A piston compressor is the oldest and most common type of industrial compressor. It uses the principle of displacement to increase the pressure of the enclosed volume of gas or air.
Piston Compressor Configurations
Piston compressors are available in a variety of configurations, including single-acting, double-acting, oil-lubricated or oil-free. One of the most common configurations is the double-acting design, which features an L-configuration with one vertical, low-pressure cylinder and one horizontal, high-pressure cylinder. Oil-lubricated designs use splash lubrication or pressure lubrication and oil-free designs feature piston rings or toothed walls.
Most piston compressors have self-acting valves that open and close based on pressure differences on both sides of the valve disk. Some larger machines are equipped with a crosshead, seals and a ventilated intermediate piece to prevent oil from being transferred into the compression chamber.
Oil-lubricated compressors
As the name suggests, these compressors normally work with splash lubrication or pressure lubrication. Most compressors have self-acting valves. A self-acting valve opens and closes through the effect of pressure differences on both sides of the valve disk.
What are oil-free piston compressors?
Oil-free piston compressors have piston rings made of PTFE or carbon, and alternatively, the piston and cylinder wall can be profiled (toothed) as on labyrinth compressors. Larger machines are equipped with a crosshead and seals on the gudgeon pins, and a ventilated intermediate piece to prevent oil from being transferred from the crankcase and into the compression chamber. Smaller compressors often have a crankcase with bearings that are permanently sealed.
So, how do reciprocating piston 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:
- crankshaft
- connecting rod
- cylinder
- piston
- 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 craftspeople 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 the 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.
Pros and cons:
Pros:
- Relatively cheap
- Easy maintenance (easy to understand the inner working) .
- Suitable for high pressures
Cons:
- Very noisy. You really need to put this type of compressor in a sound-isolating box or in its own room.
- High outlet temperature of compressed air
- High oil content in air piping.
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