York YCAV0267E, Installation Operation & Maintenance

Page: 214 / 350

214
JOHNSON CONTROLS
FORM 201.21-NM3 (616)
STANDARD IPLV CAPACITY CONTROL (CON'T)
(Loading/Unloading and starting additional compressors)
To avoid overshoot or nuisance trips on the low chilled
liquid cutout, when the temperature is below the Setpoint
– CR/2, the Chiller Control Board micro will reduce
the speed of the compressor(s) to unload the chiller by
2.0 Hz every 2 seconds. If temperature drops to within
1.0°F above the Low Chilled Liquid temp Cutout, the
Chiller Control Board microprocessor will unload the
compressors at the rate of 4.0 Hz every 2 seconds.
As the temperature rises the micro’s fuzzy logic will
factor in the rate of change before continuing to unload.
If the rate of change is rising too fast and there is poten-
tial for a positive overshoot, the Chiller Control Board
microprocessor may elect not to continue to decrease
speed.
In cases where temperature is rising too fast, when tem-
perature is within the desired CONTROL RANGE, the
Chiller Control Board microprocessor will be required
to make decisions regarding speed changes under condi-
tions where the “error” and “rate” conflict. For example,
the Chiller Control Board microprocessor may elect to
increase the speed of the compressor(s) if the error is
“0” (temperature is at Setpoint), while the rate of change
of chilled liquid temperature is “positive” (rising). The
Chiller Control Board microprocessor may also elect to
hold capacity when error is “negative” (temperature is
below Setpoint) because the rate of change of chilled
liquid is “positive” (rising). Table 4 illustrates these
conditions and the loading response from the Chiller
Control Board microprocessor.
TABLE 4 - FUZZY LOGIC LOADING/UNLOADING
VS. ERROR
NEGATIVE
ERROR
ZERO
ERROR
POSITIVE
ERROR
NEGATIVE
RATE
UNLOAD UNLOAD HOLD
ZERO
RATE
UNLOAD HOLD HOLD
POSITIVE
RATE
HOLD LOAD LOAD
HOT WATER STARTS
On a hot water start under "best" case conditions, as-
suming power has not been removed and the 120 second
timer does not inhibit starting, the design of the control
algorithm for a 2 compressor Standard IPLV leaving
chilled liquid capacity control allows full loading of a
chiller in slightly more than 14-1/2 minutes, regardless
of the number of compressors. This time period assumes
load limiting does not affect the loading sequence and
the ambient is above 40°F.
LAG COMPRESSOR OPERATION IN
LOAD LIMITING
When a single compressor is operating in current,
discharge pressure, suction pressure, VSD internal
ambient, or VSD baseplate temperature limiting for
more than 5 minutes and chilled liquid tempera-
ture is > Se CR, the Chiller Control Board
microprocessor will turn on the lag compressor to bring
the chilled liquid temperature within the CONTROL
RANGE. After 1 hour the Chiller Control Board micro-
processor will shut down the lag compressor and attempt
to control temperature with only the lead compressor to
satisfy the load.
OPERATION