York YCAS0130 Eev Operation, Mop Feature, 24VAC HEAT MOTOR, Valve Preheat Feature, Controller

FORM 201.19-NM1 (204)

8.5 EEV OPERATION

EEV OPERATION MOP FEATURE

The EEV is an electronically controlled expansion valve that meters a flow of liquid refrigerant into the evapora- tor to control superheat. The refrigerant flow direction is designated by an arrow on the expansion valve body.

24VAC HEAT MOTOR

The 24VAC Heat Motor is fed from the EEV output board in the control panel . The Heat Motor allows the micro to open and close the valve to control suction superheat.

The heat motor must be plugged into the 24 volt shielded cable feed from the EEV output board. Damage to the heat motor will occur if it is plugged into 120VAC wiring for the Pilot So- lenoid.

The controller also has an MOP feature (Maximum Op- erating Pressure) that overrides superheat control when the MOP setpoint is exceeded. This control generally will be active for hot water starts. The MOP setpoint is 60º F Saturated Suction Temperature.

The MOP feature is also used to prevent undershoot of the superheat setpoint when the suction temperature of a system being started is much higher than the return wa- ter temperature. This provides better startup superheat control for high ambient, low water temp startups when the superheat measurement is artificially high due to the warm suction line. If the return water temp sensor is in range, run time is less than 5 minutes, and suction temperature is greater than (RCHLT + 3º F), the MOP setpoint is reset to RCHLT - Superheat Setpoint. If this value is higher than the fixed MOP setpoint, the original setpoint is retained.

VALVE PREHEAT FEATURE

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CONTROLLER

When EEV is selected as the expansion valve type via DIP switch on the micro board, the EEV controller will become active. When TXV is selected, the EEV output will be fixed at 0 and the low superheat and sensor failure safeties will be disabled.

The heat motor is preheated for moderate and low ambi- ent standby conditions. When the ambient is below 25º F, the heat motor is preheated with a 25% duty cycle. This preheated value is ramped from 25% to 0% from 25º F to 50º F. When the ambient is above 50º F the heat motor is not preheated.

PILOT SOLENOID (LLSV) CONTROL

The EEV controller is a PI (Proportional plus Integral) controller. Gain scheduling varies the proportional gain based on the superheat error. As the superheat error gets smaller the proportional gain will get smaller. The integration time is adjusted to increase the controller re- sponse during start-up and low superheat conditions.

The output from the PI controller is the EEV output percentage which is shown on the display and printouts. This output is then fed into a model of the ETRE bulb/ heat motor to over and under drive the heat motor for faster valve response. The output of this ETRE model is the PWM percentage that will be sent to the ETRE heat motor. This PWM output is the percentage of a 1 second period that the 24VAC heat motor power signal is energized.

The Pilot Solenoid allows the EEV to be used in the same way as a Liquid Line Solenoid Valve. When the Pilot Solenoid is turned off, the EEV closes immediately and prevents the Heat Motor from opening the valve.

Each system has a Pump Down feature upon shut off. Manual pump down from the keypad is not possible. On a non-safety, non-unit switch shutdown, the system will fully unload. The Pilot Solenoid will be turned off and the system will run unloaded until the suction pressure falls below the cutout or for 180 seconds, whichever comes first.

YORK INTERNATIONAL 173