Hand warmers

Hand warmers are small (mostly disposable) packets which are held in the hand and produce heat on demand to warm cold hands. They are commonly used in outdoor activities. Other types of warmers are available to provide soothing heat for muscular or joint aches.

Depending on the type and the source of heat, hand warmers can last from 30 minutes (recrystallisation) up to 24 hours (platinum catalyst).

Created by Japanese inventor Niichi Matoba. Matoba received a patent for applying the principle of an oxidation reaction that produces heat by means of platinum catalysis. He then devoted his time to researching how to make the product suitable for practical use. In 1923, he manufactured a prototype of his device naming it HAKUKIN-kairo (HAKKIN warmer). A version of these original portable hand warmers is still produced in Japan.

Air activated hand warmers contain cellulose, iron, water, activated carbon, vermiculite (water reservoir) and salt and produce heat from the exothermic oxidation of iron when exposed to air. They typically emit heat for 1 to 10 hours, although the heat given off rapidly diminishes after 1–2 hours. The oxygen molecules in the air react with iron, forming rust. Salt is often added to catalyze the process.

Hand warmers can be recharged by immersing the hand-warmer in very hot water until the contents are uniform and then allowing it to cool. The release of heat is triggered by flexing a small metal disk in the hand warmer, which generates nucleation centers that initiate crystallisation. Heat is required to dissolve the salt in its own water of crystallisation and it is this heat that is released when crystallisation is initiated.

The latent heat of fusion is about 264–289 kJ/kg.

This process can be scaled up to provide a domestic heating store developed by Sunamp Scotland

Lighter fuel hand-warmers use lighter fluid (petroleum naptha). These can be re-used by simply refuelling. Typical models can generate heat for either half a day or a whole day, depending on conditions.

Battery operated hand warmers use electrically resistive heating devices to convert electrical energy in the battery. Typically hand warmers can survive for 6 hours or less, with heat outputs from 40-48C. The Rechargeable Electronic Handwarmers can be charged from a mains power supply or from a 5V USB power supply, with 500 recharge cycles indicated.

...
Wikipedia