Learn more about specific models by reading the article : Best Air Compressors Buying Guide