To alleviate this problem, a second
differential pressure transmitter can be
used or additional dampening added to
one of the pump control loops to reduce
the potential for an unstable pressure
control. However, the risk of developing
unstable pressure control can only be
reduced – it cannot be eliminated.
Figure 2. Three of Citizens Thermal’s Chilled-Water Plant Locations.
West Street Plant Illinois Street Plant IUPUI Campus North Plant
Different Approach Measures
Flow Rate
Instead of using two differential
pressure transmitters, a flow meter can
be used as the second pump control
input. The flow meter has the capability
of measuring the output of one chiller
plant and will not directly interfere with
the pressure control of the other chiller
plant. A flow meter that has no moving
parts is preferred because it provides
a smooth and steady signal. The pump
speed control setpoint is a flow, and the
signal from the flow meter is used to
adjust the variable-speed drive to control
the plant operating state. Pump staging can be accomplished independently
by control algorithms that look at both
pump speed and differential pressure.
The pump flow setpoint is a fixed
value and does not respond to cooling
load conditions. Therefore, it should
always be used in unison with another
chiller plant using the pressure control
method. This type of setpoint control
tends to load a chiller plant evenly with
a given flow setting. The only load variable becomes the plant temperature rise,
which can change throughout the day.
Temperature rise is not as dramatic and
can be reviewed more on an hourly basis
to determine if a change is required.
There is an indirect pressure feed-
back that occurs between plants. The
area of influence – the portion of the
piping system served by the flow-con-
trolled plant – changes with chilled-water
system load dynamics even though the
plant flow stays the same. This change in
the flow-controlled plant’s area of influ-
ence will cause the plant using differen-
tial pressure control to compensate. This
type of dynamic load shifting will occur
but the instantaneous pressure feedback
is disconnected (it takes time for the
flow sensor to register). The chiller plant
operating on pressure control modulates
to match the cooling load variations of
the chilled-water system.
© 2009 Google, Map Data © 2009 Tele Atlas.
be more efficient than two chillers at 90
percent load. This selection process can
be left to operations personnel with some
experience operating the plant. For this
reason, it is a good idea to install a bypass
on an all-primary pumping system. The
bypass can be controlled using a chiller
differential setpoint that allows for a minimum circulation rate to pass through the
chiller. When the cooling load drops off
quickly, the bypass will keep the chillers
on line, and the operators have a chance
to react to changing conditions.
Flow Meter Control
in Indianapolis
The Indianapolis, district cooling
system has utilized the flow meter control method with success since 2005.
This system is owned and operated by
Citizens Thermal (Citizens), a member
of Citizens Energy Group. (See the cover
story in this issue for more on Citizens
Thermal.)
Citizens’ original chiller plant, the
West Street Plant, first became operational
in 1991 and has a nameplate capacity of
32,000 tons. It is located on the south
side of the downtown area. The pumping
system selected for this plant is primary-secondary and uses a combination of electric- and steam-drive secondary pumps.
The West Street Plant operates on system
