A centrifugal compressor is a mechanical device that compresses gas via an impeller and diffuser. In a centrifugal compressor, the gas at low pressure enters axially in the impeller and discharges at a higher pressure radially. In this article, you will learn about the components, benefits, applications, and performance optimization of a centrifugal compressor.
As the gas enters the compressor, it experiences centrifugal forces due to its high-speed rotation of the impeller. This increases the pressure and speed of the gas flow through the impeller. The airflow loses its velocity after entering the diffuser section. As the gas velocity decreases, pressure increases. The impeller contributes to about 65% and the diffuser contributes to about 35% of the total pressure developed in the compressor.
The centrifugal compressor uses rotating impeller blades to transfer energy to the gas. Because the flow of gas is radial, the transfer of energy occurs due to a change of centrifugal forces that take place on the gas. Centrifugal compressors are quite efficient, reliable, and economical to maintain. These compressors deliver a high flow capacity of gas per unit.
Multiple stages of compression action allow for higher pressures to be obtained. Single-stage compressors are designed for simple applications and large multi-stage compressors are designed for complex high-temperature pressure and flow applications.
Major components that make up a compressor’s design and subsequently performance are the impeller, housing, diffuser, collector, and casing.
The impeller disc and associated blades, dictate the compression rate. The impeller disc is attached to the shaft of the compressor. About 15 to 20 curved blades are attach to this impeller disc. These blades act as a diffusion channel for the gas flowing for compression.
The impeller blades in the compressor increase the velocity of the gas through vanes attached to the rotating disk. Blades can be shaped in many configurations depending on the design requirements. Most of the multi-stage compressors use backward tilted blades for maximizing efficiency.
The centrifugal compressor components are encased in metallic housing. The housing provides a tight path to the gas around the impeller. This design ensures that the gas released at the impeller outlet has high kinetic energy. At this point, the kinetic energy converts into pressure before the gas exits the housing.
The housing must be sealed to ensure that gas does not leak at the shaft ends. From the many types of seals available, most centrifugal compressors use dry gas seals. However, for air or nitrogen compression, carbon ring seals are preferable. Dry gas seals achieve sealing by a stationary and rotating disk with a very small gap between them. When not moving, the springs press the movable seal onto the stationary disk. During compressor rotation, the groove pattern on the disks pushes it making the seal tight.
The impeller compresses the gas into the diffuser channel at very high speeds. The diffuser makes up the radial channel. It slows down the gas and converts dynamic pressure to static pressure. After the diffuser section, the gas goes into the collector.
The collector is the last component of the centrifugal compressor. After the last impeller, the collector directs the gas towards the outlet section. This outlet is also known as a volute. A collector can also have valves for controlling the compressor. A volute is a curved-like funnel structure that connects with the discharge port.
The simplest centrifugal compressor is a single-stage machine with an overhanging rotor but larger compressors have multiple stages and much more complex designs. In a multi-stage compressor, a single-stage consists of the inlet system and a return channel for subsequent stages, the impeller, the diffuser which may have vanes or it may be vaneless.
The outer shell of the compressor is commonly referred to as the casing. The casing of the compressor may be either horizontal or vertical. The rating of the casing and the compressor flanges should be sufficient to sustain the maximum discharge pressure.
The number of impellers in one casing is usually limited by rotodynamic considerations which limit the maximum amount of pressure generated in one casing. More pressure can be generated by the use of multiple stages. Another limitation for head is the limitation of discharge temperature which is typically around 350°F. For greater head pressure, cooling the gas between stages may become necessary.
Various Design Options
Depending on the flow requirements, several other additional configurations may be available. These include multibody tandems, compound compressors, back to back compressors, and integral gear types.
- Multibody tandems, which drives up to three compressor casings with a gearbox either between the driver and the compressor train or between two of the compressors.
- Compound compressors containing multiple compartments, with each compartment having its own suction and discharge nozzle. The impellers are on the same shaft and face the same direction.
- Back-to-back compressors containing two compartments. The impellers are on the same shaft, but the impellers in the first compartment face in the opposite direction from the impellers in the second compartment.
- Integral gear type compressors have overhung impellers at each end of multiple pinions, driven from a central bull gear.
Benefits of Centrifugal Compressors
Compared to rotary and reciprocating compressors, centrifugal compressors offer low weight. Their design is simple, making them easy to manufacture and design. Centrifugal compressors have few rubbing parts, are energy efficient, and can deliver high flow rates.
These advantages translate into high reliability with low maintenance costs. Unlike recriprocating compressors, vibration is typically not a concern and special foundations considerations are not required. They also provide oil free air, an essential consideration for medicial and food grade applications.
Despite the plethora of benefits, centrifugal compressors are usually not a good fit when the users requires very high compression rates. Additionally, when not run at the proper speed, surging, stalling, and choking may result.
Centrifugal Compressor Applications
Common applications are food and beverage industry where centrifugal compressors provide compressed air for food processing equipment. Oil refineries, natural-gas processing plants, refrigeration, air-conditioning, HVAC systems use them widely. Centrifugal compressors may be found in gas turbines, turbochargers, and superchargers in various types of engines. Generally speaking, where high flow rates and low to moderate pressure is required, centrifugal compressors are the best choice.
Optimizing Performance of Centrifugal Compressors
Centrifigual compressor performance presents as a curve that relates flow to pressure for a given machine. The relationship between flow and pressure is influenced by cooling water temperature, site altitude, inlet air temperature, and humidity.
As site altitude, cooling water temperature, inlet air temperature, and humidity increases, the compressor generates lower pressure at a given flowrate. As these variables decrease, the compressor generates higher pressure at a given flowrate.
As site altitude and cooling water temperature increases, the compressor generates lower flow at a given pressure. As site altitude and cooling water temperature decreases, the compressor gvenerates more flow at a given pressure.
The pressure-flow curve is influenced by site variables as discussed above, but also by general mechanical performance. Quality compressed air providers supply appropriate inlet filters, lubrication, and personnel to ensure long lasting and reliable air.
This article has been brought to you courtesy of Critical Rental Solutions, a high quality provider of compressed air solutions. For more information on selecting the proper compressor for your needs, contact [email protected]