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| A flameless ration
heater, or FRH, is a
water-activated
exothermic chemical heater included with
Meals, Ready-to-Eat (MREs),
used to heat the food.
US military specifications for the
heater require that it be capable of raising
the
temperature of an eight-ounce entree by
100° F in twelve minutes, and that it
display no visible
flame. |
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Chemical
reaction
Here is the balanced chemical reaction:
Mg + 2H2O ----> Mg(OH)2
+ H2 + Heat
The exothermic chemical reaction works by
the process of
oxidation-reduction, which is similar to
the process of rusting metals. When
iron metal rusts, it is oxidized by the
oxygen present in the air. In the case
of the flameless ration heater, the metal
used is
magnesium. Magnesium is chosen because
it is readily oxidized, which is due to its
having only two
electrons in its
valence shell. The magnesium metal is
mixed together with a small amount of iron
in a pouch. To activate the reaction, a
small amount of water is added, and the
boiling point of water is quickly reached as
the reaction proceeds. The role of iron in
the heater is most likely as a
catalyst, since it is used in a small
amount. In this case, the iron is initially
oxidized, and then reduced back to its
elemental form as the magnesium is oxidized
and sends its two valence electrons to iron.[1][2] |
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From How Stuff Works.com Most human beings much prefer a warm meal
to a cold one, especially if they're in cold
or wet conditions. Eating cold spaghetti or
cold beef stew is definitely no fun. A hot
meal, on the other hand, can lift a
soldier's spirits.
Because of the importance of a hot meal,
all military MREs come packaged with a
flameless heater. The flameless heater
uses a simple chemical reaction to provide
sufficient heat to warm the food.
Chemical heating is actually a pretty
widespread natural phenomenon. Everyone has
seen iron rust. Rust is a natural process in
which iron atoms combine with oxygen atoms
to create reddish, crumbly iron oxide. The
process is normally very slow, but we all
know that wet iron rusts faster. Iron
exposed to salty ocean water rusts the
fastest.
When iron turns to rust, the oxidation
process generates heat. But rust forms so
slowly that the heat generated is
unnoticeable. We are all familiar with much
faster oxidation reactions as well. For
example, when you "oxidize" the carbon atoms
in a charcoal briquette, they get quite hot.
We use the word burning to describe
this high-speed sort of oxidation.
The idea behind a flameless heater is to
use the oxidation of a metal to generate
heat. Magnesium metal works better than iron
because it rusts much more quickly. To make
a flameless heater, magnesium dust is mixed
with salt and a little iron dust in a thin,
flexible pad about the size of a playing
card. To activate the heater, a soldier adds
a little water. Within seconds the flameless
heater reaches the boiling point and is
bubbling and steaming. To heat the meal, the
soldier simply inserts the heater and the
MRE pouch back in the box that the pouch
came in. Ten minutes later, dinner is serve
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