Honeywell AUTOMATIC CONTROL SI Edition, Engineering Manual

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ENGINEERING MANUAL OF AUTOMATIC CONTROL
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
313
DISSIMILAR MULTIPLE CENTRIFUGAL
CHILLERS CONTROL
When a multiple chiller system consists of chillers that are
different in capacity and/or characteristics, the control problem
becomes more difficult. The more dissimilarities between chillers,
the greater the potential error in using a fixed sequence and
changeover schedule. On-line computer analysis which takes into
account the many variations in chiller conditions and different
loading conditions can significantly improve efficiency.
For optimization of a multiple chiller system, the efficiency
curves (kW per kW vs load) for each chiller at all temperature
differentials (condenser water temperature vs chilled water
temperature) from minimum load to design must be known.
Condenser water, chilled water and return water temperatures
and flows, when appropriate, are used to calculate the optimum
chiller combination and loading for each chiller. If the decision
is to add or drop a chiller, minimum off and on time restrictions
for each chiller must be considered. If the on/off restriction
prevents adding the desired chiller, a second decision is made
to pick the next most efficient combination or wait for the on/
off time restriction to expire.
The program checks water temperatures and flow constantly,
recalculates the combinations and loading, and turns chillers
on and off accordingly. If a peak power demand limitation
applies, demand is checked and the demand limit avoided.
Starting at minimum load, a typical calculation sequence selects
and starts the most efficient chiller for that load. Data is constantly
checked and as the load increases to the optimum load for adding
a second chiller (based on current load, temperature differences,
and efficiency curves for that condition), a second chiller is selected
and started. The loading of each chiller is then adjusted for
maximum system efficiency. As conditions change and the load
increases or decreases, the loading of each chiller is adjusted and
chillers are added or dropped within the limitations of the
parameters input into the computer.
ALTERNATIVE MULTIPLE DECOUPLED
CHILLER CONTROL
Another chiller staging approach measures the secondary load
(via flow and differential temperature) and stage the chillers
on and off to match the demand. Since the leaving chilled water
temperature and the entering condenser water temperature are
frequently optimized and chiller capacity varies with changes
in either temperature, the per-chiller expected should be
dynamically modified based upon manufacturers data regarding
these variations. This complicates an otherwise simple strategy.
COMBINATION ABSORPTION AND
CENTRIFUGAL CHILLER SYSTEM
CONTROL
In large buildings or campus installations using medium or
high pressure steam, a combination of a turbine driven
centrifugal chiller and an absorption chiller may be economical.
The steam is routed first to the turbine driving the centrifugal
chiller (Fig. 21). The exhaust from the turbine is then routed
to the absorption chiller. The system is symmetrical so that
the absorption chiller uses all of the exhaust steam from the
turbine. System water piping is shown in Figure 22.
With the ABSORPTION/COMBINATION SELECTOR
switch (Fig. 21) in the combination position, temperature
controller T1, with its sensor in the chilled water supply, controls
the centrifugal compressor and pressure controller P1 controls
the absorption chiller capacity control valve so that all of the
exhaust steam from the centrifugal chiller turbine is used by
the absorption chiller. This also maintains a constant back
pressure on the turbine. When the cooling load is reduced, less
exhaust steam is available from the turbine and P1 positions
the capacity control valve to reduce the capacity of the
absorption chiller until the steam balance is achieved. Whenever
the absorption chiller condenser water pump is running, pressure
controller P2 allows temperature controller T2 to control the
cooling tower bypass valve to maintain the condenser water
temperature required by the absorption chiller. The centrifugal
chiller is normally turned off when the system load is 15 to 35
percent. The cooling tower fans are controlled by a temperature
sensor in the tower leaving water.
With the ABSORPTION/COMBINATION SELECTOR
switch in the absorption position, the centrifugal chiller is off
and temperature controller T1 modulates the absorption chiller
capacity control valve to maintain the required chilled water
temperature. The pressure reducing valve (PRV) supplies steam
to the absorption chiller.