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When a capacity safety factor is used, the compressor is
selected at its maximum speed to handle design load plus safe-
ty factor. Multiplying factors for non-standard speeds are
shown in Fig. 20.
Whether or not added capacity offered by the safety factor is
incorporated at once is a matter of judgment. If it is, then the
compressor will be operated at maximum speed at the start and
any excess capacity achieved will be reflected in faster
pulldowns or lower temperatures. It is also a good practice to
drive the machine at a speed that will provide slightly more
rated capacity than is required by design load. Additional
speed-up available will then constitute reserve capacity in the
event it is needed. Motors should be sized to run the compres-
sor at maximum speed to forestall any motor changes, should
this maximum compressor speed be required in the future.
Fig. 18 — Flow Diagrams for Common Multistage Systems
(Not to be used as Piping Diagrams)