There are two ways to control the volume flow rate of the centrifugal compressor in which will change in response to changes in head or changes in cooling required. HVAC applications necessitate the compressor to operate over wide range of lift and cooling capacities.
Inlet Gide Vanes:
As the inlet guide vanes start to close, they change the gas entry angle to the impeller and reduce the gas flow and accordingly reduce the compressor capacity. As the vanes near the close position they throttle the refrigerant flow.
Varying Compressor Speed:
If the prime mover is turbine or internal combustion engine the prime mover’s speed can easily be changed. Induction motors require a Variable Frequency Drive (VFD ) to change their speed.
TO control the compressor speed, the required lift must be reduced whether by raising the supply water temperature or by reducing the condenser water temperature, in which the later is the most common way. Unless the compressor lift is lowered, varying compressor speed can’t work.
As compressor speed is changing, also the tip speed changes, by reducing the tip speed the compressor produced lift is lowered accordingly.
VFD is replacing the compressor motor starter, VFD has to be water cooled in most cases, whether condenser-water cooled (are not recommended) or chilled-water cooled, in which the later is recommended and the load to be added to the chilled water loop .
The chiller controller monitors the chiller operation conditions and manage a combination between of speed control
(VFD) and inlet guide vanes. As compressor speed control is only lowering the speed to about 60%.
VFDs act as a soft starter and they can lowering the inrush current for the motor to almost full load running amps.
Mature economical analysis with realistic load profiles and ambient wet bulb is recommended when considering using VFDs
HVACing 