Intro to Centrifugal Compressor
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INTRODUCTION TO CENTRIFUGAL COMPRESSORPrepared by: Muhammad Ridhwan Abdul Rasid, Mechanical Trainee Date: 26th March 2010
FUNDAMENTAL OF COMPRESSOR A compressor is a device that pressurizes a working fluid. One of the basic aims of using a compressor is to compress the fluid and deliver it at a pressure higher than its original pressure. Compression is required for a variety of purposes, some of which are listed below: y y y y To provide air for combustion To transport process fluid through pipelines To provide compressed air for driving pneumatic tools To circulate process fluid through a certain process
Of many types of compressors mostly used in the process industry, some of the more significant are shown below:
Figure 1: Principal Types of Compressors
The positive displacement compressors are used for intermittent flow in which successive volumes of fluid are confined in a closed space to increase their pressures. Dynamic compressor or continuous-flow compressors are machines that the flow is continuous. These include centrifugal compressors and axial flow compressors. These types of compressor are widely used in chemical and petroleum refinery industry for specifies services. They are also used in other industries such as the iron and steel industry, pipeline booster, and on offshore platforms for reinjection compressors. Compared to positive displacement type compressor, dynamic compressors are much smaller in size and produce much less vibration. The velocity of fluid then converted into additional pressure by gradual deceleration in the diffuser or volute, which surrounds the impeller. Meantime, axial compressor usually has higher efficiency and operates at the highest flow rate.
CENTRIFUGAL COMPRESSOR In centrifugal compressor, the gases flow enters the impeller in an axial direction and exits in a radial direction. The gas fluid is forced through the impeller by rapidly rotating impeller blades. The kinetic velocity energy from rotating impeller is converted to pressure energy, partially in the impeller and partially in the stationary diffusers. The diffuser consists of a vaneless space, a vane that is tangential to the impeller, or a combination of both. These vane passages diverge to convert the velocity head into pressure energy.High Velocity, Higher Pressure Gas Outlet. Gas Outlet Low Velocity, Low Pressure Gas InletFigure 2: Impeller
Below are important things that we have to know about centrifugal compressor. I. Basic Principles y Centrifugal Action In typical centrifugal compressor, the fluid is forced through the impeller by rapidly rotating impeller blades. The velocity of the fluid is converted top pressure, partially in the impeller and partially in the stationery diffuser. The example of centrifugal action is shown below.
Figure 3: Centrifugal Force
A Centrifugal Stage Most of the velocity leaving the impeller is converted into pressure energy in the diffuser. It is common for processing to have several stages in a single compressor. The low pressure fluid will enter the inlet and increase stage by stage respectively. Base on application requirement, centrifugal compressors can be design for single-stage machines, either direct-driven or geared (Figure 4), and or are design for executed in multistage configuration (Figure 5). Both single- and multistage machines are generally made up of standardized components. There are two principal casing types: (4) horizontally split casing and (5) vertically split casing (barrel-type compressors). The selection are based on table at figure 8.
Figure 4: Single-stage Compressor
Figure 5: Multi-stage Compressor
Energy Conversion The diffuser consists essentially of vanes, which are tangential to the impeller. These vane passages diverge to convert the velocity head into pressure energy.
Figure 6: Fluid Flowpath
The pressure, velocity and temperature are different at every point. Compressor operate for long periods of time, hence it is classified as steady flow process. Based on ideal gas law relationship, the temperature and pressure out for every stage are always higher than input as the volume is decreased. II. Centrifugal Applications The centrifugal compressor has many applications, requiring it to have many performance characteristics. Centrifugal compressors used in gas turbines are required to have a high pressure ratio and have a narrow operating range. Centrifugal compressors operating in the process industry have a need for a large operating range and thus operate at a small pressure ratio. The table below shows the centrifugal applications and services. Every application has different fluid being handled.
Figure 7: Application of Centrifugal Compressors
Centrifugal Compressor in Process Industry The common method of classifying process-type centrifugal compressors is based on the number of impellers and the casing design. Table below shows three types of centrifugal compressors. For each type of compressor, approximate maximum ratings of pressure, capacity and brake horsepower are also shown.
Figure 8: Casing Type Classification