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Sequence of Operations
The Chiller Sequence of Operations (SOO) has two
components.
1. Main Sequence
2. Supplementals Sequences
Main Sequence
When chiller starts from Idle state.
1. Enable signal sent from BAS either by:
a. Setting a Normally Open (NO) digital input on chiller
control to true.
NO open contact needs to close to disable. This
provides an emergency backup function where the
loss of the BMS and resulting loss of signal enables
the chiller.
b. BMS can set chiller to enable via BMS interface too.
2. Chiller is set to Cooling Enable.
3. If the chiller has control, pumps are started, and
isolation valves are opened or . Otherwise, chiller waits
for flow.
a. If no flow – annunciate No Flow Alarm.
b. Continue to wait for flow.
4. If the LCHWT is greater than or equal to the chiller
temperature setpoint plus the Low Urgency Zone (LUZ)
setpoint for the LUZ time (default – 30 seconds), start
up conditions are met and there are compressors
available to run, the chiller is set to Start state.
5. The lead available compressor(s) interlock is closed,
and 65 percent (settable) demand is sent.
a. Compressor priority is either:
•
Compressor order (left to right), or
•
Determined by runtime
•
User Settable
b. Compressor state set to Starting.
Note: The T3C application has a Delta Start parameter
the examens LCHWT against Setpoint. It divides
the difference by the parameter generating an
integer compressor start count. So, on chiller
start with LCHWT of 74°F, a Setpoint of 62°F and
the Delta Start parameter is 4, the chiller control
takes the difference (74 - 62 = 12) and divides by
the parameter (12 ÷ 4 = 3) and automatically
starts 3 compressors (if available) on the Chiller.
6. Once the selected compressor(s) is(are) started up, the
compressor and chiller states become Operational. The
chiller controller drives the chiller to setpoint using an
embedded PI-Loop.
7. If the chiller, enters the LUZ then control is by PI-Loop.
8. If chiller is above LUZ and power consumption above
settable threshold for the LUZ time, chiller enters
ARTC-SVX013A-EN
Stage-In. This state has the following steps:
a. Demand is reduced to the Running compressors
until stage in Pressure Ratio is achieved.
Compressors state is Retreating. Once PR
achieved State is Hold.
Note: On Pressure Ratio chiller start, there is a
setting in the control that monitors the
running PR. It varies by configuration and
type of chiller. It is designed to keep
compressor on the same refrigerant circuit
from disabling each other. Since TCAA
design dictates that compressors do not
share refrigerant circuits, this value is 4.0 by
default. This means that under normal
circumstances the TCAA chiller should not
display Retreating State and go directly to
Hold.
b. While maintaining Hold on running compressors,
next priority compressor is marked Start and is
send start demand.
c. When start up conditions on new compressor are
met, the chiller and all running compressors are
marked Operational.
9. If chiller drops below LUZ for the LUZ time, the chiller
will want to Stage-Out.
a. To stage out there must be more than one
compressor running. Priority compressor is sent a
demand of 0 percent and is marked Stopping.
When compressor has reached 0 rpm and IGV is
back to idle position the compressor is marked idle
and the chiller returns to Operational.
b. If only one compressor is running and the PI-Loop
cannot maintain Setpoint at minimum demand and
there is no Hot Gas Bypass Valve (Load Balancing -
HGBV), the chiller goes to Shutdown, the last
compressor is sent 0% demand and is marked
Stopping.
c. If chiller equipped with HGBV then the HGBV valve
opens, introduces artificial load to the system and
tries to maintain setpoint without cycling off the
chiller.
10. After shutdown complete, chiller and compressors are
marked Idle.
Supplemental Sequences
This section discusses control sequences that work in
conjunction with the main sequence or may work in the
background. These fall into three categories:
1. Device Sequences
2. Chiller protection
3. Optional Sequences
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