Table of Contents
Advertisement
the Load Timer has counted down to "0". The micro
will always pick the compressor with the lowest load
stage as long as it is available. In the event that both
compressors are on the same stage, priority will be cho-
sen according to lead/lag assignments.
Conditions that would prevent a compressor from load-
ing are:
• Compressor must not have loaded when the Load
Timer last counted to "0".
• The compressor must not be pumping down.
• The compressor must not be inhibited from loading
by anticipatory controls. NOTE: When a system is
inhibited from loading due to control limit exceeding
90% of the load limit thresholds, the micro will not
allow the respective system to load, or will force it
to unload, regardless of the need for cooling.
• Compressor run time is <60 seconds.
6.7 "FUZZY LOGIC" "LOADING"
ANTICIPATORY CONTROL
The fuzzy logic has the capability of decision-making
based on the rate of change of chilled liquid tempera-
ture. When the chilled liquid is within the control range,
the fuzzy logic may load the chiller, even though the
temperature is within the desired control range. This
ability allows the Micro to react to a rapid change in
temperature rise and attempt to keep the chilled liquid
temperature within the control range, rather than wait-
ing until the temperature rises above the control range
before loading takes place.
6.8 "LOAD LIMITING" ANTICIPATORY CONTROL
Anticipatory Controls are intended to prevent a system
from loading and reaching a point where a safety thresh-
old may be exceeded or forced unloading is required.
Anticipatory controls are checked by the micro each
time a system is to be loaded.
Three factors affect Load Limiting Anticipatory Con-
trol: Motor Current, Saturated Suction Temperature, and
Discharge Pressure. If any of the motor currents or
discharge pressures are greater than 90 - 99% of the
user programmed unload points, the micro will not load
the respective system, even if cooling demand requires
it. If saturated suction temperature is less than 110 -
101% of the internal limit of 4°C (24°F) for water chill-
ing or setpoint – 6°C (11°F) for brine chilling – the
respective system will not be allowed to load.
YORK INTERNATIONAL
Loading is once again permitted to occur when system
motor current or discharge pressure falls below 90% of
the user programmed unload points. The same is true
when saturated suction temperature rises above 110%
of the internal limits mentioned above.
There will be no display message to
inform the operator that a specific sys-
tem is load limited by the microproces-
sor anticipatory control.
6.9 COMPRESSOR UNLOADING AND
SHUTDOWN SEQUENCE
The micro will unload a compressor whenever the leav-
ing chilled liquid temperature falls below the low end of
the Control Range and the 30-second Unload Timer has
timed to "0". At this point, the micro will unload the
compressor with the highest stage of loading. If all com-
pressors are at the same stage of loading, the micro will
unload the lag compressor first.
If temperature remains below the low end of the Con-
trol Range, the micro will alternately unload the com-
pressors each time the Unload Timer reaches "0".
Should the fuzzy logic algorithm sense a continu-
ing trend toward unloading (temperature drops
quickly), the micro may set the unload timer to 15
seconds rather than 30. The decision to set the timer
to 15 or 30 seconds is determined by the rate of chilled
liquid temperature change.
If temperature remains below the low end of the Con-
trol Range and both compressors totally unload to Stage
"0", the micro will shut down the lag compressor when
the leaving chilled liquid temperature falls below the
"Setpoint Range/2" When a compressor shuts down due
to load, the micro will first close the liquid line solenoid
valve and allow the compressor to pump down to the
suction pressure cutout before stopping the compressor
(pumpdown on shutdown).
If load continues to drop, the lag compressor will shut
down after the chilled liquid temperature drops below
"Setpoint – Range". Before the micro shuts down the
lead compressor, it will turn the optional hot gas bypass
off and pump down the compressor.
FORM 150.52-NM2 (1001)
9
137
Hide quick links:
Advertisement
Table of Contents
