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the one operating module. The control of the variable
primary pumps must be based on the differential
pressure measured across the most remote fan coil in
the hydronic system and not the pressure differential
across the chillers (the evaporator in each module
contains a fine mesh strainer which can skew
differential pressure readings). The VFDs allow the
pumps to deliver the required flow through each
operating module.
4. The electronic isolation valve of the lead chiller module
identified in step 2 is already energized, therefore, the
system pump must produce the minimum flow required
by the lead chiller module. A system bypass with
modulating valve is provided by the customer and
installed external of the chiller at the most remote fan
coil or other suitable remote location from the chillers (a
high-quality pressure independent valve is
recommended for this bypass so as to provide accurate
bypass control regardless of system pressure
differential between supply and return headers).
5. Once there is a system load, the BMS modulates the
system bypass valve, decreasing the bypassed flow, as
the flow rate through the fan coils increase. This control
is provided by the customer external of the chiller and is
presumed to be based on the pressure differential
across the most remote fan coil or user.
6. Once the minimum flow is established and the system
demand (based on leaving fluid temperature) indicates
that there is a requirement for cooling, the lead
compressor of the lead module energizes provided all
safeties of that refrigeration circuit are met.
7. As the system demand continues to increase, the
leaving fluid temperature from the chiller slowly
increases until the differential set point is reached (set
point plus 4°F). The second compressor in the lead
ARTC-SVX008A-EN
module energizes, provided all safeties on that circuit
are met.
Note: Regardless of demand, compressors will not turn on
within 5 minutes of one another to prevent excessive
compressor cycling. Each compressor runs for a
minimum of 5 minutes regardless of the system
leaving fluid temperature.
8. As the cooling demand continues to increase, the
leaving fluid temperature from the chiller system slowly
increases. The first compressor on the lead
refrigeration circuit from the second module in the
starting sequence will be brought on-line.
9. When the leaving fluid temperature once again reaches
the set point plus differential setting, the second
compressor on the second module energizes.
10. With increased demand, the third module's lead
compressor and then its second compressor - as
necessary - will turn on but not less then 5 minutes
apart. This process occurs throughout the operating
range of the chiller system.
11. On a decrease in system cooling demand such that the
leaving fluid temperature reaches the set point minus a
differential (1°F below sent point), compressor(s) de-
energizes in the reverse order that they turned on after
all timers have been satisfied. The electronic valve on
non-operating modules closes.
12. Regardless of demand and leaving fluid temperature,
there must be no less than 5 minutes between
successive compressor(s) de-energizing to prevent
excessive compressor cycling.
13. The microprocessor provides a new lead chiller module
once a week to even the run time between the
modules.
Sequence of Operations
53
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