Dynamic Compressors: Centrifugal and Axial Compressors

A centrifugal compressor is characterized by its radial discharge flow. Air is drawn into the centre of a rotating impeller with radial blades and is pushed out towards the perimeter of the impeller by centrifugal forces. The radial movement of the air results simultaneously in a pressure rise and a generation of kinetic energy. Before the air is led to the centre of the impeller of the next compressor stage, it passes through a diffuser and a volute where the kinetic energy is converted into pressure.

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.

An axial compressor has axial flow, whereby the air or gas passes along the compressor shaft through rows of rotating and stationary blades. In this way, the velocity of the air is gradually increased at the same time that the stationary blades convert the kinetic energy to pressure. A balancing drum is usually built into the compressor to counterbalance axial thrust. Axial compressors are generally smaller and lighter than their equivalent centrifugal compressors and normally operate at higher speeds. They are used for constant and high volume flow rates at a relatively moderate pressure, for instance, in ventilation systems. Given their high rotational speed, they are ideally coupled to gas turbines for electricity generation and aircraft propulsion.