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When making comparisons about compressed air technology, it is important to understand key terms and concepts like oil lubricated air compressor, oil flooded air compressor and oil free air compressor.  In this industrial air compressor selection guide, we provide an overview and comparison between different types of compressor technologies. Understanding the types of compressors, why an oil lubricated or oil free air compressor might be preferred for a given application, and how air compressor performance is rated are essential to understanding air compressor selection.  

What Are the Types of Air Compressor Lubrication Systems?

One way to classify air compressors is by the type of lubrication. The lubrication system is key to the longevity and service of an air compressor, and it also affects the quality of air produced by the system. Your application requirements will determine whether you are best served by an oil lubricated air compressor or an oil free air compressor.

Compressor systems are ranked based on the International Organization for Standardization (ISO) standards for air purity from ISO Class 0-5, with class 0 denoting an oil free product, while the other classes indicate higher rates of oil carryover, which is generally expressed in parts per million by weight (PPMw).

Compressor-system-international-organization-for-standardization-selection-guide

Oil Free Air Compressor Lubrication

Most oil free air compressors are designed with self-lubricating materials that allow the piston to slide in the cylinder, and the grease is contained in sealed bearings. Class 0 oil free air compressors can produce oil-free air because they have no oil lubrication in the compression chamber. This class of air purity may be required for a number of applications in industries like food processing, pharmaceuticals, wastewater treatment, and chemicals.

The oil free air compressor category includes centrifugal, oil-free rotary screw and non-lubricated reciprocating compressors.  A good example is Rogers KNW Oil-free air compressor systems

Oil Lubricated or Oil Flooded Air Compressor Lubrication

Oil lubricated or oil flooded air compressors use oil-lubed pumps for air pressurization, and therefore require regular piston lubrication and maintenance. They also generally offer long service life. Oil lubricated air compressors come in a variety of types including lobe, liquid ring, scroll, screw and vane.

Oil lubricated compressors often contaminate the air produced by the compressor by some trace amount. However, just because oil-lubricated compressors use oil does not mean that the compressed air is completely oil contaminated. Installing downstream equipment such as oil filters, dryers, and air/oil separators can remove oil from the compressed air. In some applications, oil contaminated air can be desirable for the tools that run off the compressed air.

Also note that some oil air compressors do meet the ISO specification of “technically oil-free” air after compression and filtering. “Technically oil-free” air produced by an oil lubricated air compressor can still have a small degree of oil contamination after filtering.  A good example is Rogers K-series oil lubricated air compressor systems

For more detailed information, visit Achieving Technically Oil Free Compressed Air

What Are the Types of Air Compressors?

Compressors are separated into two major groups – positive displacement and dynamic – based on the mechanism used to compress air. 

How Do Positive Displacement Air Compressors Work?

Positive displacement compressors bring in air, trap it, and then reduce its volume mechanically. The most common types of positive displacement compressors are reciprocating, scroll, and rotary screw.

Reciprocating Compressors

reciprocating-oil-flooded-air-compressor

Reciprocating Compressors use a piston driven by a crankshaft to deliver high pressure air. They are generally oil-lubricated, using a splash lubrication mechanism, and they are known for:

  • small size
  • simple maintenance
  • ability to set up near the point of use
  • efficiency because they run start/stop, only producing air when needed

Scroll Compressors

scroll-oil-lubricated-air-compressor

Scroll Compressors use intermeshing spiral scrolls, with one being stationary and the other orbiting in relation to it. Usually scroll compressors are oil-free. They are known for:

  • limited size
  • relative efficiency
  • low noise levels

Rotary Screw Compressors

rotary-screw-oil-flooded-air-compressor

Rotary Screw Compressors use two mated screws to mechanically reduce the volume of the air. They are available both as oil free air compressors and as oil lubricated air compressors, depending on the application requirements, and they are known for:

  • suitability for industrial uses
  • greater durability and higher performance ratings than reciprocating compressors or scroll compressors

How Do Dynamic Air Compressors Work?

centrifugal-oil-flooded-air-compressor

Dynamic type compressors, such as centrifugal compressors, use a high-speed rotating impeller to transfer energy to the air. Dynamic centrifugal compressors are designed to be oil-free. They are known for:

  • limited capacity control modulation
  • require specialized maintenance
  • high initial costs
  • ideally suited for larger applications

How Do Oil Free Air Compressors Work?

Oil free air compressors and oil lubricated air compressors operate on much the same general principles. Both use a piston or another rotary element to draw in outside air, and through a series of steps, compress the air into a storage tank.

Operation of an oil free air compressor has a few distinctions related to the absence of oil lubrication in the piston. Some oil free air compressors have no mechanical contact inside the cylinder walls, and therefore no need for lubrication. Others use alternate materials like water or a Teflon coating to protect the mechanism without the need for lubrication. Usually, self-lubricating materials allow the piston to slide in the chamber, and sealed bearings contain grease.

Oil free compressor processes follow these general steps:

1. Drawing in Air.

When you turn on your oil free air compressor, it starts to draw in outside air through an open unloader valve and passes it through an inlet air filter. The inlet air filter removes dirt, dust, and small particles that could otherwise damage the compressor itself. When your compressor is running and delivering compressed air, the unloader valve is closed so the compressor can't draw in more air.

2. Low-pressure Compressor Element.

The filtered outside air is passed over the low-pressure compressor element. In an oil free air compressor, this element can create some heat because the air compressor lacks the flowing medium to dissipate heat.

3. Intercooler.

Pistons push the air through an intercooler, which (depending on the compressor) is either the second compression phase or the final one. The intercooler cools the air to minimize the risk of heat damage and, in two-stage pumps, to allow air to be compressed at much higher PSIs. Cooling air can lead to condensation, so intercoolers have standard filters called moisture traps to remove water from the air.

4. Higher-Pressure Compression Element.

In two-stage compressors, the cooled air is returned to the main compressor chamber of your air compressor — or the second chamber, depending on its design —for further compression by a high-pressure element. The air again becomes very hot.

5. Air Prep.

Air passes through a check valve to prevent any backflow, and then (in many models) through a pulsation dampener. A dampener reduces vibrations that occur when the compressor uses suction and opens discharge valves.

6. Aftercooler Access.

The air flows to the aftercooler, which cools the air after its final compression stages, so it can be stored before it is used.

7. Pressure Switches.

A pressure switch monitors the air in the compressor tank, and when it falls below a predefined level, the compressor will turn on to rebuild the pressurized air volume, and the process starts again.

 

How Does an Oil Lubricated Air Compressor Work?

An oil lubricated air compressor works in much the same way as an oil free air compressor, with the difference being that oil lubricated air compressors use lubricating oil to keep the piston or rotary element running smoothly and consistently without damage. In addition, the oil in these models helps to dissipate heat and maintain compression efficiency.

Most oil lubricated air compressors use either splash lubrication or pressure lubrication.

Splash Lubrication

Many reciprocating compressors have an oil bath that randomly splash-lubricates the cylinder walls and bearings while the crank rotates. An oil dipper attached to the bottom of the connecting rod splashes oil from the crankcase reservoir onto the internal parts of the compressor. Piston rings do help keep the compressed air on top of the piston and keep the lubricating oil away from the air, but some oil will become aerosolized and enter the compressed air.

Pressure Lubrication

Pressure lubrication systems add to cost, but can be an ideal solution for heavy industrial applications. In these designs, an oil pump driven by the crankshaft draws oil out of the crankcase, which is then filtered and pumped through passageways in the crankshaft directly to the connecting rod-bearing surfaces. The piston and piston bearings are also lubricated by oil sprayed onto the cylinder walls.


What Are the Other Components of a Compressed Air System?

When thinking about a compressed air system the first thing that comes to mind is a compressor, but what else goes into your system? The typical clean dry air system includes:

  • air compressor
  • wet air receiver
  • coalescing pre-filter
  • regenerative air dryer
  • particulate after-filter
  • dry air receiver
  • sometimes even a final filter before the piping takes the compressed air to the final application

 

 

Air Receivers

Compressed air receivers are used to store compressed air and limit demand on the compressor. When a receiver isn’t properly sized, it can lead to the compressor running more often than is necessary. This can lead to a decrease in the system’s overall efficiency. On top of the efficiency gains that come with properly sizing your air receiver, when the compressor runs less it will help with system reliability and longevity.

Air Dryers and Filters 

Dryers and filters are used to reduce moisture content and contaminants in the air system. When compressed air is not properly dried and filtered it can lead to contamination of the end process. In most applications, this kind of contamination is unacceptable. Another side effect of not using proper filtration and drying is degradation of the compressed air system.

Oil lubricated air compressors require specialized equipment like oil separators and filtration systems to minimize any impacts from oil carryover that could affect critical components downstream. Depending on the process needs for compressed air, it is often possible to control oil carryover by installing a coalescing filter downstream from the compressor.

Compressor Backup and Communication

If your system requires backup or additional capacity, you will have more than one compressor connected to your system. These can be set up to communicate directly through auto lead/lag or through a sequencer when connecting three or more compressors. This communication allows compressors to be turned on only as needed, which can increase system efficiency.

Compressed Air System Design and Piping 

Compressors produce a large amount of heat waste, so keep that in mind when designing your compressor room.  You will need a source of clean inlet air, discharge air ducting or consider a water-cooled machine. If you are interested in saving on energy costs you can implement a heat recovery system. Another way to design your compressor system to be more efficient is by increasing the size of your pipe, to limit friction, and shorten the distance the air has to travel.

What Is the Cost of Compressed Air?

Compressed air is sometimes referred to as the Fourth Utility in plant operation, equal in importance with the other three utilities (water, electricity, and natural gas). Since compressed air is the only utility that is actually produced within the plant, a facility has some control over the cost to produce it.

The cost of compressed air generally boils down to the initial capital investment, plus ongoing maintenance and energy costs. Energy is by far the largest of the costs over time, accounting for 76% of the lifetime cost of a compressed air system.

 average-cost-of-compressed-air

Both the initial and ongoing cost of compressed air can be affected by choosing the right compressed air system in the first place. Factors that affect the ongoing energy cost of compressed air include:

  • efficiency of the air compressor (flow control and storage)
  • type of air compressor
  • drying and purification methods
  • the appropriate sizing of the air compressor system for the actual compressed air needs

There are several ways to reduce the ongoing cost of compressed air that may be applicable in any given plant:

  1. Make sure the air compressor is turned off when not in use.
  2. Use only the amount of air pressure needed rather than increasing the pressure to compensate for clogged filters or leaks.
  3. Reduce air leaks. Clean and change filters regularly to reduce the chance of air leaks and equipment failures.

A compressed air audit can help identify opportunities for cost savings in compressed air systems, whether in filtration, reducing leaks or updating equipment.  Contact Rogers Machinery for your compressed air audit. 

For more detailed information, visit Determine the Cost of Compressed Air for Your Plant

How Is Air Compressor Performance Rated?

Knowing your air compressor’s performance ratings can ensure that you choose a compressor that best meets your needs. And keeping track of air compressor performance helps ensure that your compressed air system operates as efficiently as possible.

Compressor Performance Terminology

An understanding of some basic air compressor terminology helps when comparing various types and models, and assessing what the compressor can do and whether it fits your needs.

PSI (pounds per square inch)

Indicates the amount of pressure placed on a square inch of space, so it measures the air force the compressor can deliver. Two stage compressors can generally deliver higher PSI than single stage compressors.

CFM (cubic feet per minute)

Indicates the amount of air that a compressor can produce at a given pressure level, which is also known as the compressor’s flow rate. Compressors with higher CFM ratings can generally provide more air.

SCFM (standard cubic feet per minute)

Another way to measure the amount of air that a compressor can produce, although it is calculated at a given temperature and humidity as well as pressure. The CFM measurement for a given air compressor is a lower number than the SCFM.

HP (horsepower)

Measures the amount of work that the motor that drives the compressor pump can perform. Although higher HP generally goes along with higher CFM, newer, efficient compressors can achieve higher CFM with less HP. So higher HP does not always correlate with better performance and in fact, an efficient compressor rated for lower HP can save energy.

 

The most important performance indicators are probably CFM and PSI. In other words, a compressor needs to meet the application’s needs for sufficient air flow at the correct pressure as efficiently as possible.

Other Compressor Comparison Measurements

Some other measurements that can be useful when comparing air compressors are duty cycle and gallons.

Duty Cycle is the relationship between on time and off time. The term is specific to reciprocating air compressors, which are designed to operate in an on/off or cyclical fashion. A compressor may be rated for a 50/50 duty cycle, meaning it is designed to run 50% of the time, or 75/25 duty cycle, meaning it is designed to run 75% of the time.

Gallons are used to measure compressor storage tank size. The size of the tank can be important for tools that require a continuous flow of air. Larger tanks mean the job can be performed for a longer period of time at the air tool’s required pressure before it becomes necessary to pause and rebuild pressure in the tank.

For more detailed information, visit Measuring Performance of Installed Air Compressors

Overall Air Compressor Ratings

Overall air compressor ratings can be expressed in power ratings and Free Air Delivery ratings that are measured by the manufacturer when the compressor is new. Over time, actual compressor performance may vary, so it can be worthwhile to re-assess the performance of a compressor as installed and operating. Performance can be measured by:

  • How much electrical power the compressor consumes
  • How much air the compressor delivers
  • Pressure profile

If you suspect that your existing air compressor system is underperforming, it may be worthwhile to complete a temporary performance assessment, in conjunction with a system assessment, before investing in a new compressor.

It can also be worthwhile to put permanent metrics in place to monitor performance and trigger service or overhauls.

For more information or any questions fill out this contact form or call us at (503) 639-0808.


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