One of the benefits touted for oil-free air compressors is reduced maintenance. While there are differences in what oil-free and oil-injected compressors need, neither type will provide reliable operation with no maintenance at all. What’s more, while oil-free compressors may need less frequent attention it might be argued that they need overhauling more often.
Here we’ll discuss the maintenance and repairs needed by oil-free compressors, and how this differs from oil-injected designs. First, though, it is important to be clear about what “oil-free” means and why it can be important.
How Oil Gets Into Compressed Air
Every air compressor is a pump, the two most common types being piston and rotary screw. In a piston compressor, also called a reciprocating compressor, a piston moves up and down in a cylinder. Air is drawn in on the downstroke and compressed on the upstroke. In a rotary screw design, compression is performed by a meshed pair of helical screws that suck air in and squeeze it into a reducing volume.
In both of these designs, oil is injected into the compression chamber. Its main function is to seal the gap between the piston ring and cylinder wall or screw tips and housing. In addition, it provides lubrication and takes away heat.
As air moves through the compressor some of this oil becomes entrained. A separator after the compressor pulls out most of this oil, leaving a very small amount that is carried into the air distribution network. This may sometimes appear as a buildup on the exhaust ports of air-powered tools and equipment.
In some industries, it is essential that compressed air used in production is completely free of contaminants. Food and beverage and medical industries are the main examples. Arguably, compressed air can be filtered to the required purity, but many compressor purchasers in these industries prefer to buy a compressor that has been designed so it will not put oil into the air.
What Oil-Free Actually Means
The oil-free designation does not mean the compressor runs without lubricating oil. What it means is that no oil is introduced into the air compression chamber. Achieving this means the piston or screws need a different means of sealing and lubrication.
Reciprocating compressors, use PTFE (Teflon) seals between piston and bore. Rotary screw compressors have special sealing elements or materials applied to the screw tips. In both designs, these materials seal and lubricate motion, although there is nothing to take away heat.
Oil-Free Compressor Maintenance
The principal difference between oil-free and oil-injected compressor maintenance is that the former has no oil to check and replace. In addition, the oil separator is eliminated. However, and this is a critical point, any gearbox and driving engine will still be lubricated and that will need periodic maintenance. In addition, bearings will need greasing, if the compressor is designed that way.
Routine maintenance should address:
- Air filter replacement
- Condition of drive belts
- Unloader and check valve operation
Furthermore, as oil-free compressors have no oil to take away the heat of compression, thermal protection takes on greater importance. Always ensure external surfaces, especially fins, are kept clean for efficient heat radiation and conduction.
Oil-Free Compressor Repairs
Providing an oil-free compressor is maintained in accordance with the manufacturer’s recommendations, it should run trouble-free for a long time. However, the seals will wear and eventually need replacement. For many compressors, this can be between 2,000 and 8,000 hours of operation, depending on how the equipment is used. Some oil-free compressors will go longer, depending on the design and type of sealing material used.
A particular issue is temperature. If allowed to run too hot, thermal expansion can result in increased rates of seal wear. Cleaning and regular intake air filter changes will help prevent excessive temperatures.
Seal replacement is a big job that will take the compressor offline for several hours. It’s best done before performance has deteriorated to a point where the compressor can no longer make the required pressure or runs excessively.
Other common repairs relate to the controls and valves. As with oil-injected compressors, these can give incorrect readings or fail to work as they should. In particular, a faulty unloader valve can make a compressor hard to start due to the additional force needed to move the piston against the pressurized air.
Consider Maintenance Costs and Frequency
An oil-free air compressor that’s maintained according to the manufacturer’s recommendations should prove as reliable as its oil-injected equivalent. However, the materials used for sealing instead of oil should be considered as consumables that need periodic replacement. While this period could be a year or more depending on how the compressor is used, it is a substantial repair that the oil-injected type will not need.
The flipside of seal replacement is that there is no need for periodic oil and filter replacement. Depending on how the compressor is operated and the cost of having it down, this may help justify the cost differential between oil-free and oil-injected air compressors.