Rotary Screw Compressor FAQ Guide: Everything You Need to Know

Rotary screw compressors are a critical asset in gas operations, where reliability and efficiency can't be overlooked. This FAQ guide delivers in-depth technical insights into rotary screw compressors, equipping industry professionals with the knowledge necessary to optimize performance and make informed decisions.  

What Is a Rotary Screw Compressor? 

A rotary screw compressor is a type of gas compressor that uses a rotary-type positive displacement mechanism to compress gas. Unlike traditional air compressors, rotary screw compressors are often used in industrial applications, including natural gas compression, vapour recovery, and biogas handling. These compressors work by trapping gas between two helical screws (rotors) and compressing it as the rotors rotate. The compressed gas is then discharged at a higher pressure, making it ideal for industrial processes requiring consistent and reliable compression. 

How Does a Rotary Screw Compressor Work? 

Rotary screw compressors operate by utilizing two meshing helical screws, also known as rotors, which rotate in opposite directions. As the rotors turn, gas is trapped in the space between them and the casing. The continuous rotation reduces the volume of the trapped gas, increasing its pressure. This highly efficient process allows the compressor to handle large volumes of gas at varying pressures. The lack of valves and other components that wear out quickly ensures that rotary screw compressors can operate continuously with minimal maintenance. 

What Are the Key Benefits of Rotary Screw Compression? 

Rotary screw compressors offer several key benefits, including: 

1. High Efficiency: They are designed to operate efficiently even under continuous use, providing consistent compression with low energy consumption. 

2. Durability: Built to withstand demanding industrial environments, rotary screw compressors have a long operational life. 

3. Flexibility: Many screw compressor models offer internal capacity control, allowing operation at as low as 10% capacity using a slide valve. This flexibility is valuable for applications with variable demand. 

4. Smooth Operation: The rotary screw design ensures minimal vibration and noise, leading to smoother operation and reduced wear on components. 

5. Low Maintenance: Screw compressors require minimal maintenance due to the closed-loop oil circulation system, which typically operates without a separate pump. This reduces the need for frequent servicing. 

6. No Valves: They often feature fixed suction and discharge ports, eliminating the need for valves, which can wear out or require regular maintenance. 

7. Balanced Rotation: With no reciprocating motion and only high-speed balanced rotation, screw compressors experience less mechanical stress and have a smoother operation. 

8. Cost-Effective: Rotary compressors can achieve high rotational speeds, sometimes up to 10,000 rpm with a gearbox, allowing them to displace more gas than slower alternatives with equivalent swept volumes. This efficiency translates to less equipment needed to handle large volumes of gas. 

9. Efficient Cooling: The mixing of gas and oil within the compressor frame absorbs heat from the gas, enabling more compression ratios in a single stage without the need for an intercooler, additional cylinders, or extensive piping. 

What Are the Disadvantages of Rotary Screw Compressors?  

As is the case with all equipment, drawbacks inherent to a design must also be assessed. Here's what we think rotary screw compressors fall behind on: 

• Limited Discharge Pressures: Screw compressors are generally limited to discharge pressures of around 375 psig (2,585 kPag), depending on the manufacturer, which can restrict their use in applications requiring higher pressures. 

• Inefficiency in High Ratio Applications: As compression ratios exceed 5, screw compressors become increasingly less efficient, consuming more power for compression. While the initial capital cost may be lower than a multi-stage reciprocating compressor, the ongoing power costs can be significantly higher. 

What Are the Applications of Rotary Compressors in the Natural Gas Industry? 

Rotary screw compressors are widely used in the gas industry for applications such as: 

• Natural Gas Compression: These compressors are used to compress natural gas for transportation through pipelines. 

• Vapour Recovery: Rotary screw compressors are employed in vapour recovery units (VRUs) to capture and compress gas vapours for reuse, reducing emissions and improving efficiency. 

• Biogas Processing: In biogas plants, rotary screw compressors are used to compress biogas for storage or further processing. 

• Field Gathering: They are essential in gathering systems where gas from multiple wells is collected, compressed, and sent to processing facilities. 

What's the Difference Between Oil-Injected and Oil-Free Rotary Screw Compressors? 

The primary difference between oil-injected and oil-free rotary screw compressors lies in how they handle lubrication and cooling: 

• Oil-Injected Compressors: These compressors use oil to lubricate and cool the rotors, which helps seal and cool the compressed gas. They are typically used in applications where oil contamination is not a concern. 

• Oil-Free Compressors: On the other hand, oil-free compressors do not use oil in the compression chamber. They rely on other means for cooling and are ideal for applications where the purity of the compressed gas is critical, such as in food processing or pharmaceutical industries. 

What Are Variable Speed Rotary Screw Compressors? 

Variable speed rotary screw compressors are designed to adjust their motor speed based on the air or gas demand. Unlike fixed-speed compressors, variable speed compressors can slow down or speed up as needed, resulting in significant energy savings. These compressors are particularly beneficial in applications where the demand for compressed gas fluctuates, as they can provide the necessary compression while minimizing energy consumption. 

What Are the Common Maintenance Needs for Rotary Screw Compressors? 

Common maintenance practices include: 

• Oil Changes: Regular oil changes are necessary for oil-injected compressors to maintain lubrication and cooling efficiency. 

• Filter Replacements: Both air and oil filters should be replaced regularly to prevent contamination and ensure optimal performance. 

• Belt Inspections: Regularly inspect and adjust belts to maintain proper tension and avoid slippage. 

• Leak Checks: Regularly check for any gas leaks, as they can reduce efficiency and cause safety hazards. 

• Professional Servicing: Scheduling regular inspections and servicing by qualified technicians to address any potential issues before they lead to significant downtime. 

How Long Do Rotary Screw Compressors Typically Last? 

With proper maintenance, rotary screw compressors can have a long operational life, often lasting between 10 to 20 years or more. The longevity of the compressor depends on factors such as operating conditions, maintenance practices, and the quality of the compressor components. Investing in regular maintenance and high-quality parts can significantly extend the lifespan of the compressor. 

What Are the Advantages of Oil-Free Rotary Screw Compressors? 

Oil-free rotary screw compressors offer several advantages, particularly in applications where air or gas purity is critical: 

• No Risk of Contamination: Since no oil is used in the compression chamber, there is no risk of oil contaminating the compressed gas. 

• Lower Maintenance Costs: Oil-free compressors typically require less maintenance as there is no need for regular oil changes or oil filter replacements. 

• Compliance with Industry Standards: Oil-free compressors are often required in industries such as food and beverage, pharmaceuticals, and electronics, where strict purity standards must be met. 

What Sets Rotary Screw Compressor Technology Apart from the Rest? 

Rotary screw compressor technology improves efficiency through several design features: 

• Large Rotors: Larger rotors operate at lower speeds, reducing wear and tear while improving compression efficiency. 

• Precision Engineering: The precise meshing of the rotors ensures minimal gas leakage, maximizing compression efficiency. 

• Variable Speed Drives: Compressors with variable speed drives adjust their output to match demand, reducing energy consumption during periods of low demand. 

Rotary Screw Compressors vs Reciprocating Compressors: What's the Difference? 

Rotary screw compressors and reciprocating compressors are both widely used in natural gas applications, but they differ significantly in design and operation. Rotary screw compressors offer continuous operation, lower maintenance requirements, and smoother, quieter operation due to their lack of reciprocating motion. They are ideal for applications requiring a steady flow of compressed gas. In contrast, reciprocating compressors, which use pistons to compress gas, are better suited for high-pressure applications and offer higher efficiency at lower flow rates. However, they generally require more maintenance due to their moving parts and can be noisier during operation. 

How Do Rotary Screw Compressors Compare to Centrifugal Compressors? 

Rotary screw compressors are typically preferred over centrifugal compressors in applications where a steady, medium-pressure gas flow is required. Unlike centrifugal compressors, which rely on high-speed rotating impellers to compress gas, rotary screw compressors use meshing helical screws to compress gas more gradually.  

This makes them more efficient at lower to medium flow rates and pressures. Additionally, rotary screw compressors have fewer moving parts, leading to reduced maintenance costs and improved reliability in demanding industrial environments. While centrifugal compressors excel at very high flow rates and are often used in large-scale applications, they are less effective at handling varying loads compared to the more flexible rotary screw compressors. 

Contact Your Canadian Compression Experts for More Information Today 

Rotary screw compressors are essential for the gas industry due to their efficiency, reliability, and low maintenance requirements. Whether you want to upgrade your current system or invest in a new one, 24-7 Compression can provide expert guidance and top-quality equipment (including rentals). Contact us today to learn more about our compressor equipment and how we can support your business needs.