York Millennium YCAS 310, Installation Operation & Maintenance

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YORK INTERNATIONAL
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viation from the Target temperature. The duration of
the pulse will be a function of the deviation from Target
temperature, rate of change of leaving chilled liquid
temperature, suction pressure, discharge pressure, and
% load current. Pressure is a factor since the amount
of slide valve movement for a given pulse is depen-
dent on the discharge pressure. Motor current is a fac-
tor to assure the slide valve position changes do not
cause the compressor to exceed the current limit un-
load point. For the first two and a half minutes of com-
pressor 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 sec-
onds, 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 fails,
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 tem-
perature and the rate of change exceeds the internally
programmed 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 if fully unloaded,
when cooling demand falls, it will cycle off when leav-
ing liquid temperature drops below the low limit of the
Control 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
unload. If the second lag compressor shuts down ac-
cording 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 liquid 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.
6.2B 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 tem-
perature leaving the chilled is divided into two distinct
levels, control of the temperature after the flow from
both evaporators has been mixed and control of the
temperature 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-
perature, called the Panel Target Temperature. This is
performed by programing the acceptable deviation or
range of leaving temperatures above and below both
Target Temperatures. The tops and bottoms of the
ranges (Control Range, CR) are called the High and
Low Temperature Limits. The Target Temperatures are
midway between these programmed limits.
The Mixed Temperature Limits are programmed first
based on the desired Target Temperature for the “chiller”
application. For example, if the desired Mixed Target
Temperature is 46.0°F and the allowable deviation from
the temperature is +/– 2.0°F, then the programmed
acceptable Mixed Temperature Low and High Limits
are 44.0°F and 48.0°F respectively. This can be view
pictorially as follows: