The working principle of centrifugal compression
All air compressors exploit fluid dynamics, either through Boyle’s Law or Bernoulli’s principle. Compressors that use Bernoulli’s principle are categorized dynamic (centrifugal ) or positive displacement compressors. Bernoulli’s principle states that in a moving fluid, points of higher fluid speed will have less pressure than points of lower fluid speed. It is a seemingly counterintuitive characteristic of fluids, but we’ve come to rely on Bernoulli’s principle in aeronautics to design airplane wings. The air stream moves faster over the top of the wing, and at normal speed below the wing. Oil free centrifugal compressors use an impeller disk as the rotating element and air is drawn into the eye of the impeller disk from an inlet that’s set at a 90-degree angle perpendicular to it. The disk is divided like a pie by curved impeller vanes. Along the edge of the impeller disk is a diffuser vane and a volute casing.
As air enters the impeller eye it is re-directed radially outward by the impeller vanes and the inertia of rotation. The fast-moving low-pressure air is pushed to the edge of the disc where it finds its way through the diffuser and into the volute casing. The diffuser and the increasing diameter of the volute casing cause the air to slow down and increase in pressure that’s why we call centrifugal compressors constant pressure variable flow . This is how centrifugal compressors use Bernoulli’s principle. Oil free centrifugal Air compressors can be single-stage or multi-stage depending on what best fits your application. Single-stage centrifugal compressors will get you a 3:1 pressure ratio against atmospheric air using a single impeller and can be directly driven by Permanent Magnet Motors (high speed turbo) .
While on the other hand, Multi-stage compressors have multiple impellers which offer gradual increase in pressure over several stages. Multistage centrifugal compressors use Bull Gear with High Speed Pinions on which impellers mount to rotate at very high speeds supported by special hydrodynamic bearings, thereby imparting kinetic energy (velocity) to air. Here intermediate cooling is necessary to avoid temperature increase through inter coolers and after coolers, horizontally balanced design of multi stages had been proven to be the most reliable design for centrifugal compressors with easy access for repair and maintenance .
Half the total pressure rise is achieved by drawing static air into the impeller vanes, and the remaining half is achieved through the diffuser and volute casing. This is 1-stage compression. If the application requires more pressure, the compressed air can be put through more stages:
- 1-stage: up to 2.5 bar
- 2-stage: 2.5 bar to 5.5 bar
- 3-stage: 6 bar to 13 bar
Depending on the operating pressure either multi stage or single stage centrifugal is used mainly for large flow capacities beyond 4000 m3/h .
Centrifugal compressors performance is sensitive to ambient conditions as the air density affected by temperature , humidity and inlet pressure ,for example in winter the centrifugal compressor with lower inlet temperature will be delivering more flow but consuming more power, while in summer will be delivering less flow and consuming relatively lesser power.
Selecting and sizing the right centrifugal compressor configuration is key in order to maintain the machine in the right operating margins without reaching to the machine mechanical limits or consuming more power than and causing larger running costs .
Identifying the minimum and maximum operating flows at the rated pressure for centrifugal is essential to avoid reaching to surge condition where the machine reach to mechanical stress and high vibration that can damage the machine balancing accordingly lot protection wither in the controls and mechanical components are normally taken in place in terms of flow control via blow off valves, inlet guide vanes to control the inlet flow to the compressors.
The benefits that centrifugal compressors offer
Here are the main benefits that centrifugal compressors offer:
- Centrifugal compressors provide optimum combination of high flow with low energy consumption lowers specific energy requirements for large flows and Class 0 Oil Free air quality as per standard ISO 8573-1.
- The multi-stage compression method keeps air relatively cooler than other methods with higher pressure ratio per stage. Lower air temperature is efficient to compress and maximum machine reliability is ensured.
- Longer service intervals due to fewer rubbing parts and high-grade stainless-steel coolers with epoxy coated air path for minimum corrosion.
- Faster maintenance thanks to simplified connection like water manifolds, Horizontal split gear box and integrated components - oil lubrication system on the main shaft or no external instrument air connections need.
- Superior AGMA Q13 quality gears (aircraft quality) for longer lifetime, less mechanical losses, and lower noise. Full individual component interchangeability available.
- Exclusive backward lean impeller design offer operation flexibility with turndown ratio up to 35%.
- Accurate Servo motor-controlled Inlet Guide Vanes to offer precise positioning for optimal flow control.
- Remote monitoring and control of operation parameters, receiving alerts on mobile devices to maximize uptime.
- Compact and light unit for its class with lower total cost of ownership.
Conclusion
Each compressor type has applications for which they are best suited. Centrifugal compressors are best suited for applications where a constant and relatively stable flow rate of compressed air is required.
They are often used in applications that require flat demand flow profile with large flow capacities, with ability to have the highest air quality in terms of oil content with Class Zero oil free. Centrifugal compressors are in demand in various industries such as automotive, food, air separation, glass, metal smelting, shipbuilding, pharma, textile, power, renewable energy, wastewater treatment, the chemical industry and oil and gas.
And they are valued for their energy efficiency particularly in industrial processes that require a compressor running for long duty cycles. Since these applications require long duty cycles the energy costs will be the dominant cost of operating the centrifugal compressor. And so the centrifugal compressor models require innovations that improve energy efficiency for cost savings. This is an area that Atlas Copco has decades of expertise in.
If you’d like to browse our centrifugal compressor products and speak with someone from our team, get in touch with us today.
