Why individual assessment of compressors pays dividends

Should you be investing in a dry-running or an oil-injected compressor? Beth Wood highlights the major advantages of each design and the applications for which they are best suited.

When choosing a new air compressor, it is important to understand the differences between compressors that are fluid-cooled and dry-running.

Fluid-cooled means that a fluid – often oil, but sometimes water – 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 a cooling action and also absorbs any contaminants in the air.

In comparison, the compression chamber is not normally flooded with a fluid in dry-running systems.

As there is no fluid to deliver the cooling action, the internal compressor temperatures are significantly higher, especially when operating beyond a certain compression ratio.

To compensate, dry-running compressors employ two compression stages to attain the required pressure.

The first stage compresses the air to an absolute value of around 4 bar. The second further compresses it to within the range of 9 to 11 bar (absolute pressure).

This extended process, as well as the interim cooling, condensate separation and pulsation dampening, results in elevated energy requirements. Subsequently fluid-cooled compressors are more efficient, delivering the same output with a single compression stage and lower temperatures.

Criteria

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

It is often assumed that the quality of compressed air required should be the main criterion. This is erroneous, as compressed air treatment is almost always required to ensure quality.

In fact, 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 factor in deciding between a dry-running and fluid-injected design.

There are four key criteria that should however be considered.

PRESSURE – The range of the required pressures or compression ratios 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.

In addition, the resulting warm air is often advantageous for the compressed air applications themselves (e.g. pneumatic transport of cement).

The situation changes once the compression ratio exceeds 1:4. Beyond this value 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, easily 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.

SIZE – In addition to the compression ratio, the size of the compressors required is another important consideration when selecting an appropriate technology.

Consider the following comparison of specific output: in the range from 2 to 100kW, oil-injected compressors perform up to 20 % better than dry-running ones.

However, in the range from 100 to 250kW this advantage falls to around 10% and between 250 and 400kW, falls yet further to around 5%.

Oil-injected compressors cannot operate beyond 400kW in isolation and are installed as tandem systems up to a maximum output of around 800kW. However, dry-running compressors can individually deliver up to 900kW of output.

APPLICATION – The application for which the compressors are destined also plays a key role in selecting the most suitable technology.

If the determination is based exclusively on specific output, oil-injected compressors generally come out ahead.

However, there are some areas where dry-running compressors deliver certain technical advantages, especially when the nature of the work processes themselves requires high temperatures.

An example would be transporting cement and conveying granulates. In these cases, the hot compressed air produced by dry-running compressors – which can reach temperatures of 200°C and beyond – is a huge benefit as the heat is readily available and does not need to be supplied separately, saving energy costs.

COSTS – A more varied assessment results when the situation is examined from a cost perspective, taking into account the investment, acquisition and maintenance costs.

In terms of acquisition and energy costs, oil-injected rotary screw compressors are considerably 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 associated with each system are comparable.

In summary, oil-injected systems offer decisive advantages in terms of specific output and costs while dry-running systems provide certain benefits in specific output ranges and for some specific applications.

With a number of variables to consider, it is clear that an individual assessment is the only way to go.