York YCAS0130 - Manual (Page 159)

159 YORK INTERNATIONAL
FORM 201.19-NM1 (204)
8
to the Setpoint Temperature. The amount of load ing is
var ied by changing the amount of DC Voltage signal to
the slide valve solenoid of each compressor. Voltage
increases with load (0 - approximately 9VDC at full
load).
Slide Valve Control
The slide valve of each compressor can be moved 75
steps, where “0” equals minimum capacity and fully
load ed equals 75 steps. The amount of movement that
occurs when the micro initiates changes may vary ac-
cord ing to the error or deviation from setpoint and the
rate of change of chilled liquid temperature. Each time a
change is made, the incremental change may vary from
1 to 10 steps as determined from the micro. In cases
where internal limiting is not in effect due to pos si ble
fault conditions, the micro will load the com pres sor with
the lowest number of steps, alternating loading back and
forth between compressors until both are fully load ed.
In some cases the micro will be required to make de-
ci sions regarding loading under conditions where the
temperature “er ror” and temperature “rate” confl ict. For
example, the micro may elect to unload a compressor
if the error is “0” (tem per a ture is at setpoint), while the
rate of change of chilled liquid temperature is negative
(falling). The micro may also elect to hold capacity when
error is “+” (tem per a ture is above setpoint) because the
rate of change of chilled liquid is “-”. Below is a chart
which illustrates these conditions.
Load Timers
Fixed timers are set to minimize undershoot and over-
shoot as a result of slide valve control.
• Load Timers are always set at 10 seconds be tween
changes.
• Unload Timers are set at 5 seconds between chang-
es.
Slide Valve Position
A slide valve position (S V STEP), under the keypad
system keys, of 75 indicates that the compressor is ful ly
loaded. However due to the non-exact movement of
the mechanism, a position less than 75, possibly 60,
could also mean that the compressor is fully loaded.
A compressor may also be fully unloaded at step 35
and below. Keep this potential indicator error in mind
when attempting to de ter mine slide valve position versus
actual com pres sor capacity.
Compressor Starting & Loading Sequence
If no compressors are running, the Daily Schedule
per mits, all safeties and run permissives are satisfi ed,
the anti-recycle timers have timed out, and the leaving
liq uid temperature rises above the upper range limit of
the Con trol Range, the lead compressor will be started.
A 0VDC signal is sent to the compressor slide valve
con trol solenoid to allow the internal spring to push the
slide valve to a minimum loading position to assure it is
fully unloaded at start. At the same time, the micro will
energize (open) the pilot solenoid on the EEV. After
an ini tial period of 15 seconds, the micro will begin to
load up the lead com pres sor to bring the chilled liquid
tem per a ture to setpoint.
After 5 minutes of run time, if leaving chilled liquid
temperature is not within the Control Range, the micro
will start the lag compressor. This is not dependent on
slide valve position which after 5 minutes will typi-
cally be fully loaded at a S V Step of “75”. The lead
compressor will be reduced in capacity to a slide valve
step of 40. The lag compressor will then be load ed until
it also reaches a slide valve step of 40 while the lead
compressor is maintained at a constant load. At this
point the compressors will be alternately loaded with
loading always occurring on the compressor with the
lowest slide valve step until the leaving chilled liq uid
is satisfi ed.
Compressor Loading
The micro loads and unloads individual compressors
by varying voltage to the Slide Valve solenoid which
con trols oil fl ow to the slide valve. The slide valve load
solenoid applies oil pressure to the slide valve to over-
come spring pressure from an internal spring, in creas ing
ca pac i ty. The in ter nal spring moves the slide valve in
the op po site di rec tion against oil pres sure to de crease
ca pac i ty.
ERROR
Negative Unload Unload Hold
Zero Unload Hold Load
Positive Hold Load Load
RATE
Negative Zero Positive