York YCAV0569, Installation Operation & Maintenance

Page: 156 / 278

156
JOHNSON CONTROLS
FORM 201.21-NM2 (616)
ECONOMIZER CONTROL
ECONOMIZER CONTROL
The Economizer Solenoid controls a vapor feed to the
economizer port on the compressor from the top of the
Flash Tank. When the valve is open, refrigerant gases
off in the Flash Tank providing additional subcooling to
the liquid in the tank. The subcooled liquid is then fed
to the evaporator resulting in additional system capacity
and efficiency.
In normal operation, the Economizer Solenoid on a
compressor will be turned on whenever the VSD fre-
quency is > 120 Hz, the flash tank level is <75%, motor
current < 80%FLA, motor temperature sensors are all
less than <150°F, and the economizer timer is timed out.
Whenever the Economizer Solenoid is turned on, the
compressor load timer is set to 35 seconds and econo-
mizer timers for every system are set to 30 seconds,
unless they are already above 30 seconds.
In low ambient temperatures <40°F, run time on the re-
spective compressor is < 5 minutes, and the Flash Tank
level is <75%, the system Economizer Solenoid is turned
on. Under these conditions, the VSD frequency and the
motor temp sensor readings are not factors that could
overload the compressor. Energizing the Economizer
Solenoid also helps start a system in low ambients and
prevents low suction pressure and low oil differential
faults by increasing the load.
At ambients above 40°F, once on, the Economizer So-
lenoid will remain energized until the VSD frequency
drops below 90 Hz. Below 90 Hz, the solenoid will
be turned off, regardless of the time remaining on the
economizer timers. Under these conditions, the econo-
mizer timers will be set to “0” when the solenoids are
de-energized. Below 100 Hz, if the economizer timer
has timed out, the Economizer Solenoids will be turned
off, the unload timer will be set to 30 seconds, the econo-
mizer timer will be set to 30 seconds if <30 sec.
If a motor temperature sensor exceeds 240 °F, the Econo-
mizer Solenoid will de-energize to avoid overheating the
hot motor. When the economizer solenoid is de-ener-
gized, the compressor unload timer is set to 30 seconds
and the economizer solenoid timer is set to 60 seconds.
All other economizer timers for other systems are set to
30 seconds, if they are already < 30 seconds.
The Economizer Solenoid timer prevents the solenoid
from cycling too often.
Whenever a compressor is to be turned off, all system
Economizer Solenoids will be de-energized when
the compressor(s) ramp down. The solenoids on the
compressors that will be ramped back up, if any, will
remain off for 30 seconds before the Chiller Control
Board allows the solenoids to re-energize. Once on, the
economizer solenoid(s) must remain on for 30 seconds as
determined by the economizer timer for each system.
OPERATION