Dynamic compressor types: centrifugal and axial

While both machines are dynamic, they handle gas flow differently. Axial compressors move air parallel to the shaft, while centrifugal compressors push air radially outward from the center.

Most centrifugal compressors for air applications use open design impellers. These are manufactured from stainless steel alloys or high-strength aluminum to withstand rotational speeds between 15,000 and 100,000 rpm. At these velocities, plain oil-film bearings are used on the high-speed shaft, as standard roller bearings cannot accommodate the heat.

oil-free centrifugal compressors utilize air film bearings or active magnetic bearings to avoid contact and wear. These systems remove the need for traditional lubricants in the compression chamber. Direct-drive electric motors are often used to improve mechanical efficiency by up to 5% by removing the gearbox and associated oil systems.

Effective sealing is vital for safety. Various seal types are used depending on the application:

• Labyrinth seals: Simple and reliable for standard air.
• Mechanical and ring seals: Used for high-pressure or specialized gas.
• Dry-gas seals: Non-contact seals that use a gas film to reduce friction and leakage.

The choice between these technologies is primarily driven by mass flow requirements. Centrifugal compressors are the standard for low to medium flows, typically ranging from 5,000 to 150,000 Nm3/h (approximately 2,943 to 88,287 CFM). Conversely, axial compressors are the preferred solution for very high flow applications exceeding 150,000 Nm3/h (88,287 CFM). While axial units offer a narrower stable operating range, their ability to handle massive volumes with a smaller diameter makes them ideal for aircraft propulsion and large-scale power plant intakes.

Dynamic compressors have a specific stability range. The surge limit is the point where the flow rate is too low to overcome the discharge pressure, causing a flow reversal that can lead to mechanical damage.

The choke limit, or stonewall, occurs when gas velocity reaches sonic speed, preventing any further increase in flow. Monitoring these limits through anti-surge control systems is a critical selection factor for plant safety.

Environmental parameters significantly influence performance curves. High relative humidity reduces air density, which decreases discharge pressure at surge and lowers weight flow. Altitude also impacts density, requiring the compressor to work harder to achieve the same output pressure. For multi-stage machines, cooling water temperature is vital, as colder water improves flow and power efficiency, while warmer water reduces it.

Understanding the ISO 8573-1:2010 standard is key to selecting the correct air purity for any application or industry. It defines air quality classes based on three contaminants: particles, water, and oil. See the table below for a quick overview of purity classes.

In Atlas Copco terminology, Class 0 always refers to air produced by an oil-free compressor, not to achieving lower oil content through additional filtration stages. This distinction ensures true oil-free air at the source, reducing maintenance needs and the risk of contamination in sensitive applications.

Selecting the optimal centrifugal or axial compressor requires a detailed analysis of load profiles, environmental conditions, and long-term energy efficiency targets. Atlas Copco specialists provide comprehensive technical audits and performance mapping to identify the most effective turbomachinery configuration for specific industrial processes.