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|  MIL-U-17293D(SH)
collected or recovered from solid waste and reprocessed to become a source of
raw materials, as opposed to virgin raw materials. None of the above shall be
interpreted to mean that the use of used or rebuilt products is allowed under
this specification unless otherwise specifically specified.
3.2.2 Class 1 unit heater. The class 1 unit heater shall be constructed
of galvanized steel in accordance with ASTM A 525 with commercial thickness
zinc coating or carbon steel in accordance with ASTM A 569, except that the
heating element shall be made of copper or brass.
3.2.3 Class 2 unit heater. The class 2 unit heater shall be constructed
of aluminum, brass or 300-series corrosion-resisting steel having a perme-
ability of less than 2.0 after fabrication. The heating element shall be made
of copper.
3.2.4 Corrosion protection. Brass, copper, 300-series corrosion-resisting
steel, and galvanized steel are considered corrosion-resisting materials for
this application. When the corrosion-resistance of the 300-series corrosion-
resisting steel is degraded by fabrication processes, it shall be restored
by heat treatment.
3.2.4.1 Parts fabricated from other than corrosion-resisting materials
shall be protected against corrosion after fabrication by chemical electro-
lytic coatings such as plating, or the specified paints. The following methods
are considered corrosion protection methods when properly applied:
(a) Hot-dipped galvanized shall be in accordance with ASTMA 386
with the spelter conforming to special highgrade in accordance
with ASTM B 6.
(b) Electroplating with zinc shall be in accordance with ASTMB 633
followed by a phosphate treatment in accordance with method A
of ASTM D 2092.
(c) Hot phosphoric or chromic acid treatment, or a coating of primer
shall be in accordance with DOD-P-15328 followed by two coats
of primer in accordance with TT-P-664.
3.2.4.2 Threaded fasteners and such fastening or fittings as may be
used shall be of 300-series corrosion-resisting steel, or of steel coated in
accordance with MIL-S-1222.
3.2.5 Dissimilar metals. Where dissimilar metal contacts can not be
avoided, the materials shall be selected to minimize galvanic corrosion of the
heater.
3.3 Desire and construction.
Maintenance of the equipment shall be demonstrated
3.3.1 Maintenance.
in accordance with 4.6.
3.3.2 General construction. The unit heater shall consist of a fan and
motor, a twin header or double tube steam heating element and louvers, built
as a single unit, provided with a fan guard, constructed for overhead mounting,
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