Control method and apparatus for a centrifugal chiller using a variable speed impeller motor drive

A control method for a centrifugal chiller having a centrifugal compressor integrated into a refrigeration loop, the refrigeration loop additionally comprising a condenser and an evaporator, the compressor being adapted for the most efficient operation proximate an undesirable surge operating condition, the centrifugal compressor having an impeller and variable inlet guide vanes, the method comprising:

• generating a dimensionless plot of possible operating points of the centrifugal compressor;
• periodically sampling actual surge events experienced by the compressor during operation, to determine a region of actual surge;
• periodically positioning an operating limit curve proximate the region of actual surge on the plot to define a compressor operating area that provides for efficient operation and yet avoids the occurrence of surge events;
• defining the actual operating point of the compressor on the plot;
• and
• controlling the operating point of the compressor by selectively adjusting the impeller speed and variable inlet guide vanes to position the operating point of the compressor at the operating limit curve.

A control method for use with a control system in controlling a centrifugal compressor, the compressor having variable inlet guide vanes and at least one rotationally driven impeller and being adapted such that the most efficient operation of the compressor occurs when the variable inlet guide vanes are in a predetermined fully open position and compressor capacity is controlled solely by the speed of the impeller, the control system sensing a steady state operating condition of the compressor, defined limits compelling that the compressor be unloaded, the inlet guide vane position, the impeller speed, sensing the current operating condition of the compressor the control method comprising:

• a first control mode in which the control of the compressor capacity is a function of inlet guide vane positioning and impeller speed variations;
• a second control mode in which the control of the compressor capacity is a function of impeller speed variations with the inlet guide vanes fixed in the fully open position;
• defining an operating condition control limit region that is proximate a operating condition where a surge condition is known to occur;
• transitioning from the first control mode to the second control mode predicated upon sensing that the compressor is operating in a steady state condition and the inlet guide vanes are in the predetermined fully open position;
• and
• transitioning from the second control mode to the first control mode upon the occurrence of at least one of the following:
• a. detecting that the compressor is operating outside the operating condition control limit region;
• b. detecting that the defined limits compelling that the compressor be unloaded have been exceeded;
• c. detecting the occurrence of a surge condition.

A controller for adjusting the capacity of a variable capacity centrifugal compressor driven by a motor to maintain a setpoint, the controller adapted for adjusting said capacity in response to demand while minimizing electric power consumption and avoiding a surge conditions, comprising:

• means for determining the operating capacity of the compressor and expressing said capacity as a first variable;
• means for determining the energy input of the compressor to the refrigerant and expressing said energy input as a second variable;
• means for relating the first variable that is indicative of the operating capacity of the compressor to the second variable that is indicative of the energy input of the compressor to the refrigerant;
• means for sensing the current operating condition of the compressor;
• means for sensing surge conditions and thereby determining a region of actual surge;
• plot generation means, responsive to the operating capacity variable sensing means, the energy input constant sensing means and the surge sensing means for plotting a surge curve and an operating limits curve, the operating limits curve being proximate the region of actual surge and defining a region of operation of the compressor such that compressor operation therein avoids surge conditions yet maintains a setpoint in an energy efficient manner;
• and
• control means, responsive to the current operating condition sensing means, for relating the operating condition to the first variable that is indicative of the operating capacity of the compressor and the second variable that is indicative of the energy input of the compressor to the refrigerant said control means being adapted for controlling the compressor such that the operating condition of the compressor is within the operating limits curve proximate the region of actual surge.

An anti-surge method for a centrifugal compressor comprising the steps of:

• determining a machine coefficient;
• determining a pressure coefficient;
• plotting a compressor operating point as a function of the machine coefficient and pressure coefficient;
• monitoring actual surge events;
• determining a surge boundary control curve as a function of the monitored actual surge events;
• operating the compressor so that the compressor operating point approaches the surge boundary control curve;
• and
• modifying the boundary control curve each time a surge event is detected.

An anti-surge system for a centrifugal compressor comprising:

• a compressor;
• means for determining a machine coefficient;
• means for determining a pressure coefficient;
• means for plotting a compressor operating point as a function of the machine coefficient and pressure coefficient;
• means for monitoring actual surge events;
• means, responsive to the monitoring means, for determining a surge boundary control curve;
• means for operating the compressor so that the compressor operating point approaches the surge boundary control curve;
• and
• means for modifying the surge boundary control curve each time a surge event is detected.

A method of operating a centrifugal compressor having variable position inlet guide vanes and having impellers controlled by a variable speed motor, comprising the steps of:

• operating the variable speed motor at a constant high speed;
• modulating the variable position of the inlet guide vanes until the variable position reaches a predetermined maximum position;
• and
• incrementally reducing the speed of the motor as long as the variable position of the inlet guide vanes remains at the predetermined maximum position.

A method of operating a centrifugal compressor having impellers motivated by a variable speed impeller motor and having variable position inlet guide vanes, comprising the steps of:

• operating in a first mode of operation where the position of the inlet guide vanes is modulated and the speed of the impeller motor is held constant;
• operating in a second mode of operation wherein the position of the inlet guide vanes is maintained in a constant position and the speed of the impeller motor is modulated;
• transitioning from the first mode of operation to the second mode of operation when the position of the inlet guide vanes approaches a predetermined maximum;
• and
• transitioning from the second mode of operation to the first mode of operation when the impeller speed approaches a predetermined maximum.