Concept and Working of Centrifugal Air Compressor

Centrifugal compressors are a type of air compressors that are very useful and perform best when working with their full potential. Choke and urge happen At the high end of the curve, wet, and should be prevented. Compressor capacity is specified at the factory in actual cubic feet per minute (acfm). Centrifugal air compressors are the best choice for relatively constant applications. 

Key concepts

• Centrifugal compressor performance is most efficient when it operates at a full capacity.
• Choke and surge occur at the end of the performance curve and should be avoided.
• Compressor capacity is specified at the factory in actual cubic feet per minute (acfm).
• Centrifugal air compressors are best suited for relatively constant applications or installations that may be used for base load operations, allowing other types of compressors to be used as control mechanisms to meet peak demand. 
• Three-stage centrifugal compressors are generally more efficient than screw types and approach the efficiency level of two-stage reciprocating compressors.
• The most common design has three stages for pressures in the 100-150 psig range. 8-stage designs can reach pressures up to 1200 psig. A cooling water intercooler and interstage separator return the air temperature to approximately ambient temperature and remove condensed moisture. An aftercooler and separator cool the air from the final stage and remove excess moisture before the compressed air enters the plant air system.
• Centrifugal air compressors are dynamic machines. Air is compressed by the mechanical action of high-speed rotating propellers, imparting velocity and pressure to the air. As the air slows down in the diffuser and propeller, approximately half of the pressure energy is generated in the impeller. In contrast, the other half is obtained by converting velocity energy into pressure energy. 

An inherent characteristic of centrifugal air compressors is that efficiency increases as system pressure decreases. The slope of the flow/pressure curve depends on the design of the impeller. If the propeller blades are tilted back from the true radial position, the curve will be steeper.

Choke and surge Overview

As the pressure in the system continues to decrease, the air transfer from the compressor increases until the speed reaches the speed of sound somewhere in the compressor. At this point, the flow is choked, as further pressure reduction in the system will not lead to additional air supply.

The maximum discharge pressure of the compressor is a function of the spray line and the intersection point of the efficiency curve. As the pressure in the air system increases, the compressor delivers less air until it reaches its natural starting point. At this stage, the compressor cannot maintain a constant flow of air in the system. When this point is reached, unloading of the system takes place through the compressor until a temporary equilibrium is established between the compressor and the system.

This regression is called augmentation or augmentation. This phenomenon is almost equivalent to profile dumping. In this condition, the compressor moves from the wave point to the pressure point below it on the efficiency curve. As the compressor continues to work against the constant overpressure in the system, the performance curve moves up, and the wave repeats.

Choking or irritation is undesirable and should be avoided. Both can lead to vibration and excessive temperatures that can damage the compressor. Control systems that allow the compressor to operate without stalling or surge must be based on the prevailing environmental conditions that affect compressor performance. 

How does a centrifugal compressor work?

A centrifugal compressor converts kinetic and velocity energy into pressure energy in a diffuser. Air passes through the inlet guide vanes, which are drawn into the center of the rotating impeller by radial vanes, and then pushed outward from the center by centrifugal force. This radial movement of air increases pressure and kinetic energy. Kinetic energy is also converted into pressure as it passes through the diffuser. Several steps are required to increase the pressure sufficient for the needs of a typical industrial plant. Each stage participates in the total pressure increase of the compressor unit. Depending on the pressure required for the application, several stages can be arranged in series to achieve higher pressures.

The most common centrifugal air compressor has two to four stages to produce 100 to 150 psi and has a cooler and water separator between each stage. Centrifugal compressors are intermediate in efficiency, with typical operating costs of 16 to 20 kW/100 cc. A type of double-acting compressor that costs 15 to 16 kW/100 SCFM is a wind compressor that costs 21 to 23 kW/100 SCFM. 

Advantages of centrifugal air compressor:

• Systems up to 1500 hp are available.
• As the system size increases, the cost of HP decreases
• Provides air without lubrication
• No special mounting strap is required for installation.

Disadvantages of centrifugal air compressor:

• The initial cost is high.
• Special care is required.
• High-speed bearings (which can exceed up to 50,000 rpm) require high-speed bearings and vibration control.

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