Flameless ration heater
A water-activated chemical heater for U.S. military MREs.
A flameless ration heater (FRH), also known as an MRE heater, is a self-heating device included in U.S. military Meal, Ready-to-Eat rations since 1993. It consists of a plastic bag containing magnesium and iron powders with table salt; adding water triggers an exothermic reaction that boils the water and heats the food pouch.
Quick Facts
- First issued
- May 1990
- Research began
- 1973
- Research center
- U.S. Army Natick Research
- Development
- and Engineering Center
- Natick
- Massachusetts
- Heating time
- 12 to 15 minutes
- Target temperature
- about 60 °C
Facts from the source article.
History
Before the FRH, service members heated meal pouches by boiling them in a canteen cup over a Sterno gel or portable stove, a slow process especially in cold weather and difficult in wind or wet conditions. The visible flame was undesirable at night. Some placed pouches on a hot vehicle's engine block or exhaust manifold. Because of these difficulties, meals were often eaten cold. Research into a flameless heater began in 1973 at the U.S. Army Natick Research, Development, and Engineering Center. A water-activated magnesium-carbon chemical heating product was patented. In 1980, Natick learned the U.S. Navy had developed a magnesium-iron alloy powder for buoyancy devices and heated diving vests; this was more cost efficient, so the University of Cincinnati was contracted to develop a prototype called the Dismounted Ration Heating Device (DRHD). The inventors incorporated as Zesto-Therm Inc. and patented the ZT Energy Pad for civilian sale. In 1986 the Army evaluated the ZT Energy Pad and found it did not always heat food adequately and left a messy residue. A focus group of 26 soldiers compared heating an MRE with a Zesto-Therm pad versus the canteen cup method with a trioxane fuel bar; 100% preferred the flameless heater for being compact, disposable, and requiring no equipment to carry and clean, though it was about twice as expensive as a trioxane bar. In cold climates two or three trioxane bars were needed, making the FRH cheaper overall. Other prototypes included the Mounted Ration Heating Device (MRHD), an electrical device powered from a vehicle's supply that could heat up to four rations at once; it was generally preferred over the Zesto-Therm pads, but not all vehicles had proper connections, and a single device meant service members had to take turns.
Usage
The FRH comes in a plastic bag with printed instructions. Inside is a small quantity of metallic powders. To use, the bag is torn open, a sealed food pouch is placed inside, and about 1 US fluid ounce of water is added up to a marked line. The chemical reaction starts immediately and takes 12 to 15 minutes to heat the pouch to about 60 °C. It is recommended to place the bag inside the MRE's cardboard carton to prevent injury and to prop it upright so water does not leak out.
Chemical reaction
The heater works by an oxidation-reduction reaction: water oxidizes magnesium metal (Mg + 2H₂O → Mg(OH)₂ + H₂ + heat). This reaction is normally as slow as rusting iron, so iron particles and table salt are mixed with the magnesium. When water dissolves the salt, it forms an electrolyte, turning each magnesium and iron particle into a tiny short-circuited battery that rapidly produces heat, a process called supercorroding galvanic cells. One brand uses 7.5 grams of a powdered magnesium-iron alloy (95% magnesium, 5% iron by weight), 0.5 grams of salt, plus inert filler and anti-foaming agent. With 1 US fluid ounce of water, this mixture can raise an 8-ounce meal packet by 100 °F in about 10 minutes, releasing about 50 kJ of heat at about 80 watts. The main disadvantage is the production of hydrogen gas, which is rarely a risk in the field but may pose a fire hazard for consumer use. Alternative formulations to eliminate hydrogen gas include aluminum chloride with calcium oxide (AlCl₃/CaO) and diphosphorous pentoxide with calcium oxide (P₂O₅/CaO).
Confined space hazard
The testing used commercial grade heater meals consisting of an unenclosed flameless heat pouch, a bag of salt water, a styrofoam saucer/tray, and a meal in a sealed microwavable/boilable bowl.
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