Dunham-Bush HRSC - D, Operating & Maintenance Instructions

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A formula for calculating ramp time in minutes is
T = [(# compressors x 100) - start pt.] x Compressor
Control Interval/(Ramp rate x 60)]
The compressor control interval can be determined by
timing the frequency of load/unload pulses given to a
compressor. It is typically 2 seconds.
4.5. Manual Lead/Lag Selection
The lead compressor can be selected by the lead
B setpoint. The number stored in this setpoint
should be one less than the desired compressor
number. So a 0.0 in the setpoint specifies com-
pressor 1 as the lead, a 1.0 puts compressor 2 as
lead, etc. It is preferable to have compressor 1 as
the lead. If a compressor is locked off, the next
compressor will automatically take its place in the
lead/lag sequence.
4.6. Compressor Start Delay
A compressor start delay of one or two minutes is
incorporated into the computer to prevent two
compressors from starting at the same time and
insure that the system load requires another com-
pressor. The compressor control point will display
COFF (“clock off”) during this timing.
4.7 Power Loss Reset
This function allows the customer the option of
automatic or manual reset of the computer after
a power loss to the computer. The power loss
could be from the UVR or control switch. For au-
tomatic reset, a 0.0 is stored in the power in the
power loss B setpoint. For manual reset after
power failure, a 1.0 is stored in the set- point. A
power loss alarm is also recorded by the computer.
4.8. Anti-Recycle Timing
At the start of a compressor, a 15 minute timer is
initiated during which the compressor cannot start.
If cooling is called for during this time, a COFF is
displayed in the compressor control point.
CAUTION : This timer is bypassed by manual
control of compressors. DO NOT manually start
a compressor more than once every 15 minutes.
For an air-cooled unit, the time delay is two min-
utes when ambient temperature is above 40°F. If
ambient temperature drops below 40°F, the delay
on compressor start up is extended to 4 minutes.
After start-up, the low pressure delay is 2 min-
utes.
4.10 High Pressure Safety
If the condensing pressure of a refrigeration circuit
exceeds the high pressure A setpoint, all compres-
4.9. Low Pressure Safety
When the evaporator pressure of a given refrig-
eration circuit drops below the low pressure B
setpoint, a time delay is initiated. If the pressure
stays below the setpoint during the time period,
all compressors on the circuit will be locked off
and the low pressure alarm will be recorded by
the computer.
4.11 Freeze Safety
If the leaving water temperature of a chiller drops
below the freeze B setpoint, all of the compressors
will be locked off. The freeze alarm will be recorded
by the computer. Press RESET to clear the lockout.
4.12 High Oil Temp Safety
If the oil temperature switch of a compressor opens,
the oil temperature digital input turns ON. If the
compressor is operating, the compressor will be
locked off. The high oil temperature alarm will be
recorded by the computer and RESET must be pressed
after the problem is resolved.
4.13 Alarm Output
The alarm control point closes the digital output
relay on all safety alarms. This relay is used to turn
an alarm light on. After the problem is resolved,
press RESET to reset the system and turn the alarm
output off.
4.14 Fan Control
The fans are controlled by the microcomputer out
put relays. The computer switches fans on or off
basel on the discharge pressure and the fan
pressure setpoints. The high (A) setpoints are
where the fans turn on and the low (B) setpoints
are where the fans turn off.
4.15 Chilled Water Reset (Optional) and Customer Con-
trol Interlock
If an analog input is available, the chilled water tem-
perature setpoint can be raised automatically by a
0-5 VDC signal provided by an external controller.
The reset signal must be between 0VDC and 5VDC,
with 0VDC being no reset and 5VDC being maxi-
mum reset. The maximum temperature reset (in-
crease) desired must be stored in CWR max B
setpoint. For example, to raise the chilled water
setpoint from 44°F to 50°F (6.0°F) with a 5VDC in-
put, a 6.0 is stored in CWR max setpoint.
sors on the circuit are locked off. The high pressure
alarm will be recorded by the computer. Typical
setpoint value is 360 psig.
CAUTION: The voltage input must never exceed
5.0VDC.
Control contacts from an external controller are
used to enable or disable operation of compres-
sors. The wiring diagram specifies the terminals to
which the contacts must be wired. To enable the
compressors, the contacts must be closed. To
place the unit in stand-by mode, open the control
contact.
Note : This control must be used for automatic
control of the package. Do not use the flow
switch to control the package.
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