MIL-PRF-18953C(SH)
3.10.3.2 Balancing. The preferred method of balancing impellers is by removal of material. Provision of
sufficient rim thickness or of cast pads on the inside diameter of the impeller rim is recommended for this purpose.
If fan balancing is accomplished by adding weight, the weight(s) shall be permanently attached.
3.10.4 Vaneaxial fans.
3.10.4.1 Sizes. Sizes of Types A and X-A vaneaxial fans shall be limited to those listed in 1.2.2.
3.10.4.2 Physical interface dimensions. Physical interface dimensions of the fans shall be as shown on
Figure 1.
3.10.4.3 Watertightness. Casings shall be watertight.
3.10.4.4 Flanges. Flanges for duct connections shall be provided at each end of the fan casing. These flanges
shall be integral parts of the casing either through manufacture or welding. The flanges shall be drilled as shown on
Figure 1. An additional flanged connection may be provided at the contractor's option for stiffening large casings or
to permit use of a heavier fan race section. The extra flange shall have the same bolting as the end flanges, as well
as three unequally spaced dowels.
3.10.4.5 Direction vanes. Directional vanes shall be provided in the casing.
3.10.4.6 Motor mounting. Motors shall be face mounted. Mounting shall be designed to dampen rotor
vibration.
3.10.5 Tubeaxial fans.
3.10.5.1 Sizes. Sizes of Types L and X-L fans shall be limited to those listed in 1.2.2.
3.10.5.2 Physical dimensions. Physical dimensions of the fans shall be as shown on Figure 2.
3.10.5.3 Flanges. Flanges shall be provided at each end of the fan casing. The discharge flange shall be drilled
as shown in Figure 2.
3.10.5.4 Safety screen. A removable corrosion resistant 1-inch mesh screen shall be mounted to the inlet
flange.
3.10.5.5 Motor mounting. Motors shall be removable.
3.11 Electric equipment.
3.11.1 Motors. Fan motors shall be in accordance with Table II and 3.11.1.1 as applicable. In no case shall
motor extend beyond the fan casing. See Figure 1 for Types A and X-A fans. See Figure 2 for Types L and X-L fans.
As an alternative, motors of frame sizes 184 or larger conforming to MIL-M-17060 and motors of frame sizes
smaller than 184 conforming to MIL-M-17059 and meeting the requirements of 3.11.1.1 through 3.11.3 and Table II
are also acceptable. Commercial marine motors shall be in accordance with IEEE STD 45 with energy efficient
motors in accordance with NEMA MG 1.
3.11.1.1 General requirements. Motors shall be designed for at least 65 °C or 80 °C ambient temperature, as
specified in 6.2 (65 °C if not specified), continuous air over duty, with either face or flange mountings and the
following (see 6.2):
Service:
For Types A and L fan - hi-shock qualified motors.
For Types X-A and X-L fan - commercial marine motor.
Type:
For single-phase power either split permanent capacitor or split-phase induction with solid-
state starting switch accessible and mounted on outside of fan casing. Mechanical
centrifugal switches shall not be used.
For polyphase power - squirrel cage induction.
For Types A, X-A, L, and X-L fans - spraytight (see 3.11.1.8) (if not specified) or
Enclosure:
explosion-proof.
Continuous
Duty:
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