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MIL-P-27431B(USAF)
enclosure, an inner enclosure, an on-off toggle switch, a heater indicator light, a
power on indicator light, inlet and outlet connections, thermal fuse and a power
cable.  The heat exchanger assembly shall be designed for at least 2000 hours of
operation.
3.7.1.1 Thermostatic controls.  The thermostatic controls of the assembly shall
not be preset during manufacture and will be adjustable after assembly to a maximum
of 250 F.  Adjustments shall be for the purpose of calibration.  Controls are not to
be located in such a way to encourage adjustment except during calibration.
3.7.1.2 Inner enclosure.  An inner enclosure shall be provided around the heater
chamber.
It shall be inside the outer enclosure and be separated from it in such a
way that free air passage is permitted between the inner and outer enclosures.
3.7.1.3 Outer enclosure.  A protective outer enclosure shall be provided around
the heater chamber and inner enclosure in such a manner as to permit safe handling of
the heat exchanger assembly during operation and to permit free air passage between
the inner enclosure and the outer enclosure.  A fixed position handle shall be
provided on the outer enclosure.
3.7.1.4 On-off switch.  An MS25306-222 on-off switch shall be mounted on the
base of the outer enclosure.  The power cable specified in 3.7.1.8 shall be direct
wired to the switch and heater chamber.
3.7.1.5  Indicator lights.  A neon lamp indicator light shall be mounted on the
outer enclosure near the on-off toggle switch and shall be so connected that the lamp
will light only when power is applied to the heating element.  The lamp shall be
mounted behind a faceted green lens.  The lamp shall incorporate a series resistor
small enough to insure that the operator may readily discern that the lamp is lit
under bright sunlight conditions but large enough to insure an average rated life of
2,000 hours or more of operation.  A red "power on" indicator light shall also be
provided to the above requirements except that it shall show when the power is on and
not cycle with the heating element.  The two lights shall be
clearly marked.  At the discretion of the manufacturer, the lamps and lenses may be
mounted on the heater chamber with a corresponding opening provided on the outer
enclosure.
3.7.1.6 Inlet and outlet connections.
MS33656-5 heat exchanger inlet and outlet
connection shall be provided.
3.7.1.7 Thermal fuse.  A thermal fuse shall be provided as a part of the heat
exchanger assembly to prevent overheating of the unit in the event of thermostat
malfunction.  The thermal fuse shall be set at a predetermined value above the
operating temperature of the heat exchanger assembly and acting as a back-up safety
device.  A resetable breaker is desirable.  However, a fuse, readily accessible for
inspection and replacement is acceptable.
3.7.1.8 Power cable.  The
purging unit shall be equipped with a 20 foot, three
wire power cable conforming to
MIL-C-5756.  The wire shall be size 18 or heavier.
One wire of the cable shall be
securely grounded to the heat exchanger assembly.  The
free end of the cable shall be
equipped with a UP121M plug connector conforming to
W-C-596/142.  Substitute power
cable or plug must be approved by the contracting
activity.
6

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