Why individual assessment pays dividends

Should you be investing in a dry-running or an oil-injected compressor? Kaeser Compressors highlights the major advantages of each design.

One distinguishing factor among compressors is whether they are fluid-cooled or dry-running.

Fluid-cooled means that a fluid – often oil – is injected directly into the compression chamber. Without this internal cooling mechanism extremely high temperatures occur during the air compression process. The injected fluid therefore provides both a cooling action as well as absorbing any contaminants in the air.

In comparison, the compression chamber is not normally flooded with a fluid in dry-running systems. With no fluid to deliver the cooling action, the internal compressor temperatures are significantly higher. To compensate, two compression stages are employed to attain the required pressure; the first compresses the air to an absolute value of around 4 bar, the second compresses it to within the range of 9 to 11 bar (absolute pressure). This process, as well as the interim cooling, condensate separation and pulsation dampening, results in elevated energy requirements. Subsequently fluid-cooled compressors are more efficient.

What criteria is relevant for determining which system is optimal in any given case?

The quality of compressed air required is often assumed to be the main criterion. This is erroneous, however, as compressed air treatment is almost always required to ensure an appropriately high level of compressed air quality.

Compressed air quality is defined according to a precise, graduated scale in ISO 8573-1, where smaller numbers indicate higher quality classes. Compressed air quality classes below Class 4 (for particles), 4 (for oil) and 6 (for moisture) require that all types of compressor employ a suitable form of downstream compressed air treatment to achieve the required quality. The type of compressor technology used is therefore not the main factor in determining compressed air quality, and so should not be the main influence in deciding between a dry-running and fluid-injected design. There are four key criterion that should be considered.