Dunham-Bush WCFX-E Series, Manual

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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.