Each function has one or more subfunctions as shown in Table 2. These functions are defined in greater detail in t K Controls Operation section of this book.
Fig. 2 - Keyboard and Display Module
ELECTROWIC EXPANSION VALVE ~ The micro - processor controls the electronic expansion valve through the EXV driver module. Inside the expansion valve is a linear actuator stepper motor. To control the stepper motor’s position, the thermistor in the cooler and the thermistor in the lead compressor in each circuit are used to maintain a 20 F (1 I C) difference. Because the com- pressor sensor is after the compressor motor, which adds approximately 15 F (8.3 C) superheat, the 20 F (1 
I C) control temperature results in U”F to 5 F (2.8 C) super- heat leaving the cooler. This improves the performance of the cooler.
At initial
The control monitors the superheat and the rate of change of superheat to control the position of the valve. The valve stroke is very large; this results in very accurate control of the superheat.
The electronic expansion valve is also used to limit the maximum saturated suction temperature to 55 F (12.8 C) to keep from overloading the compressor during high cooler water temperatures. This allows the unit to start with very warm water temperatures.
THERMISTORS ~ The electronic control uses 7 ther- mistors to sense temperatures used to control the opera- tion of the chiller. Sensors are listed in Table 3.
CAPACITY CONTROL ~ The control cycles com- pressors and alternately loads and unloads cylinders to give capacity control steps as shown in Table 4. The unit controls leaving chilled water temperature. Entering water temperature is used by the microprocessor in deter- mining the optimum time to load and unload, but is not a control set point.
The chilled water temperature set point can be auto- matically reset by the return temperature reset or space and outside air temperature reset features.
