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Oil-Free Air Compressors
What is an Oil-Free Air Compressor?
An oil-free air compressor does not use oil on its cylinder walls. Oil-free designs depend on the self-lubricating materials to allow the piston to slide in the cylinder and the grease in the sealed bearings. This is quite different than an air compressor with a splash oil-lubricated pump design which relies on an oil dipper attached to the bottom of the connecting rod to randomly splash oil from the crankcase reservoir onto the bearings and internal parts of the compressor to keep them properly lubricated.
Basics of Oil-Free Air Compressors
How Oil-Less Air Compressors Work: Step-by-Step
Oil-less air compressors start by bringing in outside air through their unloader valve and passing it through an inlet air filter (or filters) in order to ensure that the air is clean by keeping out dust, dirt, and small debris. The filter will limit damage to your compressor and its internal components.
The unloader valve opens to help the compressor pump air into its chamber, placing it in the “loaded” position. When the valve closes, the compressor enters the “unloaded” condition and begins running. At this point, when your compressor is running and actively delivering compressed air, it typically won’t be able to draw in any more air.
When you turn on your air compressor and it starts to draw in air through its open unloader valve, the first destination for the air is the low-pressure compressor element. The average compressor element will operate at around 2.5 bar, making the unit operate at a temperature of up to 180 degrees Fahrenheit. That can be more than twice as high as the temperature that oil-lubricated compressors reach due to the lack of a flowing medium that takes away heat.
The air is moved through your compressor to cool the air down so it can be used in your applications. Pistons will push the air through an intercooler, where the air can cool so it can be further compressed in either the second or final phase of compression, depending on the type of air compressor.
Compressing air generates heat that limits the oxygen content of the air, thus reducing its density. Cooling the air allows dense and more oxygen-rich air to be used again by the engine, providing more fuel and improves the power output when the air compressor is working with a combustion engine. Cooling air can lead to some condensation, so intercoolers will come with standard filters (moisture traps) designed to remove moisture and water from the air.
Intercoolers are essential for two reasons. First, they cool the air down to a proper temperature to minimize the risk of any damage related to heat. Second, intercoolers allow air to be compressed at much higher PSI’s in two-stage pumps.
As aforementioned, the air will move back into the main chamber of your air compressor (or the second chamber, depending on its design) and will be further compressed by a high-pressure element. The maximum pressure you’ll achieve typically ranges from 116 to 145 psig. The air becomes very hot again due to the lack of lubrication in the surrounding elements, so it will need to be cooled once more.
As air flows to the aftercooler, it will pass through a check valve that is designed to prevent any backflow, ensuring that air continues to compress and fill your tank. Backflow will damage your equipment and cause a major failure of the air compressor. Many compressor, especially reciprocating compressors, are fitted with pulsation dampeners before the aftercooler. The dampener is designed to reduce pulsations and vibrations caused by the air compressor when it uses suction and opens discharge valves.
Detection equipment in your air compressor tank will monitor the level of air you have. When it falls below a specified level, the air compressor will turn back on and start working to rebuild the pressurized air in the tank. The pressure switch is what is used to monitor and turn the compressor off and on, and it is typically attached to the unloader valve.
Types of Oil-Free Air Compressors and Longevity
Oil-less air compressors still have oil in the device, but it won’t come into contact with the compressor. There is oil in the gearbox of your oil-less air compressor. There are three main types of compressors that you can choose from when looking for an oil-free compressor.
A reciprocating air compressor uses positive displacement, which generates air pressure via two sides that provide either suction or discharge. Positive displacement is suitable for compressing small amounts of air at high pressures and can quickly dissipate the heat from compression. Reciprocating compressors tend to have a lower output than others but they can achieve relatively high pressures. Non-lubricated reciprocating compressors will typically use a Teflon piston ring instead of oil. The Teflon ring means there’s no need for lubrication for the pistons, rings, and cylinders. Teflon rings reduce wear for these parts but need to be replaced regularly.
Rotary screw compressors use displacement and generate air compression by running a system of interlocking screws that draw in air and then pressurize it in a small area. Oil-free rotary screw compressors opt for non-contacting carbon ring seals to aid their cooling by preventing oil from entering the air stream inside. Oil-free rotary screw air compressors can see heat buildup over time, since they typically lack the ability to throttle the inlet, which will see those with large demands opt for the flooded models.
Scroll technology is an advanced air compression system which includes one mobile and one stationary spiral that works together to bring air into the chamber for compression. A decrease in air volume compresses the air in the spiral, and air then travels to the center of the compressor for cooling. There is no residual friction or wear on the system since the spirals never come into contact with each other. Some may require lubricant, but there are many common models that operate with an oil-free design.
Oil-free compressors can typically provide from 2 to 5 cubic feet per minute (cfm) of air flow and are driven by a 120-volt electric motor, which is enough to power trim nailers or complete finish work inside a structure. Oil-lubed wheelbarrow-style compressors, which can deliver 10 to 25 cfm or air. These compressors are generally engine-driven and provide enough power for heavier-duty tools or even a crew of up to 10 workers. Wheelbarrow-style oil-lubed compressors are typically used for roofing and framing work on the outside of the structure. Since they are engine-driven, they do not require power on site during this phase of construction and can power an entire crew. Many contractors like the flexibility of having both types of compressors. They may use a larger oil-lubed compressor to power outside jobs like framing and roofing and then use oil-free compressors designed for quiet operation to complete smaller jobs inside a structure.
Generally, an oil-free compressor will not last as long as an oil-lubricated model because their pre-lubrication will gradually wear away and degrade over time. Oil-free compressors also tend to use a universal motor which won’t last as long as an induction motor. If you want to extend how long your oil-less compressor will last, good maintenance is your best option. You should routinely check inside your air compressor at least once a month, looking at permanently lubricated elements such as the cylinder’s Teflon coating and Teflon or carbon ring seals.
Where are Oil-Free Compressors Used?
Myths About Oil-Free Compressors
There is an argument that oil-free compressors cannot match the longevity and dependability of oil-lubricated compressors and therefore do not offer the same value. Routine maintenance and regular oiling tend to make oiled compressors more durable and have a longer lifespan, however, since oil-free technology has developed, oil-free compressors run cooler and come better protected than ever before.
Another myth is that oil-free compressors are much louder than oil-lubricated compressors. However, today’s designs feature direct-drive, dual-piston (duplex) pumps, and advanced sound-reducing technologies that make oil-free air compressors much quieter than oil-lubricated ones.
It was also commonly believed that oil-lubricated air compressors run cooler and withstand extreme conditions. However, the dual-pump designs of oil-free air compressors allow them to achieve the desired CFM or PSI in half of the revolutions (RPM). They also use heat-dissipating aluminum cylinders to run cooler. In fact, oil-free air compressors are preferred in the extreme cold because there is no worry about the oil turning the sludge. On the other hand, oiled compressors may be desired if working in extreme heat and humidity because they can dissipate heat more quickly in a given duty cycle and resist rust build-up. However, oiled compressors may be desired if working in extreme heat and humidity because they can dissipate heat more quickly in a given duty cycle and resist rust build-up.
New Advancements in and Advantages of Oil-Free Compressor Systems
Technology advancements in oil-free compressors have made new higher-end oil-free compressors better than old oil-free products. These advancements extend product life of this style of compressor for up to 2,000 hours of total life – which is up to four times longer than the average oil-free compressor and makes product life comparable to some conventional oil-lubed styles. The advancements also make the compressor much quieter, running 10 to 15 dBA quieter than oil-free units with older technology.
These advancements in oil-free units are mostly driven by the materials used in the sealing rings on the pistons. Additionally, most high-end oil-free compressors have pumps that run at half the speed of older compressors, which also contributes to longer product life and makes the units inherently quieter because they are running slower.
Additional advantages of oil-free compressors are that their duplex pumps operate at lower RPM that reduces the noise, heat, and friction, thus extending the pump’s life cycle. Oil-free air compressors have better castings, exacting tolerances, and better frictionless. They also produce cleaner, drier air and are lighter weight and more portable. They do not require oil filters and do not need oiling maintenance.
Summary
Oil-less air compressors are a great choice if you are looking for lower costs, better environmental effects, and cleaner air. By not using oil, you can lower your carbon footprint and use less energy with smaller units. If you are in an industry that demands air purity at 100% clean levels, then an air compressor that does not rely on internal oil is your best bet. However, they also may require more maintenance if you do not routinely check the permanently lubricated elements of your oil-less air compressors. Oil-less air compressors generate a significant amount of heat. The more stress that they are put under or the higher the pressure required, the more likely it is that your air compressor will grind against itself or generate enough heat to burn out and/or warp components. Oil-free air compressors tend to have a smaller clearance than their oil-injected counterparts, so there may be less room for them to intake and move enough air to properly circulate and cool. To avoid this, you should run the compressor only at its manufactured specifications and ensure that you are providing plenty of space surrounding the unit to keep its exhaust and intake lines free from heat-causing dust and dirty. Overall, keep in mind what jobs you will be needing an air compressor for before selecting an oil-free version.
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