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Marley
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Sigma 1000/1200 Cooling Tower
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Specifications: Options
25
Specification Value
set point shall be displayed on the door
of the control panel. The bypass shall
include a complete magnetic bypass
circuit with capability to isolate the VFD
when in the bypass mode. Transfer to
the bypass mode shall be automatic in
the event of VFD failure or for specific
trip conditions allowing safe transfer of
utility voltage to the motor. Automatic
bypass with an earth ground condition
is not allowed. The bypass contactor
shall be cycled on and off while operat-
ing in bypass, to maintain the set-point
temperature of the cold water. The drive
design shall be operated as a stand-alone
system without the need for a BMS sys-
tem. Operator controls shall be mounted
on the front of the enclosure and shall
consist of start and stop control, bypass/
VFD selector switch, Auto/Manual selec-
tor switch, manual speed control, and
solid-state temperature controller. An
emergency bypass selector switch inter-
nal to the panel allowing the fluid cooler
fan motor to be run at full speed shall be
furnished. To prevent heating problems in
the fluid cooler fan motor and to assure
proper gear box lubrication the VFD sys-
tem shall de energize the motor once
25% motor speed is reached and cool-
ing is no longer required. The VFD shall
include de-icing logic with auto canceling
and adjustable time. Speed in De-Ice
mode shall not exceed 50% motor
speed. The fluid cooler manufacturer
shall supply VFD start-up assistance.
Tower vibration testing throughout the
speed range is required to identify and
lockout any natural frequency vibration
levels which may exceed CTI guidelines.
Vibration Limit Switch:
5.8 Add the following paragraph in the
Mechanical Equipment section: A
single-pole, double-throw vibration limit
switch in a NEMA 4 housing shall be
installed on the mechanical equipment
support for wiring into the owner’s
control panel. The purpose of this
switch will be to interrupt power to the
motor in the event of excessive vibra-
tion. It shall be adjustable for sensitiv-
ity, and shall require manual reset.
■ Unless specified otherwise, a Metrix
switch will be provided. A double-pole,
double-throw model is also available. If
purchased in conjunction with the Control
System, it is also factory-wired. The
requirement for manual reset assures that
the tower will be visited to determine the
cause of excess vibration.