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.

Dry-running compressors are highly advantageous for compression ratios of 1:2 and 1:4 (applications that require absolute pressure values of 0.5 to 4 bar). In this lower pressure range, the compressors deliver their output with just a single compression stage, requiring no additional compressed air cooling and, the resulting warm air is often advantageous for the compressed air applications themselves.

However, when the compression ratio exceeds 1:4, dry-running systems require a second compression stage, making the specific output of fluid-cooled systems superior (when oil is the fluid used).

Based on specific output, in the pressure range from 1:4 to 1:11, oil-injected compressors are more efficient than dry-running ones. They can also deliver a greater range of higher pressures, providing compression ratios up to 1:16. Conversely dry-running compressors require two stages beyond the relatively low ratio of 1:4 and even then are only capable of exceeding compression ratios of 1:11 in special cases.

What size of compressor is required? For example, in the range from 2 to 100 kW, oil-injected compressors perform up to 20 percent better than dry-running ones. In the range from 100 to 250 kW this falls to around ten percent and between 250 and 400 kW, falls yet further to around five percent. Oil-injected compressors cannot operate beyond 400 kW in isolation and are installed as tandem systems up to a maximum output of around 800 kW. However, dry-running compressors can individually deliver up to 900 kW of output.

What is the application for compressed air? If the determination is based exclusively on specific output, oil-injected compressors generally come out ahead. However, there are areas where dry-running compressors deliver certain advantages, especially when the nature of the work processes themselves requires high temperatures, e.g. transporting cement and conveying granulates, where the hot compressed air produced is readily available and does not need to be supplied separately, saving energy costs.

A more varied assessment results when the investment, acquisition and maintenance costs are considered. In terms of acquisition and energy costs, oil-injected rotary screw compressors are noticeably less expensive than dry-running systems, yet they are slightly more expensive when it comes to maintenance (comparison based on 4000 operating hours over five years). In all other respects the costs are comparable.

With a number of variables to consider, it is clear that an individual assessment is the only way to ensure the correct technology is selected.

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