MIL-PRF-18953C(SH)
d.
The stall and recovery region is established along the fan TP curve from the highest point in a above to
the intersection point in c above. The system resistance curve as defined in c above shall intersect or
be left of the design point as listed in 6.2.
4.8.5 Airborne noise. Fan total sound power shall be computed in accordance with AMCA 300. The fan-motor
unit shall be operated at the design point during airborne noise testing, see 3.12.1.1 and 3.12.2.1. Dual speed fans
shall be operated at high speed during noise tests. There shall be no exposed sound absorbing material on the
interior or exterior surfaces of the attached ducts. No sound attenuating devices shall be installed within attached
ducts. An orifice plate shall be installed if necessary to obtain design pressure at a known operating point. If an
orifice plate is installed, it shall be a quiet type which does not produce excessive flow-induced noise. Exceeding
level indicated in Table II in any octave band is cause for rejection.
4.8.5.1 Airborne noise detected by ear. Fan-motor units not undergoing airborne noise testing described in
4.8.5 shall be operated at free delivery. Any unusual airborne noise or any airborne noise thought to be excessive
shall be corrected if the cause is apparent. If the cause is not apparent and the condition is not corrected, the fan-
motor unit shall be tested herein, and sound power levels shall be computed to determine conformance with sound
power level requirements specified in Table I.
4.8.5.2 Airborne noise test facility. The facility wherein measurements of fan-motor unit airborne noise are
performed shall be registered in accordance with AMCA 111 as being a qualified facility in which to perform octave
band sound measurements in accordance with AMCA 300.
4.8.6 Motor heat. Motor heating shall be measured as specified in the applicable motor specification. In each
test, the motor shall be installed in the fan, and a duct shall be attached to the discharge. Maximum load shall be
obtained by means of terminal throttle on the test duct. Since neither the fan nor a test stand will normally be
available at the motor plant, the heat test may be conducted at the plant of the contractor. Motors with temperature
rises of any part in excess of specification limits or bearing temperatures in excess of 3.11.1.1 or 3.11.1.5 shall be
rejected.
4.8.7 Explosion. This test is applicable only to electrical equipment requiring an explosion-proof enclosure.
Test shall be performed in a petroleum ether atmosphere and certified by a nationally recognized laboratory. Test
shall include the conduit and conduit box actually used with the fan. Prior to the explosion containment test, the air
leakage rate of the motor enclosure shall be determined by pressurizing the interior of the motor enclosure to 10
pounds per square inch (lb/in2) and measuring the period of time in seconds for the pressure to drop from 10 to 4
lb/in2. The air leakage rate shall be recorded on the applicable motor drawing.
4.8.7.1 The motor of explosion-proof fan-motor units shall have the motor enclosure air leakage rate measured
to determine whether the rate of leakage exceeds that determined in 4.8.7. Any explosion-proof motor where there
is evidence that the predetermined rate of leakage (see 4.8.7) is exceeded when measured shall not be offered for
delivery.
4.8.8 Casing tightness. When this test is required in 6.2, the test shall be conducted as follows:
a.
Blank flanges (one with air supply and gauge connections) shall be bolted to gasketed fan flanges.
b.
Air at a pressure 50 percent higher than the maximum pressure capability of the fan shall be supplied to
the casing interior.
c.
The pressure drop in 10 minutes shall not exceed 5 percent of the test pressure.
4.8.9 Speed. The fan-motor unit shall be operated at each speed at free delivery to determine whether the speed
is at least 97 percent of the design speed at rated voltage. A speed of less than 97 percent of the design speed is
cause for rejection.
4.9 Blade tip clearance measurement. The fan blades shall be in a stationary position for the entire measurement.
Inspect visually to locate an apparent maximum clearance (radial clearance) from the blade tip to the inside of fan
casing. Measure this apparent maximum clearance and move the measurement to three different locations at 90, 180,
and 270 degrees relative to the apparent maximum clearance. Compare the four measurements to determine the true
maximum blade clearance. This maximum clearance shall not exceed ¼ of 1 percent of the casing inside diameter.
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