Compressed air is only as good as its purity. When your process is exposed to oil, it becomes increasingly difficult to keep air clean, increasing the costs you’ll face — especially as you use more and more air. To address this concern, many companies are turning to oil-free or oil-less air compressors. Today, oil-free compressors are becoming more common because they offer cost savings.
These purity and environmental benefits will often translate into other savings that may reduce your overall ownership costs. Here are a few of the things to consider if you’ve been wondering: “How do oil-free air compressors work?”
- There’s no need to collect or dispose of oil-ladened condensate.
- Downstream filters have reduced replacement needs, because they’re not filtering oil.
- Energy costs are minimized because there’s no need to increase force — some fluid-flooded units can see a downstream pressure drop due to filtration.
- Reduced oil costs, because there’s no need to continually refill your compressor.
- Typically, these units can unload within two seconds of the command to unload, and will use about 18% of their full load horsepower when unloaded.
Those savings can be very tempting. To see if you can make use of the oil-less compressors, you’ll need an understanding of how the compressors work and in which applications they work best.
Functional Steps of How Oil-Less Air Compressors Work
Understanding how oil-free air compressors work and why they last as long as they do is best illustrated by reviewing each function in a step-by-step method. Let’s walk through how an oil-less air compressor starts working and provides you with the compressed air that you need.
1. Drawing in Air
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. The filter will limit damage to your compressor and its internal components. These filters are typically fine enough that they keep out dust, dirt and small debris.
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. 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 compressor and it starts to draw in air through an open unloader valve, the first destination for the air is the low-pressure compressor element.
2. First Compressor Element
You’ve probably noticed that your air compressor can create heat, and this often has to do with the low-pressure compressor element, because it’s working without any oil.
The average compressor element will operate at around 2.5 bar, and compressing air alone can make the unit operate at a temperature of up to 180 degrees. 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 whisks away heat.
Oil-free elements will begin compressing the air and then move it through your compressor to cool the air down so it can be used in your applications.
3. Intercooler Access
After it’s initially compressed, pistons will push the air through an intercooler, where the air can cool so it can be further compressed. This will either move it to the second phase of compression or the final one, depending on the nature of your compressor.
Compressing air generates heat that limits the oxygen content of the air, thus reducing its density. Cooling the air essentially acts as a simple method for allowing denser and more oxygen-rich air to be again used by the engine, which in turn provides more fuel and improves the power output when the air compressor is working with a combustion engine.
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, and the cooling process means the second stage will face less wear.
Cooling air can lead to some condensation, and intercoolers will come with standard filters designed to remove moisture and water from the air. You’ll typically see this filter listed as a moisture trap.
After the air is cooled, it’s returned to your compressor for additional compressing.
4. Second, Higher-Pressure Compression
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 again becomes very hot due to the lack of lubrication in the surrounding elements, so it will need to be cooled once more.
5. Air Prep and Aftercooler Access
During its second phase of compression, the air will reach temperatures of around 150 degrees, requiring additional cooling before it can be used in other equipment. The aftercooler is the destination for air after its final compression stages, and this cooling allows it to be properly stored.
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 compressors — especially reciprocating compressors — are fitted with pulsation dampeners, and these are located just 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.
Pulsations can reverberate through the piping system, and these vibrations will make it difficult for your tools and machinery to measure air pressure and use it properly.
The air is finally stored or sent to your equipment for use.
6. Pressure Switches
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.
- Pressure switches are typically attached to the unloader valve, though sometimes the valve is internal.
- Pressure switches are set by the factory and arrive at predefined levels.
The Oil in Oil-Free Air Compressors
When you think about how oil-less air compressors work, you’ll need to realize that there is oil in the device, but that it won’t come into contact with the compressor. There is oil in the gearbox of your oil-less air compressor.
The gearbox of your air compressor is used to drive the two compressor elements via the electromotor. Gearboxes need lubrication to operate properly, and can be costly to replace, so maintenance checks should review them on a regular basis. The oil in your gearbox will lubricate the internal gears and bearings, as well as the bearing and timing gear located inside of each compressor element.
Oil will be pumped from an oil sump inside the gearbox, and it’s cooled via an oil cooler and oil filter before it is used to cool compressor or gearbox parts. The filters are used to remove debris during its action.
The main difference is that internal elements and parts of the gearbox will be treated with long-lasting lubrication. This puts more of a strain on the engine overtime, but requires less day-to-day maintenance.
Different Compression Cooling Elements
Understanding the general pressurization of air compressors is pretty straightforward and often operates the same across oil and oil-free units.
For the oil-free compressors we’re looking at, there are two main methods for cooling the compressor itself: Water and air. Air-cooled compressors, which tend to be rotary screw compressors, use outside air to cool the compressed air and the oil used in the gearbox. Air circulates and moves heat away from internal components as well as from the parts that house the oil, which helps remove heat from your compressor elements.
Water-cooled machines will have a two-part cooling system. Water moves near each element, heats up and flows away from the components, condenses and cools, and then moves back toward the equipment (often via gravity). These systems will have a water-cooling circuit used for the oil, intercooler and low-pressure element. The second circuit works to cool the high-pressure element and the aftercooler.
How Long Do Oil-Less Compressors Last?
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.
The Teflon coating often used to lubricate the internal cylinder simply wears off over time, and this wear can increase when an oil-less compressor is used in extreme temperatures or for long durations. You can have an oil-less compressor last for years if you’re vigilant, but this requires more time and money (typically) than an oil-lubricated air compressor.
If you want to extend how long your oil-less compressor will last, good maintenance is your best friend. Air compressors have a lot of moving parts that can cause significant wear, so ensure that you’re looking inside at least once a month. Check the permanently lubricated elements, such as the cylinder’s Teflon coating and either Teflon or carbon ring seals.
Part of how oil-less air compressors work includes a significant generation of heat. The more stress they’re put under or the higher the pressure you’re going for, 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, which means there may be less room for them to intake and move enough air to properly circulate and cool. To avoid this, you need to run the compressor only at its manufactured specifications and ensure that you’re providing plenty of space surrounding the unit to keep its exhaust and intake lines free from heat-causing dust and dirt.
Types of Air Compressors
When you’re ready to make your selection for an air compressor, it’s important to understand the three main types of compressors. The method of compression can impact their ability to output at a higher power level, be more portable or maintain more consistent pressure.
Reciprocating air compressors use 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 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, because the material reduces wear. To improve life, these units also tend to opt for aluminum pieces in place of cast iron.
Teflon rings will need to be replaced regularly, but they present little contamination risk to the compressor.
Rotary screw air compressors also use displacement and generate the 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. These seals prevent any ability of oil entering the air stream inside of the air compression unit. Sectioned off coolant is used and kept on the other side of the seal for easy cooling.
Oil-free rotary screws 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 which work together to bring air into the chamber for compression. Air is compressed within the spiral by decreasing the volume of the air, then directing the air to the center of the compressor to be cooled.
The biggest advantage of this technology is that there is no residual friction or wear on the system, because the spirals never come in contact with each other. In many cases, these compressors also work smoother, with a different sound from other compressors. Some may require lubricant, but there are many common models that operate with an oil-free design.
Choosing Oil-Free With Quincy Compressor
Oil-free air compressors are a more thoughtful choice and require careful consideration for your operations. The fundamentals of how oil-less air compressors do their work means you could be looking at lower costs, a better footprint and cleaner air.
If you’re in an industry that demands air purity at 100% clean levels, then a compressor that doesn’t rely on internal oil is a top bet. By not using oil, you’ll also be able to lower your carbon footprint, and these smaller units also tend to require less energy because they have more options for speed regulations.
Quincy Compressor offers a suite of oil-free air compressors and can make recommendations specific to industries such as electronics, pharmaceuticals, textiles, food and beverage applications. We stick to the ISO 8573-1: 2010 standard to ensure that your air purity needs are always met.
When you’re ready, reach out to your local authorized Quincy Compressor distributor to get all of the information you need about specialty models, including the QOF oil-free scroll compressor, and to receive a no-obligation price quote for your application.