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Oil-free vs. Oil-injected air compressors

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As long as there has been “oil-free” certification there have been myths floating around. The one that’s used most is the claim that oil-lubricated compressors combined with filters are “technically oil free.” And, that air quality from those oil-injected compressors with the help of that filter can be better than oil-free. While filters can reduce the level of oil particles, filters do not reduce the risk of oil contamination. Renting temporary solutions based on incorrect information could cause inadvertent damage to sensitive applications, products, and the environment.

Oil-Free vs. Oil-Injected Air Compressors

Air purity is critical for many applications where even the tiniest drop of oil, or air contaminated with oil, can cause product spoilage, product recall or damage production equipment and even environmental damage. Your specific application will determine which type of air compressor is best for you to use in your facility and the Atlas Copco Rental experts will guide you along the way.

Oil-free air compressors are not “better” than oil-lubricated compressors. The compressor you choose depends on your specific application requirements and the air quality that you need to achieve. In instances where the threat and consequences of oil contamination are too high, having an oil-free air compressor is a must such as in medical procedures, food processing, and bubble curtain applications. When the consequences of oil contamination are not as serious, such as general manufacturing and in industrial workshops, oil-injected air compressors are used.

Words that matter.

First, let’s take a moment to break down some of the above vocabulary. Oil-free and is used to describe compressors that do not use any lubrication in the compression chamber. While it describes a type of compressor, Class 0 and “technically oil-free” express how clean the air is after compression. Though they are close in meaning, there are a few important differences between the two.

  • Class 0 oil-free compressors can guarantee 100 percent oil-free air. While they may have a higher initial price point, these compressors are much safer for sensitive applications and are the cleanest choice because no contamination will reach your end-product.

  • “Technically oil-free” compressors have a high chance for contamination, because they are actually lubricated compressors that include oil-removing filters. Unlike their Class 0, oil-free counterparts, “technically oil-free” air compressors are ranked as Class 1.

Certification. Who and why.

The International Organization for Standardization (ISO)[SVB1]  sets forth worldwide standards for proprietary, industrial and commercial purposes. Compressed air has its own set of ISO standards. Depending on air ultimate air purity (determined by number of particles per cubic meter as a function of particle size), compressors can be ranked from ISO Class 0-5.


The original version of ISO compressed air purity standards (1991) was ‘made by and made for’ the filter manufacturers. The standard defined five classes of oil concentration, the best of which was Class 1. Class 1 specifies an oil concentration of 0.01 mg/m3 at 1 bar(a) 14.5psia and 20oC (68F), and conformance to these criteria is sometimes called “a technically oil-free solution.”


However, these standards were edited in 2001 and were further updated in 2010. The current standard sets limits on total oil content (aerosol, liquid and vapor), and a specific standard was introduced on measurement of oil vapor. A new class (Class 0) was added to cover more stringent quality requirements. Atlas Copco was awarded the ISO 8573-1 CLASS 0 (2010) certification and also holds the ISO 22000 certification for the oil-free production facility in Antwerp, Belgium.

How do Oil-Free Air Compressors Work?

Air compressors function based on a very simple principle: When the air is compressed, its volume decreases whereas the pressure increases. In that same vein, oil-free and oil-injected air compressors work in essentially the same way. Air compression is a two-stage process in which the pressure of air rises while the volume drops. There are, however, different oil-free air compressor technologies that work in different ways:

  • Oil-free rotary screw compressor: External gears synchronize the position of the counter-rotating screw elements, and, because the rotors do not come in contact and create friction, no lubrication is needed within the compression chamber. As a result, the compressed air is oil-free. Precision engineering within the housing minimizes pressure leakage (and drops) from the pressure side to the inlet. And because the internal pressure ratio is limited by differences in air temperature between the inlet and discharge ports, oil-free screw compressors are frequently built with several stages and inter-stage cooling to maximize the pressure reach. The gearbox driving the mechanism does contain lubricants; oil-free refers to the compression chamber itself, and the delivered air is free of foreign contaminants beyond those found inherently in the air that passes through the intake.

  • Oil-free piston compressor: a conventional piston compressor has a crankshaft, connecting rod and piston, cylinder, and valve head. At the top of the cylinder, you'll find a valve head that holds the inlet and discharge valves. Both are simply thin metal flaps, one mounted underneath the valve plate and the other on top of it. As the piston moves down, a vacuum coalesces above it. This allows outside air at atmospheric pressure to push open the inlet valve and fill the area above the piston. As the piston moves up, the air above it compresses, holds the inlet valve shut and pushes the discharge valve open. The air moves from the discharge port to the tank. With each stroke, more air enters the tank and the pressure rises. The oil-free piston compressor does not inject oil into the compression chamber, but rather has Teflon-coated rings.

  • Oil-free scroll compressor: A single spiral-shaped rotor oscillates against a similar fixed spiral and, as these spirals move against each other, the cavity trapping air between them becomes progressively smaller. This decrease in volume forces the fixed volume of intake air to increase in pressure.

What are the advantages of Oil-Free Air?

Oil-free air technology helps you avoid needing to purchase filter replacements because they are don’t have to filter any oil. This cuts the cost of oil condensate treatment and reduces energy loss from pressure decreasing filter pressure. There is also the environmental impact: By employing oil-free air, you help protect the environment and ensure better compliance with international regulations. Leaks and energy usage are minimized, and the need for condensate treatment (and the collecting/disposing of condensate) is eliminated.

What’s more, you will not risk contaminating your end-product or process with oil, so no risk of loss of reputation or negative impact to your bottom line. While our equipment is a temporary support of your own installation, you can expect the highest qualities and guaranteed absence of oil in the air you need.

Top Industries for Oil-Free Air

Which industries and critical applications employ oil-free air:

  • Automotive: high quality paint finishes, smooth running processes, better health
  • Food & beverage: healthy, better tasting high-quality end products
  • Chemical: increased product purity, better processes, less waste, increased safety
  • Electronics: uninterrupted control systems and maintenance of the ultra-clean conditions essential to high product quality
  • Medical & Healthcare: 100% reliable for any medical environment, including hospitals, dental practices, veterinary offices, or other clinical work environments
  • Oil & gas: trouble-free control systems and processes, upgraded safety, security and a higher quality end product
  • Textiles: more efficient production, reduced repair and maintenance costs, improved textile quality, less wastage
  • Pharmaceuticals: pure products, reducing contamination risks, more efficient processes, decreased waste
  • Wastewater Treatment: for every industrial or municipal wastewater treatment application

FAQ

Here are a number of questions we get asked regularly that ‘ll help you make the oil-free choice.

What are the TÜV tests required to qualify for ISO 8573-1 CLASS 0?

The Part 2 test measures aerosols and liquids. Testing can be done through partial flow (B2) or full flow (B1) methods (see below). The Part 5 test measures vapors only. Both parts are necessary to obtain ISO 8573 CLASS 0 certification. This means that all three sources of oil contamination – aerosol, vapor and liquid – have to be measured.

What is the essential difference between partial flow (B2) and full flow (B1) testing methods?

Both methods are acceptable for aerosol and liquid measurement according to ISO 8573-1 Part 2 The B2 method targets only the center of the air flow. Oil aerosols are registered but oil that sticks to the pipe wall (wall flow) is not detected. Most air compressor manufacturers still prefer this less stringent method. The B1 method examines the entire air flow to measure both aerosols and wall flow. This comprehensive test method was used on the Atlas Copco range of oil-free air compressors. Even so, no traces of oil were found in the output air stream.

Can oil-injected compressors with oil removal filters deliver oil-free air?

This solution is often referred to as “technically oil-free air”. However, even under optimum conditions and with several stages of oil removal, the air quality with regard to oil is suspect.

To achieve even barely acceptable air quality with oil-injected compressors, it is necessary to have air cooling devices and several stages of oil removal with multiple components. A failure of any of these components or inadequate maintenance can result in oil contamination of a process.

With oil-injected compressors there will always be a risk of contamination and the possibility of severe consequences for the business.

What is the impact of ambient temperature?

One aspect influencing the efficiency and purity of air systems is temperature. When using oil-injected compressors with oil removal filters, oil carryover through filter media increases exponentially according to the temperature at the filtration interface.

If the ambient temperature in the compressor room increases to 30°C, the compressor outlet temperature could be 40°C with the oil carryover 20 times the specified value. Such temperatures are not unusual even in colder countries, where the compressor room temperature is substantially higher than that outside.

Temperatures also cause an increase in the vapor content of the air, some of which can carry through to the end product. Moreover, high temperatures shorten the lifetime of activated carbon filters. An increase in temperature from 20°C to 40°C can cut filter lifetime by up to 90%. Even worse, the activated carbon filter does not warn the user when it is saturated. It will simply allow oil to pass on to processes. For Atlas Copco’s oil-free compressors, air quality is independent of temperature.

What about oil contamination in ambient air?

Ambient air has very small traces of oil coming from vehicles and industrial sources. However, in contaminated areas, oil content does not normally exceed 0.003 mg/m3. This is borne out by tests conducted by the TÜV near a factory with heavy machining activity (including turning, milling, grinding and drilling). Heavy vehicular traffic and a garbage incinerator were in the vicinity. Aspirated by an oil-free compressor, this extremely low level of atmospheric oil is almost completely washed away by the condensate in the intercooler and aftercooler, resulting in pure oil-free air for your process.