MIL-DTL-23581B(SH)
3.6.14.1 Heating unit. Heating unit shall be electric and of sufficient capacity to durably and efficiently
accomplish the drying requirements of 3.7.1. Heating elements shall be of the enclosed conduction type securely
fastened in place to withstand vibration. The elements shall be embedded in a refractory material, preferably
magnesium oxide. The resistance wire and the refractory material shall be enclosed in a seamless tubular corrosion-
resisting metal sheath which shall firmly hold the wire and refractory material in place. The sheath material may be
a nickel-chromium-iron alloy. The refractory material shall be tightly packed to prevent the wire from grounding to
the sheath when subjected to bending, forming, shock, or vibration at points where connections are made. A length
shall be left unheated to prevent damage to the connections and wire by overheating. The ends of the heating
elements shall be effectively sealed and protected to prevent the entrance of moisture. Connections to the elements
shall have locknuts and lockwashers or, if plug-in type elements are used, they shall be firmly secured in such a
manner that they will not break loose because of heat expansion and contraction.
3.6.14.1.1 Heater voltage. Unless otherwise specified (see 6.2), heater elements shall be suitable for operation
on 440-volt, 3-phase, 60 Hertz alternating current. Elements capable of operation on 220 volts, are permissible if
two elements of equal resistance are wired in series. The use of a transformer to obtain necessary voltage shall be
avoided. Heater elements shall be balanced on all three phases at rated voltage; watts input shall be within 5 percent
of the heater's nominal rated capacity.
3.6.14.2 Motors. Motors shall be of dripproof construction and of such design and capacity to operate the
equipment continuously at required speeds, under load, without exceeding allowable motor insulation temperature
requirements.
3.6.14.2.1 Motor voltage. Unless otherwise specified (see 6.2), motors shall be suitable for operation on 440-
volt, 3-phase, 60 Hertz alternating current.
3.6.14.3 Controls. Controls shall be conveniently located for the operator and shall be of dripproof
construction. Conductors and contactors shall be of a size and design to adequately handle current loads to which
subjected and provide durable service. Overload and low voltage protection shall be provided.
3.6.14.3.1 Temperature control. A thermistor type temperature controller shall control the temperature of the
air within the drying chamber by cycling the heating elements ON when the temperature falls not greater than 10 °F
below the set temperature setting and cycling the heating elements OFF when the set point temperature is attained.
The controller temperature settings shall be variable from a minimum setting of 120 °F to a maximum setting of 185
°F. The thermistor shall be securely mounted and located immediately downstream of the cylinder exhaust air flow.
A temperature display shall be provided for observation of the actual drying temperature. An indicator light shall be
provided to indicate when the heating elements are energized.
3.6.14.3.2 Timer. A timer shall control the length of the drying cycle. The timer shall have a variable setting
from 0 to 80 minutes. The timer shall either de-energize the heating elements for the last 5 minutes of the drying
tumbler cycle, or at timer completion, thermostatically control drying tumbler cooldown until the drying chamber
temperature drops to 135 °F before ending the drying cycle.
3.6.14.3.3 Safety thermostat. A high limit safety thermostat shall be installed immediately upstream of the
exhaust duct to sense the temperature of the air leaving the drying tumbler. The thermostat shall be wired such that
it will break power to the heating elements to prevent the heated air from exceeding 205 °F. The thermostat shall
automatically restore power to the heating elements when the drying tumbler conditions are safe.
3.6.14.4 Machine wiring. All machine wiring connecting components between the front cover/face plate and
devices internal to the drying tumbler shall be wired using plug connectors in order to provide convenient removal
of the front cover/face plate for disassembly/re-assembly, maintenance, and repair.
3.6.15 Machine mounting. The machine shall be mounted to a deck foundation. Resilient mountings or
springs shall not be used.
3.6.15.1 Machine mounting bolts and bolt holes. Bolt holes for mounting the Type I and Type II drying
tumblers to the deck shall be located on the machine base and bolts shall be provided. Type I drying tumbler bolt
holes and bolts shall keep the machine secured to the deck when subjected to the shock requirements specified in
3.7.5.
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