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AFDK-SVU01C-EN
AFD Operation
Adaptive Frequency Drive Control
Introduction
Achieving Efficiency
Adjustable speed impeller control is used to improve
CenTraVac™ chiller efficiency at part-load while tower
relief is available. This occurs because the addition of the
variable frequency drive gives the chiller control an extra
degree of control freedom. The combination of inlet guide
vane position and variable speed creates the possibility to
control both chiller capacity and compressor efficiency. By
manipulating speed and inlet guide vane position, it is
possible to adjust the aerodynamic loading on the
compressor to operate in a region of higher efficiency.
Challenges
There are challenges associated with achieving high
efficiency. The region of higher efficiency is near the
compressor surge boundary. Surge occurs when the
compressor can no longer support the differential
pressure required between the evaporator and condenser.
Reducing compressor speed can improve efficiency;
however, at some point the reduced impeller speed does
not add enough dynamic pressure to the discharged
refrigerant. When the total pressure ( dynamic)
leaving the compressor is less than the condenser
pressure, refrigerant will start to flow backwards from the
condenser. The flow reversal from the condenser to the
compressor discharge creates a sudden loss of the
dynamic pressure contribution from the compressor.
Refrigerant flows backwards through the compressor
creating an unpleasant audible noise. Surge is avoided
when possible because it causes a loss of efficiency and
cooling capacity if the compressor is allowed to cycle in
and out of surge for an extended period.
Solutions
The adjustable speed control algorithm of the control was
developed to operate near the surge boundary by
periodically testing to find the surge boundary and then
holding conditions at an optimal distance from surge.
Once the optimal operating condition is found the
algorithm can avoid the surge in the future. When surge is
detected, a surge recovery routine makes adjustments to
move out of surge, reestablishes stable operating
conditions, and adjusts the control boundary to avoid
surge in the future.
Chiller and AFD Sequence of Operation
The chiller/AFD sequence of operation is identical to a
standard fixed speed chiller. Chiller capacity control,
safeties, and limits work in the same manner regardless of
whether an AFD is present.
The speed control algorithm will simultaneously set Inlet
Guide Vane (IGV) position and compressor speed to
achieve a desired compressor loading command while
holding a fixed margin of safety between the compressor
operating point and compressor surge. In order to
quantify nearness to surge, a non-dimensional parameter
called “compressor pressure coefficient” is used as a
measure of surge potential. Decreasing motor speed
increases the compressor pressure coefficient. The goal of
the AFD control algorithm is to reduce speed enough to
increase the pressure coefficient to the surge boundary.
Compressor Pressure Coefficient
The non-dimensional pressure coefficient is derived
based on turbo machinery principles. Fundamentally, the
pressure coefficient is the ratio between the potential
energy based on the pressure rise across the compressor
and the kinetic energy of the refrigerant at the compressor
discharge. This normalized equation uses enthalpy
change across the compressor as a measure of potential
energy and compressor parameters such as average
impeller diameter, speed, and number of stages, to
determine kinetic energy.
The kinetic energy can be reduced by reducing the
condenser pressure. To achieve condenser pressure
reduction, reduce the temperature of the entering tower
water. To obtain the best efficiency, follow a tower relief
schedule at whenever practical. For assistance with
optimizing tower relief, contact Trane controls personnel.
Surge Boundary
Surge boundary is a non-linear, empirically derived
function of the compressor load. The compressor
pressure coefficient boundary is defined as a function of
IGV position as shown on
.
Figure 21.
Pressure coefficient surge boundary
0.0
0.6
0.7
0.8
0.9
1.0
1.1
1.2
10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0
Surge Boundary
Surge
IGV Position
Press
u
re Coefficient
AFD Speed Control
The chiller control utilizes an enhanced control method
capable of simultaneously adjusting compressor speed
