Dunham-Bush HRSC - D, Operating & Maintenance Instructions

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4.16 Hot Gas Bypass Control (option)
When hot gas bypass has been supplied with
the package, an output from the computer
controls the solenoid. The solenoid is turned on
if only one compressor is operating and the
target percent capacity of the compressor
drops below the hot gas bypass B setpoint. If
the target percent capacity then climbs above
the hot gas bypass A setpoint, the solenoid is
turned off. Typical setpoints are 45% for the B
setpoint and 80% for the A setpoint.
4.17 Compressor Power Control (No-Stop Alarm) and
Unit Schedule
The control power/schedule feeds power to the
compressor safety and control circuit and closes
the chilled water pump contacts. (See Field
Control Wiring). After power-up, the relay is
held open for 15 seconds to allow for reset of
the high temperature safety switch. The relay is
then closed to allow normal compressor control
if the unit schedule is calling for unit operation.
The computer will open this relay if it detects
that a compressor auxiliary contact digital input
stays ON for 15 seconds when the computer is
commanding the compressor to be off. A No-
Stop alarm would then be stored in the alarm
history. This would indicate that either a hard-
ware problem is forcing the compressor to run
when it should not or that the digital input is
staying on when the compressor is actually off.
When the problem is resolved, press the RESET
button to allow the machine to start.
During normal operation, this output turns ON/
OFF based on the unit schedule of operation
(see Section 3.9).
4.18 Sensor Alarm Shutdown
If the computer measures an analog sensor
value that is outside of a given boundary, the
associated compressors are shutdown. The
computer then stores the alarm code corres-
ponding to the sensor alarm. The boundary
values are as follows:
Low High
1. Water temp sensor - 190°F
2. Air temp. sensor 25°F 180°F
3. Suction pressure -20 psig See below
4. Discharge pressure 5 psig -
The high setpoint for the suction pressure sensor will
be triggered if the leaving water temperature is less
than 60°F and suction pressure is greater than 150
psig.
If one of these sensor alarms occurs, it usually indi-
cates a loose wire, blown fuse, defective sensor, or
faulty analog board.
4.19 External Shutdown Indication (No-Run Alarm)
A No-Run control point error is generated if the
computer tries to start (or run) a compressor but the
compressor is held off by an external control. The
computer determines that the compressor is
16
not operating if the associated digital input
from the compressor contactor does not
indicate that the contactor is pulled in (digital
input stays OFF), or if the amps of the com-
pressor are less than about 12 amps. If a
faulty No-run alarm occurs, monitor the digital
input and amps to see which is producing the
fault.
4.20 Fast Unload Solenoid Control
When a compressor starts, it’s fast unload
solenoid is energized for 30 seconds to ensure
that the compressor unloads fully. The load
and unload, solenoids are de-energized during
this time delay. After the time delay, normal
control of the load/unload solenoids is re-
stored.
4.21 Low Suction/Discharge Differential Alarm
If the difference between suction and dis
charge pressure is less than minimum differen-
tial pressure for 3 minutes while a compressor
is operating, all the compressors on the
affected refrigerant circuit are locked off. A
low differential pressure alarm will be recorded
by the computer and the alarm output will be
turned on. Press RESET to clear the lockout.
4.22 High Motor Temperature Safety
If the high motor temperature switch of a
compressor opens, the motor temperature
digital input turns ON. If the compressor is
operating, the ‘compressor will be locked off.
The high motor temperature alarm will be
recorded by the computer and RESET must be
pressed after the problem is resolved.
4.23 Low Chiller Flow Alarm
A low flow alarm will be generated if a com-
pressor control point is ON and the flow
switch digital input is OFF. All compressors will
be locked off and the alarm light turned on.
After resolving the problem, press RESET.
4.24 High Discharge Pressure Unload
This function unloads the compressor if
discharge pressure is approaching the high
pressure limit. If discharge pressure of an
operating compressor reaches the high
pressure setpoint minus 10 psi, the compressor
will be prevented from loading (HOLD state). If
the discharge pressure reaches the high
pressure setpoint minus 5 psi, the compressor
will begin unloading.