Product specifications

44
TYPICAL SEQUENCE OF OPERATION
The following sequence of operation describes a four-
compressor scroll packaged chiller. Operation is very similar
for a two-compressor unit.
For initial start-up, the following conditions must be met.
•All power to the package be supplied for 24 hours prior
to starting a compressor.
•Control power switch on for at least 5 minutes.
•Compressor switches on.
•All safety conditions satisfied.
•Reset pressed on the microcomputer keypad.
•Chilled water pump running and chilled water flow
switch made.
•Customer control contact closed or unit enable switch
in the “ON” position.
• Leaving chilled water temperature higher than water
temperature setpoint plus a deadband setpoint called
“Control Zone +”.
After all above conditions are met, the microcomputer will
call for compressor #1 to start. When feedback to the
compressor #1 status sensor input confirms that the
compressor has started and pumpout is complete, liquid
line solenoid #1 is energized. The first stage of capacity is
now on-line.
As discharge pressure of compressor #1 rises, fan #1 turns
ON at the “Fan Stage 1 ON” setpoint. If discharge pressure
continues to rise, the subsequent odd-numbered fans will
stage ON in increments of the “Condenser Differential_ON”
setpoint. For example, if the “Fan Stage 1 ON” is 190 psig
and the “Condenser Differential_ON” setpoint is 20, the
stage on points will be 190, 210, 230, etc. The
microcomputer may automatically increase these settings
if short cycling of fans is detected.
If discharge pressure falls, the odd-numbered fans will
stage OFF at the “Fan Stage 2 OFF” setpoint plus
corresponding number of “Condenser Differential_OFF”
setpoints. For example, if the “Fan Stage 2 OFF” is 140
psig and the “Condenser Differential_OFF” setpoint is 10,
the stage off points will be 140, 150, 160, etc.
After a minimum interstage delay of approximately one
minute, and if water temperature is not falling at a faster
rate than the value stored in the “MAX_SLOPE-” setpoint,
and the leaving water temperature is greater than the
temperature setpoint plus “Control Zone +” setpoint, the
microcomputer will call for compressor #3 to start.
However, if leaving water temperature is falling at a faster
rate than the value stored in the “MAX_SLOPE-” setpoint,
no more stages of capacity will be added at this time.
When feedback to the compressor #3 status sensor input
confirms that the compressor has started and pumpout is
complete, liquid line solenoid #2 is energized. The second
stage of capacity is now on-line.
As discharge pressure of compressor #3 rises, the even
numbered fans are activated according to the fan stage
setpoints as described above for circuit #1 fans.
The third and fourth stages of unit capacity will occur
when the following conditions are met:
1. Minimum interstage time delay on increasing load of
approximately 1 minute has expired.
2. Leaving water temperature is not falling at a faster rate
than the value stored in the “MAX_SLOPE-” setpoint.
3. Leaving water temperature is greater than the water
temperature setpoint plus “Control Zone +”.
After all above conditions are met, the microcomputer will
call for compressor #2 to start. The microcomputer than
confirms that compressor #2 has started by its feedback
to the compressor #2 status sensor input. The third stage
of capacity is now on-line.
As the load continues to increase and the conditions
described above are met, the microcomputer will call for
compressor #4 to start. After compressor #4 is
commanded to start, the microcomputer confirms that
compressor #4 has started by its feedback to the
compressor #4 status sensor input.
As the applied load decreases and the supply water
temperature falls below the water temperature setpoint
minus a deadband setpoint called “Control Zone-” stage
4 is turned off. Compressor #4 turns off.
If supply water temperature continues to fall below water
temperature setpoint minus “Control Zone-” setpoint,
stage 2 is turned off. Liquid line solenoid #2 is turned off.
When compressor #3 and 4 suction pressure falls below
the pumpdown-cutout setpoint, compressor #3 is turned
off, and the even-numbered fans are turned off. The unit
is now at 25% capacity. Note that if there is more than
one compressor on a refrigerant circuit, only the last
compressor to shut down will perform the pumpdown.
Stage 1 will shut down in a similar manner to stage 2
mentioned above.
When a refrigerant circuit is cycled off, a one-time
pumpdown of that circuit is performed. When suction
pressure falls below pumpdown-cutout setpoint, the
compressor will shut down.
Two proactive control features included in the
microcomputer are low suction and high discharge
pressures unload. If there is more than one compressor
operating on a refrigerant circuit, a compressor will be
cycled off if that circuit’s discharge pressure exceeds the
high pressure unload setpoint or if the suction pressure
approaches the low pressure trip setpoint. The cycled off
compressor will remain off for a duration of time as
specified in the “SAFETY DELAY” setpoint.
MICROCOMPUTER CONTROLLER