What is an Air Compressor?

We explain how air compressors work and what types there are

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Estimated time to read: 4 minutes

March 7, 2023

Air compressors: Vitally important

Air compressors have had more impact on our society – and especially the industrial sector – than almost any other invention. In less than 150 years since Austrian engineer Viktor Popp built the first compressor plant, compressed air has become so indispensable that it is now referred to as the ‘fourth utility’ along with electricity, water and gas.

The numbers back up this bold statement. It is estimated that 10% of all electricity consumed by the manufacturing industry is used for compressed air.

Why air is compressed

Put simply, air compressors transform regular, atmospheric air into pressurised compressed air by squeezing it into a much smaller space than it is usually in.

One of the reasons compressed air has become so indispensable is its versatility. Just about every industry relies on compressed air in some form. You’ll find it used everywhere from manufacturing plants for cars, to ships and trains, or construction sites and hospitals.

With so many different applications, it also makes sense that there are many different types of air compressors. After all, the small, quiet compressor powering a dentist’s drill should not be the same as the industrial compressor that keeps an entire manufacturing plant going. 

How air compressors work

There are two main types of air-compression technologies:

Dynamic compressors achieve the required rise in pressure by accelerating the air (or gas) with an impeller and then slowing it down in a diffuser. Dynamic compressors are primarily used for operations and applications that require very high volumes of air. 

Positive displacement compressors have a cavity that fills with regular air (or another gas at atmospheric pressure). The cavity is then made smaller. This means the same amount of air has to fit into a smaller space. The result? The air is put under pressure.

Positive displacement compressors are used much more frequently than dynamic compressors. Of course, the world of positive displacement compressors includes different types of technology too:

·       Piston compressors, also known as reciprocating compressors, use a piston that ascends and descends into the cavity to pressurise the air. They are often used for operations with a low compressed air demand. They’re relatively inexpensive.

·       Rotary screw compressors are more advanced. They use rotors to move air into increasingly small areas. The total cost of ownership is quite low. They’re more energy efficient and produce more compressed air.

·       Finally, scroll compressors feature a double rotor. However, they’re not nearly as common as the other types of positive displacement compressors.

Different air compressor types and technologies

It’s not just the method of operation that differentiate compressors. There are other factors to take into account: 

Oil-injected versus oil-free: Not every application requires top-quality, pure, clean air. For example, ‘energy air’ is fine for inflating a tyre. It’s generally okay if energy air is less clean or has traces of oil in it, which means an oil-injected compressor is perfect for producing it. Good news, since they’re more economical to buy and operate. 

However, compressed air that comes into contact with pharmaceuticals or food has to be super-clean and meet stringent regulations. This is known as ‘active air’. In these cases, an oil-free compressor is a much better choice. It produces compressed air that is completely clean.

Single-stage and two-stage piston compressors: 

Piston compressors come in two versions and are available either as single-stage or two-stage models. For small jobs that do not require a continuous air supply, a less expensive single-stage version will do. However, if you’re operating high-powered tools and need a continuous supply of compressed air, a two-stage piston air compressor is probably a better solution.

Belt drive versus direct drive: Belt-driven compressors are a better choice when reliability, ease of operation, cost-effectiveness and power matter most. If applications call for the compressor to adjust its speed and power, however, a direct drive is usually preferable.

Fixed-speed versus variable-speed drives: It takes a lot of energy to compress air. In fact, over the lifetime of a compressor, the majority of the total cost of ownership will come from energy costs.

This brings us to the difference between fixed-speed and variable speed drive compressors. As their name indicates, fixed-speed models only have one speed. They’re perfect for applications or operations that require a steady flow of air. Match the output of the compressor to the demand and you’re set.

ALUP Evoluto 55 rotary screw compressor

What if your production site has a fluctuating air demand? A variable-speed drive compressor may be a better option. The purchase price is higher. Afterwards, they consume much less energy by matching the compressor speed to the air demand. As such, they are much more efficient and offer a significantly lower cost of ownership.

Wondering which air compressor is best for your particular situation? Or do you have a complicated question about air compressors that only experts will be able to answer? Get in touch with us at ALUP. We look forward to helping you!

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