50
Vol. 2.6 © 2020 Hanbell Precise Machinery Co., Ltd. All rights reserved
VSD capacity modulation strategy & variable frequency surge formulae
VSD capacity control takes variable speed as priority and IGV as auxiliary for modulation to reach
the optimum efficiency. Thus, VSD compressor surge formulae mainly control the compressor
frequency. The expressions are as follows:
:
Z=a+bx+cy+dx
2
+ey
2
+fxy+gx
3
+hy
3
+ixy
2
+jx
2
y
where a
、
b
、
c
、
d
、
e are constants (refer to Table 3.7
VSD compressor
surge formula);
x=IGV(%) opening (10~100);
y=pressure ratio, Pr;
Z (minimum allowable operating frequency) = Frequency (Hz)
※
Insert the value of y (pressure ratio) and x (IGV opening) at the operating point into the formula
to obtain the minimum allowable operating frequency. The recommended compressor operating
frequency is Z+1 to 1.5Hz in order to reach the maximum efficiency. Reasons for setting value
range of +1 ~ 1.5Hz is:
a. To avoid forced increase in frequency at small IGV openings when IGV opening value is
inserted into the formula (could cause fluctuation of chilled water temperature).
b. To avoid surge formula deviation or inaccurate measuring sensor;
c. This formula does not have safety margin.
Model
Z= a+bx+cy+dx
2
+ey
2
+fxy+gx
3
+hy
3
+ixy
2
+jx
2
y
x=IGV opening(10~100%)，y=pressure ratio Pr，Z=Frequency (Hz)
RT-120E
a
b
c
d
e
-3.01 -5.25339E-02 7.01 1.28399E-03 -2.01
f
g
h
i
j
-2.20465E-02 -8.07113E-06 2.00 -1.42502E-03 1.35207E-04
RT-130E
RT-140E
a
b
c
d
e
-2.01 1.11192E-01 4.01 -7.54248E-04 -1.01
f
g
h
i
j
-5.47750E-02 -6.23802E-06 1.00 -1.95358E-02 9.99493E-04
RT-160E~
RT-280E
a
b
c
d
e
-3.01 -1.40837E-02 6.01 2.73888E-04 -2.01
f
g
h
i
j
-3.93628E-02 -9.02008E-07 2.00 3.62232E-03 1.55530E-04
Table 3.7 VSD compressor (50Hz) surge formula (without safety margin)
1. Start-up frequency: As high and medium pressure has not been established before start-up, it
is impossible to calculate the required minimum rotational speed based on the pressure ratio
when the compressor starts, therefore it is necessary to estimate the condensing temperature
by using cooling water inlet temperature plus a temperature difference (7~8K) to obtain the
expected condensing pressure. Then, using chilled water outlet temperature minus a
temperature difference (1~2K) as the expected evaporation temperature to obtain the
evaporation pressure. Finally, insert the pressure ratio based the estimated pressures and
IGV=10% into the formula to calculate the minimum operable fr1Hz; otherwise
start-up with the highest operating frequency.
