Introduction to centrifugal compressor structure


The commonly used steam compressors mainly include Roots compressors, axial compressors, centrifugal fans, and centrifugal compressors. Roots compressors are mainly used for low flow conditions, axial compressors are used for high flow conditions, and axial compressors are usually designed as multi-stage systems. The most commonly used steam compressors are centrifugal fans and centrifugal compressors.


Centrifugal fans can be used for low compression ratios up to Π= 1.25 working conditions. Like a centrifugal compressor, gas enters the impeller inlet axially and flows out radially under centrifugal force. The impeller and casing of the fan are welded plate structures, and reinforcement ribs are used when necessary. Usually, a gearbox is not required because the drive system can achieve the required impeller speed.


The main feature of a single-stage centrifugal compressor is the compact arrangement of the cantilever impeller, compressor, and gearbox. The motor, gearbox, and compressor are usually installed on the same base. The compressor housing adopts a cast or welded structure. Due to the high impeller line speed exceeding 400m/s, the impeller is highly stressed and therefore made of high-quality materials such as chromium nickel steel or titanium alloy.


Multi stage centrifugal compressors are used for operating conditions with high flow rate and high saturated steam temperature rise. A multi-stage centrifugal compressor is formed by arranging multiple stages on the same shaft. After leaving the first stage, the gas flows through the diffuser and interstage channel, and then enters the next stage impeller. The impeller shaft runs on the bearings inside the casing and is driven by an independent helical gear. In order to improve efficiency and avoid high temperatures that the shell cannot withstand, water can be injected into the interstage channel. In order to achieve over Π= A pressure ratio of 10 can also be used to connect single-stage machines in series. If the impeller is driven by a central drive device with several small gears, it can be called a two -, three -, or four impeller compressor.


Impeller

The impeller is a cantilever design located at the free end of the shaft. According to the design of the compressor, use semi open or closed impellers. For lower pressure rise, i.e. relative to lower blade end velocity, a closed impeller is used due to its steep characteristic curve. The impeller can be precisely milled or designed for welding. Dual phase stainless steel materials that are both corrosion-resistant and meet strength requirements are often used, as well as other CrNi alloy steels and special materials such as titanium alloy.

Volute

The accelerated gas leaves the impeller and flows into the volute and tube diffuser, reducing the flow rate and converting high kinetic energy into static pressure. The shell of centrifugal compressors is mostly cast or welded from CrNi alloy steel. The thickness of the shell and the size of the external reinforcement parts should be selected to not exceed the allowable deformation, which is particularly important for operation under vacuum conditions.

Gearbox

The helical gear of the compressor is integrated into the compressor. Therefore, there is no need for a coupling between the gear and the compressor shaft. The thrust ring is located on the high-speed small gear shaft. These thrust rings transmit the remaining axial thrust to the low-speed spindle.

Bearings and lubrication system

The bearings of the centrifugal compressor must ensure stable and vibration free operating conditions to withstand the high-speed rotation of the small gear shaft. Therefore, radial inclined oil pad bearings can be used for high-speed small gear shafts

 

Drive equipment

Various types of prime movers can be used to drive steam compressors. For each operating condition, the driving device is selected based on its efficiency and available power type. Electric motors are commonly used as driving devices, and they have significant advantages due to their standardized protection types and sizes, lower power/weight ratio, power/volume ratio, price/performance ratio, and lowest maintenance requirements.

Two types of three-phase asynchronous motors can be used: low-voltage motors and high-voltage motors. Usually, low-voltage motors can operate at 630 kW or 1250 kW power respectively when powered by 400V or 690V voltage. High voltage motors and frequency converters can be used for power up to approximately 10000 kW. Three phase asynchronous motors can operate at variable speeds using a frequency converter.

If waste steam can be recycled and reused, variable speed steam turbines can also be used as the prime mover for compressors. In this case, although the efficiency of a single-stage steam turbine is relatively low, this becomes less important due to price considerations.

 

Monitoring and security equipment

To detect abnormal situations during compressor operation, provide equipment loss warnings, and prevent mechanical damage to equipment, many monitoring and safety equipment are needed. Usually includes the following content:

-Impeller speed

-Vibration monitoring

-Oil tank level

-Oil pump

-Oil cooler

-Differential pressure of oil filter

-Oil flow rate

-Bearing temperature

-Motor coil temperature

-Motor bearing temperature

-Compressor housing temperature

-Axial displacement indicator of shaft

-Condensate discharge

-Surge protection of centrifugal compressors

Control of compressors

Evaporation devices typically heated by mechanical steam compressors can operate stably within a certain range, with parameters such as mass flow rate, pressure, temperature fluctuating over time. It is often desired that the evaporation rate can vary over a large range (i.e. partial load operation), so different heating rates must be changed. The change in processing capacity of the device is achieved by changing the temperature or pressure distribution curve. The design of compressors must consider the balance between these changes in equipment performance and design tasks. The operational performance of the equipment is described by the so-called equipment characteristic curve or performance curve, which reflects the relationship between the necessary saturated steam temperature rise and the mass flow rate of the inhaled steam. The operational performance of equipment should be largely determined through experimentation, or at least estimated. In order to optimize the operation of the steam compressor equipment, the characteristic curves of the evaporator and compressor must match.

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