York YCAS0130 - Manual (Page 134)

134 YORK INTERNATIONAL
FORM 201.19-NM1 (204)
Micro Panel Contents
MANUAL Lead/Lag selection will be automatically
overridden by the micro to allow the lag compressor to
automatically become the lead anytime the selected lead
compressor shuts down due to a lock-out, lead system
faults and is waiting to restart, system switch on the
micro board is in the OFF position, or if a run permissive
is keeping the lead system off. Automatic switch over in
MANUAL mode is provided to try to maintain chilled
liquid temperature as close to setpoint as possible.
1.22 ECONOMIZER SOLENOID CONTROL
The economizer solenoid is controlled by the micro
based on the ability of the economizer to provide extra
capacity according to system operating conditions. This
ability is primarily based on outside ambient tempera-
ture. If the ambient is low with associated low discharge
pressure, the economizer will provide very little extra
subcooling. At an ambient of 90ºF (32ºC) or above
with associated high discharge pressure, the economizer
will begin to provide appreciable additional subcooling.
The extra subcooling may be as much as an additional
20ºF to 25ºF (11.1ºC to 13.8ºC), often making the total
subcooling at the TXV over 40ºF (4.4ºC).
This should not be confused with
subcooling at the liquid valve, which
should generally be 12ºF to 15ºF (6.7ºC
to 8.3ºC). The micro monitors the dif-
ference in pressure ratio between the
discharge and suction pressure along
with the step of unloading to determine
economizer solenoid on/off points.
The micro utilizes the formula below to compute pres-
sure ratio:
PR = (DP in PSIG)+14.7
(SP in PSIG)+14.7
For the fi rst 3 minutes of operation, the micro will not
energize the solenoid. After 3 minutes of operation. If
the step of loading is above step 60 and the PR>2.2, the
micro energizes the economizer solenoid.
Once on, the solenoid will remain on until the PR<2.0
or the step of loading falls below 50. If the conditions
drop below either of these points, the micro will turn
off the solenoid.
Economizer cycling is reduced by 2 timers. The timer
with the longest remaining time will dictate when the
economizer can turn on. The fi rst timer is an "on to on"
timer which assures at least 10 minutes elapses from the
time the economizer turns on, off, and then on again for
a second time. The second timer assures that a minimum
of 3 minutes elapses from the time the economizer turns
off to the next time it is called to turn on again.