6
Operation and Maintenance Instructions
Identify and Lock-out Harmful Resonant
Frequencies
A Variable Frequency Drive (VFD) fan system, unlike traditional
xed-speed systems, is designed to operate between 25%
(13Hz) and 100% (50Hz) speeds, which creates an opportunity
for operation where resonant frequencies exist. Sustained op-
eration at resonant frequencies may lead to excessive vibration,
fatigue of structural components and/or drive system noise and
failure. Owners and operators must anticipate the existence of
resonant frequencies and lock out frequencies during start-up
and commissioning in order to prevent drive system operational
problems and structural damage. As a part of the normal start-
up and commission processes, resonant frequencies should be
identied and locked-out in the VFD’s software.
The unit’s supporting structure, external piping, and accesso-
ries contribute to the overall harmonic make-up and stiffness
of the system. The choice of VFD will also have a signicant
influence on how the system behaves. Consequently, not all
resonant frequencies can be determined in advance at the
manufacturer’s factory during nal inspection and testing. Rel-
evant resonant frequencies (if they occur) can only be identi-
ed accurately after the installation in the system.
To check for resonant frequencies in the eld, a run-up and
run-down test must be performed. Additionally, VFD carrier
frequencies should be adjusted to best align the VFD with the
electrical system. Refer to your drive’s start-up procedures for
additional information and instruction.
The procedure of checking for resonant frequencies requires
stepping through the VFD’s operating range at (2) Hz intervals
from the lowest operating frequency to full speed. At each step,
pause long enough for the fan to reach steady-state. Note
changes in unit vibration during this time. Repeat from full speed
to minimum speed. Should vibration-inducing frequencies exist,
the run-up and run-down test will isolate the resonant frequencies
which then must then be locked-out in the VFD programming.
Recirculated Water System -
Routine Maintenance
Suction Strainer in Cold Water Basin
The pan strainer should be removed and cleaned monthly
or as often as necessary. Make certain that the strainer is
properly located over the pump suction, alongside the anti-
vortexing hood.
Cold Water Basin
The cold water basin should be flushed out quarterly, and
checked monthly or more often if necessary, to remove any
accumulation of dirt or sediment which normally collects
in the basin. Sediment can become corrosive and cause
deterioration of basin materials. When flushing the basin, it is
important to keep the suction strainers in place to prevent any
sediment from entering the system. After the basin has been
cleaned, the strainers should be removed and cleaned before
refilling the basin with fresh water.
Operating Level of Water in Cold Water Basin
The operating level should be checked monthly to make sure the water level is correct. Refer to Table 2 for unit specific levels.
Model Number Operating Level
ATW
ATW
9
64
through
through
48
866
230 mm
280 mm
ESWA
ESWA
72
144
through
through
142
216
280 mm
280 mm
LSWA
LSWA
LSWA
LSWA
LSWA
20
91
116
135
174
through
and
and
and
and
87
182
232
270
348
280 mm
300 mm
300 mm
380 mm
380 mm
LRW 18 through 379 200 mm
C-ATW 67-3H through 133-6J 280 mm
eco-ATW
eco-ATW
0,9 m wide
2,3 m wide
and
through
1,2 m wide
7,3 m wide
230 mm
280 mm
eco-ATWE 2,3 m wide through 7,3 m wide 280 mm
LSCE
LSCE
LSCE
LSCE
LSCE
LSCE
LSCE
LSCE
LSCE
36
281
591
400
800
410
820
550
1100
through
through
through
through
through
through
through
through
through
385
386
770
515
1030
560
1120
805
1610
280 mm
300 mm
300 mm
300 mm
300 mm
380 mm
380 mm
380 mm
380 mm
LRC 25 through 379 200 mm
ATC
ATC
50E
M170E
through
through
165E
3714E
230 mm
280 mm
C-ATC 181 through 504 280 mm
PMCQ 316 through 1786 360 mm
eco-ATC 176 through 4086 280 mm
* Measured from lowest point on basin floor.
Table 2 - Recommended Operating Water Level
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