Dunham-Bush WCFX-E Series, Руководство пользователя

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TYPICAL SEQUENCE OF OPERATION
The Dunham-Bush Water Cooled Screw Flooded Chiller
depends mainly on its on-board controller for control.
Operation described is for a two- compressor unit and is
very similar for single or three-compressor units.
For initial start-up, the following conditions must be met:
D
Power supply to unit energized
D
Compressor circuit breakers in the "on" position
D
Control power switch on for at least 15 minutes.
D
Compressor switches on
D
Critical alarm is cleared at display panel
D
Chilled water pump running and chilled water flow
switch made
D
Leaving chilled water temperature at least 2°F above
setpoint
D
All safety conditions satisfied
After all above conditions are met, the controller will call
for the lead compressor and the condenser water pump
to start. After a one-minute delay, the first contactor (e.g.
1 M-1) is energized followed by the second contactor
(e.g. 1 M-2) after one second time delay. This provides
reduced inrush stepped start.
The compressor 15-minute anti-recycle timer is initiated
at compressor start.
The controller monitors compressor amps, volts, leaving
water temperature, and evaporator and condenser
pressures. The compressor cooling capacity is controlled
by pulsed signals to load and unload solenoid valves on
the compressor. When the compressor starts, it is fully
unloaded, about 25% of its full load capacity. As the
controller
gives it load signals, capacity gradually
increases. The rate of compressor loading is governed by
ramp control which is adjustable in the computer.
The controller responds to leaving chilled water
temperature and its rate of change which is proportional
and derivative control. If leaving chilled water temperature
is within the deadband (+/-0.8°F from setpoint), no load or
unload commands are given. If chilled water temperature
is above deadband, the controller will continue loading
the compressor until a satisfactory rate of decline is
observed. If leaving chilled water temperature is below
the deadband, the compressor is commanded to unload.
Thus the compressor capacity is continuously modulated
to match applied load and hold leaving chilled water
temperature at setpoint.
If the applied load is greater than one compressor can
handle, it will load fully and then the controller will call for
a second compressor. After one minute, the second
compressor will start in the same manner as the first.
Then both compressors will be commanded to adjust load
to 50%. They are gradually loaded up together until the
applied load is satisfied. In this way the two compressors
share the load equally.
If the applied load decreases to the point that both
compressors are running at about 40% capacity, the
controller shuts down the lag compressor and loads the
remaining compressor to about 90%. If applied load
decreases further, the remaining compressor unloads-
proportionally. If applied load decreases to less than the
minimum capacity of one compressor, the leaving chilled
water temperature will decline to 2°F below setpoint, at
which time the lead compressor will shut down. It will
restart automatically if leaving chilled water temp rises to
2°F above setpoint and both 15 minute anti-recycle and
one minute start delay timers are satisfied.
During operation, the controller monitors the difference
between condenser and evaporator pressures to insure
that a minimum of 30 psi differential is available for
compressor lubrication. If the difference falls below a
minimum of 30 psi, the controller closes refrigerant flow
control valves, starving the evaporator, causing
evaporator pressure to drop, increasing differential
pressure. This is especially helpful at startup, when warm
chilled water and cold condensing water would cause a
low head situation. This feature is called EPCAS:
Evaporator Pressure Control at Startup. It is one of
several proactive control features of the controller which
overcome potential problems while continuing operation.
Two additional proactive features are low suction and
high discharge pressure override. If operating pressures
approach trip level, compressors are unloaded as
necessary to continue operation.