York YCAS Series, Installation Operation & Maintenance

Page: 185 / 216

185 YORK INTERNATIONAL
FORM 201.19-NM8 (804)
EEV OPERATION
The EEV is an electronically controlled expansion valve
that meters a fl ow of liquid re frig er ant into the evapora-
tor to control superheat. The re frig er ant fl ow di rec tion is
des ig nat ed by an ar row on the ex pan sion valve body.
24VAC HEAT MOTOR
The 24VAC Heat Motor is fed from the EEV output
board in the con trol panel . The Heat Motor allows the
mi cro to open and close the valve to control suction
superheat.
The heat motor must be plugged into
the 24 volt shielded ca ble feed from
the EEV out put board. Dam age to the
heat mo tor will occur if it is plugged
into 120VAC wir ing for the Pilot So-
le noid.
CONTROLLER
When EEV is selected as the expansion valve type via
DIP switch on the micro board, the EEV con trol ler will
be come ac tive. When TXV is se lect ed, the EEV out put
will be fi xed at 0 and the low su per heat and sensor
failure safeties will be dis abled.
The EEV controller is a PI (Proportional plus In te gral)
controller. Gain sched ul ing var ies the pro por tion al gain
based on the su per heat error. As the su per heat er ror
gets small er the pro por tion al gain will get small er. The
in te gra tion 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 print outs.
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 out put of this ETRE model
is the PWM per cent age that will be sent to the ETRE
heat motor. This PWM output is the percentage of a 1
second pe ri od that the 24VAC heat motor power signal
is energized.
MOP FEATURE
The controller also has an MOP feature (Max i mum Op-
erating Pressure) that overrides superheat con trol when
the MOP setpoint is exceeded. This con trol gen er al ly
will be active for hot water starts. The MOP setpoint is
60º F Saturated Suction Tem per a ture.
The MOP feature is also used to prevent un der shoot of
the superheat setpoint when the suction temperature of a
system being started is much higher than the return wa-
ter temperature. This pro vides better startup su per heat
control for high ambient, low water temp startups when
the su per heat measurement is artifi cially high due to
the warm suction line. If the return water temp sensor
is in range, run time is less than 5 minutes, and suc tion
temperature is greater than (RCHLT + 3º F), the MOP
setpoint is reset to RCHLT - Su per heat Setpoint. If this
val ue is higher than the fi xed MOP setpoint, the original
setpoint is retained.
VALVE PREHEAT FEATURE
The heat motor is preheated for moderate and low ambi-
ent standby conditions. When the ambient is be low 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 am bi ent is above 50º F the heat
motor is not pre heat ed.
PILOT SOLENOID (LLSV) CON TROL
The Pilot Solenoid allows the EEV to be used in the
same way as a Liquid Line Solenoid Valve. When the
Pilot So le noid is turned off, the EEV clos es im me di ate ly
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 shut down, the system will
fully unload. The Pilot So le noid will be turned off and
the system will run un load ed until the suc tion pressure
falls be low the cut out or for 180 sec onds, whichever
comes fi rst.
8.5 EEV OPERATION
8