Basic Control Of Centrifugal Chillers

About 90% of the energy used in a centrifugal chiller is consumed by the compressor as it compresses the refrigerant gas. T he remaining 10 % is lost through electromagnetic and mechanical losses in the electric motor and windage loss due to gas stirring. As a centrifugal chiller is designed for optimum operation at the rated point, the loss in the refrigeration cycle is minimal at that point. However, as the chilled water temperature, cooling water temperature, and cooling capacity move away from the rated point, the losses increase and the performance is degraded unless properly controlled. Therefore, control is very important.

(1) Optimal control :-
In the latest A A R T series and A A R T-I series of chillers, optimization is possible through the six controllable elements outlined below, even when operation is not close to the specification point 

  1. First-stage inlet vanes :- These are aerodynamically variable blades located directly in front of the impeller to perform a capacity control function. 
  2. Second-stage inlet vanes :- These complement the first-stage inlet vane, making it possible to improve the controllability. 
  3. Variable speed control :- This controls the rotary speed of the compressor making it possible to control the pressure of the exhaust gas and air volume during suction. This is achieved by controlling the frequency of the power supplied to the electric motor (AART-I series only).
  4. High pressure expansion valve :- This circulates the amount of refrigerant required for the refrigeration cycle without waste. Only condensed refrigerant liquid flows downstream from the condenser, and not uncondensed refrigerant gas. 
  5. Low Pressure Expansion valve :- This controls only the refrigerant liquid expanded in the economizer so that it flows downstream to the compressor and not to the evaporator. 
  6. Hot Gas by pass :- T his connects the condenser and evaporator directly with piping and provides minimization control of the gas flow between the condenser and the evaporator, as occasion demands.

Numerical arithmetic control of all of these control elements is provided using an exclusive highly functional microcomputer control panel. The panel calculates the refrigeration capacity, the pressure in each section, the energy consumed, the weight f low rate of refrigerant circulated inside the chiller, the volumetric flow rate of the circulating refrigerant, the thermophysical properties of the refrigerant in each area, and the sound velocity. In addition, numerical arithmetic control includes the flow rate coefficient of each expansion valve, and the pressure and flow rate coefficients of the compressor as characteristics of the elements, and checks them against operating conditions. Optimization of the refrigeration cycle is a result obtained by optimization of each control element. With this optimization, conditions close to the design target can always be obtained even if the conditions or specifications are different.

(2) Transient Control :-
At first glance, it may seem that numerical control should concentrate on achieving optimal control given the current conditions. In actual operation, however, numerical control is usually designed to control the transition to a certain operating point. Therefore, given the target point and the current point, the equipment condition and operational values are brought to the target operating point over an appropriate interval. This makes it possible to include an appropriate transition in the control process. 
Centrifugal chillers are available in several sizes and capacities. Inputting several pieces of information on equipment composition in the form of a constant makes it possible to achieve optimization on the same level for all centrifugal chillers. 

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