Compressors rotational speed

Centrifugal compressors are dynamic compressors and are essentially constant pressure, variable volume machines and must be regulated in almost all applications. Before control can be discussed, the operating characteristics must be understood. The discussion to follow will be concerned with radial-flow compressors. Axial-flow compressors have similar characteristics curves, although the change in pressure rise is much greater with equivalent changes in compressor rotational speed. 

Compressor Rotation speed/ (r- min ) Design power/kW Designed flow/ (kg-h-i) Outlet pressure/ MPa Added power when converter efficiency drops by 1%/kW... 

N compressor rotating speed, rpm. n polytropic exponent, dimensionless. 

Compressors driven by steam mrbines, gas turbines, hot gas expanders, or variable speed electric motors usually employ variable rotation speed for capacity control. Generally, the surge limit slope varies witli rotation... 

While ricn common, engine-driven centrifugal compressors are used. This combination is desirable for low-ratio applications and in fuel cost situations where the high engine efficiency is attractive. The difference in rotating speeds (engine 300-600 rpm, compressor 3,000-5,000 rpm -t-) requires the use of a speed increaser. 

The most important characteristic of this type of compressors is that all have a fixed built-in pressure compression ratio for each stage of compression (as well as a fixed built-in volume displacement) [25]. Thus, at a given rate of rotational speed provided by the prime mover, there will be a predetermined volumetric flowrate through the compressor, and the pressure exiting the machine at the outlet will be equal to the design pressure ratio times the inlet pressure. 

Because of their cantilever design and pinion rotating speeds, bullgear compressors are extremely sensitive to variations in demand or down-stream pressure changes. Because of this sensitivity, their use should be limited to base load applications. 

Performance of a rotary positive-displacement compressor can be evaluated using the same criteria as a positive-displacement pump. As constant-volume machines, performance is determined by rotation speed, internal slip, and total back-pressure on the compressor. 

Generally, the compressor and expander maps plot referred or corrected mass flow rate, speed and torque. For instance, the mass flow rate and rotational speed are... 

The book Gas Turbine Performance by Walsh and Fletcher [10] has excellent treatment on turbomachineiy maps. An example of a compressor map is shown in Figure 8.10. This map fully defines the pressure-flow-efficiency-rotational speed relationship of the compressor. Employing the Beta-line or R-line method, maps can be digitized into tabular form as described by Walsh and Fletcher [10], The Betaline method is helpful to ensure numeric stability with such maps that would be otherwise problematic because of the near zero and infinite slope at the ends of the constant speed lines. Note that the compressor model can either be a flow element or a pressure element. That is, from speed and pressure it is possible to obtain flow and efficiency from the map, or from speed and flow it is possible to obtain pressure and efficiency from the map. The choice depends on what is more convenient, that is, what type of elements are modeled upstream and downstream. 

Rotational speeds vary from approximately 1,800 to 14,000 rpm and can be modulated over a wide range. Such variable speed is an advantage if the turbine is used to drive pumps and compressors or if it is to convert a variable steam flow into electricity, as is the case with thermal solar systems. When provided with the appropriate speed governor, steam turbines can provide excellent speed stability, which is desirable when the turbine serves as the prime mover in electric generators. 

Adjustable Speed Drive - An electronic device that controls the rotational speed of motor-driven equipment such as fans, pumps, and compressors. Speed control is achieved by adjusting the frequency of the voltage applied to the motor. 

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