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Marley
/
NC Class Stainless Cooling Tower
/
Specifications: Options
20
box which is self supporting. The inlet
attenuation will not be allowed to
impact the thermal performance effi-
ciency of the basic tower configuration.
Outlet Sound Attenuation
1.4 Add the following paragraph under
Base: The cooling tower shall be
equipped with outlet sound attenuation
baffles positioned and spaced horizon-
tally across the entire fan opening. The
baffles will be constructed of perfo-
rated sheet metal and contained within
a steel box which is self supporting
and made of the same material as the
tower.
Low Noise Fan
1.4 Replace paragraph 6.1 with the fol-
lowing: Fan(s) shall be propeller-type,
incorporating a minimum of seven
wide-chord aluminum alloy blades
and galvanized hubs. Blades shall be
individually adjustable. Maximum fan
tip speed shall be 56 m/s. Fan(s) shall
be driven through a right angle, indus-
trial duty, oil lubricated, geared speed
reducer that requires no oil changes
for the first five (5) years of operation.
The gearbox bearings shall be rated at
an L
10A
service life of 100,000 hours or
greater.
6.1 (alternate)* Fan(s) shall be propeller-
type, incorporating a minimum of
seven wide-chord aluminum alloy
blades and galvanized hubs. Blades
shall be individually adjustable. Fan(s)
shall be driven through a one-piece
multi-groove, solid back V-type belt,
pulleys, and tapered roller bearings.
Bearings shall be rated at an L
10A
ser-
vice life of 40,000 hours or greater.
*Currently available on Models NC8304 through
NC8309.
temperature without sacrificing the system’s ability to maintain a con-
stant cold water temperature. This is a relatively inexpensive solution,
and can pay for itself quickly in reduced energy costs.
• The most extreme cases may require inlet and discharge sound attenu-
ator sections—however, the static pressure loss imposed by discharge
attenuators may necessitate an increase in tower size. Two stages of
inlet or discharge attenuators supported by the tower and designed and
tested for the most stringent requirements are available as an option.
■ Falling water sound—unlike counterflow towers which allow the water
to free-fall and splash into the cold water collection basin, PVC film-
fill crossflow towers have no splashing. This allows a crossflow tower
much lower sound levels at the air inlet than a counterflow tower—
especially induced draft counterflow towers. Even with the “splash-mat-
ting” option in a counterflow cold water collection basin, a crossflow
tower air inlet is still quieter. Plus you do not have to worry about clog-
ging the splash matting. Yet another maintenance and operation advan-
tage of the crossflow configuration.
■ NC Class models with the “L” suffix in the model number are the special
low sound editions. To achieve the very lowest possible sound levels
while maintaining efficiency, the best available combination of motor,
gear ratio, fan blade count and blade profile were carefully selected for
every “L” model. All “L” models come with the Low-Noise Fan option.
If you choose the sound attenuator option, the fan will automatically
change to the Low-Noise Fan option.
■ Tip Speed—unlike thermal performance, no certification program exists
for sound. While Marley conducts actual sound tests on all its configura-
tions there are only
a few ways for the
client to ensure
they get a quiet
tower.
• One is to conduct
a field sound test
after installation.
On-site testing
after installation
can however be
inaccurate depend-
ing on the environ-
ment.
• Another is to conduct a sound test at the factory. However both can be
cost prohibitive for smaller applications.
• Specifying fan blade tip speed is one way to physically force the tower
selection to be quiet. Tip speed is easily checked by multiplying the fan
rpm by the fan circumference at the blade tip (π fan dia). Over 61m/s is
considered high by most people. 51-61 is considered typical and expect-
ed. 41-51 would be considered low noise. Below 41 is difficult to hear
above the water noise.