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UC800 AFD Operation
Adaptive Frequency Drive Control
Introduction
Achieving Efficiency
Adjustable speed impeller control is used to improve
CenTraVac™ 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 (static + 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 Tracer®
UC800 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, reestablish stabile
operating conditions, and adjust the control boundary to
avoid surge in the future.
Chiller and AFD Sequence of Operation
In the UC800, 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 UC800's AFD speed control algorithm will
simultaneously set Inlet Guide Vane (IGV) position and
38
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 part loads.
Surge Boundary
Surge boundary is a non-linear, empirically derived
function of the compressor load. For the UC800, the
compressor pressure coefficient boundary is defined as a
function of IGV position as shown on
Figure
15.
AFDE-SVU02F-EN
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