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S S e e q q u u e e n n c c e e o o f f O O p p e e r r a a t t i i o o n n s s
the pump can provide the minimum flow required
through the lead chiller (A high-quality pressure-
independent valve is recommended for this bypass
to provide accurate bypass control regardless of the
system pressure differential between the supply
and return headers).
6. Once this minimum flow is established and the
system demand (based on leaving water/glycol
temperature) indicates that there is a requirement
for cooling, the lead compressor of the lead module
will energize provided all safeties of that
refrigeration circuit are satisfied.
7. Once there is a system load, the building
management system will modulate the system
bypass decreasing the bypass flow, as the flow rate
through the fan coils increases. This control is
provided by the customer external of the chiller but
is presumed to be based on the temperature of the
water back to the chiller or the pressure differential
across the most remote fan coil unit if it has a three-
way valve.
8. As the system demand continues to increase, the
second compressor within that module will
energize provided all safeties on that circuit are
satisfied.
9. As the system demand continues to increase, the
leaving water/glycol mixture temperature from the
chiller system will slowly increase until the
differential set point is reached. When the
differential is reached, a second module is brought
on-line. The pump VFD control must be set to ramp
up quickly to provide adequate water flow to both
operating modules and eliminate nuisance alarms
such as low flow and low pressure if there is not
sufficient flow to the operating modules.
10. As the flow and demand continue to increase, and
the temperature once again reaches the set point
plus differential setting, the second compressor on
the second module will energize. This operation
continues with all subsequent chiller modules as
needed based on system demand.
11. On a decrease in system demand (and flow) such
that the leaving water/glycol mixture temperature
reaches the set point minus the differential, a
compressor will de-energize. Once both operating
compressors within an operating module have de-
energized, the modulating valve will slowly close
off flow to that module. It is critical that the
corresponding pump variable frequency drive
controller slowly ramp down the speed of the pump
to the next operating point so as not to limit flow to
the operating chillers as the valve time to close is
approximately 150 seconds.
46
12. This process occurs throughout the operating range
of the chiller system. At all "in-between" stages of
system capacity and demand balancing, the digital
scroll compressor on the primary module will
modulate its capacity within the temperature
control band limits to maintain the leaving water
temperature control as tight as possible and to
minimize the amount of compressor cycling.
13. Should the primary microprocessor fail, the
remaining modules will continue to operate in
response to demand. The secondary modules in the
chiller system each have a "distributed primary"
microprocessor that allows the system to keep
operating should there be a failure of the primary
microprocessor. The only function the distributed
primary cannot perform is the automatic rotation of
modules as described in step 2.
14. When a redundant primary microprocessor
controller is provided on a secondary chiller
module, the redundant primary microprocessor
will, without downtime and without any operational
deficiency, perform the primary microprocessor
functions should it fail. All above operational
sequences are uninterrupted and uninhibited
should the primary microprocessor fail.
15. If power is removed and then returned to a chiller,
the compressors will restart every five seconds for
the next five minutes as needed. In a power return
restart, once all the compressors have restarted
every five seconds as needed within five minutes,
any subsequent cycling of compressors will occur
every five minutes.
16. If power is removed from the chiller to take the
chiller offline and then restored later, compressors
will restart every five seconds as needed if the
chilled fluid temperature is warm enough to fall
within the start set points of each compressor and
warrant the need for cooling. If the fluid is cold and
not within the temperature set point range, no
compressor will start. If no compressor starts within
five minutes, compressors will subsequently turn
on every five minutes.
17. The quick starting of compressors occurs:
a. After a power outage and the chiller comes back
on line.
b. When chiller is off using the ON/OFF button and
it is pressed to power on.
c. When the chiller is off via the remote START/
STOP and is turned on by remote START/STOP.
d. When the chiller is off via the BMS integration
and the BMS turns the chiller on.
ARTC-SVX001B-EN
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