A greater number of impellers in a given casing will tend to reduce the stable range. Similarly, so does the number of sections of compression, or the number of casings in series.
The large majority of centrifugals use vaneless diffusers, which are simple flow channels with parallel walls, without elements inside to guide the flow. The trajectory of a particle through a vaneless diffuser is a spiral of about one-half the circumferential distance around the diffuser (Figure 200-18). If this distance becomes longer for any reason, the flow is exposed to more wall friction which dissipates the kinetic energy. As flow is reduced, the angle is reduced which extends the length of the trajectory through the diffuser (Figure 200-19). When the flow path is too long, insufficient pressure rise (head) is developed and surge occurs.
Occasionally, vaned diffusers are used to force the flow to take a shorter, more efficient path. Figure200-20 shows the flow pattern in a vaned diffuser. The vaned diffuser can increase the aerodynamic efficiency of a stage by approximately 3%, but this efficiency gain results in a narrower operating span on the head-capacity curve with respect to both surge and stonewall. The figure also shows how the path of a particle of gas is affected by off-design flows. At flows higher than design, impingement occurs on the trailing side of the diffuser vane creating shock losses which tend to bring on stonewall. Conversely, flow less than design encourages surge, due to the shock losses from impingement on the leading edge of the vane.
Despite adverse effects on surge, the vaned diffuser should be applied where efficiency is of utmost importance, particularly with small high-speed wheels.
Stationary guide vanes may be used to direct the flow to the eye of the impeller. Depending upon the head requirements of an individual stage, these vanes may direct the flow in the same direction as the rotation or tip speed of the wheel, an action known as pre-rotation or pre-swirl. The opposite action is known as counter-rotation or counter swirl. Guide vanes set at zero degrees of swirl are called radial guide vanes.
The effect guide vanes have on a compressor’s curve is illustrated in Figure 200-21. Note that pre-rotation reduces the head or unloads the impeller. Pre-rotation tends to reduce the surge flow. Counter-rotation increases the head and tends to increase the surge flow.
Movable inlet guide vanes are occasionally employed on single-stage machines, or on the first stage of multi- stage compressors driven by electric motors at constant speed. The guide vane angle can be manually or automatically adjusted while the unit is on stream to accommodate operating requirements. Because of the complexity of the adjusting mechanism, the variable feature can only be applied to the first wheel in almost all designs.