Heat pack with expansion capability

A flexible disposable heating device conformable to a shape defined by its surroundings comprising a container having:

• a first zone containing a fuel;
• a second zone containing an oxidizer;
• a collapsed third zone capable of serving as an expansion chamber;
• a first frangible separator disposed between the first zone and the second zone, the first frangible separator being manually operable to provide communication therebetween, defining a reaction chamber comprising at least one of said first and second chambers;
• a second frangible separator responsive to an exothermic chemical reaction within the reaction chamber, the second frangible separator being operable to provide vapor communication between the reaction chamber and the third zone;
• wherein communication between the first zone and the second zone allows mixing of the fuel and the oxidizing agent to initiate an exothermic chemical reaction capable of generating a vapor;
• and
• wherein an environmental parameter associated with the exothermic chemical reaction operates the second frangible separator, permitting said vapor to flow into said third zone, thereby reducing pressure in the reaction chamber.