ASHRAE Journal - August 2019 - 26

TECHNICAL FEATURE

discharge pressure -the turbine
speed can be automatically driven
down until the pressure setpoint is
satisfied, preventing the chiller from
tripping.
One feature that deserves more
mention is sensing the surge line. As
we know, running a chiller operation into the surge zone at low load
for a long period can cause significant compressor damage. Often the
operators are cautious and open
the hot gas bypass early. This can
be quite inefficient. A simplified
method to approach the surge line,
used by some panel control systems,
is to create a linear zone (Figure 3,
Page 24), which limits operation
beyond this area without use of inefficient control techniques like the
hot gas bypass. A more sophisticated
method used in a recent panel is
to employ a smart self-learning
sequence, which tests the surge line
throughout its entire operation and
stores it in memory, allowing it to
more closely approach the surge line
-further improving performance.
Measurement of turbine torque
through a non-contact digital telemetry system (Figure 4) accurately
monitors the turbine's actual loading, thereby calculating the actual
shaft horsepower in real time as well
as detecting surges.
Combined with steam condensate
measurement, overall steam usage
(lbs/h) and driveline efficiency
(lbs/hp) as well as chilled and condenser water flow, a full energy balance can be achieved.

Features Included in HMI Panels
Provisions in human machine
interface (HMI) panels allow
for starting and monitoring the
chilled water, condensate, vacuum,
26

ASHRAE JOURNAL

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FIGURE 4 Measurement of torque telemetry.

and hotwell pumps. If a hotwell
pumps fails or the level exceeds a
predefined setpoint, the second
pump can automatically start. This
interface also eliminates chilled
water flow during slow roll, thereby
minimizing the large chilled water
temperature swings during turbine
slow roll. Since steam turbines operate at higher temperatures, slow
roll minimizes the thermal stress
and expansion of components. It
also minimizes large chilled water
temperature swings by eliminating
chilled water flow. Warm chilled
water is prevented from diluting the
temperature of previously cooled
water, allowing the operators to start
the unit when required without
affecting the building's chilled water
supply temperatures. Once the slow
roll is completed, the chilled water
pumps turn on automatically, and
the turbine will ramp to minimum
rated speed. In many installations,
the original turbine hand valves can
be retrofitted with electric actuators for automatic control, thereby
sequencing their opening and closing to further improve the chiller
efficiency.
Detection of slight rotation of the
shaft when the unit is offline due

A U G U S T 2 0 19

FIGURE 5 Electric chiller driveline.

to steam leaking can automatically
start the turbine and compressor
auxiliary oil pumps running for
a predetermined amount of time
after the rotation has ceased. This
prevents damage to the bearings if
the shaft is rotated without proper
lubrication. Other options include
using analog differential pressure
sensors in lieu of flow switches for
the chilled and condenser water,
allowing configuration of low-flow
warning alarms prior to the system
tripping on low flow. Some HMI
panels also include measuring realtime and historical trends, steam
consumption from condensate
discharge, temperature sensing of
fluids and surfaces, pressure transmitters, cutouts and signals.
Interface screen panels can provide users with an intuitive graphical interface to the chiller operation.
Screen navigation and control are
accomplished with simple pointand-click actions via a touchscreen.
Usernames and passwords are
required for direct operation of
devices or changing of setpoints that
are beyond the basic operation of
the chiller. These screens include

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ASHRAE Journal - August 2019

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