Rebuilding an Air Compressor Air End: Everything You Need to Know

When is it time to rebuild the air compressor air end? Air end refurbishment is one way to extend the life of your rotary screw air compressor. How much it costs to repair the air end, and whether it is worth it, depends on the size of your air compressor and the overall condition of the machine. Learn everything you need to know in our complete guide to air end rebuilds. 

What Is the Air End for an Air Compressor?

The air end of a rotary screw or rotary vane compressor is the part of the machine that actually makes air. In other words, it is the mechanical element that compresses air from atmospheric pressure to the required pressure (PSI) for your application. It consists of the compression device (in the case of a rotary screw compressor, the screws or rotors) and the chamber in which they are contained. In a rotary screw air compressor, the air end is particularly critical because the precision and alignment of the twin rotors significantly affect the compressor's performance.

How Does the Air End Work? 

In a rotary screw air compressor, the air end consists of two interlocking rotors (or screws) within a casing. As the rotors turn, air is trapped in chambers created by the precisely machined interlocking screws. As the rotors continue to turn, these pockets of air are moved along the rotor, getting progressively smaller. This reduction in volume results in the air being compressed. 

Learn more: Full Guide to Rotary Screw Air Compressors.

In a rotary vane compressor, the air end consists of a cylindrical rotor placed within a larger cylindrical housing. The rotor has multiple slots in which movable vanes are placed. As the rotor spins, centrifugal force pushes the vanes out from the slots, creating compression cells between the vanes and the outer casing.

What Are the Components of the Air End?

The air end of a rotary screw air compressor is composed of several key components that enable it to effectively and efficiently compress air. For a screw compressor, these include: 

• Rotors: These are the central components of the air end. The rotors, often referred to as screws, consist of a male rotor with lobes and a female rotor with valleys or grooves. 
• Casing or Housing: The rotors are enclosed in a sturdy casing. This casing not only protects the rotors but also forms the compression chamber. 
• Bearings: The rotors are supported by bearings that enable them to rotate smoothly with minimal friction. These bearings must be able to withstand the high-speed rotation and the forces generated during compression.
• Lubrication System: In oil-injected rotary screw compressors, oil is introduced into the air end for cooling, lubrication, and sealing purposes. This oil is injected into the compression chamber, mixes with the air being compressed, and then separated from the air after compression.
• Seals: Seals are used to prevent compressed air and oil (in oil-injected compressors) from leaking out of the system. There are typically seals around the shafts of the rotors where they pass through the casing.
• Inlet and Outlet Valves: The air end casing has an inlet valve (or intake port) that allows outside air into the compression chamber and an outlet valve (or discharge port) where the compressed air is released.

Do All Air Compressors Have an Air End? 

All air compressors have an element that compresses air, but the term “air end” is usually associated with rotary screw air compressors. Air ends can also be found in other types of rotary compressors, such as scroll compressors or vane compressors. Other types of compressors, such as reciprocating compressors or centrifugal compressors, have similar mechanisms to the air end, but these might not be specifically referred to as an ‘air end’. Instead, they may have ‘cylinders’ (in the case of reciprocating compressors) or ‘impellers’ (in the case of centrifugal compressors).

Air End Problems: Signs That the Air End Is Failing

The air end of an air compressor is a robust component designed for durability and performance. However, like all mechanical parts, it can eventually wear down and show signs of failure. There are several common signs that the air end may be failing, including decreased CFM, increased energy consumption, excessive noise or vibration, increased operating temperatures, increased oil consumption, and loss of pressure. Remember, though, that these symptoms can also be caused by issues elsewhere in the compressor, so thorough diagnostics are necessary to confirm the source of the problem. If the air end is suspected to be failing, it's important to consult with a compressor service professional who can assess the condition of the air end and recommend appropriate actions, such as repair, rebuild, or replacement.

Decreased Performance

Decreased performance may be one of the first signs of a failing air end. If the volume of compressed air (measured in cubic feet per minute, or CFM) generated by the compressor starts to drop, it's likely that the efficiency of the air end is deteriorating. This could be due to worn rotors, damaged bearings, or problems with the seals, all of which can reduce the effective compression of air.

Increased Energy Consumption

The air end is designed to convert mechanical energy into pneumatic energy efficiently. If the compressor is drawing more electrical power (amperage) than usual for the same air output, it's a sign of decreased efficiency, possibly due to wear and tear inside the air end. More energy is needed to overcome the increased internal resistance due to this wear.

Excessive Noise or Vibration

Unusual noises or vibrations could be the result of several issues within the air end. Misalignment or damage to the rotors, worn or failing bearings, or problems with seals can create abnormal operational conditions that generate noise and vibration. These symptoms should never be ignored, as they can quickly lead to more serious damage.

Increased Operating Temperature

The compression of air generates heat, but a properly functioning air end — especially in an oil-injected compressor —  should manage this heat effectively. If the compressor is running hotter than normal, it might indicate inadequate lubrication, cooling issues, or increased friction from worn components — all potential signs of air end failure.

Increased Oil Consumption or Leakage

If the compressor is using or losing oil more rapidly than usual, it's a sign that something is wrong. In oil-injected compressors, the oil serves as a coolant, lubricant, and sealant in the air end. Damaged seals or increased wear within the air end can lead to excessive oil consumption. If oil is leaking externally, it could be a sign of a seal failure or a crack in the air end housing.

Presence of Metal Particles in the Oil

The presence of metal particles in the oil can indicate severe wear or damage inside the air end. When components like the rotors or bearings wear out, they can shed metal particles into the oil. These particles are a clear sign of a serious issue and warrant immediate inspection of the air end.

Loss of Pressure

A significant drop in the pressure output of the compressor can be a sign of air end problems. If the rotors are worn or damaged, or if the seals between the rotors and the casing are failing, the compressor won't be able to build up the same pressure as before. However, pressure loss can also be caused by issues elsewhere in the system, so a thorough diagnostic is needed.

Causes of Air End Failure

Wear and tear on the air end is a natural result of the operation of the air compressor. Over time, the rotors will lose tolerance. Normally, the rotors in a screw compressor do not directly touch each other or the sides of the compression chamber (instead, the oil forms the seal between parts). But when they lose tolerance, the rotors will start to rub against each other or against the chamber walls, resulting in additional wear and tear. Bearings, seals, and valves also deteriorate over time. The more you run your compressor, the faster this will occur. Proper maintenance will extend the life of your air end. 

There are several factors that may lead to premature failure of the air end in an oil-flooded rotary screw or rotary vane air compressor. These are some of the most common causes. 

Inadequate Lubrication

Lubrication is crucial in a rotary screw compressor. The oil serves not only as a lubricant but also as a coolant and sealant. If the oil level is too low, if the oil is not of the correct type or quality, or if the oil change intervals are not adhered to, it can result in excessive wear and heat, leading to premature failure of the air end.

Contamination

Dust, dirt, or other contaminants entering the compressor can cause significant wear on the rotors and bearings. These contaminants can enter the compressor through the air intake or with the oil, emphasizing the importance of good air and oil filtration.

Overheating

High operating temperatures can lead to a breakdown of the lubricating oil, causing varnish and sludge buildup. Overheating can also cause damage to seals and other components, potentially leading to air end failure. Overheating can be caused by inadequate cooling (for example, a blocked or malfunctioning oil cooler), high ambient temperatures, or excessive load on the compressor.

Poor Maintenance

Regular maintenance is critical for the longevity of the air end. This includes not only regular oil changes and air filter replacements but also periodic checks of the bearings, seals, and rotors for signs of wear or damage. Neglecting maintenance can lead to a variety of problems, including air end failure.

Operating Beyond Specifications

Running the compressor beyond its designed operating parameters — such as pressure, temperature, or duty cycle — can put excessive strain on the air end and lead to premature failure.

Misalignment or Imbalance

The rotors in a rotary screw air compressor need to be precisely aligned and balanced to operate smoothly. Misalignment or imbalance can result from improper installation, bearing wear, or damage to the rotors or casing. This can cause abnormal vibration and wear, potentially leading to air end failure.

How Long Does an Air End Last?

The longevity of the air end will depend on several factors, including the make and model of the compressor, operating conditions (including run speeds), and how well it is maintained. In general, an air end for an oil-lubricated rotary screw compressor can be expected to last about 80,000 hours (or ten years with continuous operation) before requiring a replacement or rebuild. However, a new air end can last as long as 100,000 hours or require a rebuild in as little as 50,000 hours or less, depending on operation and maintenance. An oil-flooded screw compressor that is not used continuously may last up to twenty years before requiring an air end replacement or rebuild. 

Oil-free rotary screw air ends generally will not last as long as their oil-lubricated cousins. The typical lifespan for the air end of an oil-free screw compressor is closer to 50,000 hours. Rotary vane compressors, on the other hand, tend to be highly durable; the air end can easily last 100,000 hours or more with proper maintenance. 

How Much Will an Air End Rebuild Extend the Life of the Compressor?

A rebuilt air end will not last as long as a brand new compressor air end, but it can help you get a lot more life out of your compressor — typically, another 40,000 to 60,000 hours, or several years of use. The lifespan of a rebuilt rotary screw air end can vary widely and depends on several factors, including the quality of the rebuild, the operating conditions, and the maintenance practices. To get the most out of your rebuilt air end, be sure to complete regular preventative maintenance, including timely oil and filter changes and periodic inspections. 

Remember also that compressor life depends on other elements, such as the motor, drive system, cooling system, oil sump, and compressor controls. Replacing the air end will not prevent a failure in these other components. When deciding between rebuilding an air end or replacing the whole compressor, be sure to consider the service life left on the motor and other critical components. 

Extending the Life of the Air End

To maximize the longevity of your air end, proper care and maintenance are essential. By performing regular maintenance, using proper lubrication, and operating the compressor within its specifications, it is possible to extend the life of the air end and improve the overall efficiency of the air compressor. Strategies for extending air end life include the following. 

Regular Maintenance

Stick to a regular maintenance schedule as recommended by the compressor manufacturer. This includes timely oil changes, filter replacements, and regular checks of all key components. 

Use the Right Lubricant

Always use the air compressor lubricant recommended by the manufacturer and keep it at the correct level. The right lubricant will reduce friction, absorb heat, and carry away contaminants, all of which will extend the life of the air end.

Keep It Clean

Cleanliness is key to the longevity of an air end. Regularly clean or replace the air filters to prevent dust and dirt from entering the system. Keep the area around the compressor clean and free of dust and debris.

Manage Heat

Overheating can accelerate wear and tear in the air end. Ensure that the cooling system (whether it's air-cooled or water-cooled) is functioning properly. Regularly clean the oil cooler to ensure efficient heat exchange.

Operate Within Specifications

Always operate the compressor within the manufacturer's specified parameters. This includes pressure, temperature, and duty cycle. Operating beyond these specifications can cause excessive wear and strain on the air end.

Regular Inspections

Regularly inspect the air end for any signs of wear or damage. Look for unusual noises, vibrations, or changes in performance, which could indicate a problem.

Proper Installation and Startup

Make sure the compressor is installed correctly, including proper alignment and balancing of the air end. Also, ensure that the startup process is carried out correctly, following the manufacturer's instructions.

Environment Control

Wherever possible, try to control the environment in which the compressor operates. Keep the compressor room well ventilated, minimize dust and contaminants, and, if possible, maintain a steady, moderate temperature.

Operator Training

Ensure that all operators are well-trained in the correct operation and maintenance of the compressor. Mistakes or neglect in operation can lead to premature wear and failure of the air end.

When Is It Time to Rebuild the Air End?

The right time to rebuild an air end generally comes when you start to notice decreased performance, increased noise or vibrations, excessive oil consumption, or increased operating temperatures. In addition, if the air end has reached its recommended operating hours (usually around 80,000 hours for an oil-lubricated screw compressor), it may be time to consider a rebuild. Always consult with a compressed air professional when these signs appear to determine the most appropriate course of action.

Rebuild or Replace? How to Make the Decision? 

Deciding when to rebuild the air end or replace the entire compressor can depend on several factors, including the age and condition of the compressor, the extent of the damage or wear to the air end, and the cost comparison between a rebuild and a new unit. Here are some considerations when deciding between compressor repair or replacement.

• Symptoms of Failure: If your compressor is experiencing decreased performance, increased energy consumption, excessive noise or vibration, increased operating temperature, or other symptoms of air end failure, it may be time to consider rebuilding or replacing the air end.
• Maintenance and Operating History: If your compressor has been well-maintained and operated within manufacturer specifications, it's more likely that a rebuild can effectively extend the compressor's life. However, if the compressor has been poorly maintained or operated under harsh conditions, there may be additional hidden damage or wear that could make a complete replacement more cost-effective.
• Cost Comparison: Compare the cost of a rebuild, including any associated downtime, with the cost of a new compressor. Don't forget to consider the potential energy savings and warranty benefits that might come with a new unit.
• Age of the Compressor: If the compressor is nearing the end of its expected life span, or if other key components (such as the motor) are also showing signs of wear or failure, it may be more cost-effective to replace the entire unit rather than rebuild the air end.
• Availability of Parts and Service: Check whether the necessary parts and skilled service technicians are readily available for a rebuild. For older models, parts may be discontinued, and technicians may not be familiar with the rebuild process.
• Future Requirements: Consider your future compressed air requirements. If your needs are expected to increase, or if you anticipate a need for better energy efficiency, it may be worth investing in a new, larger, or more efficient model.

Advantages of a Proactive Air End Rebuild 

If your air end is close to the limit of its natural life span or is showing signs of potential failure, it makes sense to schedule an air end rebuild before experiencing a failure of the air end. Timely air end refurbishment can prevent costly unscheduled shutdowns and emergency repairs. A scheduled air end rebuild can be completed during a holiday shutdown to avoid unplanned closures. Alternatively, you may consider renting an air compressor or moving production to a backup air compressor during the rebuild so operations can continue. 

Practicing scheduled or preventative replacement of the air end in a rotary screw compressor has several advantages.

• Improved Reliability: Proactively replacing the air end can help to prevent unexpected compressor failures and the associated downtime. This is particularly beneficial in industries where uninterrupted compressed air supply is critical.
• Extended Equipment Life: By replacing the air end before it fails, you can prevent collateral damage to other components of the compressor, potentially extending the overall life of the equipment.
• Cost Predictability: Scheduled replacement allows for more predictable maintenance costs, as opposed to the potential for sudden, costly repairs if the air end fails unexpectedly.
• Energy Efficiency: Over time, wear and tear on the air end can reduce the efficiency of the compressor, leading to higher energy consumption. Refurbishing the air end can help maintain the compressor's energy efficiency.
• Performance Consistency: An air end rebuild ensures that the compressor's performance remains consistent, providing the required pressure and flow rates for your operations.
• Reduction in Maintenance: A new or refurbished air end will typically require less maintenance than an older, worn one, reducing maintenance time and costs.

If the Air End Has Failed, Can It Still Be Rebuilt?

Whether a failed air end can be rebuilt largely depends on the extent and type of the failure. In some cases, if the failure is limited to specific components like bearings or seals, it may be possible to rebuild the air end by replacing these components. However, if there's extensive damage to critical components such as the rotors, or if the failure resulted in a significant amount of metal debris circulating through the system, a rebuild may not be feasible or cost-effective.

In any case, it's crucial to work with an experienced air compressor technician or service provider to accurately diagnose the failure and assess whether a rebuild is possible and worthwhile. It's also important to address the underlying cause of the failure to prevent a similar issue from occurring after the rebuild.

How Much Does It Cost to Rebuild the Air End?

The cost of rebuilding an air end compared to buying a new compressor can vary significantly depending on the make and model of the compressor, the size (HP) of the compressor, the extent of the repairs needed, labor costs in your area, and other factors. In general, a rough estimate might be that rebuilding an air end could cost about 20-25% of the cost of a new compressor. However, this can vary widely, and in some cases could be closer to 50% of a new compressor cost. For example, if you have a larger, more expensive compressor, the cost to rebuild the air end might be relatively lower compared to the cost of replacing the entire compressor, while for a smaller, less expensive compressor, the cost to rebuild might be closer to the cost of a new compressor.

However, it's important to consider more than just the upfront cost. You should also factor in the potential for increased energy efficiency with a new compressor, the warranty that comes with a new unit versus the guarantee on the rebuild, the expected lifespan of a new versus rebuilt compressor, and the potential downtime associated with each option.

Is It Worth It to Rebuild the Air End?

Whether it's worth it to rebuild the air end depends on several factors, including the condition and age of the compressor, the cost of the rebuild versus a new compressor, warranty coverage, and your operational needs. If the compressor is in generally good condition and the cost of the rebuild is significantly less than a new unit, a rebuild could be a cost-effective option. However, if the compressor is old or has other issues, it might be more cost-effective in the long run to invest in a new compressor with higher efficiency and a new warranty.

Will My Warranty Cover the Cost of an Air Compressor Air End Rebuild? 

Warranty coverage for the air end of the compressor varies widely between manufacturers, ranging from one to three years on the low end to five or even ten years on the high end. Warranty coverage may be given in years or operating hours (e.g., up to five years or 40,000 hours). If the air end fails within the warranty period, this typically indicates a manufacturing defect. The manufacturer will either cover the cost of the rebuild or provide a new replacement air end. 

Important: Failure to properly maintain and operate the compressor is likely to void the warranty. If the air end fails because it has not been maintained (e.g., inadequate lubrication) or has been run outside its normal operating conditions, the warranty will not apply. 

Fluid-Aire Dynamics offers an Extended Warranty that covers the air end, oil cooler, and oil sump tank of the air compressor for up to 20 years or 80,000 hours — even if we did not sell you the compressor originally. The warranty applies to any make or model of rotary screw air compressor that is less than 20 years/80,000 hours old, in good running condition, and installed according to the manufacturer’s specifications. All you have to do is complete the preventative maintenance at recommended intervals using materials supplied in your Extend-Aire kit and send in an oil sample every six months or 2,000 operating hours. 

Learn more: The Extend-Aire Warranty Program. 

What Happens in an Air Compressor Air End Rebuild?

An air end rebuild involves the disassembly, inspection, repair, or replacement of worn or damaged components, and reassembly of the air end. The exact process may vary depending on the make and model of the compressor and the extent of wear or damage, but here's a general idea of what typically happens. 

1. Disassembly: The air end is carefully disassembled to access the internal components. This often involves separating the rotors from the casing and removing the bearings and seals.
2. Cleaning: All disassembled parts are thoroughly cleaned to remove any oil, dirt, or other contaminants. This may require the use of specialized cleaning equipment and chemicals.
3. Inspection: The individual components are closely inspected for signs of wear or damage. This includes checking the rotors for wear or damage to the screw profiles, inspecting the bearings for signs of wear or damage, and checking the seals and casing for any defects.
4. Measurement: Critical dimensions such as rotor clearances, bearing fits, and seal areas are measured and compared to manufacturer specifications to assess the level of wear.
5. Replacement or Repair: Any components that are worn or damaged beyond acceptable limits are either repaired or replaced. This typically includes bearings and seals but may also include the rotors or other components if necessary.
6. Reassembly: The air end is carefully reassembled, ensuring all components are properly aligned and fitted. New gaskets and seals are typically installed during reassembly.
7. Testing: Once reassembled, the air end is typically bench tested to verify proper operation. This includes checking for any leaks, unusual noises, or other signs of problems.
8. Documentation: Detailed records are usually kept of all inspection findings, measurements, and work performed. This serves as a record of the rebuild and can be useful for future maintenance or troubleshooting.

It's important to note that an air end rebuild is a complex process that requires specialized knowledge and equipment. It should always be performed by a skilled technician or service provider with experience in compressor repairs. The quality of the rebuild can significantly affect the performance and longevity of the air end, so it's critical to ensure the job is done right.

Considering an Air End Rebuild?

The Aire Experts at Fluid-Aire Dynamics can help you decide between an air compressor replacement or an air end rebuild and refurbish your air end to extend the life of your compressor. Contact a service professional today to schedule an evaluation of your compressor and compressed air system.