Variable-speed and constant-speed suction throttling are the two most common control methods. Adjustable inlet guide vanes are sometimes used, primarily on single-stage units.
Turbine driven compressors typically use variable speed, with either pressure or flow as the controlled variable. Suction throttling is generally used for motor-driven compressors. Variable-speed motors and hydraulic or electric variable-speed couplings are seldom applied to centrifugal compressors due to their added cost, and because they significantly lower the efficiency of the unit.
A review of centrifugal compressor characteristics highlights the differences between these two methods:
For variable-speed control, the capacity varies directly with speed and the head varies proportional to the square of speed. Therefore, as speed is reduced, capacity and head are reduced to meet the process requirements, with a corresponding reduction of horsepower and a minimum loss in efficiency. On the other hand, constant-speed operation essentially produces a constant head.
Throttling reduces the inlet and outlet pressures but adds losses by introducing added resistance to the system.
Figure 200-70 shows typical constant-speed performance curves indicating the effect of suction throttling. Figure200-71 shows typical variable-speed performance curves. A comparison gives an indication of the difference in power requirements between the two methods.
For a capacity requirement of 80%, suction throttling requires approximately 86% horsepower. For the same 80% capacity, control by variable speed requires approximately 81% horsepower.