The Turbo Air® 3000 Centrifugal Compressor Compressor Handbook

Auto-Offline Control
Vantage Control System Logic
With Auto-Offline Control, the inlet valve modulates to satisfy the System Pressure setpoint (within the
Maximum Motor Load setpoint limits). In cases of low demand, the control throttles back to the
Minimum Flow control line setpoint. The Minimum Flow control loop controls system pressure to the
System Pressure Offset setpoint and flow to the Surge Control Offset line. The compressor inlet valve
responds to the Surge Control line. If system pressure reaches its Offset value, Vantage modulates the
compressor’s discharge blow off valve. As the blow off valve reaches point of closing the discharge
check valve, Vantage unloads the compressor. When the system pressure drops below the System
Pressure setpoint, Vantage reloads the compressor through the inlet valve control loops.
Auto-Offline Control Setpoints
· Maximum Motor Load Setpoint: Prevents motor overload condition.
· System Pressure Setpoint: Establishes the initial pressure control point.
· Surge Control Offset Line: Prevents compressor surging at minimum flow conditions.
· System Pressure Offset: Sets an upper level for allowable system pressure.
· Maximum Discharge Pressure: Prevents compressor surging at over-pressure conditions.
Auto-Offline Control Steps (see Figure C— 1)
1. Compressor loads to achieve the System Pressure setpoint (position 1).
2. If demand is low, the inlet valve throttles to the Minimum Flow line (position 2).
3. The inlet Minimum Flow control loop operates the compressor along the Surge Control Offset line
(B), between position 2 and the System Pressure Offset setpoint (position 3).
4. At the System Pressure Offset setpoint (pos.3), control switches to the Blow Off System Pressure
control loop, which begins to modulate the compressor blow off valve.
5. If blow off increases to the point that the discharge check valve closes, Auto-Offline unloads the
compressor by first fully opening the blow off valve, and then closing the inlet (pos.4).
6. The compressor reloads when the system pressure drops below the System Pressure setpoint.
Figure C—1: Auto-Offline Control Operation
C—7