70
(12) Control System Limits
The microprocessor control continuously monitors for
unsafe or unreliable operating conditions known as control
limits and safeties. If such a condition is detected the limit
control system can override Leaving Water Temperature
and concentration control requests. The override limit
would only occur when a control output (from Leaving
Water Temperature or concentration control) request is
greater than allowed by the particular limit schedule. The
limit control will then modify the output signal maintaining
chiller operation but at a reduced reliable capacity. The
control priority changes from leaving water control to the
particular limit or safety control. This allows continued
chiller operation possibly at a reduced capacity. The system
will adjust solution flow or energy input to maintain reliable
machine operation before reaching a shut off condition.
The microprocessor will shutdown the chiller if a condition
is reached that could cause damage to the machine.
When the limit or safety takes over the priority control, the
clear language display will indicate the active limit mode. A
limit mode does not necessarily mean that a problem
exists. In many cases this limit is a normal function such as
soft load during startup.
During cooling operation the microprocessor will
shutdown the chiller if the limit or safety reaches a
condition that could cause damage to the machine. Figure
21 identifies the various limit conditions that could override
Leaving Water Temperature and concentration control. The
input signal must pass through each window without
violating the conditions of each window to successfully
output the desired command.
Figure 21
Leaving
Water
Temperature
Signal
Conc
Signal
Control Limits
Control
Response
SDR.
control
ABS
entering
limit
Interstage
low or
high limit
Evaporator
Limit
Generator
solution
temperature
limit
Exhaust
Temperature
limit
ABDA
AFD
Control
Energy
Control
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