Compressed Air System

The facts about oil-free compressed air

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.

By Wayne Perry

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.

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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).

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).

Figure 2

Fluid-cooled screw compressors that use water instead of oil for the direct cooling medium also have come into use in recent years and can achieve even lower airend discharge temperatures of 104°F to 122°F. However, water has two distinct disadvantages as a cooling medium. First, its lower viscosity leads to higher back-flow losses, which limits single-stage compression ratios to between 10:1 and 13:1.

Second, the water reacts with impurities in the intake air, which means the water either needs to be changed frequently or purified. Compared to oil- or fluid-cooled screw compressors, oil-free compression systems requiring two or more stages of compression are less energy efficient and also more expensive. A lower airend temperature also results in a lower air discharge temperature, which ensures better condensate separation and eases the thermal load on the downstream compressed air treatment equipment.

The power differences between oil-free compression systems (with higher operating temperatures) and oil-cooled systems (with lower operating temperatures) have a direct effect on the operator’s budget. For example, the specific power of a modern oil-free rotary compressor in the 100 hp class at 108 psig is 19.96 kW/100 cfm, whereas the specific power for an oil- or fluid-cooled rotary compressor of the same power and pressure is 16.12 kW/100 cfm. This difference in specific power amounts to a considerable annual cost saving of about $13,409, or 23%, based on 6,000 operating hours per year with an electricity price of 10 cents/kWh.

Air cooling lowers costs

Calculations to compare the cooling costs for air- and water-cooled compressor systems usually consider only the energy consumption for the fans and ignore the additional costs for the required cooling water. A comprehensive comparison reveals the actual cost structure. Air cooling for a 4,000-hp compressor system with an air flow capacity of 15,000 cfm at 100 psig requires about 45 kW (60 hp) when taken as an average over the course of a year.

Cooling an equivalent system with water would consume between 60 kW and 70 kW (80 hp to 94 hp) when taking pumps, fans, cooling towers and other associated equipment into consideration. This difference represents an annual cost saving of approximately $20,000, based on 8,000 operating hours per year. The more cost-effective air-cooled approach for compressors with direct injection cooling also can be used in a wider range of conditions because of the considerably lower air discharge temperature.

“Oil-free” doesn’t save filter costs

One persistent myth is that oil separation and filtration systems downstream from oil/fluid-cooled compressors are responsible for energy and maintenance costs that could otherwise be avoided by using oil-free compressors. System comparisons are happily provided to support this view. But, unfortunately, these compare oil/fluid-cooled compressors equipped with a host of filters against an oil-free system fitted with only a downstream desiccant dryer.

These comparisons are misleading and bear no relevance to modern compressed air systems. An oil/fluid-cooled screw compressor doesn’t require three filters and a refrigeration dryer to maintain the air quality achieved by an oil-free compressor equipped with a single-chamber desiccant dryer. With regards to solid particle content, an oil/fluid-cooled compressor in combination with a refrigeration dryer and a 0.01-micron filter achieves class 1 air in accordance with ISO 8573, whereas an oil-free compressor with only a desiccant dryer achieves class 3. Furthermore, the combination of the oil/fluid-cooled compressor with a refrigeration dryer and filter easily achieves class 2 with respect to oil aerosol content.

However, the air treatment quality of an oil-free compressor with a downstream single-chamber desiccant dryer can’t meet ISO 8573 specification as a result of the undefined intake conditions. An objective comparison can be made only if both systems comply with ISO 8573.

Repeated measurements confirm that without filtration and additional air treatment equipment, no compressor can achieve a precisely-defined compressed air quality class. Energy consumption comparisons that don’t take this into account are, therefore, also incorrect.

Furthermore, it is not true that the pressure differential in a filter element steadily increases throughout its lifetime. The pressure differential only starts to increase significantly when the filter is at the end of its service life (Figure 3).

Figure 3

Energy losses can be avoided simply by changing a filter element 90% of the way through its recommended service life. In the end, it’s the clean air treatment equipment that determines the resulting air quality, not the compression method.

To achieve maximum reliability and efficiency, opt for the compressed air system that exhibits the lowest total life-cycle cost. Energy and maintenance costs represent between 70% and 90% of this expense, depending on the number of operating hours, and taken over the lifetime of any compressor, will add up to a multiple of the initial capital investment, potentially making a decision based on first cost a false economy. Base your investment decisions on total life-cycle costs, system compatibility and effectiveness, and reliability rather than on a particular type of compression system. Remember, air compressor technology constantly evolves.

Wayne Perry is compressed air efficiency consultant to the United Nations Industrial Development Organization. Contact him at wayne.perry@kaeser.com.