Forward Leaning Blade Radial Blade Backward Leaning Blade
INLET FLOW Q
Figure 6-.7. Effect of blade angle on stability.
comparison is true in the overallsense， but it must be remembered that thenormal operating range lies between 100% flow (.) and flow atsurge， plusa safety marginof，usually， about 10%. The right-hand tail ends of all three curves are not in the operating range. The machine must operate with a suitable margin to the left of where these curves begin their steep decent ortail-off， and in the resultant operating range， the curve for backward leaning blades is steeper. This steeper curve is desirable for control purposes. Such a curve produces a meaningful change in pressure drop across the orifice for a small change in flow. The blade angle by itself does not tell the overall performance story. The geometry of other components of a stage will con-tribute significant effects also.
Most centrifugal compressors in service in petroleum or petrochemical processing plants use vaneless diffusers. A vaneless diffuser is generally a simple flow channel with parallel walls and does not have any elements inside to guide the flow.
Paths of Particle in Diffuser
Figure 6-.8. Flow trajectory in a vaneless diffuser.
When the inlet flow to the impeller is reduced while the speed is heldconstant， there is a decrease in the relative velocity leaving the impeller andthe air angle associated with it. As the air angle decreases， the length of the flow path spiral increases. The effect is shown in Figure 6-38.
If the flow path is extended enough， the flow momentum at the diffuserwalls is excessively dissipated by friction and stall. With this greaterloss， the diffuser becomes less efficient and converts a proportionately smaller partof the velocity head to pressure. As this condition progresses， the stage will eventually stall. This could lead to a surge.
Vaned diffusers are used to force the flow to take a shorter， more efficientpath through the diffuser. There are many styles of vaned diffusers， withmajor differences in the types of vanes， vane angles and contouring， and vane spacing. Commonly used vaned diffusers employ wedge-shaped vanes(vane islands) or thin-curved vanes. In high headstages， there can be two to four stages of diffusion. These usually consist of vaneless spaces to deceleratetheflow， followed by two or three levels of vaned blades in order to preventbuild-up of boundary layer， which causes separation and surging of the compressor. Figure 6-38 indicates the flow pattern in a vaned diffuser. The vaned diffuser can increase the efficiency of a stage by two to four percen-tage points， but the price for the efficiency gain is generally a narrower operating span on the head-flow curve with respect to both surge and stonewall. Figure 6-39 also shows the effect of off-design flows.
Excessive positive incidence at the leading edge of the diffuser vane occurswhen the exit flow is too small at reduced flow， and this condition brings on astall. Conversely， as flow increases beyond the rated point， excessive negative
Design Air Angle α Good Flow Path
α
Large Air Angle α Stone Wall Condition
incidence can cause stonewall. Despite its narrowing effect on the usableoperating range on the characteristic curve， the vaned diffuser has its applica-tion in situations where efficiency is of utmost importance. Although seldomused， movable diffuser vanes or vane islands can be used to alleviate the shock losses at off-design conditions.However， as the adjusting mechanismsrequired are quite complicated， they generally are applied only to single-stage machines.
It should be noted that the illustrations of the flow paths in Figures 6-37 through 6-39 are somewhat simplistic. Each flow path is indicated by a singlestreamline. The actual flow field is far more complex， with flow separation and recalculation present.Nevertheless， these figures should help with a practical understanding of the effects of changes in velocity triangles.
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