
YORK INTERNATIONAL
106
Compressor Loading and Unloading
The micro loads and unloads individual compressor by
pulsing the slide valve solenoids which control oil flow to
the slide valve. The slide valve load solenoid applies oil
pressure to the slide valve which increases capacity. The
slide valve unload solenoid allows venting of oil pressure
which allows discharge pressure to move the slide valve
in the opposite direction, decreasing capacity.
Whenever chilled liquid leaving temperature is above
the Neutral Zone, loading pulses will be applied to open
the loading port on the control solenoid allowing oil pres-
sure to move the slide valve to increase capacity. Every
10-120 seconds, the micro will pulse the slide valve with
a 0.4-5 second pulse. The time between pulses will be
a function of the leaving liquid temperature deviation
from the Target temperature. The duration of the pulse
will be a function of the deviation from Target tempera-
ture, rate of change of leaving chilled liquid tempera-
ture, suction pressure, discharge pressure, and % load
current. Pressure is a factor since the amount of slide
valve movement for a given pulse is dependent on the
discharge pressure. Motor current is a factor to assure
the slide valve position changes do not cause the com-
pressor to exceed the current limit unload point. For the
first two and a half minutes of compressor operation,
no loading will occur.
Whenever temperature is below the Neutral Zone, un-
loading pulses will be sent to open the unloading port
on the control solenoid to relieve oil pressure on the
slide valve. This allows discharge pressure to move the
slide valve to decrease capacity. Every 6-70 seconds,
the micro will pulse the slide valve with a 0.4-5 second
pulse. The duration and length of the pulse will be a
function of the deviation from setpoint. The larger the
deviation, the longer the pulse. Limited loading/unload
may occur in the Neutral Zone, if internal rate control
limits regulating abrupt changes in leaving chilled liquid
are exceeded.
Compressor Shutdown Sequence
If the chiller is near full load and cooling demand falls,
the compressors will be unloaded equally until they
reach a programmed percentage of full load - see Sec-
tion 8.2 Lag Compressor Differential Off.
On 2 system chillers, if demand continues to fall, the
lead compressor will be maintained at constant load
and the lag compressor will be unloaded. If the lag com-
pressor reaches a fully unloaded condition, it will be shut
down under the following condiutions: a) chilled liquid
temperature drops below the Neutral Zone, b) chilled
liquid temperature drops below the Target temperature
and the rate of change exceeds the internally pro-
grammed rate limits. When the lag compressor shuts
down, the lead compressor loading is then adjusted to
match the cooling demand.
If only one compressor is running and is fully unloaded,
when cooling demand falls, it will cycle off when leaving
liquid temperature drops below the low limit of the Con-
trol Range.
On 3 compressor chillers, the process is the same as
for 2 compressor chillers except that when the mini-
mum level of equalized loading is reached, the lead and
first lag compressor are maintained at constant load
and only the second lag compressor continues to un-
load. If the second lag compressor shuts down accord-
ing to decreasing load and demand continues to fall,
then the first lag compressor will begin to unload and if
necessary shut down.
NOTE: A lag compressor may be shut down before it
is fully unloaded to avoid a Chiller fault on a
Low Water Temperature cut-out under the fol-
lowing conditions: a) if chilled liquid tempera-
ture falls below the low end of the Control Range
(CR) for more than 37 seconds, b) if chilled liq-
uid temperature drops more than
CR/4 below the low limit of the control range.
If demand continues to fall and the lead compressor is
the only compressor running, it will continue to unload
and shut down when leaving liquid temperature drops
below the low limit of the Control Range.
NOTE: The lead compressor may be shut down be-
fore it is fully unloaded to avoid a Chiller Fault
on a Low Water Temperature fault under the
following conditions: a) if chilled liquid tempera-
ture drops 2°F below the low limit of the Con-
trol Range (CR), b) if chilled liquid temperature
drops more than CR/2 below the low limit of
the Control Range.
CHILLED LIQUID TEMPERATURE CONTROL
YDAS MODELS
The four refrigerant systems on a YDAS chiller are di-
vided into two pairs, or groups, each having its own two
circuit evaporator. Control of the chilled liquid tempera-
ture leaving the chilled is divided into two distinct lev-
els, control of the temperature after the flow from both
evaporators has been mixed and control of the tem-
perature leaving the individual evaporators.
The Setpoints keys are used to program the chiller with
both the required mixed water leaving temperature for
the application, called the Mixed Target Temperature,
and with the individual evaporator leaving Target tem-
Содержание Millennium YDAS 410
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