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 durable industrial air compressors are widely used in applications that require high efficiency and continuous airflow, including conveyor systems, manufacturing equipment, process manufacturing, and more.

This comprehensive guide to rotary screw compressors will help you understand how these compressors work and determine if a screw compressor is the right choice for your application. Additionally, it covers how to install and maintain your compressor system for optimal performance.

What Is a Rotary Screw Air Compressor?

A rotary screw air compressor (sometimes termed a "twin-screw compressor") is a type of air compressor that utilizes a pair of interlocking helical screws to compress air. As the screws rotate, air is continuously compressed. Screw air compressors are commonly used for applications requiring high CFM (Cubic Feet of air per Minute) and continuous use, making them ideal for heavy industrial tasks such as powering pneumatic production lines and conveyor systems.

In contrast, a reciprocating air compressor 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 operates by forcing air through two 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 compressed into a smaller volume. This process is known as positive displacement. The compression is continuous and governed by a drive motor along with a system of inlet and discharge valves.

1. Air enters the compressor through the air intake. (An air intake filter may be necessary to prevent excess dust, debris, and dirt from entering the compressor.)
2. Air flows through the regulator valve, which sets the pressure, and then into the air end (the rotor housing).
3. The two interlocking helical screws turn rapidly, powered by the air compressor motor. In oil-flooded compressors, oil seals the rotors and housing to contain the air. In oil-free variants, special coatings prevent air leaks.
4. The rotation of the screws generates a vacuum, drawing more air into the chamber through the air intake.
5. As air moves through the chambers, it is compressed into a smaller volume.
6. Compressed air exits the air end. Typically, it passes through an aftercooler to dissipate heat and remove moisture. As the air cools, moisture condenses and is drained away. An oil/water separator removes oil from the water before disposal.
7. Air leaving the aftercooler usually undergoes drying and filtering to eliminate remaining moisture, oil mists, and particulates. Air dryers and inline filters are utilized before usage or storage.
8. Clean, dry air can be immediately used for industrial processes or stored in an air receiver tank for later use.

Types of Rotary Screw Air Compressors

There are two main types of rotary screw air compressors: oil-injected and oil-free. Oil-injected screw compressors are the most common industrial type and suitable for most applications. Notably, there are several subtypes within these categories.

• Tank-mounted vs. Base-mounted: Tank-mounted rotary screw compressors include an integrated air receiver tank for storing compressed air and may also feature an integrated air dryer. Base-mounted rotary screw compressors are stand-alone units without a tank or dryer, allowing for greater design flexibility in compressor systems.
• Fixed-speed vs. Variable-speed: Screw compressors can be fixed-speed or variable-speed. A fixed-speed compressor operates at 100% power continuously, which suits businesses needing constant airflow, but can be inefficient at reduced loads. A variable speed drive (VSD) compressor adjusts the motor speed to the actual air demand, thereby reducing energy consumption and costs.

Read more: Why Choose or Upgrade to a Variable Speed Drive Air Compressor?

Oil-Injected Rotary Screw Compressors

An oil-injected (or oil-flooded) rotary screw air compressor uses oil for lubricating the screw unit, ensuring smooth operation and cooling of the compressed air. After compression, oil is separated using an oil separator. Some oil may remain in the compressed air (oil carryover), which can further be minimized with inline filtration.

Oil-Free Rotary Screw Compressors

Oil-free (or oil-less) rotary screw air compressors employ a two-stage compression process. Air is cooled between stages using an intercooler to prevent overheating. These compressors eliminate oil carryover, producing very clean air. However, they tend to be pricier, more complex to maintain, and noisier than oil-injected models. Oil-free compressors are predominantly used in applications requiring high-purity air, yet adequate purity levels can often be achieved with dryers and filtration systems.

Read more: Do You Need an Oil-Free Compressor? Probably Not.

Rotary Screw Air Compressor Components

Here are the essential components of a rotary screw air compressor.

Air-End (Pump)

The air-end is the core of a rotary screw air compressor where air compression occurs. It includes several components:

• Air Inlet Valve: Where atmospheric air enters for compression.
• Rotors/Screws/Rollers: A pair of interlocking screws that rotate rapidly to compress air.
• Compression Cylinder: The housing for the rotors, which may be single or multi-stage. Made typically of cast iron or steel, oil-free models have intercoolers between stages.
• Discharge Valve: Where compressed air exits and may go to an air receiver tank or directly to the application.
• Bearings: Keep the rotors positioned and facilitate smooth rotation. Bearings are generally anti-friction and corrosion-resistant.
• Gaskets & Seals: Essential for an airtight system to maintain pressure and prevent air leakage.

Motor and Control System

A rotary screw air compressor also comprises several other components:

• Air Compressor Motor: Powers the rotor rotation, which can be either fixed-speed or variable-speed.
• Air Compressor Control System: Manages the motor, monitors operations, and provides control methods for operators.
• Pressure Regulator: Controls air pressure (PSI) from the compressor to maintain safe, efficient operations.

Read more: How to Adjust the Air Compressor Regulator.

Air Compressor Filters

Filters are essential for keeping contaminants out of the air supply:

• Intake filters remove contaminants from air entering the air-end.
• Inline filters eliminate particulates and oil from compressed air.
• Oil filters cleanse liquid oil in oil-lubricated compressors.

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

Air Compressor Cooling System

Due to the heat generated during compression, rotary screw air compressors require a cooling system to prevent overheating. An aftercooler serves as a heat exchanger, dissipating heat from the compressed air and removing moisture. Basic types include air-cooled and water-cooled aftercoolers.

Air-cooled models utilize a series of coils cooled by fans for heat dissipation, while water-cooled models use water to carry away excess heat. Options include:

• Closed-loop dry cooling, circulating coolant through coils.
• Open-loop evaporative cooling, using freshwater and air for cooling.

Oil-Injection System for Rotary Screw Air Compressor

An oil-injected rotary screw air compressor requires additional components for its oil injection system:

• Oil Sump: The reservoir where oil is stored for the compression system.
• Oil Filter: Cleans particulates from circulating oil.
• Oil/Water Separator: Separates liquid water from the oil generated during compression.
• Oil Cooler: Cools warm lubricant before it circulates back into the system.

Rotary Screw Air Compressor Accessories

A complete compressed air system includes various components beyond the compressor itself.

Compressed Air Dryers

Hot, compressed air contains moisture that should ideally be removed for clean, dry air in industrial processes. There are two main types of compressed air dryers:

• Refrigerated air dryers: They cool compressed air to condense excess moisture into liquid, suitable for most applications by lowering dew points to 38°F.
• Desiccant air dryers: They use adsorptive materials to remove water vapor, achieving lower dew points but at a higher cost. They are often used for ultra-dry air requirements.

Read more: Refrigerated vs. Desiccant Air Dryers – How to Choose.

Air Receiver Tank

The air receiver tank stores compressed air after it exits the compressor. Its position can be before or after the air drying system.

Read more: Air Receiver Tank Full Guidelines.

Drain Valves

Drain valves remove excess moisture from the compressed air system, which can accumulate due to cooling of hot air. These valves are crucial for managing liquid buildup in various system components.

Things to Consider When Choosing a Rotary Screw Air Compressor

Before selecting a rotary screw air compressor, ensure it’s suitable for your application. Here are five critical questions to evaluate:

1. How Large Is Your Demand?

Rotary screw air compressors are exceptionally durable and can handle substantial demands over prolonged periods. They are particularly efficient for larger demands (15 HP and more) and can run at 100% duty cycle without issue.

2. Is Lifetime Cost Important to You?

While the initial investment for a rotary screw compressor may be higher, its lifespan greatly exceeds that of piston-type compressors, which often require rebuilding after 10,000 - 20,000 hours. Rotary screw compressors can last anywhere from 80,000 - 100,000 hours.

3. Does Your Machine Operate Near Your Production Staff?

Rotary screw compressors are recognized for their quiet operation, functioning between 65 - 75 dBA. In contrast, reciprocating compressors can reach 80 - 85 dBA.

4. How Clean Does Your Compressed Air Need to Be?

Rotary screw compressors limit oil carryover to a maximum of 3 PPM, significantly reducing maintenance needs compared to piston compressors, which may reach 10 PPM or more.

5. Do You Use Your Compressed Air Continuously or Does Demand Vary?

Rotary compressors are favored for their efficiency and ability to sustain continuous high demand. For variable demands, a variable-speed drive option may be preferable as it adapts to air production needs.

Advantages and Disadvantages of Rotary Screw Air Compressors

Rotary screw compressors are prevalent in industrial applications due to their reliability, energy efficiency, and consistent performance.

Advantages of Rotary Screw Air Compressors

• Higher CFM per Horsepower: They deliver a superior volume of compressed air (4-5 CFM per HP) compared to reciprocal compressors (3-4 CFM per HP).
• Continuous Use and Consistent Airflow: These compressors function best at a 100% duty cycle, making them suitable for 24/7 operations.
• Lower Total Cost of Ownership: While initial costs are higher, operational costs tend to be lower due to less energy consumption and longer lifespan.
• Cleaner Air: They effectively minimize oil carryover, allowing for very clean air outputs.
• Quiet Operation: Their design enables quieter performance compared to other compressor types.
• Cooler Temperatures: Operating temperatures can be significantly lower, leading to less work for cooling systems.
• Space Savings: Their compact design takes up less floor space in facilities.

Disadvantages of Rotary Screw Air Compressors

• Higher Initial Capital Investment: The upfront cost is typically greater than that of piston-type compressors.
• Requires Skilled Maintenance: Maintenance complexity necessitates skilled staff.
• Requires a Clean Operating Environment: They operate best in environments with low particulate contamination.
• Not Ideal for Intermittent Applications: Designed for continuous use; not suitable for applications requiring frequent cycling on and off.

Industries that Use Rotary Screw Compressors

Rotary screw air compressors are utilized across various industries that require high CFM or continuous airflow, including manufacturing, food processing, pharmaceuticals, and more. These compressors reliably serve applications from equipment power to pneumatic conveying.

Read more: Top 10 Industrial Compressed Air Applications.

Manufacturing

Manufacturing facilities rely on screw compressors for production equipment, conveyors, and packaging lines, providing consistent airflow necessary for robotic applications.

• Power cleaning.
• Surface preparation.
• Paint lines/spray booths.
• Plastic forming.
• Cleaning and deburring.

Process Industries

Screw compressors serve the process industries by supplying air for machinery and meeting stringent purity standards in pharmaceutical manufacturing.

• Powering pumps for liquid products.
• Liquid and powder mixing.
• Product drying.
• Air curtains for gas separation.
• Equipment cleaning.

Food and Beverage

These compressors are used for both operational needs and packaging in food processing.

• Food processing.
• Food drying.
• Cooling baked goods.
• Pneumatic conveying.

Agriculture

Large-scale commercial farming leverages the efficiency of rotary screw compressors for various applications.

• Powering pneumatic conveyors.
• Agricultural spraying.
• Operating farm machinery.
• Wastewater aeration.

Oil & Gas

In the energy sector, rotary screw compressors are pivotal for powering drilling and pneumatic systems.

• Gas re-injection.
• Pumping enhancement.
• Petrochemical production.

Maintenance Requirements for Rotary Screw Air Compressors

Regular inspections and maintenance are critical to ensure optimal performance. Follow guidelines from your compressor’s manual.

• Drain excess water regularly.
• Check and change oil and filters.
• Inspect belts and lubrication systems.
• Clean and maintain coolers and heat exchangers.

Read more: Maintaining Your Air Compressor.

How Often to Change Oil?

Oil should be checked weekly and changed every 4,000 - 8,000 hours, with annual complete drains and flushes recommended.

How Often to Change Filters?

Air intake and inline filters should be changed every 2,000 hours, or more frequently in dirty environments.

How Often to Drain Water?

Water should be drained daily or more often, utilizing automated drain valves for efficiency.

Installation of Screw Air Compressor

Proper installation is crucial for a rotary screw compressor. Key considerations include:

• Space: Ensure well-ventilated locations for heat dissipation.
• Power supply: Validate proper voltage and amperage are available.
• Distribution system: Design an efficient compressed air delivery system.
• Air quality: Assess incoming air quality for necessary filtration.
• Noise: Strategically place compressors to minimize noise pollution.
• Safety: Prioritize safety measures during installation.

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

The experts at Fluid-Aire Dynamics can assist you in selecting the ideal rotary screw air compressor for your application based on your airflow needs and budget. Contact us for assistance in choosing the best compressor for your requirements.

In conclusion, rotary screw compressors have gained popularity due to their efficiency, reduced noise, and significantly lowered costs over the past decade. With their durability and low oil carryover, they represent a valuable investment for any manufacturing operation.