Man's physical universe

xanabras

AIR CONDITIONING 283

Heat conductivities are of considerable practical importance. A

study of the above table explains why a steel railroad track will be

almost too hot to touch on a hot day, while the wooden ties under the

track will feel much cooler, assuming equal absorption coefficients.

Wood feels cooler because heat is quickly conducted away from it to

the skin at points of contact which thus come down to the temperature

of the skin. Steel, being a much better conductor, requires that a much

greater mass be cooled by absorption by the body before the temperature

of the point of contact can be reduced to skin temperature. A rug

feels warmer to bare feet on a cold morning than hard wood because

less heat is required to warm up the portion of the rug in contact with

the skin, partly because the rug is a poorer conductor of heat and

partly because there are fewer points of contact. The walls of refrigerators,

the walls of buildings, hot-water pipes, ovens, etc. must be

insulated to keep heat in or out, whichever the need may be.

In other

cases good heat conductivity is desired as in the case of cookingutensils.

Waterless cooking-utensils, which cook food at relatively low temperatures

without loss of flavors through escaping steam and without

the loss of valuable minerals, vitamins, sugars, and other soluble substances

by solution in the boiling water, later to be discarded, are

excellent applications of heat conductivity. When thin aluminum or

iron utensils are used for cooking, water must be used to conduct the

heat from the very hot bottom of the pan to the food to be cooked,

because otherwise the food at the bottom of the pan would be burned

and the food toward the top would not be cooked. The insides of the

bottoms of such utensils are much hotter than in the case of waterless

cooking-utensils with thick bottoms.

In the latter case, inasmuch as

the heat is transmitted almost equally over the bottom, sides, and top

of the pan, the utensil acts as a small oven. Incidentally, it might be

added that most of the advantages of waterless cooking may be obtained

by the use of parchment paper, in which the food is tied up and

then cooked in the usual way in a pot of water.

Inasmuch as air is a very poor conductor of heat, layers of air

between windows (storm windows) and in the walls of buildings act as

good insulators. The minute air spaces in woolen textiles account for

their warmth.

Fireless cookers are devices by which the heat used to start the

cooking is prevented from loss by conduction so that the heat may thus

complete the cooking-process.

2. Convection. The transfer of heat by convection currents of liquids

or gases, which results from their expansion and consequent decrease

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