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|  MIL-U-17293D(SH)
3.3.4 Physical dimensions. The overall dimensions of each unit heater
shall not exceed those as specified on figure 1. The location and dimensions
of the supply and return connections shall be as specified on figure 1.
3.3.5 Mounting dimensions. Mounting dimensions and mounting dimension
tolerances of each unit heater shall be as specified on figure 1.
3.3.6 Heating element. The heating element shall be of the extended
surface type consisting of at least 5/8 inch outside diameter (o.d.) seamless
copper tubing expanded into fins, having twin headers or shall be of the double
tube extended surface type consisting of at least 5/8 inch o.d. seamless copper
tubing expanded into fins, an inner tube of 3/8 inch o.d. seamless copper
tubing and a single header. The seamless copper tubes shall be in accordance
with ASTM B 75, and shall have a minimum wall thickness of 0.032 for 5/8 inch
o.d. tubes and 0.031 for 3/8 inch o.d. tubes. When used in the construction of
the heating element, hairpin tubes shall have a minimum wall thickness of 0.032
inch throughout the tube. Brazed joints of the heating element shall be made
with BCUP-5 silver brazing alloy in accordance with QQ-B-654.
3.3,6.1 Fins. Fins shall be of the smooth continuous plate type, not
less than 0.012 inch thick and shall be uniformly spaced. Fins shall have
rolled collars with tubes expanded into the collars to form a permanent bond.
3.3.6.1.1 The fin surface of the heating element shall be made of copper
known commercially as "electrolytic tough pitch copper".
3.3.6.2 Ferrules. Brass, copper or 300 series corrosion-resisting steel
ferrules shall be provided where tubes pass through tube sheets. Tubes shall
be expanded into ferrules to allow wear, to occur between the ferrules and
tube sheets due to shipboard vibration and expansion and contraction of the
tubes.
3.3.6.3 Header. The header(s) shall be made of copper and shall have a
wall thickness equivalent to type L tubing in accordance with ASTM B 88 of
equivalent size. The header(s) shall be, enclosed within the casing but not in
the air stream. The supply and return connections to the header(s) shall be
secured to the enclosure to relieve the header joints from stresses due to
external pipe whip. The enclosure shall be secured to the casing by continuous
welding or rivets spaced on not more than 2 inch centers.
3.3.6.3.1 The supply and return connections shall be fitted with type A
unions in accordance with MIL-F-1183 with pre-inserted rings. Union sizes
shall be as specified on figure 1.
3.3.6.4 When tested in accordance with 4.5.2.1, the heating element
shall show no leakage.
3.3.7 Casing. The unit heater casing (when not aluminum or corrosion-
resisting steel) shall be at least 0.059 inch thick when the coil face area is
1 square foot or less, and at least 0.104 inch thick when the coil face area
exceeds 1 square foot. Casings constructed of aluminum shall be at least 0.102
inch thick when the coil face area is 1 square foot or less, and at least 0.140
inch thick when the coil face area exceeds 1 square foot. Casings constructed
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