Full Guide to Rotary Screw Air Compressors

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Is a rotary screw air compressor right for you? Screw compressors are the workhorses of industry and manufacturing. These tough industrial air compressors are widely used for applications requiring high efficiency and continuous airflow, including conveyor systems, manufacturing equipment, process manufacturing, and more.

Our complete guide to rotary screw compressors will help you understand how these compressors work and decide whether a screw compressor is the right choice for your application. Plus, how to install and maintain your compressor system for optimal performance.

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. 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.

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. The compression process is continuous and controlled by a drive motor and a system of inlet and discharge valves.

Air enters the compressor through the air intake. (An air intake filter may be needed to keep excess dust, debris, pollen, and dirt out of the compressor.)
Air goes through the regulator valve, which sets the pressure, and is directed into the air end (the rotor housing).
The two enmeshed helical screws or rotors turn rapidly, powered by the air compressor motor. In an oil-flooded compressor, oil is used to create a tight seal between the rotors and the housing walls to trap air within the rotors. In an oil-free compressor, special coatings and seals are used to prevent air leaks.
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.
As air moves through the successive chambers of the paired rotors, it is squeezed into a smaller volume, resulting in air compression.
Compressed air exits the air end unit. Usually, air will go through an aftercooler system to dissipate excess heat and remove some of the moisture from the air. As the air cools, moisture falls out of the air as condensation and is drained away. An oil/water separator is used to remove oil from the liquid water before disposal.
Air exiting the aftercooler unit will typically have to be dried and filtered to remove remaining moisture, oil mists, and particulates. Air dryers and inline filters are used to treat air prior to use or storage.
Clean, dry air can be used immediately to power industrial processes or stored in an air receiver tank for later use.

Types of Rotary Screw Air Compressors

There are two basic kinds of rotary screw air compressors: oil-injected and oil-free. Oil-injected screw compressors are the most common type of industrial air compressor and work well for most applications. Within these categories, there are also some subtypes to consider.

Tank-mounted vs. Base-mounted: Tank-mounted rotary screw compressors have an integrated air receiver tank, which stores compressed air. They may also include an integrated air dryer, making them an all-in-one solution. Base-mounted rotary screw compressors are stand-alone units that do not include an integrated tank or dryer system. They provide more flexibility in compressor system design; the air drying and storage systems can be customized for the application.
Fixed-speed vs. Variable-speed: Screw compressors may be fixed-speed or variable-speed. A fixed-speed compressor provides 100% power 100% of the time, which is ideal for businesses needing constant airflow, at reduced loads it is inefficient. A variable speed drive (VSD) air compressor ramps the motor speed and air production up and down in response to actual air demand. A VSD rotary screw compressor reduces energy use and operating expenses for businesses that have variable demand for air across shifts or seasons.

Oil-Injected Rotary Screw Compressors

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-Free Rotary Screw Compressors

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. Oil-free screw compressors are most commonly used for specialty applications requiring very high-purity air. However, in most cases, it is possible to obtain the required purity levels using dryers and air filtration.

Rotary Screw Air Compressor Components

How Do Rotary Screw Air Compressors Work?

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.
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.

Motor and Control System

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.
Pressure Regulator: An air compressor pressure regulator is a device that controls the pressure (PSI) of air coming from an air compressor. It maintains a constant output pressure to protect the system and tools from overpressure and ensure that the correct pressure is maintained for safe and efficient operation. The compressor system may have a single pressure regulator at the exhaust outlet for your compressor or individual pressure regulators for each drop or tool.

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.

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. An aftercooler acts as a heat exchanger, absorbing heat from the compressed air and carrying it away where it can be dissipated. The aftercooler cools air and removes the majority of the moisture before it goes into the drying system and filters. There are two basic types of air compressor after-coolers: air-cooled and water-cooled.

Most screw compressors are air-cooled. Air-cooled rotary screw compressors use a series of coils that are cooled by a fan. Cooling fins provide additional surface area for air to move across, increasing the heat transfer capacity.

Water-cooled rotary screw compressors use water (or, sometimes, another liquid) to carry away excess heat. Water-cooled aftercooler 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.

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 particulates 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.

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.

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.

Things to Consider When 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 costs, its life expectancy is much greater than a piston-type air compressor. Piston-type (reciprocating) compressors typically last about 10,000 - 20,000 hours before a rebuild is necessary. On the other hand, a rotary screw compressor will last between 80,000 - 100,000 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. Since 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 compressor 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. A VSD compressor will speed up and slow down in relation to your demand at any given moment. This option should definitely be considered if your demand varies, because yesterday’s controls — including online/offline, modulation, and even turn valves — have proven to be costly when compared to the benefits of a VSD.

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 to produce 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 most piston compressors cannot run at this 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 up front 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 up to 100,000 hours with proper maintenance 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 oil carryover of ~3 ppm, compared to 10 ppm or more for a typical piston compressor. 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 enclosure. 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 140-160°F. For comparison, a piston-type or reciprocating air compressor operates at internal temperatures of 150-200°F or greater. 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 a better 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.

Industries that Use Rotary Screw Compressors

Rotary screw air compressors are used across many industries that require high CFM or continuous airflow (100% duty cycle), including manufacturing, food processing, pharmaceuticals, processing plants, paint lines, packaging lines, and much more. These industry staples provide a reliable source of compressed air for many applications in manufacturing and industry, from powering equipment to pneumatic conveying, to process air.

Manufacturing

Many manufacturing facilities rely on screw compressors to power production equipment, conveyors, and packaging lines. Rotary screw compressors provide consistent airflow and pressure for robotic production applications in manufacturing sectors such as automotive, aerospace, and general fabrication. Manufacturing plants also use compressed air to lift and move heavy parts and power manual tools. Other applications in manufacturing include:

Power cleaning.
Surface preparation (e.g., abrasive blasting or vibratory finishing).
Paint lines/spray booths.
Plastic forming/blow molding.
Cleaning and deburring.

Process Industries

Process industries, including chemical manufacturing and pharmaceutical manufacturing, rely on rotary screw compressors for both process air and to power machinery and conveyors. Air for pharmaceutical and process industries may need to meet very strict standards for purity. Screw compressors can be used to power applications such as:

Operating pumps that move liquid products through stages of production.
Aeration and mixing of liquids and powders or liquid processing/agitation.
Product drying.
Maintaining air curtains to prevent mixing of gases and vapors.
Recharging desiccant dryers, nitrogen generators, or carbon filters.
Membrane filtration.
Equipment cleaning.
Packaging lines.

Food and Beverage

Food and beverage manufacturers may use screw compressors in a variety of ways, from product blow-off and cleaning to packaging lines. A rotary screw compressor provides consistent and reliable air for food and beverage applications, including:

Air-jet food processing (e.g., cutting, peeling, and cleaning of produce).
Food drying or dehydration.
Cooling of baked goods.
Carton forming, filling, and vacuum sealing for packaging.
Pneumatic conveying (e.g., bottling lines).

Agriculture

While small family farms may be more likely to use a reciprocating or piston compressor, large-scale commercial farming can benefit from the high efficiency and high CFM of a rotary screw compressor. Applications for screw compressors in farming include:

Powering pneumatic conveyors to transport grain and other harvested products from the field to storage silos.
Spraying pesticides, herbicides, and fertilizer.
Operating milking robots and other dairy machinery.
Aerating tanks for wastewater treatment.
Clearing irrigation lines.
Ventilating greenhouses.

Oil & Gas

Rotary screw compressors are widely used throughout the energy sector for a variety of purposes. In the oil & gas industry, screw compressors are used to power drilling equipment and pneumatic systems for transporting drilled oil. Compressed air may also be used to power equipment for compressing natural gas and refining petroleum. Other uses in the energy sector include:

Gas re-injection for enhanced oil recovery.
Boosting gas flow through high-pressure pipelines.
Petrochemical synthesis.

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. 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 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.

Installation of Screw Air Compressor

Proper compressor installation is very important for a screw compressor. There are several factors to consider for installation of a rotary screw compressor.

Space: The amount of space available for installation, including the location of the compressor, air receiver tank, and any other related equipment, should be taken into account. In particular, the location and surrounding space need to consider heat dissipation for the air compressor. If the compressor is located inside, make sure the compressor room is well-ventilated and there is plenty of airflow to carry excess heat away to prevent the air compressor overheating. If the compressor is located outside, proper weatherization may be needed to keep the compressor running properly in cold weather.
Power supply: The compressor must have access to a suitable power supply to operate correctly, including the correct voltage, amperage, and phase.
Distribution system: A well-designed compressed air distribution system is essential to ensure efficient and effective delivery of compressed air. The size and length of the pipes, as well as the materials used, should be selected based on the specific requirements of the application.
Air quality: The quality of the incoming air, including the presence of contaminants, should be assessed to determine if intake filtration or other air treatment is required.
Noise: Industrial air compressors can generate a significant amount of noise, so the location and design of the installation should take into account any potential noise pollution issues and reduce compressor noise where possible.
Safety: The safety of the compressor and its components, including the air receiver tank, should be a top priority during installation. This may include ensuring that the compressor is located in an area where it will not pose a hazard, installing appropriate safety features like pressure relief valves, and ensuring that the compressor meets all relevant safety regulations.

Find the Rotary Screw Air Compressors You Need with Fluid-Aire Dynamics

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

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 ten 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.