The facts about oil-free compressed air

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).

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.