How Does an Air Compressor work?

by Rob

A valuable tool for industrial and DIY use, air compressors have numerous functions. Invented in the 19th Century, automated models are widely used throughout many industries such as mining and metal work.

Direct Air have created an in-depth guide on the workings of air compressors to help provide an understanding on how these machines work and give advice for those trying to figure out which model is best for them.

Garage Air Compressor. Adjusting Pressure in a Car Tires.

The two main categories of compressor are the scroll (piston) and rotary screw (reciprocating). Below is a break down of the various elements and what they are most suited to.

Single and Dual Phase Compressors

These are the most common form of air compressor and they operate in very similar ways; the only difference is dual phase have one more step to the process.

Single phase air compressors draw air through a cylinder and it is then  compressed with a single piston movement in a vacuum system. The air is then sent into a storage cylinder until it is required.

A dual phase follows these steps but after the initial compression, it is then sent to another cylinder for compression a second time.

Typically, single phase air compressors are suited to home use while dual phase are used for more industrial tasks. Dual phase tends to be more robust and can handle more strenuous tasks.

Lubricating Air Compressors

All air compressors require lubrication, this can be done in two ways, oil based or oil free.

For air to be drawn effectively into a chamber, lubrication needs to be applied, oil free air compressors use a non-stick coating such as Teflon within the cylinder.

These are best suited to a DIY environment; they are less costly and tend to require less maintenance. However, the lifespan is typically shorter as the non-stick coating will eventually wear away.

Oil compressors are, unsurprisingly, models that require oil for their lubrication. They are used within industrial settings.

They also require more maintenance, alongside frequently topping up the oil. They are however sturdier. They are typically more costly to buy and as there are extra elements to the machine, they are far heavier than their counterpart.

Fixed and Variable Air Compressors

Variable speed compressors (VSD) automatically adjust the motor speed in relation to the demand for air. It does so by converting power into AC and then into DC using diodes.

The AC is cleaned via a capacitor and uses a transistor to convert it to DC which then act as switches. The frequency is powered by these switches which controls the motor speed.

By using a VSD, running costs and the impact on the environment are reduced as they are far more energy efficient.

These are not suitable for applications where a continuous stream of power is required. They also more costly to purchase and maintain.

Fixed speed air compressors send consistence power to the motor, the frequency is reliable, and they are cheaper is initial cost.

They are less energy efficient and therefore can be more costly to run.

Low Noise Air Compressors

A common complaint of air compressors is their noise. There are a variety of low noise air compressors available.

Decibel measurement. Gauge with green needle pointing 30 dB, concept of noise reduction

They are available in oil and oil free and achieve a lower decibel by having an extra acoustic chamber purely to contain noise.

Piston Compressors, Scroll Compressors & Rotary Screw Compressors

Previously, we have only mentioned the piston compressors. The scroll compressor is a type of piston compressor and are also known as reciprocating compressors. They are popular due to the affordability and availability.

Pistons work by having the piston travel downwards, this decreases pressure inside the cylinder. This sudden change of pressure forces the cylinder open and air is drawn in. The piston then travels up again and air is forced out at a much higher-pressure point. This continues in a reciprocating ‘scroll’ pattern.

Semi-Hermetic Compact Screw Compressor in cut section

Scroll compressors cool down quickly and are the most energy efficient. They are more costly to purchase and as they are more complex, they can be harder and more expensive to maintain.

Rotary screws work in a very similar way, but rather than the piston they use rollers. These rollers are positioned in the middle of the central shaft and one side is always in contact with the wall. The rollers rotate at an extreme speed to cause the pressure change.

They have excellent power capacity and are cheaper to purchase and maintain. However, they have very limited cooling abilities and require frequent and continuous maintenance.

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