Centrifugal Compressor Technology!
I. Main Structure of Centrifugal Compressors
1. Impeller (Bladed Wheel)
The impeller is the component that imparts kinetic energy to air. It was traditionally manufactured by precision casting. After the development of high-speed 5-axis machining centers, cutting machining has been widely adopted. This manufacturing method requires fewer working hours and delivers lower costs. Impellers are generally made of corrosion-resistant titanium alloy or stainless steel.
In principle, more compression stages bring performance closer to isothermal compression and achieve higher efficiency. Taking pressure loss of intercoolers and leakage from shaft seals into consideration, the efficiency difference between two-stage and three-stage compression is approximately 3.5%, while the efficiency gap between three-stage and four-stage compression is around 1%.
Thanks to advances in computational fluid dynamics (CFD), relevant analysis software is widely accessible, eroding the technical and data accumulation advantages of established manufacturers. The standalone efficiency of impellers has reached 95%, leaving limited room for further improvement.
For dry screw compressors, contact between screws leads to scorching and damage, which imposes extremely strict requirements on precision control. In contrast, slight contact between the impeller and volute of a centrifugal compressor seldom causes scorching or catastrophic failure. During operation, deformation of the impeller caused by centrifugal force or volute deformation eliminates the clearance at the impeller outlet. If the unit is disassembled after shutdown, minor contact marks can be observed in most cases, reflecting its inherently safe structure.
2. Diffuser
The diffuser is a device that decelerates the air accelerated by the impeller and converts kinetic energy into pressure. For bladeless diffusers, air mostly flows along the circumferential direction, lengthening the flow path to the downstream volute. Normally, fixed blades are installed to redirect airflow toward the radial direction, shortening the flow path and reducing frictional losses. Three types of flow passages are available: channel type, bladed type and bladeless type.
II. Characteristics of Centrifugal Compressors
1. Surge
Centrifugal compressors exhibit surge, a phenomenon not found in positive displacement compressors.
When the flow rate drops below a critical threshold, stall occurs in the diffuser and impeller, and downstream air begins to flow backward toward the upstream side. At this critical flow rate, all compressed air on the discharge side flows back at once. Rapid expansion of high-pressure air on the upstream side generates a loud popping noise. The backflow causes a sharp drop in discharge pressure, followed by pressure recovery and repeated backflow. This oscillating phenomenon is known as turbo surge.
Surge originates from stall in the diffuser or impeller, so measures must be adopted to prevent stall. Kinetic energy imparted by the impeller is continuously converted into pressure, and stall is triggered by excessive deceleration. As flow decreases, the diffuser — a primary decelerator — usually stalls first, though certain designs may experience impeller stall in advance.
The impeller functions as an air accelerator and is generally not expected to stall. Nevertheless, deceleration conditions may occur when analyzing the relative velocity between the impeller and airflow. Stall can be minimized by reducing the deceleration ratio of the diffuser or impeller. However, this increases radial dimensions, and frictional losses in the flow passage rise proportionally with length. Optimizing the inlet volute to realize efficient deceleration is the core technology for improving centrifugal compressor performance.
Bladeless diffusers feature longer flow passages and higher losses, yet their lower deceleration ratio makes them less susceptible to stall.
Under identical operating conditions — constant rotational speed, reduced flow rate and increased pressure ratio — the discharge temperature of dry screw compressors rises, resulting in screw scorching and damage. Hence, dry screw compressors are equipped with safety devices that trigger shutdown upon excessive discharge temperature. Centrifugal compressors are also fitted with anti-surge control systems. Even if control failure induces surge, permanent damage is rare. The unit will shut down due to excessive vibration or rising inlet temperature caused by reverse airflow.
2. Flow Regulation Range
A unique feature of centrifugal compressors is that throttling the inlet valve not only reduces intake flow, but also shifts the operating point and lowers inlet pressure to suppress excessive pressure ratio growth. The key to expanding the regulation range lies in avoiding the aforementioned surge.
To pursue maximum efficiency, designers can enhance the deceleration capacity of impellers and diffusers and reduce passage losses. However, stall will emerge rapidly as flow decreases. Fundamentally, maximizing efficiency and maximizing flow regulation range are contradictory objectives. Commercial products on the market require appropriate trade-offs.
Mature technologies for broadening flow regulation range have been applied in refrigeration compressors and automotive turbochargers, including diffuser bypass and diffuser throttling technologies to prevent diffuser stall, as well as solutions for avoiding stall at the impeller inlet.
These technologies extend the adjustable flow range of centrifugal compressors, enabling them to compete effectively with variable-frequency screw compressors as capacity-modulating equipment.
III. Advantages and Disadvantages of Centrifugal Compressors
Advantages
No wearing components, delivering the lowest maintenance cost among all compressor types.
Compared with screw compressors, clearance leakage is lower, resulting in higher single-stage efficiency.
Compared with dry screw compressors, fewer parts and shorter machining time are required; lower machining precision requirements cut production costs.
Deformation caused by thermal expansion or stress during startup and shutdown changes internal clearances. The two rotors of dry screw compressors operate with surface-to-surface contact. Minor contact triggers surface friction, thermal expansion, scorching and seizure. In contrast, the impeller blades and volute of centrifugal compressors form line contact. Even if contact occurs, the blade slides along the housing like a cutting edge without scorching. Consequently, centrifugal compressors suffer far fewer catastrophic failures than dry screw compressors.
Disadvantages
Contamination on the air flow path surfaces (impeller and diffuser) leads to performance degradation.
Performance may drop by approximately 10%, while power consumption also decreases. Air flows at hundreds of meters per second, so surface roughness greatly affects efficiency. Contamination in the diffuser — where flow velocity peaks — imposes the most severe impact. Besides contamination, corrosion also roughens surfaces and deteriorates performance.
Solutions include improving the filtration accuracy of air filters and scheduling regular internal cleaning of impellers and diffusers. Centrifugal compressors generally adopt higher-performance air filters with large filter areas to extend element service life. Some users install filters with specifications exceeding those for gas turbines (turbomachinery sharing the same drawback) to prevent performance loss. Such filters adopt automatic winding filter cartridges for easy maintenance.
Ship turbochargers have long adopted periodic water injection cleaning for impellers and diffusers, yet this method is rarely applied to land-based turbomachinery and receives little attention from end users.
High initial investment.
Screw and reciprocating compressor manufacturers rely on maintenance fees to generate profits, allowing them to set lower sales prices. Centrifugal compressors require minimal maintenance work. Even with low production costs, manufacturers have to maintain relatively high selling prices.
1. Impeller (Bladed Wheel)
The impeller is the component that imparts kinetic energy to air. It was traditionally manufactured by precision casting. After the development of high-speed 5-axis machining centers, cutting machining has been widely adopted. This manufacturing method requires fewer working hours and delivers lower costs. Impellers are generally made of corrosion-resistant titanium alloy or stainless steel.
In principle, more compression stages bring performance closer to isothermal compression and achieve higher efficiency. Taking pressure loss of intercoolers and leakage from shaft seals into consideration, the efficiency difference between two-stage and three-stage compression is approximately 3.5%, while the efficiency gap between three-stage and four-stage compression is around 1%.
Thanks to advances in computational fluid dynamics (CFD), relevant analysis software is widely accessible, eroding the technical and data accumulation advantages of established manufacturers. The standalone efficiency of impellers has reached 95%, leaving limited room for further improvement.
For dry screw compressors, contact between screws leads to scorching and damage, which imposes extremely strict requirements on precision control. In contrast, slight contact between the impeller and volute of a centrifugal compressor seldom causes scorching or catastrophic failure. During operation, deformation of the impeller caused by centrifugal force or volute deformation eliminates the clearance at the impeller outlet. If the unit is disassembled after shutdown, minor contact marks can be observed in most cases, reflecting its inherently safe structure.
2. Diffuser
The diffuser is a device that decelerates the air accelerated by the impeller and converts kinetic energy into pressure. For bladeless diffusers, air mostly flows along the circumferential direction, lengthening the flow path to the downstream volute. Normally, fixed blades are installed to redirect airflow toward the radial direction, shortening the flow path and reducing frictional losses. Three types of flow passages are available: channel type, bladed type and bladeless type.
II. Characteristics of Centrifugal Compressors
1. Surge
Centrifugal compressors exhibit surge, a phenomenon not found in positive displacement compressors.
When the flow rate drops below a critical threshold, stall occurs in the diffuser and impeller, and downstream air begins to flow backward toward the upstream side. At this critical flow rate, all compressed air on the discharge side flows back at once. Rapid expansion of high-pressure air on the upstream side generates a loud popping noise. The backflow causes a sharp drop in discharge pressure, followed by pressure recovery and repeated backflow. This oscillating phenomenon is known as turbo surge.
Surge originates from stall in the diffuser or impeller, so measures must be adopted to prevent stall. Kinetic energy imparted by the impeller is continuously converted into pressure, and stall is triggered by excessive deceleration. As flow decreases, the diffuser — a primary decelerator — usually stalls first, though certain designs may experience impeller stall in advance.
The impeller functions as an air accelerator and is generally not expected to stall. Nevertheless, deceleration conditions may occur when analyzing the relative velocity between the impeller and airflow. Stall can be minimized by reducing the deceleration ratio of the diffuser or impeller. However, this increases radial dimensions, and frictional losses in the flow passage rise proportionally with length. Optimizing the inlet volute to realize efficient deceleration is the core technology for improving centrifugal compressor performance.
Bladeless diffusers feature longer flow passages and higher losses, yet their lower deceleration ratio makes them less susceptible to stall.
Under identical operating conditions — constant rotational speed, reduced flow rate and increased pressure ratio — the discharge temperature of dry screw compressors rises, resulting in screw scorching and damage. Hence, dry screw compressors are equipped with safety devices that trigger shutdown upon excessive discharge temperature. Centrifugal compressors are also fitted with anti-surge control systems. Even if control failure induces surge, permanent damage is rare. The unit will shut down due to excessive vibration or rising inlet temperature caused by reverse airflow.
2. Flow Regulation Range
A unique feature of centrifugal compressors is that throttling the inlet valve not only reduces intake flow, but also shifts the operating point and lowers inlet pressure to suppress excessive pressure ratio growth. The key to expanding the regulation range lies in avoiding the aforementioned surge.
To pursue maximum efficiency, designers can enhance the deceleration capacity of impellers and diffusers and reduce passage losses. However, stall will emerge rapidly as flow decreases. Fundamentally, maximizing efficiency and maximizing flow regulation range are contradictory objectives. Commercial products on the market require appropriate trade-offs.
Mature technologies for broadening flow regulation range have been applied in refrigeration compressors and automotive turbochargers, including diffuser bypass and diffuser throttling technologies to prevent diffuser stall, as well as solutions for avoiding stall at the impeller inlet.
These technologies extend the adjustable flow range of centrifugal compressors, enabling them to compete effectively with variable-frequency screw compressors as capacity-modulating equipment.
III. Advantages and Disadvantages of Centrifugal Compressors
Advantages
No wearing components, delivering the lowest maintenance cost among all compressor types.
Compared with screw compressors, clearance leakage is lower, resulting in higher single-stage efficiency.
Compared with dry screw compressors, fewer parts and shorter machining time are required; lower machining precision requirements cut production costs.
Deformation caused by thermal expansion or stress during startup and shutdown changes internal clearances. The two rotors of dry screw compressors operate with surface-to-surface contact. Minor contact triggers surface friction, thermal expansion, scorching and seizure. In contrast, the impeller blades and volute of centrifugal compressors form line contact. Even if contact occurs, the blade slides along the housing like a cutting edge without scorching. Consequently, centrifugal compressors suffer far fewer catastrophic failures than dry screw compressors.
Disadvantages
Contamination on the air flow path surfaces (impeller and diffuser) leads to performance degradation.
Performance may drop by approximately 10%, while power consumption also decreases. Air flows at hundreds of meters per second, so surface roughness greatly affects efficiency. Contamination in the diffuser — where flow velocity peaks — imposes the most severe impact. Besides contamination, corrosion also roughens surfaces and deteriorates performance.
Solutions include improving the filtration accuracy of air filters and scheduling regular internal cleaning of impellers and diffusers. Centrifugal compressors generally adopt higher-performance air filters with large filter areas to extend element service life. Some users install filters with specifications exceeding those for gas turbines (turbomachinery sharing the same drawback) to prevent performance loss. Such filters adopt automatic winding filter cartridges for easy maintenance.
Ship turbochargers have long adopted periodic water injection cleaning for impellers and diffusers, yet this method is rarely applied to land-based turbomachinery and receives little attention from end users.
High initial investment.
Screw and reciprocating compressor manufacturers rely on maintenance fees to generate profits, allowing them to set lower sales prices. Centrifugal compressors require minimal maintenance work. Even with low production costs, manufacturers have to maintain relatively high selling prices.
