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Vol. 2.6 © 2020 Hanbell Precise Machinery Co., Ltd. All rights reserved
3.3.1 Fixed-frequency capacity control and surge protection
strategy
The capacity control of a fixed-frequency compressor is generally divided into constant pressure
ratio modulation and constant load modulation.
【
Constant pressure ratio modulation
】
Taking point P of Figure 3.5 at a certain constant pressure
ratio as an example, if the outlet temperature of the chilled water (CHWT) is higher than the target
temperature, then the loading should be increased by increasing IGV opening (P shifts right) until
reaching the temperature at target CHWT±0.3
℃
. If the outlet temperature of the chilled water
(CHWT) is lower than the target temperature, then the loading should be reduced by decreasing
IGV opening (P shifts left) while making sure that the operating pressure ratio does not exceeds the
limit. If further reduction of IGV opening is required when the pressure ratio reaches the limit, then
hot gas bypass valve should be opened to balance the system.
【
Constant load modulation
】
Taking point P of Figure 3.5 at a certain constant load as an example,
if the pressure ratio is required to be increased (P shifts upward) when the outlet temperature of
cooling water increases, then IGV opening should be increased while making sure the operating
pressure ratio does not exceeds the limit during the process. Start surge protection if the outlet
temperature of cooling water rises continuously. If the outlet temperature of cooling water drops,
then IGV opening should be decreased to reduce the pressure ratio (P shifts downward), this
process
Surge protection mechanism of a fixed-frequency compressor is generally divided into two modes:
unloading mode and loading mode
【
Unloading mode
】
If surge occurs during unloading mode of cooling water, then open the hot gas
bypass valve for 1-2 minutes to reduce the operating pressure ratio (raising the lower pressure).
The system can return to normal operating mode after the hot gas bypass valve is closed.
【
Loading mode
】
If surge occurs during loading mode of cooling water, then increase the opening of
IGV by 5-10% each increment. If compressor surge still persists when the IGV opening is increased
to 100% then the system should be shut down.
Caution
：
The timing of compressor surge should be flagged, typically for 10 minutes, if there are
three consecutive surge occurrences during this period, then the compressor must be shut down
and investigation is required to identify and eliminate the cause(s), e.g. high cooling water outlet
temperature due to inefficient in heat exchange of the condenser, then cleaning of the condenser is
required. Otherwise, temporarily increase the target outlet temperature of cooling water (reduce
operating pressure ratio) to allow temporary operation of the compressor until cleaning is
completed.
Fixed-frequency unit safety margin line formula
Fixed-frequency compressor allowable operating range
：
Ranging from the maximum IGV opening
(100%) to the smallest IGV opening (10%),with the maximum operation load of (105%) and
covering the operating range from safety margin line to the minimum pressure ratio.
The safety margin line formula is a polynomial of IGV opening and should be monitored when the
compressor is under capacity control to avoid operating pressure ratio from exceeding the
precaution safety margin line.
Pr=pressure ratio (safety margin line) =y=a+bx+cx
2
+dx
3
+ex
4
Where a, b, c, d, e are constants and x is the IGV opening.
