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Published byArlene Cook Modified over 9 years ago
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Sara Beth Horton
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Many different styles Two main types ◦ Iron Oxidation ◦ Salt Crystallization How do they work? What are the differences?
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Produce low level heating Sustain temperature for long periods of time Convenient packaging
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Utilizes exothermic reaction of oxidizing iron (making rust) ◦ 4Fe + 3O 2 2Fe 2 O 3 Irreversible reaction: one time use Produces temperatures from 95-140 °F Can last from 5-24 hours Wide range of styles ◦ Hand warmers ◦ Toe warmers ◦ Therapeutic heating pads
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Contain 7 ingredients ◦ Iron ◦ Oxygen ◦ Water ◦ Vermiculite / sodium polyacrylate ◦ Activated carbon ◦ Salt ◦ Cellulose/sawdust Can change the properties of the heat packs by altering the components
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Iron ◦ Powdered works best, more surface area for reaction ◦ More iron means more heat Oxygen ◦ Outer package is airtight to prevent reaction while in storage ◦ Heat pack wrapping is air permeable, transfer of air is generally not restricted Water ◦ Not present in overall reaction, but required to initially reduced oxygen
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Salt ◦ Catalyzes reaction – promotes ion formation Vermiculite/Sodium polyacrylate ◦ Reservoir for water ◦ Acts as insulation Activated carbon ◦ Disperses heat throughout pack Cellulose/Sawdust ◦ Insulation and filler
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Utilizes exothermic phase change of crystallizing salt ◦ Releases heat of fusion Reversible process: reusable heat packs Generally use Sodium Acetate ◦ Produces temperatures of ~130°F ◦ Last from 15 minutes – 3 hours
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While changing phase, compounds remain at their boiling/melting points until all of the product is fully converted If a salt solution has a melting point of 130°F: ◦ Heat solution to 150°F to make liquid phase ◦ Cool slowly down to room temperature ◦ When the system is perturbed, crystallization is initiated and the temperature instantly increases to the melting point of 130°F
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(Specifically tri-hydrate form) Melting point is 130°F Very stable when sub-cooled But still easy to initiate crystallization Non-toxic, used as a food preservative
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Temperature ◦ Melting point of tri-hydrate is 130 ◦ Can use less concentrated solution for lower average temperature ◦ Outer casing will provide insulation for lower surface temperature Duration ◦ Size is only design variable
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One-time-use vs. Reusable Variability vs. Consistency
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