MIL-PRF-24755B(SH)
4.7.2 Aerodynamic stall. The effective stall capacity shall be determined from the largest capacity
measurement of the following three stall measurement methods.
4.7.2.1 Throttled stall capacity. While the fan is operating in a performance test stand (see 4.7.1), slowly
throttle the flow at the test stand discharge. Record the capacity when the fan goes into stall.
4.7.2.2 Stall recovery capacity. This method is the capacity at which the fan recovers from a stalled condition
when the fan is operating in a stalled condition and the discharge throttle is slowly opened while moving towards
free delivery. Operate the fan in a performance test stand (see 4.7.1) at an aerodynamic stall condition resulting
from a throttled discharge. Slowly open the discharge throttle towards free delivery until the fan recovers from the
stall condition. Record the capacity when the fan recovers from the stall condition.
4.7.2.3 Stable start-up capacity. This method is capacity at which stable, stall free operation first occurs, as the
fan is started from a no-spin condition with the throttling device fixed at progressively larger openings. With the fan
in a performance test stand (see 4.7.1) with the discharge throttling device fixed at restrictive position, start the fan
from a no-spin condition. If the throttling restriction results in a fan stall condition, secure the fan, change the
throttling device position to reduce its restriction, and start the fan again. Repeat until starting the fan results in
stable, stall free operation. Record the capacity at which the fan is first able to start-up into stable, stall free
operation.
4.7.3 Airborne noise. Octave band sound power levels for a fan-motor unit shall consist of the measurement of
sound pressure levels and the computation of sound power levels in accordance with procedures and instrumentation
requirements defined in ANSI/AMCA 300, and amendments to those procedures as identified herein. The fan-
motor unit shall be operated at the design point during airborne noise testing (see 3.19.1). Dual speed fans shall be
operated at high speed during noise tests. All sound measuring instrumentation shall be laboratory calibrated within
one year of the date of test, excluding the reference sound source which shall comply with the laboratory calibration
requirements stated in ANSI/AMCA 300. A random incidence response microphone and a Type I (precision) sound
level meter conforming to the requirements of ANSI/AMCA 300 shall be used. The minimum distance between the
extreme points of the microphone swing as described in ANSI/AMCA 300 shall be 10 feet, and the speed of traverse
of the microphone shall not exceed 2.4 feet per second. Sound pressure readings shall be a time-weighted average
over an integral number (one or more) of microphone swings. Sound pressure level readings shall be averaged over
a minimum period of observation of 30 seconds. When a duct section is attached to a fan-motor unit to facilitate
measurements, it shall have the same interior, open cross-sectional dimensions as that of the fan-motor unit to which
it is attached. 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
that does not produce excessive flow-induced noise.
4.7.3.1 Total sound power. Test procedures and calculation procedures to determine inlet sound power levels
and outlet sound power levels for a fan-motor unit shall be in accordance with ANSI/AMCA 300 and the additional
test methods described in 4.7.3.1 and 4.7.3.2. For each octave band, the inlet sound power level LWi and outlet
sound power level LWo shall be summed to determine the total sound power level LW using the following equation:
LWi
LWo
LW 10 log10 10
10
10
4.7.3.2 Inlet sound power. Test procedures and calculation procedures to determine inlet sound power levels
for a fan-motor unit shall be in accordance with ANSI/AMCA 300 requirements for free inlet/ducted outlet test
configurations for fan inlet sound testing. An orifice plate shall be installed within the outlet duct if necessary to
obtain fan-motor unit operation at the design point. Acoustical treatments shall be placed within the chamber as
necessary to provide an acoustic seal at the penetration in the wall separating the chamber from the reverberant room
to ensure that no noise contributions are transmitted from the chamber into the reverberant test room.
11
For Parts Inquires call Parts Hangar, Inc (727) 493-0744
© Copyright 2015 Integrated Publishing, Inc.
A Service Disabled Veteran Owned Small Business