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Introduction to the structure of centrifugal compressors

The main types of steam compressors commonly used are Roots-type, centrifugal, and axial‑flow. Centrifugal fans are typically employed as auxiliary air‑supply equipment and are not listed alongside compressors. Roots compressors are primarily suited for low‑flow applications, while axial‑flow compressors are designed for high‑flow conditions and are usually configured as multi‑stage systems. The most widely used steam compressors are centrifugal fans and centrifugal compressors.

The main types of steam compressors commonly used are Roots-type, centrifugal, and axial-flow. Centrifugal fans are typically employed as auxiliary air‑supply equipment and are not listed alongside compressors. Roots compressors are primarily suited for low‑flow applications, while axial‑flow compressors are designed for high‑flow conditions and are usually configured as multi‑stage systems. The most widely used steam compressors are centrifugal fans and centrifugal compressors.

Centrifugal fans can be used under operating conditions with low pressure ratios, up to a maximum of Π = 1.25. Like centrifugal compressors, the gas enters the impeller axially and exits radially under the action of centrifugal force. The fan impeller and casing are constructed from welded sheet metal, reinforced with stiffening ribs as required. A gear gearbox is typically not needed, since the drive system can achieve the necessary impeller rotational speed.

The main features of a single-stage centrifugal compressor are its compact arrangement, with the cantilevered impeller, the compressor, and the gearbox mounted together. The motor, gearbox, and compressor are typically installed on a common base. The compressor casing is either cast or welded. Due to the high impeller peripheral speed—exceeding 400 m/s—the impeller is subjected to significant stresses and is therefore manufactured from high‑quality materials such as chromium–nickel steel or titanium alloys.

Multistage centrifugal compressors are used in applications requiring high flow rates and significant temperature rises of saturated steam. These compressors are configured by arranging multiple stages along a common shaft. After exiting one stage, the gas passes through a diffuser and an interstage passage before entering the impeller of the next stage. The impeller shaft rotates on bearings within the casing and is driven by a separate helical gear. To enhance efficiency and prevent excessively high temperatures inside the casing, water may be injected into the interstage passages. For pressure ratios exceeding Π = 10, single-stage units can also be connected in series. When the impellers are driven by a central drive unit equipped with several small gears, the compressor may be classified as a two‑, three‑, or four‑impeller compressor.

 

 

Impeller

The impeller is of a cantilever design, mounted at the free end of the shaft. Depending on the compressor’s configuration, either semi‑open or closed impellers are employed. For lower pressure ratios—corresponding to lower tip speeds—closed impellers are preferred, as they exhibit a steeper performance curve. Impellers may be precision‑machined or fabricated by welding. Duplex stainless steels, which offer both corrosion resistance and the required mechanical strength, are commonly used; other Cr–Ni alloy steels and specialized materials such as titanium alloys may also be specified.

Volute

After being accelerated, the gas exits the impeller and flows into the volute and the tubular diffuser, where its velocity decreases and its high kinetic energy is converted into static pressure. The casing of a centrifugal compressor is typically cast or welded from Cr–Ni alloy steel; care must be taken in selecting the casing thickness and the dimensions of external reinforcing members to ensure that allowable deformations are not exceeded, a consideration that is especially critical when operating under vacuum conditions.

Gearbox

The compressor’s helical gears are integrated within the compressor itself. Consequently, no coupling is required between the gears and the compressor shaft. Thrust washers are mounted on the high-speed pinion shaft; they transmit any residual axial thrust to the low-speed main shaft.

Bearings and Lubrication System

The bearings of a centrifugal compressor must ensure stable, vibration-free operating conditions to withstand the high-speed rotation of the pinion shaft. Accordingly, radial tilting‑pad oil‑film bearings are employed for the high‑speed pinion shaft, while the gear’s main shaft rotates on multi‑surface hydrodynamic bearings, and thrust bearings accommodate the remaining axial thrust. 
The bearings are lubricated by pressurized oil. To this end, a standardized lubrication system has been installed, comprising an oil tank, a main oil pump, an auxiliary oil pump, an oil filter, and an oil cooler.

Drive device

A variety of prime movers can be used to drive steam recompression units. For each operating condition, the drive is selected based on its efficiency and the type of available power. Electric motors are commonly employed as drives, offering significant advantages such as standardized enclosure types and dimensions, low power-to-weight and power-to-volume ratios, favorable price‑to‑performance ratios, and minimal maintenance requirements. 
Two types of three-phase induction motors can be used: low-voltage and high-voltage motors. Typically, under supply voltages of 400 V or 690 V, low-voltage motors can operate at power ratings of 630 kW or 1,250 kW, respectively. High-voltage motors, in conjunction with variable-frequency drives, are suitable for applications up to approximately 10,000 kW. Three-phase induction motors can be operated at variable speeds when equipped with a variable-frequency drive.

If waste steam can be recovered and reused, it is also advisable to use a variable-speed steam turbine as the prime mover for the compressor. In this case, although the efficiency of a single-stage steam turbine is relatively low, this becomes a secondary consideration when price is a primary concern.

Monitoring and Security Equipment

To detect abnormal conditions during compressor operation, provide early warnings of equipment wear, and prevent mechanical damage, numerous monitoring and safety devices are required. These typically include the following: 
　　
　　- Impeller speed 
　　
　　- Vibration Monitoring 
　　
　　- Fuel tank level 
　　
　　- Oil pump 
　　
　　- Oil cooler 
　　
　　- Oil filter differential pressure 
　　
　　- Oil flow rate 
　　
　　- Bearing temperature 
　　
　　- Motor winding temperature 
　　
　　- Electric motor bearing temperature 
　　
　　-Compressor housing temperature 
　　
　　- Axial displacement indicator for the shaft 
　　
　　- Condensate drainage 
　　
　　- Surge protection for centrifugal compressors 
　　

Compressor Control

Evaporation units typically heated by mechanical vapor recompression can operate stably within a certain range, even when parameters such as mass flow rate, pressure, and temperature fluctuate over time. Since it is often desirable to vary the evaporation capacity over a wide range—i.e., to operate at partial load—it is necessary to adjust the heating rate accordingly. Changes in the unit’s processing capacity are achieved by modifying the temperature or pressure distribution profile. The compressor design must strike a balance between these operational variations and the design requirements. The unit’s operating performance is characterized by so‑called equipment characteristic curves or performance curves, which depict the relationship between the required saturation steam superheat and the inlet steam mass flow rate. In most cases, the unit’s operating performance should be determined experimentally, or at least estimated. For optimal operation of a vapor‑recompression system, the characteristic curves of the evaporator and the compressor must be matched.