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 FIGURE 5-6. Five-Step Clearance Control
1.1.2 Centrifugal Compressor. Five methods are generally used to control
1.1.2.1 Inlet Guide Vanes. Centrifugal compressor control may be
accomplished by the use of adjustable inlet guide vanes. The purpose of the
guide vanes is to direct the airflow and distribute it uniformly into the eye
of the impeller. The adjustable vanes located at the first-stage inlet or the
inlet to each stage, are used to modify the pressure-volume characteristics of
the compressor. Guide vanes are manually or automatically adjustable.
1.1.2.2 Blowoff of Output. Blowoff and recirculation are occasionally used
but do not save any power because the compressor is continually operating at
full load pressure and inlet volume. Blowoff relieves excess gas to the
atmosphere, and recirculation puts throttled gas back through the compressor.
1.1.2.3 Intake Throttling. Intake throttling is widely used for control of
constant speed machines. Either inlet butterfly valves or inlet guide vanes
may be automatically or manually controlled. Power conservation is less than
with variable speed, but is still significant.
1.1.2.4 Two-Step Control. Two-step control is mainly useful during load
operations below 50 percent capacity. The amount of flow is dependent upon
two set points. Operation is at full load until the upper set point is
reached, then the unit is unloaded and operates at 15 percent flow until the
lower set point is reached and the system reverts back to full load. The
action is cyclic, maintaining an intermediate pressure value between the set
points. Two-step control is not recommended if a complete load-unload cycle
5-11
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