The facts about oil-free compressed air

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By Wayne Perry

PlantServices.com

Keywords: compressed air, oil-free and compressors

No single process or product is the panacea for producing high-quality compressed air, often just referred to as oil-free compressed air. Ultimately, the key factor is whether the technical solution fulfils the requirements of maximum reliability and efficiency.

There’s a growing demand for higher-quality compressed air in chemical and process engineering applications. As a result, the discussion on how to best produce it is constantly being fuelled. Slogans and one-sided arguments aren’t particularly helpful to compressed air users who require objective and balanced views.

No single process or product is the panacea for producing high-quality compressed air, often just referred to as “oil-free” compressed air. Ultimately, the key factor is whether the technical solution fulfils the requirements of maximum reliability and efficiency.

Definition

To clarify, the term “oil-free compressor” refers only to the compression chamber, not the compressor system as a whole or the resulting compressed air quality. Also keep in mind that “oil-free” is a vague and unhelpful phrase used by some system providers’ marketing departments solely to stir up emotion.

Those who wish to stay on safe ground, however, need to adhere to the quality classes specified in standard ISO 8571-1. Only these provide the precise definitions that form the basis for reliable comparison.

Compressor manufacturers don’t restrict their product ranges to ‘oil-free’ or oil-cooled systems. Many also produce a variety of products, which will remain the case in the future. The question then arises: Why are oil-lubricated systems used for some applications while others use oil-free equipment?

No-go without air treatment

The argument that it’s a compromise to introduce oil into the air only to remove it again later ignores several important facts. Precisely defined air quality only can be achieved by using correspondingly efficient air treatment systems, irrespective of the compression method used. Not even the most confident advocates of oil-free compression systems would deny this. For example, a business line manager in 1996 described oil-free systems as follows:

“An oil-free compressor produces identical compressed air quality to that of the intake air. Therefore, the air inside the compressor system should also remain oil-free. However this is not guaranteed to be the case if oil vapors escape when ventilating the gear casing. This is a problem that is so far unresolved for large compressors.” [1]

These views were confirmed by Pall Pharmair Dreieich company of the American Petroleum Institute. Furthermore, compressors located in industrial areas often are subjected to much higher levels of oil aerosols in the ambient air (Figure 1).

“Manufacturing facilities with inadequate or no contaminant extraction systems can even have air oil concentration levels as high as 300 mg/m3.”[2]

Positive cleaning effect

In spite of every effort to protect the environment, the composition of the intake air still remains the biggest problem in guaranteeing a constantly high air quality. The compressor acts as a giant vacuum cleaner that sucks in, and concentrates, the contaminants in the ambient air. Fortunately, the cooling fluid/oil has a welcome cleaning side-effect in addition to its other functions. The additives in the oil neutralize sulfur dioxide, for example, and the oil itself traps solid particles sucked in with the ambient air.

This results in longer filter service intervals compared to oil-free compression systems. Synthetic cooling fluids, such as polyalphaolefin, that have a greater contamination extraction capacity compared to mineral oils, have since come into widespread use enabling extended fluid change intervals.

Condensate must be treated

As condensate in an oil-free compressor precipitates in the aftercooler, the SO2 from the air reacts with the condensed water to form sulfurous acid. Measurements from compressed air systems show that the resulting condensate has a pH value below 6. In many cases, the combination of low pH and the levels of heavy metals mean that the condensate can’t simply be discharged into the wastewater system.

Additionally, sulfurous acid requires using more expensive air treatment equipment such as stainless steel heat exchangers and piping to eliminate leaks and repairs corrosion would cause.

In view of these facts, the assertion that “the condensate is always oil-free and can therefore be directly drained away without treatment”[3] can only be interpreted as an instruction to break the law. Compressed air condensate must be treated before disposal.

Direct cooling increases energy efficiency

Airend discharge temperature and its effect on energy efficiency should also be considered. The warmer the air during compression, the greater the energy requirement. These energy aspects combined with reliability favor multistage oil-free compressors over single-stage units.

For compression ratios greater than 4:1, intercooling keeps the airend discharge temperature below 392°F for each compression stage, consequently providing effective energy- and mechanical-controllability. While oil-free compressors can be cooled only indirectly via jacket-cooler, intercooler or aftercooler, screw compressors with oil or fluid injection are directly cooled. This system is highly efficient as the oil/fluid injected into the compression chamber also serves as a cooling medium.

The result is that 80% of the heat of compression can be removed from the compression chamber so the air can be discharged at the relatively low temperature of about 176°F. As a temperature increase of only 10°F leads to 1% to 2% deterioration in specific power, it’s important to keep the airend discharge temperature as low as possible. This also is why oil- or fluid-cooled screw compressors can be constructed for single-stage compression applications for compression ratios ranging from 4:1 to as high as 16:1 (Figure 2).