Microwave heating cavities and applicators - Microwave-Assisted Concrete Technology: Production, Demolition and Recycling - page 231

Magnetrons

Heating Cavity

Bandstop
Filter

Bandstop
Filter

Open Port

Open Port

Sample

Conveyor Belt

Figure 6.30 Simplified schematic representation of a typical continuous-throughput
microwave heating system. (Reproduced by permission of IOP Publishing,
from Clemente-Fernández, F.J., Monzó-Cabrera, J., et al., A new sensor-
based self-configurable bandstop filter for reducing the energy leakage
in industrial microwave ovens. Measurement Science and Technology , 2012,
23 (6):065101. doi:10.1088/0957-0233/23/6/065101.)

where P is the total power required (in kilowatts); C p is the specific heat
of the load (in kJ/kg °C); h fg is the evaporation heat for the water present in
the load, which is typically equal to 2257 kJ/kg at 100°C; R sol is the rate of
evaporation; and Δ T is the temperature rise in degrees centigrade. It should
be noted that the required microwave power estimated using Equation 6.19
does not account for heat losses. In addition, the equation assumes that
microwave heating is the only source of heating energy in the continuous-
throughput cavity. This might not always be the case as additional sources
of heat energy such as hot gases are used commonly in many commercial
industrial microwave heating systems to preheat the load. In such cases, the
equation should be modified to account for the heat energy generated by
other supplementary sources of energy.
Alternatively, a more precise method to estimate the microwave power
required by such continuous-throughput microwave heating systems to
achieve heating at a particular temperature can be estimated by consider-
ing the relationship between the field intensity, microwave frequency, and
volume and dielectric properties of the load. The microwave power dissi-
pated in a load of volume V can be estimated using the following equation:

PKfEV
r

=ε 2 (6.20)

where K = 5.56 × 10 −11 , E is the electric field intensity in the load (V/m), f
is the microwave frequency (Hz), and ε " is the loss factor of the load. By