Quantech QTC3040T, Installation Operation & Maintenance

Page: 157 / 190

157
QUANTECH
FORM QTC3-NM1
ISSUE DATE 08/03/2022
8
UNIT OPERATING SEQUENCE
The operating sequence described below relates to op-
eration on a hot water start after power has been ap-
plied, such as start-up commissioning. When a com-
pressor starts, internal timers limit the minimum time
before another compressor can start to 1 min.
1. For the chiller system to run, the Flow Switch
must be closed, any remote cycling contacts must
be closed, the Daily Schedule must not be sched-
uling the chiller OFF, and temperature demand
must be present.
2. When power is applied to the system, the micro-
processor will start a 2 min timer. This is the same
timer that prevents an instantaneous start after a
power failure.
3. At the end of the 2 min timer, the microprocessor
will check for cooling demand. If all conditions
allow for start, a compressor on the lead system
will start and the liquid line solenoid will open.
Coincident with the start, the anti-coincident tim-
er will be set and begin counting downward from
“60” s to “0” s.
If the unit is programmed for Auto Lead/Lag, the
system with the shortest average run-time of the
compressors will be assigned as the “lead” sys-
tem. A new lead/lag assignment is made whenever
all systems shut down.
4. Several seconds after the compressor starts, that
systems first condenser fan will be cycled ON
(outdoor air temperature more than 25°F (-4°C)
or discharge pressure). See Standard Condenser
Fan Control on page 162 for details concerning
condenser fan cycling.
5. After 1 minute of compressor run time, the next
compressor in sequence will start when a system
has to load. Additional compressors will be start-
ed at 60 s intervals as needed to satisfy tempera-
ture setpoint.
6. If demand requires, the lag system will cycle
ON with the same timing sequences as the lead
system after the lead system has run for five min -
utes. See the section on Capacity Control for a
detailed explanation of system and compressor
staging.
7. As the load decreases below setpoint, the com-
pressors will be shut down in sequence. This will
occur at intervals of either 60 s, 30 s, or 20 s based
on water temperature as compared to setpoint, and
control mode. See
Leaving Chilled Liquid Control
on page 158
for a detailed explanation.
8. When the last compressor in a “system” (two or
three compressors per system), is to be cycled
OFF, the system will initiate a pump-down. Each
“system” has a pump-down feature upon shut-off.
On a non-safety, non-unit switch shutdown, the
LLSV will be turned OFF and the last compressor
will be allowed to run until the suction pressure
falls below the suction pressure cutout or for 180
s, whichever comes first.
CAPACITY CONTROL
To initiate the start sequence of the chiller, all run per-
missive inputs must be satisfied (flow/remote start/stop
switch), and no chiller or system faults exist.
The first phase of the start sequence is initiated by the
Daily Schedule Start or any Remote Cycling Device. If
the unit is shut down on the daily schedule, the chilled
water pump contacts (Terminals 23 and 24 of XTBC2)
will close to start the pump when the daily schedule
start time has been reached. Once flow has been es-
tablished and the flow switch closes, capacity control
functions are initiated, if the remote cycling contacts
wired in series with the flow switch are closed.
It should be noted that the chilled water pump contacts
(Terminals 23 and 24 of XTBC2) are not required to
be used to cycle the chilled water pump. However, in
all cases the flow switch must be closed to allow unit
operation.
The control system will evaluate the need for cooling
by comparing the actual leaving or return chilled liq-
uid temperature to the desired setpoint, and regulate
the leaving or return chilled liquid temperature to meet
that desired setpoint.
SECTION 8 – UNIT OPERATION