MIL-PRF-22594C(SH)
3.11.4 Heater casing material. The casing shall be manufactured from corrosion-resisting materials and be
equal or greater in yield strength (tensile and compressive) to 0.045-inch thick 304 stainless steel when the heater
face area is 0.750 square feet or less and shall be equal or greater in yield strength (tensile and compressive) to
0.056-inch thick 304 stainless steel when the heater face area exceeds 0.750 square feet. Casing shall be of
watertight construction.
3.11.5 Enclosure for electrical connections. The enclosure for electrical connections shall be on the side of the
heater casing and shall be of spraytight construction, except the enclosure where the heating elements enter, which
shall be watertight as specified in MIL-STD-108. Provisions shall be made for the drainage of condensed moisture.
A removable cover plate shall be provided to permit access to the electrical connections, parts, and mounting of
heating elements. A separate removable cover plate shall be provided in the bottom of the enclosure to permit
installation of a cable terminal tube by the installing activity. Surfaces which mate with the removable cover plates
shall be gasketed or otherwise made suitable to provide a spraytight seal.
3.11.6 Connections (flanges) for ductwork and mounting flanges. To permit replacement, connecting flanges
for ductwork shall be provided with 13/32-inch diameter holes (or optional 3/8-inch 16 UNC 2B threaded holes), as
shown on figure 1, and shall be on 3-inch centers, working from the flange corners towards the air inlet and air outlet
center lines. When the space between the centerline and the adjacent hole exceeds 2 inches, there shall be a hole on
the centerline. Tolerance for the distance between hole centers and the location of the corner holes shall be
±1/64 inch. Mounting flanges shall be provided with ½-inch diameter holes (or optional ½-inch 13 UNC 2B threaded
holes), as shown on figure 1. Optional mounting flange center connections are shown for Size 33EH heaters and
larger; these locations are given if additional mounting connections are required to pass shock testing.
3.11.7 Intermediate supports. Spacing of intermediate supports, where required, shall not exceed 23 inches
between supports.
3.11.8 Heating element. Heating elements shall consist of a helical coiled resistance wire centered in a densely
packed insulating material enclosed in a hermetically sealed finned metal sheath. Heating elements shall be
provided with a terminal at each end of the element and a means of attaching the element to the casing such that the
casing is watertight.
3.11.9 Gaskets. Gaskets shall be of heat- and moisture-resistant material.
3.11.10 Marking of terminal board. Terminal boards shall be clearly and permanently marked.
3.11.11 Wiring diagram. A reduced size copy of the heater wiring diagram shall be permanently printed on a
material not degraded by high temperature or moisture and shall be permanently attached to the inside of the
enclosure access cover for the electrical connections.
3.12 Performance. Performance of the heater for a rated kilowatt at specified airflows and delta temperature
rise (ĆT equals leaving air DB temperature minus entering air DB temperature (10 °F)) shall be as specified in
table I. Tolerances for airflow shall be within ±5 percent and for ĆT within ±1 ºF of that specified in table I. For
test purposes, heaters are tested in accordance with table I.
3.12.1 Heating element. Heating elements shall also be in accordance with the following:
a.
Fins and hermetic seals shall be positively bonded to the metal sheath.
b.
Maximum surface temperature of sheath shall not exceed 750 °F at minimum airflow rating as
specified in table I.
c.
Moisture-resistant.
d.
Capable of continuous operation at rated kilowatt with rated airflow.
e.
21,000-hour shipboard operating life expectancy. Shipboard life expectancy of less than 21,000 hours
is defined as a test result of fusion prior to 85 watts per square inch of sheath heating surface area
(WSIA) when tested in accordance with 4.5.1.4. A shipboard life expectancy of less than 21,000 hours
shall be cause for rejection. The element does not pass if it fails by fusion prior to 85 WSIA.
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