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Drying - Heat Capacity and Thermal Conductivity


If you have deduced from the preceeding few blogs that heat is one of the major factors in drying, you are right.  But before we talk more about heat, let's "set the stage."  Heat is that stuff that increases temperature of a substance when added and reduces temperature when absorbed.  Temperature is the level of excitement of the atoms or molecules of a substance. Heat Capacity - Different materials require that different amounts of heat be added or absorbed per unit of volume to achieve a given change in temperature.  Water, for example, requires a great deal more heat to achieve a given temperature change than does something like an equal volume of air.  Likewise more heat can be absorbed from water than from air.  Water, then, can "store" much more heat than air.  Consider this simple example -  - If you have two towels, one dry and the other wet, and you put them in an oven at a temperature of 200 degrees Fahrenheit, which towel will remain hot longer once you remove them from the oven?  Anyone who has been offered one of those hot towels on a long airplane trip knows that the wet towel will retain its temperature much longer than a dry towel.  For much the same reason, you fill a hot water bottle with hot water, not hot air to keep it warm for a long period of time.  Based on weight, water has about 4 times the heat capacity of air (and remember the air requires a much greater volume to produce the same weight).  Steel has about 1/2 the heat capacity of air (per weight) while aluminum has a heat capacity about double that of steel but still not equal to that of air.  So, air has a reasonable ability to hold heat but it requires a lot more air than water to hold a specific amount of heat. Thermal Conductivity - Thermal conductivity is a measure of the capability for heat to move through a material.  Again, a simple example - - Consider an oven set to a temperature of 350 degrees Fahrenheit.  This is a common temperature used for baking in your oven at home.  Even though the temperature of the air in the oven is 350F, most of us wouldn't hesitate to stick our hand in there and move it around for a few seconds.  HOWEVER, if there is a pie baking in that oven and it has been in there long enough for the pie dish to have achieved a temperature of 350F, I think we would all pass on touching that pie dish and use a pot holder instead.  The reason we would do that is because the pot holder is made of materials that have low thermal conductivity.  In relative terms, water conducts heat about 25 times better than air.  Steel conducts heat about 4 times better than water while aluminum and copper conduct heat 3.5 and 7.5 better than steel, respectively.  So when it comes to thermal conductivity, water is a far superior conductor of heat than is air. In drying, we are looking to move heat from place to place to change temperatures to achieve particular goals.  As we continue on our discussion of drying in upcoming blogs, Heat Capacity and Thermal Conductivity are concepts that will be used to explain the mechanics of the drying process in more detail.

-  FJF  -