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|  MIL-U-17293D(SH)
of 300 series corrosion-resisting steel shall be at least 0.047 inch thick
when the coil face area is 1 square foot or less, and at least 0.076 inch
thick when the coil face area exceeds 1 square foot.
3.3.8 Louvers. Each unit heater shall be provided with louvers. The
louvers shall be constructed to permit adjustable directional air flow in the
vertical plane and to prevent overheating the fan motor when the louvers are
in the closed position.
3.3.9 Fan. The fan for the unit heater shall be of the propeller type,
with at least three blades, complete with motor, motor mount, fan mounting
panel and fan guard. The fan shall be quiet in operation and shall be dynami-
cally balanced. Rotating parts of the fan shall be designed to have a factor
of safety of not less than 8, based on the ultimate tensile strength of the
material involved. Aluminum alloy hubs shall have a liner of 300 series
corrosion-resisting steel. The fan blades shall be of such design that normal
cleaning or dusting will not result in their being bent out of alinement. The
fan blades shall be welded to the hub, or the blades and the hub tasted as a
unit. The impeller shall be secured to the motor shaft by a key and by a
castellated nut with a cotter pin.
3.3.9.1 Fan mounting. The unit heater fan shall be mounted on a tubular
steel bracket with a nonadjustable motor mounting base. The bracket shall be
permanently attached to the fan mounting panel. The tubular steel bracket, the
motor mounting base and the fan mounting panel shall be sufficiently rugged
to prevent distortion under shipboard conditions of vibration and shock. The
fan motor shall be secured to the mounting base by positive means to prevent
it from sliding under shipboard operating conditions. The fan mounting panel
shall be secured to the casing by corrosion-resisting fasteners.
3.3.9.2 Fan guard. A fan guard shall be provided, which shall be securely
fastened to the fan mounting panel and still permit ready removal. The fan
guard shall incorporate the following requirements:
(a)
prevent distortion under normal operating conditions.
(b)
Protect personnel from injury.
(c)
Minimize restriction of air flow.
(d)
Minimize vibration when the fan is in operation.
3.4 Electrical equipment. The design of the electrical equipment shall
be in accordance with the basic requirements of MIL-E-917.
3.4.1 Electric service. Fan motors shall be supplied for alternating
current (a.c.) service. Fan motors 1/4 horsepower (hp), or less, shall be
115 volts, single phase, 60 Hz. Motors that exceed 1/4 hp shall be 440 volts,
three-phase, 60 Hz.
3.4.2 Motors. Motors shall be service A in accordance with MIL-M-17059.
Motors shall be designed for 65 degrees Celsius (C) ambient temperature,
continuous duty, constant speed and shall have class A insulation and spray-
tight enclosure. Motors shall be equipped with normal internal fit ball
bearing type 120 or type III, class 1, 2, or 7 in accordance with FF-B-171. A
grounding circuit shall be established between the motor frame and the unit
heater casing, and a separate ground shall be provided via a grounding wire
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