MIL-PRF-24775A(SH)
c.
When operating temperature is stabilized, operate at this condition for at least 30 minutes, cycling
on/off at 5-minute intervals.
d.
Increase voltage to the next higher increment of 5 watts per square inch of sheath heating surface area
(WSIA) excluding the fins.
e.
Read and record voltage and wattage, and operate the element at this level for at least 30 minutes,
cycling on/off at 5-minute intervals.
f.
Repeat steps d and e until element fails by fusion.
4.5.2.11.2 Accelerated life test cause for rejection. A life expectancy of less than 21,000 hours shall be cause
of for rejection. When failure by fusion occurs prior to 85 WSIA, this constitutes a life expectancy of less than
21,000 hours.
4.5.2.11.3 Creepage and clearance. Any heater where there is evidence that the creepage or clearance distances
are not in accordance with 3.12.5.6 when measured shall not be installed in a fan-coil unit.
4.5.2.12 Unit operation. The fan motor, electric heater, and electrical control circuitry of each fan-coil unit
shall be tested at rated voltage and current to determine whether the fan, motor, heater, and controls will operate as
designed. Any fan-coil unit in which any component fails to operate properly shall not be offered for delivery.
4.5.2.13 Airborne noise. Measurement of sound pressure levels (octave band, flat, unweighted) and
computation of sound power levels (octave band) shall in accordance with procedures and instrumentation
requirements defined in AMCA 300, and amendments to those procedures as identified herein. Procedures of
AMCA 300 shall be modified to substitute the term "fan-coil unit" in place of the word "fan." All sound measuring
instrumentation shall be laboratory calibrated within one year of the date of test, excluding the reference sound
source that shall comply with the laboratory calibration requirements stated in AMCA 300. A random incidence
response microphone and a type I (precision) sound level meter conforming to the requirements of AMCA 300 shall
be used. The fan-coil unit shall be operated at fan speed providing maximum acceptable air flow delivery and
external static pressure conditions for the size unit under test, see Table I. A fan-coil unit with a dual fan speed shall
be operated at high speed. A discharge duct with orifice device shall be attached to the fan-coil unit to create design
external pressures where required, see Table I. When a duct section is attached to a fan-coil unit to facilitate noise
measurements, it shall be a rigid, heavy gauge metal duct and/or non-perforated double wall duct to eliminate noise
contributions transmitted from the duct walls. When a duct section is attached to a fan-motor unit to facilitate
measurements, the duct section shall have the same interior, open cross sectional dimensions as the opening in the
fan-coil 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 sections. The
fan-coil unit shall meet the requirements of MIL-STD-1474.
4.5.2.13.1 Total sound power levels for sizes 1 and 2 fan-coil units. Test and calculation procedures to
determine total sound power levels for a fan-coil unit shall be performed in accordance with Figure 1 of AMCA 300.
4.5.2.13.2 Total sound power levels for sizes 3, 4, 5, 6, 7, and 8 fan-coil units. Test and calculation procedures
to determine total sound power levels shall be performed in accordance with Appendix H of AMCA 300, installation
type B: free inlet/ducted outlet. A discharge duct with an orifice device shall be installed to achieve design pressure
see Table I. When an orifice device is installed, it shall be a quiet type that does not produce excessive flow-induced
noise. The length of the discharge duct shall be such than the open end of the duct does not extend more than 3 feet
beyond the edge of the casing of the fan-coil unit. The microphone travel shall be such that a direct line of sight
between the microphone and the discharge opening shall be maintained.
4.5.2.13.3 Airborne noise test facility. The facility wherein measurements of fan-coil 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.
4.5.2.14 Permeability tests. A permeability test of material used in construction of fan-coil unit shall be
conducted in accordance with MIL-STD-2142, test 501.
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