The inlet Guide Vanes are mounted in the suction of the compressor, after the suction pressure/ temperature sensors and just before the inlet of the compressor rotors.
These vanes are essentially the same as those in a standard, centrifugal compressor. In a standard, centrifugal compressor, these inlet vanes are used to reduce compressor capacity by ¨throttling¨ the refrigerant flow through the compressor, thus reducing compressor capacity.
In the Turbocor compressor, the primary capacity control is accomplished by reducing compressor rotation speed. However, under certain circumstances, when reducing compressor rotation speed would result in the compressor entering a ¨surge¨ condition, further reduction in compressor capacity is accomplished by adjusting the position of the inlet guide vanes in concert with the compressor speed reduction.
On the Turbocor Compressor, the actual position of the inlet guide vane is demonstrated via a small steel ball in a groove with a plastic cover. There is a magnet imbedded in the inlet guide vane assembly such that, as the inlet guide vanes open or close, the position of the small steel ball follows the position magnet (because the compressor housing itself is aluminum) and gives an external indication of the inlet guide vane position.
During the compressor start sequence, the inlet guide vanes are closed fully, then, typically opened to 110%. The compressor information displayed on the chiller controller indicates assumed inlet guide vane position based on signals sent by the compressor controls to the stepper motor that operates the inlet guide vanes. There is no feedback from the inlet guide vanes to indicate actual position of the inlet guide vanes to the compressor controls.
Every time a Turbocor Chiller is visited, the technician should monitor the inlet guide vane position indicated by the compressor controls and verify that position by visually inspecting the position of the small steel ball. Position should be verified in both the open (100%) and closed (0%) positions as well as during the transition from one position to the other.
If necessary, the inlet guide vanes can be accessed by isolating the compressor, removing the refrigerant gas from the compressor and pulling the suction bell housing from the compressor.
Possible causes of inlet guide vane malfunction could be a failed stepper motor, or dirt in the inlet guide vanes themselves causing the worm gear that operates the inlet guide vanes to ¨bind¨.
HINT FROM THE AUTHOR: There is also ther possibility that the insulation on the wires from the pass through plug (where the external power connection of the stepper motor is connected to the wires feeding the stepper motor itself) has been compromised. Where these wires pass through the compressor end bell, the inner edge of the hole through the end bell is not machined smooth. It is a very sharp edge. There are very specific instructions when installing the stepper motor to wrap the wires around the stepper motor itself and ¨lock¨the wires into position. This is done specifically to avoid the wires rubbing against this sharp edge. It the wires escape from their position where they are wrapped around the stepper motor, they can rub against the sharp edge on the inner surface of the compressor housing and cause a short, disabling the inlet guide vanes.