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Clean air on board: Oil-free or oil-injected compressors?

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Pneumatic air systems on board consist of numerous delicate components, sensitive to oil, dust and other airborne contaminants. Thus, air compressors must supply clean air.

But an oil-free compressor isn’t necessarily a guarantee for oil-free air in all environments.


Think twice about using oil-free compressors in a marine environment

Fundamentally, there are two main branches of rotary screw air compressors: oil-free and oil-injected. 

As the name suggests, the oil-free screw does not inject oil to cool down and lubricate the compression process (i.e. no mix of oil and air whatsoever). For that reason, they’re the go-to option for the cleanroom environments you typically associate with medical technology development, food production and laboratories, where there’s zero tolerance for internal contamination of the compressed air. 

While this design principle is perfectly reasonable for controlled environments on shore –  where the inlet air is of pristine quality – a marine environment is a different story. Why? 

The ambient air on board is already contaminated with oil. 

An air-compressor can only supply the air it takes from the surroundings. Oil-contaminated air that passes through an oil-free compressor will still be oil-contaminated. 

Therefore, you need compressors that know how to eliminate oil residue.


💡 Did you know…

Impurities such as oil, dust, particles and humidity normally stem from the ambient air on board a vessel – not from the compressor.


Oil-injected compressors remove oil by design

Equipped with active oil-removal mechanisms, oil-injected compressors like the TMC screw compressor, strip the air of both internal and ambient contaminations. It asks not where the oil originated, but simply purifies the air in two stages – by high-efficiency cyclone separation and oil-separator filtration.

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In an engine room, where the ambient air is full of dust, hydrocarbons and other contaminants, oil-injected compressors ultimately deliver air with a lower oil content than oil-free compressors do, on account of their superior oil separation/filtration capabilities. 

Granted, you could install aftertreatment equipment downstream of an oil-free compressor, too, but that would overlook all the other benefits of oil-injected air compressors.


💡 Did you know…

The only way to obtain clean, oil-free air is through proper filtration, regardless of whether the compressor is oil-free or oil-lubricated. 



Oil-injected screw compressors: Made for the sea

If the inlet air is contaminated, an oil-free compressor won’t do much good. Any type of compressor will filter the suction air to an extent, but for water and hydrocarbons in vapour form, this will have limited effect – regardless of how fine filtration that is chosen. Hence, the effects gained by using an oil-free compressor on vessels are severely limited.

And if you have to resort to aftertreatment anyway – why bother with a needlessly complicated compressor?

Oil is used for a good reason. Not only does it lubricate the screw, it’s a highly effective coolant. What we at TMC refer to as true marine air compressors, give you the largest working range – up to 14 bars in one single stage compared to 10–11 bars for oil-free alternatives – and a healthier operation overall. 

Effective cooling means the compressor runs on low internal temperatures, resulting in less wear and tear, prolonged service intervals and a significantly more compact design. 


The best choice overall

Oil-free compressors are excellent for their native domains – but not for an engine room environment. In contaminated air, an oil-free compressor is an inferior compressor type, all things considered.

No oil injection means no cooling of the compression process. Which means you have to compress the air in multiple stages using external cooling to contain overheating, resulting in a bulkier compressor, driving up costs and the physical footprint. True, an oil-free compressor saves you a little bit on oil separator filters. But the day you have to replace the air end, it's not one unit you’re replacing, but two (on account of the multi-stage compression). 

In addition you rely on 100 percent pure air at the inlet. While you can expect such conditions in a sterile lab, it’s practically impossible to achieve in an environment surrounded by hydrocarbons, heavy machinery and exhaust gases. The ambient air will necessarily contain oil vapour and particulate matter, forcing you to treat it anyway.


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