Cork insulation; a complete illustrated textbook on cork insulation ...

HEAT CONDUCTIVITY 125
It has aiso been proved that the effectiveness of insulators
depends upon their containing the greatest possible number of minute
air cells. The solid portions or thin walls of these air cells conduct
heat readily, but across the cells heat is conducted chiefly by radia-
tion. As explained in another section of this report, radiation in-
creases with the fourth power of the absolute temperature of the
heat exchanging surfaces, and this explains why in careful testing
we find that the insulating effect changes as the mean working temperature
is changed. The amount of change varies with each
material.
Units of Heat Conductivity.
Columns 6, 7 and 8 express the heat conductivity in various units
as defined. The physicist who prefers to work with the Centigrade-
Gram-Second system expresses his results in gram-calories of heat
passing in one second through a plate one centimeter square, one
centimeter thick, per one degree C. difference in temperature of the
two faces of plate.
Using this extremely small unit the conductivity even of silver
is equal to but 1 gram-calorie. For 6.9 lb. corkboard it is 0.00009275
gram-calorie. In order to eliminate from the tables at least three
of the decimals, the true numbers in column 6 are given as they
appear after multiplication by 1000. (It would be wrong to write
kilogram calories instead.)
The results of most European tests are expressed in technical,
metric system units, as shown in column 7. In this case the heat
flow is measured in kilogram-calories per hour passing through a
plate of one square meter area one meter thick, which may be writ-
ten as equivalent to 1 m' (1 meter cube) per degree C. difference in
temperature between hot and cold faces.
Finally in column 8 appear the values for heat conductivity in
technical English units, the figures as given representing the num-
ber of British thermal units (B.t.u.) passing per hour through a
plate of the material one square foot in area, one inch thick, and
per degree Fahrenheit difference in temperature of the two faces.
These last four words must be added, otherwise those who care-
lessly omit them invariably think it is understood that the difference
between warm and cold air each side of board is meant. This
mistake is cleared up in another section of this report.
Since in refrigerating plants heat must usually be removed
throughout 24 hours, it has long been the custom to use 24 hours
as the time unit for expressing the insulating effect of walls, etc.
Outside of the laboratory temperature conditions due to atmospheric
changes (sun, wind, rain) are never constant throughout 24
hours, and so the committee has decided to adopt the hourly basis
for measuring heat flow. This also conforms with the practice of
other than refrigerating engineers.