Flooded type water cooled screw chiller (PCB Control)
MCAC-CTSM-2012-11
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valves, whereas the linear one uses two NC (normal close) solenoid valve to control the increase or
decrease of loading.
When starting compressor, SV1 is energized to bypass the oil in hydraulic cylinder back to the
low-pressure suction end while SV0 is de-energized. Slider remains in its initial position due to the
spring force, and then the compressor can be sure to start at 25% loading. Once the startup
process is completed, SV0 is energized while SV1 is de-energized to increase the loading up to
100%.
To keep compressor running in steady state, SV0/SV1 is de-energized continuously to maintain the
stable refrigeration capacity output. Once loading has been changed, the system energizes
de-energizes of SV0 and SV1 to adjust output of compressor in order to fit actual loading
requirement.
When loading increases, SV0 energizes shortly to allow small amount of oil to flow into hydraulic
cylinder and force slider to move in the direction of increasing refrigeration capability. If loading
decreases, SV1 energizes shortly to allow small amount of oil to flow out of hydraulic cylinder and
cause slider to move in the direction of decreasing the refrigeration capability.
2. Description of cooling tower fan control
For Midea screw chiller, it adopts professional PCB controller to maintain the saturated condensing
temperature to a configurable set point. Cooling tower fan is controlled by cooling leaving water
temperature. During compressor startup and operating progress, cooling tower fan is OFF when
cooling leaving water temperature <23°C. When cooling leaving water temperature >25°C, cooling
tower fan is ON. When cooling leaving water temperature is between 23~25°C, cooling tower fan
keeps in initial state.
3. Description of Oil heater control
In unit shutdown period, oil heater is energized to keep normal oil temperature. When unit starts, it is
turned off.
Oil heating time limits:
When the PCB receive “Unit start” signal, the program begin to detect oil temperature.Only
when oil temperature
≥
Oil setting temperature, the unit can start.
4. Pump control
Water pump mode
When user has selected water pump mode, PCB receives operation signal and turn on
chilled/cooling water pump without starting the compressor. Chilled/cooling water flow switch is valid
and its detection method is the same with normal startup procedure.
In pump mode, if user chooses cooling mode when chilled water temperature is low and not enough
to start the chiller, both chilled/cooling water pumps keep on running until leaving chilled water
temperature condition gets satisfied.
