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By: Brad Taylor

Guide to choosing a rotary screw air compressor
Thinking about buying a rotary screw air compressor? Read this first to learn how a rotary screw air compressor works, what they are used for, and how they are maintained. Plus, five important questions to ask when determining whether a rotary screw air compressor is right for you.

What is a Rotary Screw Air Compressor?

A rotary screw air compressor (sometimes called a “twin-screw compressor”) is a type of air compressor that uses a pair of enmeshed helical screws to compress air. As the screws turn, the air is continually compressed. Rotary screw air compressors are widely used for applications that require high CFM (Cubic Feet of air per Minute) and continuous use. This is the most commonly used air compression technology for heavy industrial applications such as powering pneumatic production lines and conveyor systems.

They can be contrasted with a reciprocating air compressor, which uses pistons driven by a crankshaft to compress air. Read more: Reciprocating vs. Rotary Screw Air Compressors: What’s the Difference?

How Does a Rotary Screw Air Compressor Work?

A rotary screw air compressor works by forcing air through a pair of rotating helical screws (or rotors). The two rotors interlock as they turn, creating a series of chambers. As air is forced through the rotors, it is squeezed into a smaller volume. This reduction in volume compresses air as it moves the chambers. A rotary screw air compressor is a form of positive displacement compressor.

  1. The two enmeshed helical screws or rotors turn rapidly, powered by the air compressor motor.
  2. The revolution of the screws forces air through the chambers. This creates a vacuum, which pulls more air into the chamber through the air compressor air intake.
  3. As air moved through the successive chambers of the paired rotors, it is squeezed into a smaller volume, resulting in air compression.
  4. Compressed air exits the screw unit, where it can be captured, dried, filtered, and used or stored.

Types of Rotary Screw Air Compressors: Oil-Injected vs. Oil-Free

There are two basic kinds of rotary screw air compressors: oil-injected and oil-free.

  • An oil-injected (or oil-flooded) rotary screw air compressor uses oil to lubricate the screw unit and ensure its smooth operation. The oil also helps to cool the compressed air and prevent overheating of the unit. After the air is compressed, the oil is removed using an oil separator. A small amount of oil may be left in the compressed air (oil carryover), which can be further reduced using inline filtration. Oil-injected compressors are the most common type of rotary screw air compressor and work well for most applications.
  • An oil-free (or oil-less) rotary screw air compressor uses a two-stage compression process. Air passes through an intercooler between compression stages to bring temperatures down and prevent overheating. Oil-free rotary screw air compressors eliminate any oil carryover, producing very clean air. However, they tend to be more expensive, more complicated to maintain, and noisier than their oil-injected cousins.

Rotary Screw Air Compressor Components

These are the basic components of a rotary screw air compressor.

Air-End (Pump)

The air-end is where the work of air compression happens inside the rotary screw air compressor. The air-end consists of several component parts.

Air Inlet Valve

Where atmospheric air comes into the air-end for compression.

Rotors/Screws/Rollers

A pair of interlocking helical screws that rotate rapidly to compress air.

Compression Cylinder

The housing that contains the rotors. A rotary screw air compressor may have a single compression cylinder or multiple cylinders for multi-stage compression. Cylinders are typically made of cast iron or steel. Oil-free compressors will also have an intercooler between compression cylinders.

Discharge Valve

Where compressed air exits the air-end. Air may go from the discharge valve to an air receiver tank or directly to air-using processes.

Bearings

Bearings on both rotor ends help the rotors to stay in place and turn smoothly and rapidly. Bearings are typically anti-friction and corrosion-resistant.

Other Core Compressor Components

A rotary screw air compressor must have several other components in addition to the air-end.

Air Compressor Motor

The compressor motor powers the rotation of the rotors. The motor may be fixed-speed or variable-speed.

Air Compressor Control System

The air compressor control system controls the motor, monitors system operation and output, and provides a method for human operators to monitor and control the system.

Air Compressor Filters

Filters keep contaminants out of the air supply. Filters for a rotary screw air compressor may include:

  • Intake filters to remove contaminants from atmospheric air before it enters the air-end.
  • Inline filters to remove particulate and oil from compressed air.
  • Oil filters to remove particulate from liquid oil in oil-lubricated rotary screw air compressors.

Read more: What Type of Filtration Does Your Air Compressor Need?

Air Compressor Cooling System

Rotary screw air compressors generate a lot of heat during compression. This heat must be dissipated by a cooling system to prevent overheating. Cooling system options include:

  • Closed-loop dry cooling, which circulates a cooling liquid through a series of coils that carry heat away from the air compressor.
  • Open-loop evaporative cooling, which uses a combination of fresh water and moving air to cool the compressor.

Gaskets & Seals

The entire system must remain air-tight to maintain pressure. Gaskets and seals prevent air from escaping and maintain the pressure in the system.

Oil-Injection System for Rotary Screw Air Compressor

An oil-injected (or oil-flooded) rotary screw air compressor will have additional components as part of the oil injection system.

Oil Sump

The reservoir where oil is stored for use in the compression system.

Oil Filter

The oil filter removes particulate from the liquid oil while it circulates.

Oil/Water Separator

The oil/water separator separates liquid water (created during air compression) from the oil supply. Read more: Oil/Water Separators for Compressed Air Systems—Complete Guidelines

Oil Cooler

The oil cooler allows hot lubricant (heated by the compression process) to cool down before it is recirculated. A thermostatic bypass valve monitors oil temperature and sends too-hot oil to the oil cooler.

Rotary Screw Air Compressor Accessories

A complete compressed air system will include a number of other components in addition to the air compressor itself.

Compressed Air Dryers

Hot, compressed air coming out of the air compressor will contain excess moisture. For most applications, it is desirable to remove moisture from compressed air to ensure a supply of clean, dry air for industrial processes. There are two basic types of compressed air dryers.

  • Refrigerated air dryers work by cooling compressed air to allow excess moisture to condense into liquid water, which is drained away. A refrigerated compressed air dryer can lower the dew point of compressed air to ~38°F, which is adequate for most applications. This is the most common type of air dryer used with rotary screw air compressors.
  • Desiccant air dryers remove water vapor from the air using an adsorptive material. Desiccant air dryers are more expensive than refrigerated dryers, but they can get air much dryer—down to a dew point of -40°F to -100°F. They are used for applications requiring ultra-dry air.

Read more: Refrigerated vs. Desiccant Air Dryers—How to Choose a Compressed Air Dryer

Air Receiver Tank

The air receiver tank (or compressed air storage tank) stores compressed air after it leaves the air compressor. The air receiver tank may be placed either before or after the air drying system. Read more: Air Receiver Tank Full Guidelines

Drain Valves

Drain valves are used to remove excess liquid water from the compressed air system. When compressed air comes out of the rotary screw air compressor, it is very hot—and hot air can hold more moisture than cold air. As the air cools back down to atmospheric temperatures, excess liquid will fall out of the air as condensation. Drain valves are used to drain excess liquid out of the air compressor, air receiver tanks, air dryers, compressed air piping, and other parts of the system where liquid may accumulate.

Advantages and Disadvantages of Rotary Screw Air Compressors

Rotary screw air compressors are the most popular type of air compressor for industrial applications—and for good reason. These air compressors are the workhorses of the industry: reliable, quiet, energy-efficient, and powerful. They are an excellent choice for high-volume applications requiring a continuous supply of air.

Advantages of Rotary Screw Air Compressors

Higher CFM per Horsepower

Compared to reciprocating or piston-type air compressors, rotary screw air compressors can produce more compressed air (measured in Cubic Feet per Minute, or CFM) per horsepower. A rotary screw air compressor delivers 4-5 CFM per HP, while a reciprocating air compressor delivers 3-4 CFM per HP. In other words, if you compare a 20HP reciprocating compressor to a 20HP rotary screw air compressor, the rotary screw will produce 20-25% more air. That means they use less energy for producing the same volume of compressed air.

Continuous Use and Consistent Airflow

Rotary screw air compressors can crank out a lot of compressed air continuously—in fact, that’s what they are designed to do. A rotary screw air compressor operates best at 100% duty cycle. Since they can produce air at high CFM on a continuous basis, they are a great choice for 24/7 production operations. Depending on your application, you may be able to skip the receiver tank and just power operations directly from the air compressor. You can also count on a rotary screw air compressor for highly consistent airflow, so you can size your air compressor very close to your maximum use scenario.

Lower Total Cost of Ownership

A rotary screw air compressor costs more upfront but will save money in the long run. In addition to lower energy costs due to their higher CFM output, a rotary screw air compressor just lasts longer than a reciprocating or piston-type air compressor. A rotary screw compressor can last between 60,000 – 80,000 hours before a rebuild is necessary—about 6-8 times as long as a reciprocating air compressor.

Cleaner Air

Rotary screw air compressors deliver very clean air with low oil carryover. An oil-injected rotary screw compressor will have an oil carryover of ~3 ppm. Oil-free compressors, of course, have no oil carryover.

Quiet Operation

Are rotary screw air compressors quiet? Compared to the alternative, yes. The action of the rotary screws is quieter than the action of moving pistons. A rotary screw air compressor may also have a sound-dampening enclosing. A typical rotary screw air compressor will operate at 65-75 dBA, or somewhere between the noise level of a car engine and a vacuum cleaner. A rotary screw air compressor will also generate less vibration.

Cooler Temperatures

Typical internal operating temperatures for a rotary screw air compressor are ~ 80-99°F. For comparison, a piston-type or reciprocating air compressor operates at internal temperatures of ~150-200°F. Lower temperatures translate to less work for coolers and air dryers, further saving energy costs.

Space Savings

A rotary screw air compressor will take up less floor space than a reciprocating air compressor with similar CFM output. In manufacturing facilities where floor space is at a premium, this can be a huge advantage.

Disadvantages of Rotary Screw Air Compressors

Higher Initial Capital Investment

A rotary screw air compressor requires a higher initial capital investment than a piston-type air compressor of similar CFM output. However, a rotary screw compressor will have lower total lifetime costs due to its energy efficiency and longevity.

Requires Skilled Maintenance

Rotary screw air compressors are more complicated to maintain, requiring skilled and trained maintenance staff.

Requires a Clean Operating Environment

Air coming into the rotary screw air compressor should be as clean as possible. If you are operating in a very dirty environment or locating the air compressor outside, a reciprocating air compressor may be another choice. Intake filters can help to reduce the amount of particulate coming into the compressor if you are using your rotary screw outdoors or in a dusty facility.

Should Not Be Used for Intermittent Applications

A rotary screw air compressor is designed for continuous operation and a 100% duty cycle. It is not the best choice if your compressed air needs are intermittent and require frequent cycling of the compressor on and off. If your air demand is variable but does not drop to zero, a variable speed drive motor will help the compressor ramp air production up and down depending on your needs.

What Is a Rotary Screw Air Compressor Used For?

Rotary screw air compressors are best for applications that require continuous use (100% duty cycle) and high CFM. They are also a good choice for applications requiring very clean air. Typical applications include:

  • Robotic manufacturing
  • Pneumatic conveyor systems
  • Paint lines
  • Food processing
  • Packaging lines

Five Questions to Ask Before Choosing a Rotary Screw Air Compressor

Before selecting a rotary screw air compressor, make sure it is the right choice for your application. Here are five important questions to ask when making your decision.

1. How large is your demand?

Rotary screw air compressors are especially durable and are able to handle large amounts of demand for extended periods of time. They are still more efficient and durable than piston compressors even at smaller sizes, but the efficiency benefits are most noticeable when getting into larger demands (i.e., 15 HP and more). Rotary screw air compressors are designed to run at 100% duty cycle, day in and day out.

2. Is lifetime cost important to you?

While a rotary screw compressor may have higher air compressor maintenance costs and/or initial capital cost, its life expectancy is much greater than a piston-type air compressor. Piston-type (reciprocating) compressors typically last about 10,000 hours before a rebuild is necessary. On the other hand, a rotary screw compressor will last between 60,000 – 80,000 hours. Rotary screw compressors are known for their reliability and longevity.

3. Does your machine operate near your production staff?

Rotary screw air compressors are known for their quiet operation. Being that most rotary screw air compressors come standard with sound dampening enclosures, they generally operate between 65 – 75 dBA, which is much lower in comparison to other types of compressors. For comparison, a reciprocating or piston-type air compressor can operate at around 80 – 85 dBA.

4. How clean does your compressed air need to be?

Rotary screw air compressors are designed to have no more than 3 PPM of oil carryover, while piston compressors can have 10 PPM or more as they age. Less oil carryover means that you have to change your filter elements less often, therefore decreasing maintenance costs.

5. Do you use your compressed air continuously throughout the day? Or does your demand vary throughout the day?

  • Rotary compressors have become more and more popular because of their efficiency and their ability to keep up with constant high demand. Many rotary screw compressors are used 24/7.
  • Even if you don’t have a constant demand, rotary screw reliability and durability come in a variable speed drive version. This will speed up and slow down in relation to your demand at that given point. This option should definitely be considered if your demand varies, because yesterday’s controls including on-line/off-line, modulation, and even turn valves have proven costly when compared to the benefits of the VSD.

Maintenance Requirements for Rotary Screw Air Compressors

What kind of maintenance is required for a rotary screw air compressor? Rotary screw air compressors require regular inspection and maintenance to operate at peak performance and protect the system from excessive wear and tear. Check your owner’s manual for specific guidelines for your make and model of rotary screw air compressor. Maintenance tasks may include:

  • Draining excess water via the drain valves
  • Checking oil
  • Changing and flushing oil
  • Replacing oil filters
  • Replacing air intake and inline filters
  • Changing the oil/water separator
  • Inspecting and replacing belts or drive train parts
  • Lubricating motor bearings
  • Inspecting and adjusting controls
  • Cleaning and maintaining air compressor coolers and heat exchangers

How often is oil changed for a rotary screw air compressor?

Checking and changing the oil is one of the most important maintenance tasks for an oil-injected rotary screw air compressor. Depending on your compressor model and the type of oil used, you can expect to change the oil every 4,000 – 8,000 hours. However, if your rotary screw air compressor is operating in a dirty environment, the oil may need to be changed more often. Check oil levels and inspect the oil for contaminants at least weekly. At least once a year, oil should be completely drained and flushed. Always replace your oil filter after flushing.

How often are filters changed for a rotary screw air compressor?

Air compressor filters (including air intake filters and inline filters) should be changed at a minimum every 2,000 hours for a rotary screw air compressor. If your environment is very dirty, you may need to change filters more often.

How often does water need to be drained for a rotary screw air compressor?

Water should be drained for a rotary screw air compressor at least daily, if not more often. To reduce maintenance time, look for automated drain valves. Electric drain valves can be set to open on a timer to drain condensate regularly. A zero-loss drain valve uses a float mechanism to activate the drain, so it only opens when needed. This can reduce compressed air loss.

Other Frequently Asked Questions for Rotary Screw Air Compressors

Do you need a tank for a rotary screw air compressor?

It is possible to use a rotary screw air compressor without a tank. Tankless operation for a rotary screw air compressor works best for large operations using air continuously. In large operations, the piping system acts as residual storage as well. If your air use is more variable or prone to sudden spikes in demand, it is still advisable to use an air receiver tank.

How quiet are rotary screw air compressors?

Rotary screw air compressors are known for their quiet operation compared to piston-style (reciprocating) air compressors. A rotary screw compressor operates at about 65-75 dBA, or somewhere between a car engine and a vacuum cleaner.

How do you size a rotary screw air compressor?

One of the advantages of a rotary screw air compressor is that it can be safely sized at just a bit over your maximum demand. Unlike reciprocating air compressors, they do not require a large overcapacity. This is because the rotary screws can produce air at a very fast, reliable, and consistent rate to keep up with your demand.

Can you get a variable speed drive motor for a rotary screw air compressor?

Yes, it is possible to get a variable speed drive motor for a rotary screw air compressor. A VSD motor will allow the compressor to ramp air production up and down in response to actual need. This is advisable if your compressed air needs vary over the course of a day or shift. A VSD motor is a much better solution than turning the compressor on and off, as frequent on/off cycles can damage the rotary screw compressor. Read more: Why Choose or Upgrade to a Variable Speed Drive Air Compressor?

Do you have to use an air dryer with a rotary valve air compressor?

In most cases, yes. All air compression methods will result in condensation as compressed air cools. Excess moisture in the compressed air piping system or in the air supply can create problems for both production processes and the compressed air system itself. Read more: Getting Rid of Moisture in Your Compressed Air System

How to Choose the Right Rotary Screw Air Compressor

Choosing a rotary screw air compressor depends on your airflow requirements, production processes, and budget. The experts at Fluid-Aire Dynamics can help you select the best rotary screw air compressor for your application.

Need help choosing a rotary screw air compressor? Contact us for help.

Chicago (847) 678-8788 – Minneapolis (612) 246-3432 – Milwaukee (414) 273-1994

All in all, rotary screw compressors today have become popular because of their efficiency, noise level, and the fact that the cost for this type of compressor has dropped significantly in the last 10 years. They have low oil carryover and are durable and reliable. They come in VSD for applications with varying demands throughout the day. Rotary screw compressors are a worthwhile investment for any manufacturer looking for a good quality, overall cost-effective unit.