Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

NONIONIZING RADIATION SAFETY 769
in which the material to be heated is the dielectric. Most dielectric materials—such
as ceramics, wood, paper, plastics, etc.—absorb energy when placed in an alternating
electric field, because these “nonconductors” exhibit some degree of conductivity
as polar molecules rotate under the influence of the alternating electric field. The
loss tangent, or the degree of lossiness of a dielectric material, is a measure of energy
absorption by this mechanism. (Lossiness should not be confused with leakage of
charge through an insulater, as in the case of the pocket dosimeter shown in Fig.
6-2.) The degree of lossiness for any given material, as shown in Eq. (2.63), is a
function of the frequency of the alternating electric field. Generally, the efficiency
of dielectric heating is 50–60%. An application of industrial dielectric heating is
given in the following example:
W Example 14.22
A total of 500 kg of resin-bonded sand cores are to be baked in a dielectric-core oven
every hour. The sand mix contains 3% water, and resin that cures at 107 ◦ C. If the
specific heat of the resin-sand mixture is 1250 J/kg/ ◦ C (0.3 Btu/lb/ ◦ F), the ambient
temperature is 18 ◦ C, and the overall efficiency is 50%. Calculate
(a) the power required to heat the water in the mixture from 18 ◦ C to 100 ◦ C
(specific heat of water = 4178 J/kg/ ◦ C).
(b) the power required to vaporize the water (latent heat of vaporization of water
= 2.25 × 10 6 J/kg).
(c) the power required to heat the sand–resin mixture from 18 ◦ Cto107 ◦ C.
(d) the total power required.
Solution
(a) The required power to heat the water in the mixture from 18 ◦ Cto100 ◦ Cis
P = 0.03 × 500 kg
h (100◦C − 18 ◦ C) × 4178 J/kg
◦C 1 hour
J
×
= 1428
3600 seconds s .
(b) The required power to vaporize the water is
P = 0.03 × 500 kg
h × 2.25 × 106 J
kg ×
= 9375 J
s .
1 hour
3600 seconds