or temperature differential at a safe and conservative level. Fan rotors
shall be designed so the maximum calculated stress at any point on the
rotor, under any normal condition such as continuous operation or starting
acceleration, will not exceed 50 percent of the yield strength of the rotor
material at the maximum operating temperature. Fan rotors shall have
center plates and shroud plates of mild or alloy steel. Tie rods between
plates or blades will not be permitted. Construct so that while the rotor
is turning, the run-out of similar points on the rotor structure will not
deviate from the median path by more than 0.375 percent of the wheel
diameter. Rotors shall be designed and constructed so that the first
critical speed of the rotor is not less than 33 percent above the normal
operating speed for fans operating at temperatures up to 93 degrees C 200
degrees F and not less than 50 percent above the normal operating speed for
fans operating at temperatures of 93 degrees C 200 degrees F and above.
Fan shafts shall be forged and heat-treated steel, accurately machined,
with ground and polished journal and thrust bearing surfaces. Shafts shall
be provided with seals to minimize leakage where the shaft penetrates the
housing.
2.11.4.3
Bearings
Bearings shall be suitable for continuous heavy-duty service and shall be
self-aligning, sleeve-type journal bearings designed with adequate
lubrication for coast down without an external oil supply. Install
bearings in a horizontal split housing or pedestal of structural steel or
cast iron. Pedestals shall allow removal of bearings without removing
rotor and shall be suitable for shimming. Bearings shall be located
external to the inlet boxes. Bearings on double-inlet fans shall be
designed to withstand the thrust unbalance resulting from the shutoff of
one inlet while the other inlet is open. Protect bearings from the weather
on outdoor installations with a suitable rain hood and sun shield as
recommended by the manufacturer. Each bearing shall have bearing
temperature thermocouple as specified in paragraph entitled
"Instrumentation and Controls." Bearing lubrication systems shall be
specifically designed for air cooling except as noted below. Provide
cooling air by inlet box suction. Inlet air filters shall be provided.
Where the bearing surface rubbing speed exceeds 7.62 m/s 18,000
inches/minute bearings designed for water cooling shall be furnished. Each
bearing shall be equipped with at least two machined oiling rings and an
adequately sized integral lube oil reservoir with oil level gage.
Reservoirs shall be equipped with immersion heaters designed to
continuously maintain the lube oil in the bearing at starting temperature
over the range of ambient temperature specified in paragraph entitled
"Design Parameters." Immersion heaters shall be 120-volt AC single-phase
and shall be complete with a thermostat located to accurately control lube
oil temperature. Furnish bearing pedestals, mounting plates, and
soleplates as required to mount the equipment on the concrete foundation.
Contractor shall furnish the torque requirements of the foundation bolts
for fans.
2.11.4.4
Motor Drive
Each fan shall be furnished with a single-speed motor drive. Each fan
shall be equipped with a flexible coupling between the fan and motor drive.
Coupling shall be rated at not less than 150 percent of the motor power at
design fan speed. Design coupling to prevent any axial thrust from being
transmitted to the driver under any normal operating condition.
SECTION 15864N
Page 77
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