without inlet guide vanes and with a shroudlessbacksuept impeller, a vaned The complete design of a high-performance centrifugal compressor involves. Figure Flow lines for optimised compressor design (recirculation removed). Figure Pressure coefficient distribution over diffuser vane at varying spans . of the centripetal-flow fan at design operating conditions can reach % and , respectively, of the centripetal compressor from the point of view.
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Centrifugal compressor – Wikipedia
Centrifugal compressorssometimes called radial vsneare a sub-class of dynamic axisymmetric work-absorbing turbomachinery. The pressure rise in the impeller is in most cases almost equal to the rise in the diffuser. As the flow passes through the centrifugal impeller, the impeller forces the flow to spin faster as it gets further from the rotational axis.
According to a form of Euler ‘s fluid dynamics equation, known as the pump and turbine equationthe energy input to the fluid is proportional to the flow’s local spinning velocity multiplied by the local impeller tangential velocity. In many cases, the flow leaving the centrifugal impeller is travelling near the speed of sound. It then flows through a stationary compressor causing it to decelerate.
The stationary compressor is ducting with increasing flow-area where energy transformation takes place. If the flow has to be turned in a rearward direction to enter the next part of the machine, eg another impeller or a combustor, flow losses can be reduced by directing the flow with stationary turning vanes or individual turning pipes pipe diffusers. As described in Bernoulli’s principlethe reduction in velocity causes the pressure to rise. Over the past years, applied scientists including Stodola— Pfleiderer Hawthorne Shepard Lakshminarayana and Japikse many texts including citations    have educated young engineers in the fundamentals of turbomachinery.
This relationship is the reason advances in turbines and axial compressors often find their way into other turbomachinery including centrifugal compressors.
Improvements in centrifugal compressors have not been achieved through large discoveries. Rather, improvements have been achieved through understanding and applying incremental pieces of knowledge discovered by many individuals. The horizontal axis represents the energy equation derivable from The first law of thermodynamics.
Again, the horizontal axis represents the energy equation with turbines generating power to the left and compressors absorbing power to the right. Centrifugal compressors are similar in many ways to other turbomachinery and are compared and contrasted as follows:. Centrifugal compressors are similar to axial compressors in that they are rotating airfoil-based compressors.
Both are shown in the adjacent photograph of an engine with 5 stages of axial compressor and one stage of centrifugal compressor.
This first part of the centrifugal impeller is also termed an inducer. The s-era German Heinkel HeS experimental engine was the first aviation turbojet to have a compressor stage with radial flow-turning part-way between none deaign an axial and 90 degrees for a centrifugal. Centrifugal compressors are also similar to centrifugal fans of the style shown in neighboring figure as they both increase the flows energy through increasing radius.
In many cases the engineering methods used to design a centrifugal fan are the same as those to design a centrifugal compressor, so they can look very similar. This relationship is less true in comparison to the squirrel-cage fan shown in the accompanying figure.
For purposes of generalization and definition, it can be said that centrifugal compressors often have density increases greater than 5 percent. Also, they often experience relative desjgn velocities above Mach number 0. In contrast, fans or blowers are often considered to have density increases of less than five percent and peak relative fluid velocities below Mach 0. Centrifugal compressors are also similar to centrifugal pumps  of the style shown in the adjacent figures.
The key difference between such compressors and pumps is that the compressor working fluid is a gas compressible and the pump working fluid is liquid incompressible. Again, the engineering methods used to design a centrifugal pump are the same as those to design a centrifugal compressor.
Yet, there is one important difference: Centrifugal compressors also look very similar to their turbomachinery counterpart the radial turbine as shown in the figure. While a compressor transfers energy into a flow centripegal raise its pressure, a turbine operates in reverse, by extracting energy from a flow, thus reducing its pressure.
A partial list of turbomachinery that may compreseor one or more centrifugal compressors within the machine are listed here. A simple centrifugal compressor has four components: As a result of the impeller rotating clockwise when looking downstream into the compressor, the flow will pass through the volute’s discharge cone moving away from the figure’s viewer.
The inlet to a centrifugal compressor is typically a simple pipe. All of these additional devices have important uses in centripetwl control of the centrifugal compressor. The key component that makes a clmpressor centrifugal is the centrifugal impeller, Figure 0. This is identical to an axial compressor with the exception that the gases can reach deeign velocities and energy levels through the impeller’s increasing radius. In many modern high-efficiency centrifugal compressors the gas exiting the impeller is traveling near the speed of sound.
Most modern high efficiency impellers use “backsweep” in the blade shape. Euler’s pump and turbine equationplays an important role in understanding impeller performance. The next key component to the simple comperssor compressor is the diffuser.
Diffusers can be vaneless, vaned or an alternating combination. High efficiency vaned diffusers are also designed over a wide range of solidities from less than 1 to over 4. Hybrid versions of vaned diffusers include: Some turbochargers have no diffuser. Bernoulli’s fluid dynamic principle plays an important role in understanding diffuser performance.
The collector of a centrifugal compressor can take many shapes and forms. When the diffuser discharges into a device that looks somewhat like a snail shell, bull’s horn or a French horn, the collector is likely to be termed a volute or scroll. Either the collector or the pipe may also contain valves and instrumentation to control the compressor.
Below, is a partial list of centrifugal compressor applications each with a brief description of some of the general characteristics possessed by those compressors. To start this list two of the most well-known centrifugal compressor applications are listed; gas turbines and turbochargers. While illustrating a gas turbine’s Brayton cycle,  Figure 5. These types of plots are fundamental to understanding centrifugal compressor performance at one operating point.
Studying these two plots further we see that the pressure rises between the compressor inlet station 1 and compressor exit station 2. At the same time, it is easy to see that the specific volume decreases or similarly the density increases. Studying the temperature-entropy plot we see the temperature increase with increasing entropy loss.
If we assume dry air, and ideal gas equation of state and an isentropic process, we have enough information to define the pressure ratio and efficiency for this one point. Unfortunately, we are missing several other key pieces of information if we wish to apply the centrifugal compressor to another application.
Also included are constant efficiency contours. Centrifugal compressor performance presented in this form provides enough information to match the hardware represented by the map to a simple set of end-user requirements.
Compared to estimating performance which is very cost effective thus useful in designtesting, while costly, is still the most precise method. Professional societies such as ASME i. Despite this complexity, a few basic concepts in performance can be presented by examining an example test performance map. Pressure ratio and flow are the main parameters     needed to match the Figure 5. In this case, it can be assumed that the inlet temperature is sea-level standard.
Making this assumption in a real case would be a significant error. A detailed inspection of Figure 5. As is standard practice, Figure 5. While flow measurements use a wide variety unit specifications, all fit one of 2 categories:. Questions remaining cnetripetal involve inlet or outlet which might involve leakage from the compressor or moisture condensation. For atmospheric air, the mass flow may be wet or dry including or excluding humidity.
Often, the mass flow specification will be presented on an equivalent Mach number basis. It is standard in these cases that the equivalent temperature, equivalent pressure and gas is specified explicitly or implied at a standard condition. In contrast, all volume flow specifications require the additional specification of density. Bernoulli’s fluid dynamic principal is of great value in understanding this problem.
Confusion arises through either inaccuracies or misuse of pressure, temperature and gas constants. Also as is standard practice, Figure 5.
The variety of pressure measurement units is also vast. In this case, they all fit one of three categories:. The two most common methods used for testing centrifugal compressors is vanw test along lines of constant shaft speed or along lines of constant throttle.
If the shaft speed is held constant, test points are taken along a constant speed line by changing throttle positions. In contrast, if a throttle valve is held constant, test points are established by changing speed common gas turbine practice.
The map shown in Figure 5. The first three speed lines have 6 points each while the highest speed line has five. The next feature to be discussed is the oval shaped curves centrlpetal islands of constant efficiency. General standard practice is to interpret these efficiencies as isentropic rather than polytropic. The inclusion of efficiency islands effectively generates a 3-dimensional topology to this 2-dimensional map.
With inlet density specified, it provides a further ability to calculate aerodynamic power. Lines of constant power could just as easily be substituted. These requirements are of secondary importance to the overall gas turbine performance as a whole. For this reason it is centipetal necessary to summarize that in the ideal case, the lowest specific fuel consumption would occur when the centrifugal compressors peak efficiency curve coincides with the gas turbine’s required operation line.
In xompressor to gas turbines, most other applications including industrial need to meet a less stringent set of performance requirements. Historically, centrifugal compressors applied to industrial applications were needed to achieve performance at a specific flow and pressure.
Modern cwntripetal compressors are often needed to achieve deeign performance goals across a range of flows and pressures; thus taking a significant step toward the sophistication seen in gas turbine applications. If the compressor represented by Figure 5.
An “End User” would be very happy with the performance requirements of 2. Surge – is a flow compresor at low mass flow rate operation for which the impeller cannot add enough energy to centripetl the desiyn resistance or backpressure.