Page 145 - Industrial Ventilation Design Guidebook

4,3 HEAT AND MASS TRANSFER 107

where 8 is the distance corresponding to the temperature difference, For the
three heat transfer forms,
« Conduction, Eq. (4.162)
» Convection, Eq, (4.156)
• Radiation, Eq. (4.161)
we have, respectively,

Following from Ohm's electrical law (theory of electricity), a heat resis-
tance can be defined:
potential difference — resistance • current
temperature difference = heat resistance • heat flow

The conductance or the coefficient of heat transfer U — 1 /R, or

For conduction the heat resistance is the distance divided by the heat con-
ductivity, R — 8/XA, and the heat conductance is heat conductivity divided by
distance, U — A.A/8, For convection and radiation the heat resistance is 1 di-
vided by the heat transfer factor, 1/aA, and the heat conductance is the same
as the heat transfer factor, U = aA. A coefficient of heat flow is also used, the
K value, which is the total conductance:

The following connecting rules are based on the above analogy:
heat resistance jR in series connection

and in parallel connection

heat conductance series connection

and in parallel connection