Effects resulting from the geometric shape of the principle components of the compressor are shown in Figure200-4. Variables such as the impeller configuration and blade angle, inlet guide vane angle, diffuser size and shape, etc., can be adjusted by the machine designer for optimum performance under a specified set of operating conditions. Figure 200-5 shows impeller vector diagrams for various blade angles.
Impellers with backward leaning blades, are more commonly used for most centrifugal compressors because of their increased stable operating range ( Figure 200-6). Forward and radial blades are seldom used in petrochemical applications.
Machine output is always affected by combined losses, such as:
• Mechanical loss
• Aerodynamic loss
• Friction and shock loss
Mechanical losses, such as those from a journal or thrust bearing, affect the power input required, but do not influence the head-capacity curve. Aerodynamic losses that do influence the shape of the curve consist mainly of wall friction, fluid shear, seal losses, recirculation in flow passages, and shock losses. Shock losses are the result of expansion, contraction, and change of direction associated with flow separation, eddies, and turbulence. Friction and shock losses are the predominant sources of the total aerodynamic losses.
Figure 200-7 illustrates the affect of these combined losses in reducing the theoretical head.
Friction losses can be reduced by improving surface finishes. Shock losses may sometimes be mitigated by further streamlining of flow passages. These techniques will improve efficiency and tend to reduce the surge point, but they are costly, and there is a point of diminishing returns. The Company specification does not allow the manufacturer’s quoted performance to include efficiency improvements due to impeller polishing.