Although the families generally consist of 20 to 30
impeller sizes for a given frame size, this can be increased
to an infinite number to enhance selectability. On one hand,
it is important to keep this number to a minimum in order to
minimize the number of drawings, castings, stampings and
performance records. Although this is a good idea, it is
possible with the computerized equipment available for
analysis, drafting, and manufacturing to have an infinite
number of compression stages per compression family.
Although manufacturers' frame sizes are well-established
standards, it is feasible that an infinite number of frame
sizes could be made available with use of fabricated
components, FEM analysis, and CAD. What this means is
that a compression stage can be selected exactly at its peak
efficiency instead of compromising and selecting the nearest
frame size and the nearest impeller size (Figure 5.3).
There is still the drawback that, while the compressor
may be very efficient at the design point, efficiency drops
when operating at other than design conditions. To
compensate for this when purchasing a new compressor,