Two Basic Compression Principles: Displacement Compression and Dynamic Compression
Learn about the basic principles for air and gas compression and the two types of compressors you can distinguish.
You are using a browser we do not support any longer. To continue visiting our website, please choose one of the following supported browsers.
When you need a lot of horsepower, a dynamic compressor is the ideal choice. They are available in both axial and radial designs. They are frequently called turbocompressors. Those with radial design are called centrifugal compressors. A dynamic compressor works at a constant pressure, unlike, for example, a displacement compressor, which works with a constant flow. The performance of a dynamic compressor is affected by external conditions: for example, a change in the inlet temperature results in a change in the capacity.
Each stage takes up a part of the overall pressure rise of the compressor unit. In industrial machinery, the maximum pressure ratio of a centrifugal compressor stage is often not more than 3. Higher pressure ratios reduce the stage efficiency. Low pressure, single-stage applications are used, for instance, in wastewater treatment plants. Multi-stage applications allow the possibility of inter-cooling to reduce the power requirement. Multiple stages can be arranged in series on a single, low-speed shaft. This concept is often used in the oil and gas or process industry.
The pressure ratio per stage is low, but a large number of stages and/or multiple compressor sets in series are used to achieve the desired outlet pressure. For air compression applications, a high speed gearbox is integrated with the compressor stages to rotate the impellers on high speed pinions. The impeller can have either an open or closed design.
Open design is most commonly used for high speed air applications. The impeller is normally made of special stainless steel alloy or aluminium. The impeller shaft speed is very high compared to that of other types of compressor. Speeds of 15,000-100,000 rpm are common. This means that journaling on the high speed compressor shaft or pinion takes place using plain oil-film bearings instead of roller bearings. Alternatively, air film bearings or active magnetic bearings can be used for a completely oil-free machine. Two impellers are mounted on each end of the same shaft to counteract the axial loads caused by the pressure differences. Typically 2 or 3 stages with intercoolers are used for standard compressed air applications.
In a modern configuration of the centrifugal air compressor, ultra-high speed electric motors are used to drive the impellers directly. This technology creates a compact compressor without a gearbox and associated oil-lubrication system, thereby making it a completely oil-free compressor design. Each centrifugal compressor must be sealed in a suitable manner to reduce leakage along the shaft where it passes through the compressor housing. Many types of seals are used and the most advanced can be found on high-speed compressors intended for high pressures. The most common types are labyrinth seals, ring seals or controlled gap seals, (usually graphite seals) and mechanical seals.
Learn about the basic principles for air and gas compression and the two types of compressors you can distinguish.
There are a lot of things you have to consider when choosing an air compressor for your business. In this article we will explain what compressor is best suited for you, based on your application and needs.
Learn more about some other kinds of compressors: the vacuum pump, booster compressor and pressure intensifier.