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

106 CORK INSULATION
age wall <strong>insulation</strong>, this temperature difference would be the
difference between the temperature of the surface of the wall
and the average temperature of the surrounding air ; while
the velocity of the air across the surface of such wall must
affect the coefficient, or rate, of heat transfer by convection
and radiation.
The values for the coefficient of convection and radiation
for various materials undeY still air conditions are given in the
accompanying table, and are based upon experiments made at
the Engineering Experiment Station of the University of
Illinois.
This coefficient is generally denoted by the symbol K^,
and is called the coefficient of radiation and convection for
inside surfaces. In an actual plant, the outside walls are
exposed to the more rapid movement of the air, so that the
coefficient of radiation and convection is larger for the outside
surfaces. The symbol for this coefficient is Kg, and it is, in
general, 2.5 to 3 times the inside wall coefficient K^, due to
the greater velocity of the outside air. Thus, as a general
rule, the value of the outside coefficient, Kj, may be con-
sidered to be three times the inside coefficient, K^.
COEFFICIENTS OF RADIATION AND CONVECTION (Kl) IN B.t.u. PER
HOUR PER DEGREE TEMPERATURE DIFFERENCE F.
Material Coefficient Ki
Brick wall 1.40
Concrete 1.30
Wood 1.40
<strong>Cork</strong>board 1.25
Magnesia board 1.45
Glass 2.00
Tile plastered on both sides 1.10
Asbestos board 1.60
Sheet asbestos 1.40
Roofing 1.25
69.—Total Heat Transfer.—In its simplest form, total heat
transfer is the heat passing into, through and out of a single
wall of given area. If the surface temperatures and the
temperatures in the surrounding air are taken, the total heat
transmission may be separated into internal and external con-
ductivity, the external conductivity being sometimes called
"surface effects." In the case of a good insulator, as used for
cald storage rooms, internal conduction is the essential factor;
while in the case of a poor insulator, as the metal in a boiler
tube, good conduction is necessary and surface transmission